MAGNITUDE FREQUENCY-ANALYSIS OF SEDIMENT TRANSPORT IN THE LOWER MISSISSIPPI RIVER

Magnitude-frequency analysis of gauging station records (1950-1982) on the Lower Mississippi shows that there is a clearly defined dominant flow of about 30 000 m3/s. This lies within an effective range of channel-forming flows between 17 000 and 40 000 m3/s, which are responsible for transporting a disproportionately large percentage of the sediment load. The 33 year period of record is sufficiently long that the occurrence of an extreme high flow event does not significantly change these results. Hydrographic survey data, long-profile records and stage-discharge relationships from calibrated one-dimensional flow models indicate that the dominant discharge corresponds to 'bar-full' discharge on the Lower Mississippi and that the effective range of flows occurs between the stage that just tops mid-channel bars and that which significantly overtops the banks. Historical trends in bar growth suggest that bar-top elevations have generally risen to the dominant flow elevation over the last 30 years. In this trained river, where most of the banks are protected by revetments and the alignment is fixed by groynes and spurs, the mid-channel bars represent the major, contemporary morphological feature of the alluvial river.