Flooding & Drainage Assessment for Killingworth Local Environmental Study

Transcription

1 Flooding & Drainage Assessment for Killingworth Local Environmental Study

2 Transmittal Revision Author Verifier Issued to Date 1 Preliminary Angus Brien Ben Clark Philip Carroll RPS Group Preliminary Angus Brien Ben Clark Philip Carroll RPS Group

3 Executive Summary Northrop Consulting Engineers have undertaken a Hydrology, Flooding, Drainage and Water Resources Investigation for inclusion in the Killingworth Local Environmental Study (LES). This investigation included a review of several factors, including flooding, detention, water quality and water management practices. Strategies have been presented as part of this investigation for the purposes of examining the feasibility of re-zoning. The 1% AEP flood extents for a watercourse which appears to be a tributary of Cockle Creek running to the south west of the site has been determined. This does not have a significant impact on the subject site with the water level adjacent to the boundary greater than 500mm below the natural level on the nearest lot. Several other flooding scenarios have been proposed with a similar result. Detention and water quality requirements have been considered in accordance with Lake Macquarie City Council DCP No.1. Both these items are feasible, with an approximation of possible detention and water quality solutions included. Through a review of all the factors outlined in this report, it is considered that the subject site is suitable for re-zoning to facilitate urban development consistent with the objectives of zone 2(1) from a flooding and drainage perspective.

4 Table of Contents 1 Introduction Aim Site Characteristics Regional Characteristics Flooding Objectives Methodology Catchments Regional Catchment Local Catchments Watercourses Cockle Creek Tributary Cockle Creek Riparian Zones Flooding Peak Flow Results Flooding from Upstream Catchments Flooding from Runoff Generated on the Subject Site Flooding from Backwater Effects Potential Increased Risk of Flooding from Future Development Risk of Flooding Due to Extreme Flooding, Sea Level Rise and Climate Change Stormwater Management Strategy Outline Catchments and Existing Drainage Infrastructure Detention Connection to Existing Infrastructure Water Quality Construction Water Quality Modelled Pollutant Loading Urban Run-off Quality Control Assessment of Water Management Alternatives Detention Water Quality Maintenance of Riparian Zones and Vegetated Buffers Recommendations and Conclusions Flooding Water Management Conclusions References... 22

5 List of Figures Figure 1 - Regional Catchment... 4 Figure 2 - Local Catchments... 4 Figure 3 - Streams classified in accordance with the Strahler System... 7 Figure 4 Photos showing L: looking south to the Cockle Creek tributary along the easement to the west of the subject site. R: Vegetation at the middle of the low point Figure 5 - Extent of riparian zone and vegetate buffer... 8 Figure 6-1% AEP flood extent and sections Figure 7 - Photos L: looking north on Westcroft St R: looking north on The Boulevard Figure 8 - Possible detention locations List of Tables Table 1 - Regional sub-catchment characteristics... 5 Table 2 - Local catchment characteristics... 6 Table 3 - XP-RAFTS peak flow rates... 8 Table 4 - Comparison of XP-RAFTS flow rates with probabilistic rational method calculations... 9 Table 5 - HEC-RAS comparison with Manning's equation... 9 Table 6 - Comparison between 1% AEP peak flows and the probable maximum flood (PMF) Table 7 - Comparison of water surface elevation and flood extents 1% AEP to PMF Table 8 - Summary of detention options for the subject site Table 9 - Pollutant concentrations for pre and post development options Table 10 - Pollutant loadings for the pre and post development scenarios Table 11 - Pollutant removal targets interpreted from SQID guidelines Table 12 - Treatment train efficiencies Table 13 - Options analysis for detention Table 14 - Options analysis for water quality... 20

6 1 Introduction Northrop Consulting Engineers were engaged by RPS Group Australia Pty Ltd (RPS) to undertake a Hydrology, Flooding, Drainage and Water Resources Investigation for inclusion in a Local Environment Study (LES) for a section of land in Killingworth. This LES is to be submitted by Council as part of a proposal to rezone this section of land to facilitate urban development. 1.1 Aim The objective of this investigation was to determine the suitability of the proposed rezoning from a water management perspective. This involves the potential impact of flooding, as well as the effect of any future development on water quality and quantity, both within and downstream of the site. The discussion has been framed for a rezoning application only, as opposed to presenting detailed design solutions. Whilst the concepts presented are considered viable, they are by no means an exhaustive list of the possible alternatives. The recommendations have been prepared with consideration to and generally in accordance with the following planning instruments, policies and studies; - Lake Macquarie City Council s (LMCC) Development Control Plan No.1 (DCP No. 1) - LMCC Stormwater Treatment Framework & Stormwater Quality Improvement Device (SQID) Guidelines - LMCC Lake Macquarie Sea Level Rise Preparedness Adaptation Policy - NSW Government Floodplain Development Manual - NSW Government Floodplain Risk Management Guideline Practical Consideration of Climate Change. 1.2 Site Characteristics The site consists of the following lots in Killingworth; Lots 1-8 and Section H DP 4339 Lots Section I DP 4339 Lots 1-3 and 6-10 Section J DP 4339 Lots 1-6 and 9 Section K DP 4339 Lots 1-10 Section M DP 4339 Lots 1-10 and Section N DP 4339 Lots 1-10 Section O DP 4339 Lots 1-9 and Section P DP4339 Lot 1 DP and shall hereafter be referred to the subject site. The subject site was zoned 10 investigation land as part of the Lake Macquarie LEP The site is currently densely vegetated with some existing residential dwellings and unsealed roads and forms four discreet catchments, each draining to a different location. This is discussed in more detail in Section Page 1

7 1.3 Regional Characteristics The site forms part of the upper Cockle Creek catchment with a tributary of Cockle Creek running to the south west of the site. This tributary joins Cockle Creek to the south east of the site, just upstream of the Westside Mine. The total catchment of these two streams extends to the west of the F3 freeway up to the Sugarloaf Range, towards Wakefield to the south and approximately following the alignment of Sackville Street, Killingworth to the north. The vast majority of this catchment appears undeveloped apart from some dwellings and roads. It appears to contain dense wooded vegetation and several blue line watercourses as noted on the 1:25,000 topographic maps. A catchment plan and stream identification is included in Sections 2.3 and 2.4 later in this report. Page 2

8 2 Flooding 2.1 Objectives An assessment was undertaken to determine the effect of regional flooding on the subject site, as well as the impact of any future development on downstream drainage paths. A range of scenarios were considered including the following; - Runoff from upstream catchments causing flooding within the subject site, - Flooding from runoff generated within the subject site, - The effect of hydraulic control structures in the vicinity of the subject site, - The effect of possible future development on downstream flow paths and drainage infrastructure, - The possible increase in rainfall intensity due to climate change and new research and the effect on the above scenarios, and - Flooding of the Lake and the effect of sea level rise. 2.2 Methodology The aforementioned flooding assessment was carried out using the methodology outline below; - Regional and site catchment boundaries were determined using topographic maps obtained from the Department of Lands as well as LiDAR contours supplied by RPS. - The location of waterways was determined by identifying blue lines on the 1:25000 scale topographic maps. - Runoff generated upstream of the site as was modelled using the computer program XP- RAFTS and validated using a probabilistic rational method check. - Flood levels were modelled using the computer program HEC-RAS. Cross sections for this model were obtained from the LiDAR contours. These levels were checked using Manning s equation for channel capacity. - A sensitivity analysis was then undertaken on the results obtained from the steps above to investigate the effect of increased rainfall intensities, model parameters and peak flows. The results from this investigation have been presented and discussed below. 2.3 Catchments Regional Catchment As outlined above, the regional catchment was determined using a combination of LiDAR contours in the vicinity of the site and topographic maps further away. A diagram of this catchment is shown overleaf in Figure 1. Page 3

9 Figure 1 - Regional Catchment This has been broken down into two sub-catchments to represent the different waterways. A brief description of each sub-catchment is included overleaf in Table Local Catchments There are four distinct catchments that form a part of the subject site. A diagram of these catchments, including proposed discharge points is included below in Figure 2. A short description of each catchment is included overleaf in Table 2. Figure 2 - Local Catchments Page 4

10 Sub Size Description catchment (km 2 ) A Sub-catchment A represents the catchment of the watercourse running immediately to the south west of the subject site. It is densely vegetated with wooded vegetation. The majority of this catchment is currently undeveloped and it is zoned 7(1), 7(2) and (5). The F3 freeway crosses the western portion of this sub catchment. The terrain is undulating with slopes typically in the one to ten percent range, and an overall average slope of five percent over a flow length of 1.9km. This sub-catchment discharges to the east and into sub-catchment B. B Sub-catchment B represents the remainder of the catchment for Cockle Creek draining to the haul road at Westside Mine. There are several watercourses located in this sub-catchment which converge to form Cockle Creek. Similar to sub-catchment A, it is largely undeveloped with dense wooded vegetation. The F3 freeway also bisects this sub catchment. The terrain is undulating with elevations ranging from 370m to 14m AHD. Slopes range from very gentle at 2 percent to some extreme slopes in the order of 70 percent towards the top of the catchment. An overall average slope of 7 percent and flow length of 5.1km has been determined. This sub-catchment drains to the east underneath the haul road of Westside Mine. The concentrated flow path continues as Cockle Creek and discharges to Lake Macquarie at Speers Point. Table 1 - Regional sub-catchment characteristics Catchment Size Description (ha) Catchment 1 is located on the north-western portion of the subject site. It interfaces with the existing urban development on the north, and as such has five different discharge points. It is largely undeveloped with a few existing residential dwellings, wooded vegetation and some open fields. Slopes are in the order of eight percent. Based on the cadastre provided, it is anticipated that the catchment will drain; a) to Stephenson St, b) to the easement between Stephenson St and The Boulevard, c) to The Boulevard, d) to the easement between The Boulevard and Westcroft St and e) to Westcroft St. Page 5

11 Catchment Size Description (ha) Catchment 2 is located on the south western portion of the subject site. It is small in size with about twenty lots. It currently consists of wooded vegetation, and is steep in nature with slopes up to 25 percent. It discharges to the south west to the watercourse running adjacent Catchment 3 is located on the north-eastern portion of the site. Similar to Catchment 1 it interfaces with the existing residential grid. It also has multiple discharge points. This catchment is vegetated with trees and has moderate slopes ranging from five to fifteen percent. Based on the cadastre provided, this catchment will discharge to; a) the easement between Throckmorten St and Geordie St, b) Throckmorten St, c) Sackville St and d) the easement between Throckmorten St and The Trongate Catchment 4 is located on the south-eastern portion of the site. It discharges to the south-east. This catchment is currently densely vegetated and moderate slopes ranging from five to ten percent. It is the smallest of the local catchments with only seven lots. Table 2 - Local catchment characteristics 2.4 Watercourses A review of the 1:25,000 topographic maps from the Department of Lands indicate the presence of several blue line waterways in the regional catchment, including one immediately to the south west of the subject site. These have been classified in accordance with the Strahler System as recommended by the former Department of Water and Energy in their Government Agency Consultation Response. The Strahler system classifies watercourses from the upstream reaches of the catchment. Streams that form in these reaches are classified as first-order streams until they join with another first order stream in which case they become second order streams. A second order stream only becomes a third order stream when joined by another second order stream. This is reflected in Figure 3 overleaf Cockle Creek Tributary An unnamed tributary of Cockle Creek passes the site to the south west and discharges to Cockle Creek. This creek is a first order stream in accordance with the Strahler System and has been analysed for flood level due to its close proximity to the subject site. No water was flowing along this creek on a site inspection undertaken on 9 th March The ground did however appear boggy underfoot. The area around the creek appeared densely vegetated with tea tree and shrubs as shown over in Figure 4. No defined top of bank was observed. Page 6

12 Figure 3 - Streams classified in accordance with the Strahler System Figure 4 Photos showing L: looking south to the Cockle Creek tributary along the easement to the west of the subject site. R: Vegetation at the middle of the low point Cockle Creek Cockle Creek passes the site further to the south and heads east towards the haul road at the Westside Mine. As this point it would be classified under the Strahler System as a third order stream. 2.5 Riparian Zones Riparian zones perform important environmental functions and serve to preserve the amenity and function of the watercourse. Furthermore, as part of LMCC LEP2004 zonings, the area around the Cockle Creek tributary adjacent to the site is zone 7(1) conservation. Riparian zones and vegetated buffer zones have been included on the subject site as recommended by the former Department of Water and Energy in their Government Agency Consultation Response. The Cockle Creek Tributary is a first order stream as discussed previously. Ten metres for riparian zone plus ten metres for a buffer zone has therefore been chosen for this watercourse. No defined top of bank was observed on-site, so a top of bank in this instance has been assumed as Page 7

13 the 1% AEP flood extents (discussed later in this report). This is considered a conservative assumption at this stage. This, along with riparian zones and vegetated buffer widths, are to be confirmed by the department at DA stage for any future development. An approximate extent of the core riparian zone and vegetated buffer adopted at this stage is included below in Figure 5. Figure 5 - Extent of riparian zone and vegetate buffer 2.6 Flooding Peak Flow Results An XP-RAFTS model was set up to determine the peak runoff generated from the regional subcatchments in the 1% AEP peak storm event. The average catchment slopes determined above in Section have been used along with a Manning s roughness congruent with that observed onsite. The results are shown below in Table 3. Sub catchment Flow rate (m 3 /s) A 9.7 B A+B Table 3 - XP-RAFTS peak flow rates A probabilistic rational method analysis was also undertaken. The results have been compared and have been found to be of the same order of magnitude as those obtained from the XP-RAFTS model. This comparison has been presented below in Table 4. Full rational method calculations are available in Appendix A. Page 8

14 Sub catchment XP RAFTS Flow rate (m 3 /s) PRM Flow rate (m 3 /s) A A+B Table 4 - Comparison of XP-RAFTS flow rates with probabilistic rational method calculations The peak flow values determined from XP-RAFTS have been used to estimate the 1% AEP flood extent as outlined in the next section Flooding from Upstream Catchments The most likely cause of flooding in the vicinity of the proposed development is from upstream catchments. As such the extent of flooding has been determined to assess whether there is any potential impact on any future development. The 1% AEP flood extents have been modelled in HEC-RAS from the peak flows determined above. Cross sections for this model have been obtained using the LiDAR information provided. The creek has been modelled as a single alignment extending through sub-catchment A into sub-catchment B and terminating near the haul road to the east. The convergence of flows from the two subcatchments has been modelled as a change in peak flow at a cross section. The extent of the 1% AEP flood is shown overleaf in Figure 6. Manning s channel equation was used to check the HEC-RAS results. A summary of the comparison is shown below in Table 5. These results correlate well, with the differences most likely due to the assumptions made in determining the cross sections for the Manning s equation. Section HEC-RAS WS (m AHD) Manning s WS (m AHD) HEC RAS Extent (m) Manning s Extent (m) 1 (CH ) (CH ) (CH ) Table 5 - HEC-RAS comparison with Manning's equation From these results it is shown that whilst flooding does slightly encroach on the subject site, it is not likely to encumber any future development. Furthermore, the water level adjacent to the site calculated as 28.3m AHD is more than 500mm below the natural surface level on the nearest proposed lot on the cadastre. This satisfies the habitable floor level requirements of LMCC DCP No.1 from a flooding perspective Flooding from Runoff Generated on the Subject Site The local catchments that form the subject site are relatively small in nature; all less than five hectares. Furthermore, adequate falls are present in all catchments to facilitate drainage design. Page 9

15 Figure 6-1% AEP flood extent and sections Catchments 1 and 3 drain to multiple discharge points and therefore the peak flows generated are not expected to cause flooding. Discharge points are yet to be determined for catchments 2 and 4 as they do not interface with existing development. As such, multiple discharge points may be adopted should the need arise. It is expected that during future detailed design stages, a major/minor regime as outlined in Australian Rainfall and Runoff 1987 (AR&R) will be achievable and flooding from run-off generated on the subject site will be unlikely to cause any issues Flooding from Backwater Effects The possible impact on flood levels and extents arising from backwater effects has been considered. The nearest downstream control structure appears to be the haul road crossing Cockle Creek at the Westside Mine. Several large culverts convey water under this road. Whilst the exact configuration of this control is unknown, the crest of this road is approximately 16m AHD compared to the lowest elevation of the proposed lots at 30m AHD. Therefore backwater effects are unlikely to have an impact in this case Potential Increased Risk of Flooding from Future Development It is anticipated that all future development will be regulated by Council to limit post developed flow back to pre developed levels through the use of detention. It is envisaged that this will prevent an increase in flood risk from any potential development. This is discussed in further detail in Section Risk of Flooding Due to Extreme Flooding, Sea Level Rise and Climate Change An increase in peak flow due to increases in rainfall intensity, peak flow in extreme flood events and sea level rise due to climate change has been considered as part of this investigation. Page 10

16 Peak flow generated in a probable maximum event can be estimated anywhere from three to five times that of the 1% AEP flows. Five times has been assumed in this assessment as outlined in Table 6 below. Sub catchment 1%AEP (m 3 /s) PMF (m 3 /s) A A+B Table 6 - Comparison between 1% AEP peak flows and the probable maximum flood (PMF) Running the revised peak flows through HEC-RAS, the peak water surface elevations and flood extents increase as expected. These results are compared to the 1% AEP values in Table 7 below. Section 1% AEP WS (m AHD) 1% AEP Extent (m) PMF WS (m AHD) PMF Extent (m) 1 (CH ) (CH ) (CH ) Table 7 - Comparison of water surface elevation and flood extents 1% AEP to PMF Again, adjacent to the subject site the maximum water level is below the minimum elevation for proposed lots. As such, the PMF is not expected to significantly affect any future development. It is likely that design rainfall intensities will change not only as a result of climate change, but as further information on rainfall estimation is released as part of Australian Rainfall and Runoff 4 th Edition (due 2012). The former N.S.W Government Department of Environment and Climate Change Floodplain Risk Management Guideline Practical Consideration of Climate Change recommends that a sensitivity analysis be conducted with an increase of 10%, 20% and 30% in peak rainfall until more work is completed in relation to the climate change impacts on rainfall intensities. A sensitivity analysis was undertaken; however an approximation of the PMF was adopted, as outlined above. This equates to a 500% increase in peak flow and as such is considered the most conservative approach in this case. As outlined in LMCC Lake Macquarie Sea Level Rise Preparedness Adaptation Policy, the water level in Lake Macquarie is also expected to rise from 1.38m AHD currently, to 2.47m AHD in Due to the natural levels on site (30m AHD and above) sea level rise will not have an impact on the flood levels in the vicinity of the subject site. Page 11

17 3 Stormwater Management Strategy 3.1 Outline This stormwater management strategy has been proposed in order to demonstrate that Council s policies and other statutory requirements are feasible on this site and to identify constraints that will affect any development. The following comments have been provided with the existing drainage regime in mind, but should not be considered detailed design solutions. It is anticipated that this will be undertaken at a future development application stage. 3.2 Catchments and Existing Drainage Infrastructure Killingworth straddles a ridge line which divides the current developed area into two drainage systems. Both drain to the north under The Broadway one to the west and one to the east. The subject site is divided into two parcels of land and four catchments as described in Section The existing drainage system is comprised of a traditional pit and pipe network with some grassed swales. An inter-allotment system is provided in seven metre wide easements. The Broadway had upright kerb installed with side roads having either roll kerb or a swale system. Several inlet pits were observed capturing surface runoff from kerbs and swales and directing run-off into the piped system. Figure 7 - Photos L: looking north on Westcroft St R: looking north on The Boulevard Stormwater drainage from any proposed development is anticipated to conform to the major/minor philosophy outlined in AR&R. The minor system is designed to minimise nuisance flooding from regular events and in this case will likely be designed through a network of swales managing concentrated flow. Pit and pipe systems may also be considered. These swale/pit and pipe networks are to be designed to cater for the peak flow from the 10% AEP event. Large events up the 1% AEP event are to be managed in easements and road reserves. 3.3 Detention Detention of water is designed to minimise the effect of increased peak flows from development on waterways downstream. This prevents damage such as erosion, destruction of vegetation and downstream flooding. Lake Macquarie City Council DCP No.1 Section 2.5 nominates that; Page 12

18 Natural water bodies, waterways and vegetation are retained and protected from increased stormwater flows. (Clause P3.1) For residential developments of more than 2 dwellings or lots and for all commercial and industrial developments on-site detention of stormwater will be required. (Clause P3.2) Catchments within the subject site either discharge ultimately to a natural watercourse or have existing residential development downstream. Therefore, detention may be required for the site in order to achieve compliance with Council s policies. The following detention options summarised in Table 8 are considered for the subject site. Catchment Detention Options 1 Catchment 1 is located on the north-western portion of the subject site. As this catchment contains some residential development and drains to existing infrastructure, detention could be provided on individual lots. This at source method helps to reduce the load on existing stormwater infrastructure downstream. 2 Catchment 2 is located on the south western portion of the subject site. This catchment drains directly into a watercourse. Whilst there was no permanent flow in this watercourse observed at the time of inspection, the 1% AEP flood extent almost coincides with the discharge point. Furthermore, dense vegetation observed would help guard against erosion from increased stormwater flows. Of concern however, would be the increased frequency and magnitude of runoff associated with increased impervious area. Therefore, detention should be provided for this catchment. Options include at source treatment as per Catchment 1, or a small regional basin located in the south western corner of the subject site. 3 Catchment 3 is located on the north-eastern portion of the site. Similar to Catchment 1 it interfaces with existing development. Detention for Catchment 3 could be provided through at-source treatment on individual lots or through a regional basin on the corner of Park and Throckmorten Streets. It is considered that detention on individual lots is the most likely outcome in this case. 4 Catchment 4 is located on the south-eastern portion of the site. Detention may be provided on individual lots or in a small regional basin. In any case a small basin or trench at the outlet to act as a level spreader would be preferable. Table 8 - Summary of detention options for the subject site At source detention may be sized in accordance with LMCC Handbook for Drainage Design Criteria. This could be incorporated into tanks and gravel trenches on individual lots and as such would not affect the developable area of the site as a whole. Approximate locations and footprints of possible regional basin have been provided in Figure 8 overleaf should this option be considered. The size of these would need to be verified at a future DA stage. Page 13

19 3.4 Connection to Existing Infrastructure Connection to existing infrastructure is anticipated at all locations discussed in Section This is not expected to be an issue as detention provided upstream of these locations will have reduced flows back to pre-development levels, thus maintaining the status quo in terms of flow rate. Figure 8 - Possible detention locations 3.5 Water Quality Construction Water Quality Prior to construction, an Erosion and Sediment Control Plan should be compiled in accordance with Landcom Managing Urban Stormwater Soils and Construction (the Blue Book). Control measures may include, but are not limited to; - Cut off swales to minimise the volume of water traversing the disturbed areas. - Sediment fence to capture mobilised sediment on the downstream side of the site. - Filter socks and hay bales to capture sediment in swales and gutter prior to discharging to existing pits. - Sediment basins to capture turbid water if there is a large disturbed area. - Stockpiles limited to two metres in height and vegetated if remaining in place for a long period of time. - Re-vegetation of disturbed areas post construction. Page 14

20 3.5.2 Modelled Pollutant Loading The computer program MUSIC has been used to determine pollutant loadings as required by the LES brief. Three models have been set up; the pre-developed scenario, a possible post developed scenario and a possible treatment train to reduce the pollutant loadings as per Council s guidelines. The following pollutant loadings outlined in Table 9 have been adopted from the MUSIC program for the pre and post developed scenarios respectively. Forest Pollutant Total Suspended Solids Total Phosphorous Total Nitrogen Base (log mg/l) Std Dev (log mg/l) Storm (log mg/l) Std Dev (log mg/l) Residential Base (log mg/l) Std Dev (log mg/l) Storm (log mg/l) Std Dev (log mg/l) Table 9 - Pollutant concentrations for pre and post development options The pre-developed model has been approximated as forest with zero percent impervious area. This is not strictly correct as pollutant loadings may vary based on a number of factors such as the proximity to rural residential areas, vehicular movements through the subject site and the difference in soil type and vegetation compared to those adopted in the MUSIC values. The post developed model has been approximated as an urban area. The impervious fraction has been determined from LMCC design guidelines which suggest a 50% impervious fraction for Zone 2(1) areas in the absence of more detailed information. The results are summarised below in Table 10. As expected there is a significant increase in pollutant loads for the subject site. Pre-developed Post-developed Flow (ML/yr) Total Suspended 1,730 18,200 Solids (kg/yr) TP (kg/yr) TN (kg/yr) Gross Pollutants (kg/yr) 0 2,930 Table 10 - Pollutant loadings for the pre and post development scenarios Page 15

21 3.5.3 Urban Run-off Quality Control Lake Macquarie City Council s Stormwater Treatment Framework & Stormwater Quality Improvement Device (SQID) Guidelines are expected to be implemented in the design of any future development. Water sensitive urban design measures such as tanks, gravel trenches, grassed buffer strips, swales and constructed wetlands are recommended in this case. Alternatives such as proprietary water treatment devices could be considered; however they should not be considered as the only form of water quality control due to their expensive initial and ongoing cost. Rainwater Tanks Rainwater tanks on individual lots will perform as a primary treatment device and present several benefits including reduced potable demand as well as at-source control of roof water pollutants. Sediment and nutrients are removed from the stormwater stream via sedimentation in the tanks, thus increasing the efficiency of the treatment devices downstream. Gravel Trenches/Grassed Buffer Strips Gravel trenches may be employed in order to meet the mitigation storage requirements for impervious areas onsite. This will provide an opportunity to remove sediment and nutrients from the stormwater stream. These devices will likely surcharge over a grassed buffer strip prior to discharging to the road reserve which will provide further treatment prior to entering swales. Grass Lined/Vegetated Swales Swales further filter stormwater and replicate natural concentration of water which is congruent to the objectives of a secondary treatment device. Sediment is deposited in the vegetation and some pollutants attach to soil particles and organic matter. The use of swales on the subject site depend on the layout employed, and would best be suited if aligned approximately parallel to the contours adjacent to roadways. Constructed Wetlands Due to space limitations and the steep topography of the subject site, constructed wetlands may not be considered a viable option, however they may be considered if regional detention basins are employed. Constructed wetlands provide tertiary treatment for stormwater and can double as public open space. This has the potential to increase the amenity of the site and also provide treatment suitable to be released further downstream. In this case, extended detention can also be provided above the wetland to reduce peak flows back to pre development levels. In the interest of public safety, DCP No 1, Volume 2, Part 5 Batter Slope Treatments and Fencing Guidelines for Constructed Wetlands and Detention Basins should be consulted when designing these devices. Proprietary Devices Proprietary devices such as gross pollutant traps, pit inserts or filtration technology may be considered to supplement the treatment train at various stages. This may have benefits in terms of reducing land occupied by water treatment devices; however they are not to be considered as a replacement. Generally, these devices are expensive to install and maintain and also can have reduced efficiency when not maintained adequately. Page 16

22 In order to determine the feasibility of incorporating the above devices in the development to satisfy Council s policies, a potential treatment train was modelled in MUSIC. The SQID guidelines specify that for zone 2 developments greater than two hectares the following removal efficiency rates shown in Table 11 are required. Pollutant Removal Efficiency (percent) Pollutant Removal Efficiency (percent) Gross Pollutants Nutrients (TP/TN) Coarse Sediment Heavy Metals Medium Sediment Oil and Grease Fine Sediment Table 11 - Pollutant removal targets interpreted from SQID guidelines A possible treatment train was developed and modelled in MUSIC. The site was run through rainwater tanks (10kL per lot) discharging to a buffer strip being collected by a one hundred metre long grassed swale. Wetlands and gravel trenches were omitted from the model, but may be considered depending on the final layout. This is by no means intended as a detailed design solution, merely proof that the pollutant reductions are feasible and will not place significant restriction on any proposed development. Below in Table 12 is a summary of the reduction in pollutant load. Post-developed Treatment Train Reduction (percent) Flow (ML/yr) Total Suspended 18,200 3, Solids (kg/yr) TP (kg/yr) TN (kg/yr) Gross Pollutants (kg/yr) 2, Table 12 - Treatment train efficiencies This potential scenario shows compliance with Council s policies and demonstrates the feasibility of integrating water quality treatment devices on the subject site. 3.6 Assessment of Water Management Alternatives Detention Two main methods have been presented for on-site detention; at source control such as tanks and gravel trenches, and regional detention basins. Advantages and disadvantages of these options are presented overleaf in Table 13 taking into consideration rough life cycle costs, maintenance requirements, land take, reliability and community acceptance. All options presented achieve compliance with management objectives. Life cycle costs for this section estimate methods for providing detention such as tanks, trenches or basins and the effect on pipe sizes only. Water quality treatment costs are discussed in the next section. These costs are for order of magnitude comparison only. They have been determined from Page 17

23 past construction phase experience with residential subdivision and should not be construed as detailed costing analysis. Factors which affect these costs include, final lot layout, staging, market influences and detailed design solutions. Control At source Provision of dedicated volume in rainwater tanks or gravel trenches. Regional Basin At source + Regional Basin Life Cycle Costs $1.6 million initial $60,000/yr $2.0 million initial $150,000/yr $1.8 million initial $140,000/yr Advantages - Low maintenance. Only requirement is the clearing of gutters and periodic inspection of first flush devices. - Small land take on individual lots (5m 2 10m 2 for tanks, 20m 2 for trenches) - This option is likely to gain community acceptance as rainwater tanks will be mandatory anyway. - Reduces stormwater infrastructure since flow is reduced to predevelopment level prior to discharging from lot. - Council has access rights to maintain basin. This will likely be by front loader. - Basin is likely to be reliable given a maintenance regime. - Community acceptance will be likely for this scheme as the onus to provide detention is taken away from the individual lot. - Basin size reduced due to at source control. - Stormwater infrastructure size reduced but not to the same extent as the at source option. Disadvantages - Since infrastructure is located on individual lots, Council cannot access for maintenance which is the responsibility of the land owner. - Gravel trenches may silt up over time reducing efficacy. - Developable area is reduced due to land take of approximately 5,500m 2. - If these areas are recreational areas they may be frequently boggy. - If basin areas are recreational areas they may be frequently boggy. - Maintenance required for both basin and at source option. Table 13 - Options analysis for detention Page 18

24 3.6.2 Water Quality A similar assessment to that undertaken for detention is presented below in Table 14 for water quality. Life cycle costs presented in this section are for water quality treatment only. A suitable option for detention would need to be added to these figures. Control WSUD Treatment Train Treatment train as modelled with tanks discharging to a grassed buffer strip, then a grassed swale. WSUD Treatment Train with Wetland As above but including a wetland. Proprietary Devices Traditional kerb a gutter running through gross pollutant traps and proprietary nutrient and oil removal systems. Life Cycle Costs $490,000 initial $85,000/yr $600,000 initial $140,000/yr $860,000 initial $80,000/yr Advantages - Water quality treatment achieved through mimicking natural processes. - Council has access rights to maintain devices excluding individual tanks. This will likely be by hand. - Maintenance requirements will not be onerous on Council. - Council will reduce the amount of kerb and gutter it has to maintain. - Community acceptance likely for this scheme. - Water quality treatment well in excess of Council s guidelines. - Council has access rights to maintain devices excluding individual tanks. This will likely be by hand and long reach excavator. - Wetland may increase the amenity of the area by attracting wildlife. - Community acceptance likely for this scheme. - Small footprint. - Council has access rights to maintain devices. This will likely be by vacuum truck. Disadvantages - Since some infrastructure is located on individual lots, Council cannot access for maintenance which is the responsibility of the land owner. - Standing water may be a breeding ground for mosquitoes. - Council does not have access to maintain tanks on individual lots. - Wetland reduces developable area. - High initial and ongoing cost. - As community becomes more environmentally conscious, these devices as the sole treatment option may not be well received. - Devices will not be reliable unless they are regularly maintained. Page 19

25 Control Combination of Proprietary Devices and WSUD Treatment Train Swales discharging to proprietary devices or kerb and gutter discharging to proprietary devices discharging to wetlands Life Cycle Costs $800,000 initial $85,000/yr Table 14 - Options analysis for water quality Advantages - Reduction in WSUD element sizes meaning larger developable area. - Council has access rights to maintain devices. This will likely be by vacuum truck. - Community acceptance will be likely for this scheme. Disadvantages - High initial and ongoing cost. - Proprietary devices will not be reliable unless maintained regularly. 3.7 Maintenance of Riparian Zones and Vegetated Buffers In order for riparian zones to be successful, effective maintenance should be undertaken. Details of any rehabilitation proposal and ongoing maintenance should be included as part of the development application. Consideration in the preparation of these documents should include; - Removal of weeds and non-native vegetation. - Definition of stream banks and paths - Selection of appropriate vegetation as outlined in LMCC Estuarine Creekbank Stabilisation and Rehabilitation Guidelines and NSW Office of Water Guidelines for Controlled Activities: Vegetation Management Plans. Through proper design and management, the use of core riparian zones and vegetated buffers could improve the amenity of any future development. Page 20

26 4 Recommendations and Conclusions 4.1 Flooding - Local catchments are unlikely to result in flooding that significantly restricts any future development. - The 1% AEP flood extent from the upstream regional catchment has been determined for a tributary of Cockle Creek running to the south west of the subject site. - This extends marginally onto the subject site but does not encumber any lots based on the existing cadastre. Therefore, the 1% AEP is unlikely to significantly impact any future development. - A sensitivity analysis on this extent has been undertaken to simulate the effect of climate change, new research and the PMF on the subject site. The PMF extent is also unlikely to significantly affect any future development. 4.2 Water Management - Detention should be provided as part of any future water management plan. This may be at source including tanks and gravel trenches, regional basins, or a combination of both. Details and sizing are to be provided at a future DA stage. - Water sensitive urban design features such as buffer strips, grassed swales and wetlands should be considered as part of any development. This fits in with the current development as well as providing water quality treatment to satisfy Council s policies. 4.3 Conclusions - Based on a review of several factors, including flooding, detention, water quality and water management practices; it is considered that the subject site is suitable for re-zoning to facilitate urban and conservation development objectives from a flooding and drainage perspective. - Several strategies have been presented as part of this investigation for the purposes of examining the feasibility of re-zoning. These are by no means detailed design solutions and any future development should consider the latest information regarding water management and urban flooding estimation and design accordingly. Page 21

27 5 References Lake Macquarie City Council (2003) DCP No.1 Volume 2 Engineering Guidelines Stormwater Treatment Framework and Stormwater Quality Improvement Device Guidelines; Lake Macquarie City Council (2004) Lake Macquarie Local Environmental Plan 2004; Lake Macquarie City Council (2006) DCP No.1 Volume 1 Guidelines - Water Cycle Management Guidelines; Lake Macquarie City Council (2004) DCP No.1 Volume 1 Guidelines Estuarine Creekbank Stabilisation and Rehabilitation Guidelines; Landcom (2004) Managing Urban Stormwater - Soils and Construction; New South Wales Government (2001) Floodplain Development Manual: the management of flood liable land; New South Wales Government Department of Environment and Climate Change (2007) Floodplain Risk Management Guideline Practical Consideration of Climate Change; New South Wales Government Office of Water (2010) Controlled activities Guidelines for Vegetation Management Plans; The Institute of Engineers, Australia (1987) Australian Rainfall and Runoff - A Guide to Flood Estimation; Webb, McKeown & Associates Pty. Ltd. (2000) Lake Macquarie Floodplain Management Study; Webb, McKeown & Associates Pty. Ltd. (2001) Lake Macquarie Floodplain Management Plan. Page 22

28 Appendix A Rational Method Calculations

29

30 Appendix B Manning s Channel Calculations

31 Section 1 Input Geometry Max total width Wt m Height of water h m 0.37 Width of channel w m 40 Left batter bl xh:1v 12.5 Right batter br xh:1v 17.5 Base RL m AHD 31.8 Hydraulics Slope of HGL S percent 1 Manning's Roughness n Design Flow Rate Q m3/s Results Geometry Total width Wt m Area A m Wetted Perimeter P m WS Level m AHD Hydraulics Velocity V m/s Velocity Depth Vd m2/s Flow Rate Q m3/s

32 Section 2 Input Geometry Max total width Wt m Height of water h m 0.38 Width of channel w m 45 Left batter bl xh:1v 20 Right batter br xh:1v 20 Base RL m AHD Hydraulics Slope of HGL S percent 0.67 Manning's Roughness n Design Flow Rate Q m3/s Results Geometry Total width Wt m Area A m Wetted Perimeter P m WS Level m AHD Hydraulics Velocity V m/s Velocity Depth Vd m2/s Flow Rate Q m3/s 9.804

33 Section 3 Input Geometry Max total width Wt m Height of water h m 0.44 Width of channel w m 30 Left batter bl xh:1v 20 Right batter br xh:1v 20 Base RL m AHD Hydraulics Slope of HGL S percent 0.85 Manning's Roughness n Design Flow Rate Q m3/s Results Geometry Total width Wt m Area A m Wetted Perimeter P m WS Level m AHD Hydraulics Velocity V m/s Velocity Depth Vd m2/s Flow Rate Q m3/s 9.929