Methods for accurate estimation of net discharge in a tidal channel

Accurate estimates of net residual discharge in tidally affected rivers and estuaries are possible because of recently developed ultrasonic discharge measurement techniques, Previous discharge estimates using conventional mec hanical current meters and methods based on stage/discharge relations or ma ter slope measurements often yielded errors that were as great as or greate r than the computed residual discharge, Ultrasonic measurement methods cons ist of: 1) the use of ultrasonic instruments for the measurement of a repre sentative "index" velocity used for in situ estimation of mean mater veloci ty and 2) the use of the acoustic Doppler current discharge measurement sys tem to calibrate the index velocity measurement data. Methods used to calibrate (rate) the index velocity to the channel velocity measured using the Acoustic Doppler Current Profiler are the most critical factors affecting the accuracy of net discharge estimation, The index velo city first must be related to mean channel velocity and then used to calcul ate instantaneous channel discharge, Finally, discharge is low-pass filtere d to remove the effects of the tides. An ultrasonic velocity meter discharge-measurement site in a tidally affect ed region of the Sacramento-San Joaquin Rivers was used to study the accura cy of the index velocity calibration procedure. Calibration data consisting of ultrasonic velocity meter index velocity and concurrent acoustic Dopple r discharge measurement data were collected during three time periods. Two sets of data were collected during a spring tide (monthly maximum tidal cur rent) and one of data collected during a neap tide (monthly minimum tidal c urrent). The relative magnitude of instrumental errors, acoustic Doppler di scharge measurement errors, and calibration errors were evaluated. Calibrat ion error was found to be the most significant source of error in estimatin g net discharge. Using a comprehensive calibration method, net discharge es timates developed from the three sets of calibration data differed by less than an average of 4 cubic meters per second, or less than 0.5% of a typica l peak tidal discharge rate of 750 cubic meters per second.