A priori diagnostic for acoustic backscatter inversion to suspended sediment concentration in hyperturbid estuaries

Abstract. Accurate quantification of sediment transport in fluvial, estuarine and coastal environments depends on reliable estimates of suspended sediment concentrations (SSCs). While Acoustic Doppler Current Profilers (ADCPs) provide depth-resolved backscatter observations that have potential to become translated to SSC, conventional inversion techniques fundamentally depend on the assumption of homogeneous sediment concentration and acoustic scattering/attenuation properties along the acoustic beam. In stratified and hyperturbid conditions, this homogeneity may be compromised, even when the particle size distribution itself remains vertically stable. Strong sediment gradients trigger stratification that either amplifies signal strength at the lutocline or diminishes it by fine-sediment-induced viscous attenuation, possibly invalidating this assumption. To assess whether the assumption of homogeneous  sediment properties remains valid under hyperturbid conditions, we propose an a priori diagnostic based on the shape of the acoustic backscatter profile. Using changepoint detection, we identify slope breaks in acoustic profiles, which indicate transitions in the effective acoustic regime. These transitions likely indicate a breakdown of the homogeneity assumptions underlying acoustic backscatter inversion techniques. We apply the diagnostic to ADCP measurements (approximately 2 m above the bed) collected in an engineered navigation channel of the Ems Estuary, spanning SSCs of 0.1–100 g L^-1, this upper limit corresponding to hyperturbid conditions. Validation against in-situ water samples confirms that profiles without detected changepoints (Type 1) yield reliable SSC estimates, even at concentrations up to ~20 g L^-1. In contrast, Type 2 profiles exhibit pronounced spikes or slope breaks in the backscatter profiles that may reflect range-dependent acoustic sediment properties. This variation decouples the observed backscatter from a simple, range-invariant relationship with concentration. As a result, the inversion algorithm fails to accurately invert backscatter to concentration, leading to erroneous and unphysical SSC estimates that deviate significantly from in-situ water samples. The proposed diagnostic provides a simple step to flag unreliable profiles and support targeted sampling and interpretation in challenging hyperturbid environments, enabling more reliable estimation of suspended sediment concentrations and associated sediment transport fluxes.