ANR COASTVAR (2015>2017)
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).
Two areas were selected for the project on the basis of environmental factors and economical implications: southeastern Vietnam and West Africa (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 in West Africa 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 and in West Africa where the major cities are located near the ocean. In addition, the cultural and ecological dimensions must be considered, e.g., the dramatic erosion off Saint Louis/Barbarie sandspit in Sénégal and Ouida in Benin (UNESCO World Heritage Sites) at the rivermouth of the Niger and Mono, respectively;
(3) Feasibility and knowledge of the sites; the conduction of the project is highly facilitated by the presence of IRD researchers in Southern countries that maintain the dynamics of long-term collaboration with local partners and help with the logistic of the missions. Specifically, 2 studies were already conducted in 2012-2013, one in West Africa (INSU-IRD) and the other at Nha Trang, Vietnam (Vietnamese Ministry of Sciences and Technology).
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.
In West Africa, coastal zones represent 80% of the regional economic activity (UEMOA). These are open sandy coasts facing narrow and irregular continental shelves. Wave climate is characterized by energetic swell traveling from the South Atlantic and locally-generated short-crested waves (Degbe et al., 2010; Yao et al., 2010). The littoral drift is one of the largest in the world, comprised between 400,000 and 1,000,000 m3/year (Tastet et al., 1985; Laibi et al., 2014). The entire coast experiences a large erosion rate that reaches 10 m/year at Cotonou (Benin) (Dossou and Glehouenou-Dossou, 2007). Because most countries in West Africa are facing the same vulnerability to erosion, it has become a major regional issue (Appeaning Addo, 2009, 2011; Anthony, 2013). COASTVAR will develop an integrated regional approach in West Africa with 2 focal sites presenting the same erosion issue, being located on littoral sand barriers (Anthony and Blivi, 1999; Anthony et al., 1996, 2002), but with contrasted wave conditions from their exposition to the North or South Atlantic swell: Grand Popo in Benin (Laibi et al., 2014; Almar et al., 2014), and Saint Louis in Senegal (Anthony, 2013). The former site will be observed by remote sensing (video permanent monitoring), whereas in situ field measurements will be conducted at the second site. This choice of a regional approach is also motivated by our strong link with the recently created West African Coastal Observatory (regional coordinator is a collaborator of COASTVAR, M. Sall, Dakar).
One of the principal limitations to our knowledge of nearshore processes is a lack of appropriate observation, especially in tropical environments. Appropriate measurement of wave-related and morphological quantities in the nearshore zone is a difficult task. Traditional in-situ measurement techniques provide high sampling rates and a direct estimation of many parameters but can only provide local measurements and cannot fully describe the complex physical processes at work. A logical alternative is the deployment of large arrays of instruments, but this approach is inherently more expensive and can still only cover relatively small areas with limited spatial resolution. In Addition, instruments must be deployed in high-energy and sometimes hazardous environments (wave-breaking, strong currents), affecting the duty cycle and endangering not only the instruments but also the personnel involved. As an alternative, remote sensing techniques can provide synoptic coverage over large areas with a wide range of temporal and spatial resolutions (Holman et al., 2013). In this project, we will conduct a multi-scale and multi-method observation strategy based on both remote sensing techniques and more conventional in-situ deployments. In particular, two techniques will be applied, one of aerial drone at the event scale and the other of continuous, long-term coastal video monitoring.
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.
Contact: R. Almar, P. Marchesiello (LEGOS/IRD)
- Senechal, N., Laibi, R., Almar, R., Castelle, B., Degbe, G., DuPenhoat, Y., Chuchla, R., Honkonnou, N., 2014. Beach cusp dynamics on a reflective beach, Journal of Coastal Research, SI 70, 669-674, ISSN 0749-0208 pdf
- Lefebvre, J-P, Almar, R., Viet, N.T., Uu, D.V., Thuan, D.H., Binh, L.T, Ibaceta, R., Duc, N.V., 2014. Contribution of swash processes generated by low energy wind waves in the recovery of a beach impacted by extreme events: Nha Trang, Vietnam. Journal of Coastal Research, SI 70, 663-668, ISSN 0749-0208 pdf
- Laibi, R., Anthony, E., Almar, R., Castelle, B., Senechal, N., 2014. Morphodynamic characterisation of the human-impacted Bight of Benin sand barrier coast, West Africa, Journal of Coastal Research, SI 70, 079-083, ISSN 0749-0208 pdf
- Almar, R., Du Penhoat, Y., Honkonnou, N., Castelle, B., Laibi, R., Anthony, E., Senechal N., Degbe, G., Chuchla, R., Sohou, Z., Dorel, M., 2014. The Grand Popo experiment, Benin, Journal of Coastal Research, SI 70, 651-656, ISSN 0749-0208, pdf
- Castelle, B., du Penhoat, Y., Almar, R., Anthony, E., Lefebvre, JP., Laibi, R., Chuchla, R. dorel, M., Senechal, N., 2014. Flash rip dynamics on a high-energy low-tide-terraced beach (Grand Popo, Benin, West Africa), Journal of Coastal Research, SI 70, 633-638, ISSN 0749-0208 pdf
- Nguyen, T.V, Nguyen V.D., Vo C.H., Tanaka H., Dinh V.U, Tran T.T., Almar R., Lefebvre J-P. 2014. Investigation of Erosion Mechanics of Nha Trang Coast, Vietnam, 19th IAHR-APD 2014 Congress. 21-24 September 2014, Water Resources University, Hanoi, Vietnam
- Nguyen, T.V, Nguyen V.D., Le T.B., Duong H.T., Tran T.T., Nguyen V.T, Dinh V.U, Almar R., Lefebvre J-P. and Tanaka, H. 2014. Seasonal Evolution of Shoreline Changes in Nha Trang Beach, Vietnam. 19th IAHR-APD 2014 Congress. 21-24 September 2014, Water Resources University, Hanoi, Vietnam
- Almar, R., Bonneton, N., Bonneton, P., Lefebvre, JP, Dinh, V.U, Nguyen, T.V, Le, T.B, Nguyen, V.D. 2014. Swash hydro-morphodynamics at the low-tide terrace beach during post-typhoon recovery period, Nha Trang bay, Vietnam. 19th IAHR-APD 2014 Congress. 21-24 September 2014, Water Resources University, Hanoi, Vietnam
- Lefebvre, J-P., Almar, R., Viêt, N.T., Uu, D.V., Thuan, D.H., Binh, L.T., Ibaceta, R., 2013. Hydrosedimentary functioning of Nha Trang beach: new tools for assessing processes in the swash zone, WORKSHOP VAST/IRD Hai Phong 28-29 novembre 2013
- Degbe, C. G.E., Oyede, L.M,Almar, R. 2012. Analyse de la dynamique du trait de côte du littoral béninois. 4ème Conférence Internationale AMMA, 2-6 juillet 2012, Toulouse, France
- Ibaceta, R., Almar, R., Lefebvre, JP., Catalán, P., Cienfuegos, R., Blenkinsopp, C., Villagran, M., 2014. Estudio de la dinamica de swash altamente energetico en una playa disipativa (Mataquito, Chile) y reflectiva (Grand Popo, Benin), XXV Congreso Latinoamericano de Hidraulica, Santiago, Chile, Agosto 2014
- R. Ibaceta, R. Almar, J.P. Lefebvre, T.M. Senoo, W.S. Layrea, 2014. High frequency monitoring of swash hydromorphodynamics on a reflective beach (Grand Popo, Benin). International Conference on Coastal Engineering, Seoul, Corea, June 2014
- R. Almar, P. Catalán, R. Ibaceta, C. Blenkinsopp, R. Cienfuegos, M. Villagrán, J.C. Aguilera, B. Castelle. 2014. Comparison of swash zone measurements during energetic wave conditions at a dissipative beach. International Conference on Coastal Engineering, Seoul, Corea, June 2014
- R. Ibaceta, R. Almar, J-P. Lefebvre, T. Mensah-Senoo, W.S. Laryea, B. Castelle, N. Senechal, Y. du Penhoat, R. Laibi, N. Hounkonnou. 2014. A new high frequency remote sensing based method: application to the swash zone of a very high reflective beach under high energetic conditions (Grand Popo, Benin), Actes des XIIIèmes Journées Nationales Génie Côtier - Génie Civil, Dunkerque 2014, Paralia Editions. (pdf)
- Larnier, S., Almar, R., Cienfuegos, R., and Lejay, A., 2013, Détection de courants marins côtiers à partir de séquences vidéo, Société de Mathématiques Appliquées et Industrielles (SMAI), hal-00868401 (pdf)
- Binh, L.T., Duc, N.V., Viet, N.T, Thuan, D.H., Thin, N.V., Tung, T.T, Uu, D.V, Almar, R., Lefebvre, JP; 2013. Some preliminary results on studying the shoreline evolution of Nha Trang Bay using video-camera technique. Ocean Turbulence and its Applications 2013 Workshop (OTA-2013), Ho Chi Minh City, Vietnam