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.1 I I I I I I I I I I I I I I I I I I In our hydrology report of October 1983. we stated that the Probable Maximum Flood (PMF) could cause overtopping of the dam by six feet for five hours. The bedrock at the site appears to be erosion resistant and should withstand an event of this with only minor I'plucking". The rock has already been subjected to severe erosion activity through glacial ice gouging and plucking, thus the exposed bedrock is relatively tight and fresh. Packer permeability tests were performed in the drill holes and the results are shown in Table I. Permeabilities ranged from 1,300 to 2,300 feet per year (semi-pervious) in the rock. No seeps were observed through the abut- ments nor have any been noted by others in previous inspections. V. SEISMICITY North Central Colorado is an area of moderately low to low seismicity, at least relative to the main earthquake belts of the world. The potential for a large event (Magnitude 6.5 or greater) is present but the recurrence interval of such an event is considered to be long (1.000 to 100,000 years). The Uniform Building Code places Colorado in Zone 1 (minor damage potential). . These zones are rated from 0 to 4 with Zone 4 representing major earthquake destruction. One measurement used to equate the destructive potential of an earthquake at a particular site is ground accelerations. This value is usually expressed as a percentage of the acceleration of gravity and is an important input factor in either dynamic or pseudo-static analysis of structures. The U.S. Geological Survey (Algermissen 1982) has published a map with estimated values of acceleration for each part of the country. North central Colorado was placed in a zone where accelerations are not expected to exceed 0.04g. (for 95% probability over any 50 year period). This is a relatively low level of acceleration. -7- .