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TETRA TECH
Debris Flow Modeling &
Regulations in
Aspen, Colorado
Dai Thomas, PhD, PE
Andrew Earles, PhD, PE – Wright Water Engineers
Jim O’Brien, PhD, PE - FLO-2D
April Long, P.E. – City of Aspen
3rd May 2017
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Aspen Mountain
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Geology and Mining
It’s complicated!
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Tourtelotte Park c1890-1893
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Historic
• September 1919
– Cloudburst
– “yellow clay mud from the mountain”
• August 1964
– 1.13 inches in 1 hour
– Pioneer Gulch up to 5’ of mud
• June 1984
– Strawpile Landslide
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Strawpile Landslide
• June 1984
• Downtown Evacuated
• 28 to 62 feet deep
• ~15 acres
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Mudflow Characteristics
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Concentration by Volume (CV)
0.0
0.1
0.2
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0.5
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Wate
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Transition Zone
Mud
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Mud
Flo
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Time (hrs)
Flow
Bulked Flow
Sed. Conc.
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Mudflow Characteristics
• Saturated Soil Conditions
• Sediment
• Rainfall (10 to 25-Year Event)
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Mudflood
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• WRC – Drainage Master Plan (2001)
Existing Regulations
City of Aspen Mudflow Zones J
Areas on or within 200 feet of slopegreater than 30%
Areas south of Durant St located within2ft mudflow depth on the 100-yr mudplain map
Slopes greater than 30%
This map/drawing/image is a graphical representation of the featuresdepicted and is not a legal representation. The
accuracy may change depending on the enlargement or recustion.
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• Similar to FEMA procedure
– Duplicate Effective
• 100-Year Peak Flow Event
• Sediment Concentration 45%
– Existing Conditions
– Project Conditions
Existing Regulations
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• Evaluate downfan impacts
– No increase in mudflow depths on
neighboring properties
• Evaluate static and dynamic forces on structures
• Identify potential mitigation measures
– Store mudflow
– Convey mudflow to streets
Existing Regulations
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Example
ExistingProject
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Increase in Depth
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• Evaluate potential impact to City
• Historical Review
• Geologic Investigation
• Develop new mudflow flow (FLO-2D) model
– 2-Hour, 2-, 25-, and 100-Year Rainfall Events
– Depth and Extent of Flooding
– Hazard Mapping
– Wildfire Analysis
– Mitigation
– Economic Analysis
– Develop New Guidelines
Current Study
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• 20-foot Grid Size
• 165,214 Elements
• Based on LiDAR
mapping from City
New FLO-2D Model
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Overbank Manning’s n-values.
Land Use n-value
Urban/Structures 0.04
Roads/Streets 0.02
Mine Tailing 0.40
Grassland/Ski Runs 0.20
Light Forest 0.30
Medium Forest 0.35
Dense Forest 0.40
Manning’s n roughness
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Infiltration
Horton's infiltration parameters.
Hydrologic
Soil Group
Initial Rate
(in./hour)
Final Rate
(in./hour)
Decay
Coefficient
(1/second)
A 5 1 0.0007
B 4.5 0.6 0.0018
C 3 0.5 0.0018
D 3 0.5 0.0018
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Colorado Unit Hydrograph Procedure (CHUP)
NOAA Atlas 14
Rainfall
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Ra
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Time (hours)
2-Year
5-Year
10-Year
25-Year
50-Year
100-Year
Recurrence
Interval
(years)
Rainfall
(in.)
2 0.47
5 0.64
10 0.77
25 0.95
50 1.09
100 1.23
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Water and Sediment Hydrograph
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Bulked Flow
Sed. Conc.
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Model Output
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Mapping
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Economic Costs
Location Mud
Depth
Days of
cleanup
Cost
Streets >$300,000
Sewer Lines >$380,000
Residential 6’ 11 >$800,000
Hotel 3’ 50 >$4.5M
Commercial 3’ 3 $165,000
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• Adjust zoning areas
• 25-year (45% concentration)
• 100-year (20% concentration)
• Depth increase up to 0.5 feet
– On ground previously inundated
– Make reasonable effort to not increase.
• No depth increase on land not inundatedunder existing conditions
Possible Changes to Regulations
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• Model will be available to:
– Developers
– Engineers
• Substantial decrease in analysis cost
• Easier for City to review
• City to keep track of model changes
Current Study