Maintaining Stormwater Management Ponds
(Conservation Currents, Northern Virginia Soil and Water Conservation District)
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 703-324-5500, TTY 711.