How Wildfires Alter Sediment on Retention Structures

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Authors:

Abstract:

On June 12th 2022, the Pipeline Fire ignited on the San Francisco Peaks near Flagstaff, AZ, burning 23 steep watersheds, almost all of which previously burned in the 2010 Schultz Fire. Monsoon rains following the 2010 Schultz Fire produced significant postfire floods and debris flows, prompting construction of novel flood mitigation structures in 4 of the burned watersheds. Damaging postfire flows following the Pipeline Fire prompted construction of mitigation structures in 5 more watersheds between spring 2023 and summer 2024. These mitigation structures involve filling in recently incised channels to create a wide alluvial plain where flows lose confinement. This promotes decreases in flow velocity, water infiltration, and deposition of coarse sediment. Reducing the sediment carried by the flows could help reduce flood and debris flow hazards in the downstream community. The purpose of this study is to quantify changes in grain size distribution after flow events on these mitigation structures to gain insight into how they respond to different flood and debris flow events. To accomplish this, we collected sediment samples from the Lenox watershed throughout the 2024 monsoon season. The sediments were sieved to separate them into 6 size classes from 2 mm to less than 63um. We also characterized the coarse fraction through pebble counts. We present preliminary grain size distributions and attempt to relate changes in grain size over time to the type and magnitude of flows.