Updates to the MIDS calculator

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Version 2 of the MIDS calculator was updated in 2016. Version 3.0 includes the following changes from Version 2.

• When a user opens the calculator (either an existing file or a new file), the Site Information tab appears. In Version 3 of the calculator, the user must specify whether they are using the calculator to determine compliance with a Construction Stormwater permit. If the user answers "YES", the Stormwater disconnection (Impervious disconnection) BMP will not be available. The Stormwater disconnection (Impervious disconnection) BMP does not meet the permit requirement that "The Permittee(s) must design infiltration or filtration systems that provide a water quality volume (calculated as an instantaneous volume) of one (1) inch of runoff (or one (1) inch minus the volume of stormwater treated by another system on the site) from the new impervious surfaces created by the project." The Results tab indicates whether the calculations are for compliance with a Construction Stormwater permit. If the user selects "NO" to the question about whether the calculator is being used for permit compliance, the Stormwater disconnection (Impervious disconnection) BMP will be available. For a description of this BMP, see the next bullet.
• A Stormwater disconnection (Impervious disconnection) BMP was created. Stormwater disconnection/Impervious surface disconnection spreads runoff generated from parking lots, driveways, rooftops, sidewalks and other impervious surfaces onto adjacent pervious areas where it can be infiltrated. All pollutants in the infiltrated water are credited as being reduced. Pollutants in the stormwater that bypasses the best management practice (BMP) receive 68 percent removal for total suspended solids (TSS) and 0 percent reduction for both dissolved and particulate phosphorus. Guidance, methodology, and an example are provided on this page. A summary of the calculation methodology is found at this link. This BMP cannot be used for compliance with the Construction Stormwater permit.
• Infiltration is allowed beneath a raised underdrain and into underlying soil at a rate of 0.06 in/hr over the drawdown time (either 24 or 48 hours). This is in addition to the storage credit below the underdrain, which was provided in Version 2. Version 2 allowed infiltration beneath an underdrain located at the bottom of a BMP but not for a raised underdrain. It was thus possible to receive greater volume credit for a BMP with an underdrain at the bottom compared to the same BMP with a raised underdrain, which is incorrect. BMPs to which this applies include
  • bioretention basin (with underdrain)
  • tree trench system/Box (with underdrain)
  • swale main channel (with underdrain)
  • permeable pavement
• For permeable pavement with an underdrain, the calculation for volume credit now considers whether the subsoil was compacted during construction. The credit is calculated as follows.
  • If the subsoil is not compacted, the volume equals the bottom surface area times an infiltration rate of 0.06 inches per hour over the drawdown time.
  • If the subsoil is compacted, the volume equals the bottom surface area times an infiltration rate of 0.03 inches per hour over the drawdown time.
• For a swale main channel, if the user has a bioretention base, the user is now asked ask if the entire swale is underlain by the bioretention base. If the answer is “YES”, the volume credit given by the swale area (length times width) times the bioretention media depth times the porosity of the media. If the user answers "NO" to the question about the extent of the bioretention base, the user is asked to input the average cross-sectional area of the bioretention base beneath the swale. The volume credit is then given by the swale area (length times width) times the bioretention media depth times the porosity of the media.
• A new BMP called Underground infiltration was created. A description of the BMP, calculator inputs and methodology, and an example, are found here. To use this BMP, the user must know the storage volume in the underground pipes and in the media surrounding the pipes. The calculator does not make this calculation because of the wide variability in the type of pipes that are used for underground infiltration. An Excel spreadsheet was developed to assist users in making this calculation. This spreadsheet, as well as a description of the calculations, is found here. In addition to storage in pipes and in the media adjacent to the pipes, the user can specify storage in media beneath the pipes. The 48 hour drawdown time must be met from both the pipes and through the underlying media.