Observations of short-circuiting flow paths within a free-surface wetland in Augusta, Georgia, U.S.A.

Lightbody, Anne F., Margaret E. Avener, Heidi M. Nepf

Limnol. Oceanogr., 53(3), 2008, 1040-1053 | DOI: 10.4319/lo.2008.53.3.1040

ABSTRACT: Velocity heterogeneity is characteristic of wetland systems and results in some influent water remaining in the wetland for less than the expected residence time on the basis of volume and flow rate. This phenomenon, known as short-circuiting, alters the distribution of the chemical and biological transformations that occur within the wetland. Field observations in a 1.5-km2 constructed treatment wetland in Augusta, Georgia were used to quantify the size, distribution, velocity, and transport potential of fast flow paths, which cause short-circuiting within wetlands. The flow paths were identified by a tracer study and velocity measurements. In each of the three cells examined, between three and six fast flow paths were found, most less than 4 m wide. These flow paths had an area-averaged velocity on the order of 1 cm s-1, at least 10 times the velocity observed where water passed through vegetation. Within different cells in this wetland, 20-70% of the flow had a residence time less than one-eighth of the nominal residence time. With this degree of short-circuiting, uniform flow is a poor approximation for the flow through the wetland. In addition, tracer studies were used to make direct measurements of mixing within open-water deep zones. Lateral mixing was sensitive to the direction of the wind. The average daily maximum temperature in the densely vegetated slow-flow zones was 2.0ºC cooler than that at the surface of the open water zones and 0.9uC cooler than that of the fast-flow zones. The importance of short-circuiting flow paths in this relatively simple wetland suggests that this phenomenon is likely of even greater significance in natural wetlands because they are typically much more complex.

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