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Fairfax County faces many challenges in controlling
the quantity and quality of stormwater runoff. The way stormwater
is managed has a direct affect on the health and stability of
streams, the primary receptacles of stormwater runoff. Land
development brings with it more impervious (nonporous) surface
which increases the amount of runoff. Despite the County’s
best efforts to protect the natural environment from the effects
of development, our streams still suffer from excess erosion,
sedimentation, and pollution. One way of controlling increased
runoff is to build stormwater management facilities.
Stormwater management (SWM) in Fairfax
County typically involves ponds. A pond intercepts the runoff
before it reaches a stream. The term “pond” may
confuse those of us not in the business of stormwater management
because the term conjures up an image of a permanent pool of
water. However, a stormwater management pond can be either wet
or dry. A wet pond is exactly that. It is a basin or
depression that retains, or holds, water in a permanent pool.
While the term “dry pond” sounds like an oxymoron,
it refers to a basin or depression that detains, or slows, the
flow of water for short periods of time and is dry between storm
events. Wet ponds are often aesthetically pleasing to the eye
and may provide recreational opportunities. Dry ponds may look
less attractive or go completely unnoticed in the landscape.
Whether wet or dry, SWM ponds serve an important purpose.
They control the volume of runoff by releasing it over time.
Every pond has a pipe outlet. The outlet is generally sized
to release water over a 2-3 hour period in a heavy storm and
less time or none at all in light precipitation. If an increase
in runoff is not controlled, it may cause downstream flooding
and stream bed and bank erosion.
Some SWM ponds control not only the quantity
of runoff but also the quality of runoff. In such cases, the
SWM ponds are called BMP ponds. BMPs, or best management practices,
are techniques to manage runoff in ways that reduce water pollution.
In a BMP pond, a flow regulator is attached to the end of the
pipe to reduce the size of the outlet. A smaller outlet forces
the pond to hold the water for a longer period, allowing more
time for the sediment and attached nutrients to settle out.
Whereas a conventional SWM pond will release stormwater over
2-3 hours, a stormwater management BMP pond may release the
water over 2-3 day days.
What must an SWM pond accomplish? State
regulations require that the volume of post-development (after
construction) runoff must be equal to or less than the volume
of pre-development (before construction) runoff as measured
against a unit of time. In other words, a property downstream
of a new development must not see more runoff per second than
it did before the development took place. A properly functioning
SWM pond spreads out the flow of runoff over time to control
flooding and erosion.
In Fairfax County there are 863 SWM ponds
maintained by the Department of Public Works and Environmental
Services (DPWES), of which 317 are BMP ponds. The efficiency
of the stormwater management system is dependent on how well
the ponds are maintained.
The County expends a great deal of human and
financial resources to maintain the ponds. But even with a high
quality maintenance program, some failures still occur. Replacing
a failed dam can cost in excess of $300,000.
SWM ponds fail for a number of reasons.
The condition of the dam, which holds the water in the pond,
is one cause of pond failure. As water from the pond seeps
into the earthen dam, the dam, if not constructed properly,
could weaken to the point of washing out. The soil used to construct
the dam must be properly selected and compacted so that it will
remain in place while the pond functions. Special care must
be taken to ensure that the pipe passing through the dam does
not leak or allow seeping water to erode the soil around the
pipe. Water will always take the path of least resistance, meaning
that it will find its own pathways through a poorly constructed
dam.
In 1995 the County adopted improved dam
standards that address the seepage and leak problems. Prior
to 1995, the pieces of pipe going through the dam were fitted
together without a seal. Today, all pipe joints must have a
rubber gasket to prevent leaks. In addition, pipes must sit
on a concrete cradle to eliminate any gaps below or on the sides
of the pipe. Around the pipe and cradle there must be gravel
which is wrapped in a filter material. This forms a drainage
blanket. Any water that does seep through the dam will pass
through the drainage blanket. A perforated pipe from the drainage
blanket carries the filtered water to the stream.
Another problem affecting the function
of ponds is the condition of the trash rack. In both dry
and wet ponds, there is a screening device attached to the outlet
which prevents trash and other debris from leaving the pond
and entering the stream. If the trash rack gets so clogged with
litter, leaves, and grass clippings that the water cannot freely
pass through it, then the outlet will not function adequately.
As a result, the water will back up behind the dam. Since the
dams on dry ponds were not designed to hold back a pool of water
for an extended period, the dam may wash out.
Not all SWM ponds are maintained by the County.
There are some that are privately maintained by homeowner associations
or corporate entities. A different type, but no less important,
of a challenge for DPWES stems from the role that property owners
play in the operation and maintenance of these SWM ponds.
For more information about stormwater management
ponds, contact the DPWES Maintenance and Stormwater Management
Division at 703-934-2860.
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