Procedure for Approximate Dam Failure Floodplain Mapping

Abstract Only - In 1983, the Governor of Colorado signed into law House Bill 1416 directing the State Engineer, Jeris A. Danielson, to prepare a floodplain inundation map for each of the 238 high hazard dams in the state. Each.map was to show the ex­tent of flooding which would occur in the event of failure, downstream to a point where the dam failure flood would be contained within the 100-year floodplain. The legislation allowed only four months for completion of the project and re­quired that every political entity (city, town, county) receive a copy of the floodplain map for each dam failure which would affect properties within their juri diction. Certain poUtical entities were impacted by as many as 30 separate dam failure floodplain maps. The project represented a major challenge because of time and resource constraints.
To complete the project, the State Engineer's Office developed a procedure which combined the use of a simplified dam breach algorithm, a recognized flood routing technique, and an approximate mapping method. The dam breach algo­rithm had been used by the State Engineer earlier to establish hazard classifica­tions. Hand-held calculators and 7.5 minute U.S.G.S. topographic maps provided the necessary tools at minimal expense.
In spite of the approximate methods used, it was found that the procedure later predicted with reasonable accuracy the limits of flooding which actually oc­curred following the failure of two dams in Colorado. Flood depths and aerial extent were determined to be relatively insensitive to changes in peak failure dis­charge at the dam site. Therefore, this procedure is of value to emergency plan­ners and governments agencies as a quick and inexpensive way to estimate inundation areas and flood travel times in the event of a dam failure.