The Pilgrim and Cordery design storm was developed to provide an approach which would produce storm patterns consistent with the storm patterns of historical events. To develop the Pilgrim and Cordery Design Storm, a duration is selected and a set of events with a large rainfall for the specified duration are selected. The duration is divided into a number of time periods. The rainfall volume in each of the periods is ranked and an average ranking for each period is computed. The percentage rainfall in each period is computed and is ranked from the largest to the smallest. An average percentage rainfall is then computed for each rank in the rain period. The average percentage rainfall is then assigned to the average ranking in each period. The percentage rainfall in each period determines the hyetograph shape. It is recommended that 50 events be used in the analysis.
The storm hyetograph is determined from an analysis of local rainfall data. If the analysis has not been conducted, it would be inconvenient for an engineer to use this design storm.
The hyetograph shape that is obtained using this procedure depends on the number of internal divisions that are selected for the design storm duration. The number of rainstorms that are used in the analysis is also important. Yen and Chow (1980) show that real storm events have a very uneven temporal distribution of rainfall. For this reason, the value of the average ranking of intervals tend to a single number when a large number of rainstorms are used.