The Thomas Fire occurred between December 2017 and January 2018 resulting in burned hillsides throughout the San Ysidro Creek Watershed located in Santa Barbara County, California. Following the fire, a high-intensity storm resulted in debris flows across the destabilized watershed resulting in the destruction of eight private properties located at Randall Road & East Valley Road. The Randall Road Debris Basin Project was a collaboration between the community and the County to provide mitigation for future debris flows within those eight properties. Design for a 3.6 acre basin including debris racks were finalized in February 2021 and construction finished in August 2022.
The Project design storm hydrology was developed using pre-burn and post-burn assumptions of sediment bulking with HEC-HMS 4.2.1. Two-dimensional channel and floodplain hydraulics were analyzed with HEC-RAS 5.0.7.
The new release of HEC-RAS 6.0 in May 2021 provided an alternative method to sediment bulking for the analysis of post-burn debris flows. This study utilizes the new 2D non-Newtonian methods and internal bulking options to compare against previous sediment bulking methodology used in design of the Project. Both methods were calibrated to observed debris fields for the 2018 event.
The goal of this study is to document the coordination efforts between the community and County, acquisition of the parcels, initial modeling, and design; with a main focus on comparison of the debris flow modeling capabilities between HEC-RAS 6.0 and previous versions.
