HomeMy WebLinkAboutHazards report.pdf Arthur I. Mears, P.E., Inc.
Natural Hazards Consultants
555 County Road 16
Gunnison, CO 81230
Cell: (970)275-1548
September 1, 2015
Mr. Webb Martin
5033 Snowshoe Ln.
Vail, CO 81657
RE: Snow avalanche and debris-flow hazards at proposed deck location, 5033 Snowshoe Lane,Vail (Lot
25,Vail Meadows Filing No. 1.)
Dear Mr. Martin:
At your request I have reviewed current (July 10, 2015) plans for a new deck at your Vail residence
(described above)to determine the exposure of a proposed deck to avalanche and debris-flow hazards.
Background Information and Vail hazards maps
Your Lot 25 is indicated as being within a "moderate" snow-avalanche hazard area on a map I drew for
the Town of Vail in 1977, and within a moderate-hazard debris-flow area on a map I also completed for
the Town in 1984. The lot is in the outer limits of the runout zone of the Vail Meadows avalanche path, a
moderate-sized path starting at approximately 11,000 feet elevation and imparts the valley base and a
hill approximately 50-75 feet high at 8,800 feet. After impact with the hill the flow is deflected through
an angle of 50-to-55 degrees toward Snowshoe Lane and your residence, approximately 1,000 feet to
the north. The drainage basin could also produce debris flows during very wet conditions. During the
31-to-37 year interval since these maps were drawn no avalanche or debris-flows have reached the lot
or have reached the small valley immediately to the south (locations of the water tanks).
The Town maps referenced above that I provided for the Town were originally intended to indicate the
following: 1)the 1977 avalanche map shows Lot 25 which is north of Snowshoe Lane near the lower
end of the avalanche runout;the building would be subject to only power avalanches, not flowing snow;
2)the 1984 debris-flow map was intended to delineate an area below debris-flow hazards which would
be subject to shallow sheet flooding that accompanies debris flows in some situations. Neither
avalanche or debris-flow zones will endanger buildings or residents at this location as discussed below.
Current conditions and existing hazard
Based on our experience with major avalanches throughout the world and in Vail since 1972 we expect
that only low-density powder avalanches will be capable of reaching your residence and proposed deck
even during major, rare avalanche conditions. Because of the distance into the runout zone (about
1,000 feet),the low density of the flow and the 50-55 degrees of deflection required,the pressures
would be in the 10psf-to-20psf range. This would be similar to a wind gust and would not cause
structural damage or endanger persons on the proposed deck.
Similarly, only shallow sheet flow of muddy water a few inches deep will be capable of reaching Lot 25
and the proposed deck during a debris flow. This would not endanger the deck or persons on the deck.
Furthermore,the proposed deck will not deflect avalanches or debris flow toward adjacent public or
private property.
Mitigation requirements
Because of the lack of avalanche or debris-flow hazards, I do not recommend mitigation at the proposed
deck.
Sincerely,
Arthur I. Mears, P.E.