Runoff Estimates for Small Rural Watersheds and Development of a Sound Design Method: Volume II: Design Manual

Document Type


Publication Date

January 1976


A basin consideration in the design of culverts and other minor highway drainage structures is the estimation of the design peak flow. Frequency analysis of more than 1,000 small watersheds in the United States and Puerto Rico were used to develop the estimation method outlined herein. This method, called the Federal Highway Administration (FHWA) method, is conceptually similar to the Bureau of Public Roads (BPR) 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. In addition, the concept of risk is incorporated into the design procedure. By risk is meant the probability that one or more events will exceed a specified 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 included in the design procedure is that of the probable maximum runoff peak derived as a function of watershed area. The flow obtained from this relationship is considered to be the upper limit or ultra-safe 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.

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