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

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Presentation on theme: "Stormwater Design Adaptations for Karst Terrain in the Chesapeake Bay Watershed Photo: Virginia DCR."— Presentation transcript:

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

3 Visit: 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

4 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.”

5 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 a resources sheet, speaker contact information, and a pdf of the presentation.

6 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

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

8 Webcast Agenda 1.Karst: The Dissolving Landscape. 2.Challenges in Managing Stormwater in Karst Terrain 3.The Site Assessment Process in Karst 4.Stormwater Design in Karst Terrain 5.Stormwater and the Safe Drinking Water Act 6.Wrap-up

9 1. Karst: The Dissolving Landscape

10 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.

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12 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

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

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17 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

18 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

19 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

20 CSN TECHNICAL BULLETIN No. 1 STORMWATER DESIGN GUIDELINES FOR KARST TERRAIN IN THE CHESAPEAKE BAY WATERSHED VERSION 2.0 JUNE, 2009

21 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

22 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

23 Questions and Answers

24 2. Site Assessment for Karst Vulnerability

25 Photos: Chesapeake Bay Program

26 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

27 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

28 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

29 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

30 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

31 Photos: Chesapeake Bay Program 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

32 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

33 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 Assess Future Hotspot Status

34 Risk-Based Management Strategies for Hotspots Depending on Hotspot Severity: 1.Enhanced On-site Pollution Prevention Plans 2.Treat at least 50% of WQv prior to Infiltration 3.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)

35 SWPP Required? Restricted Infiltration No Infiltration Facilities w/NPDES Industrial permitsYes ■■ Public works yardYes ● Ports, shipyards and repair facilitiesYes ● Railroads/ equipment storageYes ● Auto and metal recyclers/scrap yardsYes ● Petroleum storage facilitiesYes ● Highway maintenance facilitiesYes ● Wastewater, solid waste facilitiesYes ● Industrial machinery and equipmentYes● Trucks and trailersYes● Airfields and aircraft maintenance areas Yes ● Fleet storage areasYes ● Industrial and Municipal Stormwater Hotspots

36 SWPP Required? Restricted Infiltration No Infiltration Parking lots (40 or more parking spaces) No● Gas stationsNo ● Highways (2500 ADT)No● Retail/wholesale vehicle/ equipment dealers No● Convenience stores/fast food restaurants No● Vehicle maintenance facilitiesNo ● Nurseries and garden centersNo● Golf coursesNo● Commercial Stormwater Hotspot Operation S

37 Questions and Answers

38 Part 3 De-centralized Stormwater Design

39 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

40 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

41 Photos: Chesapeake Bay Program 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

42 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 Storm10-year Storm100-year Storm

43 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

44 BMP Selection in Karst PreferredAcceptedDiscouragedProhibited Closed Bioretention Filter Strips Wet ponds Wet Swale Rain Tanks/Cisterns Small-scale Infiltration Dry ED ponds Large Scale Infiltration Green RoofsGrass Channel Open Bioretention Sand Filters Soil Compost Amendments Shallow Dry Swale Permeable Pavers Constructed Wetlands Roof Disconnection

45 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

46 Photos: Chesapeake Bay Program

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48 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 cm/sec Pond Excavated within Three Feet of Bedrock 24 inches of clay 1 with maximum hydraulic conductivity of 1 x 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

49 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

50 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

51 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

52 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

53 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

54 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

55 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

56 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

57 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

58 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?

59 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

60 Questions and Answers

61 Part 4. Karst and Drinking Water

62 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

63 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

64 Each State Has its Own UIC Authority Underground Injection Control Permit Agency in Each Bay State BAY STATEREGULATORY AUTHORITY MARYLANDMDE NEW YORKEPA REGION 2 * PENNSYLVANIAEPA REGION 3 * VIRGINIAEPA REGION 3 * WEST VIRGINIAWV 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.

65 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

66 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

67 Photos: Chesapeake Bay Program 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

68 Sinkhole Remediation: Lots Like a Bioretention Cross-Section

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

71 Wrap-up

72 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

73 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

74 Thanks Please take a minute to answer a multiple choice evaluation survey to improve future webcasts Or your comments directly to


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