SEDHYD-2023, Sedimentation and Hydrologic Modeling Conference

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Us Army Corp of Engineers Numerical Model Development For Post-Wildfire Flood Risk Management

Wildfire effected watersheds frequently produce significant flooding, sedimentation, and destructive non-Newtonian debris flows in response to even moderate precipitation events. This research demonstrates a post-wildfire hydrology and hydraulic numerical modeling approach, utilizing a recently developed non-Newtonian library (DebrisLib), that can be used to predict hydrology runoff sediment yields and downstream velocity and inundation conditions for representative post-wildfire flows (e.g., debris flow, ash flows, mudflows, hyperconcentrated flows). This was accomplished with U.S. Army Corp of Engineer’s (USACE) modeling software, Hydrologic Engineering Center, Hydrologic Modeling System (HEC-HMS), the two-dimensional Gridded-Surface-SubSurface-Gridded-Analysis (GSSHA) hydrology model, the two-dimensional HEC, River Analysis System (HEC-RAS), and two-dimensional Adaptive Hydraulics (AdH) numerical model. The work presented here presents a demonstration of the effectiveness of the library-based methodology, using these widely-used USACE engineering models with a comparison to Kean et al.’s datasets following the January 9, 2018 post-wildfire flooding and debris flows in Santa Barbara, California. The debris flow generating rainfall event produced extreme runoff and debris flows displacing approximately 680,000 m3 of material and sediment with observed (e.g., Kean et al., 2019) and predicted velocities of around 4.0 m/s. The hydrology modeling approach presented is a simplistic computational method that represents the current state-of-practice approach for predicting hydrology rainfall-runoff and routing following wildfires. Evaluation of the numerical models versus field data collected by Kean et al. (2019) indicate that classic Newtonian physics are inadequate for predicting post-wildfire flood runout and inundation. When the GSSHA, HEC-RAS and AdH models were linked with DebrisLib, the models sufficiently predicted velocity, depths, and floodplain inundation. This paper demonstrates the utility of a library-based framework applying conservative (fixed-bed conditions and constant sediment concentration) non-Newtonian rheology-based closures using engineering-based models for post-wildfire flood risk management and emergency response.

Ian Floyd
US Army ERDC
United States

Nawa Pradhan
US Army ERDC
United States

Jang Pak
US Army Corp of Engineers, Hydrologic Engineering Center
United States

Moosub Eom
US Army Corp of Engineers
United States

 



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