Evaluation of Flow Fields in Wetlands Using Physical Models

Arizona State University; Tempe AZ.
May 2002

Report #49 - Executive Summary -

The main objective of this project was to evaluate fluid flow in different wetland designs under controlled laboratory conditions to determine the relationships between design parameters and flow characteristics.

A physical model of a wetland to assess flow fields as a function of wetland design parameters was constructed at a scale of 20:1. The model was scaled and tested to simulate field conditions. The model was capable of testing different flow rates, emergent zone to depth zone transitional slopes, and different emergent or deep zone lengths. Flow patterns were visualized using particle-tracking velocimetry, which provided information on salient flow structures. Simultaneously with particle-tracking, the technique of laser-induced fluorescence was used to observe and quantify the mixing in various zones of flow. A series of tests were done under conditions scaled for comparison to field-scale tracer studies. Several series of tests were performed to systematically evaluate the impacts of wetland design parameters on flow fields.

The results demonstrated that establishment of recirculation in deep zones was a function of both the water depth and the slope of the transition between the vegetative zone and the deep zone. The deep zone width and deep zone length were not critical to establish recirculation. A relationship was developed to determine requirements for transitional slope and water depth to establish recirculation. All field-scale testing was done under conditions where recirculation should have been established. Consequently, analysis of field-scale tracer tests yielded a number of mixed tanks that were always equal to or greater than the number of deep zones. From macroscopic analysis, each deep zone appeared to behave as a completely mixed system consistent with established recirculation.

The lateral diffusivity created by flow through a vegetated zone does not appear to influence longitudinal dispersion in basin H1 where five deep zones were present. The longitudinal dispersion in basin H1was independent of the presence of vegetation. Other basins contained longer vegetated zones, and higher dispersion numbers were observed in these basins. This might imply that lateral diffusivity becomes more important as the vegetative zone length increases and the numbers of deep zones decreases. The presence or absence of vegetation did not affect water quality improvement in basins H1 and H2. The main effect of flow fields on the performance of wetland systems is the establishment of recirculation in deep zones. This redistributes the flow prior ro entrance into a subsequent vegetative zone and maintains residence times in the systems that are not close to theoretical.