Tips for using the MIDS Calculator for tracking pollutant loading
Contents
The Minimal Impact Design Standards (MIDS) Calculator was initially designed to assess progress toward meeting a volume-based performance goal. The default goal in the Calculator is 1.1 inches off impervious surface. The Calculator translates this into annual values for runoff volume, TSS, and total phosphorus retention. Specific information about the modeling used in developing the Calculator is found here.
The MIDS Calculator is being widely used for water quality purposes, such as tracking progress toward meeting total maximum daily loads (TMDLs). For this purpose, the MIDS Calculator has limitations, including but not limited to the following.
- default event mean concentrations for TSS and TP that are applied across the entire area being modeled
- default curve numbers for A, B, C, and D soils that are applied across the entire area being modeled
- default particulate and dissolved phosphorus fractions of 0.55 and 0.45, respectively, that are applied across the entire area being modeled
- default rain values that are linked to a zip code
- limited ability to model certain BMPs, such as proprietary devices, street sweeping, creation of natural space, improved maintenance, removal of illicit discharges, and aggressive pretreatment.
This page provides some tips for addressing these limitations to more accurately simulate pollutant loading using the MIDS Calculator.
NPDES permit considerations
Note that, where applicable, there are warnings for situations where manipulating the Calculator is not consistent with requirements in the Construction Stormwater Permit. An example would be creation of natural space for water retention or disconnection of impervious surfaces for water retention. These situations are identified with the following alert box.