Use of historical data in flood‐frequency analysis

Abstract

A flood‐frequency study for the Susquehanna River at Harrisburg, Pa., involved the use of both historical and recent flood data. The historical data started in 1786 and covered all flood peaks above a stage of 18.0 ft or approximately 340,000 cfs. The continuous record began in 1874. The use of hitherto available techniques for combining the historical with recent data was found to give inconsistent results. Analysis showed that in plotting points for the purpose of drawing a frequency curve, consistent results could be obtained only if all points were based on the same period of record. It was known that all the missing years of record since 1786 had annual peak floods of less than 340,000 cfs. It was assumed that the same distribution of peak floods below 340,000 cfs occurred during these years as is known to have occurred during those years in the recent period. The method of computation involved an adjusting of order numbers for all items below 340,000 cfs. The result is a consistent set of points and a well‐defined frequency curve.