Improvements in the Experiment Setup

Ideally a physical experiment setup is made where infiltration of overtopped water into the backfill is correctly modelled from the prototype situation. Furthermore wave propagation should not be hindered by constructions inside the scale model. Then for different horizontal positions vertical cross-sections can be defined where the water discharge should be measured without disruption of the water.

There are two options to determine the overtopping discharge at a particular location. One method is to continuously measure velocity and water depth of the water flow above the backfill. The flow over the backfill consists for a large part of air-water mixture so that also the fluid density should somehow be measured. With these

measurements one would be able to calculate the mean overtopping discharge. However to the best of my knowledge no equipment is available that can accurately measure these quantities.

A second method is to collect overtopped water that passes the location of interest. This method is commonly chosen in research that focuses on overtopping. Also in the current research this method was applied. When the overtopping discharge at one location in the scale model is required no problems arise. However when overtopping discharges need to be determined for multiple locations in the same experiment the overtopped water cannot simply be collected in collection bins. The drainage capacity in those bins would have to depend on all influencing parameters to correctly simulate the infiltration of overtopped water in the backfill material. Water pressures in the collection bins would have to be measured, a computer should in real-time calculate the infiltration speed and the discharge of the displacement pumps should be computer-controlled. It can be concluded that this results in quite a complicated and not too feasible scale model.

One could also opt for a numerical approach, simulating the water motion

computationally. Then all quantities required to calculate the overtopping discharge can be computed at the same time-step. Computationally combining the Navier-Stokes equations for external water motion, the Extended Forchheimer equation for internal water motion and adding a model for unsaturated flow theoretically could lead offer a solution. However, let alone the computational challenges, the results still would have to be validated with a physical model.

RECOMMENDED EXPERIMENT SETUP

A physical model is desired where overtopped water is collected and where the infiltration behaviour of overtopped water is correctly simulated. This can be

established by physically dividing the model into two parts. At one side the overtopped water directly behind the crest is collected, while on the other side the overtopping at a certain distance from the end of the crest can be stored. As any method also this

experiment setup has some drawbacks. Investigating the spatial distribution of overtopping for one specific wave spectrum will require multiple tests to be run. Therefore less wave conditions can be tested per value of time compared to the current experiment setup. Furthermore wave conditions as generated by the wave generator will never be exactly the same. However, because quantities are made dimensionless, it is expected this will bring about only small errors.

The recommended experiment setup is visualized in Figure 6.3-1. A vertical plane physically separates the two sides of the flume. The plane should run to the very bottom

of the flume tank as to prevent exchange of water inside the breakwater. In Appendix L the recommended experiment setup is illustrated more extensively.

FIGURE 6.3-1: IMPRESSION OF THE RECOMMENDED EXPERIMENT SETUP

It is advised to run experiments for 5 different horizontal positions behind the crest. In the current experiments at 0.60 m behind the crest scarcely any overtopped water was collected. Therefore this point is recommended as the farthest distance at which the overtopping discharge should be measured.

Per wave condition 4 experiments should be run. Refer to Table 6-4.

TABLE 6-4: EXPERIMENT PROGRAM: DISTANCES BEHIND END OF CREST

Experiment number Position chute 1 [m] Position chute 2 [m]

1 0.00 0.15

2 0.00 0.30

3 0.00 0.45

4 0.00 0.60

By measuring the overtopping discharge directly behind the crest for each experiment, a check can be performed if overtopping conditions are sufficiently similar. For these 4 experiments it is desired to simulate exactly the same overtopping conditions such that the spatial distribution of overtopping can be determined properly.

Construction drawings and some other recommendations are presented in Appendix L. POINTS OF CONSIDERATION

In Figure 6.3-1 two chutes are visible that collect overtopped water. Still displacement pumps are needed to pump the water from the chutes to floating tanks, as to prevent lowering of the water table. The pump capacity of these pumps must be sufficiently large, such that the chutes will never overflow.

Correct simulation of the infiltration of overtopping requires a correct scaling of the permeability of the backfill material. The method described in Burcharth et al. (1999) can be taken as a starting point. However it should be investigated if the flow inside the breakwater may still be regarded as one-dimensional flow.

6.3IMPROVEMENTS IN THE EXPERIMENT SETUP 105

Furthermore the boundary conditions at the landward end of the breakwater should be selected. In the recommended experiment setup the landward end of the breakwater is separated from the water behind the breakwater by means of an impermeable sheet of wood. Outflow of water at the landward end of the breakwater is not possible in this setup. This choice depends on the prototype that needs to be modelled. In the setup backfill material underneath the chutes can still be used to store overtopped water. This is a model effect and needs to be minimized.

Plastic pipelines at the bottom of the tank should ensure both sides of the flume are hydraulically connected and water can be exchanged. In that way the water level in front of the breakwater remains constant.

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