COMPARISON OF ALTERNATIVE FLOOD FREQUENCY ANALYSIS METHODS ON THE MIDDLE MISSISSIPPI

Stage-based flood frequency analysis has been proposed as an alternative approach for computation of flood probabilities and flood levels on dynamic river systems. A comparison of different methodologies that may be used to perform flood frequency analysis using stage and discharge records is presented. The main difference between the methodologies lies on the type of data processing involved before fitting the selected probabilistic distribution. In addition to the direct use of maximum annual discharges and stages, frequency analysis is performed on the basis of (1) stages obtained from converting maximum annual discharges using modern day rating curve, and (2) indexed stages – maximum annual stages converted into modern day equivalent taking into account the trend in specific gages. Comparison is made on the implication of these methodologies on the incorporation of changes in stage-discharge relationship as a result of in channel and floodplain modifications, and their potential to incorporate stochastic trends to a foreseeable planning period. The tests are performed using systematic stage and discharge records at three locations on the Middle Mississippi River, namely, Saint Louis, MO, Chester, IL, and Thebes, IL. In all the cases, the Log Pearson Type III distribution is applied. Comparison is made with published results and the Upper Mississippi River System Flood Frequency Study. The results showed that stage-based flood frequency analysis yields higher water levels than discharge-based techniques when the changes in stage-discharge relationship are taken into consideration.