· · · Repository Hydraulic Engineering Reports

Previous measurements (e.g., Klopman, 1994) have shown that in the centre-line of the flume the mean horizontal velocity profile in waves fol/owing the current bends back in the upper region of the water column compared to the current alone case. For waves opposing the current the opposite behaviour was found . In order to explain this behaviour Radder (1994) suggested the Craik-Leibovich (ell vortex-force mechanism. This hypothesis was successfully simulated numerically by Van Kester et al. 119961. However, the explanation of the mean horizontal velocity-profile changes by the CL vortex-force mechanism depends essentially on secondary circulation cells in the transverse cross-section of the flume . To test the existence of these secondary ci rculation cells in a laboratory wave-current flume, additional measurements have been performed in the present project. It was found that these secondary circulations are clearly present in the flume as well.

Changes in the mean horizontal-velocity profile in combined wave-current motion, outside the bottom boundary-layer, are essential for the correct prediction of cross-shore sediment transport outside the surf zone and the transport of dissolved matter, e.g. Klopman (1992). To be able to verify mathematical and numerical models (see Klopman, 1992) experimental data is needed. This is especially the case with the vertical structure of the wave and turbulence Reynolds stresses where data is lacking. For this reason, a laboratory experiment was conducted to study the flow kinematics under combined wave-current motion, for waves propagating in the current direction and for waves opposing the current. The wave-current facility in which the tests have been carried out is equipped with two computer-controlled wave boards, one generating waves and the other absorbing waves. Both wave boards have active wave-absorption systems which eliminate spurious waves. A constant discharge was provided by a flow-circulation circuit. Special care was taken in the design of the inflow and outflow structures, in order to introduce the current smoothly into the channel and to minimize unwanted reflections of the waves at the inflow and outflow. Flow velocities were measured in one vertical cross-section of the channel, with two laserDoppler velocimetry (LDV) systems, mounted at a fixed distance above each other. Water surface elevations were measured with six resistance-type wave-height meters, and the discharge was measured with an electro-magnetic flow (EMF) meter. Tests were performed with mono-chromatic, bi-chromatic and random waves without current, following the current and opposing the current. Also a test series was performed for a steady current without waves.

Het doel van het onderzoek is om te komen tot een inventarisatie van literatuur met betrekking tot snelheidvelden in golven. Aan de hand van deze inventarisatie kan in een later stadium inzicht worden verkregen in de toepasbaarheid van bestaande golftheorieen of worden overgegaan tot de opzet van een beschrijvend model van het snelheidsveld onder een golf, wanneer deze de kust nadert.

The present study aims to assess the prediction of wave penetration into tidal inlets under ambient current in SWAN. The performance of SWAN in the modelling of wave-current interaction was studied for two field cases, namely Port Phillip Bay Heads, Australia, and the Amelander Zeegat in the Dutch Wadden Sea. In-situ wave observations were available for both field situations. In addition, the Amelander Zeegat field case includes recently obtained marine (X-band) radar fields of a number of variables over the tidal inlet. The ambient current was shown to have a significant effect on the wave parameters. The addition of enhanced current-induced whitecapping using the expression of Deltares (2010b) has an important, but comparatively smaller impact, which is mainly seen in the significant wave height and mean period. In the Port Phillip Bay field case, which features near-idealised swell and wind sea conditions, the enhanced dissipation term clearly reduces Hmo during ebb, for which negative current gradients are found, improving the agreement with observations. For slack and flood, which feature positive or small negative current gradients, generally small impacts on wave height were found. By contrast, in the Amelander Zeegat, reductions in Hmo were found during ebb, but also for slack and flood, since negative gradients in the current field were found during all tidal phases. Although the results for Hmo improved with the application of the enhanced currentinduced dissipation, T m-1 ,O remains underestimated in the Amelander Zeegat field case. The optimal value for the proportionality coefficient Cds3 in the enhanced current-induced dissipation term has not been found yet. For some situations Cds3 = 0.8 is better, for others Cds3 = 1.6, suggesting that the parameterisation of Deltares (2010b) should be further refined. Therefore, calibration for specific situations is recommended at present. Comparison of SWAN results with the spatial fields processed from the radar observations has given valuable insight into the spatial performance of SWAN. Furthermore, qualitative agreement has been found between the observed wave lengths and computed values.

Analyse van de gegevens van de golfpalen OSII, OSIII, OSIV en BGI over de periode 1957-1962. Relatie tussen Hs, windrichting, windsnelheid, windduur, strijklengte, stroomsnelheid en waterdiepte.

Over the last five years a research program has been carried out to assess the performance of the spectral wave model SWAN in the Wadden Sea so that it may be used for the transformation of offshore wave conditions to wave boundary conditions near the sea defenses (dikes and dunes). The assessment was done on the basis of extensive wave measurements conducted in the Ameland inlet and the Dutch Eastern Wadden Sea, as well as relevant data from other inlets, lakes and estuaries. We found that the 2006 default version of SWAN (version 40.51) performed reasonably well for storm conditions, but three aspects required further attention. First, over the tidal flats, the computed ratio of integral wave height over water depth showed an apparent upper limit using the conventional Battjes and Janssen (1978) depth-limited wave breaking formulation with default breaking parameter. Using the default settings, the wave height for a given water depth would be underpredicted near the sea defenses adjacent to nearly-horizontal flats. The problem has been largely solved using a new breaker formulation. The second area of model improvement concerns two processes related to wave-current interaction, namely the so-called wave age effect on waves generated in ambient current, and enhanced dissipation due to wave steepening in negative current gradients. Third, the variance density of lower-frequency wind waves from the North Sea penetrating into the inlet was underpredicted. Best results were obtained when the bottom friction coefficient was set at a lower value than the current default for wind seas. All these improvements have lead to a wave transformation model with which reliable wave conditions in the Wadden Sea and related complex areas can be determined.

Ten behoeve van het ontwerp van zeewerende constructies van nieuwe landaanwinningswerken van de Maasvlakte 2 is onderzoek verricht naar onzekerheden in de bepaling van golfrandvoorwaarden onder ontwerpcondities. In dit deelrapport is gekeken naar het effect van stromingen op de golfcondities voor de zeewerende constructies. Hierbij is gebruik gemaakt van bestaande resultaten van stromingsberekeningen onder gemiddelde springtij situaties. Vervolgens zijn een aantal golfberekeningen uitgevoerd met het SWAN model om het effect van stromingen op de golfcondities te bepalen. Tevens is een stappenplan opgesteld voor het uitvoeren van SWAN berekeningen voor ontwerpcondities.