Evaluation of ADCP backscatter inversion to suspended sediment concentration in estuarine environments

Abstract:

Acoustic Doppler Current Profiler (ADCP), designed for measuring velocity profile, is now widely used for the estimation of suspended sediment concentration from acoustic backscatter intensity, but its application to estuarine environments has not been vigorously tested. In this study, we examined the inversion capability of two ADCPs with 600 and 1200 kHz at three Korean estuaries: macrotidal Han river estuary (HRE), microtidal Nakdong river estuary (NRE), and anthropogenically altered macrotidal Yeongsan river estuary (YRE). In particular, we examined the relative importance of the sound attenuations due to water (a_w) and sediment (a_s) in response to sediment characteristics (size and concentration) as well as changing salinity and temperature. The inverted concentration was compared with reference concentrations obtained either water samples or Optical Backscatter Sensors. In NRE and YRE, where suspended sediment concentrations were smaller than 0.2 kg/m³, the acoustic inversion performed poorly only with a_s (R² = 0.05 and 0.39 for NRE and YRE, respectively), but well with a_w (R² = 0.70 and 0.64 for NRE and YRE, respectively). Thus, it is important to accurately constrain a_w in low-concentration estuarine environments. However, we did not find that the varying a_w performed considerably better than the constant a_w. On the other hand, the acoustic inversion was poorest at HRE regardless of a_w and a_s (R² = 0.58 and mean relative error =45%). The large discrepancy appears to result from the poorly constrained, spatially and temporally varying sediment characteristics (grain size, density and concentration) due to non-local sediment transport at macrotidal HRE.