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

Contents

Background

Approach

Hazard Assessment

Rainfall and Hydrologic Response

Model Validation

Sensitivity Analysis

Conclusions

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

Background

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

Hazard Assessment

Model parameters:

Burn severity

Rainfall conditions

Topographic characteristics

Soil properties (STATSGO US Soils Database - Soil Survey Staff, 2011)

USDA Forest Service, 2010

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)

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)

Hazard Assessment

Hazard Ranking:

Sum of probability and volume rankings

Hazard rankings

(2-3=low, 4=medium,

5-6=high)

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

Rainfall and Hydrologic Response

Rainfall and Hydrologic Response

Intensity-duration threshold: I = 12.0D-0.5

Cannon et al., 2008

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

Model Validation

Probability Models

• Recorded average storm intensity values for debris flow storms

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

Sensitivity Analysis

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

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.)

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)