Water Resources Management Plan

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 - Volume Control / Infiltration Worksheet This Appendix contains a worksheet and related information that can be used for evaluating conformance to the volume control and infiltration requirements for storm water practices and determining if proposed practices meet requirements specified in Appendix C. The worksheet is used to evaluate if specific practices will meet the standards and how much volume or infiltration surface area may be needed for a specific project. The worksheet contains a step that accounts for non-infiltration practices where groundwater recharge will not directly occur, including rain barrels, cisterns, roof gardens and other storage-type practices. The worksheet user should understand that soils information for the location of each infiltration practice is needed to complete the worksheet. The hydrologic information used to develop the runoff volumes for various soils is based on modeling the impervious surfaces and pervious surface of a site separately instead of using a composite curve number. In this way, a more realistic estimate of the runoff volume for smaller storms is achieved. The key hydrologic assumptions include using a 1.0-inch rainfall (Type II distribution with AMC 2) over an average 1-acre project site. Step 1 requires the total site area in acres, the existing impervious area, the new impervious area, the redeveloping impervious area and the total impervious area after development. The new impervious area is the area where impervious surfaces will be created where there currently are not impervious surfaces. The redeveloped impervious surface includes, for example, building and parking lots that that will be demolished and rebuilt or where the subgrade soils will be exposed as part of the planned redevelopment work. The total impervious area in acres is the total surface area of buildings, roads, sidewalks, parking lots and other impervious areas on the site upon completion of the project. The total site area is equivalent to the acreage of the entire tract or parcel. Step 2 calculates the percent impervious for the proposed site plan. In Step 3 the owner or developer determines the runoff volume per acre for the proposed site using the percent impervious from Step 2 by starting on the x-axis of the chart and drawing a line up vertically until it intersects the applicable soil group line. The relationship shown in Figure 1 is based on hydrologic model simulations using a Type II, 1.0-inch, 24-hour rainfall event for various levels of impervious coverage and soils ranging from Group A to Group D. The simulations modeled impervious and pervious portions of a site separately. Therefore, while there are some differences in runoff volume from a pervious surface between group A and group D soils, the soil type is not a significant factor in the runoff volume for the modeled 1-inch storm. Step 4 calculates the total runoff volume that must be captured and treated in infiltration or filtration practices. The total volume is the total from new impervious areas and redevelopment areas. The volume from the new impervious areas is calculated by the full 1.0-inch runoff volume while the redevelopment area volume is calculated from a 0.5 inch runoff volume. Page B-1

2 October 2002 (Revised 2008) In Step 5 the portion of site runoff volume captured by filtration/storage-type (non-infiltration) practice, other than a traditional wet pond, will be accounted for. In general, this section will be used where infiltration is prohibited or site conditions are not suitable for infiltration. In some cases, a site may have a combination of filtration and infiltration BMPs. For example, the volume captured by a rainwater garden with an under drain system 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 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 1. If practices not listed are proposed, the user should enter a 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. In Step 6, users select a representative soil infiltration rate for the infiltration practices to be used at the site. If there are only filtration practices, then skip Steps 6 to 8 and go to Step 9 to determine if sufficient volume has been treated. Table 2 lists saturated infiltration rates for the four soil hydrologic soil groups and soil textures shown. If the user does not know the extent of soil conditions on the site or has not verified actual soil texture from field investigations, the most restrictive soil Group D should be used. If the user has verified on-site soil conditions and/or proposes to use the underlying soil with a permeability rate greater than those listed, data to support the infiltration rate must be submitted with the worksheet. Step 7 is used to define the types and surface areas of specific practices to be used at the project site. 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 as infiltration surface area in this section. Practices like infiltration trenches or basins, flat-sloped grassed swales, depressed parking lot islands and bio-retention areas can be included. The total surface area for infiltration practices is then calculated to obtain the Step 7 result. Step 8 calculates the volume that can be infiltrated by practices in Step 7. In Step 8, the total surface area from Step 7 is multiplied by the infiltration rate from Step 6 (and a unit conversion factor) to obtain the volume infiltrated in cubic feet. The volume infiltrated represents a volume in a 48-hour period. This step assumes that the specific BMP design will provide for the entire excess runoff volume to be captured in the infiltration basin and that the BMP will drain fully within the 48-hour period. Note that the designer must also limit the maximum ponding depth in the practice to ensure it drains in 48 hours or less. For example, a Group C soil with a 0.2 in/hr infiltration rate will allow a maximum ponding depth of 0.8 feet, or about 9 inches. Step 9 compares the total volume treated from both Step 5 and 8 to the volume need in Step 4. If the result for Step 9 shows that the volume control / infiltration requirements have not been met, the user should increase the storage capacity and/or infiltration surface area of the proposed practices in Steps 5 and 8, and recalculate the results through Step 9. Page B-2

3 Infiltration / Filtration System Evaluation Burnsville, MN (page 1 of 3) Volume / Infiltration Practice Worksheet Revised 2008 Owner / Developer Name: Site Name/Block or Outlot ID: Step 1: Determine the site areas for your project. a. Total site area in acres. acres b. Existing impervious area in acres. acres c. New impervious area in acres. acres d. Redeveloped impervious area in acres acres e. Total impervious area in acres. ( = 1b + 1c) acres Step 2: Determine the proposed percent impervious area for the site. Site percent impervious = 100 x (Step 1.e. 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 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. Or use the equation where X is the percent impervious) Runoff Volume Per Acre (cu-ft) Figure 1. Runoff Volume Per Acre Y = (27.8 * X) Percent Impervious Page B-3

4 Volume / Infiltration Practice Worksheet - Revised 2008 (page 2 of 3) Step 4. Calculate the site runoff volume required for infiltration/filtration. Site runoff volume (cu. feet) = Runoff Per Acre (cubic-feet/acre) x Total Site Area (acres) a. New Impervious Volume = Step 3 x (Step 1.c / Step 1.e) x Step 1.a. = x / x = cu.-ft b. Redevelopment Volume = Step 3 x (Step 1.d / Step 1.e) x Step 1.a. x 0.5 = x / x * 0.5 = cu.-ft c. Total Infiltration/Filtration Volume = Step 4.a + Step 4.b = cu.-ft. Step 5. Determine the runoff volume captured by filtration BMPs (used where infiltration is prohibited) For each of the non-infiltration practices you will use, if any, enter the type and quantity/id of each practice in the chart provided below. Select the volume calculation method from Table 1 that best fits the practice(s) or attach a separate sheet with calculations, calculate the volume-per-practice and a total volume for non-infiltration practices. Table 1. Example Calculations for Volume Captured by Non-Infiltration BMPs. Volume Calculation BMP Type Notes 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.) Ds = depth of soil (ft.) Dt = depth of storage between soil surface and Dt Rainwater Gardens or overflow of practice (ft.) Bioretention areas Ds Pe = effective porosity of the soil with an under drain = 0.4 (for this estimate). Typical range, most soils = Porous Paver Systems with an under drain (may be accounted for here or in Step 8 but NOT both.) Dsg V = L x W x (Dsg x Pe) Paver Natural soils Paver L = length of paved area (ft.) W = width of paved area (ft.) Dsg = depth of subgrade (ft.) Pe = effective porosity of the subgrade = 0.32 List the type (by name and ID) 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. BMP Name / ID Quantity Vol.-per-practice Total Practice Vol. = Quantity x vol.-perpractice Volume Captured (add volumes in far-right column) = Page B-4

5 Volume / Infiltration Practice Worksheet Revised 2008 (page 3 of 3) Step 6. Select or Identify Soil Infiltration Rate. If not using infiltration, skip to Step 9. For the detailed design stage, use the infiltration rate determined from site-specific soils investigation and lab testing. For planning purposes, the designer may select the infiltration rate from Table 2 that best reflects the soil type present at the site. If multiple soil-types are present, use the most restrictive soil hydrologic group when selecting an infiltration rate or complete Steps 6 to 8 for each of the different infiltration rate areas. Infiltration Rate = in/hr (circle soil hydrologic group(s) to be used in Table 2) Table 2. Soil Infiltration Rates. (Source: Infiltration rates taken from MPCA 2005 Minnesota Stormwater Manual, Version 1, Table 8.5.) Hydrologic Soil Group Infiltration Rate (in/hr) Soil Textures A 1.63 Gravel, sandy gravel, silty gravel 0.80 Sand, loamy sand, or sandy loam B 0.60 Silt loam 0.30 Loam C 0.20 Sandy clay loam D < 0.20 Clay loam, silty clay loam, sandy clay, silty clay, or clay Step 7. Identify infiltration practices and determine the total infiltration surface area. For each of the practices you will use, enter the type and quantity of each in the chart provided below and calculate the surface area per-practice and a total horizontal surface area subject to prolonged wetting for infiltration practices. BMP Name / ID 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) = (*This is the Effective Pervious Area for practices to be used for infiltration. This number should only include 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 8. Calculate the volume infiltrated by practices over 48-hours. Volume Infiltrated = Step 7 x Step 6 x 48 x 1/12 = Step 6 x Step 7 x 4 = = sq-ft. x in/hr. x 48 hrs x 1/12 ft/in = cu.-ft Step 9. Compare the excess runoff volume to the volume infiltrated by LID practices. Verify that the total volume filtered in Step 5 and infiltrated in Step 8 is greater than the excess runoff volume calculated in Step 4. If the total of Steps 5 and 8 is greater than the amount from Step 4, sufficient practices have been planned to meet the City standards. The designer should refer to the City s design standards in Appendix C of the Water Resources Management Plan for more information of design requirements for infiltration practices. If the result for Step 4 is greater than the total form Steps 5 and 8, additional filtration and/or infiltration practices are needed. Step 5 + Step 8 MUST BE GREATER THAN OR EQUAL TO Step 4 MUST BE GREATER THAN OR EQUAL TO Page B-5