Estimating and calculating bypass for the Minimal Impact Design Standards (MIDS) Calculator
The Minimal Impact Design Standards (MIDS) Calculator can be used to design and size best management practices. Typically, bmps are designed to meet a specific performance goal, such as the 1.1 inch MIDS standard or the 1 inch requirement in the Construction Stormwater General Permit.
For infiltration practices in the calculator, runoff captured by a bmp is infiltrated. Remaining water bypasses the bmp and is not treated.
For bmps with an underdrain (tree trench, bioretention, swale main channel), some water captured by the bmp is returned to the stormwater system via the underdrain. The calculator does not differentiate between water that bypasses the bmp and water returned via the underdrain, instead assuming all this water is treated. Similarly, for non-volume bmps (constructed pond, wetland, sand filter, wet swale), the calculator does not differentiate between water that has been treated and water that bypasses the bmp, instead assuming all runoff has been treated. This is shown in the adjacent image. This can result in significant errors in calculations of TSS and phosphorus, with the calculator overestimating pollutant removal.
To correct this error, the BMP parameters tab on each of these seven bmps now asks the user to input a bypass %. The default is 0 percent, which assumes the BMP is sized to meet the MIDS performance goal.
Example illustration
We ran a scenario with an infiltration trench and two biofiltration (bioretention with an underdrain) practices. One acre of impervious surface drained to each of the bmps. The infiltration trench was designed to exactly capture the MIDS performance goal of 1.1 inches (3993 ft3 of runoff). The first biofiltration practice was also designed to capture 3993 ft3 of runoff. All of this captured runoff was routed to the underdrain (no infiltration or evapotranspiration). The second biofiltration practice was designed to capture 3000 ft3 of runoff, meaning it was undersized by 25 percent (all captured water was routed to the underdrain). The results for particulate phosphorus are shown in the adjacent table.
BMP Load (lbs) Removal (lbs) Removal (%) Expected removal (%)
| Applications and treatment capabilities for infiltration basins | ||||
| BMP | Load (lbs) | Removal (lbs) | Removal (%) | Expected removal (%) |
| Infiltration trench | 1.02 | 0.94 | 92.4 | 92.4 |
| Bioretention fully sized | 1.02 | 0.82 | 80 | 75.4 |
| Bioretention undersized | 1.02 | 0.82 | 80 | 71.0 |