Bedload Transport Prediction in the Presence of a Boulder Array and at High Relative Submergence Conditions

Abstract:

The objective of this study is to develop a new predictive equation for bedload transport within an immobile boulder array and at high relative submergence conditions, which occur commonly in mountainous streams during snowmelt flows. Previous predictive methods have accounted for the effects of individual boulders comprising an array both by the stress they bear locally and by their presence as immobile area within the total bed; thereby, existing equations originally developed for unobstructed flows have been modified for use within a boulder array. However, comparison of these predictions with bedload data from laboratory flume tests conducted within an idealized boulder array show notable discrepancies.

In the last five years, new research has highlighted additional hydrodynamic complexities related to the presence of a boulder array that have important implications to bedload transport and may be contributing factors to this discrepancy. These include reach scale drag of the collective array, local bed shear stress variability, and wake eddy region flow deceleration. Based on these insights into flow characteristics, which drive bedload transport, we hypothesize that transport rates within a boulder array will be more accurately predicted after 1) improving estimates of the bed shear stress acting on mobile sediment, and 2) quantitatively accounting for the effects mobile sediment ‘hiding’ or depositing in the wake region behind individual boulders.