Localización del proyecto

The development of the breaker bar during the measurements shown in Figure 2-5 has been done starting from a flat bottom during an earlier experiment in the CIEM wave flume in Barcelona (Ribberink et al., 2014). In the current section the development of the breaker bar will be simulated. While waves only break for specific conditions ( , see Table 3-1) in Delft3D, a small initial breaker bar is required. The bottom profile after one hour of measurements has therefore been used as start bottom during the computation. The model has been run for 395 minutes. This amount arises from the measurements length (365 minutes (Ribberink et al., 2014)), excluding the first 60 minutes and including the measurements length of second campaign (90 minutes (van der Zanden et al., 2015)). This computation has been done with and without wave breaking effect.

7.1.1 Without wave breaking effect

Doing a morphological computation for 395 minutes without wave breaking effects simulates a breaker bar more offshore compared to the measured breaker bar (See Figure 7-1). Since the bed-load transport is underestimated, the breaker bar is predicted in front of the measured breaker bar. What does stand out is the steepness of the breaker bar. The modeled breaker bar has a much steeper slope compared to the measured breaker bar. In seems that bed-slope effects do not have a lot of effect on the modeled bed-load transport. This is also visible in the development of the total sediment transport (See Figure 7-2). While bed slopes increase, the total sediment transport does not decrease. The modeled sediment transports looks quite similar to the measured sediment transport. This is due to the fact that both the offshore directed suspended sediment transport and the onshore directed near/bed sediment transport are underestimated.

MODELING SAND TRANSPORT UNDER BREAKING WAVES 75 FIGURE 7-1: MORPHOLOGICAL UPDATING FOR 395 MINUTES WITHOUT WAVE BREAKING EFFECTS

FIGURE 7-2: UPDATING OF THE TOTAL SEDIMENT TRANSPORT FOR 395 MINUTES WITHOUT WAVE BREAKING EFFECTS

7.1.2 With wave breaking effect and equals 2

The result of the computation with wave breaking effect including the start and final bottom during the measurements is shown in Figure 7-3. The calibration factor for the turbulence has been set to 2. The location of the breaker is predicted more onshore compared to the measurements. This is probably due to an underestimation of the offshore directed suspended sediment transport as discussed in section 6.4.2. A deep trough is visible at location x = 68 m. This is due the sediment availability. For x > 68 m there is a fixed bottom without sediment. There is erosion for x < 68 m since there is no sediment transport towards x = 68 m.

Compared to the measurements the sediment transport is predicted poorly, this is mainly due to an underestimation of the offshore directed suspended sediment transport as indicated in 4. It could be expected that after a certain amount of time the model would reach an equilibrium situation. The total sediment transport should become equal to zero after a certain time. The total transport has not decreased after 395 minutes (See Figure 7-4). Zero total transport can be

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obtained by a balance between the offshore directed suspended sediment transport and the onshore directed near-bed sediment transport. Therefore either the near-bed sediment transport should decrease or the offshore directed suspended sediment should increase. Both, bed load- transport does not decrease (See Figure 7-5) and suspended transport does not increase (See Figure 7-6).

FIGURE 7-3: MORPHOLOGICAL DEVELOPMENT OF A BREAKER BAR FROM A NEARLY FLAT BOTTOM WITH WAVE BREAKING EFFECTS

FIGURE 7-4: DEVELOPMENT OF SEDIMENT TRANSPORT DURING THE DEVELOPMENT OF A BREAKER BAR.

Since the suspended transport was underestimated during computations without morphological updating (See section 4.2.1), it is expected that during the morphological computation the suspended sediment transport is underestimated as well. The suspension transport is probably even more underestimated as the breaker bar develops. This could either be due to the net currents or it can be due to the sediment concentrations. The net currents at the top of the breaker bar do increase, but the total Mass flux stays approximately equal (See Figure 7-7). The concentrations also increase during the morphological updating; this is due to increasing sediment diffusivity (Reference concentrations stay approximately equal) (See Figure 7-8). The

MODELING SAND TRANSPORT UNDER BREAKING WAVES 77

depth integrated sediment concentration approximately stays equal. Since the depth integrated sediment concentration and the total mass flux at the top of the breaker both stay approximately equal the suspended sediment transport will stay equal as well. In both the net currents (See Figure 7-7) and the sediment concentrations (See Figure 7-8) at the top of the breaker and sudden decreases at approximately 0.2 meter below the water surface are visible. This is due to the roller mass flux which is only applied in the top part of the water column (See 3.1.4).

FIGURE 7-5: DEVELOPMENT OF BED-LOAD SEDIMENT TRANSPORT DURING THE DEVELOPMENT OF A BREAKER BAR

MODELING SAND TRANSPORT UNDER BREAKING WAVES 78 FIGURE 7-7: DEVELOPMENT OF THE NET CURRENT AT THE TOP OF THE BREAKER BAR DURING THE DEVELOPMENT OF A BREAKER BAR

FIGURE 7-8: DEVELOPMENT OF THE SEDIMENT CONCENTRATION AT THE TOP OF THE BREAKER BAR DURING THE DEVELOPMENT OF A BREAKER BAR