Surface wavepackets subject to an abrupt depth change. Part 2. Experimental analysis

• Published in: Journal of fluid mechanics Vol. 915
• Main Authors: Li, Yan, Draycott, Samuel, Adcock, Thomas A.A., van den Bremer, Ton S.
• Format: Journal Article
• Language: English
• Published: Cambridge, UK Cambridge University Press 18.03.2021
• Subjects: Bathymetry; Boundary value problems; Experiments; Gravity; Intermediate water; Intermediate water masses; JFM Papers; Slopes; Theories; Water depth; Wave crest; Wave crests; Wave packets; surface gravity waves; coastal engineering
• ISSN: 0022-1120; 1469-7645
• DOI: 10.1017/jfm.2021.49

Surface gravity wavepackets in intermediate water depth experiencing an abrupt depth decrease are investigated experimentally. The experiments provide validation for the second-order (in steepness) theory for narrow-banded surface gravity wavepackets experiencing a sudden depth transition derived in a companion paper (Li et al., J. Fluid Mech., 2021, 915, A71). We observe the generation of free second-order sub- and superharmonic wavepackets due to the sudden depth transition, in addition to changes to the main (first-order) wavepacket and its second-order bound waves. Locally, just after the step, this leads to the superposition of different wavepacket components. Thereafter, separation occurs because of the different group speeds of the free second-order sub- and superharmonic wavepackets compared with the main packet. Experiments show that the local superposition of waves can lead to significant amplification of wave crests near the top of a step, as predicted by theory. In addition to a step, we also experimentally examine more gradual depth changes in the form of 1 : 1 and 1 : 3 slopes to explore the limits of the theory's validity. Although we find small differences in amplitude and phase comparing these steep slopes with a step, these experiments suggest that the theoretical model derived in Part 1 for wavepackets travelling over a step is applicable to slopes steeper than $1\,{:}\,3$.