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Analogue models or scale experiments of estuaries and short tidal basins are\udnotoriously difficult to create in the laboratory because of the difficulty\udto obtain currents strong enough to transport sand. Our recently discovered\udmethod to drive tidal currents by periodically tilting the entire flume leads\udto intense sediment transport in both the ebb and flood phase, causing dynamic\udchannel and shoal patterns. However, it remains unclear whether tilting\udproduces periodic flows with characteristic tidal properties that are\udsufficiently similar to those in nature for the purpose of landscape\udexperiments. Moreover, it is not well understood why the flows driven by\udperiodic sea level fluctuation, as in nature, are not sufficient for\udmorphodynamic experiments. Here we compare for the first time the tidal\udcurrents driven by sea level fluctuations and by tilting. Experiments were run\udin a 20  ×  3 m straight flume, the Metronome, for a range of tilting\udperiods and with one or two boundaries open at constant head with free inflow and\udoutflow. Also, experiments were run with flow driven by periodic sea level\udfluctuations. We recorded surface flow velocity along the flume with particle\udimaging velocimetry and measured water levels along the flume. We compared\udthe results to a one-dimensional model with shallow flow equations for a\udrough bed, which was tested on the experiments and applied to a range of\udlength scales bridging small experiments and large estuaries. We found that\udthe Reynolds method results in negligible flows along the flume except for the\udfirst few metres, whereas flume tilting results in nearly uniform reversing\udflow velocities along the entire flume that are strong enough to move sand.\udFurthermore, tidal excursion length relative to basin length and the\uddominance of friction over inertia is similar in tidal experiments and\udreality. The sediment mobility converges between the Reynolds method and tilting\udfor flumes hundreds of metres long, which is impractical. Smaller flumes\udof a few metres in length, on the other hand, are much more dominated by friction\udthan natural systems, meaning that sediment suspension would be impossible in\udthe resulting laminar flow on tidal flats. Where the Reynolds method is limited\udby small sediment mobility and high tidal range relative to water depth, the\udtilting method allows for independent control over the variables flow depth,\udvelocity, sediment mobility, tidal period and excursion length, and tidal\udasymmetry. A periodically tilting flume thus opens up the possibility of\udsystematic biogeomorphological experimentation with self-formed estuaries

Turning the tide: comparison of tidal flow by periodic sea level fluctuation and by periodic bed tilting in scaled landscape experiments of estuaries