Estimating swale performance in volume reduction

How much water can swales infiltrate? How can we accurately measure infiltration? From our study, we have found that in swales and other stormwater BMPs, sufficient measurements of infiltration parameters must be made to compensate for the large spatial variation in infiltration rates. The infiltration measurement involves estimating saturated hydraulic conductivity (K_sat) of the soil and soil suction. Individually measured K_sat values can vary by a factor of 100, indicating that large uncertainty can exist with the use of only one measurement. From our study, we found that 20 measurements at each swale can reduce the uncertainty of K_sat around the geometric mean to a factor of between 2 and 4. By using these infiltration measurements, one can estimate the amount of infiltration loss of a swale for a given design storm. A computational model has been developed to estimate the infiltration of roadway stormwater runoff into the swales for given rainfall intensities. This model is based on the Green-Ampt assumptions and takes into consideration the antecedent moisture condition of the soil, saturated hydraulic conductivity and soil suction the soil, and the geometry of the channel. Using this model, the volume reduction of a typical swale for six different types of soil and both wet and dry condition is calculated for two design storms. It was found that swales can infiltrate 22 to 100% of stormwater runoff volume depending on the soil type (clay to sand).