New Perspectives on Wave Transformation Over a Nearhorizontal Shore Platform: Comparison of a Twodimensional and Transect Approach

38 Pages Posted: 8 Dec 2021

See all articles by Raphael Louis Krier-Mariani

Raphael Louis Krier-Mariani

University of Otago - School of Geography

Wayne J. Stephenson

University of Otago

Sarah J. Wakes

University of Otago

Mark E. Dickson

University of Auckland

Abstract

Understanding wave transformations over intertidal shore platforms is necessary for modelling managing rock coasts. Previous studies have investigated waves using instruments aligned in cross-shore and longshore arrays so preclude consideration of wave directionality, which has been overlooked or only of secondary concern. We developed a two-dimensional approach to measure the wave direction of both swell (SW: 0.05  < frequency < 0.125 ) and infragravity waves (IG: frequency < 0.05 ) during transformation across a near-horizontal shore platform (NHSP). Waves were measured using a grid of 14 pressure transducers (PT) recording at 2Hz. Directional wave spectra were derived using the Extension of the Maximum Entropy Principle (EMEP) method. Both SW and IG were subject to strong directional changes over the NHSP induced by refraction on the lateral edges of the platform and reflection at the cliff toe, resulting in the dominance of oblique and reflected wave trains over the inner platform while onshore waves propagating along the transect represented only 36% of the SW and 22% of the IG. These directional changes were at the origin of complex wave energy patterns. A 77% decrease of refracted SW energy was recorded from the platform edges to the platform centre.  Two dominant IG energy patterns were identified. The first was a long-shore increase of energy toward the central axis of the platform and a cross-shore increase of energy from the platform centre to the seaward and landward extremities, this pattern was seen for 81% of the submersion period. The second pattern also reported in previous research was an increase in IG from platform edges to the shoreline, but this was only observed for 15% of the submersion period.  The two-dimensional approach developed in this paper allowed to provide field-based observations of the combined effects of wave reflection and refraction on wave energy distribution over a NHSP.

Keywords: rock coast, shore platform, wave transformation, wave energy

Suggested Citation

Krier-Mariani, Raphael Louis and Stephenson, Wayne J. and Wakes, Sarah J. and Dickson, Mark E., New Perspectives on Wave Transformation Over a Nearhorizontal Shore Platform: Comparison of a Twodimensional and Transect Approach. Available at SSRN: https://ssrn.com/abstract=3980596 or http://dx.doi.org/10.2139/ssrn.3980596

Raphael Louis Krier-Mariani (Contact Author)

University of Otago - School of Geography ( email )

Otago
New Zealand

Wayne J. Stephenson

University of Otago ( email )

P.O. Box 56
Dunedin, 9010
New Zealand

Sarah J. Wakes

University of Otago ( email )

P.O. Box 56
Dunedin, 9010
New Zealand

Mark E. Dickson

University of Auckland ( email )

Private Bag 92019
Auckland Mail Centre
Auckland, 1142
New Zealand

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