Short-term Patterns and Processes of Coastal Cliff Retreat, Rill Erosion on Colluvial Hillslopes, and a Sediment Budget for the 2018 Montecito Debris Flows
Coastal cliffs and Mountainous regions are features of geomorphic and societal significance. Determining the rates and mechanisms by which they erode are important for developing a detailed understanding of landscape evolution as well as facilitating geophysical risk analysis, event-forecasting, and environmental management at the coast and the wildland-urban interface. In the following studies, I use field-measurements and remote sensing techniques to quantify topographic change and assess the erosional processes that result in the landward retreat of coastal cliffs and the generation of post-wildfire debris flows. In Chapter 1, I use sequential terrestrial LiDAR scans to measure the volumetric change of the cliff face at several sites in Santa Barbara, CA. I find that wave action drives erosion at the cliff base, and the erosion rate is controlled by a threshold amount of wave energy received by the cliff base. These thresholds, when exceeded, produced the highest cliff base retreat rates during the study period. Retreat rates along the cliff base, middle, and top converged over a two-year period indicating that wave action ultimately drives retreat over short timescales for these soft shale cliffs, and reinforces the expectation that coastal cliff retreat rates will increase with rising sea levels. In Chapter 2, I estimate the volume and delivery rate of slurry (a water-sediment mixture) supplied to stream channels during a post-wildfire rainstorm that generated large debris flows in catchments above Montecito, CA in 2018. I find that the rapid evacuation and mixing of water and sediment during rill formation supplied an estimated 241,000 m3 of slurry with high solids concentrations of ~50% to stream channels within 12-15 minutes for debris flow generation. Colluvium on shale formations was more continuous, finer-grained, and probably less permeable than colluvium on sandstones, and these differences affected the extent and dimensions of rills. As a result, shale hillslopes supplied over two times as much per unit burn area as sandstones. The mechanism of slurry generation by rill erosion is then interpreted with the field evidence, through the lens of hydrodynamic theory and laboratory experiments conducted by others under comparable conditions. Lastly, in Chapter 3, I construct a sediment budget for the 2018 Montecito debris flows and estimate that ~910,000 m3 of sediment was mobilized from the six mountain catchments and fans within the 20-minute event. The predominate erosional processes that contributed to this total included dry ravel, rilling, sheetwash, and channel scour. The volumetric contribution and spatial distribution of sediment inputs from the individual erosion processes, along with mapping and observations from previous work, yielded insights into how the flows were generated and what led to the entrainment of such large quantities of sediment. I conclude that an integrated understanding of i) the conditions and erosional processes that deliver fine-grained sediment to stream channels, ii) the production and rate of slurry generation for debris flow mobilization, and iii) determination of the amount and nature of the sediments stored in stream channels are important for guiding assessment procedures and developing a process-based understanding of high-magnitude debris flow events that can be applied across diverse mountain environments.