Reciprocal sound transmission experiments for current measurement in a tidal river
Six repeat reciprocal sound transmission experiments for current measurement were successful carried out in the upstream region of the Qiantang River in Hangzhou city, about 90 km from the mouth of Hangzhou Bay during April to December 2009. Two Coastal Acoustic Tomography (CAT) Systems were set up at a distance of 3050 m for making a reciprocal transmission between the both banks of the Qiantang River. During the sound transmission experiments, thirty-two repeat shipboard Acoustic Doppler Profiler (ADP) surveys were also performed along the sound transmission line to get a comparison with the reciprocal sound transmission data. Range-averaged current velocities, determined from the travel time differences along the transmission line were in good agreement with those from the ADP, producing a root-mean-square difference of 0.03 m/s. The travel time differences data were well correlated with the river discharge estimated from the ADP data. The time series of river discharge during the experimental period was estimated through an empirical formula, relating to both the parameters. The variations of river discharge caused by tidal bores were well captured. The river discharge changed in the range of −7626 m3/s and 5096 m3/s, with a mean of 1246 m3/s. The above results suggest that the CAT is a powerful instrument for measuring continuously the river discharge in such tidal rivers with quite heavy shipping traffic.