The hydraulic design of constructed wetland systems is critical to their successful performance. All of the design models in current use assume uniform flow conditions and unrestricted opportunities for contact between the wastewater constituents and the organisms responsible for treatment. In the SSF wetland concept it is also necessary to ensure that subsurface flow conditions are maintained under normal circumstances for the design life of the system. These assumptions and goals can only be realized through careful attention to the hydraulic design and to proper construction methods. Flow through wetland systems must overcome the frictional resistance in the system which is imposed by the vegetation and litter layer in the FWS type and the media, plant roots, and accumulated solids in the SSF type. The energy to overcome this resistance is provided by the head differential between the inlet and the outlet of the wetland. Some of this differential can be provided by constructing the wetland with a sloping bottom; however, it is neither cost effective nor prudent to depend on just a sloping bottom for the head differential required, as the resistance to flow may increase with time but the bottom slope is fixed for the life of the system. The preferred approach is to construct the bottom with sufficient slope to ensure complete drainage when necessary and to provide an outlet that permits adjustment of the water level at the end of the wetland. This adjustment can then be used to set whatever water surface slope is required and in the lowest position used to drain the wetland. Details on these adjustable outlets can be found in a later section of this chapter.
The aspect ratio (length-to-width) selected for the wetland strongly influences the hydraulic regime and the resistance to flow in the system. In the design of some early FWS systems it was thought that a very high aspect ratio was necessary to ensure plug-flow conditions in the wetland and to avoid short-circuiting, and aspect ratios of at least 10:1 were recommended. A major problem with this approach is that the resistance to flow increases as the length of the flow path increases. A FWS system constructed in California with an aspect ratio of about 20:1 experienced overflow at the head of the wetland after a few years because of the increasing flow resistance from the accumulating vegetative litter. Aspect ratios from less than 1:1 up to about 3:1 or 4:1 are acceptable. Short-circuiting can be minimized by careful construction and maintenance of the wetland bottom, by the use of multiple cells, and by providing intermediate open-water zones for flow redistribution. These techniques are discussed in greater detail in later sections of this chapter.
In essence, a treatment wetland is a shallow body of moving water with a relatively large surface area. The hydraulic design is complicated by the fact that significant frictional resistance to flow develops because of the plants and litter in the FWS case and because of the gravel media in the SSF type. In design it is assumed that the water will move uniformly, at a predictable rate, over the entire surface area. This assumption is hydrologically complicated by the fact that precipitation, evaporation, evapotranspiration, and seepage affect the volume of water present in the wetland, the concentration of pollutants, and the HRT.
Manning's equation is generally accepted as a model for the flow of water through FWS wetland systems. The flow velocity, as described by Equation 6.12, is dependent on the depth of water, the hydraulic gradient (i.e., slope of the water surface), and the resistance to flow:
where v = Flow velocity (ft/s; m/s). n = Manning's coefficient (s/ft1/3; s/m1/3). y = Water depth (ft; m). s = Hydraulic gradient (ft/ft; m/m).
In most applications of Manning's equation, the resistance to flow occurs only on the bottom and the submerged sides of an open channel, and published values of n coefficients for various conditions are widely available in the technical literature. However, in FWS wetlands, the resistance to flow extends through the entire depth of water due to the presence of the emergent vegetation and litter. The relationship between the Manning number (n) and the resistance factor (a) is defined by Equation 6.13:
where a is the resistance factor (s-ft1/6; s-m1/6).
Reed et al. (1995) presented the following values for resistance factor a in FWS wetlands:
• Sparse, low-standing vegetation — y > 1.2 ft (0.4 m), a = 0.487 s-ft1/6, (0.4 s-m1/6)
• Moderately dense vegetation — y > 1.0 ft (0.3 m), a = 1.949 s-ft1/6, (1.6 s-m1/6)
• Very dense vegetation and litter — y < 1.0 ft (0.3 m), a = 7.795 s-ft1/6, (6.4 s-m1/6)
This range of values was experimentally confirmed by Dombeck et al. (1997). The energy required to overcome this resistance is provided by the head differential between the water surface at the inlet and outlet of the wetland. Some of this differential can be provided by constructing the wetland with a sloping bottom. The preferred approach is to construct the bottom with minimal slope that still allows complete drainage when needed and to provide outlet structures that allow adjustment of the water level to compensate for the resistance, which may increase with time. The aspect ratio (length-to-width) selected for a FWS wetland also strongly influences the hydraulic regime because the resistance to flow increases as the length increases. Reed et al. (1995) developed a model that can be used to estimate the maximum desirable length of a FWS wetland channel:
L = Maximum length of wetland cell (m). As = Design surface area of wetland (m2). y = Depth of water in the wetland (m).
m = Portion of available hydraulic gradient used to provide the necessary head (% as a decimal). a = Resistance factor (s-m1/6).
Qa = Average flow through the wetland (m3/d) = (QIN + QOUT)/2.
An initial m value between 10 to 20% is suggested for design to ensure a future reserve as a safety factor. In the general case, this model will produce an aspect ratio of 3:1 or less. The use of the average flow (QA) in Equation 6.14 compensates for the influence of precipitation, evapotranspiration, and seepage on the flow through the wetland. The design surface area (As) in Equation 6.14 is the bottom area of the wetland as determined by the pollutant removal models presented later in this chapter.
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