Grain size distribution and sediment flux structure in a river profile, measured with a LISST-SL instrument

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Abstract

New data on sediment transport in a river are reported using a LISST-SL laser diffraction instrument that simultaneously measures velocity, optical transmission, depth, temperature, and sediment particle size distribution (PSD) isokinetically. Suspended sediment concentration (SSC)is obtained from the sum of PSD. Optical transmission is seen to be nearly constant in the top 3.5m of the 4m deep river. The PSD profile, however, reveals a vertically well-mixed wash load, seen by the transmissometer, plus a coarse grain (>63jim) mode that increases in size and concentration towards the riverbed, but which is not seen by the transmissometer. The insensitivity of transmission to coarse particles is similar to turbidity data reported by Orton & Kineke 2001, and Laguionie et al., 2007. They also did not see the coarse particle mode except very close to the riverbed (0-0.1 depth). The present data show downward increasing sediment concentration and flux of coarse grains, beginning right near the surface. Near the bed, the sediment flux and concentration of coarse grains are, respectively, factors 3 and 2 higher than at the surface. The slopes of concentration profiles of different particle sizes follow Rouse (1937) closely, and are sufficiently ordered to provide consistent estimates of the friction velocity u* in the water column. SSC statistics from 0.5 to 3.5m show skewness due to the minimum concentration imposed by washload, implying potential for bias in sampling. The mass mean sediment diameter is seen to increase by a factor of 5 from surface to bottom. The apparent insensitivity of the transmissometer is explained by a factor 2.5 increase in Sauter Mean Diameter (particle volume/area) from top to near-bottom. Finally, it is seen that a mean velocity and SSC estimate at half-depth is within experimental error of column-mean values. However, PSD at half-depth is not representative of the column mean PSD.

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