Hydro-morphodynamique des plages sableuses au Vietnam
Projects (2013-2018) MOST1, ANR COASTVAR and MOST2: Etude des plages sableuses de Nha Trang
Coastal zones are essential for social and economical developments. Located at the interface between ocean and continent, the coasts are vulnerable to environmental hazard and are currently facing an intensification of risk associated with increasing human pressure and the context of global climate change. This project focuses on two regions of the world particularly exposed to coastal vulnerability: West Africa and Vietnam. The environmental conditions governing hydro-sedimentary functioning differ drastically between the two regions. Erosion in West Africa is induced all year long by energetic long swells; in contrast, Vietnam shows paroxystic events induced by typhoons. Even though the societal issues are manifest in these areas, their hydro-sedimentary functioning remains poorly known and limits social and economical development. The objective of the COASTVAR project is to advance our understanding by characterizing the morphological evolution (aerial and submerged), the driving forces and hydro-morphodynamic processes, from event to seasonal and interannual scales. Emphasis will be given to extreme events and their long-term effect, and to surf-shelf exchanges associated with the wave-induced circulation. In the first project task, innovative observational tools (video imagery and drone) will be used in addition to conventional instruments. In a second task, deep-water wave conditions will be downscaled to the beach, then nearshore configurations of a 3D coupled wave-current model will be set up. In a third task, the ECORS beach evolution predictor (PEA SHOM-DGA), which was yet only tested in mid-latitude environments, will be applied for the first time to tropical coastal systems. Our objective here is to obtain a generic operational tool that can be applied to any coast in the world. The research developed in the COASTVAR project has a strong dual aspect. First, it will provide the first high quality survey and forecasting system for the selected regions (waves, currents and bathymetry), which will be highly relevant to military action. Then, it will propose tools to anticipate coastal risks (erosion and submersion), quantify vulnerability and exposure of people to hazard, and lay solid grounds to improve coastal management.
Figure: Image du systeme vidéo de la plage de Nha Trang, Vietnam, (WRU/MOST/IRD-LEGOS) durant le typhon Haiyan, Nov. 2013
Context, position and objectives of the proposal
Context, social and economic issues
Southern countries of tropical coastal regions are currently facing an intensification of coastal risks under increasing demographic pressure and uncontrolled exploitation of resources associated with rapid economic development. The natural environmental vulnerability of the coastal zones (i.e. extreme events of tropical storms, erosion, flooding) conjugated with demographic pressure increases hazard for human activities and represents a limitation for coastal development (maritime transport, tourism, urban development). There is a need for a greater understanding and estimation of present dynamics and future evolution (ALOC-GG 2011 report, global warming impact, Stive, 2004). This project aims at understanding the processes responsible for the observed large coastal variability and quantifying their impact.
Processes governing the hydro-sedimentary system evolution are complex, resulting from interactions between the continuously changing oceanic forcing and morphologic response from the event-scale to longer-term seasonal and interannual scales (project PEA ECORS (short-term), Senechal et al., 2011; project ANR VULSACO (long-term), Idier et al., 2013). Even though temperate coastal environmental hazard is currently under the scope of the research community (e.g., role of the North Atlantic Oscillation on European coasts - project ANR CECILE, coastal vulnerability and submersion - ANR MISEEVA - ANR Cocorisco), the available literature is scarce regarding the coastal processes that affect tropical coastlines (Short, 2013). The few studies have so far mainly followed a descriptive or naturalistic approach, despite peculiarities: strong seasonality with monsoon impact on continental sediment discharges and wave conditions, paroxysmal tropical storms (typhoon- Vietnam) or rather year-persistent long-travelling swells (> 20 s) coming from high latitudes and impacting reflective, mostly erosive coasts (Gulf of Guinea). Little is known on the functioning of these specific tropical hydro-sedimentary systems.
This project will complement our knowledge of coastlines impacted by tropical forcing, with emphasis on event-scale processes and their long-term impact. More specifically we address the following scientific questions: What is the rapid beach response to extreme energetic events, and how are post-event low-energetic conditions implicated in coastal recovery? What is the role of transient nearshore circulation in the exchange of sediments between nearshore and offshore zones? What is the integrated impact (e.g., sequence of storms, Coco et al., 2013) of these events on the long-term evolution? What is the forcing variability and the coastal response to multi-scale forcing? The interest of the COASTVAR project is emphasized by the context of global warming as model projections suggest an increase of vulnerability of tropical coastal zones to environmental hazards.
In this project, the study of remote coastal systems is addressed through acquisition of an extensive dataset provided by reliable and innovative observation techniques (video monitoring network, drone, conventional in-situ observations) together with the application of state-of-the-art modeling approaches (tridimensional wave-current models, beach evolution predictor).
The study area for the project was selected on the basis of environmental factors and economical implications: southeastern Vietnam (Figure 1). Like in most developing countries, these two coastal zones are currently facing intensification of coastal vulnerability exacerbated by increasing demographic pressure. Three main criteria were considered in the choice of these regions:
(1) Contrasted environmental conditions governing the hydro-sedimentary functioning; erosion is year-persistent and driven by long swell (open ocean) whereas it is associated with extreme typhoon events in Vietnam, dominated the rest of the year by wind-waves (fetch-limited China Sea)
(2) Social and economical issues particularly acute along the 3400 km stretch of Vietnamese coastline. In addition, to the evident cultural and ecological dimensions that must be considered
(3) Feasibility and knowledge of the sites; the conduction of the project is highly facilitated by the presence of IRD and Vietnamese partners.
In Vietnam, the continental shelf is wide and the rugged coastline offers a sandy/mud mix environment. Wave climate is highly seasonal, varying from southerly to northerly swells. In average, 4 to 6 cyclones hit the coast every year and the resulting surge of up to a few meters causes submersion and important erosion that can reach 8 m/day (e.g., typhoon Haiyan in 2013; Lefebvre et al., 2014). The event-scale impact of typhoons and storm surges on the coastline is not compensated by wind-wave-induced alongshore drift of sediments (Imamura & To 1997; Lefebvre et al., 2014). The problem may even be enhanced by sea level rise, which would extend the destructive wave action further landward (Hanh & Furukawa 2007). The coastline of Vietnam could thus be one of the most severely impacted in East Asia by global warming (Nicholls et al. 1999). Although no accurate correlation can yet be made with climate changes, the number of typhoons reaching the coast of Vietnam increased in the period from 1960 to 2008 (SCBCIA 2009). COASTVAR will focus on the semi-closed Nha Trang Bay on the south coast of Vietnam, which has been identified by the Vietnamese government as a priority for the development of tourism.
Figure: Presentation of the study regions in COASTVAR: Nha Trang, Vietnam.
Another goal of the project is to simulate and analyze nearshore processes using numerical models. It will be challenging to reproduce the observed beach evolution under tropical environmental forcing (e.g., typhoons, continuous high-energy swell). This is extremely relevant to coastal management and to assess long-term sustainability of coastal developments. A series of state-of-the-art models will be used:
- XBEACH (part of the ECORS sequence): newly available phase-averaged, infragravity phase-resolving model (Roelvink et al., 2009).
- ROMS: an innovative modeling system using tridimensional wave-current interactions (McWilliams et al., 2004). It will be used to investigate the transient nearshore currents and key exchanges between the nearshore and the shelf circulation.
- WW3: an international community wave model. Regional configurations (China Sea and Tropical Atlantic Basin) will be conducted to propagate waves from the offshore deep-water generation areas to the shore. These configurations will be passed on to the partners for operational use in forecast mode.
Objectives, originality and novelty of the project
This exploratory project is an important step to not only improving our understanding of coastal dynamics in vulnerable and non-documented tropical environments, but more generally providing fundamental insight in the functioning of coastal zones. It will enable us to validate 3D wave-current models and 2D morphological models in challenging conditions. As such, COASTVAR has the following 3 key scientific objectives:
(1) Bridging the knowledge gap in event-scale beach evolution and more specifically on hydro-sedimentary mechanisms linking the upper beach and surf-zone dynamics. This includes the study of extreme events and crucial but mostly unknown beach recovery.
(2) Identifying the hydrodynamic processes involved in cross-shore exchanges between nearshore and shelf zones and the role of transient currents (i.e., resulting from alongshore or rip currents). There is still no consensus on the origin and effect of these currents, on their 2D or 3D dynamics and their relation to the stratification of shelf waters.
(3) Understanding the sources of coastal variability at various scales, in particular the link between short-term (event scale) and long-term evolution (e.g., following a sequence of storm). This includes determining the long-term impact of individual events (i.e., resilience?) and the coastal response to oceanic forcing with seasonal modulation of wave conditions (e.g., monsoon) or regional climatic modes.
Methodological objectives of the project
(1) Providing a high quality, multi-scale observational dataset of tropical nearshore environments, including a documentation of sea states, sediment fluxes and beach morphodynamics.
(2) Testing and developing innovative remote sensing observational tools: permanent video monitoring systems coupled with drone aerial survey during the experiments; Validation will be performed using conventional techniques,
(3) COASVAR will offer a unique dialogue space for two modeling communities, 2D and 3D approaches, through comparative tests and developments. The 2D approach of the nearshore modeling community includes transient 2D circulation effects on morphology and sandbars/upper beach interaction during extreme events. The 3D approach brought about by the shelf/offshore modeling community involves scale interactions and 3D wave-current coupling in stratified shelf environments.
LEGOS-IRD, Toulouse, France
SHOM, Brest, France
EPOC, Bordeaux, France
IUEM-LDO, Brest, France,
CEREGE, Aix en Provence, France
Main Vietnamese Collaborators
Water Resources University, Hanoi, Vietnam
Hanoi University of Sciences-VAST, Hanoi, Vietnam