Stormwater Design Adaptations for Karst Terrain in the Chesapeake Bay Watershed

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

2. The Chesapeake Bay Stormwater Training Partnership Visit: www.cwp.org/CBSTP To learn how you can have access to: Discounted Webcasts Free One-day design workshops Intensive master stormwater design seminars Direct On-site technical assistance Self guided web-based learning modules

3. Welcome to the Webcast • To Ask a Question – The lower left-hand corner of the screen contains a chat box. Click on the “Public” tab and type your question in the box and click on the arrow to submit it. We will try to answer as many questions as possible during the webcast. • To Answer a Poll Question – Polling questions will appear throughout the webcast. To answer a poll question, click on the radio button to the left of your answer and click submit. Do not type your answer in the chat box. • To Adjust How the Slides Appear on Your Screen – On the top of your screen, click on the small down arrow next to the button that looks like . Scroll down to “Zoom” and click on “Auto Fit.”

4. 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.

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 www.cwp/org/CBSTP

6. Speaker Info Jim Lawrence Opequon Targeted Watershed Project 408 Marion Street Winchester, VA 22601 jiml@crosslink.net Tom Schueler Chesapeake Stormwater Network117 Ingleside AvenueBaltimore, MD 21228 watershedguy@hotmail.com www.chesapeakestormwater.net

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. 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

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 • Jim Lawrence, Virginia Tech • WilOrndorff, Virginia DCR • Alana Hartman, WV DEP • Michael Schwartz, Freshwater Institute • Wayne Webb, Winchester VA • Sherry Wilkins, WV DEP • 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

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

35. Commercial Stormwater Hotspot OperationS

36. Questions and Answers

37. Part 3 De-centralized Stormwater Design

38. 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

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

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

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

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