The Metronome


Metronome tilting flume for tidal systems with reversing flows

Creating sufficient energy gradient for periodically reversing sediment motion is a challenge in experiments. The major problem is driving the tidal flow as follows. Assuming an experimental tidal system with a depth of 2 cm, the tidal water level amplitude can be at most about 1~cm. Given a typical aspect ratio of the estuary mouth of W/h>100, this means that the width of the experiment should be about 1 m. However, with a minimum slope of 0.01 m/m, the distance from the mouth with sufficient gradient to move sediment is also about 1 m given the maximum water surface amplitude. This, then creates a short tidal basin rather than a long estuary. When the tidal amplitude is enlarged, net export of sediment results so that the tidal system excavates until it is in static equilibrium as probably happened in a number of the experiments of Osborne Reynolds in 1889. By tilting the entire flume periodically, sufficiently strong reversing flow is driven for sediment transport similarity. With relatively small setups this was shown to result in dynamic tidal systems with channel and shoal patterns that are similar to those in nature.

The basic components are a steel basin that tilts over the short central axis, motion control, water recirculation, and optical imaging. The steel basin has inner dimensions of 20.00 m long by 3.00 m wide and 0.40 m deep. The flume has two end tanks for water supply, water outflow and for sediment trapping. The tilting axis is directly below the steel floor to minimise longitudinal motion and the entire flume setup is symmetrical about this axis. The basin was constructed from 4~mm steel plates cut and folded such that the sidewalls are suitable for a gantry to screed the bed and set up measurement equipment, and are a structural part of the basin to minimise bending. Further stiffness was accomplished by a ribbed structure and steel beams along and across the flume. Finite element modelling on the design showed that the maximum expected bending of the flume was 2~mm under loads larger than expected in typical experiments. This model was also used to select the required power of the actuators and motion control and to estimate the loading and required reinforcement of the floor. The steel basin was further curved upwards slightly such that it is straight under the expected water and sediment loading. The end tanks were designed to function as constant head tanks, with sediment traps at the inside of a movable weirs. Small actuators control the motion of the weirs. The largeactuators to tilt the flume operate in pairs with motion mirrored at the tilting axis. The motion and forces are monitored and internal safety controls prevent values above this that might be damaging. The motion at periods and amplitudes as used in this paper is typically 0.01 mm accurate. The actuators keep repeatable positions at all times, also during rest, such that the flume does not deform.