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Elevation Limit to Rainfall Flooding
Flood Hydrology and Spillway Design

Prior to recent USGS studies, it was widely believed that large rainfall floods could occur at any elevation in the Rocky Mountains. One such rainfall flood was the Big Thompson flood of 1976, which killed 140 people and caused over $35 million in damages. For more than a decade USGS hydrologists have documented the size of many contemporary and prehistoric floods on rivers throughout the Rocky Mountains. Analyses of this large, detailed, data set by USGS hydrologists indicates that there is an elevation limit to rainfall flooding. Floods in river basins above about 5,500 feet in the Northern Rocky Mountains (or 7,500 feet in the Southern Rocky Mountains) are comparatively small and result from snowmelt rather than high-intensity rainfall. This research has led to substantially lower estimates of the 100-year flood (and the probable maximum flood) in high altitude basins throughout the Rocky Mountains.

These results have important implication for floodplain management, implementation of flood-warning systems, and the design of hydraulic structures in flood plains. Dam safety guidelines developed before this USGS research was done suggested that many dams in the Rocky Mountains were underdesigned. The cost of rebuilding spillways throughout Colorado to meet these guidelines was expected to be $184 million. One of the dams thought to have an underdesigned spillway is Olympus Dam in Estes Park Colorado, located at 7,500 feet above sea level. The spillway is designed for a flood of 22,000 cubic feet per second (cfs). The guidelines for spillways in this area would have required a redesign to accommodate a flood of 84,000 cfs. The USGS research showed that no floods in this basin had ever exceeded 5,000 cfs in the last 10,000 years. Thus the USGS was able to demonstrate that the costly spillway reconstruction at this high altitude was not necessary and spillway criteria for the Rocky Mountain region are being rewritten to reflect these findings. These downward revisions of flood risk means that spillway modifications will not be necessary at some dams and that reservoir storage set aside for flood control can be used for water storage for municipal, industrial, irrigation, recreation, or habitat related uses. Thus, the USGS findings not only result in a savings of redesign costs but result in more beneficial storage in the existing reservoirs.

For additional information and references, see the project description,
Paleohydrology and Climate Change, or contact:
Robert D. Jarrett
U.S. Geological Survey, P.O. Box 25046, MS418, Denver Federal Center, Lakewood, CO 80225-0046
Phone: 303-236-6447

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Department of the Interior, US Geological Survey
National Research Program || Last Updated: 02/03/2006
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