Dissertation Defence: Ditch and Deflection Berm Design to Mitigate Sediment-Laden Flows from Small Watersheds Affected by Wildfires

Kaushal Gnyawali, supervised by Dr. Dwayne Tannant, will defend their dissertation titled “Ditch and Deflection Berm Design to Mitigate Sediment-Laden Flows from Small Watersheds Affected by Wildfires” in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Civil Engineering.

An abstract for Kaushal Gnyawali’s dissertation is included below.

Examinations are open to all members of the campus community as well as the general public. This examination will be offered in hybrid format.  Registration is not required to attend in person, but please email dwayne.tannant@ubc.ca to receive the Zoom link for this exam.

Abstract

Post-wildfire debris flows in small watersheds (less than 5 km²) pose persistent hazards to rural communities, First Nations, and road and highway infrastructure across south-central British Columbia. Existing design approaches, developed for larger watersheds under different geological and climatic conditions, do not reliably apply to small, burned watersheds in the BC Interior. This thesis develops a methodology to assess post-wildfire debris-flow hazard, design terrain-based mitigation structures, and evaluate structural performance under design flow conditions. The empirical foundation is a systematic dataset of post-wildfire flow events compiled through fieldwork across south-central BC.

Based on this evidence base, regionally calibrated debris-flow volume-estimation models were developed alongside design hydrographs derived from the SCS Curve Number method with sediment bulking. Post-wildfire flows were simulated in 2D using HEC-RAS with Newtonian and Bingham (non-Newtonian) rheology on high-resolution terrain. An analytical framework was developed to evaluate existing terrain-based mitigation structures — including riprap-lined channels and deflection berms — against post-wildfire design flows. The framework was further applied to guide the design of new storage basins, deflection berms, and diversion ditches. Volume and discharge estimates carry substantial uncertainty, reflecting limited regional data, the perishable nature of post-wildfire field evidence, and rapid landscape recovery that obscures deposits between successive events. The tools are calibrated for screening-level assessment and are applicable during the first post-fire recovery period using remotely sensed inputs.

Wilson Creek, West Kelowna, BC—affected by the 2023 McDougall Creek wildfire—serves as a comprehensive application of the integrated methodology. A two-zone mitigation system was designed, comprising an upper storage basin with an earth berm and a lower diversion system. Structural performance was assessed through numerical simulation. The methodology can be implemented using standard earthworks equipment and freely available software, including the Python tools developed as part of this work, making this approach practical for the rural and First Nations communities most exposed to post-wildfire debris-flow risk in BC.