Bed-Material Load Computations for Nonuniform Sediments

The nonuniformity of bed material affects the bed-material load calculations. A size gradation correction factor $K_d$ is developed to account for the lognormal distribution of bed material. The use of $K_d$ in conjunction with bed-material load equations originally developed for single particle sizes improves the accuracy of transport calculations for sediment mixtures. This method is applicable to laboratory flumes and natural rivers with median diameter $d_{50}$ of bed material in the sand size ranges. The improvement on transport rate by $K_d$ factor is significant for data with standard deviation ${\sigma }_g$ of bed material greater than 2, while the correction is negligible for data with ${\sigma }_g$ less than $1.5$. Sediment in transport also follows a lognormal distribution with a median diameter $d_{\mathrm{50t}}$ generally finer than the corresponding $d_{50}$. As the size gradation increases, $d_{\mathrm{50t}}$ becomes much finer than the corresponding value of $d_{50}$. The relationship between $d_{\mathrm{50t}}$ and $d_{50}$ is defined as a function of ${\sigma }_g$ and agrees well with field data. The previously recommended use of $d_{35}$ as representative size of the bed material is found not to be generally applicable.