USGS - science for a changing world

GMEG - Geology, Minerals, Energy, & Geophysics Science Center

BIGFOOT: BIG-storm FOOTprint on California and future hazards



Landslide Maps

As we cannot date hillslope erosion events at present, we propose to generate process-based sediment budgets summarizing the influences of bedrock geology, soil depth, particle size, and relative mass-wasting susceptibility in the context of landscape response to large storms, offshore sedimentation, and marine ecology. Regional geologic structure influences the spatial variability of rock and soil strength properties and weathering (often concentrated within specific geologic units). Tectonic deformation and material properties of the underlying material influence both the topographic form and the spatial location of landsliding and concentrated erosion. As such, the geologic framework should provide a means to categorize the rates and processes of mass wasting. The region along the California coast from Point Conception to Oxnard has been widely recognized as being susceptible to landsliding, in part because of the rapid tectonic deformation rates and associated uplift of marine terraces. We aim to tie spatial patterns of bedrock geology, landsliding and erosion with triggering rainfall characteristics, fluvial responses to urbanization and wildfire, and offshore records of sedimentation.

The task aims to document the spatial distribution of landslide and erosion processes throughout different geologic units and triggering events. The primary objectives are to examine the integrated influence of:

  • landscape form as defined by high-resolution topography (LiDAR) both airborne and terrestrial,
  • geologic materials through field and laboratory analyses, and
  • triggering rainfall characteristics. The many different types, sizes and depths of landslides, as well as numerous faults in the region are especially detailed and will benefit from the use of LiDAR imagery for recognition and detailed evaluation of subsequent seasonal movement.

Highlights and Key Findings:

Collaborating with NOAA/NWS, we deployed terrestrial rain gages and assisted with citing a portable truck mounted C-band Doppler weather radar observe rainfall over the Station fire burn area near Los Angeles, California. This research effort improved the definition of rainfall conditions that trigger debris flows from steep topography within recent wildfire burn areas. The higher resolution portable radar outperformed the local permanent NWS network radars such that the network radars underestimated hourly precipitation totals by about 50%. This advance represents a more accurate estimate of rainfall rates and a higher resolution of spatial variability. These efforts assist with debris-flow warning decisions as part of a joint NOAA-USGS collaboration.

To understand sediment transport processes characteristic of post-wildfire erosion and debris-flows, and to map their geomorphic process signatures on high-resolution topographic base maps, we surveyed steep, low-order drainage basins using repeat terrestrial laser surveys (TLS). We generated a time-series of bare-earth digital elevation models (DEMs) for 6 burn areas in southern California. We've documented how patterns of rain splash, overland-flow scour, and rilling contribute to erosion and debris-flow generation. We fortuitously surveyed topography using TLS within hours both preceding and following the first appreciable storm of the winter season following wildfire. The associated landscape change and geomorphic process boundaries continued to vary over time in response to changing rainfall/runoff relationships, storm magnitudes, and stripping of hillslope cover with conversion to sediment supply limited conditions.


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