Jebbe J. van der Werf

Sediment transport in nonlinear skewed oscillatory flows: Transkew experiments

New experiments under sheet flow conditions were conducted in an oscillating water tunnel to study the effects of flow acceleration on sand transport. The simulated hydrodynamic conditions considered flow patterns that drive cross-shore sediment transport in the nearshore zone: the wave nonlinearities associated with velocity and acceleration skewness and a negative mean current, the undertow. Net transport rates were evaluated from the sediment balance equation and show that (1) the acceleration skewness in an oscillatory flow produces a net sediment transport in the direction of the highest acceleration; (2) the net transport in the presence of an opposing current is negative, against the direction of the highest acceleration, and reduces with an increase in flow acceleration; and (3) velocity skewness increases the values of the net onshore transport rates.

Operational model to simulate storm impact along the Holland coast

Abstract: We have set-up an operational model system to simulate storm impact along the Holland coast. It consists of four coupled numerical models to simulate flow, waves and nearshore morphodynamics with meteorological forcing from numerical weather models. Water levels are well predicted, both during a calm and a more stormy month. Wave heights are well predicted for the stormy period, but overpredicted for the calm period. Water levels and wave heights during the January 1976 storm event are strongly underestimated, probably due to the too coarse resolution of the meteorological forcing. If we use the observed water levels as input, the XBeach transect models generally predict the dune erosion along the Holland coast due to the 1976 storm well. The operational model system can be used to make 3-day forecasts of water levels, wave heights, flow velocities and dune erosion.

Wave-Related Transport and Nearshore Morphology

This paper discusses the simulation of nearshore bar migration using the CROSMOR profile model. Two sediment transport formulations have been used : the method of Van Rijn and that of Van der Werf. The model has been applied to two large-scale laboratory experiments in the Delta flume of Delft Hydraulics. Both transport models can represent the trends observed in the laboratory experiments.

Testing of the new SANTOSS transport formula in the Delft3D morphological modelling system

Abstract: Recently, the SANTOSS practical sand transport model was developed. This paper describes the implementation and testing of this formula in the Delft3D morphological modeling system. This is done based on a compressive set of hydrodynamic and sand transport from two largedata Delta Flume -scale experiments. It is shown that the measured net transport is a delicate balance between offshoredirected suspended load due to undertow and onshore- -directed near-bed transport due to wave asymmetry and skewness effects. The Delft3D model is able to reproduce the importance of current-related suspended load the for the highwave cases (leading to offshore breaker - migration), as well as the bar onshore net transport for the lower-wave case (onshore bar migration). The mismatch between measured andcalculated near -bed transport is most apparent near the breaker bar. 1. Introduction Engineering morphological models (e.g. Delft3D, Mike, Telemac) are frequently used in coastal engineering practice to predict coastal evolution due to the combined influence of natural processes and human interferences. The resolved physics are fairly simple to keep computational times within practical limits, and therefore these models include a large number of parameterisations to account for unresolved processes. These parameterisations are usually not very well-founded on experimental data nor on fundamental understanding of the underlying hydrodynamic and sand transport processes. This especially applies to wave-driven cross-shore processes that are highly variable in time and space. As a consequence, the performance of engineering models in predicting beach and shoreline evolution is poor. For example, Van Rijn et al. (2011) showed that Delft3D systematically overpredicted measured beach erosion. This mismatch was so severe for accretive, low-wave conditions that the initial bed level was a better prediction of the final bed level than the Delft3D computation. 100 17/03/201514 pp

37. 3D COMPUTATIONS OF WAVE-DRIVEN LONGSHORE CURRENTS IN THE SURF ZONE

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.