A stochastic approach to designing wetlands for stormwater pollution control

Abstract

Central to wetland design for stormwater pollution management is the proper control of the hydrologic regime of these wetlands and wet detention basins. The paper proposes a stochastic approach in determining the appropriate wetland size for stormwater treatment in recognition of the inherent variability of stormwater runoff and pollutant load generation within a storm event and between storm event. The determination of wetland size would include the use of stochastic generation of stormwater runoff and continuous wetland behaviour simulation. A typical design chart summarising the interaction between wetland size, prescribed retention period of stormwater and wetland hydrologic effectiveness for Melbourne is presented. The chart enables the selection of the appropriate storage volume and outlet characteristics on the basis of long-term average effectiveness of the wetland to retain stormwater over a desired minimum period rather than a prescribed level of performance for an individual probablistic event.

How hydrological and hydraulic conditions affect performance of ponds

“…In some cases this is impossible due to lack of space, or unnecessary due to small amplitudes of flow peaks, but it should at least be considered. The term "hydrological effectiveness", introduced by Wong and Somes (1995), describes this aspect, and is defined as the ratio between the volume entering the pond and the total volume of runoff. The second aspect is to capture and retain flow peaks, which demands a retention volume.…”

The hydraulic performance of ponds of various layouts

“…Hydraulic eciency can be described as how well the incoming water distributes within the pond (Wong & Somes, 1995). All of the design models in current use assume uniform¯ow conditions and unrestricted opportunities for contact between the incoming water and the organisms responsible for treatment.…”

A Randomization Process for Modeling Constructed Wetlands with an Optimization Example

“…Several authors have indicated that nutrient removal performance is governed by hydraulic efficiency (how well the water distributes throughout the system) with optimum efficiency described as a uniform velocity profile (e.g. plug flow; Wong and Somes 1995;Walker 1998;Persson et al 1999;Persson 2000). Walker (1998) studied wetland basins with various length to width ratios and concluded that basins with low length to width rations (0.5:1) resulted in poor residence time and basins with length to width ratio greater than 4:1 resulted in performance similar to plug flow.…”