Prediction of depth‐integrated fluxes of suspended sediment in the Amazon River: particle aggregation as a complicating factor

Abstract

International audienceLarge rivers have been previously shown to be vertically heterogeneous in terms of suspended particulate matter (SPM) concentration, as a result of sorting of suspended solids. Therefore, the spatial distribution of suspended sediments within the river section has to be known to assess the riverine sedimentary flux. Numerous studies have focused on the vertical distribution of SPM in a river channel from a theoretical or experimental perspective, but only a few were conducted so far on very large rivers. Moreover, a technique for the prediction of depth-integrated suspended sediment fluxes in very large rivers based on sediment transport dynamics has not yet been proposed. We sampled river water along depth following several vertical profiles, at four locations on the Amazon River and its main tributaries and at two distinct water stages. Depending on the vertical profile, a one-to fivefold increase in SPM concentration is observed from river channel surface to bottom, which has a significant impact on the 'depth-averaged' SPM concentration. For each cross section, a so-called Rouse profile quantitatively accounts for the trend of SPM concentration increase with depth, and a representative Rouse number can be measured for each cross section. However, the prediction of this Rouse number would require the knowledge of the settling velocity of particles, which is dependent on the state of aggregation affecting particles within the river. We demonstrate that in the Amazon River, particle aggregation significantly influences the Rouse number and renders its determination impossible from grain-size distribution data obtained in the lab. However, in each cross section, the Rouse profile obtained from the fit of the data can serve as a basis to model, at first order, the SPM concentration at any position in the river cross section. This approach, combined with acoustic Doppler current profiler (ADCP) water velocity transects, allows us to accurately estimate the depth-integrated instantaneous sediment flux

Optical Methods Applied to Hydrodynamics of Cohesive Sediments

“…These results indicate that flocs of both rivers are strong flocs (low values of n), which is logical because shear rates at the Usumacinta and Grijalva rivers are high (for comparison u* at Amazon River varies between 0.07 and 0.1 [1]). It is also observed that the value of Z R in the Usumacinta river is more sensitive to changes in the value of n. It was observed that for large depths it is more difficult to define the n value as it can change even in the same cross section of the river at different levels.…”

“…The formation of flocs in large rivers is the reason why Rouse equation cannot be used with particle sizes from classical granulometric measurements in conjunction with non-cohesive settling velocity equations. Recently, researchers working in the Amazon River and tributaries made similar observations [1]. Their conclusion was that granulometric measurements performed did not represent the real particle size because cohesive sediments agglomerate to form flocs [5,6,9] and after sampling, these flocs are destroyed and could not be measured appropriately in laboratory.…”

“…A large part of the suspended sediment charge in large rivers is constituted by cohesive sediments as shown in the Amazon River sampling of suspended sediments [1]. A characteristic of cohesive sediments is to form aggregates (flocs) that behave in a very different way that non-cohesive sediments.…”

“…It was possible to obtain appropriate values of the Rouse parameter Z R, which was shown to be difficult to obtain with classical granulometric and Coulter counter methods used in river studies [1].…”

Novel analytical strategies for tracing the organic carbon cycle in marine and riverine particles

“…Further, interpreting point measurements at one location of the river network is limiting because large rivers systems like the Amazon are heterogeneous in space and time. Within the Amazon cross-section, hydrodynamic sorting causes large grain-size sediments to settle faster and thus concentrate at deeper depths (Rouse 1950;Gibbs, 1967;Curtis et al, 1979;Bouchez et al, 2011b), leading to compositional differences between the deeper, coarser sediments and the shallower, finer sediments (Bouchez et al, 2011a;Bouchez et al, 2014). Finally, seasonality presents another challenge to measuring fluxes.…”