Abstract

Rock type governs hillslope morphology and evolution, but the mechanisms controlling this relationship are not well known. Along the eastern edge of the Northern California Coast Range lies the eastward dipping (50 degrees) Mesozoic turbidite sequence of mudstones, siltstones, sandstones and conglomerates of the Great Valley sequence. Hillslopes are strongly aligned to strike and channel dissection ranges from major valleys to closely spaced rills on some hillslopes. Average rainfall is about 540 mm, leading to a vegetation cover of a blue-oak woodland savannah, the density of which is strongly influenced by aspect. In February 2017, a storm produced some 678 shallow soil landslides within a 16 km2 area, with a median size of just 7.22 m2. Differential high resolution airborne lidar surveys (between 2015 and 2017) enabled us to map landslide runout and guided extensive field investigations. Only a small percentage of the landslides produced flows that reached the channel and the estimated volume of sediment transported via shallow landsliding for this event was 4150 km3, orders of magnitude less than erosion rates estimated from cosmogenic radionuclide dating. The primary impact of this storm was instead significant sediment storage evacuation in the channel network. Landslide density was greatest in areas underlain by mixed siltstone and sandstone. The majority of landslides occurred in areas expected to be unstable by the SHALSTAB model. Surprisingly, in February 2019, another, smaller storm event triggered 25 landslides on a 0.36 km2 hillslope where in 2017 49 landslides had previously occurred. Differential ultra-high resolution airborne lidar surveys (average 75 points/m2 in 2017) showed that the majority of landslides that occurred within our study area in 2019 occurred on this hill. Field observations of exfiltration from fractured bedrock within old landslide scars and macropores > 5 cm wide along scar crowns suggest that shallow subsurface flow may play an important role in localizing and scaling the scars, but does not yet explain why the 2019 landslides were not triggered by the larger storm in 2017. Despite the limited sediment production by these landslides, their runoff and subsequent erosion may be important in generating distinct rills.

Citation

Sanders, M., Nelson, M.D., Bryk, A.B., Huang, M.-H., Fauria, K., and Dietrich, W.E. (2019): The role of small shallow landslides in landscape evolution as revealed by high resolution differential lidar surveys and field mapping. American Geophysical Union 2019 Fall Meeting, San Francisco, CA, 9-13 December 2019 Abstract #EP43D-2399.

This Paper/Book acknowledges NSF CZO grant support.