Techniques for accurate estimation of net discharge in a tidal channel

Abstract

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 mechanical current meters and methods based on stage/discharge relations or water slope measurements yielded errors that were as great or greater than the computed residual discharge. Ultrasonic measurement methods consist of: (i) the use of ultrasonic instruments for the measurement of a representative velocity used for in situ estimation of mean water velocity, and (2) the use of the acoustic Doppler current discharge measurement system 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 velocity must first be related to mean channel velocity and then used to calculate instantaneous channel discharge. Finally discharge is low-pass filtered to remove the effects of the tides. An ultrasonic velocity meter discharge-measurement site in a tidally affected region of the Sacramento-San Joaquin rivers was used to study the accuracy of the index velocity calibration procedure. Calibration data consisting of ultrasonic velocity meter index velocity and concurrent acoustic Doppler discharge measurement data were collected during three time periods. Two sets of data were collected during a spring tide (monthly maximum tidal current) and one set was collected during a neap tide (monthly minimum tidal current). The relative magnitude of equipment errors, acoustic Doppler discharge measurement errors, and calibration errors were evaluated. Calibration error was found to be the most significant source of error in estimating net discharge. Using a comprehensive calibration method, net discharge estimates developed from the three sets of calibration data differed by less than an average of 4 cubic meters per second (less than 0.5 percent of the maximum tidal flow). Typical maximum flow rates during the data-collection period averaged 750 cubic meters per second.