Cyclic steps and hydraulic jumps along the topsets of experimental deltas

We report laboratory experiments devoted to the morphodynamics of fluvial deltas. Along the topsets of the prograding deltas, we observe the formation and upstream migration of cyclic steps and hydraulic jumps. The hydraulic jumps exert key controls on the cyclic step morphology, curtailing bed erosion by decelerating the flow and also delaying redeposition by mixing the suspended sediment throughout the flow depth. To characterize the associated water flow, sediment transport, and bed evolution, we image the backlit channel through its transparent sidewalls. A wide-angle camera records the cyclic steps and hydraulic jumps at regular time intervals, yielding repeat measurements of the slowly evolving sediment bed and free surface profiles (Fig. 1). In parallel, a high-speed camera records the rapid flow of suspended sediment particles through the jump (Fig. 2). Velocity and sediment concentration profiles are extracted, to characterize the mixing processes and energy dissipation that occur in the jump region. To complement the experimental measurements, the results of preliminary modeling attempts will also be reported.

Keywords: delta long profile evolution; cyclic steps; hydraulic jumps over erodible beds.

Fig. 1 Side view of the constant-width experimental channel, showing the formation of cyclic steps along the delta topset.

Fig. 2 High-speed visualization of the toe of the hydraulic jump: (a) Raw image frame; (b) Measured velocity field, with velocity profiles (orange) and sediment bed (red), zero horizontal velocity (green), and water free surface profiles (blue).