Abstract
Low tide terrace (LLT) beaches are characterised by a moderately steep beach face and a flat shallow terrace influencing the local hydro-morphodynamics during low tide. The upper beachface slope (β) and the terrace width (Lt) are the main morphological parameters that define the shape of LTT cross-shore beach profiles. This work aims at better understanding the behaviour of β and Lt and their link with the incoming wave forcing. For this purpose, our results are based on 3.5 years times series of daily beach profiles and wave conditions surveys at two different microtidal LTT beaches with similar sediments size but different wave climate, one at Nha Trang (Vietnam) and the other one at Grand Popo (Benin). While they look similar, two contrasting behaviour were linked to two sub-types of LTT regimes: the first one is surf regulated beaches (SRB) where the swash zone is highly regulated by the surf zone wave energy dissipation on the terrace, and the second is swash regulated beaches (SwRB) acting in more reflective regime where the terrace is not active and the energy dissipation is mainly produced in the swash zone, the terrace becomes a consequences of the high dynamics in the swash zone. Finally, extending the common view of an equilibrium beach profile as a power law of the cross-shore distance, the ability of a simple parametrized cubic function model with the Dean number as unique control parameters is proposed and discussed. This simple model can be used for the understanding of LLT environments but it can not be extended to the whole beach spectrum.
Highlights
Accepted: 14 December 2021Sandy beaches are loose deposits of sand accumulated at the shoreline that have been carried alongshore and cross-shore by currents and waves
This paper aims at improving our knowledge on low tide terrace (LTT) beach morphodynamics by analysing two field data set at LTT environments with two different wave climate
From the morphological scaling at Nha Trang and Grand Popo, two different morphological responses of the inner-surf zone were identified depending on wave climate
Summary
Sandy beaches are loose deposits of sand accumulated at the shoreline that have been carried alongshore and cross-shore by currents and waves. The location of theses beaches is a function of sand sources, coastal processes and geology [1]. The modal beach morphology of sandy beaches changes in response to environmental conditions (waves and tide forcing, sediments, geology, etc.). This has been studied for a long time and has led to the development of several beach classification models. All of them include the occurrence of distinct beach morphologies linked to the key environmental conditions like wave climate, tidal regime and beach sediment characteristic [2,3,4]. Wright and Short (1984) [2]
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