Flume Experiments on Lower-Flow-Regime Bed Forms in Coarse Sand

Abstract

Thirty-five runs were made in a recirculating channel 11.5 m long and 0.92 m wide with four sizes of very well sorted coarse sand to study geometry, migration, and hydraulics of bed configurations in equilibrium with steady uniform open-channel flows at flow depths of about 15 cm. Equilibrium energy slope and sediment transport rate were measured as well. Bed phases (kinds of bed configurations) studied were ripples, low-flow-velocity flat beds with sand transport, and dunes ( identical with megaripples). Current velocities in none of the runs were high enough for development of an upper-flow-regime flat bed. A plot of mean flow velocity versus sand size for all the runs shows that at low flow velocities the ripple field narrows with increasing sand size and pinches out at about 0.7 mm; the field for flatbed transport narrows with decreasing sand size and pinches out at about 0.5 mm, just above the ripple field. At high flow velocities, dunes are the stable phase for all sand sizes studied. Dune configurations are divided into two subphases. Two-dimensional dunes (2D dunes), with fairly straight, continuous, even crests and no strong localized scour in troughs, are formed at relatively low flow velocities; three-dimensional dunes (3D dunes), with strongly sinuous, discontinuous, uneven crests and strong, localized scour pits in troughs, are formed at relatively high flow velocities. 3D dunes tend to be higher than 2D dunes, have larger height/spacing ratios, and show less variability in height, spacing, and migration rate. Dunes can be viewed as kinematic shock waves; differences between 2D dunes and 3D dunes lie in the differing importance of shock-wave coupling and of sand transport in bed-form troughs. In both geometry and hydraulic ralationships, the 2D dunes and 3D dunes in our flume runs correspond to the relatively regular megaripples at low flow velocities and relatively irregular megaripples at high flow velocities described by other authors from intertidal environments.