Deputy Director Southern California Coastal Water Research Project
New, unbiased and transferable metrics based on quantitative monitoring data have been developed to evaluate the hydrologic performance of stormwater control measures (SCMs). Hydrologic performance evaluation is essential to measure success towards minimizing combined sewer overflows or where infiltration is a feasible end-result for stormwater quality management, amongst other scenarios. The proposed data-driven metrics enable post-construction performance comparisons between BMPs of different types, or amongst BMPs of the same type, while deconstructing metrics provide information to support maintenance and planning. The primary concept behind the proposed metrics is to evaluate performance relative to the assumptions and calculations used to design the SCM, i.e., to benchmark performance against expectations for its installation at a specific location. Hydrology serves as the basis for sizing most SCMs. Ratios of monitored storm size to the design storm, and retained runoff volume to design volume create categorical distinctions for objectively determining success or failure. Likewise, ratios of measured rainfall intensity to design rainfall intensity and peak discharge experienced to allowable discharge are appropriate benchmarks for SCMs with treated discharge. Because the performance metrics couple monitoring data with design assumptions, deconstructing the metrics reveals information to support maintenance, watershed planning, or the design or construction itself. For example, bypass is expected to occur during storms in excess of the design storm; its repeated occurrence under other storm conditions may indicate a maintenance problem, substantive changes in drainage patterns in the watershed, or illicit connections. SCMs that routinely do not manage as much runoff as expected may indicate that runoff is not routed correctly to the SCM or the SCM was built smaller than intended. Conversely, SCMs that consistently manage more runoff than designed may be overdesigned, causing unnecessary expense. Data primarily from the International Stormwater BMP Database are used to demonstrate the proposed hydrologic performance metrics.
