Sediment removal is typically the major purpose of wetlands designed for treatment of urban stormwater flow from parking lots, streets, and landscapes. In essence, the wetland is a stormwater retention basin with vegetation, and the design uses many of the basic principles of sedimentation basin design. The presence of vegetation fringes, deep and shallow water zones, and marsh segments enhances both the treatment and habitat functions. These wetlands have been shown to provide beneficial responses for BOD, TSS, pH, nitrates, phosphates, and trace metals (Ferlow, 1993).
At a minimum, a stormwater wetland system (SWS) will usually have some combination of deep ponds and shallow marshes. In addition, wet meadows and shrub areas can also be used. Because the flow rate is highly variable and the potential exists for accumulation and clogging with inorganic solids the SSF wetland concept is not practical for this application, so the marsh component in the SWS system will typically be FWS constructed wetlands. These may be configured as shown in Figure 6.5 or in alternative combinations. Key components include an inlet structure, a ditch or basin for initial sedimentation, a spreader swale or weir to distribute the flow laterally if a wet meadow or marsh is the next component, a deep pond, and some type of outlet device that permits overflow conditions during peak storm events and allows slow discharge to the "datum" water level in the system. The "datum" water level is usually established to maintain a shallow water depth in the marsh components. Use of drought-resistant plant species in the marsh components would permit complete dewatering for extended periods.
Typha, Scirpus, and Phragmites can withstand up to 3 ft (1 m) of temporary inundation, a factor that would establish the maximum water level before overflow in the SWS if these species are used. The maximum storage depth should be about 2 ft (0.6 m), if grassed wet meadows and shrubs are used. The optimum storage capacity of the wetland (the depth between the "datum" and the overflow level) should be a volume equal to 0.5 in. (13 mm) of water on the watershed contributing to the SWS. The minimum storage volume, for effective performance, should be equal to 0.25 in. (6 mm) of water on the contributing water shed. The storage volume for these, or any other depths, can be calculated with Equation 6.1:
V = Storage volume in stormwater wetland (ft3; m3). C = Coefficient = 3630 for U.S. units; 10 for metric units. y = Design depth of water on watershed (mm). Aws = Surface area of watershed (ac; ha).
The minimum surface area of the entire SWS, at the overflow elevation, is based on the flow occurring during the 5-year storm event and can be calculated with Equation 6.2:
Asws = Minimum surface area of SWS at overflow depth (ft2; m2). C = Coefficient = 180 for U.S. units; 590 for metric units. Q = Expected flow from 5-year design storm (ft3/d; m3/d).
The aspect ratio of the SWS should be close to 2:1, if possible, and the inlet should be as far as possible from the outlet (or suitable baffles can be used). The spreader swale and inlet zone should be sufficiently wide to reduce the subsequent flow velocity to 1 to 1.5 ft/s (0.3 to 0.5 m/s).
In essence, the SWS performs as a batch reactor. The water is static between storm events, and water quality will continue to improve. When a storm event occurs, the entering flow will displace some or all of the existing volume of treated water before overflow commences. It is possible, using the design models presented in previous sections, to estimate the water quality improvements that will occur under various combinations of storm events. It is necessary to first determine the frequency and intensity of storm events. These data can then be used to calculate the hydraulic retention time during and between storm events; it is then possible to determine the pollutant removal that will occur with the appropriate design model.
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