Debris Flows/Mudflows

Debris flows pose a property and life safety risk within and below drainages and sloping terrain throughout the CZU burn area. Residents occupying homes in these areas should be aware of the heightened risk of debris flows for several rainy seasons following a fire before returning to baseline risk levels, which remain elevated due to the geology, climate and topography of the Santa Cruz Mountains.

  • Debris Flow Hazards – Frequently Asked Questions (FAQs)
    • What is a Debris Flow?
      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.
    • What makes Debris Flow so dangerous?
      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.
  • WATCH: "Debris Flow" Event – Foreman Creek, Boulder Creek – December 13, 2021
  • CA Department of Conservation Debris Flow Facts
  • Debris Flows Explained: A short New York Times Video: The Science of Mudslides
  • Debris Flows in Montecito: A short video showing the devastating impacts of a 2018 debris flow event.

Evacuations and Emergency Planning

County geologists are working with local, state and federal partners to assess debris flow risks in neighborhoods throughout the burn area and prepare evacuation plans should upcoming winter storms trigger a debris flow event. Residents should be prepared to evacuate. Be aware that temporary housing accommodations must be approved by the County and ensure that first responders are aware of those residents’ locations should evacuations or rescue operations become necessary.

Rebuilding in Debris Flow Hazard Areas

Residents rebuilding within potential Debris Flow Hazard areas must address potential slope instability in the design of their rebuild projects. Due to the time and expense required for site-specific geologic evaluation of rebuild sites, the County Board of Supervisors has issued a Rebuild Directive that alleviates eligible “in-kind” replacement home projects from the requirement to prepare a full geologic report as outlined in the Santa Cruz County Geologic Hazard Ordinance (County Code Chapter 16.10). To assist rebuilders in meeting California Building Code (CBC) requirements to address slope instability, the County has partnered with the Community Foundation of Santa Cruz County who generously funded the Atkins Engineering Flood & Debris Flow Hazard Study to further characterize debris flow hazards and provide data that may be utilized by geotechnical (soils) engineers to meet the design requirements of the CBC.

Debris Flow Studies and Reports

State Partners

Rapid evaluations of geologic hazards within and below the CZU burn area were conducted by state partners immediately following the CZU Fire and identified areas of heightened risk of debris flows.

County Mapping

The areas at risk of potential impact from debris flows were refined by field mapping conducted by County geologic staff to develop a map of potential hazard areas for use in evacuation planning and geologic hazard screening for rebuild proposals, which will be updated as additional information is evaluated by the County Geologist.