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BIGFOOT: BIG-storm FOOTprint on California and future hazards

 

 

Mapping Footprint of Prehistoric Big Storms and Commensurate Areas of Flooding

A new record of California's past from yearly varves in the Santa Barbara basin reveals that every several hundred years, exceptionally large storms cause extreme flooding. Satellite observations over the last decade suggest these largest events may be atmospheric rivers, narrow (< 200 km) jets of moisture spun outwards from the tropics. Where these jets collide with California, their high moisture content leads to exceptionally heavy rainfalls, amongst the largest 3 day totals in the nation's history. As a consequence, lasting jet impacts may lead to floods of record at stream gages. Although we believe we understand the geometry and crude dynamics of such storms well enough to simulate them (Arkstorm project), their impact on the landscape remains largely a mystery. The last very large sequence of atmospheric rivers to hit northern and southern California occurred in 1861/62, long before most scientific data (there were 4 daily rainfall gages in the state at the time). Outside of recorded high-water marks at Agua Mansa and the American River below the modern Folsom dam, our understanding of the effects of these storms on the landscape is anecdotal. Widespread flooding is reported in northern and southern California, and press reports indicate substantial damage to infrastructure. Outside of these fragments from the past, we are largely ignorant of the magnitude and extent of past atmospheric rivers on California's landscape.

  • Do the events in the Santa Barbara core represent 1861/62-scale storms, or are they far larger as some have suggested?
  • How do the effects of 20th century storms (e.g., 1969, 1997/98, 2005) on the landscape scale up to larger storms?
  • Are floods and debris flows from these modern storms good analogs for the effects of past atmospheric rivers on the landscape, or do we need to look to geologic evidence to map and quantify the effects of the largest storms?

This task explores the onshore geologic record of big storms using fieldwork and remotely sensed imagery to map, date and model deposits from historic and pre-historic storms. It will attempt to address the questions above by quantifying the effects of historic storms, and finding storm metrics that allow us to scale the effects of the storms up to the larger, pre-historic storms recorded in the Santa Barbara basin. Where dated flood and mass wasting deposits can be recovered, a map of these deposits they will provide the most authoritative guide to future hazards from large storms, especially in an era of climate change when flood frequency curves are no longer stationary.

Highlights and Key Findings:

Scientists used digital maps of historic shallow landslides in southern California, USA, and nearby rainfall records to construct a relation between rainfall intensity and the fraction of unstable model cells that actually fail in historic storms.

Faulted gravels near Ojai, CA.Faulted gravels near Ojai, CA.

It was found that this fraction increases as a power law with the 6-hour rainfall intensity for sites in southern California. We used this relation to forecast shallow landslide abundance for a dynamic numerical simulation storm for California (Arkstorm), representing the most extreme historic storms known to have impacted the state.

   

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