a description is given of the development and testing of a method of estimating flood flows from natural catchments at desired return periods. the work is based on rainfall and run-off data recorded on a continual basis for several years at four natural catchments adjacent to motorways and supplemented by results from an additional natural catchment which was studied in an earlier investigation. the initial phase of the study was a statistical analysis of the rainfall and stream flow data to determine the significant factors from which an equation relating a "time of concentration" to certain catchment features was deduced. peak rates of flow may then be calculated using the bilham rainfall formula, the average annual rainfall of the catchment and the catchment area. the method was based on data from natural catchments with an underlying stratum of clay or boulder clay and, when tested on similar gauged catchments, gave reasonable agreement between the calculated and observed flows. a preliminary study of other natural catchments of greater or lesser permeability was made and the difference between the observed and calculated flows was explained by considering the catchment properties. hence a modifying factor is incorporated in the equations. the method is primarily for the calculation of peak rates of flow and is not intended for the determination of complete hydrographs. (a).

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