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Fire and Rain

Yet another winter storm is slated to send more rain to Southern California this week and we at RedZone think it warrants a little inside info on the risks of post-fire debris flows. Winter rains mean a higher potential for land-moving rainfall rates in areas where fires have scorched the landscape. After 2018’s tragic events that occurred in the Thomas fire burn scar, officials have not taken the potential of these continued storms lightly. Last winter, residents were evacuated near the Holy, Thomas, Cranston, Napa/Sonoma, and Woolsey fires (all burn scars from the last couple years).

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Fire and Rain

Yet another winter storm is slated to send more rain to Southern California this week and we at RedZone think it warrants a little inside info on the risks of post-fire debris flows. The nation is experiencing its second El Nino effect in three years with it forecast to last in to the summer. What that means, is a higher potential for land-moving rainfall rates in areas where fires have scorched the landscape. After last years tragic events that occurred in the Thomas fire burn scar, officials have not taken the potential of these continued storms lightly. So far this winter, residents have been evacuated near the Holy, Thomas, Cranston, Napa/Sonoma, and Woolsey fires (all recent burn scars from the last couple years).

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Incoming Heavy Rains Increase Debris Flow Risk Yet Again

The National Weather Service (NWS), Los Angeles Office is warning the region that sustained heavy rains are incoming for the majority of the rest of the week. The beginning of the rain is set to arrive today (Tuesday) and last well into Thursday. The “atmospheric river” storm is expected to bring between 5 and 10 inches of rain in the foothills and mountains, significantly more total rainfall than the 1/9 debris flow, which brought between 3 and 6 inches to the region. The NWS says this storm is projected to have the heaviest rainfall and the longest duration of this winter storm season. “All models indicate high confidence in rainfall totals and the duration of the storm.”

Rainfall forecast through 3/26

National Weather Service Precipitation forecast for the Greater Los Angeles and Santa Barbara Areas through the weekend. Recent Fires are seen in black on the map as worry grows about debris flow potential.

Debris Flow Risk for Santa Barbara County Burn Areas

We caught Monday’s press conference from Emergency Officials with Santa Barbara County, who are stressing the seriousness of the renewed threat flooding and especially of debris flow.  Opening the discussion, Meteorologist, Mark Jackson warned, “A key worry with this storm is rainfall rates that can trigger debris flows. It’s not necessarily the total amount of rain that occurs; it’s how fast that rain falls.” Well, the latest meteorological models by the National Weather Service indicate that there is potential for rainfall intensity of between .5 to .75 inches per hour, which is enough to trigger debris flows at any time during the storm.

Recent Burn Areas tweet

NWS Los Angeles warning via tweet today that debris flows near recent fires are likely across the region

As a result, many residents downslope of Thomas and other fires in the region (seen as black in the map above & highlighted in the tweet below) have been evacuated or at least cautioned. Santa Barbara County is also managing and updating an evacuation map found here.  In addition, Rob Lewin, director of the Santa Barbara County Office of Emergency Management, warned the amount of rain and the intensity is enough to cause flooding even without the impact of the recent fires. “We could experience localized flooding and road closures which are not isolated to the burn areas. The threat of rock falls, mud slides and debris flow is high,” he noted.

“A key worry with this storm is rainfall rates that can trigger debris flows. It’s not necessarily the total amount of rain that occurs; it’s how fast that rain falls.” – Mark Jackson, NWS Meteorologist

Storm Facts:

  • Heaviest Rainfall for Ventura County to San Luis Obispo County is expected Wednesday afternoon to Thursday morning (March 21 – March 22)
  • Heaviest Rainfall for LA County is expected Thursday into Thursday Night (March 22)
  • Rainfall totals for the Coasts/Valleys will be 2-5 inches, for the Foothills/Mountains more like 5-10 inches.
  • Flash flooding and significant debris flows near recent burn scars are likely
  • Urban and small stream flooding expected
  • Slight chance of main stem river flooding
  • Road closures anticipated due to rock fall in the mountains
  • Residents in the Extreme and High Risks areas of Santa Barbara County are required to evacuate at noon on Tuesday (March 20)
  • Mandatory evacuations prompted for areas below the Thomas, Whittier, and Sherpa Fires

Sources:

National Weather Service

Santa Barbara County Emergency Services

KSBY News

Burn Scar

Debris Flow Devastates Montecito, CA Immediately After The Thomas Fire

While the threat of the Thomas fire just recently diminished in Ventura and Santa Barbra Counties, residents were weary to hear that they are now being threatened by mudslides originating from within the burn area. Rain started Monday afternoon in the areas of Santa Barbara and Ventura Counties, with the peak rainfall being around 2:30 AM Tuesday morning. The rain came down in amounts up to an inch per hour over the burned area, which the incident commander reported as being a critical factor in the amount of sediment and debris being carried by this amount of water. The devastating debris flow ranged from Cold Springs Canyon to Toro Canyon, and wreaked havoc all the way down to Highway 101. The debris flow was so strong in some locations that it pushed homes off of their foundations and carried them several hundred feet.

First responders have been preparing for this incident since Monday morning by preemptively staging resources in the areas that were forecasted to be impacted the most severely. This strategic placement of resources was followed by officials releasing evacuation zones. The warning stated that all residents within mandatory evacuation zones should leave by 12 noon on Monday in preparation for the heavy rains that were forecasted for the area. Since the early morning hours of Tuesday, first responders have been in a search and rescue mode: still actively engaged in performing helicopter and contact rescues. The threat from this debris flow still remains and first responders are warning residents to stay away from the area if at all possible.

Debris flow

An explanation of a debris flow, and its power.

 

Over the last couple weeks of the Thomas Fire the Federal Burned Area Emergency Response (BAER) Team has been evaluating the blaze’s impact to the fire area and watersheds by predicting debris flow hazards should rains like this impact the area. The orange, red, and dark red areas were determined to have the highest probability of debris flow during a heavy rain event. The BAER team is currently embedded with the Santa Barbara and Ventura Office of Emergency Services to assist them in implementing response plans for communities downstream of the fire.

BAER Debris Flow

The map above displays estimates of the likelihood of debris flow (in %), potential volume of debris flow, and combined relative debris flow hazard.

Santa Barbara/Ventura Flooding at a Glance

  • 17 confirmed deaths related to the storms (These numbers are subject to change as the incident continues)
  • 13 missing people
  • At least 25 injured
  • 50 rescues via helicopter hoists have been performed during today’s search and rescue operations.
  • 1-6.5 inches of rainfall over the Thomas Fire area.
  • Search and rescue efforts still remain priority with approximately 75 percent of the primary search completed in the debris flow area.

If you liked the material in this blog, you can read similar material RedZone has covered here.

Sources

http://www.cnn.com/2018/01/09/us/southern-california-evacuations-rain-flooding/index.html

http://www.latimes.com/local/lanow/la-me-ln-rainfall-mudflow-20180109-story.html

http://santabarbara.onerain.com/map/?sensor_class=10|2880&view_id=5&view=8bc6e88f-eeab-4281-9d92-3d723016e945

https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=178