Turning the tide: comparison of tidal flow by periodic sea level fluctuation and by periodic bed tilting in scaled landscape experiments of estuaries
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 fl...
| Main Authors: | , , , , , , , , , |
|---|---|
| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2017-11-01
|
| Series: | Earth Surface Dynamics |
| Online Access: | https://www.earth-surf-dynam.net/5/731/2017/esurf-5-731-2017.pdf |
| Summary: | 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. |
|---|---|
| ISSN: | 2196-6311 2196-632X |