Snow Avalanche Hazard Assessment and Mitigation Study

Wasatch Drain Tunnel Office Building

Snowbird Village,

Salt Lake County Utah

Proposed

building site

Table of Contents

1. Avalanche Hazard Assessment

2. Snow Avalanche Impact Forces

a. Scope of Work, and Design Criteria

b. Design Snow Avalanche Loads and Velocities

c. Powder Avalanche Blast Loads

3. Appendices: A-C pages 1-29

a. Google Earth Image of Site

b. Table I Summary of Avalanche Dynamic Models

c. Superior Slide Path:

i. Photo of Extents

ii. ALI5 Superior 300- year output

iii. ALI5 Superior 30- year output ( 2m crown)

iv. ALI5 Superior 30- year output (1.5m crown)

v. PCM Model of Superior

d. Blackjack Area: Appendix D pages 1- 64

i. Photo of Blackjack Area

ii. ALI8-Blackjack Area 2.5m 300-year

iii. ALI8-Blackjack Area 2.0m 300-year

iv. ALI8-Blackjack Area 1.5m 30-year

v. ALI8-Blackjack Area 1.0m 30-year

vi. ALI8-Blackjack Area 1.0m 30-year – narrower starting zone

vii. Profile2-Blackjack Area 300- year 300 m width

viii. Profile2-Blackjack Area 30- year 250 m width

ix. Profile2-Blackjack Area 30- year 200 m width

x. Profile 2-Blackjack Area PCM model

Avalanche Hazard Assessment

CTS Engineering has been contracted by Salt Lake County Service Area #3 to perform a snow avalanche

study to address the requirements of Salt Lake County Code section 19.75.083 for the building site at

the existing entrance to the Wasatch Drain Tunnel. The proposed building site is located on an

unincorporated portion of Salt Lake County in the vicinity of the Base Area of Snowbird Resort. CTS

Engineering is partnered with Dave Scroggin of IGES, and Liam Fitzgerald (Lead UDOT Avalanche

Forecaster) to prepare this study. Figure 1 indicates the location of the building site in a Google Earth

image. The purpose of the proposed new structure is to provide new office space for the Service Area#3

employees adjacent to the drain tunnel service entrance. This new building will limit the exposure of

the employees to snow avalanche danger since they will not have to travel across the Snowbird parking

lot to access the drain tunnel. The new Building is approximately 60 ft. x 40 ft. x 18 ft. tall located at the

front of the current drain tunnel entrance.

Qualifications of Engineering Team:

Joseph D Crilly: MSCE, PE, SE; over 15 years of performing Avalanche Dynamic modeling for projects in

the town of Alta, and other locations in the Wasatch Range. Completed three graduate level classes at

the University of Utah based on Avalanche dynamics, snow mechanics, and snow avalanche control

systems. In depth knowledge of AVAL-1D; PCM methods, and NGI-method for statistical methods for

predicting snow avalanche runout distances.

Dave Scroggin:

Liam FitzGerald: Professional avalanche worker since 1968, 42 years working as avalanche professional

in Little Cottonwood Canyon (27 years as Snow Safety Director at Snowbird, 15 years as Supervisor of

UDOT Highway Avalanche Safety program). Consultant for avalanche related problems in ski areas,

helicopter skiing operations, mining operations, and oil exploration, in North America, Europe, and Asia.

Avalanche analysis for residential and commercial building projects in Lt. Cottonwood Canyon. Instructor

(past) National Avalanche School, American Avalanche Institute.

The study was performed in four separate phases:

I-Field studies:

• To determine whether the proposed building site is within a snow avalanche area.

• Analysis of terrain, to help determine the volume of snow that could become involved in the

design avalanche.

• Analysis of terrain to determine the direction of flow and possible runout distance of avalanche

debris.

• Analysis of tree damage and vegetation patterns to help determine direction of travel, and flow

height of avalanche debris, and some assumptions of the return interval of large avalanche

events.

II-Mapping:

• Detailed mapping of the area of avalanche release, and the flow of avalanche debris, with

contour/elevation references.

III-Avalanche Modeling:

• A detailed analysis of the likely area of avalanche release, contour lines, and slope angle, to

determine the velocity of the Design Avalanche, as well as impact pressures at the building site.

VI- Review of Past History:

• A review of recorded avalanches that have affected the proposed building site, as well as

anecdotal reports of historical avalanche events in the area, to help determine the Return

Interval of avalanche events capable of reaching the proposed building site.

The building site is located on the north of the existing parking structure in the Cliff Lodge’s parking area

at Snowbird in Little Cottonwood Canyon. This site is in the confluence of two snow avalanche paths,

Superior and Little Superior from the north, and Black Jack Area from the east of the site.

Figure 1

The County’s avalanche hazard ordinance provisions reference the Colorado Avalanche Handbook (Snow

–Avalanche Hazard Analysis for Land-Use Planning and Engineering A.I. Mears Bulletin 49 Colorado

Geological Survey) and in part requires snow avalanche event analysis using a 25-year return period to

determine whether the site falls into a ‘red’ hazard zone (in which building is not prohibited) vs. a ‘blue’

hazard zone (in which construction of habitable structures is permitted only with appropriate structural

or other mitigation). Our team modeled the avalanche paths affecting the site using AVAL-1D, a

sophisticated numerical computer model, which solves a system of partial differential equations that

model snow avalanche flow; this program is produced by the Swiss Federal Institute of Snow and

Avalanche Research. This program models the 30-year return period, which effectively addresses the

25-year return period. The model also allows evaluation of a 300-year return period, which we used to

address the building design parameters.

To determine the 30-year and 300-year snow avalanche runout distances, site velocities, and site impact

pressures; snow volumes which are directly proportional to snow avalanche slab thicknesses were

estimated and coordinated with visual observations of Liam Fitzgerald and Dave Scroggin. This

information was then modeled using AVAl-1d. This computer model has been calibrated to the runout

distances measured in the Wasatch Range, and path topography was determined by path profiles and

slope analysis performed by IGES. Although the 30-year return period is slightly more conservative than

the 25-year specified by the County Code, the volume of snow input into the model, (which has a great

determination of the run out distance and impact pressure), is representative of a twenty-five year

event. Therefore we will refer to the thirty year event for our cut off point on the red zone blue zone

demarcation.

Our study determined that the proposed building site is in an area that is likely to be overrun by

avalanche debris more than once every 25 years, as well as an area where impact pressures from the

maximum avalanche event that could occur once in 300 years would exceed 600 psf. The

determinations of both Return Interval and Impact Pressure would suggest that the proposed building

site is in a “Red Zone” according to section 19.75.083 of the Salt Lake County Code.

We are requesting building approval for this location based on the following multi-faceted mitigation

procedures:

• Existing local structures constructed and occupied under similar “red” zone hazards.

• Institution controls:

o Active Avalanche Controls implemented which affect the avalanche paths affecting the

building location.

o Active Inter-Lodge Travel controls which affect the occupants travel outside the building

during high avalanche periods.

• Building engineering and grading to minimize structural impact forces and resist resulting forces.

Existing Conditions: Adjacent areas such as the Town of Alta, and the Snowbird Village or Base Area,

have numerous inhabited buildings (Lodges, private residences, commercial buildings, etc.) located

within avalanche runout zones, and which are threatened by avalanche events more frequent the a “25-

year” event.

Institutional Controls: The same avalanche paths that affect buildings within the Town of Alta and the

Snowbird Village, also affect the canyon road (State Road 210). The Utah Department of Transportation

has developed a program to address the avalanche hazard affecting the canyon road. That program

includes continually monitoring snowpack and weather information, developing an Avalanche Forecast

based on weather and snowpack conditions, and using explosives to test snowpack stability and to

initiate avalanches. Once it has been determined that avalanches maybe likely or possible, the canyon

road is closed to travel and explosives are used in an attempt to reduce the avalanche hazard. This

method of monitoring conditions, developing a forecast, and artificial release of avalanches has proven

to be very successful in reducing the avalanche hazard to acceptable levels that allow for thousands of

vehicles to travel up and down the canyon road each day. The efforts that are carried out to provide for

safe travel along the canyon road, also helps reduce the avalanche hazard to numerous buildings at the

Town of Alta and the Snowbird Village. Although data is limited, field observations and dynamic

modeling both indicate regular, thorough avalanche control work lengthens the return interval of large

and destructive avalanches in controlled paths but it does not however, eliminate snow avalanches

completely. This suggests that not only the canyon road but inhabited structures adjacent to the road

will be reached on a less frequent basis under existing mitigation efforts than if no avalanche control

work were carried out.

Due to the frequent development of hazardous avalanche conditions, and regular avalanche control

work in Little Cottonwood Canyon (including above the Town of Alta and the Snowbird Village)

restrictions are implemented to control travel between, and outside of buildings during certain times.

These restrictions known as Inter-Lodge Travel Restrictions are implemented by the Alta Town Marshal,

and the Unified Police Department, Salt Lake County1. These restrictions are made after consultation

with or recommendations from UDOT Avalanche Forecasters. Inter-Lodge Travel Restrictions require all

occupants to remain inside of buildings during periods of high avalanche hazard (determined by the

avalanche forecast) or when avalanche control work is being carried out. Inter-lodge Travel Restrictions

are lifted once avalanche control work has been completed and/or it is felt that the avalanche hazard

has been reduced to acceptable levels.

Avalanche occurrence, and Return Interval, is directly associated with snowpack and weather

conditions. Monitoring of these conditions is an integral part of the Avalanche Forecast. Little

Cottonwood Canyon, including the areas above Alta and Snowbird has the benefit of continuous

monitoring of these conditions as well as continual updates of the avalanche forecast throughout the

avalanche season, by trained avalanche forecasters. Whereas at times, snowpack and weather

conditions are misinterpreted, and the avalanche hazard underestimated, most of the time avalanche

forecasters have a good assessment of the hazard to the canyon road, and to occupied buildings. It is a

widely held belief, (and supported by historical data), that the Inter-lodge Travel Restrictions

1 UPD has delegated the implementation of Inter-Lodge Travel Restrictions within the Snowbird Village to

Snowbird Snow Safety personnel.

implemented within the Town of Alta and the Snowbird Village provide protection to occupants of

buildings located within avalanche run out zones by requiring them to remain inside during those times

when avalanches would be most likely to occur.

Salt Lake County Service Area #13, as a public utilities facility, receives information directly from UDOT

Avalanche Forecasters as to when Inter-lodge Travel restrictions will be implemented. Salt Lake County

Service Area #13 will develop its own protocol for compliance with Inter-lodge Travel restrictions.

Enforcement of Inter-lodge Travel restrictions, including those affecting the proposed building site will

be carried out by the Alta Town Marshal, UPD Salt Lake County, and the Utah Department of

Transportation.

Engineering: Due to the snow avalanche hazard to the structure the structure will be engineered by a

licensed structural engineer well versed in avalanche dynamics to withstand 300-year return period

snow avalanche forces. These forces will be mitigated through the use of grading, concrete deflection

walls, and a reinforced concrete structure. The avalanche modeling report of design forces at the site

follows.

The combination of existing structures in the vicinity of the building site, institutional controls (inter-

lodge, SR-210 road closures, and an educated, avalanche-aware community), site-specific snow

avalanche hazard analysis, building structural mitigation by a licensed structural engineer maintain

appropriate building and life safety for this site. These methods have been effectively applied in Little

Cottonwood Canyon at Snowbird Ski Resort and the Town of Alta to prevent and minimize structural

damage and/or injury to persons. It is for these reasons we are requesting building approval at this site.

Sincerely,

Joseph D. Crilly, SE

4625 South 2300 East, Suite 105, Holladay, UT 84117

Phone 801-274-2831 Fax 801-274-2832

joe@ctsengineering.net

www.ctsengineering.net

COMMITMENT TO ENGINEERING EXCELLENCE AND SERVICE

Snow Avalanche Hazard Report February 22, 2013

Client: Keith Hansen General Manager Salt Lake County Service Area #3 P.O. Box 320067 Snowbird, Utah 84092-0067 . Property Location: Wasatch Drain Tunnel Snowbird, Utah Scope of Work: This report completes sections 1 through 5 of the scope of work listed below.

1. Description of the snow avalanche hazard at the building site referenced above.

2. 30 and 300-year snow avalanche loads on proposed structure.

Consultant:

CTS Engineering, Joseph D. Crilly, S.E.

Design Criteria:

This analysis is based on a return event frequency of 300 years, a one third percent chance of occurrence in any year. The return periods chosen are based on the two requirements. The county ordinance requires a one-hundred year return period for design forces for structures in a snow avalanche runout zone, and second the avalanche software AVAL 1-D is calibrated for a 300 year return period. The use of the greater return periods allows the forces provided by this report to be considered strength design level forces to be used by the building’s structural engineer. The higher return period therefore exceeds the 100-year return period required by county ordinance.

CTS Engineering

Preliminary Design Snow Avalanche Loads and velocities: Forces are for impact on a vertical element perpendicular to flow,

(300 year return period)

1. Avalanche velocity at the proposed building location a. From Superior 75 mph b. From Blackjack Area 71 mph

2. Design flow depth a. From Superior 14.4 ft b. From Blackjack Area 13.1 ft

3. Snow Density at impact 1.8728 lbm/ft^3

4. Reference impact pressure at structure (ρ V2) a. From Superior 7100 psf b. From Blackjack Area 6330 psf

5. Roof deposition load 271 lb/SF

6. Impact forces Pn roof : ρ x (V x sin(impact angle))2 Normal to roof surface. 7. Friction forces on roof Ps = 0.5 * Pn roof 8. Powder blast on the west elevation: 50 lbs/ SF

(Impact forces are dynamic in nature and should be multiplied by a factor of 1.0< F <2.0. Rise time for the above impact forces is approximately 0.5 to 1 second. See Dynamics of Structures Penzien or other similar reference of impulse loading.) Roof deposition load is static. Attached figure I and II specifies loading as projected on the structure. Snow avalanche forces are in addition to IBC roof snow loads, where geometry will allow these to occur simultaneously. The structure’s roof configuration should combine both slope roof snow loads and roof deposition load. Combining these forces is common engineering practice in structures impacted by avalanches. Overturning forces should also be considered by the design engineer. Snow avalanche forces can act simultaneously with soil pressure loads. Powder Avalanche Blast Loads: The location of proposed structure is such that the affects of powder avalanche from Superior will impact the west elevation of the structure.

CTS Engineering

Summary The occupants of this structure should be aware of all the potential hazards at the site. People outside the structure should also be aware of the hazards posed by snow avalanches, and heed all local ordinances, and safety recommendations. Please call with any questions, (801) 274-2831. Sincerely,

Joseph D. Crilly, S.E.

Figure I

CTS Engineering

Figure II