The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): Towards degraded delta resilience? Manon Besset, Edward J. Anthony, Philippe Dussouillez, Marc Goichot To cite this version: Manon Besset, Edward J. Anthony, Philippe Dussouillez, Marc Goichot. The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): Towards degraded delta resilience?. Comptes Rendus Géoscience, Elsevier Masson, 2017, 349 (6-7), pp.238-247. 10.1016/j.crte.2017.09.002. hal-01765609 HAL Id: hal-01765609 https://hal.archives-ouvertes.fr/hal-01765609 Submitted on 13 Apr 2018 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. C. R. Geoscience 349 (2017) 238–247 Contents lists available at ScienceDirect Comptes Rendus Geoscience w ww.sciencedirect.com External Geophysics, Climate and Environment The impact of Cyclone Nargis on the Ayeyarwady (Irrawaddy) River delta shoreline and nearshore zone (Myanmar): Towards degraded delta resilience? a, a a b Manon Besset *, Edward J. Anthony , Philippe Dussouillez , Marc Goichot a Aix-Marseille University, CNRS, IRD, CEREGE UM34, 13545 Aix-en-Provence, France b Lead, Water and Energy Security, WWF Greater Mekong, 18, Tu Xuong street, Ward 7, District 3, Ho Chi Minh City, Viet Nam A R T I C L E I N F O A B S T R A C T Article history: The Ayeyarwady River delta (Myanmar) is exposed to tropical cyclones, of which the most Received 11 May 2017 devastating has been cyclone Nargis (2–4 May 2008). We analysed waves, flooded area, Accepted after revision 22 September 2017 nearshore suspended sediments, and shoreline change from satellite images. Suspended sediment concentrations up to 40% above average during the cyclone may reflect fluvial mud supply following heavy rainfall and wave reworking of shoreface mud. Massive Handled by Isabelle Manighetti, recession of the high-water line resulted from backshore flooding by cyclone surge. The Rutger De Wit, Ste´phanie Duvail, shoreline showed a mean retreat of 47 m following Nargis. Erosion was stronger and Patrick Seyler afterwards (À148 m between August 2008 and April 2010), largely exceeding rates prior to Nargis (2000–2005: 2.14 m/year) and over 41 years (1974–2015: 0.62 m/year). This Keywords: À À implies that resilience was weak following cyclone impact. Consequently, the increasingly Ayeyarwady River delta Cyclone Nargis more populous Ayeyarwady delta, rendered more and more vulnerable by decreasing Delta shoreline erosion fluvial sediment supply, could, potentially, become more severely impacted by future Delta resilience high-energy events. D Brielta vulnerability C 2017 Acade´mie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/ 4.0/). 1. Introduction However, human impacts, coupled with the effects of climate change, are rendering many deltas economically River deltas are commonly highly productive environ- and environmentally vulnerable, resulting in weakening of ments that offer rich and biodiverse ecosystems and a wide their resilience (Brondizio et al., 2016; Ericson et al., 2006; range of ecosystem services, such as coastal defence, Syvitski et al., 2009). This vulnerability is increasing drinking water supply, recreation, green tourism, and because of reduced sediment flux from rivers and various nature conservation. Deltas are characterized, however, by other modifications caused by human interventions. A low topography, and are, thus, particularly vulnerable to better understanding of delta dynamics and vulnerability catastrophic river floods, tsunami, and cyclones. Deltas is needed in order to implement viable delta restoration may develop morpho-sedimentary resilience to these and rehabilitation strategies. high-energy events. This commonly occurs through re- Myanmar is the largest country in Southeast Asia, with 2 organization of delta morphology, notably channel pat- an area of 676,600 km , and the country relies heavily on terns, and changes in sedimentation (Anthony, 2016). the economic advantages provided by the Ayeyarwady (Irrawaddy) River basin and its delta (Fig. 1). As in most developing countries, population growth has been ex- tremely rapid, increasing the risk potential associated with * Corresponding author. E-mail address: [email protected] (M. Besset). river flooding, storm surge activity, and coastal erosion. http://dx.doi.org/10.1016/j.crte.2017.09.002 C 1631-0713/ 2017 Acade´mie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). M. Besset et al. / C. R. Geoscience 349 (2017) 238–247 239 Fig. 1. Shaded-relief map showing physical features of Myanmar and the Ayeyarwady River basin (a, b), and Google Earth image of the Ayeyarwady delta (c) with indications of alongshore kilometric points (from east to west) used to identify shoreline sectors. The inset in (c) shows tropical storms and cyclones over the Ayeyarwady River delta from 1870 to present. Data from International Best Track Archive for Climate Stewardship (IBTrACS), Knapp et al. (2010). About 15 million, out of Myanmar’s current population of historic times (Fritz et al., 2009), the other recorded events 51.4 million, reside in the Ayeyarwady delta, according to being mainly of categories 1 to 2 (Fig. 1c). These high- the provisional results of the 2014 census (official energy events may also cause storm surges that further estimates of nearly 60 million), and this represents an aggravate coastal flooding and damage (Dube et al., 2010). increase by nearly 150% in about 30 years (Brakenridge Prior to Nargis, the 10 May 1968 cyclone gives a measure of et al., 2017). The delta offers large areas of rich agricultural the deadly damage susceptible to be caused by cyclones in land and is one of the most productive rice-growing areas Myanmar (Brakenridge et al., 2017). This cyclone killed in the world. About 60% of the delta is currently under rice 1037 people, left 17,537 livestock dead, close to 300,000 cultivation (Frenken, 2012). people homeless, and 57,663 homes destroyed (USAID, Myanmar is, however, among the 15 nations that 1968). together account for 80% of the world’s population exposed Tropical cyclone Nargis caused the worst natural to river flood risk (Ward et al., 2013; Winsemius et al., disaster in the recorded history of Myanmar (Fritz et al., 2013). This risk is particularly pertinent to the vast 2009; Topich and Leitich, 2013; Wolf, 2009). According to Ayeyarwady delta (Brakenridge et al., 2017), the world’s these authors, fatality estimates exceeded 138,000, at least tenth largest delta (Coleman and Huh, 2004). The Bay of 2.4 million people were severely affected and over a Bengal is affected by tropical cyclones and Myanmar lies in million people were left homeless. An estimated 90–95% of the pathway of these high-energy events, over 25 of which the buildings in the delta, and much farmland, livestock, have affected the Ayeyarwady delta (Fig. 1c) since 1870 and fisheries were lost, and the overall economic damage (Knapp et al., 2010). Tropical cyclonic storms make landfall estimated at over 10 billion US$ (Fritz et al., 2009). These in Myanmar between May and October, and generate extreme casualties and damages resulted from a cyclone heavy rainfall and severe flooding (Brakenridge et al., surge that ranged from 1.9 to 5.6 m, and that led to flooding 2017). Tropical Cyclone Nargis (2–4 May 2008), a category- up to 50 km inland (Fritz et al., 2009). Using satellite 5 event just prior to landfall in Myanmar, is the latest and images, Brakenridge et al. (2017) mapped the vast land strongest meteorological event to have affected the delta in areas affected by the Nargis coastal storm surge and 240 M. Besset et al. / C. R. Geoscience 349 (2017) 238–247 unusual inland rainfall. Major flooding was restricted to monsoon winds become attenuated due to the southward the delta and lower floodplain areas. migration of the ITCZ. Maximum wave heights can reach In this paper, we address the issue of the effects of this 5 m during cyclones. Longshore currents generated by high-energy event on shoreline stability and on coastal and southwest monsoon winds and waves are, together with nearshore sedimentation. We then discuss these effects in tidal currents, important in transporting both bedload the context of the current vulnerability of the Ayeyarwady material (sand) along the beaches bordering the delta, and delta, increasingly subject to the impacts of human suspended sediment, which dominates the load of the activities in both the river catchment and delta, and Ayeyarwady. Longshore currents flow dominantly from considered as a delta ‘in peril’ by Syvitski et al. (2009). west to east in response to the dominant wave approach from the southwest. Reversals of these currents are locally 2. The Ayeyarwady River basin and delta observed, especially near the mouths of the larger delta distributaries where wave refraction commonly leads to The Ayeyarwady River (Fig. 1b) has the second largest bi-directional currents with beach sand transport materi- watershed in Southeast Asia, after the Mekong. The river alized by spits. covers slightly over 61% of the territory of Myanmar, and The Ayeyarwady delta is associated with relatively 2 its delta (Fig. 1c) has formed 20,570 km of low, fertile large semidiurnal tides ranging from 3 to 6 m.