HomeMy WebLinkAboutDRB140202_4.11.14 Rockfall Hazard report_1402336140.pdfArthur I. Mears, P.E., Inc.
Natural Hazards Consultants
555 County Road 16
Gunnison, CO 81230
Cell: (970) 275-1548
Tel/Fax: (970) 641-3236
April 11, 2014
Michael Suman
Suman Architects
141 E. Meadow Dr., Suite 211
Vail, CO 81657
RE: Elk Meadows Subdivision
Dear Mr. Suman:
As requested, I evaluated the rockfall hazard to 3 lots (lots 2,3 and 5) in the Elk Meadows Subdivision of
Vail. They are identified on the “partial topographic map” by PLC dated “09/15/13.” Changes to lot
locations may invalidate the results of this study. These lots were designated ‘high severity” rockfall
hazard on Town of Vail hazard maps produced in 1984. We recommend removing these lots from
rockfall hazard designation for the reasons discussed in this report.
1. Evidence of previous rockfall activity on the slopes was not found. The 30° to 35° slope directly
north of the subdivision was investigated by me on site on April 9, 2014. Snow cover had
melted and vegetation had not leafed out, consequently visibility was good. The slope from
Buffehr Creek Road 8,300’) to a possible source outcrop at 8,950 feet was investigated. Rockfall
boulders were not found on the slope above the 3 lots.
2. Evidence for rockfall in the valley. Extensive grading and construction has taken place in the
valley adjacent to Buffehr Creek Road in the past 30 years. Construction and grading would
have obscured evidence of rockfall deposits at the base of the slope. However, additional high-
quality U.S. Forest Service photos dated “10-12-50,” more than 2 decades before construction,
also shows no evidence of rockfall deposits at the base of the slope.
3. Rockfall source area characteristics. The
primary source for rockfall is a light grey, fine-
grained sandstone member of the Minturn
Formation (Figure 1). The bedding planes of rock
units dip to the north at approximately 20°. Joints
at spacings of 2 – 5 feet are parallel to the beds.
Some rocks , which tend to be tabular in shape,
have fallen to the base of the slope and stopped
(see figure 2). Rocks of this typical shape are not
likely to roll or bounce down the slope as rockfall.
Sections of the cliff do not appear in danger of
toppling onto the steep slope and descending as
rockfall.
Figure 1. Sandstone source 650 feet above site
4. Buffehr Creek Road. The road is
approximately 30 feet wide above
the subdivision would serve to
stop and/or dissipate rockfall
energy if the very improbable
(refer to points 1, 2 and 3 above)
rockfall event reached the road.
It is not possible to state that the rockfall
hazard and risk is zero below this or any
other sandstone outcropping. However,
based on my experience in Vail during the
past 40 years and my experience with
rockfall hazard analysis, mapping and
mitigation at numerous sites in Colorado, I believe the hazard is minimal at the proposed building
locations evaluated in this study.
As a result, I do not recommend mitigation for rockfall hazard at lots 2, 3 and 5 of the Elk Meadows
Subdivision.
Limitation and disclaimer
You as my client should know that while our company can and does attempt to uphold the highest
professional standards, the state of scientific and engineering knowledge is incomplete, and does not
always permit certainty. The complex phenomena involved in rockfall cannot be perfectly evaluated and
predicted, and methods used to predict rockfall behavior change as new research becomes available.
While we can and will offer our best professional judgment, we cannot and do not offer any warranty or
guarantee of results.
Sincerely,
Arthur I. Mears, P.E.