Date of Award

5-2025

Document Type

Thesis

Degree Name

Master of Science in Geology

Department

Geological Sciences

First Reader/Committee Chair

Cato, Kerry

Abstract

The Yucaipa Ridge is a section of the San Bernardino Mountains within the Transverse Ranges of Southern California. During the summer and early fall of 2020, Yucaipa Ridge experienced severe vegetation damage from both the Apple and El Dorado wildfires. Burned slopes exhibit a higher susceptibility to geomorphic change within several years of burning from the introduction of meteoric water onto the slope. Yucaipa, Oak Glen, and other nearby communities are currently at risk of damage by hyperconcentrated flows and debris flows during the seasonal rainy season.

Debris flows and hyperconcentrated flows (a.k.a. mudflows; are grouped and referred to in this paper as debris flows) are fast moving, mass-wasting movements that can reach up to about 56 kilometers per hour (35 miles per hour). Debris flows are triggered by an increase in soil saturation which creates seepage forces associated with temporary high ground water which, in turn, triggers soil failure after the denudation of vegetation on a slope following a fire. The benefits of the vegetation interrupting and slowing the water flow as well as vegetation’s ability to absorb water are eliminated by the fire. Mud and debris flows begin on steep slopes. Gravitational kinetic energy moves water as sheet flow on the surface and seepage below the surface. The water flow erosion, rills and other surficial instability in the soil, which become concentrated into channelized flow as the water and debris increases in velocity. Within the channels, a slurry of loose soil, rock, and organic matter descend the slope while continuing to pick up velocity until the flow is capable of moving boulders and trees. The flow stops, or decreases in velocity, when the kinetic energy is dissipated by obstructions and friction within lower, less-steep slopes. There, the coarsest-grained sediment is first deposited while the water and finer-grained components of the hyperconcentrated flows continue further downstream. This lower sediment content flow may continue for miles downstream.

Fluvial drainage channels in the wildfire impact area, including those selected for this study, have undergone a Values at Risk (VAR) assessment deeming the probability of flows to be likely. This study monitored and evaluated channels during pre- and post-storm events of burned and unburned slopes. With the advent of small Uncrewed Aerial System (sUAS) drones, data collection during these events helped to document the flows that occurred. The data collected can be used to map the possible future outcomes, e.g., post-fire debris flows. These estimates of future outcomes can be applicable and useful throughout the entire west coast and other fire prone regions and localities. Observing soil erosion, formation of rills, noting channel segment changes, determining where material is deposited and in what amount, and finding hydrologic connectivity are integral in establishing a greater understanding of post-fire debris flow thresholds within a slope. Building and contributing to a database of debris flow information with agencies such as CGS, Cal Fire, USGS, and others will greatly serve working professionals in the future in hazard mitigation strategies and natural disaster preparedness.

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