|Dynamica van golfgedreven langsstromen|
Reniers, A. (1999). Dynamica van golfgedreven langsstromen. Communications on Hydraulic and Geotechnical Engineering, 99.2. TU Delft. Faculty of Civil Engineering and Geosciences: Delft. xviii, 132 pp.
Part of: Communications on Hydraulic and Geotechnical Engineering. Delft University of Technology. Department of Civil Engineering: Delft. ISSN 0169-6548, more
Dynamics; Longshore currents; Marine
Laboratory experiments have been performed to examine the shear instability of longshore currents. Minimizing the recirculation resulted in minimal alongshore set-up gradients, indicating that the measured current velocity profiles corresponded to purely wave-driven currents only.
Based on the experiments we conclude that, for purely wave-driven longshore currents, the maximum longshore current velocities occur where breaking is most intense, i.e. on the bar and near the shoreline. Lateral mixing smoothes the current profile, but does not significantly affect the position of the maximum current velocity, which in the case of a barred beach clearly remains near the crest. This holds for the cases of short waves, long waves and random waves on barred beaches.
Next we have used the mean wave-and flow data for verification and validation of a numerical model for uniform alongshore conditions using existing model equations. Comparison of computational results with measurements showed good agreement for wave transformation, set-up and longshore current velocity profiles. Lateral mixing was required to obtain a good match with the data, also in the case of random waves on a barred beach. The transition effect, caused by the surface roller, was essential in the correct prediction of the location of maximum wave forcing and improved the correspondence with the measured longshore current velocity in the trough.
The thus verified numerical longshore current model has been extended with an additional forcing of the longshore current due to the presence of a steady alongshore pressure gradient. Next the model was applied to examine longshore current measurement data obtained during the DELILAH field experiment. The analysis showed that bathymetry induced alongshore pressure gradients were present during DELILAH experiment. A relatively small alongshore pressure gradient over the trough, acting in the same direction as the wave forcing, has a significant effect on the longshore current velocity distribution. If this alongshore pressure gradient varies weakly in the alongshore direction, the current velocity can be obtained from the local force balance. With increased modulation, the inertia effect gains importance and the local approach is no longer valid. In the cases of opposed forcing, the computed longshore current velocity profiles matched the measured longshore currents, with velocity maxima over the bar, analogous to the laboratory data.
Summarising, the preceding conclusions imply that in absence of an alongshore pressure gradient the maximum longshore current velocity occurs near the areas of most intense breaking, and that the occurrence of velocity maxima in the trough, as often observed in the field, can be ascribed to the presence of an alongshore pressure gradient (excluding wind effects).
Using a spectral analysis technique based on the Maximum Entropy Method the longshore current dynamics during the laboratory experiment could be analysed in detail. Using the zero-mode edge-wave dispersion as a reference, this analysis clearly showed the generation of alongshore exponentially growing shear instabilities. The dispersion curves thus obtained were almost linear, with a phase speed of order of 70 %of the maximum longshore current velocity. The downstream shear instability conditions for the various tests differed considerably, indicating that the growth is a delicate balance of forcing and damping. The results also showed that there is a significant influence of the beach profile on the occurrence of shear instabilities. They did occur on a barred beach but were not observed on a non-barred beach though the back shear was of similar magnitude. This was explained by the fact that the position of the maximum longshore current velocity in the case of a planar beach is closer to shore, rendering a more stable longshore current.
A comparison between measurements of the longshore current dynamics and computational results obtained from linear and non-linear modelling has been performed. In general the correspondence between computations and measurements is good. This is especially so for the frequency-alongshore wave number signatures of the shear instabilities, implicating a good match of the computed and measured phase speeds. Still, uncertainties in the prediction of the initial growth rate exist if a linear stability analysis is used. Both in the measurements and in the non-linear model computations, the observed shear instabilities were very robust all along the beach. As a case of interest the bottom friction was reduced, no longer requiring a good match with the measured longshore current velocity profile, resulting in more energetic vorticity motions. The interaction of the shear instabilities with the longshore currents has been examined. A detailed analysis of the measured cross-shore momentum flux showed an alongshore increasing cross-shore flux, becoming of comparable order as the mixing associated with breaking wave induced turbulence. However, no significant downstream changes in the mean longshore current velocity profile were detected. The same behaviour was observed in the numerical computations over the domain of the laboratory basin. This is explained by the fact that the shear instabilities take only effect near the downstream end, showing a strong increase over a short distance making inertia effects important, whereas the wave forcing is present all along the beach.
The computations with an extended numerical basin indicate that equilibrium for the shear instabilities was not yet reached during the experiment. The cross-shore component of the shear instability velocity reaches an equilibrium at approximately 35 m from the inflow opening (whereas the available length of the basin was 32 m including outflow effects), as does the mean longshore current profile. The latter does now exhibit the anticipated smoothing of the velocity profile, overcoming the earlier mentioned inertia effects.
The changes in the longshore current due to the presence of shear instabilities are significant and therefore important to any related quantities such as the transport of sediment. Using the results obtained from a linear stability analysis in an iterative procedure, the shear instability momentum transfer can be incorporated in the computation of longshore currents on an alongshore uniform beach.