As much-needed rain fell on the Santa Clarita Valley Monday, officials encouraged residents to prepare for potential mudslides and debris flow along vulnerable hillsides impacted by recent wildfires.
Especially at-risk areas included slopes damaged by the Rye Fire in December 2017 and the Sand Fire in July 2016, where barren hills make it easier for dirt and rocks to slide off during or after a storm.
The danger for debris flow will be at its peak early Tuesday morning, as the winter storm brings high winds, showers and potential thunderstorms to the area.
“The heaviest rain times will be from 2 a.m. to 11 a.m. for LA County,” National Weather Service (NWS) Weather Specialist Stuart Seto said. “We have these strong south winds and they’re meeting up with this cold air from the north and that’s why the rainfall has increased.”
During the two-day storm, the Santa Clarita Valley is forecasted to receive anywhere from 2 inches to 3 inches of rain with winds between 15 to 25 miles per hour (mph) and gusts as high as 40 mph.
These conditions prompted the NWS to issue to warning for SCV through Tuesday: a Wind Advisory and Flash Flood Watch.
“Rainfall rates between one half and one inch per hour are possible during the peak of the storm,” the NWS said. “Such rain rates are capable of producing flash flooding. Recent burn areas will be especially vulnerable where dangerous mud and debris flows are possible.”
Debris Flow
During normal conditions, vegetation helps hillsides absorb rainwater; however, after a wildfire, burned vegetation can block water absorption and cause rainfall to run downstream and increase the chance of flooding and mudflow, according to the National Flood Insurance Program.
A recent debris flow hazard assessment from the United States Geological Survey (USGS) indicates that the Rye Fire burn area perimeter basin is one of these areas at risk.
The assessment found that the basin has a 0 to 20 percent likelihood of experiencing debris flow following a storm with a peak 15-minute rainfall intensity of 24 millimeters per hour (mm/h).
Other areas at risk include roads, canyons, trails and residences affected by the Sand Fire, which broke out more than a year ago.
The Los Angeles County Department of Public Works’ Storm Water Engineering Division 2017- 2018 Storm Season Debris and Mudflow Potential Forecast noted that several of these areas could experience moderate debris and mudflows.
“Some streets may be completely blocked by debris,” the mudflow forecast for the burn area said. “Depending on location and terrain, some structures may be endangered, in addition to those advised to be prepared to evacuate with any forecast of rain.”
However, as of Monday evening, no major mudslides or debris flows were reported for the area.
USGS Tips to Stay Safe During Storms:
- Stay alert! Many debris-flow and flood fatalities occur when people are sleeping. Listen to the radio for warnings of intense rainfall. NOAA Weather Radio All Hazards tone alert will let you know of hazards in your area. Be aware that intense bursts of rain may be particularly dangerous, especially after longer periods of heavy rainfall.
- If you are in an area susceptible to flooding or debris flow (or has experienced flooding or debris flow in the past), consider leaving if it is safe to do so. Remember that driving during heavy rainstorms can be hazardous.
- If you are near a stream or a channel, listen for any unusual sounds that might indicate moving debris, such as trees cracking or boulders knocking together. A trickle of flowing mud or debris may precede larger flows. Be alert for any sudden increases or decreases in water flow and for a change from clear to muddy water. Such changes may indicate debris-flow activity upstream, so be prepared to move quickly. Don’t delay! Save yourself, not your belongings.
- Keep in mind that rises in water levels during flash floods and debris flows may occur much more rapidly, and may be significantly larger, than those produced when the watershed is not burned.
- Be particularly alert when driving. Bridges may be washed out, and culverts overtopped. Do not cross flooding streams!! Turn Around, Don’t Drown! Embankments along roadsides are particularly susceptible to landsliding. Watch the road for collapsed pavement, mud, fallen rocks, and other indications of debris flow.
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