Observation of monsoon and typhoon-driven hydro-morphodynamics at a tropical low-tide terraced beach: a case study at Nha Trang, Vietnam

Abstract : Most of the coast of Vietnam is currently eroding, an imbalance in the sediment budget that may be caused by overuse of river sediments for construction, subsidence associated with overuse of groundwater, over-exploitation of the littoral zone (coastal squeeze), and possibly by an increase in typhoon frequency and winter monsoon events. Assessment of sediment budget is needed and requires accurate evaluation of sediment transport in the coastal environment. In my PhD study, I use two close-range camera systems for nearshore monitoring: a land-based nearshore camera system for observing short-, medium- and long-term coastal hydro-morphodynamics and a drone for monitoring short-term events. Specifically, I used a camera system installed in Nha Trang beach, Viet Nam, from 05/2013 to 08/2016. The cross-shore profiles, shoreline positions and wave characteristics (height and period) extracted from the video data are calibrated with in-situ measurement from two field experiments during the Haiyan typhoon event. Then, the results on short-term and seasonal shoreline changes are analyzed -- and published. The study shows a marked seasonal evolution of Nha Trang shoreline and dramatic impact of cat-5 typhoons Nari and Haiyan with changes of 4 to 8 m in each case. However, the recovery to individual events is fast, as opposed to the effect of winter monsoon events. Our continuous video observations show for the first time that long-lasting monsoon events have more persistent impact (longer beach recovery phase) than typhoons. Using a shoreline equilibrium model, we estimate that the envelope of intra-seasonal events rather than monthly-averaged waves drives the seasonal shoreline behavior. Finally, the shoreline study suggests that the interplay between intensity and duration of intra-seasonal events may be of key significance. In the second part of this PhD study, a video-based bathymetry inversion technique is applied to long-term data with varying wave environment from swell to wind wave conditions. A main result is the overall stability and high resiliency demonstrated for the low tide terrace (LTT) beach of Nha Trang, with rapid exchange of sediment between the terrace and the upper beach during typhoons, monsoon events or seasonal cycles. The high resiliency of these tropical environments may provide faster beach recovery compared with mid-latitude configurations. At the same time, the study details the treatment techniques needed for a long-term dataset, and focuses on the limitations of the method in the case of wind waves. For the first time, a tide gauge is used to quantify the error produced by video estimates, which provides a quality criterion. More generally, this study provides guidelines for users of video-based depth inversion methods and a base for standalone error assessment, which is essential for operational and data assimilation systems. A comprehensive error estimate, such as the one proposed here, is thus an excellent step for improving the assimilation approach. The study is concluded with an exploratory chapter on the use of drones for remotely assessing a continuous profile of topography and bathymetry. Instead of using a fixed-wing drone, which is not best suited for stationary applications, in this study, we use a rotary-wing drone with a very-high resolution camera. The study shows that the technique developed for shore-based video cameras can be successfully applied to a video system attached to a rotary-wing drone. Drone estimates are successfully compared with bathymetric surveys in Nha Trang. In addition, a preliminary sensitivity analysis suggests some guideline for future users.
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Submitted on : Thursday, October 10, 2019 - 3:37:23 PM
Last modification on : Friday, October 11, 2019 - 8:22:39 PM

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Hai Thuan Duong. Observation of monsoon and typhoon-driven hydro-morphodynamics at a tropical low-tide terraced beach: a case study at Nha Trang, Vietnam. Ocean, Atmosphere. Université Toulouse III - Paul Sabatier, 2019. English. ⟨tel-02310953⟩

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