<br />. above and below an elevation limit for substantial rainfall-producing flows. Above about 7,500 feet (ft) in
<br />Colorado, maximum unit discharge is about 100 cubic feet per second per square mile (ft3/slmi2) for small,
<br />high-elevation basins, As basin size increases, unit discharge decreases. These low magnitude unit
<br />discharges result in only minor out-of-bank flooding in Colorado, These peak discharges are caused
<br />primarily by snowmelt runoff, relatively small amounts of rainfall (as compared to lower-elevation rainfall
<br />intensities and amounts and rainfall-produced flooding), or a combination of rainfall on snowmelt. In
<br />Colorado, there are no documented large rain and snowmeltfloods (Jarrett, 1987). Below about 7,500 ft in
<br />Colorado, peak flows as large as 4,000 ft3/slmi2 have occurred, (part of a spectrum of values of almost
<br />6,000 ft3/slmi2 that have occurred in New Mexico to about 2,000 ft3/slmi2 in the Northern US Rockies), The
<br />elevation limit varies from about 8,000 ft in the Southern Rockies to about 5,500 ft in the Northern Rocky
<br />Mountains, This gradient reflects an increasing distance from the prevalent precipitation-moisture source in
<br />the Gulf of Mexico and Pacific (Collins and others, 1991; Jarrett, 1993). The maximum unit discharge is 100
<br />ft3/slmi2 above the elevation limit throughout the Rocky Mountains. Quick (University of British Columbia,
<br />unpublished data, 1993) indicated that there is an elevation limit for rainfall flooding in the Canadian Rocky
<br />Mountains that is consistent, but at a lower elevation, with the elevation limit for northern U.S, Rocky
<br />Mountains. These results, which are supported with analysis of extreme rainfall data and paleoflood
<br />studies (Jarrett, 1987, 1990b, in review; Jarrett and Costa, 1988; Grimm, 1993; Waythomas and Jarrett,
<br />1994; Pruess, 1996; Jarrett and Waythomas, in press; Brien, 1996), contrast dramatically from published
<br />values of PMP and PMF for the Rocky Mountains, Because of these differences, concerns and questions
<br />. of extreme-rainfall and flood design values for structures located in floodplains (dams, bridges, sitings of
<br />hazardous-material facilities) have been raised in the Rocky Mountains and elsewhere in the United
<br />States. Several comparative studies to estimate maximum flooding have recently been done that
<br />demonstrate these differences in flood hydrology,
<br />
<br />.
<br />
<br />Olympus Dam is located on the Big Thompson River at Estes Park, Colorado, at an elevation of 7,500
<br />ft. The spillway was designed for a PMF of 22,500 ft3/S, However, a revised PMF (Bureau of
<br />Reclamation, written commun., 1984), based on the new PMP estimates, is 84,000 ft3/S. Paleoflood
<br />investigations by Jarrett and Costa (1988) indicated that the largest natural f100dflow in the Big Thompson
<br />River upstream from Olympus Dam is 5,000 ft3/S or less during at least the past 10,000 years (since
<br />glaciation). This paleoflood information and a risk-based assessment of the existing and revised PMF
<br />values by the Bureau of Reclamation, resulted in a decision not to modify the spillway for Olympus Dam.
<br />Recently, site-specific PMP/PMF and paleoflood investigations have been conducted in Colorado for
<br />Horsetooth Reservoir near Fort Collins (Waythomas and Jarrett, 1994; Jarrett and Waythomas, in press),
<br />for the Williams Fork Basin (North American Weather Consultants, unpublished report, 1994), for the
<br />Muddy Creek Basin (North American Weather Consultants, 1995; Boyle Engineering Corporation,1995,
<br />Jarrett, in review), for Bradbury Dam on the Santa Yenz River, California (Levish and others, 1994), and
<br />for Causey Dam on the South Fork of the Ogden River, Utah (Ostenaa and Levish, 1995). Increasingly,
<br />site-specific PMP/PMF studies, in combination with paleoflood investigations are being conducted to
<br />assess the design of new spillways or the need to modify existing spillways. Chang non and McKee
<br />(1986) estimated the cost for modifying just the 162 high-risk dams in Colorado to the revised PMP
<br />
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