Laboratory study on the evolution of gravel-bed surfaces in bed armoring processes

Abstract

Cluster microforms in bed armoring have important implications for bedload transport. In this study, two sets of flume experiments were carried out to study the evolution of the cluster microforms in the bed armoring and their influences on the bedload transport. Dynamically stable armor layers were formed by applying a staged hydrograph on a mobile sediment bed consisted of poorly sorted sand and gravel, thus enabling the investigation of the cluster microforms associated with different flow characteristics simulated in these experiments. The size and the planimetric positions of the large particles on the bed surface were extracted using photogrammetric technique, allowing analysis of surface grain size distribution and particle clustering pattern of the bed. The results showed a tendency towards a coarsened and clustered bed surface, which is likely responsible for the declining bedload transport rate. The cluster microforms evolved with increasing bed shear stress in the following sequence: flat bed - pebble cluster - line cluster - heap cluster - reticulate structure. We found a bed armoring statistics that can partially explain the declining bedload transport rate under constant discharge. Nonetheless, additional parameters are needed to simulate the fluctuation in the bedload transport rate under changing flow conditions.