Comparing Landslide-Derived Sediment Yield to Anthropogenic Sources at Hollister Hills Off-Highway Vehicle Park in Central California

Abstract:

Earthflows are pervasive in the rolling hills bordering the San Andreas Fault in the Bird Creek watershed south of Hollister, California. Studying the pathways and transport rates of landslide-derived colluvial sediment is important for understanding the role of natural colluvial processes in water quality impairment. We provide a case-study of a landslide complex located on Hollister Hills State Off-highway Vehicle Park to determine if naturally-occurring landslides contribute sediment to streams on a time frame and scale that competes with anthropogenic non-point-source sediment sources currently managed under the Clean Water Act. We determined suspended sediment yield from stream gauging, and conducted geomorphic analyses based on RTK GPS and SfM photogrammetry.

Hudner Slide is a large, complex earth flow that has blocked Hudner Creek (an ephemeral tributary to Bird Creek). Survey markers in the slide body moved an average of 1.77 m in a two-year period including heavy rainfall in 2017. Sediment ponded and aggraded up-valley from the Hudner slide toe for several years, but the down-valley side of the slide toe was steeper than the pre-existing valley gradient, which led to gullying. The gully head gradually cut through resistant toe material and has recently accessed the highly erodible ponded sediment. The gully head advanced approximately 55 m in 2017, liberating approximately 2,950 tonnes of sediment to Bird Creek, a volume exceeding the 180 tonnes of suspended sediment derived from both natural and anthropogenic sources in a typical year. Our study shows that landslide-derived sediment yield can far exceed anthropogenic sources on a short time scale. Over a longer time frame, continued gullying at the toe of the slide will likely trigger a positive feedback of sediment production and downstream transport by reactivating the Hudner slide, ponding sediment, and renewed gullying.