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, ~-i 2.. PROPOSAL FOR EVALUATING EXTREME PRECIPITATION FOR THE MOUNTAINOUS AREAS OF COLORADO INTRODucnON The state engineer's Regulations for Dam Safety and Dam Construction require that spillways for dams be adequate to handle floods based upon Probable Max/mum Precipitation (PMP). PMP is the theoretically greatest depth of precipitation for a given duration, that is physically possible over a drainage basin at any specific time of year. This is essentially a no risk standard that is in accord with the national standards for dam safety, and Colorado case Jaw, where failure of a dam could be catastrophic to the public health and welfare. PROBLEM The sources of extreme rainfall (PMP) 1 data for the mountainous areas of Colorado are presently the National Weather Service." Recent studies by the US Geological Survey (Jarrett-Costa), reveal a difference in quantity between the level of flooding predicted by the weather service publications, and runoff observations for areas above 7500 feet in Colorado. Also, studies presently being done (December 1993) by the Denver Water Board for their Williams Fork Dam appear to indicate that extreme rainfall for this basin is significantly less than predicted by the weather service. Another study, of the Grizzly Creek watershed near Aspen (1992), provided a conservative reduction of about 20 % in the PMP in relation to the weather service publications. PROPOSAL Because of these apparent differences, and the significant cost associated with designing/constructing spillways to handle floods caused by extreme precipitation (EP), the state engineer is proposing that the Colorado Water Conservation Board fund a study of the extreme precipitation problem in the mountainous aretls of Colorado. # of dams affected. 81 Class I, 69 Class II Total = 150 # of owners affected. 81 Volume of storage affected. 3,879,000 Acre Feet According to a thesis by David Chagnon, Colorado State University, Department of Atmospheric Science (1986), the total economic impact of reducing extreme precipitation ranges from $10-$16 Million per inch of change in rainfall for about 150 dams in the area affected by HMR 55A. (1996 costs at 3% inflation for 10 years are $13.5 - $22 Million per inch of change in rainfall.) A 20% reduction in estimates of about 3 inches (conservative analysis) could result in a total savings of $40 - $60 million dollars (1996 dollars). 1 Hydrometeorological Reports No. 55A (June 1988) for areas east of the continental dMde; and No. 49 (1984) for areas west of the divide. "US Department of Commerce, National Oceanic and Atmosphere Administration (NOAA)