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In producing the dam break flood forecast, the SMPDBK model first com- putes the peak outflow at the dam, based on the reservoir size and the tem- poral and geometrical description of the breach. The computed floodwave and channel properties are used in conjunction with routing curves to determine how the peak flow will be diminished as it moves downstream. Based on this predicted floodwave reduction, the model computes the peak flows at speci- fied downstream points with an average error of +10 percent or less. The model then computes the depth reached by the peak flow based on the channel geometry, slope, and roughness at these downstream points. The model also computes the time required for the peak to reach each forecast point and, if the user entered a flood depth for the point, the time at which that depth is reached as well as when the floodwave recedes below that depth, thus providing the user with a time frame for evacuation and fortification on which the preparedness plan may be based. , . The SMPDBK model has performed well in test simulations of the flooding produced by the failure of Teton Dam and the Buffalo Creek "coal waste" Dam, as well as in numerous theoretical dam failure simulations where the progression of the floodwave was not significantly altered by backwater effects created by downstream dams or bridge embankments, the presence of which can substantially reduce the model's accuracy. Its speed and ease of use recommend it well for use in emergencies. However, emergencies are not the only situations where it can be useful; planners, designers, emergency managers, and consulting engineers responsible for predicting the potential effects of a dam failure may employ the model in situations where backwater effects are not significant for pre-event delineation of areas facing danger should a particular dam fail. I. INTRODUCTION The devastation that occurs as impounded reservoir water escapes through the breach of a failed dam and rushes downstream is quick and deadly. . This potential for disastrous flash flooding poses a grave threat to many communities located downstream of dams. Indeed, a report by the U.S. Army (1975) indicates 20,000 dams in the U.S. are "so located that failure of the dam could result in loss of human life and appreciable property damage ..... This report, as well as the tragic destruction resulting from the failures of the Buffalo Creek coal-waste dam, the Toccoa Dam, the Teton Dam, and the Laurel Run Dam, underscores the real need for accurate and prompt forecasting of dam-break flooding. Advising the public of downstream flooding during a dam failure emer- gency is the responsibility of the National Weather Service (NWS). To aid NWS flash flood hydrologists in forecasting the inundation resulting from dam-failures, the numerical NWS Dam-Break Flood Forecasting Model (DAMBRK) (Fread 1977,1980) was developed for use with large, high-speed computers to model the outflow hydrograph produced by a time-dependent, partial dam breach, and route this hydrograph downstream using the complete one- dimensional unsteady flow equations while accounting for the effects of downstream dams, bridges, and off-channel storage. However, in some situa- tions the real-time use of the DAMBRK model may be precluded because warning-response time is extremely short or adequate computing facilities are not available. , 2