USGS Debris Flow Hazard Mapping Completed
WINTHROP, WA– The Cub Creek 2 and Cedar Creek Fire Burned Area Emergency Response (BAER) Team coordinated with the US Geological Survey (USGS) during its evaluation of the burned area to assess potential post-fire hazards, including debris flows.
The USGS utilized the BAER team’s soil burn severity map to model potential debris flow hazards within the burned area. The resulting Debris Flow Hazard Map displays the combined relative debris flow hazard for areas burned by the Cub Creek 2 and Cedar Creek Fires. The combined relative hazard metric is a combination of the likelihood of debris flow (in %) and potential volume of debris flow (in m3).
Probabilities are calculated based on a typical storm for this area, which are described by both their intensity and volume. On the Okanogan-Wenatchee NF, in any given year, there’s a 20% probability of a 15 min intensity / 32 mm per hour storm occurring during a one-hour rainstorm. That means that over a 15-minute period, we could expect to receive 8 mm (~0.32 inches) of rain.
“Having a storm that produces about a third of an inch of rain over a 15-minute period is not uncommon for this forest, so we decided to use that as our “model storm” for analysis,” said hydrologist Molly Hanson. “At that intensity, there are multiple watersheds within the burned area that have the potential to produce debris flows.”
Many watersheds within the burned area have a relatively high probability of experiencing a debris flow. On the Cub Creek 2 Fire, multiple basins that feed into Eightmile Creek, Falls Creek, and the main stem of the Chewuch River are at high risk (>60% probability) in response to expected storm intensity. On Cedar Creek, multiple basins in Wolf Creek, Methow River Valley, and along Highway 20 on the north of the fire also have a high probability of experiencing a debris flow (see accompanying maps).
This information is extremely important as we all need to improve our safety awareness of the areas, especially where there may be an increased risk of hillslope erosion, flooding sediment delivery to streams, and a higher probability of debris flows – all of which are potential risks to human life, safety, and property.
While the USGS delivers debris flow hazard potential to BAER teams for multiple storm designs, their website only displays results using a standard storm design for all fires across the country: 15 min / 24 mm/h. Please be aware of the differences between their visualization and local implementation due to site-specific hydrological experience.
USGS debris flow combined hazard maps are available in both JPEG and PDF formats and can be viewed and downloaded under the "Related Information" header below or the "Maps" tab of this InciWeb site. Maps will also be available at http://www.centralwashingtonfirerecovery.info/
SPECIAL NOTE: Everyone near and downstream from the Cub Creek 2 and Cedar Creek Fires burned area should remain alert and stay updated on weather conditions. Flash flooding could occur quickly during heavy rain events. Current weather and emergency notifications can be found at the National Weather Service (NWS), Spokane (OTX) (https://www.weather.gov/otx/) website.
Central Washington USFS BAER information is available at http:/www.centralwashingtonfirerecovery.info