Debris flows are fast-moving, deadly masses of mud, rocks, boulders, entire trees - and sometimes, homes or vehicles. They are similar to landslides but often carry, or “flow,” over a large area.
Debris flows will often be called "mudslides" or "mudflows". Many people use the terms interchangeably, but to scientists, each is a different kind of landslide and debris flows are the most powerful and dangerous of the three. They move faster than a person can run.
Debris flows occur most commonly during intense rain after wildfires. A debris flow doesn't need a lengthy rain or a saturated slope. It can start on a dry slope after only a few minutes of intense rain, due to soils that can no longer absorb water and the loss of stabilizing vegetation.
"Intense” rain means a burst of rain at a fast rate, about half an inch in an hour. That means even storms with light overall rainfall totals can create problems. With debris flows, the rainfall rate matters more than total rainfall.
Debris flows are fast and unpredictable. They travel faster than you can move. While the County is mapping areas of risk, debris flows can be unpredictable in where they will start or stop. It could begin in a stream channel, then jump out and spread through a neighborhood. A debris flow may happen where others have occurred, or in a place that has never seen one before.
Leave. The best thing you can do is evacuate before the storm. Many of us are used to protecting our properties with sandbags, culverts, k-rails, or retaining walls, however such efforts to alter its path are likely to become part of the debris flow.
Staying at home will endanger, rather than protect, you. If you are told to evacuate, leave. Learn your evacuation zone, make sure you are signed up for emergency alerts, put new batteries on your radio, follow local news and stay connected to County social media. First responders will do everything they can to assure you evacuate safely, but being able to receive communications is vital during emergencies. The only sure way to protect you and your family from a debris flow is to get out of their way.
Leave as soon as an evacuation order is issued. Do not wait to be ordered by authorities to leave. If you wait until you are sure a debris flow is coming, it will be too late to leave safely.
Visit our website at https://www.santacruzcounty.us/FireRecovery/DebrisFlowsMudflows.aspx for an evacuation map and to look up your address.
When a wildfire burns a slope, it increases the chance of debris flows for several years. The danger does slowly drop over time, but the risk is highest in the seasons immediately after a fire because organic materials have not had a chance to reestablish.
Never underestimate a debris flow. People die just about every year in debris flows, but often the affected area is small, so the story only makes local news. In numerous cases, debris flows have led to more death and injuries than the fires that preceded them.
Sign up for emergency alerts using the Code Red app and pay attention to weather forecasts for the vicinity of the burn area. The County also maintains some debris flow information and maps here: https://www.santacruzcounty.us/FireRecovery/DebrisFlowsMudflows.aspx
Yes! The warning signs including rushing water and signs of flowing mud, as well as unusual sounds such as breaking tree limbs and boulders tumbling. Survivors have even said it sounds like freight train. Consider making plans ahead of time to stay elsewhere during significant storm events and stay out of harm’s way.
Get to higher ground. Don’t attempt to cross a debris flow, but if a debris flow catches you by surprise, your best hope of surviving is to move up if possible. The top story of your home or even the roof (if it can be accessed safely) are your best bet.
Only when the conditions create safety risks for you and your neighbors, but you may be told to evacuate more than once and you could get frustrated, especially if a debris flow does not occur. Debris flows are unpredictable. Evacuation orders will be issued based on conditions and the extreme risks posed by debris flows.
Many insurance policies that cover other kinds of storm damage do not cover debris flow damage. Check your coverage now.