Water Resources of the United States

PROJECT ALERT NOTICE (CA) Flooding, debris flows, and mudslides in California from AR storm

Date: Tue, 09 Jan 2018 14:47:41 EST

Summary: California is experiencing its first major storm event of the year, an AR that poses flooding, debris flow, and mudslide hazards for recent burn areas in northern and southern CA.

An atmospheric river storm event is bringing precipitation to nearly all parts of California today (Tuesday, January 9, 2018), This is California’s first major storm of the year, and is expected to bring 1-2 inches of rainfall to most parts of the State, and more than 6 inches of rainfall to areas along the central and southern coast from the North Bay down to Los Angeles. High elevation areas, particularly in the Sierra Nevada, expect snowfall accumulations of up to 2 feet. High winds are also expected with this storm system. The National Weather Service (NWS) has issued flash flood warnings and wind advisories for numerous counties state-wide, with warnings making particular mention of potential hazards in areas downstream of recent burn areas. These hazards can include, flash flooding, landslides, mud and debris flows, and fallen trees from high winds.

Today, many streams will rising in response to this event with peak streamflows expected later today or tomorrow. Currently, the NWS California Nevada River Forecast Center predicts that most flood forecast points in California will remain below critical stages in response to this storm event, but local flooding is possible, particularly in and near recent burn areas. More discussion on expected flooding in burn areas is provided below. The U.S. Geological Survey (USGS) Water Watch reports for California show 25 real-time gages have streamflow much above normal for this day of year.

The USGS California Water Science Center (CAWSC) currently has 19 crews deployed from the following service areas: Santa Maria, CA (3 crews); Redlands, CA (3 crews); Santa Cruz, CA (4 crews); Poway, CA (5 crews); Ukiah, CA (3 crews); and Sacramento (1 crew). Flood response by the CAWSC will be assessed continuously as this storm develops and as basins respond. Additional crews are planned to deploy over the next few days in response to these events. Currently, all streamgages are transmitting data. No equipment damage or safety issues have been reported.

The USGS is working closely with officials from various Federal, State, and local agencies to monitor this storm, and any future storms, with particular attention to recent burn areas. In the greater North Bay area, the USGS operates more than two dozen streamgages and approximately half a dozen water-quality monitoring stations at locations within or downstream of the 2017 Pocket, Tubs, Nuns, and Altas Fires in Sonoma, Napa, and Lake County. Six USGS streamgages in those fire-affected areas are NWS flood forecast points. Along the southern coast, the USGS operates more than a dozen streamgages at locations within or downstream of the 2016 Sherpa Fire, and the 2017 Thomas Fire and Whittier fire in Ventura and Santa Barbara County. Two USGS streamgages in those fire-affected area are NWS flood forecast points.

The 2017 Thomas Fire burn area in Ventura County is one of particular concern because of the numerous steep canyons in the burn area with high probability of debris flow which pose serious downstream hazards. A USGS debris flow probability map for the Thomas Fire (accessed at https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=178), shows many areas in the Thomas Fire burn scar that have an 80-100 percent chance of debris flow probability in response to a short duration high intensity storm event, defined as a peak 15-minute rainfall intensity of 1 inch per hour. The LA Times reports 5 fatalities overnight from flooding and "waist-high” mudflows in a Montecito, CA, a residential area located downgradient from an area burned by the Thomas Fire. Areas in the Thomas Fire burn area have already received over 5 inches of rain since yesterday, with another 2-3 inches or more in areas expected before the storm passes late this evening. Caltrans reports numerous road closures in Ventura County and Santa Barbara County due to mudslides.

The CAWSC will remain in contact with cooperators and officials from Federal, State and local agencies and updates will continue to be provided during the course of this event.

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