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

2. Stormwater Training Partnership
The Chesapeake Bay
Stormwater Training Partnership
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Direct On-site technical assistance
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5. 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

6. Chesapeake Stormwater Network 117 Ingleside Avenue Baltimore, MD 21228
Speaker Info
Jim Lawrence
Opequon Targeted
Watershed Project
408 Marion Street
Winchester, VA 22601
Tom Schueler
Chesapeake Stormwater Network 117 Ingleside Avenue Baltimore, MD 21228

7. Webcast Agenda Karst: The Dissolving Landscape.
Challenges in Managing Stormwater in Karst Terrain
The Site Assessment Process in Karst
Stormwater Design in Karst Terrain
Stormwater and the Safe Drinking Water Act
Wrap-up

8. 1. Karst: The Dissolving Landscape

9. 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.

11. 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

14. very ancient and, in many areas, is deeply buried by residual soils
The Karst terrain of the Bay Watershed behaves differently than other regions
very ancient and, in many areas, is deeply buried by residual soils

16. Unique Development Conditions where Karst is Found
Most is in the Ridge and Valley Province
Extremely large lot ex-urban development
Individual development projects are small
Limited public water and sewer service
Runoff reduction practices are new
Limited experience by contractors, designers and reviewers

17. 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

18. 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

19. CSN TECHNICAL BULLETIN No. 1
STORMWATER DESIGN GUIDELINES
FOR KARST TERRAIN IN THE
CHESAPEAKE BAY WATERSHED
VERSION 2.0
JUNE, 2009
Workshops or charrettes such as the one pictured in the slide above are important tools that should be used early in the planning process of retrofits to inform and educate stakeholders.

20. 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

21. 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

22. Questions and Answers

23. 2. Site Assessment for Karst Vulnerability

25. 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

26. 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

27. Techniques for Assessing Subsurface Conditions
Electric resistivity tomography
Seismic refraction
Gravity surveys
Electromagnetic (EM) inductance/conductivity surveys
These surveys identify suspect areas to be further evaluated by borings

28. 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

29. 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

30. 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

31. 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

32. Assess Future Hotspot Status
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

33. Risk-Based Management Strategies for Hotspots
Depending on Hotspot Severity:
Enhanced On-site Pollution Prevention Plans
Treat at least 50% of WQv prior to Infiltration
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)

34. Industrial and Municipal Stormwater Hotspots
SWPP Required?
Restricted Infiltration No
Infiltration
Facilities w/NPDES Industrial permits
Yes ■
Public works yard ●
Ports, shipyards and repair facilities
Railroads/ equipment storage
Auto and metal recyclers/scrap yards
Petroleum storage facilities
Highway maintenance facilities
Wastewater, solid waste facilities
Industrial machinery and equipment
Trucks and trailers
Airfields and aircraft maintenance areas
Fleet storage areas

35. Commercial Stormwater Hotspot OperationS
SWPP Required?
Restricted Infiltration No
Infiltration
Parking lots (40 or more parking spaces) ●
Gas stations
Highways (2500 ADT)
Retail/wholesale vehicle/ equipment dealers
Convenience stores/fast food restaurants
Vehicle maintenance facilities
Nurseries and garden centers
Golf courses

36. Questions and Answers

37. Part 3 De-centralized Stormwater Design

38. Guiding Philosophy for Stormwater 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

39. 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

40. 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

41. 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 Frequency
2-year Storm
10-year Storm
100-year Storm
100
0.33
0.43
0.50 80
0.38
0.51
0.62 60
0.55
0.66
0.74 40
0.73
0.80
0.85 20
0.91
0.92
0.93
1.00

42. 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

43. BMP Selection in Karst Preferred Accepted Discouraged Prohibited
Closed Bioretention
Filter Strips
Wet ponds
Wet Swale
Rain Tanks/Cisterns
Small-scale Infiltration
Dry ED ponds
Large Scale
Infiltration
Green Roofs
Grass Channel
Open Bioretention
Sand Filters
Soil Compost Amendments
Shallow Dry Swale
Permeable Pavers
Constructed
Wetlands
Roof Disconnection 43

44. 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

47. 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

48. 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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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

57. 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?

58. DISCOURAGED PRACTICES
Wet Ponds
Dry ED Ponds
High cost for testing and liners, and future sinkhole liability
PROHIBITED PRACTICES
Wet Swales (don’t work)
Large Scale Infiltration Basins (CDA more than 20,000 SF of IC)

59. Questions and Answers

60. Part 4. Karst and Drinking Water

61. 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

62. 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

63. Each State Has its Own UIC Authority
Underground Injection Control Permit Agency in Each Bay State
BAY STATE
REGULATORY AUTHORITY
MARYLAND
MDE
NEW YORK
EPA REGION 2 *
PENNSYLVANIA
EPA REGION 3 *
VIRGINIA
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.

64. 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

65. 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

66. 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

67. Sinkhole Remediation: Lots Like a Bioretention Cross-Section

69. The Keys to Karst
Understand the landscape below ground
Detailed site planning and assessment
Decentralized stormwater design
Remove pollutants prior to underground discharge

70. Wrap-up

71. Webcast Resources
More info, glossary and useful weblinks are provided in Technical Bulletin No. 1 which will be ed to you
Also check out the CSN website at

72. 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

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