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To alleviate this potential problem and attempt to improve upon the accuracy and versatility of existing simplified dam breach modelling procedures, the NWS has developed the Simplified Dam-Break (SMPDBK) Flood Forecasting Model. With this model, the user may within minutes produce forecasts of the dam-break floodwave peak discharges, stages, and travel times. It should be noted, however, that the use of the NWS SMPDBK model is not limited to NWS flash flood hydrologists. Planners, designers, civil defense officials, and consulting engineers who are concerned with the potential effects of a dam failure and who have limited time, resources, data, computer facilities, and/or experience with unsteady flow models may also wish to employ the model to delineate the areas facing danger in a dam- break emergency. . This document presents an outline of the NWS SMPDBK model's conceptual basis. Appendix I gives a step-by-step guide and example of the computa- tions involved in the model. Appendix II presents the FORTRAN computer code for the automated (mini/micro-computer) version of the model and an example run of that version. Appendix III presents the BASIC computer code for the microcomputer or programmable hand-held computer version of the model. Appendix IV presents the HP41C hand-held computer version of the model. II. MODEL DEVELOPMENT The SMPDBK model retains the critical deterministic components of the numerical DAMBRK model while eliminating the need for large computer facilities. SMPDBK accomplishes this by approximating the downstream channel as a prism, neglecting the effects of off-channel storage, concern- ing itself with only the peak flows, stage, and travel times, neglecting the effects of backwater from downstream bridges and dams, and utilizing dimensionless peak-flow routing graphs developed using the NWS DAMBRK model. The applicability of the SMPDBK model is further enhanced by its minimal data requirements; the peak flow at the dam may be calculated with only four readily accessible data values and the downstream channel may be defined by a single "average" cross-section, although prediction accuracy increases with the number of cross-sections specified. Three steps make up the procedure used in the SMPDBK model. These are: (1) calculation of the peak outflow at the dam using the temporal and geo- metrical description of the breach and the reservoir volume; (2) approxima- tion of the channel downstream of the dam as a prismatic channel; and (3) calculation of dimensionless routing parameters used with dimensionless routing curves to determine the peak flow at specified cross sections down- stream of the dam. 2.1 Breach Description . . Most investigators of dam-break flood waves have assumed that the breach or opening formed in a failing dam encompassed the entire dam and occurred instantaneously. While th!..s_"ssu_m.Qtion mav_l>E!_vali<L...fo.L.lLf~__. concrete arch dams, it is not valid for...the exceedingly large number of earth dams. Because earthen dams generally do not fail completely nor instantaneously, the SMPDBK model allows for the investigation of partial failures occurring over a finite interval of time. And, although the model assumes a rectangular-shaped breach, a trapezoidal breach may be analyzed by specifying a rectangular breach width that is equal to the average width of 3