Influence of bed material size heterogeneity on bedload transport uncertainty

The bed material grain size distribution of gravel bed streams is often spatially heterogeneous. The heterogeneity is actually a random parameter, even for a '' well-mixed mixture,'' which potentially causes the transport rate for a given bed material to become an uncertain variable. The cause of bed material heterogeneity is the nonuniformity of the bed material, which is analyzed in this paper using examples from field observations and experimental data. The Monte Carlo simulation method is applied to study the uncertainty of the bedload transport rate using Wilcock's experimental data (P. R. Wilcock et al., 2001; P. R. Wilcock and J. C. Crowe, 2003). Each realization of the Monte Carlo simulation employed a randomly generated grain size distribution field for the entire simulation domain. With sufficient realizations the simulation results were adequate to show that the transport rates were a random variable and the mean transport rates did not fall on a single-valued curve when the local heterogeneity was taken into account. The results indicate that the bedload transport rate of nonuniform sediment has an intrinsic uncertainty that can result solely from the bed material. The results also partially account for the scatter of the fractional transport rate within Wilcock's experimental data. This study presents an important concept in understanding the uncertainty associated with estimates of sediment transport.