Frequency analyses of more than 1,000 small watersheds in the United States and Puerto Rico were used to develop the estimation method for design of peak flow for ungaged watersheds. This method, called the Federal highway Administration (FHWA) method, is conceptually similar to the Bureau of Public Roads (BRP) method developed by W. D. Potter. The FHWA method relates the runoff peak to easily determined hydrophysiographic parameters and is intended for use on watersheds smaller than 50 square miles. The concept of risk is incorporated inot the design procedure. The risk is the probability that one or more events will exceed a specific peak flow within the usable lifetime of the drainage structure. The return period of the design flood peak can then be modified according to the risk the designer is willing to take. Another concept dealing with the probable maximum runoff peak derived as a function of watershed area is included. The flow obtained from this relationship is considered to be the upper limit of the design flow that may realistically be expected to ever occur. As such it may be appropriate to use in situations where the consequences of failure are extremely great.
Fletcher, Joel E.; Huber, A. Leon; Haws, Frank W.; and Clyde, Calvin G., "Runoff Estimates for Small Rural Watersheds and Development of a Sound Design method. Volume II, Recommendations for Preparing Design Manuals and Appendices B, C, D, E, F, G, & H" (1977). Reports. Paper 474.