Changing history at st. louis-adjusting historic flows for frequency analysis

摘要

In 1997, the Corps of Engineers embarked on a major re-evaluation of flood discharge and stae frequencies along the Lower Missiuri and Upper Mississippi Rivers. The first step in the overall study, to be completed in 2000, is to develop a preliminary set of peak discharge data that is relatively homogenous. This requries that the effect of known biases, like historic flood discharge over-estiamtes and reservoir effects, be removed. For this data set, flood peak discharges at St. Louis prior to 1931 were adjusted downward to reflect the over-estiamtes made throughout the period when floats were primarily used for velocity measurements. Flood peak discharges recorded after 1957 were adjusted upward to remove the partial control by flood reduction reservoirs which came on line throughout the period to the early 1980s. Preliminary evaluations were made of peak discharge frequency at St. Louis using Bulletin 17B techniuqes and compared to the current discharge-frequency estiamte for a no reservoir condition. The results using the revised data show a varying impact, ranging from almost nothing at common frequencies to about a 10