||In the past, the suspended sediment concentration (SSC) was mainly measured by the optical backscattering device (OBS) and water sample filtration. However, there has been a new development that user is based on the acoustics backscattering (ABS) to measure the SSC in the world. The acoustic instruments have some advantages that the optical ones do not have. For example, acoustic instruments are not effected by high turbidity, biofouling and high viscosity in the water. Acoustic instruments have high spatial and temporal resolutions. And they can immediately indicate the SSC changes than the water sample filtration method. Therefore, in this study we used the multi-frequency acoustics instrument (AQUAscat-1000) to investigate the relations of the suspended sediment size and concentration to the acoustic characteristics.|
The results are separated into two parts: (1) The calibration process in the flume. (2) The acoustic results in the field experiment. In the first part, we examined the range of preferred bin size with respect to the different frequencies as well as the condition in the flume, and determine that conclude the 20 mm is the best range for our case. In addition, the gain should be used when the maximum SSC is less than 100 mg/l and vice versa. Therefore, in the field case around the river mouth, the signal gain should be turned off due to high concentrations. According to the sensitivity of the backscatter intensity of different frequencies to the suspended particle sizes, multiple frequencies are needed to derive the SSC when the sediment size becomes smaller. The last point in this part is the comparison of the results between the acoustics and optical instruments. When the suspended particles in the water column are transparent, the optical measurement of the SSC is underestimated, but the acoustic method is not.
The result of the field experiment in 2009 showed that the AQUAscat-1000 is a better instrument to quantify SSC than the optical instruments. The SSC increase caused by the bottom sediment re-suspension was due to the wave shear. In the 2011 experiment, although the acoustic results overestimated the SSC at some points but they still had higher relation and significance with water sample data than the optical measurements. The SSC increase caused by the bottom sediment re-suspension was due to the current shear.