GEOLOGIC HAZARDS DUE TO LANDSLIDE DAMS IN THE COLD SPRINGS AND HOT SPRINGS WATERSHEDS, MONTECITO, COUNTY OF SANTA BARBARA, CALIFORNIA

The January 9, 2018 (1/9/18) debris flows that devastated the community of Montecito was the catalyst in forming the non-profit RenewSB-Partners in Community Renewal. RenewSB funded this study to identify sites for debris basins and to develop a long-term master mitigation plan. Aerial photos, LiDAR, and field mapping were employed to identify and categorize various types of landslides and temporary landslide dams in the Cold Springs and Hot Springs watersheds. Landslide dams can magnify debris flow hazards and pose an outbreak flood hazard when the dams are overtopped and fail catastrophically.

Numerous bedrock landslides mantle the slopes and constrict creek channels, forming noticeable knickpoints in channel profiles. Most of these slides block channel flows creating temporary lakes that eventually overtop and are eroded within 1 to 12 hours, regardless of their volume. The resulting outbreak floods often increase peak discharge far in excess of what could be generated by concentration of overland flows.

Most of the breached landslide dams appear to be the result of deep-seated landslides in confined bedrock canyons, formed by high rates of uplift within erodible sedimentary rocks. Tertiary units exposed in the watersheds include, from youngest to oldest, the Coldwater Sandstone, Cozy Dell Shale, Matilija Sandstone, and Juncal Formation. The distribution of the landslides is generally greater in the Cozy Dell Shale and Coldwater Sandstone.

Historic records suggest that there have been eight debris flow and debris laden flood events over the last 106 years: in 1914, 1926, 1964, 1969, 1971, 1995, 2018, and 2019. These flows vary in magnitude from small, affecting one or two watersheds, to large (1/9/2018 event), impacting five adjacent watersheds. Newspaper accounts of the 1914 event describe debris dams forming only to subsequently collapse. A 1926 newspaper account describes a collapse of a dam resulting in a flood wave in the adjacent San Ysidro watershed. Six landslide dams were identified in the Cold Springs and Hot Springs watersheds, triggered by the 1964 Coyote Fire-Flood sequence. These landslides and related dams will be further analyzed to better understand their relative age and frequency of occurrence, so probabilistic methods can assess the hazards they pose over the next 100+ years.