Turning the tide: comparison of tidal flow by periodic sealevel fluctuation and by periodic bed tilting in the Metronome tidal facility

Abstract. Analogue models or scale experiments of estuaries and short tidal basins are notoriously difficult to create in the laboratory because of the difficulty to obtain currents strong enough to transport sand. Since Osborne Reynolds' experiments over a century ago, experimental tidal flow has been driven by periodic sealevel fluctuations. Recently we discovered a novel method to drive periodic tidal currents: periodically tilting the entire flume. This leads to sediment transport in both the ebb and flood phase which in tidal systems with dynamic channel and shoal patterns. Here we compare the tidal currents driven by sealevel fluctuations with those driven by tilting. We use a bespoke flume of 20 m by 3 m with rough bed: the Metronome. Experiments were run in a straight flume with different tilting periods and with either one or both boundaries open at constant head with free inflow and outflow. Also experiments were run with flow driven by periodic sealevel fluctuations. We recorded surface flow velocity along the flume with Particle Imaging Velocimetry and water levels along the flume. Furthermore we compared the results to a one-dimensional model with shallow flow equations for a rough bed. We found that Reynolds’ method results in negligible flows along the flume except for the first few meters, whereas flume tilting results in nearly uniform, reversing flow velocities along the entire flume that are strong enough to move sand. A periodically tilting flume therefore has the potential to simulate estuaries as observed in a pilot experiment and opens up the possibility of systematic biomorphological experimentation with self-formed estuaries.