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Are Predicative Indicators in a Standard Regional Model Accurate in Determining Peak Discharge and Other Flooding and Debris-flow Hazards from Recently Burned Basins?
Objective and Application:
The primary objective of this work is to evaluate the effectiveness of GIS-based methodologies for quickly ranking the relative degree of hazard posed by debris flow and sediment-laden streamflow from recently burned basins and, based on these evaluations, to develop data-calibrated models for estimating peak discharges and the probability of debris-flow occurrence from recently burned basins.
The models developed here will be useful both in pre-fire planning and post-fire response. Application of models before the occurrence of wildfires can be used to identify sensitive basins and thus serve to direct protection efforts during a fire incident. Post-fire, these models can be used not only to guide and prioritize post-fire data collection and field mitigation efforts, but also to make effective and appropriate mitigation decisions.
Study and Sampling Design:
The first element of our study is to apply GIS-based quick assessment methodologies developed by Cannon and Gleason (USGS and Colorado Geological Survey) and the Montana Bureau of Mines and Geology for ranking the relative degree of hazard posed by debris flow and sediment-laden streamflow from selected basins burned during the summer of 2000 in Regions 1 and 4.
Monitoring of the response of the recently burned basins to spring snowmelt and summer thunderstorms using available and supplemental monitoring data will be used to evaluate the effectiveness of the quick-assessment methodologies, and to make modifications for improvement. In addition, post-event field examinations will be used to identify the transport processes produced (debris flow, sediment-laden flooding, streamflow) in response to each significant rainfall event. This monitoring configuration allows us to collect data both on the cause (rainfall) and the effect (peak discharges and flow processes) of post-wildfire runoff events.
In addition to the data on the basin response to rainfall events, for each basin we will characterize the geologic materials (rock type, soil characteristics, and grain-size distribution of burned surficial soils) from geologic maps, soil surveys, and field samples collected after the fire, basin morphology (area, average drainage gradient, relief ratio, aspect, and distributions of hillslope gradients) measured from 10- and 30-m DEMs, the presence or absence of water-repellent soils determined from field tests, and the area burned at high and moderate severities determined from the burn intensity map of the fire.
Where possible, USFS personnel will be used for the monitoring and characterization of basin response to storm events. This project will augment existing monitoring efforts by USFS, USGS, and BLM.
Data analysis:
The peak discharge and rainfall data collected will be used to develop rating curves for estimating peak discharges from recently burned basins. In addition, using the data collected from the spring and summer monitoring period, application of a multiple logistic regression analysis will be used to develop a model for debris-flow susceptibility. Such an analysis returns the probability of a positive binomial outcome (in this case, debris-flow occurrence) as a function of the dependent variables. This model, when applied to individual drainage basins, can then be used to generate probabilistic maps of debris-flow hazards following wildfires.
Products:
- Modified methodologies for the rapid assessment of relative hazards posed by sediment-laden streamflow and debris flows from recently burned basins based on data readily available before or after wildfires.
- A model for debris-flow susceptibility that can be applied to recently burned watersheds to determine the probability of debris-flow occurrence relative to different rainfall events.
- A GIS-platform database consisting of channel-response (peak discharge and response process), basin characteristics, and rainfall data compiled for burned basins in Regions 1 and 4.
- Rating curves for estimating peak discharges from recently burned basins in Regions 1 and 4.
Information Dissemination:
Reports that describe the techniques and results from this project will be published in peer-reviewed journals as well as other professional formats accessible to the Forest Service.
| Project Activity | Cost: | Agency: |
| FY2001: |
| Basin geology and configuration data compilation Region 1: | 20K | MBMG | | GIS-based rapid assessments Region 1: | 10K | MBMG | | Basin geology and configuration data compilation Region 4: | 20K | USGS | | GIS-based rapid assessments Region: | 10K | USGS | Monitoring in Region 1: (this funding requests augments an on-going project)
Rain and crest stage gages/ Event Response | 20K | USGS, USFS* | Monitoring in Region 4: (this funding requests augments an on-going project)
Rain and crest stage gages/ Event Response | 40K | USGS, USFS* | |
*The status of the existing monitoring activity is unclear to date. This portion of the budget may need to be readdressed. |
| FY2002: | |
Development of improved rapid assessment methodologies | 40K | USGS, MBMG | | Debris-flow susceptibility model development | 40K | USGS, MBMG | | Rating curve development | 10K | USGS | |
Lead Scientists and field personnel:
Susan Cannon, USGS (303)273-8604 cannon@usgs.gov
Robert Wintergerst, Forest Service (406)494-0259 rwintergerst@fs.fed.us
John Metesh, Montana Bureau of Mines and Geology (406)496-4159 Jmetesh@mtech.edu
Robert Davis, USGS (406)457-5921 rdavis@usgs.gov
David Clark, USGS (208)387-1392 clarkd@usgs.gov
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