Designing Dry Swales for Stormwater Quality Improvement Using the Aberdeen Equation

Abstract

This case study presents a semiempirical method for designing water quality swales to treat stormwater runoff that is an alternative to current mostly anecdotal design approaches. Water quality swales are intended to reduce pollutant concentrations; they are not just flow conveyance systems. The design presented herein is a two-part process: (1) hydraulic design, and (2) treatment design. A hydraulic design feature unique to water quality swales includes maximum flow depths typically lower than grass height. Frequency analysis is used to estimate the water quality design storm intensity, and the design peak flow rate is estimated using the Rational method. Subsequently, Manning’s equation is used to determine the swale cross-section and slope. A relatively high roughness coefficient (n=∼0.35) is applied because the water is not intended to overtop the vegetation. This case study used the Aberdeen equation to calculate pollutant removal efficiencies if particle-size information was available. The method was applied to field-monitored swales in Auckland, New Zealand and Knightdale, North Carolina, US, and was found to accurately predict sediment capture. The conceptual approach presented here can be used to estimate reductions in total suspended solids by swales. However, the method needs to be validated with appropriate monitoring data in estimating removal of metals and other particulate-bound pollutants, but it is not applicable to the dissolved fraction of pollutants.