ALBERT

Description: 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.

Description: 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. Our recently discovered method to drive tidal currents by periodically tilting the entire flume leads to intense sediment transport in both the ebb and flood phase, causing dynamic channel and shoal patterns. However, it remains unclear whether tilting produces periodic flows with characteristic tidal properties that are sufficiently similar to those in nature for the purpose of landscape experiments. Moreover, it is not well understood why the flows driven by periodic sea level fluctuation, as in nature, are not sufficient for morphodynamic experiments. Here we compare for the first time the tidal currents driven by sea level fluctuations and by tilting. Experiments were run in a 20  ×  3 m straight flume, the Metronome, for a range of tilting periods and with one or two boundaries open at constant head with free inflow and outflow. Also, experiments were run with flow driven by periodic sea level fluctuations. We recorded surface flow velocity along the flume with particle imaging velocimetry and measured water levels along the flume. We compared the results to a one-dimensional model with shallow flow equations for a rough bed, which was tested on the experiments and applied to a range of length scales bridging small experiments and large estuaries. We found that the Reynolds method results in negligible flows along the flume except for the first few metres, whereas flume tilting results in nearly uniform reversing flow velocities along the entire flume that are strong enough to move sand. Furthermore, tidal excursion length relative to basin length and the dominance of friction over inertia is similar in tidal experiments and reality. The sediment mobility converges between the Reynolds method and tilting for flumes hundreds of metres long, which is impractical. Smaller flumes of a few metres in length, on the other hand, are much more dominated by friction than natural systems, meaning that sediment suspension would be impossible in the resulting laminar flow on tidal flats. Where the Reynolds method is limited by small sediment mobility and high tidal range relative to water depth, the tilting method allows for independent control over the variables flow depth, velocity, sediment mobility, tidal period and excursion length, and tidal asymmetry. A periodically tilting flume thus opens up the possibility of systematic biogeomorphological experimentation with self-formed estuaries.