A "Typical" Landslide Distribution from Above-Average Winter Storms in the San Francisco Bay Area: a New Landslide Inventory from the East Bay Region
Abstract
Strong winter storms during January and February 2017 caused widespread landsliding in San Francisco's East Bay region (California, USA). We used high-resolution satellite imagery to map the landslide distribution (individually distinguishing source area polygons from runout zone polygons) over a 1,050 km2 area encompassing most of the East Bay Hills region. We identified approximately 8,900 landslides (landslide concentration equal to 8.5 landslides/km2) and conducted field work at 163 of these sites to verify the inventory and gain insight into salient aspects of the landslides themselves. Comprised primarily of shallow, debris and earth, slides and flows (Varnes, 1978), most landslides occurred in undeveloped areas (i.e., parks and open space). However, many ( 1,400, i.e., 16% of the total) affected structures and infrastructure, with several larger, deeper landslides destroying homes, and many shallower, but steep, debris slides and falls blocking roads that subsequently required months to reopen.
Landsliding in the San Francisco Bay region is often associated with El Niño events (e.g., 1997-1998). However, significant storms outside these conditions have also caused widespread landsliding (e.g., January 1982). The present landslide distribution occurred during a weak La Niña year (2016-2017), but still as a result of above-average precipitation (compare 820 mm in 2016-2017, to 550 mm average in San Francisco, and 930 mm in 1981-1982 and 1,200 mm in 1997-1998). Examining the long-term climatology of the region reveals that the 2016-2017 season was one of 26 years since 1849 (i.e., in the upper 15% of years) that exceeded the mean plus one standard deviation precipitation level (740 mm). In many of these rainy years (e.g., 1972-1973, 1981-1982, 1997-1998, 2005-2006), landsliding was also widespread. Thus, although the number and resulting impacts of the 2016-2017 landslides was significant, the landslide distribution that occurred during this time could also be considered typical with respect to what should be expected from years with above average precipitation. The event serves as a reminder to emergency managers and the public that landslides remain an important hazard to be aware of and prepare for in the future.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMNH14A..03C
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGYDE: 4302 Geological;
- NATURAL HAZARDSDE: 4303 Hydrological;
- NATURAL HAZARDSDE: 4315 Monitoring;
- forecasting;
- prediction;
- NATURAL HAZARDS