Stormwater Design for Karst Terrain Key Challenges Guiding Design Philosophy Site Investigations Stormwater Hotspots Sinkholes and UIC Permits CSN Technical Bulletin No. 1 available at www.chesapeakestormwater.net
What is karst Karst is a landscape type that is characterized by the presence of sinkholes, springs, caves, and a pinnacled, highly irregular soil-rock interface. The karst landscape is a consequence of the presence of soluble bedrock, most commonly carbonate rocks (e.g. limestone, dolomite, and marble).
The Regional Karst Landscape In our area the karst features are ancient, having been subjected to many cycles of periodic flush and fill with thick, cohesive residual sediments. Catastrophic (vault) collapses are relatively uncommon in our region, and in most cases are due to human modification of the landscape. Sinkholes form in our region almost exclusively due to soil raveling (i.e. cover collapse sinkholes), primarily influenced by the intrusion of surface water; depression of the water table may have an effect, but is usually not the principle factor.
Cover Collapse in Cohesive Sediments
Cover Collapse in Granular Sediments
Karst Water Basics The regional carbonate rocks lack primary porosity, therefore their water bearing potential is dependent on secondary porosity (solution enlarged fractures, joints and bedding plane partings). The principle aquifer resides in solution-enlarged fractures and fissures in the limestone bedrock below the water table (phreatic zone). Typically there is generally little or no near surface aquifer located at the subsoil-bedrock interface; however, perched surficial aquifers are sometimes present lying above localized layers of relatively impermeable soils.
Environmental Issues Soils in karst terrains are moderately to poorly permeable, yet there is little surface runoff. Thus, rainwater is diverted underground through sinkhole insurgences ( swallows ) and/or by diffuse recharge through the overburden into numerous small fractures in the limestone. Contaminants can pass rapidly through the subsurface system with little or no modification other than advective dissipation. Long residence times, confined aquifers, and lack of natural filtration creates special needs regarding groundwater protection in karst.
Endangered Species Act Developers may offer higher standards of stormwater management to avoid formal consultation with the Fish and Wildlife Service or DGIF. Presence of legally protected species is sometimes enough to spook developers and kill a project.
Unique Development Conditions in the Ridge and Valley Karst Terrain Extremely large lot development Individual development projects are small Surface/subsurface drainage poorly understood Limited public water and sewer service Runoff reduction practices are new Limited experience by contractors, designers and reviewers
Key Karst Challenges Major increase in surface runoff Variable subsurface vulnerability Increased ponding or infiltration form sinkholes Leads to groundwater contamination Can cause practice or infrastructure failure Confusing surface drainage patterns (losing streams)
Contents of Technical Bulletin No. 1 Why Karst is Different Preliminary and Detailed Site Investigations Assessing Stormwater Hotspot Risk Stormwater Design Principles for Karst Design Criteria for Specific Practices Sinkhole Remediation in Stormwater Practices Karst Glossary
Is carbonate rock present at my site? YES Preliminary Investigation Section 3.1 Are karst features or landforms present? YES. Detailed Karst Investigation Section 3.2 Can site layout avoid karst risk? YES? Site Plan Layout Section 5.1 Is the site a stormwater hotspot? YES? Hotspot Risk Analysis Section 4.1 Can groundwater risks be minimized or prevented? YES? Hotspot Management Section 4.3 Do I have borings for proposed drainageway & stormwater practices? NO? Soil Borings Section 3.3 Have I computed the water quality volume needed for the site? NO? BMP Sizing State SWM Manual Do I have acceptable runoff reduction practices to fully meet the WQv? NO? BMP Design Section 6 Treat remainder in stormwater pond? YES? Special Pond Design Section 3.4 & 6.3 Have I karst-adjusted post development flows for flood control? NO? Runoff Modeling Section 5.3 Does my site discharge to an adequate channel? NO? Channel Routing Section 5.4 Does my site discharge to a karst swale? YES? Karst Swale Protection Section 5.5 Does my site discharge to a sinkhole, cave or losing stream? YES? UIC Permit & Sinkhole Best Practices Section 4.3 Conduct sinkhole inspection remediation as part of routine stormwater maintenance YES? Sinkhole Remediation Section 7.0
Two Stage Site Assessment: General Karst Vulnerability and Specific Geotechnical Investigation
Guiding Philosophy for Stormwater Design in Karst Detailed on-site geotechnical survey first LID practices work well in karst Limited use of infiltration in karst terrain Avoid big contributing areas and deep trenches/pools Define stormwater hotspots and ensure full treatment before discharge No discharge to sinkhole w/o full water quality treatment Underground injection permits for sinkholes and deep infiltration
Designating Stormwater Hotspots Future status of development determines how much treatment is required and whether runoff can be infiltrated or discharged to a sinkhole
Table 1: Potential Stormwater Hotspot and Site Design Responses Potential Stormwater Hotspot Operation SWPP Required? Restrictd Infiltration No Infiltration Facilities w/npdes Industrial permits Yes Public works yard Yes Ports, shipyards and repair facilities Yes Railroads/ equipment storage Yes Auto and metal recyclers/scrap yards Yes Petroleum storage facilities Yes Highway maintenance facilities Yes Wastewater, solid waste facilities Yes Industrial machinery and equipment Yes Trucks and trailers Yes Airfields and aircraft maintenance areas Yes Fleet storage areas Yes Parking lots (40 or more parking spaces) No Gas stations No Highways (2500 ADT) No Retail/wholesale vehicle/ equipment dealers No Convenience stores/fast food restaurants No Vehicle maintenance facilities No Nurseries and garden centers No Golf courses No
BMP Selection in Karst Preferred Bioretention (closed) Rain Tanks/Cisterns Green roofs Dry swale (closed) Sand filters Filter Strips Accepted Grass Channel Soil Restoration Small-scale Infiltration Permeable pavers (closed) Constructed wetlands (lined) Discouraged Wet ponds Dry ED ponds Prohibited Wet swale Large scale infiltration
Bioretention Design Considerations in Karst Terrain: Line bottom with impermeable filter fabric Use underdrain to daylight safely Add sump stone layer below underdrain to increase RR Keep contributing drainage areas small Increase setbacks to buildings and infrastructure Shallow excavation OK (2 to 4 feet)
Process for Developing Better Stormwater Guidance in Karst Terrain Peer Review of CSN Technical Bulletin No. 1 Nov 3 rd Workshop in Ransom, WV Incorporate into state and local guidance Other issues