Project Acronym: HyIV-Deltares-09
1 Title of
Research
Project
2 Facility Location
3 Summary
GLOBEX: wave transformation on a gently sloping beach
Scheldegoot, Deltares, Delft, The Netherlands
The overall aim of the GLOBEX project was to improve our understanding of the
non-linear water motion on gently sloping beaches. To that end, GLOBEX focused
on three aspects of wave non-linearity on a fixed 1:80 sloping beach: (1) the
energy-flux balance of infragravity waves, aiming to identify the physical
mechanism(s) leading to infragravity-wave dissipation, (2) non-linear wave
propagation, aiming to test and improve existing predictors of non-linear wave
celerity, and (3) boundary-layer dynamics, with the objective to extend our
knowledge of non-linear boundary layer streaming and the vertical structure of
velocity skewness and asymmetry.
We collected a data set of sea surface elevation and near-bed velocity of 8 wave
conditions (3x random, 3x bichromatic, 2x monochromatic waves) that is unique in
terms of spatial resolution (0.074-0.75 m) and extent (from the wave maker into
the swash zone) and in terms of temporal resolution (128 Hz) and duration (> 1
hour). The measurements were supplemented with extensive video imagery and
terrestrial laser scanning. Highly detailed vertical profiles of horizontal flow were
obtained at two locations in the surf zone. The collected data set is highly suited to
answer the research questions underlying the three GLOBEX aims, to validate
numerical wave models and to design and test new time series analysis techniques
that aim to unravel the complexity of nearshore wave dynamics. To illustrate the
high spatial resolution of the data, Figure 1 shows a time-space diagram of the seasurface elevation of one of the random-wave cases.
Main scientific results achieved so far include:
(1) Incoming infragravity waves loose a substantial part of their energy. Preliminary
analysis suggests that this dissipation is due to infragravity wave breaking and nonlinear energy transfer from high- to low infragravity frequencies.
(2) The celerity of individual waves can vary by 90% of the mean celerity over the
duration of a wave group. The variability depends on the relative infragravity-wave
surface elevation (with respect to the mean water depth), and is therefore
maximum when approaching the swash zone. Moreover, individual short waves
converge toward the infragravity-wave crest while propagating through the surf
zone.
(3) Vertical profiles of short-wave velocity skewness and asymmetry remain
constant in the free-stream but rapidly change within the boundary layer. For all
conditions the velocity asymmetry decreases within the boundary layer, while the
skewness increases. A linear relationship seems to exist between the near-bed
velocity skewness and the velocity asymmetry and skewness in the free-stream.
(4) Swash-swash interactions are important in controlling swash oscillations.
(5) Infragravity waves dominate the inner-surf and swash zone and, presumably,
are of critical importance to the sediment transport in these zones.
Data analyses and numerical modeling are ongoing (dd. October 2013) within
several PhD and postdoc projects at the institutes of the various GLOBEX partners.
FIGURE 1. Two-minute time-space diagram of the sea-surface elevation measured
during condition A2 (offshore Hs = 0.2 m, Tp = 2.25 s). Warm (red) colours
correspond to values above 0 (the still water level), cold (blue) colours to values
below 0. The black line fluctuating around x ~ 84 m is the run-up located
determined with the swash wire.
4 Contact
Information
Dr. Hervé Michallet
LEGI, CNRS, Université de Grenoble, BP53, F-38041 Grenoble CEDEX, France.
herve.michallet@legi.grenoble-inp.fr
Prof. Dr. Gerben Ruessink
Department of Physical Geography, Faculty of Geosciences, Utrecht University, P.O.
Box 80.115, 3508 TC Utrecht, Netherlands
b.g.ruessink@uu.nl
5 Photos
6 Keywords
Wave non-linearity, wave dynamics, infragravity waves, boundary-layer flow, wave
celerity
7 Reference to An overview of the GLOBEX project can be found in:
publications
Ruessink, G., H. Michallet, P. Bonneton, D. Mouazé, J.L. Lara, P.A. Silva and P.
Wellens, 2013. GLOBEX: wave dynamics on a gently sloping laboratory beach.
Proceedings Coastal Dynamics 2013. Available on-line (dd. 25-10-2013) at
http://www.coastaldynamics2013.fr/index.php?page=10&lang=eng
Here, six additional conference proceedings describing preliminary GLOBEX results
can be found: Almar et al., De Bakker et al., Rocha et al., Ruju et al., Tissier et al.
and Van der A et al.
8 Status
Measurements were collected in March-May 2012. Data analysis and modeling are
ongoing (dd. October 2013) within PhD and postdoc projects at the home institutes
of the various project partners.