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Long wave generation and behavior on a plane sloping beach: a study of the behavior of long wave motion driven by short wave groups
Janssen, T.J. (1999). Long wave generation and behavior on a plane sloping beach: a study of the behavior of long wave motion driven by short wave groups. MSc Thesis. Delft University of Technology. Faculty of Civil Engineering and Geosciences: Delft. 141 pp.

Thesis info:

    Long waves
    Wave generation
    Wave groups
Author keywords
    Wave behaviour; Golfgedrag

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  • Janssen, T.J.

    This thesis studies the relation between short wave groups and long wave motion in the shore-normal case on a uniformly sloping bottom. The short wave groups can be described through two time scales; the time scale of the individual waves and the time scale of the groups. The energy of the individual waves is dissipated through wave breaking and in the nearshore region the energy is found concentrated in the lower frequency part, the time scale of the groups. The long waves are assumed to be generated due to variation in the radiation stress on the time scale of the groups.

    The research study comprised the physical modeling of a mild, impermeable, uniformly sloping bottom at Queen's University Coastal Engineering Research Laboratory in a 3D environment. At the end of the sloping bottom a breakwater was constructed. The test program consisted of irregular and bichromatic tests. Emphasis was on irregular tests with a JONSWAP energy spectrum. Although in this thesis only the shore normal case was anaIyzed, also tests were performed with the breakwater at different angles thus reproducing situations with oblique incoming waves.

    For the analysis the second order effects in the basin were investigated. The parasitic wave component, generated due to lack of second order corrections in the paddle control signal, was estimated through the evaluation of the short wave spectrum at the paddle. Since long waves are known to (partially) reflect at the shoreline and to re-reflect at the paddle, an algorithm was developed to separate incoming and outgoing long waves on a slope using Bessel functions. Through the separation of incoming and outgoing waves at the offshore stations it was possible to estimate re-reflections coming from the paddle. Both the parasitic and the re-reflected components were determined and subtracted from the data to analyze their effects on the measurements.

    The experimental data show that the phase relation between the long waves and the short wave groups changes while travelling on the slope. The long wave is negatively correlated to the short wave groups at offshore positions. On the slope the long waves are found to lag the short wave groups. In the nearshore region positive correlation was found between the short waves and the long waves. Further testing is necessary to determine the relation and effects of the different possible mechanisms that cause the phase shift between the short wave groups and the long waves coming from offshore on a uniformly sloping bottom.

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