Discharge determination by Acoustic Doppler Current Profilers (ADCP): a moving bottom error correction method and its application on the River Amazonat Obidos

Since 1995, hydrologists of the HiBAm (Hydrology and Geochemistry of the Am azon Basin) Research Program carried out several hundred discharge measurem ents in the Amazon basin. Implementation of modern discharge measurement te chniques using ultrasonic devices (ADCP), give evidence of a systematic err or linked to the displacement of the river bottom due to high water velocit y close to the bottom. This error leads to an underestimation of discharge value. It was possible to establish a correlation between the water velocit y close to the river bottom and the error between real position and positio n computed by ADCP when the boat returns to its starting point after a two- way crossing of the river. When there is no bottom displacement, i.e. durin g low flow period, this return position error is weak (less than 50 m). Thi s has allowed quantification of river bed load speed, or bottom displacemen t speed. A correction method was developed on the basis of this correlation . This method, systematically applied to ADCP discharge measurements obtain ed at Obidos hydrometric station, allowed all measured discharges to be cor rected, especially for 1997 and 1999 floods. Another method, based on the a nalysis of rear trajectory of the boat (obtained from topographic measureme nt or GPS positioning) compared with the ADCP computed trajectory, is under study.