PERMFILE64516

• Permit No: M2001035
• IBM Index Class Name: Permit File
• Doc Date: 7/8/2001
• Doc Name: PROBABILISTIC SEISMIC HAZARD ANALYSIS TROUT CREEK DAM PROJECT NEAR BUENA VISTA CHAFFEE CNTY COLO
• Media Type: D
• Archive: No

Michael ~5t En
ineerin
Geolo
Ground Water H~o
g
g
gy.
gy
and A55oclate5, InC. Scismotectonics and Earthquake Engineering,
Consulting Engineers Geotechnical Engineering,
and Geologists Engineering Risk Management
April 29, 1998
P.O. Box 696
Castle Rock, CO 80104-0696
[phone]
FAX [phone]
E-Ma-l: MWWAI®lbm.net
Spann Engineering, LLC
Attn: Mr. Stephen Spann, P.E
4801 S. Galapago Street
Englewood, CO 80110
Il1l
SUBJECT: Probabilistic Seismic Hazard Analysis, Trout Creek Dam Project neaz
Buena Vista, Chaffee County, Colorado. Project No. 98279
Gentlemen:
The attached report presents results of a probabilistic seismic hazard
analysis (PSHA) for the proposed Trout Creek dam located on the east side of the
upper Arkansas Valley graben neaz Buena Vista, Chaffee County, Colorado. The
project, as presently envisioned, will include a 70-foot high, roller-compacted
concrete dam and appurtenant facilities. Existing geologic studies indicate faults
on the west side of the upper Arkansas Valley at the base of the Sawatch Range are
seismogenic and capable of producing lazge magnitude earthquakes (M,,=7f) at an
epicentral distance of about 10 km from the dam site. A deterministic seismic
hazazd assessment, using hypothetical maximum credible earthquakes (MCEs) and
widely-accepted attenuation relationships, produces unreasonably high peak
bedrock accelerations at the site. Deterministic studies, however, do not account
for the probability of occurrence of hypothetical maximum credible earthquakes
and, thus, aze overly conservative in azeas of low historic seismicity and long
recurrence intervals characteristic of central Colorado.
The primary products of the PSHA for Trout Creek dam aze estimation of
site-specific peak ground acceleration and development of response spectra for use
in engineering analyses. The results of the PSHA aze expressed as a relationship
between peak and effective (bedrock) accelerations for probabilities of
non-exceedance in time periods of interest. For any specified level of risk, the
corresponding peak and effective bedrock accelerations from all earthquake sources
can be determined. Zero period bedrock accelerations may then be used to generate
standard response spectra or to scale response spectra from historic earthquakes
compazable to those expected to occur in central Colorado.
Based on results of the PSHA, we conclude peak horizontal bedrock
accelerations at the Trout Creek dam site will be 0.355 g with a 90% probability of
non-exceedance in 5268 yeazs or an annual exceedance probability of 2 x 10-',
corresponding to a return period of 50,000 yeazs. This value for peak ground
acceleration corresponds to the U.S. Bureau of Reclamation's Design Standazd 13
for seismic design. The effective peak horizontal acceleration, 0.302 g, at the Trout
Creek site is calculated as 85% of the peak horizontal acceleration (0.355 g).
Brench O(flce: P.O. Box 555. Mon-Lson, CO 80465-0555, Tele/Fax [phone]