Water Resources Management Plan Appendix B

Transcription

1 B u r n s v i l l e M i n n e s o t a Water Resources Management Plan Appendix B Appendix B Page B-1

2 Burnsville, Minnesota STORM WATER LOW-IMPACT DEVELOPMENT GUIDE MANUAL SHORT ELLIOTT HENDRICKSON INC. September 2001

3 Burnsville, Minnesota STORM WATER LOW-IMPACT DEVELOPMENT GUIDE MANUAL TABLE OF CONTENTS I. Introduction...1 A. Objective...1 B. Storm Water Management Goals...1 Conventional Storm Water Goals...1 Low-Impact Development Goals: Mimic Existing Site Hydrology...2 II. Low-Impact Development Tool Box...3 A. Roof Top BMPs...4 B. Parking Lot BMPs...5 C. Street (curb to curb) BMPs...6 D. Sidewalk and Boulevard BMPs...7 E. Pervious Area BMPs...8 F. Park and Open Space BMPs...9 G. Pond BMPs...9 III. Hydrologic Evaluation of Proposed LID Practices...9 A. Instructions for Worksheet A: Evaluating Known LID Practices...10 B. Instructions for Worksheet B: Unknown Area for LID Infiltration Practices...15 References...19 LIST OF APPENDICIES Appendix A. Links to Low-Impact Development, Best Management Practice Fact Sheets Appendix B. Summary Tables: Design Considerations for LID Practices Appendix C. Example LID Infiltration Worksheets Appendix D. Links to Additional Low-Impact Development BMP Websites ii

4 Burnsville, Minnesota STORM WATER LOW-IMPACT DEVELOPMENT GUIDE MANUAL LIST OF FIGURES Figure 1. Typical Green Roof Cross-Section...4 Figure 2. Example of Green Roofs in an Urban Area...4 Figure 3. Example of a Green Island Rain Garden in a Parking Lot with Curb Cuts...5 Figure 4. Typical Catch Basin Filter Insert...6 Figure 5. Typical Grated Infiltration Trench...7 Figure 6a. Plan View of a Typical Bio-retention Area...8 Figure 6b. Cross-Section of a Typical Bio-retention Area...8 Figure 7. Typical Grassed Swale in a Residential Area...9 Figure W-1. Runoff Volume Per Acre vs. Percent Impervious Area (1.5-inch Rainfall)...12, 16 LIST OF TABLES Table W-1. Example Calculations for Volume Captured by Non-Infiltration LID Practices...13, 17 Table W-2. Soil Infiltration rates...14, 18 iii

5 Burnsville, Minnesota STORM WATER LOW-IMPACT DEVELOPMENT GUIDE MANUAL I. Introduction A. Objective This document is intended to provide a method to evaluate proposed storm water low-impact development (LID) practices proposed within the City of Burnsville (City). This document is intended for a broader audience than a similar document prepared specifically for the Heart of the City (HOC) project area. One of the primary goals is to provide a better understanding of the basic benefits, limitations and storm water control functions of LID techniques. An overview of the types of LID options available and references to more information on storm water LID techniques and applications are provided. Section I of this document summarizes the City s storm water management goals. Section II establishes a tool-box of LID techniques including a discussion of site considerations for each type of LID practice. Section III outlines an evaluation process and provides two worksheets for evaluating proposed storm water best management practices against the City s storm water LID goals. B. Storm Water Management Goals The City has two general levels of storm water management goals. The first level focuses on conventional storm water management as discussed in the City s Comprehensive Storm Water Management Plan Update (2001). The second level focuses on the more innovative LID techniques recommended specifically for projects in the HOC and more generally throughout the City. Specific goals and requirements of each level are discussed in more detail below. Conventional Storm Water Goals The City of Burnsville, like many Minnesota cities, has adopted the criteria established by the National Urban Runoff Program (NURP), which are widely accepted and recommended for design of storm water ponds. The City has also adopted the Minnesota Pollution Control Agency s (MPCA) urban best management practices (BMPs) described in the document Protecting Water Quality in Urban Areas, Best management Practices for Dealing with Storm Water Runoff from Urban, Suburban and Developing Areas of Minnesota, March 1,

6 Storm water runoff rate and pollutant loading controls based on NURP and MPCA criteria are typically met by utilizing existing and proposed storm water detention ponds and wetlands. A major design element for pond in the conventional system is to control discharge rates for the 100-year storm event. Storm water management in the City is also addressed in the Black Dog Watershed Management Organization (BDWMO) Plan (Barr, 2000) which indicates that runoff should be controlled to existing conditions if already developed and controlled to predevelopment conditions if not developed. NURP, MPCA and BDWMO criteria form the basis for the conventional storm water rate and pollutant loading standards throughout the City. Low-Impact Development Goals: Mimic Existing Site Hydrology Low-Impact Development goals are intended, in part, to encourage development that allows the post-development hydrology to more closely mimic existing site hydrology and to promote the use of infiltration BMPs to obtain additional water quality benefits. Different than the conventional storm water system described above, the LID techniques are intended and designed primarily for the smaller (1- to 2-year) rainfall events. Because of their greater frequency of occurrence, these smaller events or water quality storms often make the greatest contribution to annual runoff volume and have a greater impact on the water quality of receiving waters. Collectively, the smaller storm events also have a greater impact on groundwater recharge in the watershed. Implementing LID techniques will also extend the life of and reduce the maintenance requirements for the downstream pond system. The City s specific goals for LID practices are to: Mimic existing hydrology and approach zero runoff for the smaller water quality rainfall events (1.5 inch rainfall over 24 hours). Take advantage of high-infiltration rates soils, where present. Minimize connected impervious areas (route impervious areas like roofs and parking lots to pervious areas instead of directly to the conventional storm sewer system). Minimize the reliance on conventional, centralized storm sewer collection systems and centralized detention pond treatment. Section II of this document contains additional details on the benefits, site and design considerations, limitations and costs for a range of LID techniques. The benefits and limitations of LID techniques are generally well documented. References and design information are provided in Appendices A, D and F. LID practices provide several important benefits, including: Improving the ability to mimic or replicate pre-development hydrologic functions, thus reducing negative water quality and quantity impacts of development. Reducing construction and long-term maintenance costs of the overall storm water infrastructure. Encouraging flexibility and innovation in site planning and design. Improving the aesthetic value of development and re-development. LID practices also pose several limitations, including: 2

7 Significantly reduced effectiveness of infiltration practices in fine-textured soils. Variations in effectiveness based on climate and seasonal effects. Maintenance requirements can be high for some practices. Practices generally have limited impact on larger storm events. II. Low-Impact Development Tool Box This section contains a listing, or Tool Box, of several common LID techniques related to various surface types from which runoff originates. In addition to the discussions in this section, references to several more detailed fact sheets are provided in Appendix A and tables describing important LID considerations are located in Appendix B. The fact sheet referenced in Appendix A provide more detailed information on the selected BMPs including discussions of applicability to different climates, site and design considerations, pretreatment needs, limitations, maintenance requirements, effectiveness and additional cost considerations. Tables B1-B7 in Appendix B summarize site-specific issues such as water quality and quantity control, snow melt, aesthetics, maintenance requirements and costs for various storm water LID BMPs. More detailed design guidance is also available through several sources including those listed at the end of this document and the websites listed in Appendix D. Several of the websites include more detailed design and construction information on selected practices. One of the more detailed design guides for the types of LID practices discussed in this guide is Infiltration/ Biofiltration Management Practices (O Reilly, 1999). This tool box is not intended to be all inclusive of the wide range of LID practices and techniques available. The intent is to provide a basic understanding of the goals, benefits and limitations of LID techniques, and at the same time encourage design innovation that takes advantage of the site-specific features. The tool-box is categorized by surface type to help match the specific areas (or surface types) present in the area to be developed or re-developed. The types of surfaces presented include roof tops, streets, parking lots, sidewalks and boulevards, pervious areas and parks and open spaces. A category for ponds is also provided. A. Roof Top BMPs A range of flat roof and pitched roof practices are available. BMPs specific to flat roofs include green roofs, interior vegetated walls, gardens and fountains. Practices applicable to both flat and pitched roofs include rain barrels, cisterns, dry wells and rain gutters. Some of the benefits of green roof techniques include cooling the ambient air temperature, reducing heating and cooling costs and extending the life of roofing materials. Table B1 (Appendix B) provides an overview of the performance, maintenance and cost considerations of various flat and pitched roof top BMPs. Additional considerations and details on many of these practices can also be found in the fact sheets in Appendix A. 3

8 Figure 1. Typical Green Roof Cross-Section. (Source: Low-Impact Development Design Strategies: An Integrated Design Approach, Prince George s County Maryland, June 1999) One example of a roof top BMP is a green roof or vegetated roof cover for which a cross-section is illustrated in Figure 1. Vegetated roofs capture and hold precipitation in the vegetative canopy and in the soil substrate or root zone. These roofs require consideration of the load bearing capacity of the roof structure. As shown in Table B1 (Appendix B), green roofs vary in controlling discharge flow rates while controlling runoff volumes very well. Snow removal is recommended, maintenance requirements are moderate to high and cost are low to moderate. Figure 2. Example of Green Roofs in an Urban Area. (Source: Green Roof Project, Northwest EcoBuilding Guild, 2001) 4

9 The aesthetic value of a green roof is a major benefit. Green roof practices have proven very effective throughout several European countries and have been in use there for more than 25 years. Demonstration projects have also been implemented in larger cities in the United States including Philadelphia (EPA, 2000), Chicago, Los Angeles, and Maryland. A local example of a green rooftop is the Phillips Eco-Enterprise Center in Minneapolis (Metropolitan Council 2001). An example of a green roof in an urban area is shown in Figure 2. B. Parking Lot BMPs Parking lot BMPs can be categorized into three groups, pavement, storage and infiltration practices. Pavement BMPs include several different types porous pavements and turf pavements. These would typically be used in less frequently used overflow parking areas. Storage BMPs include dry extended storage ponds, catch basin inserts and standing storage. Infiltration BMPs include grassed swales, curb cuts/infiltration basins, filter tanks, infiltration trenches, french drains, dry wells and bio-retention areas. Table B2 (Appendix B) provides an overview of the performance, maintenance and cost considerations of various parking lot practices. Additional considerations and details on many of these practices can be found in the fact sheets in Appendix A. Figure 3. Example of a Green Island Rain Garden in a Parking Lot with Curb Cuts. (Source: Storm Water Manager s Resource Center Website, Center for Watershed Protection, Inc., 2000) 5

10 One example of a parking lot BMP is the combination of curb cuts and infiltration basins. In this practice, the curbs are cut to allow runoff to enter an infiltration area that is lower in elevation than the surrounding parking surface. A photograph of a curb cut/infiltration system or green island is shown in Figure 3. These systems are very well suited to areas having soils with high infiltration rates. As shown in Table B2 (Appendix B), the curb-cut/infiltration basin combination varies in controlling discharge flow rates while controlling runoff volumes very well. These practices also perform well in removing sediment and dissolved pollutants in parking lot runoff. Maintenance requirements are low to moderate and costs are relatively low. This practice can also provide a great deal of aesthetic value. Improved aesthetics can be obtained by installing curb sections that slope into a depressed parking lot island, thus eliminating the cut portion of the typical curb and gutter. C. Street (curb to curb) BMPs Street BMP options can include many of the same practices mentioned for parking lots. Additional practices for streets include layout and design of shorter street sections and narrower roads to reduce the amount of impervious surface. The same cautions that apply to porous and turf pavements for parking areas apply to streets. That is, they should only be used for lesser used streets and alleys. Table B3 provides an overview of the performance, maintenance and cost considerations of various street practices. Additional considerations and details on many of these practices can be found in Appendix A. Figure 4. Typical Catch Basin Filter Insert. (Source: Kristar Enterprises Inc., 2001) 6

11 An example of a street BMP is placing a catch basin filter insert just under the grate of the street storm sewer inlets. An example filter insert is shown in Figure 4. Filter inserts provide removal of sediment, debris trash and petroleum products from street runoff. Filters do not provide for infiltration capacity or runoff storage capacity, although they can slow down the runoff and reduce flow rates entering the storm sewer system. As shown in Table B3 (Appendix B), subcatchment filter inserts rate low to moderate for both maintenance requirements and costs. An important consideration is determining who is responsible for carrying out activities and bearing the costs of maintenance. D. Sidewalk and Boulevard BMPs Sidewalk and boulevard area BMPs include porous pavements, vegetated filter strips, infiltration basins, bio-retention areas and grated sand infiltration trenches. Layout and design of sidewalks on only one side of the street may also be an effective LID technique. Table B4 (Appendix B) provides an overview of the performance, maintenance and cost considerations of several sidewalk/boulevard practices. Additional considerations and details on many of these practices can be found in Appendix A. Figure 5. Typical Grated Infiltration Trench. (Source: Storm Water Manager s Resource Center Website, Center for Watershed Protection, Inc., 2000) One common example of a sidewalk BMP is a grated infiltration basin or trench that has a circular or rectangular area surrounding a single tree or series of trees. As described in Table B4, this practice can be aesthetically pleasing, low maintenance and has a low to moderate cost. This practice also provides a high level of sediment and dissolved pollutant removal. Reductions in runoff volume are significant in areas having soils with high infiltration rates. A grated infiltration trench (Figure 5) for use in streets or parking areas is similar to those for sidewalk or boulevard applications. 7

12 E. Pervious Area BMPs Pervious areas can include maintained practices such as dry extended detention ponds, grassed channels, plunge pools, wet ponds, bio-retention areas and grassed swales. Natural pervious areas include wetlands and pond/wetland systems. Table B5 (Appendix B) provides an overview of the performance, maintenance and cost considerations of various pervious area practices. Additional considerations and details on this type of practice can be found in Appendix A. Figure 6a. Plan View of a Typical Bio-retention Area. Figure 6b. Cross-Section of a Typical Bio-retention Area. (Source: Low-Impact Development Design Strategies: An Integrated Design Approach, Prince George s County Maryland, June 1999) Pervious area practices are very well suited to park-type areas where a relatively large portion of the development or area will be pervious. In addition to the infiltration benefit of pervious area 8

13 BMPs, they present an excellent opportunity to incorporate trees and landscape plantings for improving the aesthetic value of these areas. A plan view and a cross-section of a typical bioretention area are illustrated in Figures 6a and 6b, respectively. A typical grassed swale is shown in Figure 7. F. Park and Open Space BMPs Figure 7. Typical Grassed Swale in a Residential Area. (Source: Storm Water Manager s Resource Center Website, Center for Watershed Protection, Inc., 2000) Park BMPs are similar to those listed for pervious areas. Table B6 (Appendix B) provides an overview of the performance, maintenance and cost considerations of various park and open space practices. Additional considerations and details on these practices can be found in Appendix A. As described above for pervious areas, parks and open spaces provide great opportunities for infiltration-lid practices as well as enhancing the aesthetic value of a development. G. Pond BMPs Typical pond BMPs are extended dry ponds, ponds and pond/wetland systems. Ponds are primarily intended to provide runoff rate and volume control for larger storm events and, on an annual basis, pollutant loading control. Table B7 (Appendix B) provides an overview of the performance, maintenance and cost considerations of various pond practices. 9

14 III. Hydrologic Evaluation of Proposed LID Practices This section contains two worksheets and related information that can be used for evaluating the infiltration capacity of proposed LID practices and determining if proposed practices meet the City's LID storm water management goals. Example worksheets are also provided in Appendix C to help understand how to apply the worksheets to a specific development project. The first worksheet, Worksheet A, is used to evaluate if specific LID practices will meet the City s LID goals. Worksheet B, is used to determine how much LID volume or infiltration surface area may be needed to meet the City s LID goals. Both worksheets contain a step that accounts for non-infiltration LID practices where groundwater recharge will not occur. Practices such as rain barrels, cisterns, roof gardens and other storage-type LID practices can be accounted for in this step. The worksheet user should be aware that the hydrologic information used to develop the worksheets is specific to the Heart of the City Project area. The user should be cautioned that results may not may reflect the site under evaluation if the hydrologic characteristics are significantly different than those used for the developing the worksheets. The key hydrologic assumptions include: 1.5-inch rainfall, Type II distribution with AMC 2 An average 10-acre block area (average block width = 479 feet) Average slope of 2.1 percent Soil infiltration rates of , , and inches per hour A. Instructions for Worksheet A: Evaluating Known LID Practices Worksheet A is used when the extent and types of LID practices to be used are known. Worksheet A contains ten steps to determine if the proposed LID practices will be sufficient to meet the goal of zero runoff for the 1.5-inch rainfall event. Step 1 requires the total site area in acres and the proposed direct impervious area after development. In most cases, the proposed direct impervious area in acres is the total surface area of buildings, roads, sidewalks, parking lots and other impervious areas in the block under consideration. Where the project consists of redeveloping an existing developed area, the total site area and impervious area for this evaluation may be limited to the areas that do not drain directly to existing storm sewer systems. In some cases, areas such as existing roads from which routing runoff to a LID practice is not feasible may be excluded from the total site area and direct impervious area. In other words, this number should reflect the total site area that could be captured or treated in LID practices. If areas are excluded the developer should consult with City staff the total site area. This number should be based on an evaluation of the proposed site plan. Step 2 simply calculate the percent impervious for the proposed site plan. In Step 3 the developer determines the estimate runoff volume per acre for the proposed block using the percent impervious from Step 2. The relationship shown in Figure W-1 is based on data obtained using the XP-SWMM hydrologic model. Inputs to the model included a block width of 10

15 479 feet (the average of individual block widths for the seven Town Center blocks), soil infiltration rates ranging from , , and inches per hour and using a Type II, 1.5-inch, 24-hour rainfall event. All infiltration rates modeled for the 1.5-inch rainfall resulted in the linear relationship shown in Figure W-1. Users are cautioned that the relationship shown in Figure W-1 is based on inputs specific to site features for the Heart of the City project area. The user should verify that similar site conditions (average block width and soil conditions) are present before applying the worksheet to a given site. Step 4 calculates the total site runoff using the result from Step 3 and the total site are listed in Step 1. This number represents the estimated total runoff volume in cubic feet that would occur without the benefit of any LID practices implemented on the site. In Step 5 the portion of the site runoff volume captured by all non-infiltration LID practices is accounted for. Four example types of practices are shown in Table W-1 (Worksheet A) describe how to calculate the estimated volume captured by each practice. For example, the volume captured by a green roof or roof garden is equal to the volume of rainfall that enters the planting soil plus the volume stored above the soil and below the outlet or overflow point. The volume captured in the soil is estimated as the volume of soil multiplied by the effective porosity of the soil. The effective porosity is estimated from the ranges of values presented in Applied Hydrology (Chow, et. al., 1988). For this example, the volume captured is simplified in that it does not account for soil under-drains that would reduce the volume held in the soil to field capacity after some time. For each non-infiltration practice to be used, the user should list the type, quantity and capacity of each practice in the chart provided below Table W-1. If practices not listed are proposed, the user should enter the volume estimate in the chart and describe the method used to determine the volume. The result for Step 5 is the sum of the volumes captured by the different practice types. Step 6 simply calculates an excess runoff volume after subtracting the Step 5 result from the Site Runoff volume obtained in Step 4. This value represents the volume that must be infiltrated by LID practices to meet the zero runoff goal for the 1.5-inch rainfall event. In Step 7, users select a representative soil infiltration rate for the infiltration practices to be used at the site. Table W-2 lists saturated infiltration rates for the four soil hydrologic soil groups and soil textures shown. If the user does not know the extent or has not verified actual soil texture from field investigations, the most restrictive soil Group should be used. This will be Group B or the 0.25 in./hr. infiltration rate. If the user has verified on-site soil conditions and/or proposes to use the underlying soil with a permeability greater than 2.0 in./hr., a Group A soil, or 0.50 in./hr. infiltration rate, may be used in the worksheet. In the same context, if for example half of the soils will be Group B and half will be Group A, the user could select an infiltration rate of in./hr. in Step 7. In Step 8, user lists, in the chart provided, the infiltration-lid practices they will use and the surface area each practice will cover. As noted in the worksheet, this step considers the total surface area of LID practices where the focus will be infiltration and is not intended to include 11

16 general pervious areas not subject to intensive infiltration. Areas like a parking lot islands located above the elevation of the parking surface or boulevard grass sloping into the street curb and gutter system should not be considered in this section. Practices like grated infiltration trenches, grassed swales, depressed parking lot islands and bio-retention areas should be included in the chart. The total surface area for infiltration practices is then calculated to obtain the Step 8 result. Step 9 calculates the volume that can be infiltrated in 24 hours by the practices listed in Step 8. In Step 9, the total surface area from Step 8 is multiplied by the infiltration rate from Step 7 (and a units conversion factor) to obtain the volume infiltrated in cubic feet. The volume infiltrated in the step represents a volume in a 24-hour period corresponding to the 24-hour duration of the 1.5-inch storm event. Step 10 compares the volume results from Step 9 and Step 6. In order to meet the goal of zero runoff for the 1.5-inch storm event, the volume infiltrated must be greater than or equal to the excess runoff volume from Step 6. If the practices you evaluated in Step 8 result in meeting the goals, you should proceed tot he more detailed design stage for specific BMPs. Again, the factsheets referenced previously in this document provide more detail on this design and construction stages. If the result for Step 10 shows that the LID goals have not been met, the user should increase the storage capacity and/or infiltration surface area of the proposed LID practices in Steps 5 and 8, and recalculate the results through Step 10. A second option is to complete Worksheet B to determine how much LID storage volume and infiltration surface area is needed. 12

17 Low-Impact Development Infiltration Evaluation for Burnsville, MN,, (Based on Heart of the City Hydrologic Information) (Sheet 1 of 3) Worksheet A: Evaluating Known LID Practices Developer s Name: Site Name/Block No.: Step 1: Determine proposed impervious and pervious areas for the site. a. Total site area in acres. acres b. Proposed direct impervious area in acres. acres Step 2: Determine the proposed percent impervious area for the site. Percent impervious = 100 x (Step 1.b. Step 1.a.) = 100 x ( ) = % Step 3: Determine the post-development runoff per acre from Figure W-1. *Runoff per acre in cubic-feet = cu. feet (*Obtained from Figure W-1 by drawing a line up from the percent impervious determined in Step 2 until it intersects the diagonal line, then drawing a line to left from that point to the total runoff per acre value.) Figure W-1. Runoff Volume Per Acre vs. Percent Impervious Area (1.5-inch Rainfall). Step 4. Calculate the site runoff volume for the 1.5-inch rainfall event. Site runoff volume (cu. feet) = Runoff Per Acre (cubic-feet/acre) x Total Site Area (acres) Site Runoff = Step 3 x Step 1.a. = x = cu. feet

18 Low-Impact Development Infiltration Evaluation for Burnsville, MN,, (Based on Heart of the City Hydrologic Information) (Sheet 2 of 3) Worksheet A: Evaluating Known LID Practices Step 5. Determine the runoff volume captured by non-infiltration LID practices. For each of the non-infiltration LID practices you will use, enter the type and quantity of each practice in the chart provided below. Select the volume calculation method from Table W-1 that best fits the practice(s), calculate the volume-per-practice and a total volume for non-infiltration LID practices. Table W-1. Example Calculations for Volume Captured by Non-Infiltration LID Practices. Volume Calculation Practice (cubic-feet) Notes Rain Barrels V = x D 2 x H or V = gallons per barrel x D = diameter of barrel (ft.) H = height of barrel (ft.) Cisterns V = L x W x H or V = gallons per tank x L = length of tank (ft.) W = width of tank (ft.) H = height of tank (ft.) Sub-catchment Inserts No storage volume provided Some detention in flow. No volume reduction provided. Green roofs roof gardens V = L x W x [(Ds x Pe) + Dt] L = length of soil area (ft.) W = width of soil area (ft.) Dt Ds Soil Ds = depth of soil (ft.) Dt = depth of storage between soil surface and overflow of practice (ft.) Pe = effective porosity of the soil = 0.4 (for this estimate). Typical range, most soils = List the type and quantity of each practice to be used in the following chart. Calculate the total volume captured for the Step 5 result. Add additional sheets if more than 3 types of practices to be used. Practice Quantity Vol.-per-practice (cubic-feet) Total Practice Vol. = Quantity x vol.-per-practice (cubic-feet) Volume Captured (add volumes in far-right column) = cu. feet Step 6. Calculate the excess runoff volume to be infiltrated. Excess Runoff Volume (cubic-feet) = Site Runoff (cubic-feet) - Volume Captured (cubic-feet) Excess Runoff Volume = Step 4 - Step 5 = - = cu. feet Step 7. Determine the runoff volume capable of being infiltrated within 24 hours. Select the infiltration rate from Table W-2 that best reflects the soil type present at the site or the soil conditions that will be used for the proposed LID infiltration practices. If multiple soil-types are present, use the most restrictive soil hydrologic group when selecting an infiltration rate for Step 7. Infiltration Rate = in/hr (circle soil hydrologic group(s) to be used in Table W-2)

19 Low-Impact Development Infiltration Evaluation for Burnsville, MN,, (Based on Heart of the City Hydrologic Information) (Sheet 3 of 3) Worksheet A: Evaluating Known LID Practices Table W-2. Soil Infiltration Rates. (Source: Urban Hydrology for Small Watersheds, SCS, June 1986) Infiltration Rate Soil Hydrologic Group (in/hr) Soil Textures A 0.50 Sand, loamy sand, or sandy loam B 0.25 Silty loam or loam C 0.10 Sandy clay loam D 0.03 Clay loam, silty clay loam, sandy clay, silty clay, or clay Step 8. Identify LID infiltration practices and determine the total infiltration-lid surface area. For each of the infiltration LID practices you will use, enter the type and quantity of each practice in the chart provided below and calculate the surface area per-practice and a total surface area for infiltration LID practices. Attach additional sheets if needed. Practice Quantity Area-per-practice (square-feet) Total Practice Area = Quantity x area-per-practice (square-feet) *Infiltration LID Surface Area (add areas in far-right column) = sq. feet (*This is the Effective Pervious Area for LID practices to be used for infiltration. This number should only included those areas available to accept runoff from impervious surfaces. Areas such as parking lot green islands located above the elevation of the parking surface should not be included here. This number should also not include the wetted area of permanent ponds.) Step 9. Calculate the volume infiltrated by LID practices during the 24-hour storm event. Volume Infiltrated = Step 8 x Step 7 x 2 = x x 2 = cu.-feet Step 10. Compare the excess runoff volume to the volume infiltrated by LID practices. Verify that the volume infiltrated in Step 9 is greater than the excess runoff volume calculated in Step 6. If the Step 9 result is less than the Step 6 result, try Worksheet B to determine how much surface area is needed. A second option is to increase the LID capacities in Steps 5 and/or 8, and redo Steps Step 9 MUST BE GREATER THAN OR EQUAL TO Step 6 MUST BE GREATER THAN OR EQUAL TO

20 B. Instructions for Worksheet B: Unknown Area of LID Infiltration Practices Worksheet B is used when the extent of LID practices that will be used are not known, but the user wants an idea of how much surface area will be needed for infiltration practices. Worksheet B calculates a minimum surface area needed for LID infiltration practices. Worksheet B also allows the user to consider runoff volume captured by non-infiltration LID practices. In cases where the user is taking a preliminary look at what might be needed for a given development, they would most likely assume a volume captured of zero in Step 5. Steps 1 to 7 of Worksheet B are identical to Steps 1-7 of Worksheet A. Please refer to the relevant discussions and instruction in Part A above for more information on these steps. Step 8 calculates the required infiltration-lid surface area. Again, depending on what practices, if any, the user assumes in Step 5, this result may or may not account for rain barrels, roof gardens and other storage-type LID practices. This is important when completing Step 9, because if the surface area requirements are difficult to achieve, the user can also return to Step 5 and increase the volume capture by non-infiltration practices. In Step 9, the user takes the result from Step 8 and proceeds to fill in the chart provided until the total surface area in the chart equals or exceeds the result obtained in Step 8. Upon completing Step 9 and meeting the LID goals, the user should proceed to the more detailed design stages for the specific LID BMPs. Again, references to more detailed design and construction information are provided in the appendices.

21 Low-Impact Development Infiltration Evaluation for Burnsville, MN, (Based on Heart of the City Hydrologic Information) (Sheet 1 of 3) Worksheet B: Unknown Area for LID Infiltration Practices Developer s Name: Site Name/Block No.: Step 1: Determine proposed impervious and pervious areas for the site. a. Total site area in acres. acres b. Proposed direct impervious area in acres. acres Step 2: Determine the proposed percent impervious area for the site. Percent impervious = 100 x (Step 1.b. Step 1.a.) = 100 x ( ) = % Step 3: Determine the post-development runoff per acre from Figure W-1. *Runoff per acre in cubic-feet = cu. feet (*Obtained from Figure W-1 by drawing a line up from the percent impervious determined in Step 2 until it intersects the diagonal line, then drawing a line to left from that point to the total runoff per acre value.) Figure 1. Runoff Volume Per Acre vs. Percent Impervious Area (1.5-inch Rainfall). Step 4. Calculate the site runoff volume for the 1.5-inch storm event. This is the volume that needs to be captured or infiltrated by LID practices to meet the zero runoff goal. Runoff volume = Step 3 x Step 1.a. = x = cu. feet

22 Low-Impact Development Infiltration Evaluation for Burnsville, MN,, (Based on Heart of the City Hydrologic Information) (Sheet 2 of 3) Worksheet B: Unknown Area for LID Infiltration Practices Step 5. Determine the runoff volume captured by non-infiltration LID practices. Go directly to Step 7 if: a) you want to know the maximum infiltration area you will need; or b) if your plans will include only infiltration-lid practices. Practices considered in Steps 5 and 6 are noninfiltration practices and will reduce the volume that needs to be infiltrated during the 1.5-inch storm. For each of the non-infiltration LID practices you will use, enter the type and quantity of each practice in the chart provided below. Select the volume calculation method from Table W-1 that best fits the practice(s), calculate the volume-per-practice and a total volume for non-infiltration LID practices. Table W-1. Example Calculations for Volume Captured by Non-Infiltration LID Practices. Volume Calculation Practice (cubic-feet) Notes Rain Barrels V = x D 2 x H or V = gallons per barrel x D = diameter of barrel (ft.) H = height of barrel (ft.) Cisterns V = L x W x H or V = gallons per tank x L = length of tank (ft.) W = width of tank (ft.) H = height of tank (ft.) Sub-catchment Inserts No storage volume provided Some detention in flow. No volume reduction provided. Green roofs roof gardens V = L x W x [(Ds x Pe) + Dt] L = length of soil area (ft.) W = width of soil area (ft.) Dt Ds Soil Ds = depth of soil (ft.) Dt = depth of storage between soil surface and overflow of practice (ft.) Pe = effective porosity of the soil = 0.4 (for this estimate). Typical range, most soils = List the type and quantity of each practice to be used in the following chart. Calculate the total volume captured for the Step 5 result. Add additional sheets if more than 3 types of practices to be used. Practice Quantity Vol.-per-practice (cubic-feet) Total Practice Vol. = Quantity x vol.-per-practice (cubic-feet) Volume Captured (add volumes in far-right column) = cu. feet Step 6. Calculate the excess runoff volume to be infiltrated. Excess Runoff Volume (cubic-feet) = Site Runoff (cubic-feet) - Volume Captured (cubic-feet) Excess Runoff Volume = Step 4 - Step 5 = - = cu. feet

23 Low-Impact Development Infiltration Evaluation for Burnsville, MN,, (Based on Heart of the City Hydrologic Information) (Sheet 3 of 3) Worksheet B: Unknown Area for LID Infiltration Practices Step 7. Determine the runoff volume capable of being infiltrated within 24 hours. Select the infiltration rate from Table W-2 that best reflects the soil type present at the site or the soil conditions that will be used for the proposed LID infiltration practices. If multiple soil-types are present, use the most restrictive soil hydrologic group when selecting an infiltration rate for Step 7. Infiltration Rate = in/hr (circle soil hydrologic group(s) to be used in Table W-2) Table W-2. Soil Infiltration Rates. (Source: Urban Hydrology for Small Watersheds, SCS, June 1986) Infiltration Rate Soil Hydrologic Group (in/hr) Soil Textures A 0.50 Sand, loamy sand, or sandy loam B 0.25 Silty loam or loam C 0.10 Sandy clay loam D 0.03 Clay loam, silty clay loam, sandy clay, silty clay, or clay Step 8: Calculate the minimum surface area needed for LID infiltration practices. Required LID Surface Area (square-feet) = Excess Runoff Volume (cubic-feet) [Infiltration Rate (in/hr) x 24 (hrs) x 1/12 (ft/in)] Required Infiltration-LID Area = Step 6 [Step 7 x 2] = [ x 2] = square feet or: Step 8 sq. ft. x acres/sq. feet = x = acres Step 9. Identify LID infiltration practices and compare total infiltration-lid surface area. Identify infiltration practices you will use in the chart provided below. Calculate the area per-practice and a total surface area for infiltration practices. The total surface area for this step must be equal to or greater than the result from Step 8 to meet the zero runoff goal. Attach additional sheets if needed. Practice Quantity Area-per-practice (square-feet) Total Practice Area = Quantity x area-per-practice (square-feet) *Infiltration LID Surface Area (add areas in far-right column) = sq. feet (*This is the Effective Pervious Area for LID practices to be used for infiltration. This number should only included those areas available to accept runoff from impervious surfaces. Areas such as parking lot green islands located above the elevation of the parking surface should not be included here. This number should also not include the wetted area of permanent ponds.)

24 REFERENCES 1. City of Burnsville, Minnesota. April 9, Environmental Assessment Worksheet, Heart of the City, Town Center. 2. Dahlgren, Shardlow, and Uban, Inc., Paul Madson & Associates, Maxfield Research Group, Inc. November Design Framework Manual for the Heart of the City, Burnsville, Minnesota. 3. City of Burnsville, Minnesota Comprehensive Storm Water Management Plan Update. 4. Minnesota Pollution Control Agency. March 1, Protecting Water Quality In Urban Areas, Best Management Practices for Dealing with Storm Water Runoff from Urban, Suburban and Developing Areas of Minnesota. 5. O Reilly, N., Hey and Associates, Inc. May Infiltration/Biofiltration Management Practices. 6. Chow, V. T., Maidment, D. R., and Mays, L. W Applied Hydrology. 7. Barr Engineering Black Dog Watershed Management Organization Plan. 8. Environmental Protection Agency, Office of Water Vegetated Roof Cover: Philadelphia, Pennsylvania. EPA-841-B D. 9. Metropolitan Council. July Minnesota Urban Small Sites BMP Manual: Best Management Practices for Cold Climates.

25 APPENDIX A LINKS TO LOW-IMPACT DEVELOPMENT BEST MANAGEMENT PRACTICE FACT SHEETS Summary of Fact Sheets Available at Storm Water Managers Resource Center, Bioretention Porous Pavement Grassed Filter Strip Grass Channel Infiltration Basin Infiltration Trench Sand and Organic Filter On-lot Treatment Vegetated Roof Cover (

26 APPENDIX B SUMMARY TABLES: DESIGN CONSIDERATIONS FOR LOW-IMPACT DEVELOPMENT PRACTICES

27 Table B-1. Summary of Considerations for Roof Top LID Techniques. LID Water Quantity Water Quality Technique Rate Volume Sediment Soluble Snow Melt Aesthetics Maintenance Costs Flat Roof Practices (<15% pitch) Rain Barrels NA NA NA Low- Low Low Dry Wells NA NA NA Low Low- Sub- Catchment Inserts Removal Rec. Low- Low- Green Roofs Variable NA NA Removal Rec. - Low- Green Walls, Gardens NA NA NA - Cisterns NA NA NA Low Low Pitched Roof Practices (>15% pitch) Rain Barrels NA NA NA Low- Low Low Dry Wells NA NA NA Low Low- Sub- Catchment Inserts Green Walls, Gardens, Fountains Rain Gutters to Green Areas Rain Gutters to Infiltration System Removal Rec Low- Low- NA NA NA - Cisterns NA NA NA - NA NA NA - NA NA NA - Low- Low- *BMP performance is rated on a scale of Low,,, or NA (not applicable), based on past history regarding common concerns associated with storm water runoff. For example, a BMP with a rating regarding Volume possesses the capability to retain larger volumes of runoff than that of a Low rating. Low Low Low

28 Table B-2. Summary of Considerations for Parking Lot LID Techniques. LID Water Quantity Water Quality Snow Technique Rate Volume Sediment Soluble Melt Aesthetics Maintenance Costs Porous - - Removal - Pavement Rec. Turf - - Removal - Pavement Rec. Standing Variable Low Low Removal Low Low Low Storage Rec. Grass Swales Variable NA Low Low Curb Cuts, Variable NA Low- Low Infiltration Basins Filter Tanks NA - Infiltration Variable NA Low Low- Trenches Underground Infiltration, French Drains, Dry Wells NA Low- Low- Low- Bio-Retention NA Low- Sub- Removal Low- Catchment Rec. Inserts Dry Extended Storage Ponds NA Low Low- *BMP performance is rated on a scale of Low,,, or NA (not applicable), based on past history regarding common concerns associated with storm water runoff. For example, a BMP with a rating regarding Volume possesses the capability to retain larger volumes of runoff than that of a Low rating. Low- -

29 Table B-3. Summary of Considerations for Street (Curb to Curb) LID Techniques. LID Water Quantity Water Quality Technique Rate Volume Sediment Soluble Road Sections (Layout & Design) Porous Pavement (Alleys) Low NA NA Removal Rec. - - Snow Melt Aesthetics Maintenance Costs NA Low Low Partial Removal Rec. - Infiltration Basins Removal Rec. Low Low- Infiltration Trenches Removal Rec. Low Low- Grass Swales NA Low Low Grass - - NA Low Low Channels Standing NA NA NA Removal NA Low Low Storage Rec. Vegetated Filter Strip NA Low Low- Dry Extended Storage Ponds NA Low Low- - Bio-Retention NA Low- Low- Sub- Catchment Inserts Removal Rec. Low- Low- Table B-4. Summary of Considerations for Sidewalk and Boulevard LID Techniques. LID Water Quantity Water Quality Snow Technique Rate Volume Sediment Soluble Melt Aesthetics Maintenance Costs Porous - - Removal - - Pavement Rec. Vegetated Filter Strips NA Low Low- Infiltration Basins Removal Rec. Low Low- Bio- Retention NA Low- Low- Grated Infiltration Trenches Variable NA Low Low- Sidewalks (on one side of street) Low NA NA Removal Rec. NA Low Low *BMP performance is rated on a scale of Low,,, or NA (not applicable), based on past history regarding common concerns associated with storm water runoff. For example, a BMP with a rating regarding Volume possesses the capability to retain larger volumes of runoff than that of a Low rating.

30 Table B-5. Summary of Considerations for Pervious Area LID Techniques. LID Water Quantity Water Quality Snow Technique Rate Volume Sediment Soluble Melt Aesthetics Maintenance Costs Maintained Pervious Areas Dry Extended Storage Ponds NA Low Low- - Grass - - NA Low Low Channels Artificial NA Channels, Dams, Plunge Pools Wet Pond - NA Low Grass Swales NA Low Low Natural (un-maintained) Pervious Areas Wetland NA Low Low Pond- Wetland System NA Low Low Table B-6. Summary of Considerations for Park and Open Space LID Techniques. LID Water Quantity Water Quality Snow Aesthetics Maintenance Costs Technique Rate Volume Sediment Soluble Melt Maintained Areas Dry Extended Storage Ponds NA Low Low- - Wet Pond - NA Low Artificial Channels with Plunge Pools NA Natural (un-maintained) Areas Wetland NA Low Low Table B-7. Summary of Considerations for Pond Techniques. LID Water Quantity Water Quality Snow Aesthetics Maintenance Costs Technique Rate Volume Sediment Soluble Melt Extended dry pond NA Low Low- - Wet pond - NA Low Pond-wetland system NA Low Low *BMP performance is rated on a scale of Low,,, or NA (not applicable), based on past history regarding common concerns associated with storm water runoff. For example, a BMP with a rating regarding Volume possesses the capability to retain larger volumes of runoff than that of a Low rating.

31 APPENDIX C EXAMPLE LID INFILTRATION WORKSHEETS

32 Low-Impact Development Infiltration Evaluation for Burnsville, MN, Heart of the City (Sheet 1 of 3) Worksheet A: Evaluating Known LID Practices Developer s Name: LOW-IMPACT DEVELOPERS, INC. Site Name/Block No.: HOC TOWN CENTER, BLOCK 1 Step 1: Determine proposed impervious and pervious areas for the site. a. Total site area in acres acres b. Proposed direct impervious area in acres acres Step 2: Determine the proposed percent impervious area for the site. Percent impervious = 100 x (Step 1.b. Step 1.a.) = 100 x ( ) = 90 % Step 3: Determine the post-development runoff per acre from Figure W-1. *Runoff per acre in cubic-feet = 4350 cu. feet (*Obtained from Figure W-1 by drawing a line up from the percent impervious determined in Step 2 until it intersects the diagonal line, then drawing a line to left from that point to the total runoff per acre value.) Figure W-1. Runoff Volume Per Acre vs. Percent Impervious Area (1.5-inch Rainfall). Step 4. Calculate the site runoff volume for the 1.5-inch rainfall event. Site runoff volume (cu. feet) = Runoff Per Acre (cubic-feet/acre) x Total Site Area (acres) Site Runoff = Step 3 x Step 1.a. = 4350 x 5.92 = 25,752 cu. feet

33 Low-Impact Development Infiltration Evaluation for Burnsville, MN, Heart of the City (Sheet 2 of 3) Worksheet A: Evaluating Known LID Practices Step 5. Determine the runoff volume captured by non-infiltration LID practices. For each of the non-infiltration LID practices you will use, enter the type and quantity of each practice in the chart provided below. Select the volume calculation method from Table W-1 that best fits the practice(s), calculate the volume-per-practice and a total volume for non-infiltration LID practices. Table W-1. Example Calculations for Volume Captured by Non-Infiltration LID Practices. Volume Calculation Practice (cubic-feet) Notes Rain Barrels V = x D 2 x H or V = gallons per barrel x D = diameter of barrel (ft.) H = height of barrel (ft.) Cisterns Example below: L=6, W=6, D=6 V = L x W x H or V = gallons per tank x L = length of tank (ft.) W = width of tank (ft.) H = height of tank (ft.) Sub-catchment Inserts No storage volume provided Some detention in flow. No volume reduction provided. Green roofs roof gardens V = L x W x [(Ds x Pe) + Dt] Example below: L=190 W=70 Dt=2 =0.167 Ds=12 =1 Dt Ds Soil L = length of soil area (ft.) W = width of soil area (ft.) Ds = depth of soil (ft.) Dt = depth of storage between soil surface and overflow of practice (ft.) Pe = effective porosity of the soil = 0.4 (for this estimate). Typical range, most soils = List the type and quantity of each practice to be used in the following chart. Calculate the total volume captured for the Step 5 result. Add additional sheets if more than 3 types of practices to be used. Practice Quantity Vol.-per-practice (cubic-feet) Total Practice Vol. = Quantity x vol.-per-practice (cubic-feet) Sub-catch inserts Cisterns Green roof 1 7,536 7,536 Volume Captured (add volumes in far-right column) = 7,968 cu. feet Step 6. Calculate the excess runoff volume to be infiltrated. Excess Runoff Volume (cubic-feet) = Site Runoff (cubic-feet) - Volume Captured (cubic-feet) Excess Runoff Volume = Step 4 - Step 5 = 25,752_ - 7,968_ = 17,784 cu. feet Step 7. Determine the runoff volume capable of being infiltrated within 24 hours. Select the infiltration rate from Table W-2 that best reflects the soil type present at the site or the soil conditions that will be used for the proposed LID infiltration practices. If multiple soil-types are present, use the most restrictive soil hydrologic group when selecting an infiltration rate for Step 7. Infiltration Rate = in/hr (circle soil hydrologic group(s) to be used in Table W-2)

34 Low-Impact Development Infiltration Evaluation for Burnsville, MN, Heart of the City (Sheet 3 of 3) Worksheet A: Evaluating Known LID Practices Table W-2. Soil Infiltration Rates. (Source: Urban Hydrology for Small Watersheds, SCS, June 1986) Infiltration Rate Soil Hydrologic Group (in/hr) Soil Textures Half of area = A 0.50 Sand, loamy sand, or sandy loam Half of area = B 0.25 Silty loam or loam C 0.10 Sandy clay loam D 0.03 Clay loam, silty clay loam, sandy clay, silty clay, or clay Step 8. Identify LID infiltration practices and determine the total infiltration-lid surface area. For each of the infiltration LID practices you will use, enter the type and quantity of each practice in the chart provided below and calculate the surface area per-practice and a total surface area for infiltration LID practices. Attach additional sheets if needed. Practice Quantity Area-per-practice (square-feet) Total Practice Area = Quantity x area-per-practice (square-feet) Grated infiltration trenches (tree rings) Bio-retention area of approximate dimensions 50 x 500 (1/2 of area below 28 soil depth) 1 25,000 25,000 *Infiltration LID Surface Area (add areas in far-right column) = 25,502 sq. feet (*This is the Effective Pervious Area for LID practices to be used for infiltration. This number should only included those areas available to accept runoff from impervious surfaces. Areas such as parking lot green islands located above the elevation of the parking surface should not be included here. This number should also not include the wetted area of permanent ponds.) Step 9. Calculate the volume infiltrated by LID practices during the 24-hour storm event. Volume Infiltrated = Step 8 x Step 7 x 2 = 25,502 x x 2 = 19,126 cu.-feet Step 10. Compare the excess runoff volume to the volume infiltrated by LID practices. Verify that the volume infiltrated in Step 9 is greater than the excess runoff volume calculated in Step 6. If the Step 9 result is less than the Step 6 result, try Worksheet B to determine how much surface area is needed. A second option is to increase the LID capacities in Steps 5 and/or 8, and redo Steps Step 9 MUST BE GREATER THAN OR EQUAL TO Step 6 19,126 MUST BE GREATER THAN OR EQUAL TO 17,784 OK

35 Low-Impact Development Infiltration Evaluation for Burnsville, MN, Heart of the City (Sheet 1 of 3) Worksheet B: Unknown Area for LID Infiltration Practices Developer s Name: LOW-IMPACT DEVELOPERS, INC. Site Name/Block No.: HOC TOWN CENTER, BLOCK 1 Step 1: Determine proposed impervious and pervious areas for the site. a. Total site area in acres acres b. Proposed direct impervious area in acres acres Step 2: Determine the proposed percent impervious area for the site. Percent impervious = 100 x (Step 1.b. Step 1.a.) = 100 x ( ) = 90 % Step 3: Determine the post-development runoff per acre from Figure W-1. *Runoff per acre in cubic-feet = 4350 cu. feet (*Obtained from Figure W-1 by drawing a line up from the percent impervious determined in Step 2 until it intersects the diagonal line, then drawing a line to left from that point to the total runoff per acre value.) Figure W-1. Runoff Volume Per Acre vs. Percent Impervious Area (1.5-inch Rainfall). Step 4. Calculate the site runoff volume for the 1.5-inch rainfall event. Site runoff volume (cu. feet) = Runoff Per Acre (cubic-feet/acre) x Total Site Area (acres) Site Runoff = Step 3 x Step 1.a. = 4350 x 5.92 = 25,752 cu. feet