Measuring discharge in a river with tidal bores by use of the coastal acoustic tomography system

Abstract

Eleven repeat reciprocal sound transmission experiments for current measurements were successful carried out from April 2009 to May 2010 in the Qiantang River, China. Ubiquitous are tidal bore intrusions that occur during spring tides in Hangzhou Bay. The experimental site is about 90 km away from the mouth of Hangzhou Bay. The coastal acoustic tomography (CAT) system consisted of two acoustic transceiver units, one on each side of the river, separated by 3050 m. During the sound transmission experiments, 42 shipboard acoustic Doppler profiler (ADP) surveys were also performed along the sound transmission line to obtain validation data for reciprocal sound transmission data. The average current along the vertical section, determined from the travel time difference between the reciprocal data, was in excellent agreement with the ADP results, with a root-mean-square difference of 0.04 m s−1. The sudden variations in river discharge caused by the passage of tidal bores were captured well by the CAT. The river discharge (QCAT) measured by CAT had a mean value of 2170 m3 s−1 but varied between −9097 m3 s−1 and 7985 m3 s−1 during the bore events. For the periods with no tidal bores, the water level data were well correlated with the steady river discharges estimated from the CAT data. By using the empirical relationship between these two variables, the annual river discharge (QWL) was predicted in the range from 914 m3 s−1 to 11,982 m3 s−1, with a mean of 2737 m3 s−1. A comprehensive discussion of the potential sources of error is presented. The resultant QWL error (621 m3 s−1) is small compared with the range of QWL (11,068 m3 s−1), implying that the proposed method for estimating river discharge is acceptable.The present study provides an efficient way for the long-term monitoring of river discharge in large tidal rivers with heavy shipping traffic, such as the Qiantang River.