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===Pollutant Reduction=== | ===Pollutant Reduction=== | ||
[[File:Schematic pollutant reductions permeable pavement.jpg|thumb|300px|alt=schematic showing how pollutant reductions are achieved for permeable pavement|<font size=3>Schematic showing how pollutant reductions are achieved for permeable pavement in the MIDS calculator</font size>]] | [[File:Schematic pollutant reductions permeable pavement.jpg|thumb|300px|alt=schematic showing how pollutant reductions are achieved for permeable pavement|<font size=3>Schematic showing how pollutant reductions are achieved for permeable pavement in the MIDS calculator</font size>]] | ||
− | Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting the ''Volume reduction capacity of BMP'', which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves (add link to addendum) developed from multiple modeling scenarios. The performance curves use the ''Volume reduction capacity of BMP'', the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction. While oversizing a BMP above the | + | |
+ | Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting the ''Volume reduction capacity of BMP'', which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves (add link to addendum) developed from multiple modeling scenarios. The performance curves use the ''Volume reduction capacity of BMP'', the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction. While oversizing a BMP above the ''Required treatment volume'' will not provide additional credit towards the performance goal volume, it may provide additional pollutant reduction. | ||
A 100 percent removal is credited for all pollutants associated with the reduced volume of stormwater since these pollutants are either attenuated within the media or pass into the underlying soil with infiltrating water. Stormwater that is not infiltrated is assumed to flow through the filter media and out the underdrain. A 74 percent TSS, 82 percent particulate phosphorus, and 0 percent dissolved phosphorus removals are applied to the filtered stormwater. A schematic of the removal rates can be seen in the sidebar. | A 100 percent removal is credited for all pollutants associated with the reduced volume of stormwater since these pollutants are either attenuated within the media or pass into the underlying soil with infiltrating water. Stormwater that is not infiltrated is assumed to flow through the filter media and out the underdrain. A 74 percent TSS, 82 percent particulate phosphorus, and 0 percent dissolved phosphorus removals are applied to the filtered stormwater. A schematic of the removal rates can be seen in the sidebar. |
For permeable pavement, stormwater runoff captured by the BMP and stored below the underdrain is infiltrated into the underlying soil between rain events. All pollutants in the infiltrated stormwater are credited as being reduced. Pollutants in stormwater captured by the BMP but entering the underdrain are treated as they pass through the filter media and out the underdrain.
For permeable pavement systems, the user must input the following parameters to calculate the volume and pollutant load reductions associated with the BMP.
The following are requirements or recommendations for inputs into the MIDS calculator. If the following are not met, an error message will inform the user to change the input to meet the requirement.
The water underneath the underdrain must meet the drawdown time requirement specified. The drawdown time requirement is checked by comparing the user defined drawdown time with the calculated drawdown time(DDTcalc) given by
\(DDT_{calc} = D_U / (I_R / 12)\)
Where
Required treatment volume, or the volume of stormwater runoff delivered to the BMP, equals the performance goal (1.1 inches or user-specified performance goal) times the impervious area draining to the BMP, plus any water routed to the BMP from an upstream BMP. This stormwater is delivered to the BMP instantaneously following the Kerplunk method.
The volume reduction achieved by a BMP compares the capacity of the BMP to the required treatment volume. The Volume reduction capacity of BMP is calculated using BMP inputs provided by the user. For this BMP the volume reduction credit is equal to the amount of water that can be instantaneously captured by the BMP in the media below the underdrain. The capture volume (V) is therefore given by
\(V= (A_U + A_B)) / 2 * n * D_U \)
Where:
The Volume of retention provided by BMP is the amount of volume credit the BMP provides toward the performance goal. This value is equal to the lesser of the Volume reduction capacity of BMP calculated using the above method or the Required treatment volume. This check makes sure that the BMP is not getting more credit than necessary to meet the performance goal. For example, if the BMP is oversized the user will only receive credit for the Required treatment volume routed to the BMP, which corresponds with meeting the performance goal for the site .
Pollutant load reductions are calculated on an annual basis. Therefore, the first step in calculating annual pollutant load reductions is converting the Volume reduction capacity of BMP, which is an instantaneous volume reduction, to an annual volume reduction percentage. This is accomplished through the use of performance curves (add link to addendum) developed from multiple modeling scenarios. The performance curves use the Volume reduction capacity of BMP, the infiltration rate of the underlying soils, the contributing watershed percent impervious area, and the size of the contributing watershed to calculate a percent annual volume reduction. While oversizing a BMP above the Required treatment volume will not provide additional credit towards the performance goal volume, it may provide additional pollutant reduction.
A 100 percent removal is credited for all pollutants associated with the reduced volume of stormwater since these pollutants are either attenuated within the media or pass into the underlying soil with infiltrating water. Stormwater that is not infiltrated is assumed to flow through the filter media and out the underdrain. A 74 percent TSS, 82 percent particulate phosphorus, and 0 percent dissolved phosphorus removals are applied to the filtered stormwater. A schematic of the removal rates can be seen in the sidebar.
NOTE: The user can modify event mean concentrations (EMCs) on the Site Information tab in the calculator. Default concentrations are 54.5 milligrams per liter for total suspended solids (TSS) and 0.3 milligrams per liter for total phosphorus (particulate plus dissolved). The calculator will notify the user if the default is changed. Changing the default EMC will result in changes to the total pounds of pollutant reduced.
A permeable pavement BMP can be routed to any other BMP, except for a green roof and a swale side slope or any BMP that would cause stormwater to be rerouted back to the infiltration basin already in the stormwater runoff treatment sequence. All BMPs can be routed to the permeable pavement, except for a swale side slope.
The following general assumptions apply in calculating the credit for a permeable pavement system. If these assumptions are not followed, the volume and pollutant reduction credits cannot be applied.