Bed-Material Load Computations for Nonuniform Sediments

The nonuniformity of bed material affects the bed-material load calculations. A size gradation correction factor Kd is developed to account for the lognormal distribution of bed material. The use of Kd 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 d50 of bed material in the sand size ranges. The improvement on transport rate by Kd factor is significant for data with standard deviation σg of bed material greater than 2, while the correction is negligible for data with σg less than 1.5. Sediment in transport also follows a lognormal distribution with a median diameter d50t generally finer than the corresponding d50. As the size gradation increases, d50t becomes much finer than the corresponding value of d50. The relationship between d50t and d50 is defined as a function of σg and agrees well with field data. The previously recommended use of d35 as representative size of the bed material is found not to be generally applicable.