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Beyond standard practice, but cost-effective
Development usually comes with increased stormwater runoff due to impervious surfacesSurfaces that promote runoff of precipitation volumes instead of infiltration into the subsurface. The imperviousness or degree of runoff potential can be estimated for different surface materials. like roofs and parking lots. To earn this credit with previously undeveloped sites, you’ll need to avoid any increase in runoff, while on mostly impervious developed sites, you’ll need to reduce runoff. You will probably need to go beyond standard practice to achieve this credit, and you might see increased costs, although an integrated approach can make this credit cost-effective....
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4 Comments
SSc6.2 - Calculate % of Annual Rainfall Treated by BMP
I have not found a good example of how you calculate the % of Annual Rainfall Treated by BMPBest Management Practice. I'm guessing that it has to do with the tributary area draining to each particular BMP. So if 5.0 acres of runoff is draining to a bioretention area, and the total area of the project LEED boundary is 10.0 acres, then the % of Annual Rainfall treated by the bioretention area is 50%. Can anyone confirm thIs is correct? Thanks.
The response to your question is a little more complicated than just calculating areas of watershed.
First, the effectiveness of the BMPBest Management Practice must be determined and the percentage of annual rainfall. LEED provides some guidance concerning the average annual rainfall depth, where LEED suggests that the depth of treatment for a humid area is 1-inch, semi-arid is 0.75 inch and arid is 0.5 inch. The IMP (Integrated Management Practice) treatment train must be designed to remove 80% of TSSTotal suspended solids (TSS) are particles that are too small or light to be removed from stormwater via gravity settling. Suspended solid concentrations are typically removed via filtration. for this run-off volume (rainfall depth x area x run-off coefficient x BMP effectiveness). Table 2, page 104 of the LEED Reference Guide provides some guidance on the average TS removal for different BMP’s. This process will provide you with the level of conformance that you can reasonably expect to meet for your project for the BMP’s that you have selected.
If a portion of the project site is not providing any treatment, then the reduction in TSS that you are achieving is reduced by the portion of the site for which you have no treatment. If the portion of the site that is being treated is being treated to a higher level than 80% TSS removal, you can claim some credit against the portion of the site that is not being treated.
So, you are correct that if 5.0 acres of runoff is draining to a bioretention area and the total area of the LEED boundary is 10.0 acres, then the % of Annual Rainfall treated by the bioretention area is 50% assuming a similar run-off coefficient for all of the site boundary. However, it is possible that you are not meeting the requirement for 80% TSS reduction for 50% of the site, depending on the size and effectiveness of your bioretention area, or it is possible that you are achieving more than 80% reduction for 50% of the site.
Soil Infiltration
You bring up a good discussion. Typically when SSc6.1 is evlauated, you can use the Rational MethodA formula that can be used for calculating stormwater flow rates. Q = CIA, where C represents a coefficient for physical drainage area, I is the rainfall intensity, and A is area. The method is suitable for watersheds smaller than 300 acres in size. to determine the peak rate and volume of runoff for the 1 and 2-year, 24 hour storms or the NRCS method is used for volume of runoff. Both of these methods consider infiltration in developing their respective results, NRCS through soli types, and Rational Method through the runoff coefficient.
However, your question seems to imply that you would like to consider the use of a BMPBest Management Practice or LID strategy to help meet the peak flow and volume reduction criteria in your discharge calculations. Certainly, the soil infiltration will have an effect on how effective these BMP's are in minigating the flow and volume; if the soil is a clay that does not infiltrate, the BMP will still be mostly full when the next storm hits and the BMP will not be effective in reducing the flow or volme. On the other hand, if the soil allows stormwater to infiltrate quickly, it will be very effective. Many times, the soil can be modified to provide greater infiltration.
The end result is that the only way that I know to determine the iniltration rate for a BMP is to have a permeability test completed using standard procedures. Our experience is that even this approach may only be useful during the early life of the BMP, and the permeability of the soil may change basee on the sediment that the BMP is collecting. Maintenance is the key to long term BMP effectiveness.
Soil infiltration
Can someone direct me toward appropriate calculations to determine infiltration rates for various soil types? It seems that this calculation would be critical in determining whether the proposed low-impact strategies will be sufficient, however I can't seem to find the needed information.
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