Wave-Driven Longshore Currents in the Surf Zone Hydrodynamic validation of Delft3D

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.