What everyone should know about Storm Water Ponds
Storm water ponds are extremely important components of your community’s drainage system. They are designed to provide two critical services:
-prevent flooding by suppressing surges of storm water runoff that wash from lawns, buildings, and paved surfaces.
-protect water quality by holding water long enough for natural processes to remove sediment and some pollutants from the water before it is discharged to nearby rivers or beaches.
Many communities with pond concerns have been mostly associated with erosion and pond banks settling and forming a shelf. There are a variety of causes of the erosion which range from mowing to close to bank edges, to the wave action created from fountains in the ponds. Shorelines of stormwater ponds are prone to erosion because there are many forces working to undermine the stability of the bank. In many areas, property lines and HOA easements are drawn relative to the pond shore. The loss of shoreline is the same as moving the easement which reduces the usable real estate for the waterfront owner. Most communities consider it the responsibility of the property owner to manage the waterfront of the property, so stopping shoreline erosion is up to the homeowner, but may be restricted by the HOA bylaws. This means that the HOA has to determine what shoreline management strategies it will allow the homeowner to use to prevent erosion of their property.
Why are our banks eroding faster than the neighboring community?
Bank Slope: The slope of the bank is the single most important factor that determines rate of erosion. Steep banks with slopes greater than 3 to 1 (3 feet of width for every foot in height) are almost certain to erode and undercut over time if they are not stabilized.
Soil Type: The consistency of the soil also determines rate of erosion. Sandy soils have less structural integrity than soils made of clay. In the coastal plain (where the vast majority of South Carolina’s stormwater ponds are located), soils consist of little or no natural rock and are often composed mostly of sand or clay.
Fountains: Surface fountains agitate the water surface, creating more wave action. In most cases wave action dissipates before it reaches the bank, but ponds with large fountains or fountains that are located near the shore are likely to experience accelerated shoreline erosion.
Nuisance Wildlife: Animals also cause erosion, especially when large numbers are confined to small ponds. Concentrations of ducks and geese are the most common destabilizers of pond banks because they eat shoreline vegetation and trample the bank. They also churn sediments near the bank as they sift for invertebrates in the mud. Another problematic animal is the muskrat because it burrows into the bank to make its den, and it consumes shoreline vegetation. Dens can collapse unexpectedly and create large holes in the bank slope.
Lack of Protection: The most common way to manage shorelines in storm water ponds is with turf grass that is mowed to the water’s edge. This strategy provides very little structural integrity to the bank. Turf grass is not an aquatic plant, and its roots do not grow well in saturated soils. Turf grass will not send roots down below the water surface where the most significant erosion is taking place. As a result, pond banks with grass mowed to the edge are almost certain to undercut with time.