Retrofitting existing stormwater management ponds and facilities helps the County capture and treat additional volumes of rainfall runoff generated by impervious urban areas of the watershed.
Depending on the time of original construction and the regulatory standards in place at that time, older ponds were often designed to handle rainfall runoff for shorter storm events and smaller volumes of runoff than present-day ponds.
The following techniques are used to retrofit existing stormwater management ponds:
Increasing the volume of the pond
Sometimes it's possible to excavate a pond to create a larger footprint or deeper facility. More capacity to store water allows control structures to be modified to capture water from more frequently occurring events.
Modifying the outflow
Pond outflow is controlled by control structures, such as riser pipes, valves, weirs, or pipes. These structures are sized to restrict storm flows and allow only a certain volume of water to flow through at a certain rate. It is possible to change the timing and rate of flow by modifying the structure that controls the outflows, that is, by changing the size and elevation of weirs, openings, valves, and other orifices. Sometimes additional risers or low-flow control structures are added to fine-tune controls. Retrofits often allow ponded runoff to be stored for longer periods to better mimic the predevelopment flows downstream.
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Using bypass weirs/structures
Additional flow controls can be installed to control large stormwater flows within the stream channel. These controls, such as bypass weirs, allow steady, spring-fed, non-storm flows to remain in the natural channel while excess storm runoff is directed to a stormwater pond facility for water quality or quantity treatment.
Adding wetlands or a permanent pool
Shallow excavations are carved into the bottom of dry ponds to create wet micro-pools or wetlands. Most of the water drains between rain events, but these small pools remain full. In some dry ponds, a permanent pool of water is created throughout the pond by excavating or by modifying the control structure to create the wet pond. Wetlands and wet pools improve pollutant removal and create aquatic habitat.
Increasing time of travel
Older stormwater ponds did not adequately treat small, frequent storm events. They routed water quickly through stormwater facilities with concrete channels and direct flow paths to outflow structures. During its retrofit projects, DEP often removes such concrete pilot channels and carves out a winding path through the stormwater facility. This increases the time it takes for storm flows to go through the facility; it also allows more time to remove pollutants from the stormwater.
Over time, well-functioning ponds tend to collect sediment, which settles at the bottom of the ponds. Sediment reduces a pond's capacity to store water. After 10 or 20 years, many ponds require dredging. The pond is drained and allowed to dry out. Then the sediments are removed with excavators.
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