Infragravity waves on gently sloping beaches

Introduction

Infragravity waves are 20-200 s waves that are formed by short wave (2-20 s) groups. While the short waves break close to shore, the infragravity wave is able to propagate into even shallower water. As infragravity-wave energy dominates the water motion close to shore, it can be important in beach and dune erosion.

INRGRAV_Anouk
Figure 1: Short waves (solid line) travel in groups, and together they induce an infragravity wave (dashed line).

To be able to predict the erosion/accretion of beaches caused by infragravity waves, sediment suspension and transport by infragravity waves have to be fully understood, and therefore the hydrodynamics as well. One of the aspects that currently is receiving a lot of attention is the energy dissipation of the infragravity wave in very-shallow water. Over the last decades it was thought that infragravity waves travel to the beach, perfectly reflect at the shoreline and travel seaward again, and thus conserve all their energy. However, more recently both field and laboratory data indicate that infragravity waves dissipate energy close to shore, in particular on low-sloping beaches. The energy dissipation strongly affects infragravity-wave induced sand transport, and hence the magnitude of beach erosion. Several dissipation mechanisms have been put forward, however there is still little field evidence supporting either of these hypotheses.

Objectives

The present PhD project is aimed at understanding the processes related to infragravity wave transformation in the nearshore, from the generation of the infragravity waves to ultimately their role in beach erosion.

The main objectives are to:

  • Fully understand the infragravity wave characteristics. This includes their generation, propagation and dissipation.
  • Identify the infragravity-wave dissipation mechanism. Is it bottom friction, non-linear energy transfer back to short waves, or infragravity-wave breaking?
  • Study infragravity-induced sediment suspension and transport.
  • Quantify their role in beach evolution under storm conditions.

Methods

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Figure 2: Fieldwork on Ameland, the Netherlands in 2010.

Several different approaches are applied, including field and laboratory data analysis and numerical modeling.

Two field data sets of pressure, velocity and sediment concentrations are available, both obtained on rather mild sloping beaches (Ameland in 2010, see figure 2, and Egmond beach in 2011). During the fieldworks a cross-shore transect with pressure transducers (PTs), Electromagnetic Flowmeters (EMFs), Sontek Acoustical Doppler Velocitymeters (ADVs) and Optical Back-Scatter sensors (OBSs) were positioned in the intertidal zone of the beach.

The laboratory data were obtained as part of the GLOBEX project, where a high resolution data set (both in space and time) was collected on a low-sloping (1:80) fixed laboratory beach. (see Figure 3) The experimental program comprised eight wave conditions: 2 monochromatic conditions, 3 bichromatic conditions, and 3 random wave conditions.

Additional modeling studies will be done with SWASH and/or XBeach.

Results

The PhD project started in March 2012. Over the last year, field data obtained on Ameland in

Figure 3: GLOBEX laboratory experiments in 2012 in the Scheldegoot at Deltares, the Netherlands.
Figure 3: GLOBEX laboratory experiments in 2012 in the Scheldegoot at Deltares, the Netherlands.

2010, and at Egmond beach in 2011 have been analyzed, and an article with those results is currently under review (topic: objective I and II). In this article we show that for low-sloping beaches, infragravity waves indeed dissipate a considerable part of their energy close to shore. The dissipation was frequency dependent; low infragravity frequencies conserved a large part of their energy, whereas high infragravity frequencies dissipate their energy completely. We demonstrate that for these data sets, the most likely mechanism causing the energy dissipation in infragravity wave breaking. However, further research is needed to verify this, and to identify the zone over which this occurs

This process will therefore be explored in more detail by analyzing laboratory data. With the high resolution data set of GLOBEX, a quantification of the energy fluxes and non-linear energy transfers will be examined by means of bispectral analysis. This is still under progress. Preliminary results can be found in the Coastal Dynamics 2013 proceedings. (www.coastaldynamics.fr – de Bakker et al. 2013)

For more information please contact: Anouk de Bakker, Marion Tissier or Gerben Ruessink.