Lidar Observations of the Swash Zone of a Low-Tide Terraced Tropical Beach under Variable Wave Conditions: The Nha Trang (Vietnam) COASTVAR Experiment

Luís Pedro Almeida,Paula Gomes Da Silva,Nadia Senechal,Nguyen Trung Viet,Duong Hai Thuan,Jerome Ammann,France Floc’H,Philippe Grandjean,Erwin Bergsma,Melanie Biausque,Chris Blenkinsopp,Charles Caulet,Rachid Benshila,Patrick Marchesiello,Rafael Almar

doi:10.3390/jmse8050302

Abstract

A field experiment was conducted at a tropical microtidal intermediate sandy beach with a low tide terrace (Nha Trang, Vietnam) to investigate the short-term swash-zone hydrodynamics and morphodynamics under variable wave conditions. Continuous 2D Lidar scanner observations of wave height at the lower foreshore, subsequent run-up and swash-induced topographic changes were obtained. These data were complemented by detailed real-time kinematic GPS topographic surveys. Variable wave and tide conditions were experienced during the field experiment with relatively large swell waves (offshore significant wave height, Hs = 0.9 m to 1.3 m; peak wave period, Tp = 8 to 12 s) concomitant with spring tides at the beginning of the period, followed by mild wind waves (offshore Hs under 0.5 m and Tp 5 s) and neap tides. This resulted in the following morphological sequence: berm erosion followed by rapid neap berm reformation and beach recovery within a few days. New insights into the link between intra-tidal swash dynamics and daily beach profile evolution were found using the Lidar dataset. While waves directly cause morphology changes on a wave-by-wave basis, tidal levels were found to be a key factor in determining the morphological wave-effect (accretive or erosive) due to modulated interaction between surf and swash hydro-morphodynamics.

Highlights

Beaches continuously adapt to wave forcing at multiple temporal [1,2,3] and spatial scales [4] and the morphological changes commonly observed in these environments are related to the hydrodynamics of the swash zone
As part of the COASTVAR project, a field experiment was undertaken between 26th November and 4th December 2015 at Nha Trang Beach, a sandy beach located in a semi-enclosed bay in southeast Vietnam (Figure 1)
Field measurements performed at Nha Trang beach show that under relatively constant offshore wave forcing, wave height at the base of the foreshore and swash motions were distinctly different between low and high tide as the swash zone oscillated between reflective and dissipative conditions

Summary

Introduction

Beaches continuously adapt to wave forcing at multiple temporal [1,2,3] and spatial scales [4] and the morphological changes commonly observed in these environments are related to the hydrodynamics of the swash zone. Tidal modulation of breaker type (e.g., plunging breakers at high tide and spilling breakers at low tide) are known to occur on this type of beach [5,6]; the extent to which tidal variation affects the swash-zone hydrodynamics and, the morphological response is still unclear [7] These theoretical concepts provide a basis for understanding the swash dynamics on this type of intermediate beach, the details of the process are highly complex, and limited field observations exist for validation. Hydrodynamics and morphology measured using Lidar during a nine-day field experiment are used to assess swash asymmetry and the role of wave and tidal variability on swash-zone morphology at an intermediate low-tide terrace beach

Study Area

Daily Morphological Evolution

Tidal Modulation of Waves and Run-Up Heights

Daily Evolution

Conclusions