2012 southwest wildfire hydrology and hazards workshop evan friedman and dr. paul santi colorado...
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DEBRIS-FLOW HAZARD ASSESSMENT AND
MONITORING WITHIN THE 2010 MEDANO FIRE
BURNED AREA,
GREAT SAND DUNES NATIONAL PARK AND
PRESERVE, COLORADO
2012 Southwest Wildfire Hydrology and Hazards Workshop
Evan Friedman and Dr. Paul Santi
Colorado School of Mines
3 April 2012
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Contents
Background
Approach
Hazard Assessment
Rainfall and Hydrologic Response
Model Validation
Sensitivity Analysis
Conclusions
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Background The Medano Fire burned 6000 acres at Great Sand
Dunes NP during the summer of 2010
Debris-flow hazard assessment for park resource managers
Regression models to predict probability and volume of post-fire debris flows (Cannon et al., 2009)
Debris-flow monitoring used to validate models, and establish rainfall thresholds and relative timing
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Background
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Approach
Perform initial hazard assessment using design storms
Monitoring of rainfall and hydrologic response (rain gauges, pressure transducers, deposit surveys)
Validate predictive model results using actual storms
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Hazard Assessment
Model parameters:
Burn severity
Rainfall conditions
Topographic characteristics
Soil properties (STATSGO US Soils Database - Soil Survey Staff, 2011)
USDA Forest Service, 2010
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Hazard Assessment Debris-flow probability
prediction:
Three logistic regression models: A, B, and C (Cannon et al., 2009)
2-year and 10-year, 1-hour design storms
Models did not agree on magnitude, and thus were averaged
Probability rankings (low=1, medium=2, and high=3)
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Hazard Assessment
Debris-flow volume prediction:
One regression model (Cannon et al., 2009)
2-year and 10-year, 1-hour design storms
Volume rankings (low=1, medium=2, and high=3)
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Hazard Assessment
Hazard Ranking:
Sum of probability and volume rankings
Hazard rankings
(2-3=low, 4=medium,
5-6=high)
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Rainfall and Hydrologic Response
8/22/11 Debris Flows Basins 24 and 25
8/28/11 Debris Flows Basins 15, 23, 24, and 25
8/22/11 Debris Flow Basin 32
8/22/11 Debris Flow Basin 7
8/28/11 Debris Flow Basin 7
7/22/11-8/14/11 Un-calibrated Data
6/16/11-8/14/11 Debris Flows Basin 7
6/22/11-8/15/11 Missing Data & Debris Flow Basin 32
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Rainfall and Hydrologic Response
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Rainfall and Hydrologic Response
Intensity-duration threshold: I = 12.0D-0.5
Cannon et al., 2008
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Rainfall and Hydrologic Response Debris-flow and
flood timing:
5:10pm Debris Flow
4:19pm Debris Flow
4:55pm Debris Flow
5:10pm Debris Flow
4:19pm Debris Flow
4:55pm Debris Flow
5:10pm Debris Flow 4:19pm
Debris Flow 4:55pm Debris Flow
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Model Validation
Probability Models
• Recorded average storm intensity values for debris flow storms
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Model ValidationVolume Model
• Recorded total rainfall amounts for debris flow storms
Model predictions one order of magnitude higher than measured
Previous validations: similar models over-predict volumes for relatively small basins
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Sensitivity Analysis
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Conclusions
Peak rainfall intensities for short periods within storms better predict debris-flow occurrence than average storm intensity in this setting
Models A and C were successful at predicting high probability of debris flows in this setting
The volume model predicts volumes within one order of magnitude higher than measured for relatively small basins in this setting
Percentage of basin area burned at moderate to high severity is the most significant variable for debris-flow probability in the western US
Probability models are sensitive to soil property variables, thus representative values from the range of STATSGO data
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Probability ModelsProbability = ex/(1+ex)
Model A: x = -0.7 + 0.03a - 1.6b + 0.06c + 0.2d - 0.4e + 0.07f
a = % basin area w/ slope >30%
b = Ruggedness (change in basin elev./sq. root of basin area)
c = % basin area burned at moderate and high severity
d = Clay content (%)
e = Liquid limit (%)
f = Avg. storm intensity (mm/hr.)
Model B: x = -7.6 - 1.1a + 0.06b + 0.09c - 1.4d + 0.06e
a = Ruggedness (change in basin elev./sq. root of basin area)
b = % basin area burned at moderate and high severity
c = Clay content (%)
d = Organic matter (%)
e = Avg. storm intensity (mm/hr.)
Model C: x = 4.8 + 0.05a + 0.2b - 0.4c - 1.5d + 0.07e
a = % basin area burned at moderate and high severity
b = Clay content (%)
c = Liquid limit (%)
d = Hydrologic group (based on soil infiltration rate and depth to confining layer)
e = Average storm intensity (mm/hr.)
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Volume Model
Ln V = 7.2 + 0.6(Ln A) + 0.7(B)1/2 + 0.2(T)1/2 + 0.3
V = volume (m3)
A = area of basin w/ slopes >30% (km2)
B = area of basin burned at moderate and high severity (km2)
T = total storm rainfall (mm)