Observations of Bed State and Ripple Geometry Resulting from Oscillatory, Mean, and Combined Flow Forcing in a Large-Scale Laboratory Flow Tunnel

Laboratory observations of the wave bottom boundary layer and the dynamics of movable, rippled beds have largely been limited to small-scale studies. The utilization of a large-scale laboratory flow tunnel allows for more comprehensive observations of sediment response to simulated wave, current, and combined wave-current forcing.

Several experiments were conducted in the University of New Hampshire's rigid-lid environmental flows water tunnel (EFWT), which has a sediment bed comprised of fine grain silica sand with a median grain size diameter of 0.22 mm. To simulate wave, current, and combined wave-current flows, the tunnel was operated in oscillatory, recirculating, and combined oscillatory-recirculating modes, respectively, and observations were made of the resulting bed state and ripple geometry.

Ripple formation and migration generally consistent with field conditions were observed under the oscillatory and combined flow regimes. The range of resulting ripple wavelengths and heights corresponded with those typically observed in the nearshore environment. Bed load transport was observed under most flow conditions, and in some cases, progressed to plug flow when the magnitude of the hydrodynamic forcing was increased. Bed form geometry was resolved with an LED projector and the bed stress and pressure field were estimated with a pressure sensor array and an acoustic Doppler profiler.

Preliminary observations using the EFWT suggest the tunnel is capable of simulating wave, current, and combined wave-current flows typical of the nearshore environment. Ripple scales and migration rates consistent with those observed in the field provide additional support for this claim. As such, the EFWT may be used for subsequent evaluation of sediment dynamics under a variety of hydrodynamic forcing conditions and may thus prove useful for future studies of the wave bottom boundary layer, sediment transport, and scour.