1. Bridges
      2. 7.3.4 Dirt and Gravel Roads
      3. 7.4 Karst Areas
      4. 7.4.1 The Nature of Karst

Pennsylvania Stormwater Best Management Practices Manual
Chapter 7
In new development, the roads and driveways often comprise the greatest amount of impervious
area, sometimes as much as 70% of the total impervious area. Techniques that seek to manage
the roadway runoff where it is generated, and reduce piping and conveyance of stormwater,
should be implemented to the greatest extent possible.
Grit and oil removal BMP’s should be considered for addressing stormwater discharging from
scuppers serving bridge decks. If the inclusion of grit and oil removal BMP’s is not feasible due to
design constraints, more frequent “street cleaning” of the bridge deck should be made part of the
project’s Operation and Maintenance plan.
7.3.4 Dirt and Gravel Roads
A significant portion of the state is served by unpaved roadways constructed of various types of
gravel base, constructed over time and with locally available materials. While not constructed
with AC impervious pavement, theses roadways serve as stormwater conveyance pathways,
creating significant erosion in the process and requiring constant maintenance to restore
Pennsylvania has over 18,000 miles of unpaved roads. These roads consist of dirt and or gravel,
and have historically been undermaintained compared to paved roads. These roads are
frequently a source of pollution to streams and rivers in a drainage area, especially for sediment.
This pollution occurs as precipitation carries sediment eroded from these roads and adjacent
banks along the road surface and into open water. Statewide, while runoff from these roads is
not the major source of pollution in streams, close proximity of rural roads to high quality streams
is common, and these roads often parallel streams and discharge directly into them. Others have
culverts that convey large amounts of water before discharging at high rates, following long
downhill grades to a stream crossing. Adequate drainage is essential to the longevity of these
roads, but environmentally sensitive practices for discharge of this drainage will benefit the health
of the surrounding environment.
Pennsylvania’s Dirt & Gravel Road Pollution Prevention Program was formed in 1997 to “fund
environmentally sound maintenance of unpaved roadways that have been identified as sources of
dust and sediment pollution.” This program strives to reduce erosion, sediment, and dust
pollution by using improved maintenance techniques that benefit both dirt and gravel roads and
the environment. This program is centered on using local control as a method of stopping
pollution. To date, at least 1400 projects have been completed under this program, and over
3,500 people have participated in the program’s two day “Environmentally Sensitive
Maintenance.” Training course. Eligible dirt and gravel road sections are those identified by
County Conservation District personnel as having a sediment source from the road polluting a
Program initiatives include identifying and replacing pipes running beneath unpaved roads that
are undersized and contribute to “ponding” on the road. The program also has developed a GIS
(Geographical Information Systems) database, which tracks the location and status of all the dirt
and gravel roads in PA, and allows the local entities to submit electronic reports directly to the
State Conservation Commission. In 2000, data from over 17,000 miles of unpaved roads was
compiled and resulted in over 11,000 verified pollution sites found. In addition to this, the
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Pennsylvania Stormwater Best Management Practices Manual
Chapter 7
program is undergoing an aggregate study in Centre County, PA to determine the most
economical and durable stone for gravel roads.
Local Municipalities and state agencies have jurisdiction of over 90% of dirt and gravel roads, and
because the cost of paving these roads is often too high for the road owner, there are several
best maintenance practices that can be employed to maintain an unpaved road in an
environmentally sensitive manner. Recommendations include:
Working with the natural landscape in the design of roads (minimize cut and fill)
Identifying existing drainage patterns and designing to minimize disturbance
Crowning the road to drain the water away from the center
Using graders with scarified blades as preferred equipment to reshape a road
Sizing roadside ditches appropriately and outletting appropriately within an infiltration
Driving Surface Aggregate mix should have increased abrasion resistance, be angular on
the surface with increased fines to provide stability and facilitate compaction (stone quality
Vegetating roadside banks to prevent erosion
Using snowplow shoes when clearing snow and re-shaping the road after snow season
Preserving soil stabilizing vegetation in ditches and observing appropriate roadside
vegetation management practices along road corridors
Limiting driving speeds
Reduced road maintenance costs (grading, regrading, & re-graveling), and reduced
sedimentation in water affecting aquatic life and drinking water reservoirs, should result from the
implementing these measures, and are consistent with the various BMPs discussed in this
manual. A detailed listing of technical bulletins and further information on “Environmentally
Sensitive Maintenance” practices for dirt and gravel roads is available from the Center for Dirt and
Gravel Road Studies at Penn State University (www.dirtandgravelroads.org).
7.4 Karst Areas
7.4.1 The Nature of Karst
Surface-Water Interaction:
Water is a key to sinkhole collapses. Taking water away from where
it was or putting a new, concentrated source of water where it wasn’t before can speed the
development of sinkholes. Examples of new sources of water could be drainage from rain gutters,
pavement, collection ditches and ponds. Treatment basins or lagoons must be diligently lined in
karst to prevent a sudden drainage out of the bottom and into the groundwater. Leaky water and
sewer pipes can cause the soil underneath to wash away and are often the trigger for sinkholes.
However, an existing sinkhole under a pipe can cause the initial leak. The greater the volume of
water and the faster it moves into the karst system, the more soft material is washed from the
voids. Weather events can also trigger sinkholes. In Pennsylvania, sinkholes can “pop” when a
heavy rain event comes after a prolonged drought.
Karst areas present problems to those attempting to work with conventional hydrologic models.
Typically, modeling of a karst site or watershed via SCS or other traditional methods provides
poor representation of runoff rates, with regard to both flooding and over-design of conduits and
stormwater management facilities. This is largely because standard hydrologic modeling methods
lack allowances for losses into sinkholes, fractures, crevices or caves that may exist in the
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