Index scoring tool for rating BMP performance released

SCCWRP has developed a scoring tool that quantifies the degree to which structural BMPs (best management practices) contribute to improving the health of downstream aquatic ecosystems – a novel method for rating the performance of existing BMPs that has the potential to reduce reliance on less insightful, legacy evaluation approaches.

The Multi-Metric BMP Performance Index, which was publicly unveiled in April, generates a quantitative performance score that provides insights into whether individual BMPs are performing at levels that contribute to meeting watershed-specific water-quality goals, or whether further management action is needed for the BMPs to perform at these levels.

The index’s assessment approach is unique in that traditional BMP performance assessments tend to focus on whether BMPs are meeting static expectations as specified in BMP design manuals and TMDLs, and/or meeting performance targets established by regulators – but not necessarily whether the BMP is essentially “pulling its own weight” in protecting the downstream ecosystem.

Researchers’ ultimate goal is to help managers conduct more insightful, quantitative BMP performance evaluations than are currently possible, ensuring managers gain actionable information that helps maximize the value of BMPs already built, as well as better plan for future BMPs and runoff water-quality management strategies.

Already, some stormwater management agencies have started beta-testing the Multi-Metric BMP Performance Index, including the Riverside County Flood Control and Water Conservation District. SCCWRP is preparing to test-drive the index with additional stormwater agencies this summer.

In Southern California, stormwater managers have spent billions of dollars implementing and managing bioswales, bioretention systems, infiltration galleries and other types of structural stormwater BMPs. But they have had limited access to rigorous tools for determining whether the BMPs are performing as intended.

Existing BMP performance evaluations don’t focus on estimating how a BMP influences aquatic health, ecosystem services and other beneficial uses, even though protection of these beneficial uses is the primary reason that managers invest in BMPs.

Instead, managers generally focus on whether a BMP has been built according to specifications and standards – either spelled out in a BMP design manual or in regulatory objectives. These types of evaluations offer comparatively little insight into BMPs’ role in protecting downstream ecosystems. Performance can vary widely from one BMP to another, even when they’re the same BMP type, and from one storm to another – a consequence of the wide range of ways that BMPs are designed, built and maintained, plus the wide range of levels and types of pollutants that BMPs treat.

For example, through a traditional BMP performance evaluation, a stormwater manager might determine that the BMP is reducing pollution loading by a certain percentage. While this pollution removal figure might be deemed adequate for complying with a regulatory objective or management program, this assessment would not shed light on the central question of whether the BMP’s design, construction and maintenance enable it to adequately contribute to restoring or protecting the downstream beneficial uses.

Researchers’ breakthrough with the Multi-Metric BMP Performance Index was developing a robust method for quantifying two discrete aspects of performance for an individual BMP:

» The water-quality assessment benchmarks field-collected pollution removal data for the BMP with water-quality objectives associated with the watershed where the BMP is located. The goal of this assessment is to determine if the BMP is contributing adequately to achieving downstream water-quality improvement goals. Data from existing BMPs also can be used to support planning and management strategies for other watersheds; the user simply benchmarks by the locally relevant water quality objectives, i.e., as if the same BMP were being used to achieve different water quality goals.

» The hydrology assessment benchmarks field-collected data on the runoff volumes being retained by the BMP with corresponding design specifications associated with that particular BMP, including the BMP’s specific dimensions and water capture estimates. The goal of this assessment is to determine if the BMP is capturing runoff as intended by planning and design. Data from existing BMPs also can be used to help plan future BMPs – specifically, by identifying beneficial design features and/or systematic design or maintenance condition issues.

Because mangers are often concerned about multiple pollutants at the same time (e.g., BMP sites with downstream TMDLs for multiple contaminants), a water-quality index score can be generated for each individual pollutant for which monitoring data are available.

The index uses a two-step process to generate an index score for each individual pollutant: First, benchmarked monitoring data are binned into categories. Then, these categories are consolidated using a weighted averaging approach to generate the final index score for that pollutant. A similar two-step process (benchmarking, binning, and applying a weighted average) is used to generate the hydrology index score.

Final scores for individual pollutant or hydrology performance are associated with performance categories that, in turn, are linked to a specific set of recommended follow-up management actions. Recommended actions range from “take remedial action” to “check data” to “take no action.” Examples of specific remedial actions include: “inspect for maintenance condition,” “investigate design/construction,” and “additional treatment needed.”

A key advantage of the Multi-Metric BMP Performance Index is that managers can evaluate water quality index scores for any number of individual pollutants, or can combine them together and also include the hydrology assessment as part of an overall performance index assessment.

The tool’s BMP-specific approach to benchmarking performance enables apples-to-apples comparisons between BMPs of the same type, as well as across BMPs of different types (i.e., because each individual BMP is assessed relative to its own design specifications or intentions).

An initial version of the BMP performance index tool was originally developed last year in partnership with the Los Angeles County Department of Public Works. But given the tool’s widespread applicability across Southern California and beyond, SCCWRP has been focusing in recent months on adapting and expanding the tool for broader use.

The Multi-Metric BMP Performance Index is publicly accessible via this ShinyApps web portal.

For more information, contact Dr. Elizabeth Fassman-Beck.