Modeled Flow Duration Variations, Pollutant Discharges, and Costs for Different Stormwater Controls

Runoff volume and pollutant discharges increase with development, with associated detrimental receiving water effects. These increases can be partially controlled by installing stormwater control practices, such as wet detention pond at outfalls, using conservation design controls such as grass swales and bioretention devices, and by improved development practices that reduce the amounts of impervious areas. The volume of runoff and the pollutants associated with the different source areas within a watershed can be used to identify the most likely suitable stormwater controls for the area. The reductions in runoff volume and pollutant discharges, and the costs associated with installing these control practices, are presented in this paper for an example 228 acre watershed located in Jefferson County, AL. This site consists of 75% commercial lands and 25% residential lands. The Source Loading and Management Model for Windows (WinSLAMM) was used to calculate the reduction of these pollutants and runoff volume, the associated variations in flow durations, and the costs involved with retrofitting different combinations of a wet detention pond, grass swales, and bioretention devices in the example watershed.

It is well known that the volume of runoff from a watershed increases with development because of the increase in the amount of impervious areas that prevents the infiltration of rainwater. This increased runoff volume, and associated peak flows, is a common cause of increased streambank erosion and other problems in receiving waters. An effective combination of stormwater management and site development practices can be used to reduce peak flows and water volume and pollutant discharges, with subsequent benefits to the receiving waters. Stormwater controls can include wet detention ponds, bioretention facilities, and grass swales, while development characteristics include the amount of impervious cover and how they are connected to the drainage system. The stormwater controls add extra costs to the development costs. Costs must consider their design and construction costs, plus maintenance costs. The magnitude of these costs are dependent on a number of complex factors including local site conditions, site topography, time of year, accessibility to equipment, economies of scale, type of control measure, existing and proposed future land uses, environmental considerations, government regulations, public preferences, and degree of technical assistance available. However, some of the stormwater controls (those that reduce the peak discharge rates during critical design storms) can also reduce the costs of other components of the conventional drainage system. This presentation will discuss how runoff flow-duration, pollutant discharges, and costs, can be compared for different development scenarios using recent modifications made to the Source Loading and Management Model, WinSLAMM (Pitt 1986; Pitt and Voorhees 2002).

A number of local watersheds are being monitored by the Storm Water Management Authority (SWMA) of Jefferson County, AL, as part of their NPDES stormwater permit. Table 1 lists five of these sites and their calculated annual average volumetric runoff coefficients, TSS concentrations, percent impervious values, and the expected biological conditions of the receiving waters due to expected hydromodifications of the receiving waters from the land development. The expected biological conditions of the receiving waters were calculated by WinSLAMM to be “poor.” It is interesting to note that the highly impervious watersheds (ALJC001 and ALJC012), which have mainly industrial and commercial land use respectively, have higher values of Rv (~0.6) but lower values of TSS concentrations, compared to the watersheds dominated by residential land uses (ALJC009 and ALJC010). The residential watersheds are closer to the threshold between fair and poor biological conditions (an Rv of about 0.25) than the industrial and commercial watersheds. These biological conditions in the nearby receiving waters have been verified by biologists from the Jefferson County Storm Water Management Authority during their stream investigations. It is therefore possible that stormwater controls that reduce the runoff discharges could be effective in improving receiving water biological conditions in these residential areas, but it would be much more difficult in the industrial and commercial watersheds, as expected.

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