Water Resources Engineer Craftwater Engineering, Inc.
Southern California MS4 permits introduced Watershed Management Programs (WMPs), an alternative compliance pathway for agencies that can provide deemed compliance for water quality priorities, if approved and properly implemented. WMP development is informed by a Reasonable Assurance Analysis (RAA), which uses modeling and analysis to predict the performance of control measures in meeting water quality goals. In 2021, Ventura County was incorporated into this permit type and recently developed WMPs for the Calleguas Creek, Malibu Creek, Santa Clara River, Ventura River, and Ventura County Coastal watersheds. In addition to MS4-responsible areas, these watersheds contain agricultural and open space influences that impact receiving water quality; thus, strategic planning was needed to identify MS4 responsibilities to address stormwater pollutants. Recognizing the lack of dedicated stormwater funding, the WMPs were developed using the following framework to identify direct solutions to water quality issues: 1) use watershed science to refine water quality priorities, 2) focus on MS4-specific load reductions, 3) leverage enhancements to existing minimum control measures, and 4) identify feasible project-led compliance pathways. Key RAA considerations, including what pollutants are evaluated, where load reduction targets are assessed, and when numeric targets are assessed, were explored. Understanding these factors, which significantly influence water quality outcomes and the amount, type, and costs of subsequent control measures, was key for agencies to defensibly show compliance with activities most effectively utilizing their resources. Acknowledging that WMP development may be incorporated in more permits, this presentation will share best practices for conducting RAAs and implications on WMP recommendations.
