Velocity and discharge measurements at selected locations on the Mississippi River during the great flood of 1993 using an Acoustic Doppler Current Profiler

A relatively new device overcomes the difficulty of traditional methods in measuring discharge during floods because of the time required to measure velocities across wide and deep flow areas. The Acoustic Doppler Current Profiler (ADCP) makes it possible to measure a large number of velocities and discharges in a very short time, but several limitations of the instrument need to be understood before it can be used effectively. The ADCP uses sound waves to measure water depth and velocity profiles at positions along the transect of a channel. The device determines water depth by measuring how long it takes for an acoustic signal to be reflected off the channel bottom and simultaneously calculates the velocity profile of the channel by measuring the Doppler shift of sound waves reflected off sediment particles (which are assumed to have the same velocity as the water) at multiple depths. The collection of velocities, discharges, and depths measured at each position is called an ensemble of data, and a transect of the channel consists of many ensembles of data. Distances between positions where ensembles of data are taken are not constant and vary depending on the speed of the boat on which the ADCP is mounted. For the purposes of evaluating the carrying capacity of floodplains, an ADCP was used to measure velocity and discharge data at six locations on the Mississippi River during the Great Flood of 1993. Data collected for both the main channel and the floodplain at river mile (RM) 218 are presented in this article. Discharge measurements were very consistent for most transects of the river but measurements of velocity and position were not. Measurement errors may have been due to a direction sensor misreading, but the errors had little or no effect on calculation of discharge.