Stormwater

Design for Karst

TerrainKey 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 TerrainExtremely large lot development Individual development projects are small Surface/subsurface drainage poorly understood Limited public water and sewer serviceRunoff reduction practices are newLimited 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 DifferentPreliminary and Detailed Site Investigations Assessing Stormwater Hotspot Risk Stormwater Design Principles for KarstDesign Criteria for Specific PracticesSinkhole Remediation in Stormwater PracticesKarst Glossary

Is carbonate rock present at my site?

Are karst

features or landforms present?

Can site layout avoid karst

risk?

YES Preliminary InvestigationSection 3.1

Is the site a stormwater

hotspot?

Can groundwater risks be minimized or prevented?

Do I have borings for proposed drainageway

& stormwater

practices?

YES. Detailed Karst Investigation Section 3.2

Have I computed the water quality volume needed for the site?

YES? Site Plan Layout Section 5.1

YES? Hotspot Risk AnalysisSection 4.1

Do I have acceptable runoff reduction practices to fully meet the WQv?

YES? HotspotManagementSection 4.3

NO? Soil BoringsSection 3.3

NO? BMP SizingState SWM Manual

NO? BMPDesignSection 6

Have I karst-adjusted post development flows for flood control?

NO? Runoff ModelingSection 5.3

Does my site discharge to an adequate channel?

Does my site discharge to a karst

swale?

Does my site discharge to a sinkhole, cave or losing stream?

NO? Channel RoutingSection 5.4

YES? Karst Swale ProtectionSection 5.5

YES? UIC Permit & Sinkhole Best Practices Section 4.3

Treat remainder in stormwater

pond?YES? Special Pond Design Section 3.4 & 6.3

Conduct sinkhole inspection remediation as part of routine stormwater

maintenanceYES? SinkholeRemediationSection 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

HotspotsFuture 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

InfiltrationNo 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 3rd

Workshop in Ransom, WV

•

Incorporate into state and local guidance•

Other issues