1. 7.4.3 Basic Principles
      2. Avoid Concentrating Water.
      3. 7.4.4 BMP Considerations
    1. Increased storage

Pennsylvania Stormwater Best Management Practices Manual
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consideration of potential contaminants that can be introduced by the proposed project. The
worst scenario is to ignore karst features entirely and thus significantly increase the potential for
costly delays, repairs, catastrophes and legal proceedings.
Stormwater control plans that utilize infiltration in karst are more common in areas such as
Kentucky (Crawford, 1989) and Tennessee (McCann & Smoot, 1999) but have generally been
avoided by hesitant or inexperienced developers in Pennsylvania. Non-infiltration plans may
seem safer and more economical even with the increased cost, but, an additional, long-term
“cost” is associated – lowering of the groundwater table, reducing the potential groundwater
resources of an area, and increasing the risk of a sudden, catastrophic ground collapse (via a
failed impoundment, swale, retention structure, etc.). Use of infiltration BMPs, especially
watershed-wide, is the best method for stormwater control in most karst areas. (Crawford,
1989)(McCann & Smoot, 1999) Future research in this area should identify additional innovative
solutions to these stormwater management challenges.
Basic Principles
Successful stormwater management in karst areas can be achieved by developing a strategy for
the site that will be best suited to function within the tolerance limits of the natural system. Every
effort should be made to maintain the pre-development hydrologic regime and utilize existing
karst drainage features in a safe way. The risk of sinkholes, subsidence problems and potential
groundwater contamination issues should be of utmost consideration. As previously noted in
Chapter 3, watershed-wide stormwater planning that considers and incorporates the existing
karst drainage will achieve the best overall results.
The following basic principles must be considered in karst areas:
Identification, understanding and consideration of geologic information are crucial.
An initial site assessment is critical to identify karst and existing drainage features. It is
recommended that a broader area be reviewed to spot regional trends in geology and
drainage. A thorough site assessment should include, but not be limited to, the following:
o Review of aerial photographs, geologic literature, sinkhole maps, borings (if
available), existing well data, and municipal wellhead or aquifer protection plans.
o Site reconnaissance, including a thorough field examination for features such as
limestone pinnacles, sinkholes, closed depressions, fracture traces, faults, springs
and seeps. Special attention should be paid to confirmation of features located
during literature review.
o Drilling of boreholes.
o Determination of groundwater elevations, especially with respect to the bedrock
surface, and flow direction. To assess seasonal changes, it is necessary to obtain
groundwater measurements over several months to a year.
o Geophysical surveys to locate subsurface anomalies. Consult a professional
experienced in geophysical methods and karst areas before conducting these
Observe the site under different weather conditions especially during heavy rain events and
through different seasons. Identify and map the natural drainageways.
A site design in karst areas should be supported by a geotechnical or hydrogeologic report
conducted by a qualified and/or licensed professional (i.e., soil scientist, geologist,
hydrogeologist, geotechnical engineer, etc.). The report should include:
o Site reconnaissance discussion.
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o Identification and mapping of karst features and hydrogeologic conditions of the
o Identification and mapping of existing drainage patterns and features.
o Discussion of groundwater hydrology.
o Survey of soil characteristics and thickness and analysis of the site’s capability for
infiltrating stormwater.
o A discussion of how infiltration will be handled to avoid contamination of the
groundwater aquifer.
o A plan view drawing of the site, noting the locations of important features. This
plan should delineate areas available for infiltration, areas not suitable for
infiltration and areas where development should not occur.
o A contingency plan to be used if unexpected conditions or unmapped karst
features are encountered during site excavation.
Refer to the Case Studies in karst areas contained in Chapter 9 for examples. More information
is available from Virginia DCR Technical Bulletin No. 2, Memon and others, 1999 and Ralston,
and others, 1999)
Maintain natural conditions within the stormwater plan to the maximum extent possible.
Maintain the natural water balance for surface flows and groundwater recharge. (See also
section 5.4.3). Existing drainage patterns and features, both natural and artificial, should be
taken into consideration. Use these pre-development drainage ways to the maximum extent
possible. Avoid building on or adjacent to these drainage features.
Maintain groundwater levels and hydrostatic pressure to the maximum extent possible – avoid
large groundwater withdrawals, elimination of recharge areas or concentrated injection (in
reference to time as well as location). Fluctuating groundwater levels will undermine the
structural stability of the subsurface.
Establish a buffer zone around karst features that are not used for infiltration - areas of
historic or active sinkholes or surface depressions and related geologic features such as
fracture zones and faults - grading water away from these features. Establish filter berms
(with gabions or vegetation, for example), etc. to prevent contamination from overland flow
and discourage access to these areas. (McCann & Smoot, 1999)
Designate aquifer recharge areas. Promote safe infiltration. Direct recharge into
groundwater aquifers without proper filtration of sediments and pollutants is prohibited.
sinkholes may be utilized as injection wells, but must be properly constructed.
Casing must be firmly seated into competent bedrock and grouted into place. Sediment and
pollution controls must be incorporated. EPA categorizes these structures as Class 5
Underground Injection Wells. With adequate planning and design, these infiltration structures
can be used successfully in karst areas (McCann & Smoot, 1999, case studies). A permit
from EPA must be obtained to construct and operate a Class 5 Underground Injection Well.
Replicate natural hydrologic loading rates as much as possible when designing infiltration
BMPs. Minimize impervious surfaces. Drastically increasing or decreasing the loading rate
may promote or accelerate sinkhole development. (Loading rate is the ratio of drainage area
to infiltration area.)
Avoid Concentrating Water.
Employ methods to reduce runoff volumes and velocity.
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Implement numerous infiltration BMPs throughout the site instead of just one.
Stormwater should not be conveyed into concentrated runoff flow paths. Broad and shallow
flow dispersion is most effective. Minimizing impervious surfaces should aid in decreasing
runoff, in general. (Virginia DCR)
Impounded water causes soil saturation and loss of cohesion, and produces stress from the
weight of the water. Differences in hydraulic head and steep hydraulic gradients can result in
sinkhole development. For these reasons, shallow basins with overflow channels are
preferred over one large, deep basin. Basins, if they must be used, must have synthetic
liners to prevent failure and sudden loss of water into a subsurface drain.
Diligence and site maintenance can influence the ultimate success of the stormwater plan.
Seal all exploratory boreholes to eliminate surface water entry.
Minimize earth disturbance when installing stormwater structures. Disturbing the upper,
cohesive soils can lead to subsidence and future collapses. (Newton, 1987)
Management of stormwater structures usually ends after construction. In karst, however,
BMPs need to be inspected, cleaned, maintained, and possibly repaired. Sinkholes should
be promptly and properly repaired. Inspection and maintenance schedules must be
addressed in the plans.
Pay specific attention to the integrity of piping of all types. Evidence of pipe leakage or
sagging should be immediately addressed because these areas quickly become the focus for
soil loss into subsurface voids that leads to subsidence and sinkhole collapse.
All stormwater management designs for karst areas must include details for sinkhole repair
during and after construction. The sinkhole repair plan should appear on the construction
drawings and also be made a part of the site’s Operation and Maintenance Plan. The
sinkhole repair plan should be flexible to accommodate a variety of failure modes and
locations. A qualified individual should oversee the repair work.
7.4.4 BMP Considerations
The conventional stormwater BMPs presented for traditional development activities are generally
applicable and effective in karst areas. However, these are not necessarily the
effective or
appropriate. (McCann & Smoot, 1999) (Virginia DCR) The following are some conventional
examples of karst area BMPs:
Increased storage
Dry detention pond
Wet retention with lined settling ponds
Shallow detention ponds
Vegetated Roof
Increased infiltration
Runoff spreaders
Porous pavement
Improved sinkholes / Class V injection well (See Crawford, 1989, Chapter 3)
Perforated pipes
Bioretention cells / rain gardens
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