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h2. TheProcesses influenceand ofenvironmental thecharacteristics near-shorethat profileinfluence on the barduration cyclesof inthe Noordwijkbar and Egmondcycle
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Sandbars migrate on average in offshore direction. They decay at a certain water depth and the released sediment is transported onshore to form a new bar, generating a continuous bar cycle. It is important to have detailed knowledge about sandbar behaviour, because morphodynamic changes in the future can then be predicted. There are already some models that predict bar behaviour relatively well, but the factors of influence that determine the duration of the bar cycles are not clearShore-face nourishments are a common way to protect the coast from eroding. These nourishments interact with near-shore bars, so bar dynamics is an important process that must be understood to perform the nourishments as efficient as possible. The general bar behaviour is clear. Bars move onshore with calm conditions and offshore with storm conditions. The latter is dominant. The bars decay at the end of the bar zone and a new bar forms onshore. However, the factors that influence the bar cycle duration are not completely understood yet. For example, the durationbar ofcycle theduration barfor cycles in Noordwijk and Egmond is about 4 and 15 years respectively, with conditions that are only slightly different. For example, the waves of Egmond are on average 20 cm larger and the near-shore profile until a water depth of 15 meter is steeper at Egmond.
Preliminary work, performed by E. van der Deijl (in 2013) showed already. The goal of this report is to find which processes are responsible for the variation in bar migration speed at Noordwijk and Egmond. A Unibest-TC model is used to simulate 10 years of morphological development for both sites. Several simulations are performed with variable wave climate, profile and D50. The results are that it is probablymainly the steeper near-shoreprofile zone of Egmond that causes thea longer bar cycle duration. MyEspecially researchthe willsteeper focusslope moreof onthe thisbar near-shore profile, to determine which part of the profile is mainly responsible for this. Is it the the steeper bar zone of Egmond, the steeper and higher bars, or is it the steeper offshore part of Egmond (offshore from the bar zone) that increases the bar cycle duration? I will also try to find morphological and hydrodynamical reasons for this. Why causes these different profiles a significant different bar-cycle duration?
A calibrated UNIBEST-TC model for Noordwijk is used for this research. This model is adapted for profile changes in order to find the answers of the research questions.
zone gives a lower bar migration speed. This is regulated by the water depth. Initially, at the same water depth, the steeper profile of Egmond gives more sediment transport. This is due to a smaller range of wave breaking above the bar top. However, the bars of Egmond arrive earlier in deeper water, so that wave breaking, sediment transport and offshore bar migration becomes smaller at Egmond. It must be taken in mind that these models are used to find the factors that influence the bar cycle and not to model to morphological development as good as possible. This means that there must be focussed on trends and not on absolute values.
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