File:Bypass 1.png

File:Bypass 1.png

In condition A, a biofiltration practice (bioretention with underdrain) is sized to meet the MIDS performance goal of 1.1 inches (3993 ft3 of runoff from 1 acre of impervious surface). Some TSS and TP are filtered by the media, and this filtered water is captured by the underdrain and returned to the stormwater conveyance, where it is routed to the downstream infiltration practice. TP and TSS routed to the downstream BMP are 0.60 and 46.3 pounds, respectively. In condition B, the biofiltration basin is sized to treat 25 percent less runoff, with the result that less water is treated and less pollutant removed. TP and TSS routed to the downstream BMP are 0.66 and 50.8 pounds, respectively. However, no bypass was included. In Version 3, this bypass would be routed to the BMP but the pollutants in the bypass would not be. In condition 3, the User specifies that 25 percent bypassed the undersized biofiltration practice, with the results that 0.96 and 122 pounds of TP and TSS, respectively, are delivered to the infiltration practice.

Note the bypass only applies to BMPs that are undersized.

The percent bypass can be determined using the MIDS Calculator. To calculate bypass percent, run a scenario with the undersized BMP and a scenario with a correctly sized BMP and calculate the volume difference, as a percent. Input this value as the bypass percent in the Calculator.