Representative sediment sizes in predicting the bed-material load for nonuniform sediments

This paper evaluates the applicability of representative sediment size in terms of median grain size and the effective sediment sizes for the transport of non-uniform natural sediment mixtures. Natural sediment from the Krasak River, Yogjakarta was used. Bed samples were collected from three different river sections, i.e. upstream (U), midstream (M) and downstream (D). The median diameters, d50 and sediment geometric standard deviation, σg for sections U,M and D were 1.63 mm, 0.91 mm and 0.69 mm, and 4.8, 2.3 and 1.5 respectively. Particular attention was paid to the effect of varying representative sediment sizes on the threshold sediment motion, where four threshold equations were employed. The threshold value varies when σg=4.8σg=4.8 and has little variation for sediment mixtures with σg<2.4σg<2.4. Analysis is extended to prediction of bedload transport using eight selected formulae which account for both with and without the threshold criteria of sediment motion. To account for the non-uniformity of the bed materials, a comparison between the calculated bedload discharge using d50 was made with the bedload prediction when a representative sediment diameters were used. Analysis shows that an improvement of the statistical analysis was only observed for predictions calculated using Brownlie (1981) and Camenen and Larson (2005) expressions when effective sediment sizes were employed for non-uniform sediment mixtures. Employing the effective sediment sizes de or dem to calculate the bedload discharge does not necessarily improve the bedload prediction and the use of d50 is seen as sufficient.