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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

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Transition Zone

Mud

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Mud

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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

0

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in.)

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

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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