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

BIGFOOT: BIG-storm FOOTprint on California and future hazards

 

 

 

What footprint do big storms leave on the landscape, both onshore and offshore?

In tectonically active landscapes such as the western Transverse Range (WTR) in California, the regional geologic structure influences the spatial variability of rock and soil strength properties and weathering.

Outline of BIGFOOT project focus

As part of the broad San Andreas transform plate margin, the WTR has undergone dramatic Neogene and Quaternary transpressional faulting, folding, and clockwise vertical-axis rotations of crustal blocks. Geologically youthful manifestations of this tectonism include regions of concentrated seismicity and recent to active crustal uplift and associated basinal subsidence at local to regional scales. Tectonic deformation and the resulting spatial distribution of material properties inherent to the underlying rocks and weathering products influence both the topographic form and the spatial distribution of landsliding, concentrated erosion, and downslope transport and deposition within the fluvial network. As such, the geologic framework provides a means to categorize the rates and processes of sediment sources and deposits in response to big storms. The region along the California coast from Point Conception to Oxnard has been widely recognized as being susceptible to landsliding and flooding, in part because of the rapid tectonic uplift rates generating the steep topographic front of the Santa Ynez Mountains that strongly influence the local orographic rainfall.

During "atmospheric river" meteorological conditions, this region is likely to experience widespread erosion and flooding, as evidenced by punctuated depositional events in the Santa Barbara Basin.

Composite satellite image of integrated water vapor (IWV) from Special Sensor Microwave Imager (SSM/I) depicting substantial plume capable of generating heavy rainfall and regional flooding [Ralph et al. (2005)]

Composite satellite image of integrated water vapor (IWV) from Special Sensor Microwave Imager (SSM/I) depicting substantial plume capable of generating heavy rainfall and regional flooding [Ralph et al. (2005)]

Through a series of geomorphic process maps highlighting landscape responses to such big storms, we aim to tie the spatial patterns of geology and tectonics, landsliding and erosion, fluvial and alluvial responses, and onshore/offshore records of sedimentation.

The overall project objectives are to investigate geologic hazards including their causes, processes, frequencies, rates, and spatial distributions through conventional geologic mapping and thematic mapping. The geohazards addressed will include:

  1. effects of storms of varying magnitudes with a focus on big storms ("atmospheric rivers" with ~200 yr. frequency),
  2. hillslope erosion, landslide processes, and regolith distribution,
  3. flooding and fluvial erosion and deposition,
  4. seismic hazards constrained by neotectonic (Pliocene through Holocene) style, kinematics, and rates of deformation.

Such geohazard studies will afford the opportunity to investigate the interactive role of climate and tectonics on landscape evolution over millenial to million year time scales to see if we can distinguish their respective influences on landscape change.

 

Why is this important? The need for high-quality geologic mapping and related studies has been proven repeatedly in southern California by the customary demand for these products.

For instance, during the aftermath of wildfires there is heightened concern over accelerated magnitudes and rates of flooding, sediment transport, and landsliding during winter rains. Since 2008, three high-profile wildfires (Gap, Tea, and Jesusita) along the steepland urban interface of the Santa Barbara region prompted numerous agencies (e.g., NOAA/NWS, City of Santa Barbara Public Works Department and Water Resources Division; Santa Barbara County Flood Control and Water Conservation District), land use managers, locals, and consultants to request USGS geologic mapping and hazard information. Ongoing need is demonstrated by interest expressed by the NPS and NOAA/NWS in obtaining surficial geology and hazard susceptibility maps for coastal Parklands (Channel Islands National Park and Santa Monica Mountains National Recreation Area). Such need is increasing as the region's large population places increasing demands on the landscape for:

  • water-storage and waste-disposal
  • living space where housing already is encroaching on steep hillslopes (e.g., La Conchita) and wavebattered sea cliffs and beach fronts
  • recreational space where ecosystems already are stressed

Example of post-fire hillslope monitoring of hydrologic response for runoff, erosion, and landsliding at Gap fire field site

Example of post-fire hillslope monitoring of hydrologic response for runoff, erosion, and landsliding at Gap fire field site

   

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