Storm Water Ponds


        Sediment loading from its watershed is considered the principal challenge to water quality within the Brandywine. The desire to control bank erosion and sediment runoff has generated huge amounts of good intention, endless frustration and considerable concern about legal requirements for sediment reduction in many townships located within the Brandywine drainage. Sediment runoff is exacerbated by urbanization, increases in the proportion of “impervious” land cover (e.g., roads, driveways, rooftops), and increasingly frequent soil disturbance associated with new home, business and road construction (whereas the very large amounts of sediment runoff associated with colonial farming practices in the watershed have declined more recently).
history_fig12ab.jpg

Fig. 12a–b. (a) Dry basin in West Whiteland Township, showing two smaller inflow structures and a single, larger outflow in the foreground. (b) Stormwater pond in Newlin Township, with cement inflow at the far end. Note the elongate shapes of both basins. Mowed grass on steep slopes and the absence of other vegetation are typical.


        One method of sediment control is the construction of swales, intended in part to trap sediment run–off. Such basins may be “wet” (with permanent water), “dry” (filling with water only during storm events), or “mixed” (functioning more like a wetland, and capable of sustaining “hydrophytic” vegetation characteristic of water-logged soil). The terminology has been quite inconsistent, but wet basins are usually termed “wet detention basins” (we refer to them here simply as stormwater ponds). Dry swales by contrast are often termed “retention basins”. Examples of a dry retention basin and wet pond are shown in Figure 12a–b.
        Although wet and dry basins differ, a major purpose of each is to allow the large amounts of sediment, nutrients and toxins carried by rainwater to remain within the basin, thus protecting water quality downstream. Design features are intended to slow down passage of the rainwater through the basin (increase its hydraulic residence time). These features often include large length–to–width ratios (extending the distance between inflow and outflow structures), the presence of baffles to prevent water from heading directly from inflow to outflow, the positioning of the outflow high enough on the basin slope to prevent water from smaller storm events from leaving the basin, and optimizing the cross-sectional area of the outlet pipe to regulate flow.
        Interestingly, although sediment retention is the primary goal of wet (storm water) ponds, there appears to be little monitoring of basin pond condition by the townships or county, although increasing sediment accumulation (and thus loss of water volume) over time gradually reduces basin effectiveness.
        Aesthetics are rarely considered in constructing/managing stormwater ponds, which are perhaps as distinguishable by the absence of their riparian amenities as by the presence of cement inflow and outflow structures. The engineering firm Princeton Hydro has been a regional leader in attempting to reverse the more traditional, single-minded view of storm water ponds as simply pollution control devices.