Post-Fire Debris Flow Hazards View (Public Use)

Los Angeles County Public Works’ (Public Works’) Debris Flow Forecasts are based upon a review of watershed conditions, weather forecasts from the National Weather Service (NWS) and other weather predicting sources, and the functionality of flood control facilities. The purpose of the Debris Flow Forecasts is to provide emergency response agencies and the public with an assessment of the magnitude of potential debris flows associated with forecasted storms in recently burned watershed areas. What Areas Are Covered by Debris Flow Forecasts Public Works generally prepares Debris Flow Forecasts for all areas in which a major fire has occurred within the last 4 years. Each forecast includes qualitative descriptions of the estimated severity of debris flow impacts (see below). Debris Flow Forecast Determinations Approximately 36 to 48 hours prior to the arrival of a forecasted storm, Public Works will begin tracking the storm closely and will make a preliminary assessment of the potential for debris flows. Public Works considers several of factors when preparing Debris Flow Forecasts, including: * The degree of watershed recovery * Estimated rainfall totals * Estimated rainfall intensities * Whether thunderstorms are being forecasted and if so, their likelihood. * The degree of confidence in the forecasts, including the forecasters’ level of confidence in the models upon which their forecasts are based and the accuracy of the forecasts for prior storm events. * National Weather Service advisories for flood watch, warnings, and advisories, including summary of greatest impacts for mud flows/rock falls impacts. * Estimated soil moisture in the burned watersheds. * Whether the forecasted storm is the first storm after a dry or relatively dry period, or will occur soon after one or more previous storms. * If there have been recent storms over the fire area, whether the previous storms produced enough rainfall to saturate or nearly-saturate the soil in the fire area. Beginning 24 hours prior to the storm’s arrival, Public Works makes a Debris Flow Phase estimate (as described below) for each fire area. It should be noted that, notwithstanding Public Works' best engineering judgment, debris and mudflows are highly unpredictable events. The purpose of the following Debris Flow Phase descriptions is to assist storm response Divisions, members of the public and emergency response agencies in their emergency planning: NO PHASE (light rainfall –minimal debris flow potential) Rain is forecast by the NWS, but the amount of rain and/or watershed conditions are not anticipated to produce appreciable amounts of debris on streets or specific public infrastructure locations above those normally observed under unburned watershed conditions. Reports of debris on private property may be received by emergency response agencies. Few, if any, structures are anticipated to be impacted, other than those advised to be prepared to evacuate with any forecast of rain. PHASE I (light to moderate rainfall – light to moderate debris flow potential) Small isolated debris and mudflows possible at specific public infrastructure locations. Some streets may be flooded or partially/significantly blocked by debris at these locations. Reports of debris on private property may be received by emergency response agencies. A few structures may be impacted, in addition to those advised to be prepared to evacuate with any forecast of rain. PHASE II (moderate to heavy rainfall – moderate to heavy debris flow potential) Moderate debris and mudflows possible at more widespread locations. Several streets may be completely blocked by debris. Depending on location and terrain, some structures may be impacted, in addition to those advised to be prepared to evacuate with any forecast of rain. PHASE III (heavy rainfall – large amounts of debris flow potential) The potential exists for significant debris and mudflows to be widespread over specific areas. Significant numbers and/or extents of streets may be blocked and considered unsafe for travel. Many structures could be impacted by debris and mudflows. Extreme caution should be exercised below uncontrolled canyons due to sudden debris flows. Debris Flow Forecast Postings Debris Flow Forecasts are posted to the Internet on Public Works’ Fire Disaster Information website: [https://pw.lacounty.gov/wrd/Fire/]. In addition, residents as well as public agencies can sign up for Public Works’ eNotify system to receive Phase alerts: [https://pw.lacounty.gov/go/enotify/]. Similarly, residents can sign up for “Alert LA County” to receive emergency notifications.