Evaluation of Stormwater Control Measure Performance Uncertainty

Abstract

The percentage removal metric is widely used in engineering practice for stormwater control measure (SCM) selection and design, because it has been adopted by regulatory agencies and because it is straightforward to use for load reduction calculations. Various studies have shown that the metric is poorly suited to the episodic and stochastic nature of stormwater flows, but the extent of uncertainty introduced by the use of the metric has not been quantified. This study evaluated SCM performance uncertainty for total phosphorus (TP), total nitrogen (TN), and total suspended solids (TSS) load reduction in 14 regulator-defined SCM categories, based on data from 176 SCM monitoring studies aggregated using a subjective weighting scheme. Uncertainty was characterized according to the cumulative distribution of percentage removal values from these studies, and defined as the probability that SCMs will perform equal to or below credit values. SCM uncertainty based on these probabilities ranged from 0 to 0.95 and was generally between 0.31 and 0.84, although data were limited for several SCM categories. Probabilities that percentage load reduction was less than or equal to zero ranged from 0 to 0.61, and were generally between 0 and 0.30 across all SCM categories. Probabilities that TSS removal was less than or equal to 80% were generally between 0.35 and 0.77.