Burned Area Emergency Response (BAER) Update - Fork Complex
Incident: Fork Complex Wildfire
Release Date: Sep 24, 2015
Contact(s): BAER Information: (415) 881-1871
In order to assess the probability and potential volumes of debris flows in the Fork Complex burned area, the Shasta-Trinity National Forest BAER assessment team obtained the assistance of the US Geological Survey (USGS). Their ongoing research has developed empirical models for forecasting the probability and the likely volume of such debris flow events. To run their models, USGS uses geospatial data related to basin morphometry, burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to a design storm. A 10-year design storm (a 10% chance of occurrence in any given year) was selected to evaluate debris flow potential [probability] and volumes. Debris-flow hazards from a given basin can be considered as the combination of both probability and volume.
From the USGS website found at http://landslides.usgs.gov/hazards/postfire_debrisflow/:
“Wildfire can significantly alter the hydrologic response of a watershed to the extent that even modest rainstorms can produce dangerous flash floods and debris flows. The USGS conducts post-fire debris-flow hazard assessments for select fires in the Western U.S. We use geospatial data related to basin morphometry, burn severity, soil properties, and rainfall characteristics to estimate the probability and volume of debris flows that may occur in response to a design storm.”
USGS debris flow modeling estimates that in the burned area of the Barker Fire, the Little Baker watershed basin is the only basin that has a combination hazard class of high. The remaining Fork Complex burned areas have two relatively small watershed basins that are estimated to have a high combination hazard class. One of these two basins is within the Gardner Gulch, located in the southeast corner of the Peak Fire; the second basin is a small basin on the west end of the Rail Fire, just below the Kingsbury Range and above Highway 3. Both of these basins are located above private properties adjacent to the Shasta-Trinity National Forest boundary.
The USGS debris flow hazard assessment is a part of the Shasta-Trinity National Forest’s BAER watershed response assessment report. In addition to the team’s post-fire watershed response findings, the BAER team’s report contains a description of the potential risks to human life, safety, property, cultural resources, and critical natural resources along with recommended short-term emergency stabilization treatments for burned federal lands. When the BAER team’s report is finalized it will be posted on the Shasta-Trinity NF Post-Fire BAER InciWeb page at http://inciweb.nwcg.gov/incident/4601/.
USGS Fact Sheet 176-97, entitled “Debris Flow Hazards in the United States” contains information used to interpret the debris flow map and analysis that was incorporated into the BAER assessment team’s anticipated soil erosion and hydrologic response findings.
In 2005, USGS prepared a report published as USGS Fact Sheet 3106 entitled “Southern California—Wildfires and Debris Flows” that describes how the agency “developed its methods that are used to estimate the locations, probability of occurrence, and size of potentially destructive debris flows. Public officials can use this information to plan and execute emergency response and post-fire rehabilitation.”
According to the USGS, “Analysis of data collected from studies of debris flows following wildfires can answer many of the questions fundamental to post-fire hazard assessments— what and why, where, when, how big, and how often?” This information is extremely important to assist the public with increasing their safety awareness of the areas where there may be a higher increase in flooding, sediment and soil erosion, and a high probability of debris flows –- all of which are potential risks to human life, safety, and property.
SPECIAL NOTE: Everyone near and downstream from the Fork Complex fire areas should remain alert and stay updated on weather conditions that may result in heavy rains over the burn scar. Flash flooding may occur quickly during heavy rain events. Current weather and emergency notifications can be found at the National Weather Service, Eureka Office (http://www.wrh.noaa.gov/eka/) website.