Modeling the hydraulic effect of transverse deep zones on the performance of short-circuiting constructed treatment wetlands

Short-circuiting, which occurs when a large fraction of water traveling through a system exits well before the residence time, reduces the performance of constructed treatment wetlands. Recent field and laboratory observations have shown that short-circuiting within vegetated marsh areas results from narrow, continuous fast flowpaths that experience longitudinal dispersion but do not exchange fluid with slow flowing regions of dense vegetation on either side. Based on these observations, this paper develops an analytic stream tube model with dispersion to represent flow through the marsh areas of a short-circuiting constructed treatment wetland under steady flow conditions. The model also includes transverse deep zones, which rapidly dissipate fast flowpath momentum and introduce lateral mixing through wind-driven circulation. Modeled results reveal that a transverse deep zone that replaces a portion of a wetland marsh can offset the adverse impact of short-circuiting flowpaths through two separate mechanisms. First, lateral mixing can dilute the water that has traveled through a fast flowpath. Second, deep zones reduce the probability that fast flowpaths align throughout the entire wetland. The model results suggest that deep zones may improve wetland performance when properly sized and located, even when they do not directly contribute to contaminant removal.