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Recent study has shown that 3D computations of the morphological development of a coast shows irregularities compared with the 2DH (depth-averaged) computations. Therefore a validation of the surf zone currents computed using the 2DH (depth-averaged) and 3D approach in Delft3D is made. The 2DH and 3D approach are compared using an idealized case and validated using data from the laboratory experiment performed by Reniers and Battjes and data from SandyDuck97 field measurements. The 3D approach underestimates the wave-driven longshore current compared with the 2DH approach. The longshore current computations in the 3D approach are dependent on the thickness of the computational layer just above the bed. In the 3D approach the bed shear stress is computed using the quadratic friction law and the velocity in the computational layer just above the bed as input, and the assumption of a logarithmic distribution of the longshore current. The dependency is caused by the assumption of a logarithmic velocity distribution in the computation of the bed shear stress. Due to wave breaking enhanced turbulence this assumption is not valid. Computing the bed shear stress using the velocity in the computational layer just above the edge of the wave boundary layer solves the layer dependency. This new method of computing the bed shear stress in particular and the longshore current computations by Delft3D in general are extensively validated. The 2DH and 3D approach agree well with the measurements for both the laboratory and the field data. For the laboratory experiments the longshore currents are underestimated in the bar trough. The wave height is the bar trough is overestimated, which might causes the underestimation of the longshore current since too little wave energy is dissipated. It is recommended to further examine the translation of wave forces to a current. For the field experiments the longshore currents are generally overestimated near the coast. The wave height computation showed a reasonable agreement with the measurements but also a systematically overestimation. More attention should be paid into accurately modelling the wave height and the wave height decay. Also the vertical distribution of the current velocity is compared with data from the SandyDuck97 measurements and showed a reasonable agreement.

The present report describes some of the results obtained during experiments in the Large Wave Flume of the Fluid Mechanics Laboratory of the Delft University of Technology. The experiments are part of the PhD-work of M. Boers. They follow on the LIP llD-experiments, carried out in the Delta Flume of Delft Hydraulics in spring 1993. During the LIP 11D-experiments much information was obtained about physical parameters in the surf zone such as wave heights, wave set-up and velocities. The experiments have the following objectives: To add measuring data to the LIP llD-data To obtain data which can reconstruct the mass, momentum and energy balances To obtain detailed information about regions with steep gradients of wave heights and wave set-up (onshore slope of breaker bar and toe of foreshore) To obtain information about the breaking behaviour of waves To measure bed shear stresses To measure turbulence motions The objective of this report is to distribute the results of the measurements among researchers working in the field of coastal engineering. Further, it gives information about the accuracy of the measuring data. The results of the experiments are described in two reports. Report 1 (the present report) describes the experimental set-up [Chapter 2], wave height measurements and the video recordings of the wave breaking [Chapter 3]. The results of velocities and shear stress measurements are described in Report 2 [Boers, 1996]. Some of the results are already published by Boers and Van de Graaff [1995]. The results of the analysis of the measurements is presented in many figures. Data are also available in files [Enclosure A].

The main problem dealt with in this thesis is the calculation of certain effects caused by random waves breaking on a slope. The solution to this problem is greatly complicated by the fact that wave breaking is a highly nonlinear process. The flow field is further complicated by far stronger in homogeneities than those occurring outside the breaker zone, by air entrainment and by generation of turbulence. No realistic deductive treatment of it has been developed so far. Even for the simpler case of periodic waves, empirical knowledge of certain macroscopic properties of the breakers is still an integral part of calculations relating to the surf zone. An attempt has been made in this thesis to apply this knowledge in a formulation incorporating the stochastic nature of wind-generated waves. The computations are of two distinct categories, those relating to comparatively gentle slopes and those relating to comparatively steep slopes. A summary of the results will be given in the following. The energy variation is calculated in chapter 5 by clipping a fictitious wave height distribution, which theoretically would be present if breaking did not occur, at an upper limit which is determined from an adapted breaking criterion for periodic waves. The computed results are in fair agreement with measurements carried out on a plane slope. Knowledge of the energy variation permits the radiation stresses to be evaluated, which in turn are necessary for the calculation of the set-up and the longshore current velocity profiles. A comparison of the calculated set-up profiles with empirical data has not given conclusive results. Good agreement has been found with field data, but not with laboratory data, which locally showed a systematically smaller rise towards the shore than would be expected on the basis of the measured or calculated wave height variations. However, there is some uncertainty with respect to the system used for measuring the set-up in the laboratory, so that is not known to which extent the differences are real or apparent.