Stormwater Design Adaptations for Karst Terrain in the Chesapeake Bay Watershed Photo: Virginia DCR.

Stormwater Design Adaptations for Karst Terrain

in the Chesapeake Bay Watershed

Photo: Virginia DCR

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Welcome to the Webcast

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Welcome to the Webcast

• To Complete the Webcast Survey – After the webcast, we will have a short multiple choice survey to get feedback on your experience. Please take a few minutes to fill the survey out so we can identify areas for improvement.

• Resources – After the webcast, we will email a resources sheet, speaker contact information, and a pdf of the presentation.

Webcast Archive

• We will archive the webcast in a few weeks where it can be accessed at the Chesapeake Bay Stormwater Training Partnership website at www.cwp/org/CBSTP

http://www.cwp/org/CBSTP

Speaker Info

Tom Schueler

Chesapeake Stormwater Network

117 Ingleside Avenue

Baltimore, MD 21228

[email protected]

www.chesapeakestormwater.net

Jim Lawrence

Opequon Targeted

Watershed Project

408 Marion Street

Winchester, VA 22601

[email protected]

http://www.chesapeakestormwater.net/

mailto:[email protected]

Webcast Agenda

1. Karst: The Dissolving Landscape.2. Challenges in Managing Stormwater in

Karst Terrain3. The Site Assessment Process in Karst4. Stormwater Design in Karst Terrain5. Stormwater and the Safe Drinking Water

Act6. Wrap-up

1. Karst: The Dissolving Landscape

What Is Karst?

Karst is a dynamic landscape characterized by sinkholes, springs, caves and a pinnacled, highly irregular soil rock interface that is the consequence of the presence of underlying carbonate geology such as limestone, dolomite or marble.

Surface and Subsurface Hydrology in Karst

• Soils in karst terrain are moderately to poorly permeable, yet there is little surface runoff.

• Rainwater is diverted underground through sinkhole insurgences and/or by diffuse recharge into numerous small fractures in the limestone.

• Contaminants can pass rapidly from surface to subsurface waters with little or no modification

• Short residence times, confined aquifers, and lack of natural filtration creates special need for groundwater protection

The Karst terrain of the Bay Watershed behaves differently than other regions

very ancient and, in many areas, is deeply buried by residual soils

CWP

Unique Development Conditions where Karst is Found

Most is in the Ridge and Valley ProvinceExtremely large lot ex-urban development Individual development projects are small Limited public water and sewer serviceRunoff reduction practices are newLimited experience by contractors,

designers and reviewers

Why Karst is Different

• Post development runoff rates greatly increase

• Karst is interspersed with non-karst at sites

• Highly variable subsurface conditions

• Surface/sub-surface drainage patterns poorly understood

• Confusing surface drainage patterns (losing streams)

• Lower stream density and more karst swales

Why Karst is Different

• Impervious cover dramatically increases site runoff• Increased ponding or infiltration of the runoff can create

new sinkholes• Pollutants in runoff increase risk of groundwater

contamination• Rural development relies on wells for drinking water • Increased sinkhole formation can damage local

infrastructure (roads, buildings and BMPSe• Sensitive endangered species found underground

CSN TECHNICAL BULLETIN No. 1

STORMWATER DESIGN GUIDELINES

FOR KARST TERRAIN IN THE

CHESAPEAKE BAY WATERSHED

VERSION 2.0

JUNE, 2009

Purpose of Technical Bulletin

• Limited, conflicting and disjointed guidance available to local planners and engineers

• Consensus among diverse karst working group over last year

• Document can be incorporated by reference into state and local ordinances and development review policies

• Doesn’t eliminate all risk, but reduces it sharply compared to status quo

Special Thanks for the Karst Working Group

• Chris Anderson, Page County, VA

• Twila Carr, West Virginia DEP

• Bob Denton, Potomac Environmental Services

• Tom Devilbiss, Carroll County, MD

• Mike Eller, EPA Region 3

• Jim Lawrence, Virginia Tech

• Wil Orndorff, Virginia DCR

• Alana Hartman, WV DEP • Michael Schwartz,

Freshwater Institute• Wayne Webb,

Winchester VA • Sherry Wilkins, WV DEP

Questions and Answers

2. Site Assessment for Karst Vulnerability

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I s carbonate rock present at my site?

Are karst features or landforms present?

Can site layout avoid karst risk?

YES Preliminary InvestigationSection 3.1

I s the site a stormwater hotspot?

Can groundwater risks be minimized or prevented?

Do I have borings for proposed drainageway & stormwater practices?

YES. Detailed KarstInvestigation Section 3.2

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

YES? Site Plan Layout Section 5.1

YES? Hotspot Risk Analysis Section 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 karstswale?

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 maintenance

YES? SinkholeRemediationSection 7.0

The Flow Chart From Hell

























































The Flow Chart From Hell

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Preliminary Site Investigation

• Assess whether site is vulnerable to karst problems

• Analysis of geologic and topographic maps, aerial photos and field visit by experienced professional

• Screen for proximity to known caves and sinkholes

• Product is site map showing location of suspected karst features & decision

• While caves/sinkholes are diagnostic, their absence does not mean karst is not a problem

I f YES, Conduct Preliminary I nvestigation


Detailed Site Investigation

• Used to develop a karst feature plan that identifies the location and elevation of subsurface voids, cavities and fractures

• Scope reflects the size and complexity of the development project

• Used to determine the nature and thickness of subsurface materials

Are karstfeatures or landforms present?

I f YES, Conduct Detailed Site I nvestigation

Techniques for Assessing Subsurface Conditions

• Electric resistivity tomography• Seismic refraction• Gravity surveys • Electromagnetic (EM)

inductance/conductivity surveysThese surveys identify suspect areas to be

further evaluated by borings

Key Data to Collect at Site

• Bedrock characteristics (type, contacts, faults, and structure)*

• Overlying soil characteristics (type, thickness, infil rate, water table, geologic parent)*

• Verification of geological contacts between karst and non-karst formations

• Photo-geologic fracture trace map

* these are often spatially variable

Key Data to Collect at Site

• Locations of bedrock outcrops, sinkholes, cave openings, closed depressions, springs and other karst features

• Perennial, intermittent or ephemeral streams (flow behavior and surface/subsurface discharge points)

• Site scale “watershed” boundaries (1 ft or less contours)

• Public or private wells within ¼ mile of site

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Adjust Site Plan Layout• Avoid karst when possible in site layout

and when locating building pads and roads

• Minimize site disturbance and major cut/fill

• Minimize drainage alteration and protect existing flow paths (karst swales)

• Minimize site IC to reduce runoff

• Any existing sinkholes should be recorded and protected with buffers or easements

• Sensible location of wells and septic systems


I f YES Adjust Site Plan Layout

Karst Swale Protection (KSP) Areas

• Centered on drainage-way or swale with width of 50 to 300 feet.

• Credit: Subtract twice the KSP area from the contributing IC area when computing WQv.

• No disturbance during construction• Protected by conservation easement after

construction• Adjacent filter strips and internal spreaders can

further enhance its performance

• A hotspot is future operation or activity on part or all of the site that generates highly polluted runoff and/or has a greater risk of spills leaks and discharges

• Localities can designate which types of development with potential to become hotspots

• If a site is designated as a hotspot, it influences how much runoff must be treated and whether it can be infiltrated or discharged to a sinkhole


I f YES, AssessHotspot Risk & Management

Assess Future Hotspot Status

Risk-Based Management Strategies for Hotspots

• Depending on Hotspot Severity: 1. Enhanced On-site Pollution Prevention Plans2. Treat at least 50% of WQv prior to Infiltration3. Prohibit Infiltration and Use Sand Filters Instead

• These rules also apply to other practices where infiltration may be expected after little or no treatment (e.g., dry ED ponds, grass swales, filter strips)

SWPP Required?

Restricted 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 ●

Industrial and Municipal Stormwater Hotspots

SWPP Required?

Restricted Infiltration

No Infiltration

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 ●

Commercial Stormwater Hotspot OperationS


Part 3 De-centralized Stormwater Design

Guiding Philosophy forStormwater Design in Karst

• Treat runoff in a series of small runoff reduction practices across the site

• Disperse flows to avoid ponding, flow concentration or extended soil saturation

• LID practices work well in karst with CDA less than a half acre

• Avoid big contributing areas and deep trenches/pools• Define stormwater hotspots and ensure full treatment

before discharge• Increase setbacks from buildings and other

infrastructure

Take Soil Borings

• At key locations near buildings, roads, conveyance and at centralized stormwater facilities

• Number and depth of borings depends on the karst feature plans and local requirements

• More guidance is contained in the Technical Bulletin

I f No, Conduct Soil Borings

Do I have borings for proposed drainageway & stormwater practices

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Modeling large storm events in karst

• Adjustments are need to Curve Numbers when using TR-55 or TR-20 models

• These apply to predevelopment runoff computations

• Post-development runoff rates should be computed based on site impervious cover

• Do a second adjustment to curve numbers to reflect the effect of runoff reduction practices

• These prevent super-sized detention ponds

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

No?, make runoff modeling adjustments

Curve Number Adjustments for Karst

Multipliers for Adjusting Predevelopment Runoff Quantities for Karst Impact

Adapted from Laughland (2007) and VA DCR (1999)

% of Drainage

Area in Karst

Design Storm Return Frequency2-year Storm 10-year Storm 100-year

Storm100 0.33 0.43 0.5080 0.38 0.51 0.6260 0.55 0.66 0.7440 0.73 0.80 0.8520 0.91 0.92 0.930 1.00 1.00 1.00

Step 6 Select Most Appropriate BMPs

• Stormwater Practices in State Manuals are Classified as being: • Preferred• Adequate • Discouraged• Prohibited

• All require some design adaptation for karst


Select the most appropriate BMPS for karst terrain

BMP Selection in KarstPreferred Accepted Discourag

edProhibited

Closed Bioretention

Filter StripsWet ponds Wet Swale

Rain Tanks/Cisterns

Small-scale Infiltration

Dry ED ponds

Large Scale Infiltration

Green Roofs Grass Channel

Open Bioretention

Sand FiltersSoil Compost Amendments

Shallow Dry Swale

Permeable Pavers

ConstructedWetlands

Roof Disconnection

Special Pond Design Criteria

• Use of larger ponds highly discouraged in karst, especially wet ponds

• Temporary detention water elevations should not exceed six feet

• Liners required, with thickness and material based on proximity to bedrock/groundwater sensitivity

• Maintenance protocol to inspect and remediate sinkholes


Yes?, then provide special pond design features

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Required Groundwater Protection Liners for Ponds in Karst Terrain (WVDEP, 2006 and VA DCR, 1999)

Pond Excavated at least Three Feet Above Bedrock

24 inches of soil with maximum hydraulic conductivity of 1 x 10-5 cm/sec

Pond Excavated within Three Feet of Bedrock

24 inches of clay1 with maximum hydraulic conductivity of 1 x 10-6 cm/sec

Pond Excavated Near Bedrock within wellhead protection area, in recharge area for domestic well or spring, or in area with high fracture density or significant geophysical anomalies.

Synthetic liner with a minimum thickness of 60 ml.

1 Clay properties defined in the Technical Bulletin

Bioretention Design in Karst

• Wide and shallow• Use underdrain to daylight if bedrock is w/in 3 feet of

bottom• Add sump stone layer below underdrain to increase RR • Keep contributing drainage areas small (1/2 acre or less)• Increase up an down gradient setbacks to buildings and

infrastructure • Shallow media depths OK (2 to 3 feet)

PREFERRED

Rain Tank Design in Karst

• Well suited to provide alternate water source in rural communities

• Above ground tanks are preferred to below ground

• Tanks can be combined with automated irrigation, rain gardens or other practices to increase overflow treatment

• Tank overflows should extend 15 feet from foundation PREFERRED

Rooftop Disconnection Design for Karst

• Need to provide a 15 foot setback from foundation

• Some kind of flexible pipe, buried just underground

• Connect the pipe to a small depression, rain garden or mini-dry well to spread out runoff

• Discharge to a turf filter corridor, filter strip or karst swale

• Minimum 40 feet if it reconnects to IC or storm drain system

PREFERRED

Filter Strip Design for Karst

• Good idea to treat adjacent runoff to the boundaries of a karst swale protection area (KSP)

• Use shallow gravel diaphragms or short grass berms to help spread runoff

• Keep disturbance in filter strip area to a minimum and maintain native meadow or forest

• Drainage to each individual strip should be kept to a ½ acre or less

PREFERRED

Dry Swale Design for Karst

• Try to locate them on predevelopment flow path

• Bottom invert should be at least two ft above bedrock

• Filter bed media can be less than 18 inches

• Lateral set back from roads • Can often dispense with

underdrain• Tie it into an adequate

channel or discharge to karst swale protection area

PREFERRED

Sand Filter Design for Karst

• Recommended practice to treat runoff from hotspots

• Bottom invert should be two feet above bedrock

• Minimum depth of sand bed can be reduced to 18 to 24 inches

• Filter bottom should be closed and be water tight

PREFERRED

Grass Channel Design for Karst

– Incorporate soil compost amendments along bottom to improve treatment

– Check dams are discouraged since they pond too much water. Spreaders that are flush with ground surface can spread flows evenly

– The minimum depth to the bedrock layer can be 18 inches.

– The grass channel may have off-line cells and should be tied into an adequate discharge point.

ADEQUATE

Small-scale Infiltration Design• Max CDA of 20,000 square feet • Maximize the surface area of the infiltration

practice so that it is wider than it is deep • Soil borings must show at least three feet

of vertical separation exist between their bottom invert and the bedrock layer.

• In many cases, bioretention is preferred to infiltration in karst areas.

• 15 ft down-gradient and 25 feet up-gradient setback

• Prohibited if the contributing drainage areas is classified as a severe stormwater hotspot.

ADEQUATE

Permeable Paver Design for Karst

• Best to use micro and small scale applications

• Larger applications should have an impermeable bottom liner and under drains

• Use carbonate source of rocks for stone reservoir to preserve buffering capacity

ADEQUATE

Constructed Wetland Design for Karst

• Will generally need liner to hold water

• Shallow, linear, and multiple cell configurations are preferred

• Regenerative conveyance systems are worth testing (with sand and organic lenses)

• Use them leading to or in close proximity to KSPs

ADEQUATE?

DISCOURAGED PRACTICES

• Wet Swales (don’t work)• Large Scale Infiltration Basins (CDA more

than 20,000 SF of IC)

PROHIBITED PRACTICES

• Wet Ponds• Dry ED Ponds• High cost for testing and liners, and future

sinkhole liability


Part 4. Karst and Drinking Water

The Safe Drinking Water Act

• This federal act regulates the infiltration of stormwater in certain situations under the underground injection control (UIC) program

• State or federal permits are required in order to protect underground sources of drinking water from contamination

• Two kinds of stormwater discharge can be subject to UIC regulations in karst terrain– Certain kinds of underground stormwater disposal

(BMPs)– Discharges to an existing “improved” sinkhole

Underground Injection Permits

• Class V permits are needed for shallow wells

• Any bored, drilled, driven shaft or dug hole that is deeper than its widest dimensions, or an improved sinkhole, or a subsurface fluid distribution system.

• Must notify appropriate state or federal permit authority

Each State Has its Own UIC Authority

Underground Injection Control Permit Agency in Each Bay State

BAY STATE REGULATORY AUTHORITYMARYLAND MDENEW YORK EPA REGION 2 *

PENNSYLVANIA EPA REGION 3 *VIRGINIA EPA REGION 3 *

WEST VIRGINIA WV DEP* in states where EPA administers the UIC program, Class 5 wells are “rule- authorized”, meaning that they do not require a permit, but the operator must contact the agency to inventory their well.

What is an improved sinkhole?

• “ a naturally occurring karst depression or other natural crevice that has been modified by a man-made structure to direct fluids into the subsurface “

• Man-made structures are pipes, swales, ditches or other devices that channel water toward and/or into an existing sinkhole

• Both the increased runoff from a development and the storm drain system qualify as discharge to an improved sinkhole


Stormwater Discharges to Improved Sinkholes

• Sinkhole should be registered as Class V injection well• Analyze for any public/private drinking wells w/in

1500 feet of sinkhole • Dye tracing advised if wells are present• Prevent increased runoff volumes from discharging to

sinkhole, but maintain predevelopment stormwater volume

• Treat full water quality volume prior to sinkhole discharge using runoff reduction practices (one inch)

• Maintenance of stormwater practices a condition of underground injection permit

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What if a Sinkhole is Created in a Stormwater BMP?

• Maintenance Inspection • Notification to Local Authority• Sinkhole Investigation *• Sinkhole Stabilization *• Final Grading

* The complexity of the methods

depend on the depth and areal

extent of the new sinkhole

Sinkhole Remediation:

Lots Like a Bioretention Cross-Section

The Keys to Karst

• Understand the landscape below ground

• Detailed site planning and assessment

• Decentralized stormwater design• Remove pollutants prior to

underground discharge

Wrap-up

Webcast Resources

• More info, glossary and useful weblinks are provided in Technical Bulletin No. 1 which will be e-mailed to you

• Also check out the CSN website at www.chesapeakestormwater.net

Upcoming Webcasts – for 2010

• June 13 Stormwater Design for Redevelopment Projects in MD (Register thru MAWP)

• September 1 Permeable Pavement Design, Installation, and Maintenance *

• October 20 Rooftop Disconnection, Filter Strips & Rainwater Harvesting *

* Register at http://www.cwp.org/CBSTP

http://www.cwp.org/Webcasts

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Stormwater Design Adaptations for Karst Terrain in the Chesapeake Bay Watershed Photo: Virginia DCR.

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