After suffering through a disaster caused by the devastating July 2015 Blue Creek fire that burned 10% or 6,500 acres of a parent watershed, questions arose from the federal, state, and local emergency managers. What impact did this massive fire have on the increased runoff and sediment flow? How could it impact the thousands of residents downstream?
The Blue Creek drainage is a tributary of Mill Creek, which flows through the community of Walla Walla and includes a USACE-owned off-stream storage flood control dam, reservoir, and concrete-lined channel. An assessment was necessary to quantify the post-wildfire flood risk through a hydrology and hydraulic analyses because rain events over burned areas can produce significantly greater runoff and sediment.
The project raised the flood risk awareness of community officials and emergency planners that have the most direct contact with affected landowners at risk. The culmination of all the data collection, modelling, and analyses resulted in a straightforward, 25-page report that outlines the estimated increase in flows in the Blue Creek and downstream Mill Creek, the anticipated sediment impacts, and some possible mitigation actions. The analyses demonstrated that dozens of people that live alongside Blue Creek will be subject to the greatest risk for flash flooding. The effects of potential increased Blue Creek runoff are dampened downstream due to higher channel capacity of Mill Creek and the flood control project, so the risk is not as high for the 32,000 people that live downstream of the USACE flood control project.
The results of the assessment sent a clearer message to the county, city, and conservation district officials who have been eager to address the post-wildfire flood potential. Landowners are better prepared to take steps to mitigate risks. Likewise, the USACE will closely monitor incoming sediment into Mill Creek flood control project since this was a major concern identified from the study.
The coordination among federal, state, and local partners ensured local decision makers had a watershed-wide understanding of the increased flood risks, which will better inform their actions. Finally, the information from the study will allow the state to update the State of Washington Hazard Mitigation Plan.