Hydrodynamics in the inner surf and swash zone

Started: October 2014
Supervisor: Alsina, J.Swan, C.

Description of Research

The study of high-frequency wave groups and the associated low-frequency wave motion is important for the design of coastal structures and it is also considered a dominant forcing of the coastal morphological evolution. In particular, it is well known the mechanism of bound long wave generation from the radiation stress gradients within the wave groups in steady conditions [Longuet-Higgins and Steward, 1962]. During short-wave groups propagation over a sloping bottom, the short waves shoal and the incoming bound long wave, which travels with the group, gains energy at expenses of the short wave groups [Battjes et al., 2004].

The aim of this work is to extend existing experimental works to gentle slope (1:100) conditions and small ratios of beach slope to long-wave angular frequency. Bi-chromatic wave conditions have been generated and the energy transfer from the primary short-waves to the bound wave will be studied. 

Preliminary Results

Preliminary results from 2 different bi-chromatic cases (same spectral energy content) are illustrated in Fig. 1, where Fig. 1.a, presents the measured short-wave height propagation obtained from spectral analysis and the wave height associated to the bi-chromatic components (f1 and f2) during shoaling and wave breaking. Fig. 1.b, on the other hand, illustrates the wave height associated to the wave group frequency separated in reflected and incident bound long waves.

It is shown a remarkable drop of wave energy associated to the component f1 in comparison with the f2 component. From x=33 m to the breaking position, the differential drop of the amplitude associated to f1 between run 1 and run 2 matches with the different growth of the incoming bound wave.  This suggests a larger energy transfer from f1 to the group frequency for the shorter wave group frequency.

The preliminary analysis also suggests modifications in the wave group modulation associated to the selective f1 energy drop, with larger modulation reduction as the wave group frequency reduces. The wave group modulation reduction may affect the wave breaking location and its spatial distribution. 

Fig 1
Panel a: Cross-shore distribution of wave height, including the wave height at the primary components f1 and f2. Panel b: Cross-shore distribution of the incoming bound long wave and its reflected free long wave. Panel c: Schematic layout of the wave flume with the measuring locations.

Background

Enrique is a graduate with a MEng in Civil Engineering and an MSc in Environmental Hydraulics, both from the University of Granada. In addition to his current PhD research into “hydrodynamics in the inner surf and swash zone”, he has previously worked in the area of estuarine research, in collaboration with the group of ‘Puertos y Costas’ from the University of Granada. The results of this research were published in Padilla et al., 2015. 

Enrique Padilla

Enrique PadillaPhD Candidate - Fluid Mechanics
Department of Civil & Environmental Engineering 
Imperial College London SW7 2AZ 
e.padilla14@imperial.ac.uk