Structural BMP use in karst settings | Minnesota Stormwater Manual

**Stormwater BMP selection in karst settings.**
BMP	Suitability in karst regions	Karst considerations	Construction stormwater permit restriction¹
Impervious area disconnect	Preferred	Strongly recommended for most residential lots less than 6,000 square feet. Discharge point from the disconnect should extend at least 15 feet from any building foundation.	No
Bioretention with underdrain (biofiltration)	Preferred	Requires 3 foot minimum separation distance between practice bottom and bedrock. If this is not possible, an underdrain should be used to convey the water away. It’s recommended that the drainage area to an individual bioretention BMP be kept less than 20,000 square feet. Larger designs that rely on exfiltration of treated runoff into the soils below are not recommended.	No
Rain tank/cistern	Preferred	Above ground tanks are preferred to below ground. Overflow of tank should extend at least 15 feet from building foundation.	No
Rooftop disconnect	Preferred	15 feet foundation set back. Runoff should be spread diffusely across landscape.	No
Green roofs	Preferred	Runoff should be spread diffusely across landscape.	No
Dry swale or grassed channel	Preferred	Line with underdrains. Incorporate compost amendments into the bottom of the channel to improve runoff reduction for vegetative uptake, and transpiration. Do not incorporate check dams unless the swale or channel incorporates an underdrain. Otherwise small areas of infiltration would develop in the zones upgradient of the check dam where the water is captured without adequate outlet. Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst	Yes if designed for infiltration
Media filter	Preferred	Recommended practice in areas of shallow bedrock and soil.	No
Vegetative filter	Preferred	Recommended practice in areas of shallow bedrock and soil.	No
Soil compost amendment	Adequate		No
Small scale infiltration/micro-bioinfiltration	Adequate	Create multiple small sized infiltration BMPs to infiltrate impervious areas of 250 to 2,500 square feet. Not recommended for sites identified as a stormwater hotspot. Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst	Yes
Permeable pavement	Adequate	Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst	Yes if designed for infiltration (no underdrain)
Infiltration trench or basin	Adequate	Surface area to depth ratios of practices may need to be larger than typical basin designs to minimize depth of the BMP. Confirm suitability with supporting geotechnical investigations and calculations. Not recommended for sites identified as a stormwater hotspot. Incorporate pre-treatment to limit risk of groundwater contamination in the event of future failure of the BMP associated with development of a karst feature. Local review authority should be consulted for approval. Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst	Yes
Constructed wetlands	Adequate	Requires larger surface area to drainage area ratios to limit the depth of the wetland. Bedrock should act like a liner and help to maintain a permanent pool, unless fracture zone is present. Consider using liner and liner cells. Warning: If the CSW permit applies, liners are required in areas of active karst	No
Dry extended detention (ED) ponds and wet ponds	Adequate	Requires larger surface area to drainage area ratios to limit the depth of the basin or pond. If analysis shows that the soils can support the weight of a pond then a liner should be used to prevent infiltration Warning: If the CSW permit applies, liners are required in areas of active karst	No
Wet swale	Discouraged	Not feasible	No
Large scale infiltration	Discouraged	Use small scale infiltration practices instead. Warning: If the CSW permit applies, Section 16.20 prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst	Yes
Sources Karst Working Group, 2009; Minnesota Stormwater Wiki; Tennessee Permanent Stormwater Management and Design Guidance. ¹Section 16.20 of the CSW Permit prohibits prohibits permittees from constructing infiltration systems in areas within 1,000 feet upgradient or 100 feet downgradient of active karst features.