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*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 | *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) | **bioretention basin (with underdrain) | ||
− | **tree trench system/ | + | **tree trench system/box (with underdrain) |
**swale main channel (with underdrain) | **swale main channel (with underdrain) | ||
**permeable pavement | **permeable pavement | ||
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**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 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. | **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 | + | *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 bioretention area 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 [https://stormwater.pca.state.mn.us/index.php?title=Requirements,_recommendations_and_information_for_using_underground_infiltration_BMPs_in_the_MIDS_calculator 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 [https://stormwater.pca.state.mn.us/index.php/Guidance_on_calculating_volumes_for_underground_infiltration_BMPs_in_the_MIDS_calculator 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. | *A new BMP called ''Underground infiltration'' was created. A description of the BMP, calculator inputs and methodology, and an example, are [https://stormwater.pca.state.mn.us/index.php?title=Requirements,_recommendations_and_information_for_using_underground_infiltration_BMPs_in_the_MIDS_calculator 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 [https://stormwater.pca.state.mn.us/index.php/Guidance_on_calculating_volumes_for_underground_infiltration_BMPs_in_the_MIDS_calculator 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. | ||
*The BMP previously named ''Infiltration basin/Underground Infiltration'' was renamed ''Infiltration basin/infiltration trench (aboveground)''. No other changes were made to the aboveground infiltration BMP. | *The BMP previously named ''Infiltration basin/Underground Infiltration'' was renamed ''Infiltration basin/infiltration trench (aboveground)''. No other changes were made to the aboveground infiltration BMP. |
Version 2 of the MIDS calculator was updated in 2016. Version 3.0 includes the following changes from Version 2.