Post-Fire Debris Flows are Expected, Communities are Evacuating, and it’s Time to Sample Because That’s What We Do! Key Takeaways from Evaluating Contaminant Flux of the 2018 Holy Fire
The Holy Fire burned approximately 23,000 acres of the Cleveland National Forest in August 2018. The steep slopes and high-to-moderate burn severity of the once forested lands created all of the ingredients for post-fire debris flows. Loss of vegetation, hydrophobic soils, changes in soil erosiveness, and a greatly altered landscape stability meant that mud and debris flows were a real and imminent concern for not only the safety of the community, but the potential water quality impact to downstream waterbodies. The subsequent 2018-2019 winter storm season came with above average rainfall combined with large high-intensity storms of size and duration that have not been seen for nearly a decade. The result was damaging debris and mud flows, creating increased safety concerns, and rapid mobilization needs. For the Riverside Permittees this presented a very challenging environment to assess the water quality impacts, but hey that’s what we do!
The Riverside County Flood Control and Water Conservation District (District) undertook the coordination and oversight of the post-fire preparation and response, including development and implementation of a post-fire water quality monitoring study on behalf of the Permittees within the Santa Ana River Region of Riverside County. Due to its uniquely overlapping service area, the District serves as the Principal Permittee in the Santa Ana River Region as regulated by the municipal separate storm sewer system (MS4) NPDES Permits issued by the Santa Ana Regional Water Quality Control Board. In coordination with Alta Environmental, with feedback from the Lake Elsinore/Canyon Lake TMDL Task Force, and following the guidance included in the Southern California Stormwater Monitoring Coalition (SMC) Post-Fire Water Quality Monitoring Plan, the District developed a post-fire monitoring to assess the potential water quality impacts of the Holy Fire.
The SMC Post-Fire Water Quality Monitoring Plan is organized around three priority management questions. The District’s sampling design focused on addressing one priority management question. “How does post-fire runoff affect contaminant flux?”. The study was designed to assess contaminant concentration and flux by sampling stormwater runoff from the terminal end of burned catchments and comparing the data to reference sites. The goal was to assess the effects of the Holy Fire on the hydrologic response, sediment loads, and contribution of pollutant loads (metals, nutrients, and organic contaminants) from post-fire runoff to downstream receiving waters. In a region where storms are difficult to predict and average annual precipitation varies from 10 inches to 14 inches, monitoring implementation takes creative thinking and planning. In order to successfully target the capture of post-fire storm flow the mobilization criteria required real-time modifications of the approach.
This presentation highlights the results and findings of evaluating post-fire pollutant loading and flux. Data presented will show measurable changes between storms and in general provide insight on the impacts of storm water discharge quality from burned forestry lands. The presentation also describes the challenges of conducting post-fire monitoring in terms of mobilization, safety, and adapting to the unknowns when monitoring post-fire runoff. The presentation of this dynamic and challenging project includes dramatic videos and aerial imagery to captivate the audience and put them near the debris flows, while being far from harm’s way. With more frequent extreme weather patterns and increased fire risk throughout California, data from post-fire monitoring will be of be of great importance to agencies responsible for safety, environmental, and risk management.