Stormwater Management in Karst TerrainRegional Perspectives for VA and WV

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Stormwater Management in Karst Terrain-Regional
Perspectives for VA and WV

• R. K. Denton Jr., C.P.G.
• Specialized Engineering
• Ranson, WV

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Karst Water Basics

• Carbonate rocks lack primary porosity, therefore
  there water bearing potential is dependant 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).
• There is generally no near surface aquifer
  located at the subsoil-bedrock interface, with
  water percolating directly from the surface
  soils, regolith and epikarst into the phreatic
  zone.

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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.

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Considerations for Stormwater Management Planning
in Karst

• Proposed management structures usually correspond
  with the topographic low-point(s) of a site,
  often where water is already migrating into the
  subsurface.
• Covered Karst presents special challenges to
  design.
• Too rapid infiltration can cause basin failures,
  but restricted infiltration can deplete the
  aquifer!

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Virginia DCR Guidelines

• Any design in regions suspected to include karst
  topography should be supported by a thorough
  subsurface geotechnical or geological
  investigation.
• Recommends onsite adsorption of runoff encouraged
  through engineered improvements upstream of the
  stormwater management facility, which may include
  green space or bioretention practices.

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Virginia DCR-Pond Management

• Storm water management facilities in karst areas
  to detain 10 year storms and release at a
  one-year flow rate.
• Sedimentation forebay use is desirable, and
  facilitates easier maintenance and inspection.
• All ponds that impound water should be lined with
  10-6 permeability clay or an impermeable
  geo-membrane liner, unless geophysical studies
  show an absence of underlying voids.

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VA DCR-When the water leaves the pond, what next?

• Flow dissipation at the basin outlet is needed
  unless discharge is to a pre-existing, adequate
  channel.
• Broad swales lacking well-defined streambeds do
  not qualify as adequate channels, and discharge
  to them from stormwater impoundments should be
  dissipated.
• Nevertheless, discharge to outlet channels that
  are losing streams or dry runs can exacerbate
  the development of downgradient sinkholes.

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Berkeley County, WV

• For pond sites located within 500 feet of a
  mapped or field located sinkhole, a geotechnical
  study shall be preformed with recommendations as
  to possible safeguards required to protect the
  groundwater. Ponds located within 500 feet of a
  sinkhole shall not direct any discharge toward
  the sinkhole.

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Shenandoah County, VA

• Requires 100-feet vegetated buffers around all
  sinkholes.
• Has generally adopted the VA-DCR guidelines for
  stormwater management in karst.

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Areal Survey Techniques

• Structural Analysis-detects presence of faults,
  folds, or other structures that may create
  solution features.
• Fracture Trace Assessment-detects potentially
  significant subsurface conduits.
• Topographic Assessment-Two-foot contour maps are
  used to locate springs, sinkholes, or areas of
  closed, descending contours that may be impacted
  by a stormwater structure.
• Field Verification is critical.

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Subsurface Characterization Techniques

• Electrical Resistivity Survey-
• Seismic Survey-
• Air-track drill-
• Rock coring-
• GPR is typically not used due to interferences
  from clay-rich soil.

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Liners-the good, bad and ugly

• The use of in-situ, clay-rich soil can result in
  basin failure due to the preservation of
  pre-existing rock fabric heterogeneity in the
  residual matrix.
• Impermeable geomembrane in conjunction with
  compacted, low-permeability clay (10-6 ) is
  recommended.
• Inspection are critical.

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Acknowledgements
William Jones, P.G. (Environmental Data) Will
Orndorff (VA-DCR) Malcolm Fields, PhD (US-EPA)
Randy Ondorff (USGS) University of Tennessee
Geo-Hazards Program Karstwaters Institute
National Speleological Society ASFE Berkeley
County Department of Engineering Shenandoah
County Planning Commission David Hubbard
(VA-DMMR) William White, PhD (Penn State
Univ.) In memory of John Laughlin, P.E., friend,
mentor, and advocate of enlightened karst
practices in the eastern panhandle of WV.