Characterization of Performance Predictors and Evaluation of Mowing Practices in Biofiltration Swales

Abstract

General site maintenance is important for keeping a well functioning swale; however, it is not always easy to determine how well a site is currently functioning. Swale performance can be evaluated by measuring its hydraulic residence time (HRT), because a HRT of at least nine minutes has been found to be most effective at removing stormwater pollutants (Metro, 1992). Yet, determining the performance of an existing facility based on its HRT is also not easily accomplished. The first part of this study, therefore, attempts to develop an easy method for evaluating performance by relating easily measured swale characteristics to HRT. The second part of this study is intended to remedy the lack of information on stormwater maintenance practices by examining and quantifying the benefit of mowing on pollutant removal in swales. This study also explored whether mowing more than once during the growing season produces better removal efficiencies. Determining the effect of mowing on swale water quality can help guide planning for future maintenance needs and potentially save money that could be used to address other maintenance concerns. Identifying the practices and characteristics that influence biofiltration swale performance is important to modifying current practices to alternatives that could provide greater benefit. Determining the real value, if any, of mowing for vegetation cover and pollutant removal could save much time, money, and effort without compromising swale performance. In addition, being able to quickly evaluate a swale’s current performance based on a few readily measurable features could focus retrofitting efforts on the sites that are likely performing the worst.