Acoustic backscatter and suspended sediment calibration in a large river. Impact of bimodal grain size distribution and fine fraction concentration

SZUPIANY, RICARDO N.; LOPEZ WEIBEL, CECILIA; LATOSINSKI, FRANCISCO; GUERRERO, MASSIMO; AMSLER, MARIO L.; BOLDT, JUSTIN; OBERG, KEVIN

Iquitos

Simposio; 8th IAHR Symposium on River, Coastal and Estuarine Morphodynamics; 2015

Universidad Nacional de la Amazonía Peruana

Traditional methods currently used to quantify sediment transport,especially in large rivers characterized by depths and widths greater than 10 mand 300 m respectively, require that the vessel remains anchored for sediment samplingand flow velocity measurements. These methods entail long measurement times andlaboratory processing of samples, with a consequent increase in associatedcosts. Moreover, these traditional techniques present low spatial-temporalresolution hindering abetter understanding of the associated variability. On the other hand, the timeconsuming of sampling operations may be highly unsafe in a large river characterizedwith high flow velocities, depths and inland navigation. Due to these reasons, during the last decade, a significant numberof surrogate technologies has been proposed (based on optical, laser andacoustic principles) aimed to meet operational as well as research objectives.Regarding acoustic technologies(especially the use of the acoustic Doppler current profile instruments, ADCP),they have been recognized as potential tools for the quantification of sedimenttransport in natural streams, that entails the analysis of the echo intensitylevel (EIL) of the return signal (backscatter) (Dienes, 1999; Thorne and Meral,2008; Guerrero et al, 2011; Latosinski et al, 2014; among others). Moreover,the ADCP has important advantages such us: it is not intrusive, it measuresflow velocity simultaneously to EIL and enables suspended sediment fieldinvestigation with high temporal and spatial resolutions.Despite the significant progress made so far, further analyses arestill required regarding the impact of particle size distributions in a cross sectionof a river channel that typically combines suspended sediment from the riverbed(i.e., sand) and washload (i.e., silt and clay). These fractions differently affectthe backscatter and corresponding attenuation. Additionally, an accurate anddetailed field sampling with high spatial resolution (i.e., point samplingalong verticals) is necessary to further validate the acoustic theory undernatural condition. This contribution presents the relationships between theconcentrations of suspended sediment from the riverbed (Css), washload (Cw) and EILrecorded from two ADCPs working at different frequencies and with changing energysupply. For this purpose, sediment concentrations and grain size distributionwere sampled simultaneously with the two ADCPs in a large secondary channel of theParana River, the sixth largest lowland river in the world.