SMG-6 - Low Impact Design and Techniques Table of Contents

Transcription

1 SMG-6 - Low Impact Design and Techniques Table of Contents SMG-6.1 General... 1 SMG-6.2 Low Impact Design vs Conventional Approaches to Stormwater Management... 1 SMG-6.3 Low Impact Design Techniques... 2 SMG-6.4 Clustering and Lot Configuration... 2 SMG-6.5 Reducing Impervious Surfaces... 5 SMG-6.6 Minimum Site Disturbance... 6 SMG-6.7 Constructing Biofiltration Practices... 9 SMG-6.8 Stormwater Reuse... 9 SMG-6.9 Creating Natural Areas SMG-6.10 Design Procedures for Incorporation of Low Impact Design into Site Design.. 10 SMG-6.11 Measuring Success SMG-6.12 Other Types of Development SMG-6.13 Costs of Low Impact Design SMG-6 Appendix A Case Studies: Low Impact Design Implementation SMG-6 - Appendix B - Site Resources SMG-6 Appendix C Bush Revegetation SMG-6 Appendix D Low Impact Design Costing SMG-6 Appendix E New Zealand Case Studies: Cost Comparison on Low Impact Design for Subdivisions SMG-6 Appendix F International Case Studies: Cost Comparison on Low Impact Design for Subdivisions Page 1 Updated 01/11/2012

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3 SMG-6.1 General Low Impact Design (LID) is an inter-disciplinary design approach to stormwater management which seeks to deliver integrated solutions and achieve multiple benefits from land development. The term LID, adopted in New Zealand, comes from the United States of America. In Australia a similar design approach is referred to as Water Sensitive Urban Design (WSUD) and in the United Kingdom it is known as Sustainable Urban Drainage Systems (SUDS). The definition and principles of LID (Auckland Council 2012) are: "Low Impact Design seeks to protect, enhance and ultimately utilise natural systems and processes for enhanced stormwater management, ecosystem services and community outcomes" The principles for Low Impact Design include: a) Promote Inter-Disciplinary Planning and Design: This requires input from a range of disciplines early in the design process e.g. planners, ecologists, engineers, so that multiple benefits can be achieved. b) Protect the Values and Functions of Natural Ecosystems: Existing site resources such as wetlands and riparian buffers, as well as having intrinsic value, can also serve to reduce the effects of stormwater quality and quantity. Retention and protection of site resources are further discussed in SMG-6 Appendix B Site Resources. c) Avoid Stormwater Effects as Close to Source as Possible: This is discussed in SMG-6.3 Low Impact Design Techniques below and can also be achieved by following the treatment train approach as further described in SMG-8 Choosing a Stormwater Management Device. d) Utilise Natural Systems and Processes for Stormwater Management: Natural systems can provide both quality and quantity benefits. This is undertaken by processes such as infiltration, evapotranspiration, filtration and microbial action (Refer to SMG-9 Appendix B Stormwater Treatment Processes). Devices such as raingardens, filter strips and wetlands can be constructed in order to provide these treatment capabilities. Further information on the capability of these devices, including design parameters is provided in SMG-9 Detailed Stormwater Management Practice Design. SMG-6.2 Low Impact Design vs Conventional Approaches to Stormwater Management Traditionally stormwater infrastructure has consisted of hard engineering approaches such as concrete pipes and channels. While this can be an efficient means of conveying stormwater to manage flood risk, they also have limitations in terms of concentrating flows and contaminants. Low Impact Design (LID) approaches focus on the principles of avoid, remedy and mitigate, preventing stormwater runoff generation at source and using natural process to manage quality effects. Page 1 Updated 01/11/2012

4 However, LID is a design approach, which examines each site on its own merits to achieve the best outcome. It is therefore a best-fit for situation approach and may include a combination of conventional structures and natural systems. SMG-6.3 Low Impact Design Techniques There are a number of key site design techniques which are considered to be Low Impact. These are: a) Clustering and Lot Configuration. b) Reducing Impervious Surfaces. c) Minimum Site Disturbance. d) Constructing Biofiltration Practices. e) Water Reuse. f) Creating Natural Areas. SMG-6.4 Clustering and Lot Configuration How a site is developed and to what degree the entire site must be utilised will have a significant impact on sediment and stormwater runoff from the site. Conventional land development encourages sprawl, while other approaches to land development can provide significant sediment reduction during construction and post-construction stormwater benefits. Cluster development encourages smaller lots on a portion of a site, allowing the same site density but leaving more site area in open space and disturbing less of the natural ground cover. Clustering entails designing residential neighbourhoods more compactly, with smaller lots for narrower single-family homes, as are found in traditional villages and small towns. Cluster development can provide for protection of site natural areas, while at the same time reducing total site imperviousness by reducing the areal extent of roads. SMG-6 Figure 1 House Clustering Schematic for a Residential Subdivision Page 2 Updated 01/11/2012

5 Stormwater management is optimised when stormwater objectives are integrated into site planning from the earliest stage. The process translates into concentrating or clustering development so that the most environmentally sensitive areas of the site are left undisturbed or are subject to minimal disturbance although there may be aspects of site design that cannot be readily incorporated within a conventional understanding of clustering. Most of the discussion here focuses on various aspects of clustering that have evolved during recent years. Clustering offers tremendous potential in terms of stormwater benefits and overall resource protection. While clustering is an important approach to site development it can also provide huge benefit when considered at a catchment level. SMG-6 Figure 2 Example of Clustering on a Residential Development Although some density bonuses may be offered which increase density, clustering in a strict sense usually begins after the basic determination of how much of what type of use, a certain number of single-family residences for example, already is to be permitted parcel-byparcel. In some cases, parcels may be combined to produce a broader development pattern, but a typical clustering design should reflect the existing pattern of ownership if it is to function properly. In some cases, the clustering concept may be structured to include different types of development, including single family and apartment concepts. As a Low Impact Design (LID) approach, clustering is important. From a stormwater management perspective, clustering minimises stormwater and contaminant loading generation from the outset and therefore is preventive in nature. To maximise positive stormwater effects, clustering works well when used in conjunction with other low impact design approaches and practices. In many cases, a tight clustering approach to site design facilitates these other approaches and practices and even makes them possible. In order to achieve maximum benefit such as shown in SMG-6 Figure 3 Conventional Approach to Site Development versus an LID Approach, substantial design flexibility must be maintained. Clustering can be made to work effectively on a small site or a large one, but clearly the standards imposed on a 40 hectare site need to be different, possibly significantly different, than the standards imposed on a 4 hectare site. Clustering may involve lot design and arrangement only or clustering may transcend lot design and even involve changing Page 3 Updated 01/11/2012

6 types of residences e.g. smaller lot sizes and changes in setback provisions. The challenge is to create a clustering system, which maximises clustering benefits such as open space preservation even as developer incentives are maximised as well. SMG-6 Figure 3 Conventional Approach to Site Development versus an LID Approach (blue areas denote stormwater management areas) These techniques are discussed further within the guidelines however it should be noted that subdivision layout and configuration is regulated by the District and City Plans. SMG Clustering Considerations Other important issues to keep in mind when considering clustering include: a) Are meaningful open space requirements established? b) Do these open space requirements vary with site size, type of use allowed, etc.? c) How is open space controlled and managed over the long term? d) Have water supply and wastewater provisions been incorporated? Page 4 Updated 01/11/2012

7 e) Have private property management systems been incorporated to the maximum extent feasible? f) Does the need for a private property management association discourage use of a clustering option? SMG Clustering Benefits Benefits achieved from clustering can be considerable such as: a) Reduction in imperviousness. b) Reduction in contaminant loadings. c) Preservation of special values and sensitive features. d) Habitat protection and associated wildlife benefits. e) Protection of aesthetic values. f) Passive recreation and open space maintenance. g) Reduction in costs, both development and operational Although reduced imperviousness is dealt with separately later, it is such an important benefit from clustering that it deserves special mention. Holding all other aspects of the development constant (number of units, types of units), clustering significantly reduces impervious coverage. Impervious reduction is achieved mostly through reduced road construction and reduced driveway lengths. Given the direct relationship between imperviousness and stormwater generation, impervious area reduction can be expected to result in a comparable reduction in stormwater generation, both total volume and rate. SMG Clustering Costs Clustering has the potential to reduce costs through reduced land clearance, reduced road construction (including kerbing), reduced pathway construction, fewer street lights, less street tree planting, less landscaping, reduced sanitary sewer line and water line footage, reduced stormwater pipelines, reduced sizing or need for stormwater management ponds and other related infrastructure reductions. Further details on cost, as well as cost comparison case studies can be found in: a) SMG-6 Appendix D Low Impact Design Costing. b) SMG-6 Appendix E New Zealand Case Studies: Cost Comparison on Low Impact Design for Subdivisions. c) SMG-6 Appendix F International Case Studies: Cost Comparison on Low Impact Design for Subdivisions. SMG-6.5 Reducing Impervious Surfaces Impervious surfaces are an essential factor to consider in stormwater management both from a quality and quantity standpoint. Impervious surfaces (roads, roofs, footpaths) prevent the passage of water through their surface into the ground. Water must then be transported across the surface to a point of discharge. Site-by-site and catchment-by-catchment Page 5 Updated 01/11/2012

8 increased impervious cover means increased stormwater generation with increased contaminant loadings as well. Consequently actions that can be taken that reduce impervious cover become important stormwater management strategies. While still meeting minimum road width requirements, residential subdivisions can reduce the width of roadways or design the roadways to limit the total length needed to service individual properties. In conjunction with imperviousness, roof spouting may be directly connected to streets or reticulation systems when providing splash blocks and discharging the water across grass and away from impervious surfaces that will allow for a greater amount of water to infiltrate into the ground. In many cases these Low Impact Design approaches can stand-alone and be used development-by-development although reduction in imperviousness also can be used in tandem with other approaches and practices. Options may include: a) Street / Footpath width reduction. b) Use of cut kerbs to disperse stormwater flow that would then travel into a vegetated swale or similar. c) Use of permeable paving. SMG-6 Figure 4 Example of a cut kerb in Albany, Auckland An important factor in limiting impervious surfaces and separating roof drainage from direct connection to streets is the need for community awareness to ensure continued function of these practices. Homeowners often change or otherwise redirect lot drainage to impervious surfaces which undoes a lot of low impact design benefits. Council have a number of education/community awareness initiatives to support this. SMG-6.6 Minimum Site Disturbance Minimum site disturbance is an approach to site development where clearing of vegetation and disturbance of soil is carefully limited to a prescribed distance from proposed structures and improvements. In most cases, the concept is appropriate for sites with existing native Page 6 Updated 01/11/2012

9 vegetation, although existing vegetation can also be dune vegetation, pasture grasses and coastal grasses. Tree cover need not consist solely of stands of mature native vegetation as scrub provides significant quantity and quality benefits as well. The objective of minimum site disturbance is to maximise existing vegetation and to minimise creation of an artificial landscape. At issue here are both construction phase impacts as well as the long term operation of the development. By doing this not only are the disturbed site impacts avoided as the result of substantial reduction in areas to be disturbed, but also natural areas of vegetation are preserved, retaining all of their functions and ecological values. An example of minimum site disturbance is shown in SMG-6 Figure 5 Comparison of Individual and Combined Driveways. Forested areas provide for rainfall interception by leaf canopy. In addition, an organic forest litter develops on the forest floor that acts very much as a sponge to capture the water and prevent overland flow. Trees provide for uptake and storage of nutrients. They also moderate temperatures during the summer and provide wildlife habitat, thus providing other environmental benefits. Wetlands are valuable resources and provide numerous benefits including flood control, low stream flow augmentation, erosion control, water quality and habitat. They are very productive ecosystems whose maintenance would have significant water quantity and quality benefits. Where they exist on a development site, they could become an important element in site design. From a construction standpoint, leaving areas in natural ground cover can have a significant benefit by reducing downstream sediment delivery. Sediment yield from disturbed soils can be 2000 times greater than yields from forested areas. Leaving site areas undisturbed is an important low impact approach component. Page 7 Updated 01/11/2012

10 SMG-6 Figure 5 Comparison of Individual and Combined Driveways The first step in developing a minimum site disturbance programme is to establish a variety of standards and criteria that define the approach. a) Establish a "limit of disturbance" (LOD) based on maximum disturbance zone lengths. i) Such maximum distances should reflect construction techniques and equipment needs together with the physical situation such as slopes as well as the building type being proposed e.g. a 4.0m LOD distance may be workable in low-density residential development where a 10.0m limit may be more appropriate for larger projects where larger equipment use is necessary. ii) LOD distances may be made to vary by type of development, size of site and specific development features involved. A special exception procedure should be provided to allow for those circumstances with unusual constraints. b) Integrate minimum site disturbance requirements fully into the project review process. Procedurally, the LOD should be established early on in the reviewing process. Page 8 Updated 01/11/2012

11 c) Require the LOD to be staked out in the field for contractor recognition. In addition, site disturbance can be minimised by locating buildings and roads along existing contours, orienting the major axis of buildings parallel to existing contours, staggering floor levels to adjust to grade changes, allowing for steeper cuts and grades provided that proper stabilization, erosion and sediment controls are in place and designing structures including garages to fit into the terrain, lot by lot. SMG-6.7 Constructing Biofiltration Practices The use of vegetative swales, buffer strips and rain gardens can provide a significant water quality benefit in addition to reducing the total volume of stormwater runoff. The primary processes involved in their performance are filtering of pollutants contained in stormwater runoff, evapotranspiration and infiltration of runoff into the ground. Further discussion on these processes is provided in SMG-9 Appendix B Stormwater Treatment Processes. Kerb cuts or openings can be placed in the kerb to allow water to pass off the paved surface into a biofiltration practice. This would allow for both objectives (traffic control and stormwater) to be attained. Further information on Biofiltration practices can be found within DS-5 Stormwater of the Infrastructure Development Code. SMG-6.8 Stormwater Reuse Using stormwater generated from roof areas or even from impervious surfaces for domestic or industrial purposes can reduce the total volume and peak flow rates of stormwater being discharged as water being reused will be separated from catchment stormwater delivery. Water reuse is potentially a very valuable tool in reducing stormwater runoff volumes and would have other beneficial effects such as reduced use of groundwater and reduced potable water needs. Further information on stormwater reuse can be found within DS-5 Stormwater of the Infrastructure Development Code. A brochure called Using Rainwater has been developed by Council. It is aimed at householders and contains information on the use of rainwater and raintanks as an auxiliary water supply. This is available from the Council website. Page 9 Updated 01/11/2012

12 SMG-6 Figure 6 Water Reuse at a Hospital Site SMG-6.9 Creating Natural Areas In many site development situations, the predevelopment condition may be pasture or other highly modified hydrological condition. Re-establishment of native bush as riparian cover, steep slope protection or general site revegetation as open space would have significant stormwater management benefits for both water quantity and water quality. The area, if well designed and constructed, could become an attractive amenity to a community and enhance the value of the properties. Further details on the creation of natural areas can be found in SMG-6 - Appendix C: Bush Revegetation. SMG-6.10 Design Procedures for Incorporation of Low Impact Design into Site Design Council supports the implementation of Low Impact Design (LID) as an evolutionary step in our use of land. SMG Design Procedure in Overview provides a simple means for considering a Low Impact Design approach through the use of checklists. SMG Design Procedure in Overview The Low Impact Design (LID) procedure shown in SMG-6 Figure 7 Low Impact Design Approach is based on using an analysis of existing site conditions as a baseline from which to start from. These existing site conditions provide an inventory of the full range of natural systems - water, soil, geology, vegetation, and habitat - as well as cultural and archaeological factors. These systems range from the very macro in scale for resources of City wide significance, down to micro scale site-specific conditions such as steep slopes or the presence of first or second order streams. The more this complex system is documented and understood from the start, the better the earthworks and building programme can be fitted on the site with reduced impact. Extra design effort up front will pay important dividends in the long run. LID requires a major departure from the conventional mind-set of site disturbance and stormwater disposal, which is a reactive approach with an end of the line process forcibly imposed through a consent requirement. LID is based on understanding natural system opportunities, which enable us to achieve essential stormwater quantity and quality management objectives integrated into the development design from the very beginning. Page 10 Updated 01/11/2012

13 LID requires that a series of questions be answered which, from an earthworks and stormwater perspective, are preventive in nature. If these questions are addressed, the reduction of stormwater runoff can be maximised. In most situations, sediment control and stormwater mitigation will still be required due to site disturbance and the increased volume and peak rates of runoff. On-site mitigation efforts in stormwater management should attempt to use less impacting forms of management such as incremental stabilisation and vegetated swales or filters where practical, but more structural forms of management such as ponds or wetlands will still possibly be required although their sizes will be reduced from the conventional site development approach. A relevant analogy to the benefits of LID is the relationship that erosion control has to sediment control. Erosion control during construction is preventive in that it reduces the total amount of sediment generated. Sediment control attempts to reduce downstream delivery of sediment through the use of mitigative practices. LID is similar to erosion control as it is preventive. Conventional stormwater management is like sediment control in that it attempts to reduce adverse impacts rather than preventing them. The most important aspect shown in SMG-6 Figure 7 Low Impact Design Approach is the conceptual earthwork and stormwater management plans being done concurrently with the entire site design process. All of the preventive and mitigative steps link into the conceptual process. The building programme, site design including earthworks, and stormwater management concept are integrated and optimised. This integration of erosion and sediment control and stormwater management issues into site design from the start of the design process is essential to LID. Page 11 Updated 01/11/2012

14 SMG-6 Figure 7 Low Impact Design Approach SMG Low Impact Design Checklist Items The following presents a series of questions or checklist items that should be answered as the design process proceeds. The most important aspect of LID is that it achieves a new way of thinking about site design. One key theme that permeates the overall process is to have a minimum earthworks strategy. The less site modification that occurs, the less adverse effects will occur. There are individual sections to the process depending on the aspect of site design that is being considered. Those processes include the following items: a) General Context Information. b) Low Impact Design Ancillary Benefits. Page 12 Updated 01/11/2012

15 c) Site Natural Features Analysis. d) Receiving Environment and Stormwater Issues. e) Hydrological Factors. f) Building Considerations. g) Lot Configuration. h) Impervious Surface Reduction. i) Minimisation of Site Disturbance. j) Calculation Considerations. k) Mitigation Considerations. l) Catchment Planning Considerations These considerations are appropriate at a site or subdivision level of development. SMG General Context Information SMG-6 Table 1 General Context Information Information The surrounding land context (rural, urban, vegetation, etc.) The site position in a catchment (top, middle, bottom) Site size Structure plan, district plan, network consent, Infrastructure Development Code indicate ranges of development for this site and adjacent ones Considered SMG Low Impact Design Ancillary Benefits SMG-6 Table 2 Ancillary Benefits Information Yes No Urban design components Crime Prevention Through Environmental Design Energy efficiency Ecology Landscape amenity Page 13 Updated 01/11/2012

16 Urban Design Information Yes No Context: Fit within the catchment Character: Create vision and identity Choice: Range of options within the development Connections: Pathways through the development Creativity: Innovation for the future Custodianship: The creation of stewardship Collaboration: Involvement of key catchment stakeholders Crime Prevention through Environmental Design (CPTE75 Information Yes No Access: Safe movement and connections Surveillance and sightlines: See and be seen Layout: Clear and logical orientation Activity mix: Eyes on the street Sense of ownership: Showing a space is cared for Quality environments: Well-designed, managed and maintained environments Physical protection: Using active security measures Energy Efficiency Information Yes No Water use options available (roof, mains, grey water etc) Control over the amount of water use and water use options? Site design optimises solar exposure for living environments but allows for shading and cooling in summer months Ecology Information Yes No Rehabilitation potential for ecological systems Enhanced/capitalised biodiversity of flora and fauna communities Viability of ecological systems and processes Landscape connectivity Page 14 Updated 01/11/2012

17 Landscape Amenities Information Yes No Conservation: Protection of significant landscape features View Protection: Access to existing views Coherence: Are existing landscape shapes maintained Connectivity: Seamless transitions between development and open spaces Scenic Appeal: Enhancement of landscapes. Access and Safety: Allowance for sightlines and orientation. SMG Site Natural Features Analysis Wetlands SMG-6 Table 3 Site Natural Features Analysis Feature Yes No Streams (including intermittent ones) Floodplains Riparian buffers Existing site vegetative cover Soils Depth to groundwater Steep slopes (>33% or 18 o ) Other natural site features Cultural or archaeological locations Page 15 Updated 01/11/2012

18 SMG Receiving Environment and Stormwater Issues SMG-6 Table 4 Receiving Environment Factors Questions to Answer Yes No Does the site drain directly to tidewater or the coast Does the receiving system have sensitivities e.g. as detailed in the City Plan/State of the Receiving Environment Reporting/Stormwater Management Plans Are there known downstream flooding problems Does the site contain 1st or 2nd order streams Are any site streams perennial Does the site stormwater discharge to ground Circle the appropriate Receiving System and Stormwater Issue to determine priorities: SMG-6 Table 5 Receiving Environments and Stormwater Issues Receiving System Flooding Issues Stream Erosion Issues Water Quality Streams May be a priority High priority if the High priority depending on receiving stream is a location within a natural, earth catchment channel Ground Not an issue Not an issue depending on overflow High priority Estuaries Not an issue Not an issue High priority Harbours Not an issue Not an issue Moderate priority Open Coast Not an issue Not an issue Lower priority Lakes Not an issue Not an issue High priority Page 16 Updated 01/11/2012

19 SMG Hydrological Factors SMG-6 Table 6 Hydrological Factors Questions to Answer Yes No Have you identified the route taken by stormwater runoff from the source to the receiving environment Is the pathway stable enough for stormwater drainage to enter it without eroding Can the pathway provide for stormwater treatment Can site soils be used for infiltration of runoff Can roof or site runoff be used to reduce overall stormwater runoff volume Can revegetation of native vegetation be used to reduce runoff Do impervious surfaces drain directly to receiving waters SMG Building Considerations SMG-6 Table 7 Building Considerations Consideration Yes No Can the type of units be modified (single family to apartment) Is there flexibility in lot density Is there flexibility in individual lot requirements Is there any flexibility in the dwelling type e.g. single storey dwelling to apartment block Page 17 Updated 01/11/2012

20 SMG Lot Configuration SMG-6 Table 8 Lot Configuration Consideration Yes No Is there flexibility to reduce lots sizes? Are there opportunities for clustering? Can lots be configured to avoid important natural features? SMG Impervious Surface Reduction SMG-6 Table 9 Impervious Surface Reduction Consideration Yes No Is there potential to reduce road lengths and widths? Is there potential to reduce driveway lengths and widths? Is there potential for shared driveways or parking spaces? Is there potential for reducing parking ratios or parking sizes? Can cul-de-sacs and roundabouts be designed to minimise imperviousness? Is there potential to reduce kerbing or use of kerb cuts to reduce flow concentrations? Page 18 Updated 01/11/2012

21 SMG Minimisation of Site Disturbance SMG-6 Table 10 Site Disturbance Minimisation Considerations Consideration Yes No Has maximum total site area, including both soil and vegetation been protected from clearing or other site disturbance Can disturbance of important natural, cultural or archaeological features be minimised Are areas of open space maximised In terms of individual lots, has maximum lot area, including both soil and vegetation, been protected from clearing or other site disturbance Do structures correspond to site features such as slope, both in terms of type of structure, placement on lot, elevation, etc. Have vegetation opportunities been maximised throughout the site Have revegetation opportunities been maximised in important natural areas SMG Calculation Considerations SMG-6 Table 11 Design Calculation Considerations Consideration Yes No As a result of a decrease in the total disturbed area, are numbers of sediment ponds minimised as far as practicable, and subsequently their size and areal extent also minimised Total sediment yield from the site during construction has been minimised as far as practicable from the conventional approach Could impervious cover be minimised as far as practicable from conventional development Page 19 Updated 01/11/2012

22 Have Runoff Coefficient Values been reduced as for as practicable from conventional to LID Have total runoff volumes been affected Has the predevelopment time of concentration for site runoff been maintained as far as possible SMG Mitigation Considerations SMG-6 Table 12 Mitigation Considerations Consideration Yes No Has the stormwater management plan been integrated into the overall site design Has prevention been minimised through LID considerations Has mitigation been maximised through vegetative and soil based practices such as swales, rain gardens or infiltration practices Can unpreventable impacts be mitigated through conventional stormwater management controls SMG Catchment Planning Considerations SMG-6 Table 13 Catchment Planning Considerations Consideration Yes No Greenfield Have all perennial and intermittent streams been identified Has catchment geology been done and are areas with instability identified Has a topographical map of the catchment been done Page 20 Updated 01/11/2012

23 Have overall development densities been assigned Can development occur on the contour to limit earthworks Can development clustering be done to protect streams and reduce earthwork requirements while achieving development densities Can headwater streams be protected from hydrological change Can total volumes of runoff be reduced from a conventional approach to reduce impacts on receiving systems Brownfield Have all perennial streams been identified Has the storm drain system been identified and capacities determined Can redevelopment within the catchment provide opportunities for reduction in impervious surfaces Has consideration been given to source control of contaminants Can redevelopment provide for stream rehabilitation or daylighting Can redevelopment provide for vertical growth and create additional open space SMG-6.11 Measuring Success SMG-6 Table 14 Effectiveness of LID Approaches summarises the general effectiveness of each of the site design approaches discussed in this guideline. It indicates that most practices are at least moderately effective at providing two or three environmental benefits. Certain practices, notably natural landscaping and cluster development are at least moderately effective in achieving all four of the desired environmental objectives. However, the table also implies that a site design should incorporate several management practices in an integrated fashion to be highly effective in controlling adverse environmental impacts. Page 21 Updated 01/11/2012

24 LID practice Reduced street widths Reduced building setback Natural drainage Natural detention Natural landscaping Cluster development Runoff rate reduction Moderately effective Moderately effective Moderately effective Very effective Moderately effective Moderately effective SMG-6 Table 14 Effectiveness of LID Approaches Runoff volume reduction Moderately effective Moderately effective Moderately effective Limited effectiveness Runoff contaminant reduction Moderately effective Moderately effective Moderately effective Very effective Habitat Protection Limited effectiveness Limited effectiveness Limited effectiveness Moderately effective Very effective Very effective Very effective Very effective Very effective Very effective The effectiveness of natural landscaping and cluster development will depend on how well these approaches are integrated into the overall landscape and drainage plan SMG-6.12 Other Types of Development Low Impact Design (LID) is commonly thought of as being applicable for residential development as residential development covers the most land area and is the most common form of site development. But commercial, industrial, and even horticultural (especially protected cropping) activities as shown in SMG-6 Table 15 Appropriateness of LID for Selected Development Types can successfully apply LID to reduce their impact on receiving systems. The approach is identical to residential LID in that existing site resources are delineated, site development is integrated into the site, and LID approaches to stormwater treatment are investigated. Not all LID approaches are suited to all development types. For example, site revegetation is most easily done on larger residential lot development, but is less feasible on small commercial development. Page 22 Updated 01/11/2012

25 LID Practice Reduced street widths Reduced building setbacks Natural drainage Natural landscaping Cluster development SMG-6 Table 15 Appropriateness of LID for Selected Development Types Low Density Residential Generally appropriate Generally appropriate Generally appropriate Generally appropriate Generally appropriate Medium Density Residential Generally appropriate Generally appropriate Occasionally appropriate Occasionally appropriate Generally appropriate Multi-Family Residential Generally appropriate Occasionally appropriate Generally appropriate Generally appropriate Occasionally appropriate Commercial Industrial Occasionally appropriate Occasionally appropriate Occasionally appropriate Occasionally appropriate Generally appropriate not Any site being developed can incorporate LID. The greatest potential use of LID on commercial and industrial sites lies with use of the treatment train approach to stormwater management implementation. That is an element of LID but only one element of a broader context. SMG-6 Figure 8 LID on a Commercial Site is for a small commercial development and demonstrates several LID design principles. Roof runoff reused on site and parking lot runoff enters swales, which are then directed towards a stormwater detention pond having wetland attributes. The site design promotes water reuse, water quality treatment, plus providing control of water quantity peak discharges. As can be seen, the project does have to provide structural stormwater management control but the work done by the controls is augmented by water tanks and swales in addition to benefits provided by the wetlands vegetation. The same issues related to residential site development requirements exist for commercial and industrial development in terms of kerbing, parking requirements, level of imperviousness, or revegetation opportunities. Almost every site can have steps taken to reduce downstream impact from conventional development approaches. One key important element is that combinations of approaches should be employed in an integrated fashion to maximise cumulative benefits. Page 23 Updated 01/11/2012

26 SMG-6 Figure 8 LID on a Commercial Site SMG-6.13 Costs of Low Impact Design Further details on the cost involved in the construction and maintenance of Low Impact Design Techniques as well as cost comparison case studies on a subdivision level can be found in: a) SMG-6 Appendix D Low Impact Design Costing. b) SMG-6 Appendix E New Zealand Case Studies: Cost Comparison on Low. Impact Design for Subdivisions. c) SMG-6 Appendix F International Case Studies: Cost Comparison on Low Impact Design for Subdivisions. Page 24 Updated 01/11/2012

27 SMG-6 Appendix A Case Studies: Low Impact Design Implementation Several examples of Low Impact Design implementation will be shown in case studies on residential subdivisions whose size and density are typical of Tauranga City. There are any number of examples that could be used but these two were considered as typical. SMG-6 Apx A.1 Case Study One Residential subdivision having the following: 40 hectare site 2,000 m² lots 142 lots Overall imperviousness 30% Conventional storm drain pipe system 100% site disturbance This is a conventional approach that is shown on SMG-6 Figure A.1a Typical Subdivision Development Approach Showing Stormwater Management Ponds. SMG-6 Figure A.1a Typical Subdivision Development Approach Showing Stormwater Management Ponds The site was considered from a Low Impact Design (LID) context to see if benefits could be realised in terms of open space maximisation and reduced hydrological change. The following two approaches were considered as shown in SMG-6 Figure A.1b Two Different LID Approaches to Site Development. Page 25 Updated 01/11/2012

28 SMG-6 Figure A.1b Two Different LID Approaches to Site Development When the site runoff for the site is considered for the conventional development, parkway approach and village cluster approach shown in SMG-6 Table A.1 Subdivision Annual Site Runoff, the following table is calculated. SMG-6 Table A.1 Subdivision Annual Site Runoff (m 3 ) Pre- Development Conventional Development LID Approach (Village) Even Better (Parkway) Precipitation 432, , , ,373 Runoff 18, ,515 82,671 67,397 Recharge 154, , , ,556 Evapotranspiration 259, , , ,441 As can be seen, the LID approaches provide a substantial reduction in stormwater runoff being discharged on an annual basis with greater levels of groundwater recharge and evapotranspiration from the conventional approach. These numbers only indicate runoff differences from the modified development approach. They do not consider the benefits of using water tanks for non-potable water use or rain gardens, infiltration practices, swale or filter strips as overall site stormwater management components. Using those practices in conjunction with an LID approach to site land use would result in a similar site runoff to the predevelopment condition. Page 26 Updated 01/11/2012

29 SMG-6 Apx A.2 Case Study Two Case Study Two provides for residential development in a more urban situation. The conventional development approach consists of the following: 14.2 hectare site 128 lots Average lot size 765 m 2 Overall imperviousness 69% Reserve 1.09 ha Earthworks area 9.5 ha Earthworks volume 62,000 m 3 Conventional storm drainage system 100% site disturbance SMG-6 Figure A.2a Predevelopment Site Conditions and Conventional Development Approach shows the predevelopment and conventional site development approach. SMG-6 Figure A.2a Predevelopment Site Conditions and Conventional Development Approach The Low Impact Design approach shown in SMG-6 Figure A.2b LID Design Approach provided for the following: 138 lots Reserve area of 2.34 ha Average lot size 650 m 2 Earthworks area 7.6 ha Earthworks volume 53,000 m 3 Overall impervious = 51% Conventional storm drainage system 80% site disturbance Page 27 Updated 01/11/2012

30 SMG-6 Figure A.2b LID Design Approach In terms of a water budget, the following hydrological information is presented in SMG-6 Table A.2a Hydrological Results. SMG-6 Table A.2a Hydrological Results Storm Peak flow (m 3 /s) Runoff (mm) % increase 2-year, 24-hour storm Predevelopment Conventional Low Impact year, 24-hour storm Predevelopment Conventional Low impact Considering the water budget from an increase in total runoff and impacts on stream base flow over a given year, SMG-6 Table A.2b Changes in Annual Stream Flow and Base Flow provides the following information: Page 28 Updated 01/11/2012

31 SMG-6 Table A.2b Changes in Annual Site Runoff and Base Flow Site Development Rainfall (m 3 ) Site Runoff (m 3 ) Base flow (m 3 ) status Predevelopment 181,638 31,449 39,088 Conventional 181,638 99,160 12,254 Low impact 181,638 81,945 19,090 As can be seen, there is still a significant increase in site runoff both storm related and annually, but use of an LID approach can reduce site runoff and, in conjunction with stormwater practices, reduce downstream adverse effects. Page 29 Updated 01/11/2012

32 SMG-6 - Appendix B - Site Resources SMG-6 Apx B.1 General Just as it is important to recognise the value of receiving systems, it is also important to recognise resources that are available on individual sites. Site resources are those natural features or site characteristics, which to a large extent, provide a benefit to receiving systems through their existence. They provide a benefit to the general public by their continued function to reduce peak rates and volumes of stormwater runoff, provide for water quality treatment and prevent damage to improved or natural lands either on site where the site resources exist or downstream of those resources. Site resources have intrinsic and other values for habitat and biodiversity regardless of their stormwater functions. They can include a wide variety of items but those discussed here are considered primary resources that should be recognised and considered in site development and use. The following site resources are important primarily for their stormwater management benefits, although some of the benefits are less obvious than others, all provide a benefit: a) Terrestrial Ecology and Landscape Form. b) Wetlands. c) Floodplains. d) Riparian Buffers. e) Vegetation Cover. f) Soils. g) Slopes / Topography. h) Other Natural Features. i) Linkage with Site Development. Site resources often overlap e.g. a riparian buffer may lie within a floodplain or a forested area. They are discussed individually throughout this section although their benefits may be and generally are, cumulative. SMG-6 Apx B.1.1 Terrestrial Ecology and Landscape Form It is often said that the three principal economic factors that drive real estate prices are location, location and location. The same is true of natural resources and site resources. Where natural features are located on a site is just as important as the characteristics of the natural features themselves. The importance of the position of ecological features in the landscape has spawned an entire field of study called "landscape ecology". There are several basic principles of ecology that can be used to improve the quality of receiving environments. These principles apply to all site resources: Page 30 Updated 01/11/2012

33 a) Retain and protect native vegetation (native forest, regenerating native scrub and forest, wetlands, coastal forest/scrub) - these ecosystem types have important intrinsic values, and provide different habitats for native flora and fauna and different ecological functions. b) Allow natural regeneration processes to occur (e.g. pasture => scrub => forest; wet pasture => wetland). c) Undertake weed and pest control to improve the natural values of native vegetation, allow natural processes and seed dispersal mechanisms to occur. d) Replant and restore with native plants to provide vegetation cover, which is characteristic of what would once have been there and/or which reflects other local remnants in the area. e) Restore linkages with other natural areas or ecosystems (e.g. using waterways and riparian areas, linking fragmented forest remnants, linking wetland ecosystems and freshwater ecosystems to terrestrial forest/scrub remnants). Native species need extensive areas of vegetation to survive. f) Our knowledge on ecological effects is limited so we need to provide a greater factor of safety in protecting natural areas. SMG-6 Apx B The Ecological Values of Site Resources It is important to retain natural areas (including scrub, forest, and wetlands) on a site for their biological diversity and intrinsic values, that include the following: a) They are important for their values as characteristic examples of biodiversity. b) The diversity of species or ecosystem types that they contain. c) Containing rare or special features or unusual ecosystem types. d) Their value as habitats for indigenous species and the level of naturalness. e) Their ability to sustain themselves over time (e.g. available seed sources, active regeneration, bird dispersal processes active, level of weeds and pests and outside influences controlled). f) Being of adequate size and shape to be viable. g) If they are buffered or they provide a buffer to habitats or natural areas, from outside influences (e.g. scrub on edges of native bush, intact sequences from estuarine to terrestrial, from freshwater to terrestrial, from gully bottom to ridge top); and provide linkages with other natural areas in an area (corridors for native birds, invertebrates). The following factors provide a set of basic principles for the determination of ecosystem significance for the evaluation of ecological significance of native vegetation: h) Representativeness. i) Rarity and Naturalness. j) Diversity and Pattern. k) Size and Shape. Page 31 Updated 01/11/2012

34 These are paraphrased from the Protected Natural Areas Programme survey methodology (Myers et al, 1987). SMG-6 Apx B Representatives It is important to protect what is common and characteristic of the ecology of an area. Natural areas that are representative of the ecological communities once formerly present in a given area (e.g. an Ecological District) are significant. It is not only rare and unusual features that are important. Most natural areas have been reduced dramatically from their former extent; so remaining representative examples of each different type of ecological community are valuable. There has been a move away from protection of only rare species and their habitats to protecting ecosystems that are good examples of the landscape character. Protection of substantial parts of ecosystems is usually needed to assure the survival of their constituent parts, such as individual species. It is easy to ignore or place less importance on elements of ecosystem functioning, which are not obvious. Many evaluations are based on visual assessment e.g. a comparison of pasture to mature forest. But there are many other important elements of ecosystem integrity that are not so readily apparent, including water cycle, chemical factors, energy flow and biotic interactions. SMG-6 Apx B Rarity and Naturalness It is easy to underestimate the value of rare species. Rarity is an indicator of the scarcity of numbers of a species or other element of biodiversity. The presence of a rare or special or unusual feature in a natural area adds to its ecological value. Rare species reflect the highest degree of ecosystem complexity and function and are the most sensitive to impact. Unfortunately, their rarity makes them impractical for use with most assessment studies done as part of development projects. Naturalness is important to the survival of species, communities and other components of biodiversity, many of which will not survive outside a natural environment. Naturalness in ecosystems is inversely proportional to the degree of disturbance by humans or introduced species (e.g. weeds). SMG-6 Apx B Diversity and Pattern A fundamental aim of nature conservation is to protect natural biological diversity. The diversity of a natural area refers to the species of plants and animals present as well as its communities, ecosystems, and physical features. Generally the ecological value of a natural area increases with its diversity and the complexity of its ecological patterns. Wetlands, floodplains, and mature forests are key resources in sustainable design because they are generally the oldest and least disturbed site resources. Ecosystem function increases over time. Page 32 Updated 01/11/2012

35 Long-term ecological viability is the ability of natural areas to retain their inherent natural values over time. This includes the ability of a natural area to resist disturbance and other adverse effects and for its component plant and animal species to regenerate and reproduce successfully. Complex ecosystems often have a messy or "wild" appearance to them as their complexity increases. A mature forest can take hundreds of years to develop so seeing one indicates a lack of recent disturbance. SMG-6 Apx B Size and Shape Size and shape of the area affect the long-term viability of a natural area s ecological components and functions. With increase in size, the diversity and resistance to disturbance of an area generally increases. The shape of a natural area influences its resistance to external effects (e.g. a compact shaped area is less vulnerable to edge effects than a complex one). Ecosystem function increases as the size of the natural area gets larger. The inverse is also true that ecosystem function is reduced when roads and urban development fragment natural systems. But small fragments and patches of native vegetation are still important and may be the only remnants left of a certain type in an area. They may provide habitatht for relict population, or rare species may provide seed source for local revegetation. The smaller an area of bush is, the greater the edge effects, the lack of microclimates for certain species, and the more likely weed invasion will be. Much of the City was covered by forest prior to human settlement. This forest had maximum ecosystem function due to its age, size, and complexity. Human influence on the land has shrunk or eliminated this network of connected woodlands to a fraction of its former size. The effect of area size on ecosystem function is, to some degree, a matter of geometry; the various dimensions of the tract change in proportion to the area of the tract. A tract reduced in area by a factor of one hundred reduces by onetenth the distance to the centre of the tract and increases ten times the dominance of the perimeter habitat (edge/area ratio). Tract size has important implications for species that require interior habitat. The tract can become so small that the interior habitat and the species that depend on it are eliminated. Urbanisation causes a shift in the aquatic community from one dominated by pollution sensitive species towards one dominated by contaminant tolerant species. This ecological principle also applies to the terrestrial environment where the adverse impacts tend to be subtler in nature and more variable from site to site. SMG-6 Apx B.1.2 Wetlands Wetlands, as defined in the Resource Management Act, include permanently or intermittently wet areas, shallow water, and land water margins that support a natural ecosystem of plants and animals that are adapted to wet conditions. They occur on Page 33 Updated 01/11/2012

36 land-water margins, or on land that is temporarily or permanently wet. Wetlands are a major habitat for at least eight species of indigenous freshwater fish as well as frogs, birds, and invertebrates. Wetlands have unique hydrological characteristics that can be irreversibly modified by activities such as drainage. There can be few other vegetation classes that have suffered so severely during human times than have wetlands. The reasons for this are many, but can be attributed largely to their position on flat land, suited to agriculture, and to the generally low esteem in which such vegetation has been held by the average layperson. These changes have occurred despite the manifest value of wetlands as wildlife habitats, as regulators of flooding, their intrinsic values, for recreation, and for scientific research. Nevertheless a far larger area than remains today has been lost through drainage, fire, topdressing, and flooding. The problem with wetlands is that they are rarely seen as being a valuable resource. They are usually difficult to access, and therefore are rarely visited. Their wildlife is usually secretive and their plants are seldom spectacular or flamboyant. Their values as a source of mined material or as pastoral land or for horticulture are only realised after the wetland has been destroyed. Their ability to assist in water control is often only recognised after both floods and water shortages have occurred following their destruction. SMG-6 Figure B Carmichael Constructed Wetlands Nationwide freshwater wetlands covered at least 670,000 hectares before European settlement but have now been reduced by drainage for pasture to around 100,000 hectares. Although several thousand wetlands still survive, most are very small and have been modified by human activities and invasive species. It is likely that some characteristic wetland types have been lost completely, while very few examples are left of others such as kahikatea swamp forest and some kinds of flax swamp. Page 34 Updated 01/11/2012

37 New Zealand s wetlands are as varied as the terrain that shapes them. It is important to recognise that even without the presence of humans; wetlands systems are modified and eliminated by a natural ecological ageing process referred to as succession. The filling and conversion of wetlands into more terrestrial type ecosystems occurs naturally at a relatively slow rate. The intervention of man into the process vastly accelerates the conversion process. In their natural condition, wetlands provide many important functions to man and the environment. SMG-6 Table B.1.2 Summary of Wetland Functions and Values summarises the major functions and values of wetlands. SMG-6 Table B.1.2 Summary of Wetland Functions and Values Function/Value Description Flood control Attenuation of peak flows Storage of water Absorption by organic soils Infiltration to groundwater Flow attenuation Maintenance of stream flow during droughts Erosion control Increased channel friction Reduction in stream velocity Reduction in stream scour Channel stability by vegetative roots Dissipation of stream energy Water quality maintenance Sedimentation Burial of contaminants in sediments Adsorption of contaminants to solids Uptake by plants Aerobic decomposition by bacteria Anaerobic decomposition by bacteria Habitat for wildlife Food Shelter/protection from weather and predators Nursery area for early life stages Fisheries habitat Galaxids, eels, freshwater mussels, crayfish Food chain support Food production from sun (primary production) Recreation/aesthetics Enjoyment of nature Hiking, boating, bird watching Education Teaching, research Page 35 Updated 01/11/2012

38 In addition to the listed beneficial values, the water quality benefits of wetlands can be expanded. Natural wetland systems have complex mechanisms and the following listing of benefits describes the major processes occurring in wetlands that allow them to provide water quality enhancement functions. These functions include: a) Settling / burial in sediments. b) Uptake of contaminants in plant biomass (absorption). c) Filtration through vegetation. d) Adsorption on organic material (surface accumulation). e) Bacterial decomposition. f) Temperature benefits. g) Volatilisation. SMG-6 Apx B.1.3. Floodplains Floodplains occupy those areas adjacent to stream channels that become inundated with stormwater during large rainfall/runoff events. For the most part, in Tauranga City, rainfall (in conjunction with inadequate drainage capacity) is the main cause of flooding although surges by wind driven currents can exacerbate the problem, or in unique situations, cause the flooding problem. Flooding problems result from two main components of precipitation: the intensity and duration of rainfall, and its areal extent and distribution. Many studies have shown that paving and drainage systems in urban areas increase flooding, particularly as many urban areas are located along floodplains and former wetlands. Flooding in and of itself is not a problem. Floods have been around since the beginning of time and are a natural part of the water cycle. Problems are caused when man interacts with the floodplain. Thus, flood hazard potential relating to human health, property damage, and social disruption are strongly influenced by human activity on the floodplain. There are several key catchment characteristics which impact on flood frequency and depths. Page 36 Updated 01/11/2012

39 SMG-6 Figure B.1.3 Bruce Road Flooding, May 2005 SMG-6 Apx B Catchment Size and Slope The abundance of rainfall in Tauranga City feeds small first and second order streams. These streams and their associated floodplains are the conveyance means of getting water downstream, through the catchment, and to sea level. Smaller catchments have a rapid response time to storm flows where larger catchments have a longer response time as storm flows take time to travel through the drainage system. SMG-6 Apx B Surface Conditions and Land Use Until the nineteenth century, 75% of the country was covered in temperate rainforest. Replacing two-thirds of it with exotic grasses has dramatically increased the rate at which rain reaches the ground surface and flows overland into the stream system. Urbanisation, with its impervious surfaces has an even more profound effect on flood flows. Not only do flood flows increase in size and number, but also their speed of onset is increased, particularly in the first 20% of change from rural to impervious cover. This makes intensive, short-duration rainfalls more flood prone. In addition, time of year can impact on flood levels via intensity of rainfall and saturated condition of soils. SMG-6 APX B Topography The channel form and associated floodplain in part determine the size of flood, particularly its depth and areal extent. A small catchment and wide floodplain will result in a shallow, but widespread flood. On the other hand, a deep channel and steep slopes will result in deeper flooding, but on a small areal extent. Page 37 Updated 01/11/2012

40 The many benefits that floodplains provide are partly a function of their size and lack of disturbance. But what makes them particularly valuable ecologically is their connection to water and the natural drainage systems of wetlands, streams, and estuaries. The water quality and water quantity functions provided by the floodplain overlap with the landscape functions of tract size and ecosystem complexity to make them exceptionally valuable natural resources. Floodplains provide a wide range of benefits to both human and natural systems. These functions and values can be broadly placed in three categories: a) Water Resources. b) Living Resources. c) Societal Resources. SMG-6 Apx B Water Resources Floodplains provide for flood storage and conveyance during periods when flow exceeds channel boundaries. In their natural state they reduce flood velocities and peak flow rates by out of stream bank passage of stormwater through dense vegetation. They also promote sedimentation and filter contaminants from runoff. In addition, having a good shade cover for streams provides temperature moderation of stream flow. Maintaining natural floodplains will also promote infiltration and groundwater recharge, while increasing or maintaining the duration of stream base flow. Floodplains provide for the temporary storage of floodwaters. If floodplains were not protected, development would, through placement of structures and fill material in the floodplain, reduce their ability to store and convey stormwater when the need for floodplain storage occurs. This, in turn, would increase flood elevations upstream of the filled area and increase the velocity of water travelling past the reduced flow area. Either of these conditions could cause safety problems or cause significant damage to private property. SMG-6 Table B Values of the Roughness Coefficient n in Floodplains provides values of roughness coefficients that have been established for floodplain areas for the purposes of hydraulic calculations to determine flow velocities and elevations. They indicate the value that vegetation has on the movement of flood flow and can be considered in the context of retardance factors. The higher the roughness value is, the greater the retardance to flow movement through it. Page 38 Updated 01/11/2012

41 SMG-6 Table B Values of the Roughness Coefficient n in Floodplains Type of Ground Cover Normal n Pasture, no brush Short grass High grass Cultivated areas No crop Mature row crops Mature field crops Brush Scattered brush, heavy weeds Light brush and trees Medium to dense brush Trees Heavy stand of timber, little undergrowth Heavy stand of timber, flood stage in branches As can clearly be seen, the denser and taller the vegetation, the greater the frictional resistance to stream flow. SMG-6 Apx B Living Resources Natural floodplains are fertile and support a high rate of plant growth, which supports and maintains biological diversity. They provide breeding and feeding grounds for fish and wildlife. In addition, they provide habitat for rare and endangered species. Ground cover in natural wetlands tends to be composed of leaf and dense organic matter. Organic soils have a lower density and higher water holding capacity than do mineral soils. This is due to the high porosity of organic soils or the percentage of pore spaces. This porosity allows floodplain soils generally to store more water than mineral soils would in upland areas. Page 39 Updated 01/11/2012

42 SMG-6 Apx B Societal Resources Floodplains provide areas for active and passive recreational use. They increase open space areas, and provide aesthetic pleasure. They also contain cultural and archaeological resources and provide opportunities for environmental and other studies. Human development historically has occurred around waterways for food and transportation. Many walkways exist in reserves and those walkways tend to be adjacent to stream channels. SMG-6 Apx B.1.4 Riparian Buffers Although reduction of contaminants is a generally recognised function of riparian buffers, they also contribute significantly to other aspects of water quality and physical habitat. Habitat alteration, especially channel straightening and removal of riparian vegetation, continues to impair the ecological health of streams more often and for longer time periods than contaminants. When considering riparian buffers, it is helpful to detail the variety of benefits that are gained by their protection or implementation. SMG-6 Figure B.1.4 Example of a riparian buffer around a stream Riparian buffer systems provide the following benefits: a) Temperature and Light. b) Habitat Diversity and Channel Morphology. c) Food Webs and Species Diversity. d) Contaminant Removal. e) Importance to Wildlife. f) Channel Stability and Flood Flow Protection. Page 40 Updated 01/11/2012

43 SMG-6 Apx B Temperature and Light The daily and seasonal patterns of water temperature are critical habitat features that directly and indirectly affect the ability of a given stream to maintain viable populations of most aquatic species. Considerable evidence shows that the absence of riparian cover along many streams has a profound effect on the distribution of many species of macroinvertebrates and fish. In the absence of shading by a forest canopy, direct sunlight can increase stream temperatures significantly (up to 12 o c), especially during periods of low stream flow in summer. Riparian buffers have been shown to prevent the disruption of natural temperature patterns as well as to mitigate the increases in temperature following upstream deforestation. SMG-6 Apx B Habitat Diversity and Channel Morphology The biological diversity of streams depends on the diversity of habitats available. Woody debris is one of the major factors in habitat diversity. Woody debris can benefit a stream by: a) Stabilising the stream environment by reducing the severity of the erosive influence of stream flow. b) Increasing the diversity and amount of habitat for aquatic organisms. c) Providing a source of organic carbon. d) Forming debris dams and slowing stream velocities. Loss of the riparian zone can lead to loss of habitat through stream widening where no permanent vegetation replaces forest, or through stream narrowing where forest is replaced by grass. In the absence of perennial vegetation, bank erosion and channel straightening can occur. The accelerated stream flow velocity allowed by straight channels promotes channel incision as erosion of sediment from the stream bottom exceeds the sediment load entering the stream. This process can eventually lead to the development of wide, shallow streams that support fewer species and are more susceptible to temperature fluctuations. SMG-6 Apx B Food Webs and Species Diversity The two primary sources of natural food energy input to streams are litter fall from streamside vegetation and algal production within the stream. Total annual food energy inputs are similar under shaded and open canopies but the presence or absence of a tree canopy has a major influence on the balance between litter input and primary production of algae in the stream. Having a stream exposed to sunlight for most of the day promotes algal growth and promotes proliferation of algal grazing species. This proliferation reduces species diversity. The diversity of the macroinvertebrate community in a stream protected by a riparian buffer has a much greater diversity than does a stream not having a riparian canopy. This diversity is important in that it is in such a small area that goes from low land wetter soil conditions to upland fairly rapidly and thus promoting very Page 41 Updated 01/11/2012

44 different vegetative types. Also, riparian buffer areas are adjacent to streams and therefore floodplains. By periodic out of bank flow, floodplains are depositional zones for fertile sediments. SMG-6 Apx B Contaminant Removal Riparian vegetation removes, sequesters, or transforms nutrients, sediments, and other contaminants. The removal function depends on two key factors: a) The capability of a particular area to intercept surface and/or groundwater borne contaminants. b) The activity of specific contaminant removal processes (filtration, adsorption, biological uptake, etc.). New Zealand studies have shown that the majority of nitrate removal in a pasture catchment takes place in the organic riparian soils, which receive large amounts of nitrate laden groundwater. The location of the high organic soils at the base of gullies caused a high proportion of groundwater to flow through the organic soils although they occupied only 12 percent of the riparian zone area. Sediment trapping in riparian forest buffers is facilitated by physical interception of surface runoff that causes flow to slow and sediment particles to be deposited. Channelised flow is not conducive to sediment deposition and can, having higher velocities, cause erosion in the riparian buffer. From a sediment deposition perspective, the following main processes occur: c) The forest edge fosters large amounts of coarse sediment deposition within a few metres of the field/forest boundary. d) Finer sediments are deposited further into the forest. e) The reverse occurs during out of bank stream flow where sediments carried from upstream in the catchment are deposited in the riparian buffer. The lowest velocities are at the outer edge of the buffer and the finer sediments are deposited there. SMG-6 Apx B Importance to Wildlife The following are important factors relating to wildlife: a) The greater availability of water to plants, frequently in combination with deeper soils, increases plant production and provides a suitable site for plants that would not occur in areas with inadequate water. This increases plant diversity. b) The shape of many riparian areas, particularly their linear meandering nature along streams, provides a great deal of habitat. c) Riparian areas frequently produce more edge within a small area. This edge effect provides diverse nesting and feeding opportunities for wildlife. d) Riparian areas along intermittent and perennial streams provide travel routes for wildlife. Page 42 Updated 01/11/2012

45 e) Although vulnerable to negative edge effects, such as weeds, riparian vegetation maintains habitat required for life cycle completion by riparian species and many instream species. f) Usually riparian margins are the remnants of more extensive natural areas, which is something to build upon for restoration. SMG-6 Apx B Channel Stability and Flood Flow Protection Streams are dynamic systems that are characterised by change. In-stream stability and streambank erosion at a given point are heavily influenced by the land use and condition in the upstream catchment. However, vegetation is essential for stabilising stream banks, especially woody vegetation. Forested buffer strips have an indirect effect on streambank stability by providing deep root systems that hold the soil in place more effectively than grasses, and by providing a degree of roughness capable of slowing runoff velocities and spreading flows during large storm events. While slowing flood velocities may increase flood elevations upstream and in the buffer, downstream flood crest and damage may be significantly reduced. These processes are also critical for building floodplain soils. SMG-6 Apx B.1.5 Vegetation Cover New Zealand s vegetation cover has changed considerably in the past 700+ years, with the most dramatic changes occurring in the past century. Before human settlement, the Tauranga area had a covering of native podocarp/broadleaf, beech forests with some grasslands. Maori cleared some forest for cultivation and hunting, and later European settlement had a greater impact as forests were felled or timber and pasture, and exotic animals and plants were introduced. Due to the conversion of bush areas to pasture and then to an urban environment, very small areas of the City have native vegetation. Forests have a number of components whose characteristics determine its effectiveness in terms of water quantity and quality. These characteristics include: a) Stormwater Runoff Reduction. b) Soil Structure. c) Organic Litter Layer. d) Forested Areas. SMG-6 Apx B Stormwater Runoff Reduction Woody vegetation and forest floor litter have a significant impact on the total volume of rainfall converted to runoff. Runoff volumes from forested areas are much less than volumes from other land uses. This lesser volume in runoff acts to minimise downstream erosion and instability problems. Page 43 Updated 01/11/2012

46 SMG-6 Apx B Soil Structure Forest soils are generally regarded as effective nutrient traps. In New Zealand, most nutrients are retained (and recycled) in the leaf litter and shallow soil layers. Roots are usually quite shallow. The ability of a forest soil to function in removing nutrients in surface and groundwater is partially dependent on soil depth, ground slope, density of vegetation, permeability, extent and duration of shallow water table, and its function as a groundwater discharge zone. SMG-6 Apx B Organic Litter Layer The organic litter layer in a forest buffer provides a physical barrier to sediment movement, maintains surface porosity and higher infiltration rates, increased populations of soil mycorrhizae (a symbiotic relationship of plant roots and the mycellium of fungi - aids in decomposition of litter and translocation of nutrients from the soil into the root tissue), and provides a rich source of carbon essential for denitrification. The organic soil provides a reservoir for storage of nutrients to be later converted to woody biomass. A mature forest can absorb as much as 14 times more water than an equivalent area of grass. The absorptive ability of the forest floor develops and improves over time. Trees release stored moisture to the atmosphere through transpiration while soluble nutrients are used for growth. SMG-6 Apx B Forested Areas Trees have several advantages over other vegetation in improving water quality. They aggressively convert nutrients into biomass. They are not easily smothered by sediment deposition or inundation during periods of high water level. Their spreading root mats resist gullying and stimulate biological and chemical soil processes. They produce high amounts of carbon needed as an energy source for bacteria involved in the denitrification process. A forest s effectiveness in pollution control will vary with the age, structural attributes and species diversity of its trees, shrubs and understory vegetation. To consider the involvement of a forested area in water quality treatment, there are a number of functions that define that performance. These functions can be broadly defined as physical and biological functions and include the following: a) Sediment Filtering: The forest floor is composed of decaying leaves, twigs, and branches, which form highly permeable layers of organic material. Large pore spaces in these layers catch, absorb, and store large volumes of water. Flow of stormwater through the forest is slowed down by the many obstructions encountered. Suspended sediment is further removed as runoff flows into the vegetation and litter of the forest floor. This sediment is readily incorporated into the forest soil. With a well-developed litter layer, infiltration capacities of forest soils generally exceed rainfall and can also absorb overland flows from adjacent lands. Page 44 Updated 01/11/2012

47 b) Nutrient Removal: Forest ecosystems serve as filters, sinks, and transformers of suspended and dissolved nutrients. The forest retains or removes nutrients by rapid incorporation and long term storage in biomass, improvement of soil nutrient holding capacity by adding organic matter to the soil, reduction in leaching of dissolved nutrients in subsurface flow from uplands by evapotranspiration, bacterial denitrification in soils and groundwater, and prevention from erosion during heavy rains. SMG-6 Apx B.1.6 Soils SMG-6 Apx B Biological Factors Influencing Soil Development Soils possess several outstanding characteristics as a medium for life. It is relatively stable structurally and chemically. The underground climate is far less variable than above-surface conditions. The atmosphere remains saturated or nearly so, until soil moisture drops below a critical point. Soil affords a refuge from high and low extremes in temperature, wind, evaporation, light, and dryness. These conditions allow soil fauna to make easy adjustments to the development of unfavourable conditions. On the other hand, soil hampers movement. Except for organisms such as worms, space is important. It determines living space, humidity, and gases. A wide diversity of life is found in the soil as shown in SMG-6 Figure B Soil Life Forms. The number of species of bacteria, fungi, protists, and representatives of nearly every invertebrate phylum found in soil is enormous. It has been estimated that approximately 50% of the earth s biodiversity occurs in soil. Dominant among the soil organisms are bacteria, fungi, protozoans, and nematodes. Prominent among the larger soil fauna are earthworms. Earthworm activity consists of burrowing through the soil. Burrowing involves ingestion of soil, the ingestion and partial digestion of fresh litter, and the subsequent egestion of both mixed with intestinal secretions. Egested matter is defecated as aggregated castings on or near the surface of the soil or as a semiliquid in intersoil spaces along the burrow. These aggregates produce a more open structure in heavy soil and bind light soil together. In this manner earthworms improve the soil environment for other soil organisms by creating larger pore spaces and by mixing organic matter with the mineral soil. Biological processes in soil development are the most complex soil forming factors. Lichens secrete organic acids that dissolve rock surfaces and successions of plants add nitrogen from the atmosphere. Dead roots, stems and leaves decompose and the products are absorbed back into the soil. The vegetative community also has a very strong impact on soil development. The microclimate of a forest is very different from that of grassland. Tree roots penetrate further into the ground than do grass roots and bring up minerals from deeper areas and thus incorporate them into the organic layer. The total amount of soil organic material depends not only upon vegetation, but also upon topographic and climatic influences. Peat formation can occur in basin situations where the water table is high. High levels of organic matter are also found in soils in cool, wet climates. Page 45 Updated 01/11/2012

48 SMG-6 Figure B Soil Life Forms SMG-6 Apx B.1.7 Slopes / Topography The presence of shallow root depths does not resist slope slippage on steeper slopes. Soil slippage is also directly related to the steepness of the slope, the type of soil and the underlying geology. Without deeper-rooted plants holding a slope, in situations where native vegetation has been replaced by grassed lawn, slopes in excess of 33% (180) may start to creep. Slopes greater than 45% (240) may see the onset of mass movement. In the case of Onerahi Chaos Breccia (Northland Allochthon where the soils are highly sheared and crushed variably calcareous and siliceous mudstones prone to slippage), slopes as flat as 1:8 can be unstable. Due to the shallow nature of the soils, most movement tends to occur in the first 1.5 metres. Leaving native vegetation on these steeper slope areas is very important to maintain slope stability. Recent studies in New Zealand have assessed the susceptibility of different vegetation types to landslides during rainstorms. In a study north of Gisborne landslide densities were 16 times greater under pasture than indigenous forest and 4 times greater under pasture than regenerating scrub. A survey of storm damage in Tertiary sandstone/siltstone hill country reported that landslides in pasture were 3-4 times greater than in indigenous forest. Finally, in a detailed study of the relationship between slope morphology, regolith depth, and landslide incidence in eastern Taranaki hill country, identified a 10 times increase in erosion rate for modal slopes of degrees following deforestation. Page 46 Updated 01/11/2012

49 SMG-6 Apx B.1.8 Other Natural Features There are other natural conditions that exist on sites beyond those discussed to this point. Those discussed earlier are the primary ones in terms of overall importance but there are others and consideration of their importance is in order. SMG-6 Apx B Depression Storage Of the rainfall that strikes roofs, roads, pathways, and pervious surfaces, some is trapped in the many shallow depressions of varying size and depth present on practically all ground surfaces. The specific magnitude of depression storage varies from site to site. Depression storage commonly ranges from 3 to 19 mm for flat areas and from 12 to 30 mm on grasslands of forests. Significant depression storage can also exist on moderate or gentle slopes with some estimation for pervious surfaces being between 6 to 12 mm of water and even more on forestland. Typical depths on moderate slopes can be 1 to 2 mm for impervious surfaces, 2 to 4 mm for lawns, 4 mm for pastures and 6 mm for forest litter. Steeper slopes would obviously have smaller values. When using traditional hydrologic procedures, depression storage is contained in an initial abstraction term. The term includes all losses before runoff begins. It includes water that is ponded, retained by vegetation, evaporation, and infiltration. It is highly variable, but generally is correlated with soil and cover parameters. Prior to urbanisation, catchments have a significant depressional storage factor. Passing through agricultural or wooded areas after significant rainfall clearly demonstrates the existence of depressional storage. The urbanisation process generally reduces that storage in addition to significantly modifying the land s surface. The combination of site compaction, site imperviousness, and reduced depression storage causes dramatic increases in downstream flood potential and channel erosion. Information from the Mahurangi catchment north of Auckland indicates that longterm average annual predicted runoff varied from less than 300 mm (18% of rainfall) to greater than 600 mm (greater than 35% of rainfall). The 300 mm coincided with subcatchments under permanent forest cover. The 600 mm coincided with subcatchments in predominantly pastoral land use and on low infiltration soils. There is a clear statement in these statistics that significant volume reductions in runoff exist in forested catchments as opposed to volumes of runoff from pastoral land cover. SMG-6 Apx B Natural Drainage Systems Natural site drainage features exist on every site. The most common of these features is having an existing flow path for stormwater runoff. Water doesn t travel down a hill in a straight line. Straight lines are something that humans have developed to accelerate the passage of water downstream as quickly as possible. During site development, the tendency is to place water in conveyance systems, open and enclosed, which follow the shortest distance to site outfalls. Page 47 Updated 01/11/2012

50 Shortening the flow distance effectively increases the slope that water travels on, accelerates the flow of water, and increases the ability of water to scour downstream receiving systems. When water travels over a meandering flow path, energy is dissipated, which reduces the erosion potential. Shortening flow lengths reduces energy expended and increases the available erosion producing energy. Stream channels will meander regardless of the degree of human alteration. Replicating existing flow paths and lengths, to the extent possible, promotes channel stability and increases function and value. The additional functions provided by meandering channels over straight channels are also simply related to the length of the aquatic resource and the time that the water is in contact with the various biotic and abiotic processing mechanisms. The additional length of meandering channels provides a greater total quantity of aquatic resource, and the associated functions and values they provide. SMG-6 APX B Uncompacted Vegetated Areas A common approach to site development is to clear most, if not all, of the site being developed. Existing vegetated areas of the site are often cleared even when in nonessential locations. Clearing and grading of areas that will remain pervious results in significant compaction of those areas. This compaction reduces expected infiltration rates and increases overland flow. A key issue with respect to urban development is the issue of significant soil compaction. The activity of heavy earth moving equipment on a construction site causes significant compaction of soils whose surface is designed to remain pervious. Landcare Research (Zanders, 2001) did some investigation on urban soils and found that earthworks had a significant adverse effect on water flow through soils after cut and fill operations were conducted. The earthworks allowed virtually no percolation of water through Horizon 2 soil profile. There are three options to address this concern: a) Where cuts or fills of at least one metre are intended to facilitate site development, the expected permeability of the soil may be reduced. Stormwater management calculations that detail post construction hydrology should use a modified approach to soil classifications. b) In areas of significant site disturbance, and where there is less than one metre of cut or fill, soil classifications are not modified, but consents should contain a construction requirement that significantly disturbed soils in areas where those soils remain pervious should be chisel ploughed. Chisel ploughing will break the surface crust of the disturbed soil and allow for a greater infiltration rate. This would then provide a good foundation for the placement of topsoil and prevent slippage of the topsoil when on slopes that become saturated. c) Keep equipment out of areas preserved for open space Page 48 Updated 01/11/2012

51 SMG-6 Apx B.1.9 Linkage With Site Development The only way that site development can occur in a manner that integrates existing site resources is to identify those site resources present on the site prior to initiation of site design. The first step in site resource integration is in conducting an inventory of site resources and detailing them on a plan. A simple checklist as shown in SMG General Context Information which is based on the items presented here. The checklist could include wetlands, floodplains, riparian buffers, vegetative cover, soils, steep slopes and other natural feature, all of which have been discussed throughout this section. A checklist should also include: a) Archaeological sites. b) Cultural sites. This plan should be included as a part of the stormwater management plan submitted to Council. A narrative should also be submitted to detail what steps have been considered and/or provided to integrate existing resources into the stormwater management plan. SMG-6 Apx B.2 Natural Mechanisms for Stormwater Pollution Removal Although many stormwater related contaminates can be reduced if not eliminated through preventive design approaches driven by water quantity reduction objectives, not all contaminants can be eliminated. In such situations, an array of natural pollutant removal processes is available for use and should be exploited to the maximum. Because these processes tend to be associated with, even reliant upon, both vegetation and soil processes, they can be readily incorporated into other low impact design approaches. Such natural contaminant reduction/elimination processes include: a) Settling / Deposition b) Filtering c) Biological Transformation and Uptake / Utilisation d) Chemical Processes SMG-6 Apx B.2.1 Settling / Deposition The kinetic energy of stormwater washes all types of matter, particulate form and other, from land cover surfaces. Particulates remain suspended in stormwater flows as long as the energy level is maintained. Heavier particulates require more kinetic energy in order to remain in suspension. As the energy level declines - as the storm flow slows, these suspended particulates begin to settle out by gravity, with larger, heavier particulates settling out most quickly and the smallest colloidal particulates requiring considerably more time for settling. To the extent that time can be maximised, more settling can be expected to occur, holding all other factors constant. Therefore, approaches which delay stormwater movement or approaches Page 49 Updated 01/11/2012

52 which reduce kinetic energy in some manner (e.g., energy dissipaters) serve to maximise settling and deposition. SMG-6 Apx B.2.2 Filtering Another natural process is physical filtration. As contaminants pass through the surface vegetative layer and then down through the soil, larger particulates are physically filtered from stormwater. Vegetation on the surface ranging from grass to underbrush removes larger contaminant particulates. Stormwater sheet flow through a relatively narrow natural riparian buffer of trees and undergrowth has been demonstrated to physically filter surprisingly large proportions of larger particulates. Both filter strips and grass swales rely very much on this filtration process. Filtration may also occur as stormwater infiltrates into the topsoil strata. SMG-6 Apx B.2.3 Biological Transformation and Uptake/Utilisation Although grouped as one type, this category includes a complex array of different processes that reflect the remarkable complexity of different vegetative types, their varying root systems, and their different needs and rates of uptake of different contaminants. An equally vast and complex community of microorganisms exists within the soil mantle, and though more micro in scale, the myriad of natural processes occurring within this realm is just as remarkable. Certainly both phosphorus and nitrogen are essential to plant growth and therefore are taken up typically through the root systems of the various vegetative types, from grass to trees. SMG-6 Apx B.2.4 Chemical Processes As stormwater infiltrates into the soil mantle and moves toward groundwater aquifers, various chemical processes also occur. These processes include adsorption through ion exchange and chemical precipitation. Cation exchange capacity (CEC) is a rating given to soil, which relates to a particular soil s ability to remove contaminants as stormwater enters the soil mantle (through the process of adsorption). Adsorption will increase as the total surface area of soil particles increases; this surface area increases as soil particles become smaller, and as soil becomes tighter and denser (clay has more surface area per unit volume than does sand). Low impact design techniques offer an array of natural processes and techniques that substantially increase contaminant removal potential above and beyond mitigation being provided by many of the structural stormwater practices. Through a combination of vegetative-linked removal combined with using soils on a site, contaminants entrained in stormwater runoff are removed and in some cases eliminated. In this way, contaminants are prevented from making their way into either surface or groundwaters. The various design details are discussed later in this guideline. Page 50 Updated 01/11/2012

53 SMG-6 Appendix C Bush Revegetation SMG-6 APX C.1 General Bush revegetation reduces site runoff through: a) Leaf Canopy Interception. b) Evapotranspiration. c) Soakage. d) Flow Retardance. SMG-6 Figure C.1 Bush Revegetation SMG-6 Table C.1 Stormwater Management Function - Bush Revegetation Water Quality Metals Sediment Flood Protection Stream Channel Erosion Protection x Relating to land use, stormwater runoff is greatest from impervious surfaces. Less runoff is generated from pasturelands. Native bush that is protected from grazing and having litter and brush covering the ground generates the least amount of stormwater runoff. Page 51 Updated 01/11/2012

54 When land is being converted from rural to residential, commercial or industrial land use the total volume and peak rate of stormwater runoff are increased. As pastureland has a greater volume of stormwater runoff than does bush, conversion of existing pastureland into bush can reduce future runoff and mitigate for the effects of increased impervious surface generation. SMG-6 Apx C.2 Basic Design Parameters The approach is based on extent of area that is set aside for re-establishment of bush. Key considerations related to re-establishment are the following: a) Existing areas of bush that can be extended. b) Natural site features. c) Slope. d) Location of waterways. Providing additional bush to existing bush areas would increase the value of existing bush by increasing bush interior areas. This would reduce fringe vegetation that could become a weed maintenance problem. When sizing bush restoration for various lot sizes, the level of imperviousness will be very important. As lot size reduces from 1 hectare to 2,000 m 2, the proportion of the site that is impervious will increase the required bush area. Under an assumption of a 1 ha lot having 600 m 2 of imperviousness; it will take 3500 m 2 of bush to compensate for that impervious surface. If a lot is 0.5 ha and the imperviousness of the lot remains at 600 m 2 the amount of bush to compensate for the impervious surface is still 3,500 m 2 but that will represent approximately 70% of the site area rather than 35%. If the site area goes below 0.5 ha bush cannot compensate for 600 m 2 imperviousness. of The approach can be used on a subdivision or catchment wide basis, where area can be set aside, converted to bush and overall subdivision or catchment stormwater runoff reduced. It is not only an individual site practice. Revegetation does not have to totally mitigate for impervious surfaces but it can reduce stormwater runoff increases and reduce the amount of work that other practices have to accomplish to minimise adverse impacts. SMG-6 Apx C.3 Targeted Contaminants While native bush vegetation having a good ground cover can provide contaminant reduction benefits, its main purpose is the reduction of stormwater runoff volumes. Organic matter on the bush floor will remove metals and assist in removal of sediments but residential land use in rural areas does not generate large contaminant loads. Commercial and industrial land use may increase contaminant loads but other practices provided in the guidelines would provide greater levels of treatment. Page 52 Updated 01/11/2012

55 SMG-6 Apx C.4 Advantages Native bush grows over time and maintenance concerns diminish. Where other stormwater management practices need maintenance to ensure long-term performance, bush revegetation improves its hydrological function over time and maintenance obligations become minimal. Native bush also provides benefits for wildlife habitat, shading and cooling during summer. It can act as a windbreak and can be an aesthetic amenity. SMG-6 Apx C.5 Limitations Native bush planting can have fairly high maintenance needs during the first 2-3 years of growth relating to weed control and possible watering needs during drought conditions. Native bush can also be seen as limiting site usage. If some livestock were a desired activity on the site, they must be excluded from access to the bush areas to ensure that bush growth is not adversely affected. When planted in widths of less than 20 metres, weeding can remain a problem for years. SMG-6 Apx C.6 Design Sizing Bush re-establishment is based on SMG-6 Table C.6 Bush Planting Requirements. SMG-6 Table C.6 Bush Planting Requirements Proposed site impervious area (m 2 ) Area of bush required (m 2 ) 100 1, , , , , ,500 The calculations, using an annual runoff spreadsheet approach that calculates storm and base flow under various landuse scenarios (Beca Carter Hollings & Ferner, 2000), work out to be fairly consistent. For every 100 m 2 of imperviousness beyond the first 100 m 2 of imperviousness there is a 500 m 2 requirement for bush establishment. Recognising the significant areal extent of bush replacement, it may be best to isolate various impervious surfaces and address them separately. That would allow for several practices to provide site management without using too much of a given portion of the site to any one practice. Page 53 Updated 01/11/2012

56 SMG-6 Apx C.7 Case Study A house on 1 hectare is being constructed and the footprint for the house and driveway is 550 m 2 of imperviousness. The site, as shown in SMG-6 Figure C.6 Bush Revegetation for Runoff Control, has a house, driveway, septic system and needs 3,250 m 2 to compensate for impervious surfaces. Since the roof of the house has a water tank that was designed as in the water tank design section then the 250 m 2 can be excluded from the bush revegetation approach. In that case, the impervious surface is now 290 m 2 so the bush replacement area is now 1,950 m 2, which is a significantly reduced area. Using practices in conjunction with one another can significantly reduce the size of a practice if it is used to address all of the areas. SMG-6 Figure C.7 Bush Revegetation for Runoff Control Page 54 Updated 01/11/2012

57 SMG-6 Appendix D Low Impact Design Costing SMG-6 Apx D.1 General There are a number of significant stormwater problems that are related to continued growth, development and redevelopment within Tauranga City such as: a) Increased flooding potential. b) Increased volume and rates of stormwater which overflows reticulation systems and increases stream channel erosion. c) Degradation of water and sediment quality. d) Costs associated with long term maintenance of constructed stormwater practices built to mitigate adverse effects. For a number of years now, Low Impact Design (LID) has been offered as a solution to address the effects of stormwater discharges. LID is a philosophy regarding site design and development rather than just about managing stormwater at its source through vegetative practices such as swales or rain gardens. It is premised on stormwater being considered up front as part of the design process, rather than being dealt with as an afterthought. By addressing stormwater issues at this initial, concept design stage, opportunities may be created for the protection, remediation or enhancement of natural resources. Worldwide there has been considerable research undertaken to document the environmental protective and social benefits of LID. However, a key impediment to implementation has been the perception that LID costs more to implement both in the short term (i.e. construction and development costs), and long term (i.e. operating and maintenance costs). The purpose of this appendix is to investigate costs associated with LID in order to aid decision making processes. SMG-6 Apx D.2 Quantifying the Cost of Low Impact Design Council strongly supports a Low Impact Design (LID) approach to stormwater related issues. In order to pursue an LID approach there are two key questions that need to be answered: a) Is LID an economically viable method of development? b) Could LID assist in reducing the costs of stormwater management resulting from conventional development? SMG-6 Apx D.3 Understanding and Determining Cost Cost estimation plays a key role in all development activities. For developers, the bottom-line reality of cost usually outweighs marginally increasing environmental improvements that were gained from using alternative technologies. For Council, the cost of long term maintenance of stormwater infrastructure is an important issue. Page 55 Updated 01/11/2012

58 In addition, in New Zealand there are two key pieces of legislation which direct councils and the development community to consider cost as a decision making criteria: a) Resource Management Act (1991) (RMA) - Sets the statutory framework requiring the management of stormwater discharges. Stormwater Discharge Permits are issued under Section 15 of the RMA which controls the discharge of "contaminants or water into water". The RMA also requires that development "avoid, remedy or mitigate" the effects of these discharges on the receiving environment. However, it requires this in the context of understanding the social, cultural, environmental and economic implications of different development options. b) Local Government Act (2002) (LGA) - Sets the legislative framework for how councils must function. Under the LGA, councils prepare a Long Term Plan (LTP) which identifies community outcomes for implementation over the next 10 years. Providing for good stormwater management (i.e. building and maintaining stormwater networks to reduce flooding, stormwater contamination and stream erosion) is a key part of most LTPs. SMG-6 Apx D.4 Methods of Assessing Low Impact Design Economics Despite the importance of cost as a tool in the decision-making process, until recently, there has been scant research undertaken in New Zealand on quantifying costs of stormwater management, and on developing consistent methodologies for determining cost. Internationally, there are three methods that are usually used to assess the economics of Low Impact Design: a) Life Cycle Cost Analysis. b) Cost Comparisons. c) Cost-Benefit Analysis (North Carolina State University, undated). SMG-6 Apx D.4.1 Life Cycle Costing Analysis A life cycle costing (LCC) approach has been suggested as an important way to estimate costs associated with stormwater devices (Taylor, 2003; NZWERF, 2004; SKM, 2004). AS/NZS 4536:1999 defines LCC as "the process of assessing the cost of a product over its life cycle or portion thereof". The life cycle costing process assesses the acquisition and ownership costs of an asset over its life span, from the planning and design stage, to the construction stage, to the usage and maintenance stage and finally through to disposal. A cradle-to-grave time frame is warranted because future costs associated with the use and ownership of an asset are often greater than the initial acquisition cost, and may vary significantly between alternative solutions (Vesely et al., 2006). Life cycle costing is therefore able to describe the type, frequency and level of cost associated with a specific stormwater practice across the life span of that practice as shown in SMG-6 Figure D.4.1a Phases in the Life Cycle of a Stormwater Practice and Potentially Associated Costs. Page 56 Updated 01/11/2012

59 As a result, life cycle costing has a number of benefits and supports a number of applications and analyses (Lampe et al. 2005): a) It allows for an improved understanding of long-term investment needs. b) It helps Council make more cost-effective choices at the project scoping/ feasibility phase. c) It provides for an explicit assessment of long-term risk. d) It reduces uncertainties and helps Council determine appropriate development contributions. e) It assists Council in its budgeting, reporting and auditing processes. SMG-6 Figure D.4.1a Phases in the Life Cycle of a Stormwater Practice and Potentially Associated Costs (Taylor, 2003) Decision making on the use of low impact stormwater devices needs quality data on the technical and financial performance of these devices. The financial performance depends on the sum and distribution, over the life cycle of the device, of the acquisition and maintenance costs which include design, construction, use, maintenance, and disposal. Life cycle costing can be used for structuring and analysing this financial information. However, while life cycle costing is an important tool in understanding the costs associated with infrastructure development, it is only one parameter in the evaluation process as shown in SMG-6 Figure D.4.1b The use of Life Cycle Costing in an Evaluation Process such as the Design of a Stormwater Management Practice and needs to be considered in the context of social, cultural and environmental goals. Page 57 Updated 01/11/2012

60 SMG-6 Figure D.4.1b The use of Life Cycle Costing in an Evaluation Process such as the Design of a Stormwater Management Practice (Taylor, 2003) Life cycle costing can be done using either a statistical, or unit cost approach. A statistical approach is based on developing a statistically significant relationship between the size of a practice and its acquisition and/ or maintenance costs. Unit costing, however, involves identifying individual elements of the acquisition and maintenance phase and costing them using average tender rates (Ira et al., 2008). The life cycle costing model which is being developed for New Zealand by Landcare Research, "Costing of Stormwater Treatment for New Zealand" (COSTnz), primarily uses unit costing. SMG-6 Apx D.4.2 Cost Comparisons The second way of quantifying costs associated with Low Impact Design (LID) is to undertake cost comparisons of conventional developments and compare these with costs associated with LID developments. There have been a number of these types of studies done both here in New Zealand as well as in the United States. However, given that LID is a relatively new concept and many of the projects discussed and presented are pilot projects, there is little data on the long term maintenance aspects of LID and, therefore, the comparisons tend to focus solely on differences in construction related costs. The types of costs quantified during these analyses include: Page 58 Updated 01/11/2012

61 a) Clearing and earthworks. b) Pavement construction. c) Stormwater reticulation. d) Wastewater reticulation. e) Water reticulation. f) Trenching. g) Concrete works. h) General and day works. These costs are generally compared for the conventional and alternative site designs and assessed to determine whether or not a developer s profit margin will be increased or decreased as a result of a low impact development. Owing to the focus of cost comparison case studies on construction costs, combining them with life cycle costing assessments would assist in presenting a more complete picture of the actual long term costs involved in low impact design. SMG-6 Apx D.4.3 Cost-Benefit Analyses A cost-benefit analysis considers not only the full range of costs associated with undertaking life cycle costing but also considers the economic benefits of a project. The analysis is more complex and time consuming than life cycle costing but it does assist in highlighting that there are occasions where the economic benefits of undertaking low impact design projects can outweigh any additional expected costs (North Carolina State University, undated). Environmental goods and services (e.g. clean air, good water quality, healthy fish, etc) are not easily measured in monetary terms as they are not tradable commodities. As a result, it becomes increasingly more difficult to attempt to quantify their value to a community, or the loss of value resulting from degradation. The estimation of these values is called non-market valuation (North Carolina State University, undated) and is an important part of the cost-benefit analysis process. Ward & Scrimgeour (1991) used this non-market valuation method to quantify some of the aspects of Auckland s Waitemata Harbour, which would have some similarities to Tauranga Harbour. The total economic benefits derived from the Waitemata Harbour amounted to approximately $442 million (in 1991 dollars) annually. Scenarios were also considered to calculate future losses as a result of deterioration in water quality. The study shows that there are significant financial benefits to implementing stormwater quality treatment to maintain the harbour values. Due to the lack of research regarding non-market benefits in New Zealand, this information cannot be discussed further. Rather, it focuses on using life cycle costing and cost comparison to determine the costs associated with low impact design. Page 59 Updated 01/11/2012

62 SMG-6 Apx D.5 Economic Benefits of Low Impact Design Many reports describe the environmental and social benefits of Low Impact Design (LID) however, few of these studies quantify these benefits in economic terms (ECONorthwest, 2007). In fact, when asked about needs for further research into LID, many researchers cite the need for measuring and quantifying LID benefits in economic terms. Vesely et al. (2005) goes further by saying that negative economic impacts of conventional controls should also be quantified economically; otherwise management decisions will continue to be biased towards conventional controls. The authors state that: "Exclusive reliance on profitability and market value will favour the conventional approach to stormwater management by disregarding both the negative environmental externalities associated with this approach, and the positive environmental externalities associated with the low impact approach. Even when an attempt is made to include environmental benefits such as water savings, market distortions prevent the true manifestation of the associated impact. New Zealand costs and rates reflect the historically free treatment of water in this country." (page 12). SMG-6 Apx D.6 Example Benefits to Tauranga City Benefits to Tauranga City relate mainly to property value and maintenance aspects, and can include: a) Protection of Rate s Revenues: Protecting water quality and having property abutting open green spaces helps to protect or increase real estate values which in turn protect rate s revenues. b) Reduced Expenditure: Low Impact Design (LID) can reduce Council expenditures on stormwater piped infrastructure and expensive retrofits. This can also lead to reduced costs associated with the need to provide large scale flood management and water quality treatment practices. c) Reduced Maintenance Costs: Reduced infrastructure allows for reduced maintenance costs of pipe infrastructure and system-wide operations. d) Increased Life Span then a Reticulated System: Vegetated systems normally have longer life spans than piped infrastructure (North Carolina State University, undated). e) Avoidance of Costs: LID can also lead to Council avoiding or reducing costs associated with reduced downstream flooding and flood damage. Avoided costs of flood damage were quantified by the American Forests Report ( ) on the economic benefits of stormwater services provided by trees. SMG-6 Table D.6 The Avoided Stormwater Construction Costs Attributed to Trees highlights the amount that councils can save (one-off construction costs of flood basins) by having trees within catchment areas (ECONorthwest, 2007). Page 60 Updated 01/11/2012

63 SMG-6 Table D.6 The Avoided Stormwater Construction Costs Attributed to Trees (American Forests CITYgreen model (ECONorthwest, 2007) US$) Urban Area Houston, Texas Atlanta, Georgia Vancouver, Washington/ Portland-Eugene, Oregon Washington D.C Metro Area New Orleans, Louisiana San Antonio, Texas San Diego, California Puget Sound Metro Area, Washington Detroit, Michigan Chesapeake Bay Region Amount that trees save in one-time stormwater-construction costs $1.33 billion $2.36 billion $20.2 billion $4.74 billion $0.74 billion $1.35 billion $0.16 billion $5.90 billion $0.38 billion $1.08 billion Source: American Forests SMG-6 Apx D.7 Example Benefits to Developers Economic benefits of Low Impact Design (LID) to developers include: a) Increased Property Value: Properties abutting natural open space are generally found to be more desirable to potential home buyers. LID developers find that property values are increased and sales are better. Powell et al. (2005) reported that clustered housing sold at an average of US$17,000 more than housing on conventional subdivisions, and the Triangle Greenways Council found the average house price adjacent to green space was US$5,000 more than houses located away from green areas. North Carolina State University (undated) report that LID lots sold for US$3,000 more than lots in conventional subdivisions. b) Increased profit margins - LID, through clustering, often allows for an increased number of lots and reduced construction costs, thereby increasing profit margins for developers (Auckland Regional Council, 2000). Similar results have been found for LID developments in the US. A LID subdivision at Gap Creek (Arkansas) resulted in a US$2.2 million economic benefit to the developer (refer SMG-6 Table D.7 Projected Economic Benefits of a LID Development at Gap Creek, Arkansas, when Compared to a Conventional Subdivision Design). Reduced construction costs are normally the result of less paving, lower site preparation, less grading and less earthworking activities. Page 61 Updated 01/11/2012

64 SMG-6 Table D.7 Projected Economic Benefits of a LID Development at Gap Creek, Arkansas, when Compared to a Conventional Subdivision Design (Shaver 2009) Projected Results from Total Development Total Site Conventional Development LID Development Lot yield Linear street length 6,635 metres 6,439 metres Linear collector street lengths 2,243 metres 0 metres Linear drainage pipe 3,078 metres 2,052 Drainage structures: Inlets/boxes/headwalls Estimated cost per lot $12,907 $10,512 Actual Results from First Phase of Development Phase 1 Conventional development (Engineers LID Development (actual figures) estimated figures) Lot yield Total cost $1,028,544 $828,523 Total cost per lot $16,326 $11,507 Higher lot yield Higher lot value Lower cost per lot Enhanced marketability Added amenities Total economic benefit Economic and Other Benefits from LID Development 17 additional lots $3,000 more per lot over competition $4,800 less cost per lot 80% of lots sold in first year 9.5 hectares of green-space parks More than $2,200,000 added to profit SMG-6 Apx D.8 Example Benefits to Homeowners and the City in General Examples of economic benefits of Low Impact Design (LID) to homeowners and the community include: a) Increased Property Values: LID can potentially reduce downstream flooding. Studies (North Carolina State University, undated; ECONorthwest, 2007) show that a marginal reduction in flooding increases the value of properties within floodplain areas by approximately 5%. Property values also increase due to the proximity of houses to green open spaces. Page 62 Updated 01/11/2012

65 b) Increased Amenity Values: In Seattle their analysis of retrofitted "green streets" (such as SEA Street) added up to 6% to property values (North Carolina State University, undated). Prior to the earthquakes of 2011, the City of Christchurch also report that properties adjacent to daylighted and restored streams saw an increase in $20,000 as a result of the increased amenity value of the land surrounding the properties. c) Avoidance Costs, to the City, of having Clean Water, Reduced Flooding and the Protection of Natural Systems: Although this is difficult to quantify, through the implementation of LID, Council could avoid costs by reducing its reliance on large constructed stormwater systems in two ways. Firstly, by having improved water quality by dealing with stormwater at the source and secondly, by reducing the volume of water discharged through reduction in impervious areas. SMG-6 Apx D.9 Costs of Low Impact Design Each of the following components of a Low Impact Design (LID) development is discussed in relation to its construction and maintenance costs: a) Minimising Site Disturbances. b) Cluster Development. c) Reducing Impervious Areas through Streetscape Design. d) Creating or Enhancing Natural Areas. e) Water Reuse. f) Using Vegetative Practices (rain gardens, swales, filter strips). g) Using Infiltration to reduce runoff. Where available, New Zealand specific data has been used to compile this information and has been supplemented with information from international studies. SMG-6 Apx D.9.1 Minimising Site Disturbances From a cost perspective, minimising disturbances has two key implications. Firstly, by reducing the amount of earthworks as well as clearing and grading, there will be reduced costs associated with the construction activity e.g. excavation, stockpiling, disposal, replacement of soils and material. Therefore, if the amount of clearing and grading required is reduced, the cost of the construction process would be reduced. Further savings would be made as there would be a reduced need for sediment and erosion control on the site. The Conservation Research Institute (2005) reports that minimising disturbances on site and cluster development can reduce the need to earthwork by 35% 60% when compared to a conventional subdivision design. Secondly, by reducing disturbance to naturally vegetated areas, one is able to reduce the volume of stormwater generated by the site development and therefore reduce the work that would need to be done by constructed stormwater practices. This in turn reduces the cost associated with constructing and maintaining these devices. Page 63 Updated 01/11/2012

66 SMG-6 Apx D.9.2 Cluster Development Conventional development encourages sprawl, whilst Low Impact Design (LID) approaches encourage smaller lots on a portion of the site in order to achieve the same, or even an increased, density (Shaver, 2009). Clustering development means reducing road lengths, house setbacks and sprawling development. Consequently, the amount of stormwater piping, impervious road area and lengths for utilities (power, water and sewage) would also be reduced, whilst open space areas are increased. SMG-6 Apx D Construction Class Impervious surfaces and associated infrastructure are expensive to construct. If the length required for piping reduces, the cost of trenching and installing pipes as well as the materials for the pipes themselves will be drastically reduced. The cost of road construction is also a significant part of the development budget. If the road length is reduced, the construction costs of the development would also be reduced. A study in the United States found that by clustering (and therefore reducing road and service lengths), large-lot residential developments are able to save around 25% and smaller lots e.g. 2000m 2, achieve smaller savings of around 10% (Conservation Research Institute, 2005). Clustering, like minimising disturbances, also allows for more vegetated areas to be preserved on site therefore, also reduces the cost associated with constructing and maintaining downstream devices. SMG-6 Apx D Maintenance Costs Studies have not yet quantified the effects of clustering and reducing impervious areas on long term maintenance. However, one can infer that if lengths of road, utilities and piping and sizes of constructed stormwater practices were to be decreased, costs associated with maintaining and repairing these roads and services, would decrease correspondingly. SMG-6 Apx D Clustering Conclusions By reducing road lengths and corresponding lengths of services, costs during the development phase of a project can be reduced, and savings ranging from 10% 25% can be achieved. Although there is no quantifiable data on long term maintenance, it can be surmised that as the length of a service decreases, so the cost of maintaining that service would decrease correspondingly. Page 64 Updated 01/11/2012

67 SMG-6 Apx D.9.3 Reduced Impervious Areas through Streetscape Design Reducing impervious areas allows for a reduction in the effects of stormwater discharges and the work required by constructed stormwater practices. SMG-6 Apx D Construction The discussion on costs relating to reducing impervious areas through streetscape design and parking is similar to that in SMG-6 APX D.9.2 Cluster Development. Reducing road widths and parking lot ratios can lead to substantial savings during the construction phase of a project. This saving not only relates to the reduced impervious area that needs to be constructed but also to the reduction in size of any stormwater practices required to mitigate stormwater effects from remaining impervious areas. Schueler (1995) reported that the cost of constructing a single car parking space can be US$1,100. He also worked out that when maintenance costs are included, lifetime savings for each parking space eliminated can be US$5,000 to US$7,000. The elimination of kerbing is another practice advocated through Low Impact Design. Kerbing concentrates flow creating point source discharges and causing effects on the downstream aquatic environment. If kerbing is installed, there would be a need to install street sumps as well as a piped network. By eliminating kerbing and pipes, stormwater can be transported in conveyance systems such as swales or discharged diffusely across filter strips. A study was originally undertaken by Backstrom et al. (cited in Conservation Research Institute, 2005) for swale and pipe systems in Europe and they showed that piped systems tended to be 34% to 80% more expensive than swales. It should be noted that kerb and channel components did not form part of this analysis and it can therefore, be inferred that when this aspect is taken into account, conventional piped systems are even more costly than what is shown here. It should also be noted that where swales require driveway crossings, the cost differential shown above would be reduced (the more crossings required, the higher the cost of the swale). SMG-6 Apx D Maintenance The difference in maintenance costs between conventional kerb and channel reticulated systems and low impact design swale conveyance systems have not been quantified. Some inferences however, can be made. If constructed correctly, given that swales are above ground and tend to have long life spans, maintenance and replacement costs are likely to be less than underground piped systems. As swales move towards more naturalised systems (e.g. using native species such as Oioi for planting rather than grass), these maintenance costs would be further reduced due to the reduced need for mowing and care of plants (Conservation Research Institute, 2005). Page 65 Updated 01/11/2012

68 SMG-6 Apx D Reduced Impervious Areas Conclusions International studies show that reducing parking areas, road widths, road lengths, and kerb and channel systems would reduce construction costs. This is also backed up by New Zealand unit cost data on the cost of road, parking, kerb and pipe construction. In addition, there seems to be agreement in the literature that swale systems (used for conveyance) are also cheaper than piped systems. Whilst it can be inferred that maintenance costs of swales or filter strips are likely to be less than piped systems, further research is needed before the actual long term costs can be fully quantified. SMG-6 Apx D.9.4 Creating or Enhancing Natural Areas By creating and re-establishing native bush areas on a site (such as riparian cover to protect steep slopes, or general revegetation), significant stormwater benefits from a water quantity and quality perspective, can be achieved (Shaver, 2009). SMG-6 Apx D Construction There is often the perception with Low Impact Design (LID) developments that landscaping costs are far higher than in conventional subdivisions because of all the "vegetated" areas within the site. Literature on this aspect states otherwise. Because many of these vegetated areas are natural areas that have not been disturbed, regraded or replanted, costs involved in the creation of natural areas are often actually lower than in conventional developments. Conventional developments will normally regrade the whole site and then begin the process of landscaping and replanting (often with grass). This leads to additional costs associated with grading the site as well as replanting costs. If pastoral areas are to be revegetated, as part of a bush replanting scheme to offset increases in impervious areas, then costs of planting these areas may well be more expensive than the costs of landscaping a conventional subdivision. However, the bush replanting costs need to be placed in perspective with their stormwater function. By planting areas back into native bush, the need for large scale constructed stormwater practices, such as ponds or wetlands, becomes defunct. Therefore, the stormwater management costs for the site would be dramatically reduced as highlighted in SMG-6 Table D Difference in Cost of Bush Replanting and a Stormwater Wetland, Constructed to Offset Increases in Stormwater Peak Flows, for an Impervious Catchment Area of 5000m². Page 66 Updated 01/11/2012

69 SMG-6 Table D Difference in Cost of Bush Replanting and a Stormwater Wetland, Constructed to Offset Increases in Stormwater Peak Flows, for an Impervious Catchment Area of 5000m² Stormwater Practice Area Required to offset 5000m 2 imperviousness Construction Cost (utilising low cost estimates) Bush replanting 24,900 m 2 $622,500 Stormwater Wetland 1075 m 2 $864,000 (Surface area at 100 (from COSTnz) year ARI storm event volume) SMG-6 Apx D Maintenance Maintenance is required for all types of landscaping, be it grassed areas, flower beds or replanted native bush areas. In New Zealand there has been little data collected on the different costs associated with maintaining different types of vegetation. The COSTnz Model differentiates between routine general maintenance i.e. mowing and maintaining healthy vegetation cover and aftercare of bush / native plants. The Model estimates that costs associated with mowing are in the order of 20c to 50c per square metre and that aftercare of bush areas are around 25c to 29c per square metre. This would infer that there is not a significant difference in the actual maintenance cost incurred for different types of vegetation. It is important to realise that the mowing of grassed areas is long term and involves on-going costs however, aftercare of native bush areas will reduce with time. The Conservation Research Institute (2005) reported that, within the United States, maintenance costs of native vegetation are significantly less after the first 5 years of establishment and will decrease further with time as highlighted in SMG-6 Table D Ten Year Maintenance Costs for Native Plants vs Turf Grass. SMG-6 Table D Ten Year Maintenance costs for native plants vs turf grass (taken from Conservation Research Institute 2005). Costs are given in US Dollars. 10 Year Average Maintenance Cost per Acre* Landscape Treatment Low End Estimate High End Estimate Turf Grass $5,550 $6,471 Native Landscaping $1,600 $1,788 Difference (Savings from Native Landscaping) * 1 acre = 0.4 hectares $3,950 $4,683 Page 67 Updated 01/11/2012

70 SMG-6 Apx D Creating or Enhancing Natural Areas Conclusions New Zealand and international research shows that protecting natural areas on site and replanting areas with native bush in order to offset increases in impervious areas, is more cost effective both in the long and short term than conventional development approaches. SMG-6 Apx D.9.5 Water Reuse Using water from roof areas for potable and non-potable uses can significantly reduce the volume of water discharged to downstream receiving environments. More recent Low Impact Design (LID) subdivisions have even used tanks to drain parking areas in order to reuse the water for gardening purposes. Conventional subdivisions, on the other hand will generally connect to mains water supply for both potable and non-potable water uses. In New Zealand, a 25,000 litre rain tank can cost between $3,000 and $3,500. Additional costs are incurred if the rain tank is installed underground or requires concrete bedding. Costs for pumps, piping, connections, electrical work and filters can add an additional $3,800 to $5,000 to the cost of installation. The costs associated with the maintenance of rain tanks include replacement of filters, electrical work on pumps and replacement of pumps as well as general cleanout/ inspections. It is difficult to quantify the cost differences to home owners of constructing and maintaining water tanks for reuse purposes. Vesely et al. (2005) noted that due to the low value of water within New Zealand, savings gained from using water tanks for non-potable water within cities are often very small. The study (Vesely et al., 2005) examined retrofitting a small residential subdivision (Glencourt Place in Auckland) with rain water tanks in order to investigate the environmental benefits of, and costs associated with, water reuse. Twenty new rain tanks were installed and the water was then utilised for gardening, laundry and toilet flushing purposes. A life cycle costing analysis was undertaken in order to compare the cost of the tanks versus a conventional piped system. The report found that retrofitting the tanks incurred similar costs to that of upgrading the downstream pipe. Over the life cycle, the LID approach cost between 4% and 18% more than the conventional approach depending on the level of the discount rate that was used in the analysis. However, the inclusion of water savings benefits to each household did diminish this difference (the maximum difference between options being 6%). For both approaches the total acquisition costs i.e. design and construction costs, dominated the analysis. Boubli and Kassim (2003) investigated the costs of installing rain tanks in two separate subdivisions in Sydney Australia and compared them with the costs of a conventional subdivision. They found that for the Pioneer Street subdivision, LID was cost neutral when compared with the conventional design. However, at Heritage Mews, the LID option offered significant savings (approximately 25%). The authors found that the larger the site and the larger the capacity of the rain tanks, the greater the opportunity for savings. The study only examined the construction costs of rain tanks. Page 68 Updated 01/11/2012

71 Coombes et al. (undated) also investigated the cost of rain water tanks in the Lower Hunter region of Australia. They found that the installation of rain tanks was a more economically viable solution than conventional piped infrastructure which was connected to mains water supply. They quantified that at a household level, the tanks were 0.9% more economically efficient that traditional water supply. From these studies it can be inferred that water is more highly valued as a commodity in Australia, than in New Zealand. Water reuse could also provide a valuable source for non-potable water in the event of a supply problem or civil defence emergency. SMG-6 Apx D.9.6 Use of Vegetative Practices The use of rain gardens, vegetated swales and filter strips can provide significant water quality benefits whilst also reducing the total volume of stormwater runoff. Even if kerbing is required, kerb cuts or diverting inflow, will allow water to pass off the paved surface into a biofiltration practice. This then disconnects the impervious area from the piped reticulation network (if there is one), assisting in mitigating effects of stormwater discharges. SMG-6 Apx D Construction Overseas studies (Conservation Research Institute, 2005) generally state that rain gardens, swales and filter strips allow for reduced stormwater infrastructure costs as they will often reduce the need for piped reticulation systems. Swales can replace piped systems and can be used as conveyance channels whilst providing for a degree of water quality treatment. With respect to rain gardens, the Conservation Research Institute (2005) states that lot level costs can be decreased by 25% 30% when using rain gardens rather than a conventional detention pond and pipe system. However, this will depend on whether outflow is designed to infiltrate into the ground, or still require a piped system to discharge stormwater. The Stormwater Centre factsheet on bioretention (accessed on 22 August 2009) states that bioretention is relatively expensive. This is mainly due to the fact that rain gardens consume a fair amount of land for the catchment area treated (approximately 5% surface area to catchment area). In Australia Lloyd et al. (2002) undertook a study to compare costs of biofiltration systems with a conventional reticulated system (no treatment) and a reticulated system and wetland to provide treatment. The study found that using biofiltration would increase construction costs by approximately 22% over the "no treatment" option as opposed to a 47% increase with the wetland scenario. In New Zealand, Ragunathan (2007) compared costs of rain gardens, swales and conventional stormwater ponds for a subdivision in the Papakura District near Auckland. The subject site has a 14 ha catchment with a proposal for 250 new lots. The study used a life cycle costing analysis to determine costs associated with different development options. The following options and costs were presented: Page 69 Updated 01/11/2012

72 a) 36 rain gardens, average lengths 16.0m, surface widths 4.0m, total acquisition cost $1.2 million. b) 2.1km length of swales, average lengths of 36m and 5.4m wide, total acquisition cost $342,100 ($30/m 2 = low estimate) to $1.7 million ($416/m 2 = high estimate). The low estimate related to swales without subsoil drains whilst the high estimate included subsoil drains. c) 1 stormwater pond with a permanent wet water surface area of 2950 m 2, wetland planting of 1500m 2 and riparian planting of 500m 2, total acquisition cost $244,900 (low estimate) to $431,100 (high estimate). SMG-6 Apx D Maintenance Long term maintenance is particularly crucial with regard to the effective functioning of all stormwater practices. There is very little information on the effective life span or maintenance costs, of these vegetative devices. Because many low impact projects are still within their pilot stage, there has as yet not been the opportunity to monitor and assess them in the long term. Some studies state that rain garden maintenance costs are lower since homeowners are often responsible for the costs rather than councils. However, it is still a cost which needs to be considered. It also raises the question of quality of maintenance in the long term as well as the potential future costs to councils if onsite devices are not properly maintained. Having said that, no studies were found that show that maintenance costs for rain gardens or swales are greater than maintenance costs for ponds or wetlands. Swales are relatively inexpensive to maintain, given that the key maintenance requirements relate to mowing, clearing debris and the occasional regarding of the swale. Rain gardens on the other hand are slightly more expensive to maintain given the likelihood of having to clean them out and replace the filter media at least once within its working life span. The rain garden would also need to be replanted following replacement of the filter media. Luckily, these costs occur fairly infrequency (around every 20 to 25 years). COSTnz model scenarios show that maintenance costs for ponds and rain gardens are very similar. This finding equates to that presented in overseas literature. In general, annual maintenance costs for rain gardens are generally in the order of 6% 7% of the total acquisition costs, whilst swale maintenance costs are approximately 3% - 4%. SMG-6 Apx D Vegetative Practices Conclusions At present the literature on the life cycle costs of vegetative practices such as rain gardens, swales and filter strips is relatively scant and in some instances it is contradictory in nature. However, a common theme regarding swales does emerge, i.e. they are more cost effective to construct and maintain than rain gardens, ponds or wetlands. Further research into the total acquisition and maintenance costs of already constructed swales and rain gardens will assist in obtaining a clearer picture of the long term costs when compared to the costs of conventional pond and wetland systems. Page 70 Updated 01/11/2012

73 SMG-6 Apx D.9.7 Using Infiltration to Reduce Runoff Costs for infiltration practices relate to the type of infiltration practice being constructed. SMG-6 Apx D Construction From a construction or total acquisition cost standpoint, infiltration practices tend to be substantially cheaper than other types of stormwater practices, as, like swales, their existence negates the need for a piped stormwater system. This is particularly so with infiltration trenches. On the other hand, costs associated with the purchase and installation of permeable paving can be quite high (depending on the product). Total acquisition costs are therefore dependant on the type of infiltration practice being constructed. SMG-6 Apx D Maintenance The real concern with infiltration practices is with their long term functioning. The Stormwater Centre factsheet on infiltration (accessed on 22 August 2009) states that infiltration practices have a fairly high failure rate and therefore will have a shorter life span than other types of stormwater practices. They estimate that maintenance costs of infiltration practices are between 5% and 10% of the construction cost. Costing scenarios using COSTnz found comparable results with annual maintenance costs being around 5% - 6% of the total acquisition cost. The majority of this expenditure relates to unclogging and reestablishing the surface of the infiltration practice. SMG-6 Apx D.9.8 Other Cost Considerations Other costs to consider include: a) Land Costs. b) Future Ownership. SMG-6 Apx D Land Costs During the process of life cycle costing or other methods used to quantify costs associated with stormwater practices, land costs are often excluded. In general, they are excluded owing to the fact that they tend to be very high and will often skew the analysis. However, land costs do play a significant role in the decisionmaking process. In economic analyses land costs are often expressed as an opportunity cost. An opportunity cost is defined as "the benefit that could have been gained from an alternative use of the same resource" (English Collins Dictionary English Definitions and Thesaurus, 2009). In essence, what this means is that an increase Page 71 Updated 01/11/2012

74 in benefits leads to lower opportunity costs, whilst fewer benefits leads to higher opportunity costs. Much of the international literature states that Low Impact Design (LID) reduces land costs and that the opportunity cost will decrease as benefits increase with LID. LID puts a focus on the multi-functional use of space, thereby decreasing the opportunity cost by allowing for more uses and benefits from the land. This can occur when treatment is provided at a sub-catchment level, land for large constructed practices such as stormwater ponds and wetlands can be used for other uses such as recreation, additional lots, nature areas, etc. More commonly, however, using a LID approach means that stormwater is managed at the source and land is required at the individual property scale for treatment or attenuation. Thurston (2006) states that individual owners actually face increasing opportunity costs as land within their own backyards is now required for stormwater management as opposed to play or utility gardening areas. As the lot size decreases, so the opportunity cost will increase. Home owners are often not only faced with using an increased proportion of their land for stormwater management but are also responsible for the cost associated with the maintenance of that stormwater practice. The acceptability of these two issues within New Zealand communities has not been investigated. SMG-6 Apx D Future Ownership As mentioned above, in many instances Low Impact Design (LID) subdivisions include on-site stormwater management and the burden for this management and maintenance therefore lies with the individual home owner. Although research into long term maintenance costs of LID techniques is limited (mainly owing to the fact that the majority of subdivisions have only recently been completed), it does highlight that there is the potential for significant savings that relate to maintenance costs for councils. A word of caution should be issued here for four reasons: a) Studies often do not quantify the maintenance costs incurred by the home owner. b) Studies do not include the cost to councils should homeowners not undertake maintenance or should on-site stormwater practices fail causing environmental degradation. c) Studies do not take into account council costs associated with monitoring and compliance of on-site stormwater management. d) Studies do not take into account the legal and administrative costs associated with non-compliance. The issues of on-site costs and private ownership are ones that will need further research in the future. SMG-6 Apx D.9.9 Conclusions In most cases both New Zealand and international literature highlight that there opportunities for cost savings resulting from a Low Impact Design (LID) approach Page 72 Updated 01/11/2012

75 however, the majority of the studies have focused on construction related costs. As a result the literature tends to highlight construction cost savings more strongly than long term maintenance costs. In many instances it does appear that, from a maintenance perspective, savings are likely. However, the actual value of these savings still needs to be quantified. In addition, further research into the actual construction and maintenance costs of vegetative practices, such as rain gardens and swales, would assist in determining their cost effectiveness when compared to practices such as ponds or wetlands. SMG-6 Apx D.9.10 Cost Comparison Case Studies In addition to examining actual costs involved with Low Impact Design (LID), it is useful to undertake cost comparisons between conventional subdivision designs and LID designs in order to attempt to quantify cost savings. The majority of cost comparisons presented in the available literature focus on developer-related costs and do not investigate differences in long term maintenance costs between the different approaches. SMG-6 Appendix E New Zealand Case Studies: Cost Comparison on Low Impact Design for Subdivisions includes 3 case studies undertaken by the Auckland Regional Council for the development of their Low Impact Design Manual (2000). SMG-6 Appendix F International Case Studies: Cost Comparison on Low Impact Design for Subdivisions includes 6 case studies from the United States. Page 73 Updated 01/11/2012

76 SMG-6 Appendix E New Zealand Case Studies: Cost Comparison on Low Impact Design for Subdivisions SMG-6 Apx E.1 Case Study One: Heron Point Heron Point is a 7.4 ha site in Auckland and is located adjacent to a marine/ harbour environment as shown on SMG-6 Figure E.1a Pre-development Site Plan. The predevelopment landuse is predominantly pasture and there is a small, intermittent stream which flows through the site. The site borders a tidal estuary. Owing to the topography, the site discharges via sheet flow to the sea. SMG-6 Table E.1a Key Differences between Conventional and LID Designs summarises the key differences between the conventional and Low Impact Design (LID) subdivision designs. SMG-6 Figure E.1a - Pre-development Site Plan Page 74 Updated 01/11/2012

77 SMG-6 Table E.1a Key Differences between Conventional and LID Designs Conventional Yield Reserve 1.09 ha 2.34 ha Lot Size Range 700 m m m m 2 Average Lot Size 766 m m 2 Stormwater Treatment Online pond Offline Earthworks Area Earthworks Volume 50,000 m 3 30,000 m 3 Site Impervious % 70% 56% LID SMG-6 Figure E.1b Conventional Subdivision Design Plan and SMG-6 Figure E.1c LID Subdivision Design Plan show differences in the subdivision layout. SMG-6 Figure E.1b - Conventional Subdivision Design Plan Page 75 Updated 01/11/2012

78 SMG-6 Figure E.1c - LID Subdivision Design Plan On examination of the costs for both scenarios it can be seen that if the difference in cost is calculated as a percentage, then the LID scenario is 14% more cost effective than the conventional scenario. SMG-6 Table E.1b Comparison of Construction Costs: Conventional vs LID Development highlights significant savings as a result of reduced clearing and earthworks, pavement construction, stormwater piping and general dayworks. In addition, a site valuation was undertaken for both scenarios. The developers profit for the conventional design was found to be 39%, whilst that of the LID design was 38%. An explanation as to why the profit for the LID design is lower is not provided, however, it could relate to the smaller lot sizes. Page 76 Updated 01/11/2012

79 SMG-6 Table E.1b Comparison of Construction Costs: Conventional vs LID Development Item Conventional Subdivision ($) LID Subdivision ($) Clearing and earthworks 347, ,000 Pavement construction 447, ,000 Sanitary sewers 196, ,000 Stormwater drainage system 394, ,000 Water reticulation 126, ,000 Trenching/ducting/cabling 46,000 45,000 Retaining wall ,000 Dayworks and general 288, ,000 Total 1,844,000 1,590,000 From a stormwater perspective annual runoff volumes and the percentage increase in peak flows were also reduced under the LID scenario. Refer SMG-6 Table E.1c Annual Stormwater Runoff Volumes and Percentages Related to Pre-development Conditions. SMG-6 Table E.1c Annual Stormwater Runoff Volumes and Percentages Related to Predevelopment Conditions Land Use Annual Runoff Volume (m 3 ) Percentage increase from Predevelopment Condition (%) Predevelopment 11,311 Conventional Development 44, LID Development 37, (16% less than conventional development) SMG-6 Apx E.2 Case Study Two: Palm Heights Palm Heights is a 27.7 ha site in Auckland shown on SMG-6 Figure E.2a Predevelopment Site Plan. It is steeply sloping with a number of gullies streams draining through the site, draining into a larger stream. The pre-development landuse is a mixture of native and exotic bush, and pasture. Page 77 Updated 01/11/2012

80 SMG-6 Table E.2a Key Differences between Conventional and LID Designs summarises the key differences between the conventional and Low Impact Design (LID) subdivision designs. SMG-6 Figure E.2a - Pre-development Site Plan SMG-6 Table E.2a Key Differences between Conventional and LID Designs Conventional LID Yield Reserve 3.75 ha 8.61 ha Average Lot Size 600 m m 2 Stormwater Treatment On-stream ponds Off-line Earthworks Area 23.7 ha 18.8 ha Earthworks Volume 330,000 m 3 235,000 m 3 Site Impervious % 54% 39% SMG-6 Figure E.2b Conventional Subdivision Design Plan and SMG-6 Figure E.2c LID Subdivision Design Plan show differences in the subdivision layout. Page 78 Updated 01/11/2012

81 SMG-6 Figure E.2b - Conventional Subdivision Design Plan SMG-6 Figure E.2c - LID Subdivision Design Plan Page 79 Updated 01/11/2012

82 On examination of the costs for both scenarios it can be seen that the LID scenario, as a percentage, is 23% more cost effective than the conventional scenario. SMG-6 Table E.2b Comparison of Construction Costs: Conventional vs LID Development highlights significant savings from reduced concrete works as a result of the LID subdivision. Marginal savings also result from reduced earthworks, pavement construction, stormwater drains and trenching. A site valuation was also undertaken for both scenarios. It was found that the developers profit for the conventional design was 26%, as opposed to 18% for the LID design. In this case, even though costs were lower the LID option did not appear to offer a financial incentive over the conventional design. Under the LID scenario this is likely to be the case because of the reduced average lot size, as well as the reduction in the lot yield. SMG-6 Table E.2b Comparison of Construction Costs: Conventional vs LID Development Item Conventional Subdivision ($) LID Subdivision ($) Clearing and earthworks 1,800,000 1,719,000 Pavement construction 1,362,000 1,134,000 Sanitary sewers 850, ,000 Stormwater drainage system 1,178,000 1,050,000 Water reticulation 492, ,000 Trenching/ducting/cabling 338, ,000 Concrete works 1,036, ,000 Dayworks and general 162, ,000 Total 7,218,000 5,536,000 From a stormwater perspective, annual runoff volumes and the percentage increase in peak flows were reduced under the LID scenario. Refer SMG-6 Table E.2c Annual Stormwater Runoff Volumes and Percentages Related to Pre-development Conditions. SMG-6 Table E.2c Annual Stormwater Runoff Volumes and Percentages Related to Predevelopment Conditions Land Use Annual Runoff Volume (m 3 ) Percentage increase from Predevelopment Condition (%) Predevelopment 77,202 Conventional 209, Development LID Development 170, (19% less than conventional development) Page 80 Updated 01/11/2012

83 SMG-6 Apx E.3 Case Study Three: Wainoni Downs Wainoni Downs is a 14 ha site in Auckland as shown on SMG-6 Figure E.3a Predevelopment Site Plan. The average slope of the site is 5%, however, the site is adjacent to the coastal margin and in this vicinity the slopes are up to 30%. There is a stream which flows through the site and discharges to the marine receiving environment. The pre-development landuse includes an esplanade reserve to the north and west of the site. The site itself consists of a mixture of small stands of native trees and pastoral areas. SMG-6 Table E.3a Key Differences between Conventional and LID Designs summarises the key differences between the conventional and LID subdivision designs. SMG-6 Figure E.3a - Pre-development Site Plan Page 81 Updated 01/11/2012

84 SMG-6 Table E.3a Key Differences between Conventional and LID Designs Conventional LID Yield Reserve 1.09 ha 2.34 ha Average Lot Size 766 m m 2 Stormwater Treatment On-stream ponds Offline Earthworks Area 9.6 ha 7.6 ha Earthworks Volume 62,000 m 3 53,000 m 3 Site Impervious % SMG-6 Figure E.3b Conventional Subdivision Design Plan and SMG-6 Figure E.3c LID Subdivision Design Plan show differences in the subdivision layout. SMG-6 Figure E.3b - Conventional Subdivision Design Plan Page 82 Updated 01/11/2012

85 SMG-6 Figure E.3c - LID Subdivision Design Plan On examination of the costs for both scenarios it can be seen, when converted to percentages, that the LID scenario is 25% more cost effective than the conventional scenario. SMG-6 Table E.3b Comparison of Construction Costs: Conventional vs LID Development highlights that under the LID scenario there are significant savings as a result of reduced earthworks, pavement construction and general dayworks operations. As with the other two case studies, a site valuation was undertaken for both scenarios. The developers profit for the conventional design was 15%. In this case however, the LID profit for developers was 23%. This shows that the LID option is the more financially attractive option. SMG-6 Table E.3b Comparison of Construction Costs: Conventional vs LID Development Item Conventional Subdivision ($) LID Subdivision ($) Clearing and earthworks 1,425, ,000 Pavement construction 1,390,000 1,111,000 Sanitary sewers 500, ,000 Stormwater drainage system 855, ,000 Water reticulation 210, ,000 Trenching/ducting/cabling 338, ,000 Concrete works 1,123,000 1,123,000 Dayworks and general 460,000 60,000 Total 5,963,000 4,478,000 Page 83 Updated 01/11/2012

86 From a stormwater perspective, annual runoff volumes and the percentage increase in peak flows were reduced under the LID scenario. Refer SMG-6 Table E.3c Annual Stormwater Runoff Volumes and Percentages Related to Pre-development Conditions. SMG-6 Table E.3c Annual Stormwater Runoff Volumes and Percentages Related to Predevelopment Conditions Land Use Annual Runoff Volume (m 3 ) Percentage increase from Predevelopment Condition (%) Predevelopment 31,449 - Conventional Development 99, LID Development 81, (17% less than conventional development) SMG-6 Apx E.4 Case Study Conclusions Three New Zealand case studies have been presented: a) SMG-6 APX E.1 Case Study One: Heron Point. b) SMG-6 APX E.2 Case Study Two: Palm Heights. c) SMG-6 APX E.3 Case Study Three: Wainoni Downs. These examine the difference in construction costs and developer profit margins, for conventional and Low Impact Design (LID) subdivisions. In order for LID to become a "mainstream" development option, it cannot impact on the profitability or the practicality of the project. From an economic perspective only Palm Heights had a significantly less desirable outcome for a developer. In this particular case, in order to provide protection for the watercourses, the LID design incorporated significantly smaller lots and the valuation anticipated that there would be less demand for those smaller sites in a greenfield area. Page 84 Updated 01/11/2012

87 SMG-6 Appendix F International Case Studies: Cost Comparison on Low Impact Design for Subdivisions SMG-6 Apx F.1 General The majority of cost comparison case studies that have been done internationally have been undertaken in the United States of America (US). The US Environmental Protection Agency (USEPA) undertook a significant portion of this work through their investigation into the costs associated with Low Impact Design (LID) (2007). Seventeen studies were evaluated for the report, and the elements of LID found within each development were quantified These are shown on SMG-6 Table F.1a Summary of LID Practices Employed in the USEPA Case Studies. In these same case studies, costs were analysed for a conventional development layout and a LID layout. In some of these cases, initial costs were higher under the LID design, however, in the majority of cases LID developments showed significant cost savings as shown in SMG-6 Table F.1b Cost Comparisons between Conventional and LID Approaches. Only Kensington Estates showed far higher LID costs. The USEPA Report (2007) explains that the reason for this dramatic increase in cost was owing to a new, permeable paving, product which was used for all parking spaces within the subdivision. This product dramatically increased the LID costs. SMG-6 Table F.1a Summary of LID Practices Employed in the USEPA Case Studies (USEPA 2007) LID Techniques Name Bioretention Cluster Building Reduced Impervious Area Swales 2nd Avenue SEA Street Permeable Pavement Vegetated Landscaping Wetlands Green Roofs Auburn Hills Bellingham Parking Lot Retrofits Central Park Commercial Redesigns Crown Street Gap Creek Garden Valley Kensington Estates Laurel Springs Mill Creek Page 85 Updated 01/11/2012

88 Poplar Street Apartments Portland Downspout Disconnection* Prairie Crossing Prairie Glen Somerset Tellabs Corporate Campus Toronto Green Roofs Project a SMG-6 Table F.1b Cost Comparisons between Conventional and LID Approaches (USEPA,2007) 2nd Avenue SEA Street Conventional Developmental Cost LID Cost Cost Difference b Percent Difference b $868,803 $651,548 $217,255 25% Auburn Hills $2,360,385 $1,598,989 $761,396 32% Bellingham City Hall $27,600 $5,600 $22,000 80% Bellingham Bloedel Donovan Park $52,800 $12,800 $40,000 76% Gap Creek $4,620,600 $3,942,100 $678,500 15% Garden Valley $324,400 $260,700 $63,700 20% Kensington Estates $765,700 $1,502,900 -$737,200-96% Laurel Springs $1,654,021 $1,149,552 $504,469 30% Mill Creek c $12,510 $9,099 $3,411 27% Prairie Glen $1,004,848 $599,536 $405,312 40% Somerset $2,456,843 $1,671,461 $785,382 32% Tellabs Corporate campus a $3,162,160 $2,700,650 $461,510 15% Some of the case study results do not lend themselves to display in the format of this table (Central Park Commercial Redesigns, Crown Street, Poplar Street Apartments, Prairie Crossing, Portland Downspout Disconnection, and Toronto Green Roofs). b Negative values denote increased cost for the LID design over conventional development costs. c Mill Creek costs are reported on a per-lot basis. Page 86 Updated 01/11/2012

89 The USEPA report (2007) and case studies in SMG-6 Appendix F International Case Studies: Cost Comparison on Low Impact Design for Subdivisions clearly show that, from a construction cost perspective, LID can be significantly cheaper than conventional subdivision design. However, further research is needed to quantify these studies in terms of the profit margins for developers as well as the costs involved in the long term maintenance of stormwater infrastructure for local councils. SMG-6 Apx F.2 United States Case Studies: Cost Comparison on Low Impact Design for Subdivisions The following case study comparisons are presented: a) SMG-6 APX F.3 Case Study One: Gap Creek. b) SMG-6 APX F.4 Case Study Two: Auburn Hills. c) SMG-6 APX F.5 Case Study Three: Chapel Run. d) SMG-6 APX F.6 Case Study Four: Buckingham Green. e) SMG-6 APX F.7 Case Study Five: Tharpe Knoll. f) SMG-6 APX F.8 Case Study Five: Pleasant Hill Farm. The case studies presented are the result of a costing analysis undertaken by the USEPA and the State of Delaware. The information used for this appendix was collated and presented by Earl Shaver (2009) in a report to the Auckland Regional Council on developer-related costs associated with Low Impact Design. Where available site plans, aerial photographs, basic site information and more detailed cost information has been provided. SMG-6 Apx F.3 Case Study One: Gap Creek Gap Creek is a site in Arkansas, USA. Although not confirmed, the predevelopment site likely consisted of bush and forested areas as seen in the aerial photograph below. The site is 52 hectares in size. The developer took an alternative approach to site development by implementing a variety of practices to reduce the environmental impact of the development. More specifically the developer ensured streets were at the natural site grade, native vegetation and natural drainage features were preserved where possible and a network of buffers and greenbelts were developed to protect sensitive areas. Kerbing was eliminated and streets were narrowed from 10.9 metres to 8.2 metres. Page 87 Updated 01/11/2012

90 SMG-6 Figure F.3 - Aerial image of Gap Creek (Google Earth) SMG-6 Apx F.3.1 Cost Estimates As shown in SMG-6 Table F.3.1 Project Cost Results from the Gap Creek Development, the Low Impact Design (LID) approach to site design resulted in significant economic benefits derived from a combination of lower development costs, higher lot yield and greater lot values. The developer also maximised the number of lots that abut open space areas thereby increasing marketability and increasing property values. It should be noted that the LID costs in SMG-6 Table F.3.1 Project Cost Results from the Gap Creek Development are actual costs and result in a net saving to the developer of US$2,200,000. Page 88 Updated 01/11/2012

91 SMG-6 Table F.3.1 Project Cost Results from the Gap Creek Development ($US) Total Site Conventional Development LID Development Lot yield Linear street length 6,635 metres 6,439 metres Linear collector street lengths 2,243 metres 0 metres Linear drainage pipe 3,078 metres 2,052 Drainage structures: Inlets/boxes/headwalls Estimated cost per lot $12,907 $10,512 Actual Results from First Phase of Development Phase 1 Conventional development (Engineers estimated figures) LID Development (actual figures) Lot yield Total cost $1,028,544 $828,523 Total cost per lot $16,326 $11,507 Higher lot yield Higher lot value Lower cost per lot Enhanced marketability Added amenities Total economic benefit Economic and Other Benefits from LID Development 17 additional lots $3,000 more per lot over competition $4,800 less cost per lot 80% of lots sold in first year 9.5 hectares of green-space parks More than $2,200,000 added to profit SMG-6 Apx F.4 Case Study Two: Auburn Hills Auburn Hills is a 34.2 ha site in Wisconsin, USA. Although not confirmed, the predevelopment site likely consisted of bush and agricultural land. he site was developed as a Low Impact Design (LID) subdivision so the conventional approach shown here is for demonstration purposes only. Through the LID design, 40% of the site was preserved as open space (this included existing wetlands, green space and natural plantings and walking trails). Stormwater management was provided via open swales and bioretention. The conventional subdivision proposed 126 lots, whilst the LID subdivision approach had 113 single family homes and 13 attached dwellings. The conventional and LID site design approaches are shown in SMG-6 Figure F.4a Conventional vs LID Subdivision Design. Page 89 Updated 01/11/2012

92 SMG-6 Figure F.4a - Conventional vs LID Subdivision Design Page 90 Updated 01/11/2012

93 SMG-6 Figure F.4b - Auburn Hills During Construction SMG-6 Apx F.4.1 Cost Estimates Given that the development was undertaken as a Low Impact Design (LID) subdivision, conventional costs were estimated in order to allow for a comparison of the site construction costs. LID costs are therefore actual costs incurred by the Page 91 Updated 01/11/2012

94 developer (Bielinski Homes). In addition, the developer also compared costs relating to development-related finances such as consent fees required by local authorities, professional expenses for site analyses and site planning and design, financing expenses such as loans and legal fees and real estate taxes that developers are required to pay. The cost comparison is shown in SMG-6 Table F.4.1 Conventional Development Costs vs LID Development Costs. SMG-6 Table F.4.1 Conventional Development Costs vs LID Development Costs Description Conventional LID Development Cost Savings (%) Development ($) Site preparation 699, , Stormwater management 664, , Sanitary sewer 671, , Water distribution 858, ,160 9 Utilities 290, , Site paving and footpaths 771, , Landscaping 225, ,000-7 Construction Cost Subtotal 4,180,585 3,039, Consent fees 80,000 47, Professional services Financing expenses 218, , , , Real estate tax 69,500 69,500 0 Finance subtotal cost 726, , Total Costs 4,906,835 3,618, Summary 3,090 linear metres of roads in LID versus 3,505 linear metres in conventional development Construction cost per metre of roadway: $1,171 for conventional development versus $1,401 for LID development Construction cost per unit: $26,030 for LID development versus $38,943 for conventional development 113 lots/13 duplex apartments in LID versus 126 lots in conventional development Page 92 Updated 01/11/2012

95 One of the key features of the LID development was the use of clustering to minimize earthworks, impervious area and maximize open space. This clustered design therefore allowed for reduced clearing and grading costs, reduced paving and footpaths costs and lower costs associated with reduced street length and width. Stormwater savings were realised primarily through the use of vegetated swales. The swales provided stormwater conveyance and treatment and also lowered the cost of conventional stormwater infrastructure. Whilst there was an increase in landscaping costs resulting from additional open space under the LID scenario, overall construction costs were still lower than for the conventional design. It is of interest to note that the consenting fees appear to be less under an LID design. Clearly within the State of Wisconsin there are incentives for developers to utilise LID principles as part of their subdivision design. It is unlikely that developers would see similar consenting cost reductions here in New Zealand until LID becomes more mainstream within council plans and policies. Overall the subdivision s LID design retained more natural open space for the benefit and use of the homeowners and aided stormwater management by preserving some of the site s natural hydrology. A site valuation was not done for this project and the actual profit relating to it was not reported. SMG-6 Apx F.5 Case Study Three: Chapel Run Chapel Run is a 40 ha site in Delaware, USA as shown on SMG-6 Figure F.5a Site and Pre-development Land Use. The predevelopment land use was of pasture and bush and the site drains into streams that drain to a tidal estuary. SMG-6 Figure F.5a - Site and Pre-development Land Use Page 93 Updated 01/11/2012

96 The conventional development proposed 142 lots with an average lot size of 2,000 m 2. Earthwork areas and volumes were not calculated however, the site imperviousness was calculated to be 29%. There was no provision for any reserve land within the conventional design no reserve area on the site and stormwater management was proposed through the use of stormwater ponds (shown in blue on SMG-6 Figure F.5b Conventional Subdivision Design). SMG-6 Figure F.5b - Conventional Subdivision Design The LID subdivision also proposed 142 lots, however, the average lot size had been reduced to 1,000 m 2. Stormwater treatment was provided by swales and infiltration practices. Community open space / green areas were allowed for and comprised 50% or 20 hectares of the subject site as shown on SMG-6 Figure F.5c LID Subdivision Design. This allowed for a commensurate reduction in earthworks area and volume. Site imperviousness was calculated as 15%. Page 94 Updated 01/11/2012

97 SMG-6 Figure F.5c - LID Subdivision Design From a stormwater context, the preservation of bush areas and open space along with the infiltration practices and vegetated swales ensured that post-development peak flow rates and volumes did not exceed pre-development levels as shown on SMG-6 Table F.5 Annual Stormwater Runoff Volumes and Percentages Related to Pre-development Conditions. SMG-6 Table F.5 Annual Stormwater Runoff Volumes and Percentages Related to Predevelopment Conditions Land Use Annual Runoff Volume (m 3 ) Percentage increase from Predevelopment Condition (%) Predevelopment 18,761 Conventional 249, Development LID Development 67, (73% less than conventional development) Page 95 Updated 01/11/2012

98 SMG-6 Apx F.5.1 Cost Estimates SMG-6 Table F.5.1 Cost Data for Conventional vs LID Subdivision provides a detailed breakdown of costs related to a conventional subdivision and a Low Impact Design (LID) approach. Where calculated, the LID scenario shows significant financial savings over a conventional approach. Again, as with the other case studies presented this is likely due to the use of clustering, thereby reducing road lengths, drainage systems and concrete works. A site valuation was not undertaken for this case study. SMG-6 Table F.5.1 Cost Data for Conventional vs LID Subdivision Item Standard Subdivision ($) LID Subdivision ($) Clearing and earthworks Not calculated Not calculated Pavement construction 2,008, ,000 Sanitary sewers Individual site wastewater Individual site wastewater Stormwater system drainage 400, ,000 Water reticulation Individual wells Individual wells Trenching/ducting/cabling Not calculated Not calculated Concrete works 52,000 13,000 Dayworks and general Not calculated Not calculated Reforestation 0 36,855 Total 2,460, ,735 SMG-6 Apx F.6 Case Study Four: Buckingham Green Buckingham Green is a 7.7 ha site in Delaware, USA. The predevelopment land use was a mixture of bush, pasture and recovering bush areas that drained into streams as shown on SMG-6 Figure F.6a Predevelopment Land Use. The streams eventually discharge into a tidal estuary. Page 96 Updated 01/11/2012

99 SMG-6 Figure F.6a - Predevelopment Land Use The conventional development proposed the creation of 55 lots with an average lot size of 600 m 2 shown on SMG-6 Figure F.6b Conventional Subdivision Design. The reserve area was 1.6 hectares and site imperviousness 23%. Earthworks and total earthworks volumes were not calculated independently. SMG-6 Figure F.6b - Conventional Subdivision Design Page 97 Updated 01/11/2012

100 As with the other case studies presented, the LID subdivision proposed a clustered approach comprising 55 lots (shown on SMG-6 Figure F.6c LID Subdivision Design). Stormwater treatment is provided by swales and infiltration practices. Community open space increased from 1.6 to 4 hectares (i.e. 52% of the site), with a commensurate reduction in earthworks area and volume. Site imperviousness was reduced to 21%. The stormwater approach used revegetation, infiltration and vegetated swales. SMG-6 Figure F.6c - LID Subdivision Design From a stormwater context, the preservation of bush areas and open space along with the revegetation, infiltration practices and vegetated swales ensured that postdevelopment peak flow rates and volumes did not exceed pre-development levels as shown on SMG-6 Table F.6 Annual Stormwater Runoff Volumes and Percentages Related to Pre-development Conditions. SMG-6 Table F.6 Annual Stormwater Runoff Volumes and Percentages Related to Predevelopment Conditions Land Use Annual Runoff Volume (m 3 ) Percentage increase from Predevelopment Condition (%) Predevelopment 1,206 Conventional 19, Development LID Development 18, (7% less than conventional development) Page 98 Updated 01/11/2012

101 SMG-6 Apx F.6.1 Cost Estimates A detailed breakdown of costs comparing the conventional subdivision and LID approach is provided in SMG-6 Table F.6.1 Cost Data for Conventional vs LID Subdivision. A site valuation was not done for this project. Once again the LID approach provided the developer with significant cost savings. SMG-6 Table F.6.1 Cost Data for Conventional vs LID Subdivision Item Standard Subdivision ($) LID Subdivision ($) Clearing and earthworks Not calculated Not calculated Pavement construction 405, ,500 Sanitary sewers Not calculated Not calculated Stormwater system drainage 87,000 15,100 Water reticulation Not calculated Not calculated Trenching/ducting/cabling Not calculated Not calculated Concrete works 49,400 0 Dayworks and general Not calculated Not calculated Reforestation 0 6,092 Total 541, ,692 SMG-6 Apx F.7 Case Study Five: Tharpe Knoll Tharpe Knoll is a 13.4 hectare site in Delaware, USA. SMG-6 Figure F.7a Predevelopment Landuse highlights that the predevelopment landuse was a mixture of native bush and horticultural land. There are also streams on the site which drain to a tidal estuary. Page 99 Updated 01/11/2012

102 SMG-6 Figure F.7a - Predevelopment Landuse The conventional development proposed 23 lots. The average lot size equated to approximately 4,000 m 2 and site imperviousness was estimated to be 12.6%. The reserve area on the site was 1.5 hectares. Earthworks and total earthworks volumes were not calculated independently. SMG-6 Figure F.7b - Conventional Design Page 100 Updated 01/11/2012

103 The Low Impact Design (LID) subdivision used clustering to maximize the open space area and was able to ensure that 50% (or 6.7 hectares) of the site was set aside for this purpose. The development approach still allowed for 23 lots however, the average lot size was reduced to 2,000 m 2. Site imperviousness equated to approximately 7.4%. Stormwater treatment was provided by swales and revegetation practices. SMG-6 Figure F.7c - LID Subdivision Design From a stormwater context, the preservation of bush areas and open space, along with the revegetation, infiltration practices and vegetated swales ensured that postdevelopment peak flow rates and volumes did not exceed pre-development levels as shown on SMG-6 Table F.7 Annual Stormwater Runoff Volumes and Percentages Related to Pre-development Conditions. SMG-6 Table F.7 Annual Stormwater Runoff Volumes and Percentages Related to Predevelopment Conditions Land Use Annual Runoff Volume (m 3 ) Percentage increase from Predevelopment Condition (%) Predevelopment 5,180 Conventional 22, Development LID Development 12, (33% less than conventional development) Page 101 Updated 01/11/2012

104 SMG-6 Apx F.7.1 Cost Estimates A detailed breakdown of costs comparing the conventional subdivision and Low Impact Design (LID) approach is provided in SMG-6 Table F.7.1 Cost Data for Conventional vs LID Subdivision. A site valuation was not done for this project. Once again the LID approach provided the developer with significant cost savings. SMG-6 Table F.7.1 Cost Data for Conventional vs LID Subdivision Item Standard Subdivision ($) LID Subdivision ($) Clearing and earthworks Not calculated Not calculated0 Pavement construction 464, ,525 Sanitary sewers Not calculated Not calculated Stormwater system drainage 77,600 54,050 Water reticulation Not calculated Not calculated Trenching/ducting/cabling Not calculated Not calculated Concrete works 19,500 5,200 Dayworks and general Not calculated Not calculated Reforestation 0 10,825 Total 561, ,800 SMG-6 Apx F.8 Case Study Five: Pleasant Hill Farm Pleasant Hill Farm is a 34 hectare site in Delaware, USA. The predevelopment landuse also comprised a mixture of horticultural land and bush, and the streams running through the site drain to a tidal estuary. Page 102 Updated 01/11/2012

105 SMG-6 Figure F.8a - Predevelopment Landuse The conventional development proposed 90 lots with a total impervious area of 26.2% as shown on SMG-6 Figure F.8b Conventional Subdivision Design. The average lot size proposed was 1700 m 2 and the reserve area (comprising 13.8 hectares) was located within the floodplain area. Earthworks and total earthworks volumes were not calculated independently. SMG-6 Figure F.8b - Conventional Subdivision Design Page 103 Updated 01/11/2012