Journal of Coastal Conservation: Planning and ...

Journal of Coastal Conservation: Planning and Management A GIS-based coastal monitoring and surveillance observatory on tropical islands exposed to climate change and extreme events: the example of Mayotte Island, Indian Ocean. --Manuscript Draft-Manuscript Number:

JCCO-D-13-00025R1

Full Title:

A GIS-based coastal monitoring and surveillance observatory on tropical islands exposed to climate change and extreme events: the example of Mayotte Island, Indian Ocean.

Article Type:

SI: Human-altered coastal systems

Keywords:

tropical islands; mangroves; coral reefs; beach morphodynamics; monitoring; observatory; GIS; Coastal Management; Mayotte Island; Indian Ocean.

Corresponding Author:

Matthieu Jeanson Dinard, FRANCE

Corresponding Author Secondary Information: Corresponding Author's Institution: Corresponding Author's Secondary Institution: First Author:

Matthieu Jeanson

First Author Secondary Information: Order of Authors:

Matthieu Jeanson Franck Dolique Edward J Anthony

Order of Authors Secondary Information: Abstract:

The global change currently observed is deemed to generate accelerated coastal erosion and an increase in frequency and intensity of extreme weather events. Populated tropical island coasts are particularly vulnerable. Awareness of this vulnerability has prompted recourse to the construction of operational observatories on the coastal dynamics of several French tropical islands, including Mayotte. The aims of this initiative are to characterise the coastal morphology of tropical islands and to monitor their rhythms and mechanisms of evolution, adaptation and resilience in the face of extreme climate and wave events (cyclones, storms, surges, strong swells…). Based on this, appropriate defence and/or adaptation strategies can be developed and implemented. Mayotte Island is a fine example of the implementation and utility of such an observatory. The island, in the southwest Indian Ocean, is characterised by a highly diversified coral reef-lagoon complex comprising pocket beaches and mangroves subject to increasing pressure from strong island demographic growth. The operational observatory set up on the island incorporates a Geographical Information System (GIS) based on a network sourced by various field measurements and observations conducted on the coastal forms on the basis of a predefined protocol and methodology. Field experiments include hydrodynamic measurements, topographic surveys, and observations, and these are coupled with the analysis of aerial photographs and regional meteorological data in order to gain a better understanding of the coastal morphology and of the evolution of the reef-lagoon complex. The results fed into the observatory and analysed through the GIS provide interactive maps of the coastal landforms and their evoluton and dynamics over various timescales. Within a local framework of strong socio-economic and demographic pressures, and a more global context of environmental change, this observatory should lead to a better understanding and prediction of the morphodynamics of the coast of Mayotte, while providing data to the public at large, to researchers, and to stakeholders involved in decision-making in the face of the major and rapid environmental and socio-economic Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation

changes liable to affect the fragile reef-mangrove systems and pocket beaches. Response to Reviewers:

REVIEWER #1 This is a worthwhile contribution to the volume. It has a presentation of a program to gather data appropriate to assist in the decision-making process. As I read through the statements, I find that the text could be structured a bit better to establish the specific as well as the general attributes of a monitoring network. As it is, pieces of information are incorporated throughout the manuscript as tidbits, but seemingly not part of an overriding structure. With an improved organization, the information could better expound the function of a monitoring program and the scientific principles that it uses and the application of its information. Scientifically, there are different reasons for monitoring different variables. There are different techniques and different scales. I am trying to understand the specific purpose of this manuscript. Is it a general piece of information that is meant for general public consumption, or is it a portrayal of the scientific basis for conducting a monitoring program? At present, it seems to fall between the two end products. Present and defend the program. Provide the basis for the decisions on variables, on sites, on data portrayal. Provide the rationale for the science as well as its value in management. At times it is an explanation of the monitoring program, and at other times it is a presentation of the data record. They probably need to be differentiated and handled separately. Response Both of the themes raised by the reviewer are addressed by the manuscript, but have now been clearly separated, making for a better-organised manuscript along the lines suggested by the reviewer. We have also indicated value-added aspects of monitoring in terms of management. The eye-opening remarks by the reviewer have been very helpful in this manuscript reorganisation. To do this, we have recast the manuscript in terms of the following sections: (1) introduction; (2) a description of Mayotte Island; (3) the rationale and goals of the coastal observatory; (4) observatory design and methodology, declined into three aspects: field sites and monitoring, aerial photographic analyses, and GIS data organisation and dissemination; (5) examples of generated datasets and their interpretation; (6) discussion, conclusions and perspectives. A new figure, Figure 4, depicts the structure, and organisation of the observatory. These sections correspond to the recommendations of the reviewer, and enable a clear separation of the various facets of the observatory: the reasons for its implementation, a brief history of the stages of implementation, the methods, timescales and expected output of field monitoring, the GIS data input, treatment and output, and examples of generated datasets, their interpretation and management implications. We are sure the reviewer will be pleased with this revision. The section labeled Methodology needs considerable upgrading. It includes a wandering dialogue of elements of the monitoring, but little about the methodology of site selection, of profile sites (are there single profiles, or a collection of profiles at some interval to create digital elevation models?). If single profiles, volume measures are not possible. Which variables are collected in the seasonal monitoring program?? Response We have considerably upgraded the Methodology section and clearly indicated aspects regarding site selection. We have corrected the volumes as m3/m of change within each profile. All the variables collected in the seasonal monitoring programme are indicated. Are beach/dune systems subject to aerial photo analysis or only the mangrove edge??? Response No, only the mangrove edge has been subject to aerial photo analysis. There are no significant dunes in Mayotte. What kind of instrumentation is used to collect wave and current measurements?

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Response The instruments used were actually indicated in the original submission but were missed out by the reviewer! If this is to be a description of the methodology, much more detailed information about what is being monitored, how it is being monitored, and the quality of the products should be presented, or certainly should be referenced. Response We have clearly presented what is being monitored and added a reference to published work on specialized hydrodynamic data. After the discussion of methodology, I expected to see some data sets derived from the variables that are mentioned throughout the manuscript up to this point. Instead, the first item is mangrove edge shift, not part of the methodology. Indeed, there is no organized discussion of the data that have been accumulated. This is the area that needs serious re-writing, harking back to the observation program and the measurements that have been accumulated. Fig 6 is a data set and represents a product of the monitoring. Good. There is also a location of a current meter (first mention of instrumentation related to hydro measurements. Response Very good point! The crux of the revision! These recommendations are followed to the letter as stated above. Are volumes relating to Figure 6 derived from the three profiles??? Response Yes, over the period indicated in Fig. 6c. Results should really present some of the data sets or the information being gathered. The interpretation and the application for management may follow. But, the paper needs to set up the scenario for the acquisition of data, the presentation of data, and the utilization of the data. At present, that trilogy is not followed. Response We have now clearly respected this scenario in the revision, and management aspects relating to the acquired datasets have been included. On page 7, there is discussion about changes to a profile, and the values are stated in volumes. Profiles are area measurements. And, as the figures display, there is a gradient of profile change from the ends to the middle. BE careful in presenting and describing the data sets. The short term net change is a loss, what does that portend for the beach in this area? Isn’t that the message from the data set?? Where does management go with these data??? Response We have now specified that the volume changes concern profiles expressed in m3/m. The implications of this loss are now set in the longer-term morphodynamics of the beach, and cast in terms of a management perspective. Are there protocols for data collection, for data processing, for data portrayal in the observation program??? Response These protocols are fully explained in the revised text (see section (4) Observatory design and methodology, and especially subsection GIS data organisation and dissemination. Figure 4, a new figure, proposes a chart depicting the structure, and organisation of the observatory. Figure 8 is a very good example of the output of a data gathering program. Perhaps, it could be the focus of the paper. That is, there are products associated with the variables that are being collected. Each of the components of the program is designed to create a data set and an application of that data set, and perhaps a publication as Powered by Editorial Manager® and ProduXion Manager® from Aries Systems Corporation

well as a web site entry. This figure seems to be the culmination of the observational program (I still do not understand the role of these authors relative to the observation program). Response The direct role of the authors in the inception and implementation of the programme has been specified. Reviewer #1 also suggested a number of minor text corrections that have been carried out. REVIEWER #2 Reviewer #2 raised a number of minor points of improvement in the pdf that have been attended to. These included tempering the use of the term ‘extreme events’, excessive use of the name Mayotte, a clearer presentation of the structure and organization of the observatory (now provided in new Fig. 5 - see also response to reviewer #1), deletion of the figure on hydrodynamic datasets (this has been done) and a reference to initiatives on coastal vulnerability monitoring within the framework of the SOPAC programme in the South Pacific islands.

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Title Page (author details)_rev

A GIS-based coastal monitoring and surveillance observatory on tropical islands exposed to climate change and extreme events: the example of Mayotte Island, Indian Ocean. Matthieu Jeanson1, Franck Dolique2, Edward J. Anthony3. 1

Laboratoire de Géomorphologie et Environnement Littoral, Ecole Pratique des Hautes Etudes - UMR 8586 Prodig, 15 Boulevard de la Mer, 35800 Dinard, France. 2

Université Antilles-Guyane, Campus de Martinique, BP 7207, 97275 Schoelcher Cedex, France.

3

Aix Marseille Université, Institut Universitaire de France, CEREGE, UM 34, Europôle Méditerranéen de l’Arbois, B.P. 80, 13545 Aix en Provence Cedex, France.

*Manuscript (without authors' details) Click here to view linked References

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A GIS-based coastal monitoring and surveillance observatory on tropical islands exposed to climate change and extreme events: the example of Mayotte Island, Indian Ocean.

Abstract The global change currently observed is deemed to generate accelerated coastal erosion and an increase in frequency and intensity of extreme weather events. Populated tropical island coasts are particularly vulnerable. Awareness of this vulnerability has prompted recourse to the construction of operational observatories on the coastal dynamics of several French tropical islands, including Mayotte. The aims of this initiative are to characterise the coastal morphology of tropical islands and to monitor their rhythms and mechanisms of evolution, adaptation and resilience in the face of extreme climate and wave events (cyclones, storms, surges, strong swells…). Based on this, appropriate defence and/or adaptation strategies can be developed and implemented. Mayotte Island is a fine example of the implementation and utility of such an observatory. The island, in the southwest Indian Ocean, is characterised by a highly diversified coral reef-lagoon complex comprising pocket beaches and mangroves subject to increasing pressure from strong island demographic growth. The operational observatory set up on the island incorporates a Geographical Information System (GIS) based on a network sourced by various field measurements and observations conducted on the coastal forms on the basis of a predefined protocol and methodology. Field experiments include hydrodynamic measurements, topographic surveys, and observations, and these are coupled with the analysis of aerial photographs and regional meteorological data in order to gain a better understanding of the coastal morphology and of the evolution of the reef-lagoon complex. The results fed into the observatory and analysed through the GIS provide interactive maps of the coastal landforms and their evoluton and dynamics over various timescales. Within a local framework of strong socio-economic and demographic pressures, and a more global context of environmental change, this observatory should lead to a better understanding and prediction of the morphodynamics of the coast of Mayotte, while providing data to the public at large, to researchers, and to stakeholders involved in decision-making in the face of the major and rapid environmental and socio-economic changes liable to affect the fragile reef-mangrove systems and pocket beaches. Keywords: tropical islands, mangroves, coral reefs, beach morphodynamics, monitoring, observatory, GIS, coastal management, Mayotte Island, Indian Ocean. Introduction The coastal zone is a theatre of interest to a multitude of actors and stakeholders. The concept of ‘integrated coastal zone management’ expresses a need for collective action on natural and societal processes that are susceptible to threaten the sustainability of environmental quality and coastal activities (Cicin-Sain and Knecht, 1998). A necessary preamble to the efficient management of the coastal zone, however, is a sound knowledge of the way this zone functions and evolves in time. Significant increase in anthropogenic pressure, limited space, and vulnerability to climate change, sea-level rise, and high-energy events, are a number of 1

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constraints that are specific to islands, especially in the tropics (Pelling and Uitto, 2001; Mimura et al., 2007). Accelerated sea-level rise as well as modification of numerous physical and biogeochemical processes such as the acidification of oceans, the increase in ocean surface temperatures, changes in oceanic circulation, in wave climates and in salinity are now well documented (Church et al., 2006; Solomon et al., 2007; Doney et al., 2009; Cazenave and Llovel, 2010; Mori et al., 2010; Church et White, 2011). These changes will have important impacts on tropical coastal ecosystems, especially coral reefs and mangroves (Hughes et al., 2003; McLeod and Salm, 2006; Hoegh-Gulberg et al., 2007; Gilman et al., 2008; Wilkinson, 2008; Hoegh-Gulberg and Bruno, 2010), but also on adjacent or related coastal morphosedimentary systems lying at elevations close to sea level such as lagoons, beaches and barriers. The management of tropical coasts will need to integrate over short to medium timescales the evolution of dynamic parameters characterized by random phases of respite and erosion that will become accentuated in the future in response to climate change (Woodworth et al., 2004; Sheppard et al., 2005; Titus, 2005; Nicholls et al., 2007; Storlazzi et al., 2011). Tropical island coasts can be particularly vulnerable to climate change in the future (Hay, 2013; Forbes et al., 2013), although marked differences will be expected between those with high rocky coasts and those, more vulnerable, with low depositional coasts. It is therefore necessary, for decision-makers, managers, and the scientific community, to monitor and quantify the reactions of coasts to the consequencs of climate change. The understanding gained on the functional mechanisms and resilience of coastal systems constitutes a key contributory element in attempts to identify vulnerability levels in order to implement improved coastal management and risk evaluation plans (UNESCO, 2003; Boruff et al., 2005; Vinchon et al., 2009; Romieu et al., 2010). Awareness of these challenges has prompted the construction of an operational observatory on the coastal dynamics of several French tropical islands. The rationale for the setting up of one of these observatories, that of Mayotte, in the Indian Ocean (Fig. 1), and the aims, structure and organisation of this observatory are presented, together with examples of results on coastal monitoring that provide a better framework for future coastal conservation and management. Mayotte acquired the status of a French Department, identical to that of metropolitan departments, in 2011. Situated at approximately 13° S and 45° E about 300 km northwest of Madagascar in the north of the Mozambique Channel and about 450 km away from the African continent, Mayotte forms, together with the islands of Anjouan, Mohéli and Comore, the Comoros Archipelago (Fig. 1a). Mayotte Island Geomorphology and coastal landforms Mayotte has an area of 374 km², and is composed of two main volcanic islands, Grande Terre and Petite Terre, culminating at a peak elevation of 660 m at Mount Bénara. The territory also comprises about 30 islets of volcanic or coral reef origin spread out in a lagoon behind a ring of remarkable coral reef barriers, comprising an almost continuous barrier with a circumference of 157 km, and 40 km of submerged reefs, giving a total reef ring with a circumference of 197 km. Mayotte lagoon is 3-15 km wide, up to 80 m deep, and, with an area of nearly 1500 km², is one of the largest reef lagoons in the Indian Ocean. The volcanic 2

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and tropical context, and the vast reef-lagoon system have resulted in remarkable coastal geomorphic diversity. The total shoreline length of Mayotte is 265 km, and is an intricate alternation of cliffs separating variably indented pocket beaches of sand and sandy mud, the sheltered low-energy backshores of many of which are colonised by mangroves (Fig. 1b). Climate and oceanography Mayotte experiences a humid tropical climate comprising two seasons, a hot rainy monsoon season, and a cooler, dry trade wind season. Mean annual rainfall is about 1500 mm. Pressure gradients between the Intertropical Convergence Zone and anticyclones in the Indian Ocean generate, respectively, hot and humid monsoon winds from a north to northwest sector during the southern hemisphere summer (November to April) and cool, dry trade winds from south to southeast during the southern hemisphere winter (May to October). The latter form the dominant winds (Fig. 1c). These wind conditions are directly reflected in the hydrodynamic regime of the reef-lagoon system, which is characterised by marked contrasts in the seasonal wave climate. Waves are from the north during the southern hemisphere summer and from the south during winter. Tides on the island are semi-diurnal and the tidal range mesotidal, with a mean spring range of about 3.2 m. Mayotte also lies on the track of tropical storms and cyclones (Fig. 2). These occur exclusively during the rainy season, and essentially at the start and end of the season. The island is, however, relatively sheltered by Madagascar. The cyclone return period is about 1520 years (mean wind speeds exceeding 117 km/h). Notwithstanding, the cyclone risk is clearly present as attested by the devastating historical events of 1898, 1934, 1953, and that of 1961, to date the most powerful to have hit the island. The last major events, Cyclone Kamisy, and Tropical Storm Feliska, occurred successively in 1984 and 1985. Winds associated with Kamisy attained 150 km/h in Pamandzi and an estimated 200 km/h in Sada, 80% of which was destroyed. There are hardly any records of the impacts of these extreme events on the coast of Mayotte and efforts drawing attention to the risks of marine flooding caused by cyclones are very recent (Audru et al., 2010). Environmental and socio-economic stakes The socio-economic context of Mayotte over the last two decades has been one of accelerated development and strong demographic growth (Fig. 3a). The island’s population in 2012 was 212,600, yielding a density of 568 inhabitants/km², i.e., over five times that of metropolitan France. This represents a significant increase compared to the 1958 figure of 23,000 inhabitants (INSEE, 2012). The strong population growth is due to a high birth rate (41.2‰) and sustained immigration from the other, much poorer, islands of the Comoros Archipelago. The population forecast for 2017 ranges from 260,000 to 320,000. Caught up in this context of strong demographic growth, the coastal zone has been subject to increasing anthropogenic pressures and ancillary economic and tourism development (Fig. 3b-c). These pressures bear on the quality of the coastal landforms of Mayotte. The coral reef system exhibits important signs of degradation and is considered today as one of the most endangered in the Indian Ocean (Ahamada et al., 2008). The effects of anthropogenic pressure exacerbate those of climate change and impair the resilience of these coral systems in the face of both natural and 3

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human perturbations (Wickel and Thomassin, 2005; Thomassin et al., 2011; Nicet et al., 2012). Among these, pressures from fishing and sediment input from terrestrial catchments are particularly important in Mayotte. Similarly, observations conducted since the end of the 1990s on the evolution of the mangroves associated with the island reef system show retreat in surface area associated with coastal erosion (Lebigre, 1997; Laulan et al., 2006). Rationale and goals of the coastal observatory In the early 2000s, calls started coming from the local administrative authorities and environmental conservation entities for an integration of development planning within a viable and sustainable economic strategy. The problems raised by the degradation of the coastal zone and the reef-lagoon system of the island were increasingly bringing pressure to bear on local and national authorities regarding the necessity of setting up and implementing a sound policy of integrated and sustainable coastal management. The successful implementation of projects aimed at environmental conservation needed to be supported by a fine understanding of the functional dynamics of the coast, which imply interactivity between ‘natural’ (morphosedimentary processes) and anthropogenic factors (human activities and land use in the coastal zone). Under these conditions, the establishment of an operational network, in the form of a coastal observatory, for monitoring the coastal zone, became necessary in order to reinforce observational and analytical capacities. The idea of setting up of this observatory was initiated by the Bureau de Recherches Géologiques et Minières (BRGM for French Geological Survey) in partnership with environmental services run by the French government and by the local island authorities. The roadmap of this venture recommended a preliminary characterisation of the island’s coastal geomorphology, enabling the identification of both eroding and vulnerable sites. Between 2006 and 2009, detailed morphodynamic studies of representative beach and mangrove sites were conducted (Jeanson, 2009), with the underlying aim of setting up this observation network (Dolique et al., 2007). These efforts debouched in the implementation in 2008, in collaboration with the Institut de Recherches pour le Développement (IRD for French Overseas Institute for Development), of a Geographical Information System named MANGUIERS (MANGroves mahoraises, sUrveillance par Instrumentation Et Recherches Sédimentaires) funded by the French Ministry of Overseas Affairs. Following this, plans are now underway for devolving the running of the observatory to the Mayotte Marine Park. This park was created by the French government in January 2010. Closed linked with the idea of an observatory, the notions of management and controlled development of the coastal zone were central themes in the establishment of a Management and Sustainable Development Plan (MSDP) for the island in 2009 and in the creation of the Mayotte Marine Park, both of which define the main orientations and the management policy of the island and its reef-lagoon system for the decades to come. The rationale for these initiatives became further reinforced as Mayotte voted massively in 2011 to become part of France. As an entity in the European Union, this overseas island territory is subject to EU directives regarding environmental conservation. The goals of the observatory are: (1) to enhance our understanding of the functioning of the coast and its resilience relative to high4

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energy events, and (2) to translate the acquired information into a framework liable to inform the general public, researchers, and decision-makers, with a view towards the implementation of decisions aimed at rational coastal management and conservation. The structure and functional framework of the coastal surveillance network are summarised in Fig. 4. This observatory comprises three major tasks: (1) the constitution of a data bank on the state of the Mayotte coast, (2) the observation, measurement and characterisation of vulnerable coastal landforms and their changes at various temporal and spatial scales based on the acquisition of field data, (3) the analysis and integration of the generated data into userfriendly tools accessible to decision-makers, the general public and scientists. Observatory design and methodology The capacity of recovery of tropical coasts and the processes involved in their reversion to their initial state (resilience) are still rather poorly known and increasingly require observations, monitoring and measurements from which may eventually be derived conceptual and deterministic models of behaviour that can be fed into management schemes. An observatory approach requires determination of threshold levels, sensitivity to adaptation, and the processes and rhythms of resilience of diverse coastal environments (mangroves, sandy beaches, coral reefs). The protocol for achieving these objectives was therefore based on the observatory programme initiated in 2006, and covering various aspects such as monitoring waves and currents, short-term beachface stability and longer-term cycles of erosion, and mangrove surface area variations. It was expected that such monitoring should throw light, for instance, on the recovery potential of the sediment budget of a beach or characterise mangrove recovery following storm damage. Such elements should contribute to better anticipation and/or definition of response strategies and to the evaluation of the financial costs facing the local stakeholder communities in their efforts at ensuring better protection and management of urban growth zones, tourist infrastructure, and halieutic ressources, in the context of strong demographic growth. Beyond fieldwork and data collection, a fundamental element of this surveillance network resided in the constitution of a data bank on the coast. This has necessitated the implementation of a GIS enabling the diffusion and exploitation of data collected by the observatory. The field monitoring techniques, longer-term aerial photographic interpretation method and the GIS data organisation, storage and diffusion are briefly reported here. Field sites and monitoring The choice of field sites monitored within the framework of the observatory has concerned all ‘vulnerable’ coastal morphotypes representive of the diversity of the island: sandy beaches, mangroves, cliffs and strongly human-modified shores. Resistant rocky shores subject to hardly any change at the scale of a life time, and thus not considered directly vulnerable at this temporal scale, were not included in the monitoring phase, although these shores were mapped. The implementation of the observation network in 2006 started with 10 sensitive sites (Fig. 5). A total of 28 representative sites, including the initial 10 sensitive sites, were subsequently selected on the basis of specific characteristics such as exposure to waves, present stability/instability status (erosion or accretion), sedimentological composition, and environmental, patrimonial and economic value (Jeanson, 2009). Beaches of high tourism 5

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potential, especially those listed as major development sites in the framework of the MSDP, and mangroves, have thus been priority sites. Mangroves play a crucial role in favouring the deposition of suspended sediments brought down to the sea by streams, thus filtering and limiting the deposition of fine sediments in the lagoon and over reefs. Mangrove dynamics and evolution thus have a direct bearing on the quality and quantity of local fish resources. A total of 62 cross-shore profiles at the selected sites have been monitored since 2008, twice a year, in February and September, months representing respectively the trade wind and monsoon seasons. Profile variations are calculated for each survey interval, starting with a baseline provided by the initial survey, thus enabling appreciation of seasonal morphological changes. Each profile was initially benchmarked, using high-resolution GPS, with reference to the local island elevation system (IGN 1950) set up by the French Institut Géographique National (IGN) (Fig. 6a). An additional benchmark was established further inland for each profile as a precautionary measure in the event of the destruction or disappearance of the initial benchmark resulting from a severe storm or cyclone. The profile surveys were conducted (Fig. 6b) using a total elecronic station (LEICA TC 407). The measurement error margin has been estimated from field tests at ± 2 cm for sandy beaches and coral reefs, and at ± 5 cm in the less firm substrates associated with mangroves and muddy environments. In some sites, additional denser topographic evaluations of the intertidal beach have been carried out to capture spatial trends in morphology and change over time through the generation of digital elevation models (DEMs). In order to highlight the potential shoreline impact of seasonal variations in offshore wave climate and the modulation of wave energy between the outer barrier reef and the pocket beaches, data have been collected on the Indian Ocean wave regime in the vicinity of Mayotte, and energy dissipation analyses over the outer barrier reef and across the lagoon to the pocket beaches carried out based on brief (three days to two weeks) field campaigns of hydrodynamic measurements (wave and current parameters) using instruments deployed in a number of selected sites under both fair-weather and storm conditions (Fig. 6c). Several types of current meters (ADCP/RDInstruments, ADV/Nortek, Midas DWR/Valeport) were deployed, and the configuration of each experiment discussed by Jeanson et al. (2013). The data recorded in the course of these experiments: (1) feed a database on wave and current characteristics such as significant wave heights, breaker heights, periods, energy spectra, and provenance, and cross-shore and longshore currents; (2) provide a template of fair-weather conditions to which data acquired during rough weather conditions and exceptional events may be compared; (3) have been used to define the long-term stability/instability status and morphodynamics of the numerous pocket beaches of the island (Jeanson et al., 2013). Aerial photographic analyses In addition to the topographic surveys, longer-term changes in shoreline position were also analysed from IGN aerial photographs (Fig. 6d) from 1949 to 2008 specifically for 28 mangrove sites. The data were provided in digital format by the Environment, Agriculture and Forestry Service. The analysis of changes in shoreline positions from vertical aerial photographs involves various sources of errors such as scale variations, identification of 6

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common reference points, digital computation and determination of the shoreline itself (Moore, 2000). However, unlike sandy coasts where shoreline position may not be easily identified, the seaward limits of mangroves are commonly quite easily identified. Their landward limits are more difficult to identify from aerial photographs, necessitating recourse to field verifications. Differences in quality and resolution of the various series of aerial photographs were a further source of difficulty in shoreline delimitation. The estimated error margin associated with the determination of shoreline position from aerial photographs is ± 6 m. This margin is the sum of errors in the precision of the orthophotographs (± 0.5 m), the identification of landmarks on the aerial photographs (± 1.5 m), the identification of invariable elements between different series of photographs (± 3 m), and the residual error calculated for the whole set of landmarks (± 1 m). The photographs were analysed using the image analysis modules of Er-Mapper® and the spatial analysis modules of MapInfo®. GIS data organisation and dissemination The GIS is articulated around two cornerstones: Scan25 (1999) and the Ortho databank (IGN 2003) of the French mapping agency, the Institut Geographique National. Further embedded in these two referentials are thematic layers incorporating various items of collected external information such as island relief and shoreline contour, and various characteristics of the reeflagoon environment such as the distribution of reefs and seagrass colonies. Data acquired from field monitoring and crossed with other data sources, such as the regional wave climate, are converted into geographic information and incorporated into various tables such as point tables (e.g., benchmark locations), linear tables (e.g., topographic profiles) or polygons (e.g., mangrove area). The data and tables were all standardised and referenced in the local RGM04 geodetic projection (Réseau Géodésique de Mayotte 2004) using MapInfo®. Access to data and data diffusion stand out as two essential elements in the eventual success of the Mayotte coastal observatory regarding good management practice. The observatory network needed to build up a database that should contribute to a better understanding of the Mayotte coast and its evolution. This was accomplished through the construction of the GIS ‘Manguiers’. The network also addresses the needs of local, regional and national stakeholders involved in coastal planning and management. The database should subsequently improve coastal hazard mapping and contribute as a support to decisions on protection and adaptation necessary to sound coastal management planning within a framework of sustainable development. The GIS ‘Manguiers’ collects, archives, standardises, and diffuses data generated by the surveillance network. It fulfills several objectives, including: - collation of existing information on geographic and environmental aspects of the coast; - pooling of efforts aimed at data acquisition and development of protocols of data collection and treatment; - making available to researchers, technicians and decision-makers knowledge acquired on the functioning of the coastal environment;

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development of tools aimed at visualising and anticipating present and future coastal evolution trends and providing aid in decisions taken by coastal managers.

The results and field observations collected by the surveillance network are integrated into the GIS as downloadable files. The GIS is available to local management organs such as the Directorate of the Environment and of Coastal Management (DEAL), the General Council (administrative organ of Mayotte Department) and the Marine Park Authority of Mayotte, and is updatable as new data are acquired. Finally, data diffusion by the GIS and the observatory contributes to knowledge acquisition and sharing with existing observation networks involved in coastal matters, such as the Coral Reefs Observatory and the Marine Turtles Observatory. Examples of generated datasets and their interpretation Two examples of coastal datasets generated within the framework of the activities of the observatory, and their interpretation in terms of processes involved in change, are briefly discussed here. Changes in mangroves: long-term trends and morphodynamic processes Analysis of the aerial photographs highlights significant variations in mangrove evolution trends between 1949 and 2008. Of the 23 monitored sites, only one has been stable, whereas 11 underwent an increase in area and 11 others a decrease. The mangroves showed an overall area loss of 6 % over the 59-year period of comparison. The mild loss in mangrove area over the 59-year period of comparison needs to be viewed against the important demographic upsurge of the island over about the same period, which suggests a rather limited impact of human activities on mangrove retreat. The collected data showed, however, marked differences between advancing northern and eastern shores of the island and retreating southern and western shores, these differences involving complex morphodynamic interactions (Jeanson, 2009), as well differences in anthropogenic pressures between these two sectors. Cross-shore profiles and hydrodynamic surveys show that progressive mangrove retreat due to downcutting promotes erosion of the muddy substrate over which waves rework and concentrate sand into well defined bars. These sand bars progressively migrate shorward as swash bars that are built up into beach ridges behind the subsisting mangrove fringe. Continuous beach ridge accretion leads to burial and asphyxia of mangrove root systems, leading to mangrove mortality. At the same time, the decrease in the width of the mangrove fringe enhances wave energy transmission across this fringe. The ridges are built up by swash processes but are also subject to active overwash processes that lead to landward ridge migration. The resulting deposits are morphologically similar to the classic cheniers identified in other tropical muddy mangrove environments such as those of the coasts between the Amazon and the Orinoco in South America (Augustinus et al, 1989; Anthony et al., 2010), in West Africa between Sierra Leone and Guinea (Anthony, 1989), and in the Gulf of Carpentaria in Australia (Woodroffe and Grime, 1999). The Mayotte system differs in that the mangrove areas being relatively small, reorganisation of the mud and sand fractions ends up in irreversible dispersal of the latter and eventual mangrove elimination to the benefit of developing cheniers. By influencing the 8

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dissipation of incident wave energy, the distribution and architecture of subsisting mangrove communities further play an active role in modulating geomorphic and grain-size sorting processes. Cheniers located behind a subsisting mangrove fringe, for instance, are subject to lower incident wave energy than those where the mangrove fringe has been eliminated. This difference further induces variations in grain-size sorting of the cheniers. Knowledge of the way these coastal changes are triggerred by mangrove downcutting should inform coastal managers, via the observatory, on the necessity of maintaining a wide, energy-dissipating mangrove cover. Pocket beach morphodynamics and their link with the regional wave climate The setting up of the monitoring network has also enabled the characterisation of the morphodynamics of several beach sites (Jeanson et al., 2013). Figure 7 shows, for instance, the results from Mtsanga Gouela beach on the southwest coast of the Island (Fig. 5). This 350 m-long and 60 to 80 m-wide pocket beach is almost entirely submerged at high water during spring tides. The beach is bound at its extremities by two small volcanic headlands and is linked lagoonward to a subhorizontal 300 m-wide fringing reef platform (Fig. 7a). The topographic profiles surveyed on this beach between February 2005 and February 2008 (Fig 7b) show seasonal longshore morphological variations that revolve around mobilisation of the sand mass from one end of the beach to the other. Profile P100 showed accretion during the southern hemisphere summer and erosion during the winter whereas profile P300 exhibited inverse behaviour, eroding in summer and accreting in winter. As a result, the monitored profiles show a succession of well-defined morphotypes that ranged from a convex to a concave beach morphology depending on accretion and erosion phases. Elevation differences between two successive (seasonal) surveys attain up to 1.5 m for profile P100 and over 1.6 m for profile P300, and can involve the total relocation of the sand mass in the profile. The calculated profile variations in m3/m of beach can thus be very large (Fig. 7c). Between February 2007 and June 2007, for instance, profile P100 experienced a net loss of 51.7 m 3/m of sand, whereas between June 2007 and February 2008, the same profile registered a net sand gain of 54.4 m3/m. Profile P300 showed a gain of 40.7 m3/m over the period February-June 2007 and a loss of 57.5 m3/m between June 2007 and February 2008. The central profile P200, a typical transit sector between the two extremities of the beach, showed much more subdued changes, of the order of 0.7 m for elevation and a maximum of 15 m3/m for volume. The hydrodynamic measurements carried out on Mtsanga Gouela beach clearly suggest a link between this seasonal ‘rotation’ process and the seasonal wave regime (Jeanson et al., 2013). During the southern hemisphere winter, trade winds from the south generate waves that impinge on this beach. The oblique wave incidence generates longshore drift from south to north, resulting in erosion of profile P100 and accretion of P300. In contrast, during the summer months, monsoon winds from the north generate north-northwesterly impinging waves that generate longshore drift from north (P300 in erosion) to south (P100 in accretion). Overall, the net loss exhibited by the profiles does not, therefore, portend long-term erosion but simply reflects a short-term situation within the framework of beach rotation. Analysis of aerial photographs of this beach over the last 50 years shows net stability. Understanding the underlying morphodynamic framework of rotation informs, however, on the quasi-seasonal to 9

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annual mode of evolution of this beach. It also informs on the relative stability of the beach sediment budget and the necessity of proscribing activities, such as illicit sand extractions, that can adversely affect this budget in the future. This seasonal beach rotation pattern has been observed on various other beaches, including Ngouja, Sada, Trevani, and the beaches fringing the cliffs of Pamandzi (Fig. 5). This trend is, however, absent in many other beaches dominated by cross-shore transfers, and where longshore processes are subdued by seasonal sheltering or swash-alignment to waves, especially where strongly protruding headlands limit directional variation in wave approach. Examples include Mzouazia, Mronabéja, Mbouini, and Acoua beaches (Fig. 5). In some of the more mobile of these swash-aligned pocket beaches, the sediment dynamics involve crossshore bar migration between the low-tide reef platform and the upper beach. Examples include Dapani, Bandrele, Tsingoni, and Mounyambani beaches (Fig. 5). Where mangroves are present on some of these barred beaches, their destabilisation by exceptional wave energy events or by human interventions eventually favours the development of cheniers as described earlier. Discussion, conclusions and perspectives The main objective of the Mayotte coastal observatory is to provide a decision-support tool in the management of the coast of the island. This is a particularly crucial point in this newly created French overseas island department presently subject to strong development pressures and therefore in need, more than ever before, of solution approaches of this type. The results thus far obtained by the surveillance network throw light on the way hydrodynamic processes on the coast interact with the sediments and mangroves to generate morphological change. These results are then integrated into a readily accessible GIS as downloadable files. Figure 8 shows, for instance, screen shots of the diversity of datasets acquired via various techniques, and interactive maps generated by the analysis of mangrove area evolution since 1949 from aerial photographs. These results also highlight the complex time and space-varying morphodynamic interactions involved in these changes in a reef environment. The interest elicited among political and management actors by the research work undertaken on the beaches and mangroves of the island within the framework of this observatory suggests that the tools developed by the network will be useful to the management and conservation of the coast. The surveillance network should favour interaction between scientists and civil actors by improving information diffusion and communication towards local coastal managers through the organisation of technical seminars and through the GIS, including the provision of a regularly updated website and interactive maps. The observatory will also propose actions aimed at informing and educating the public, and in enhancing public awareness of the stakes involved in sustainable coastal management. Although recommendations to stake holders are not currently dispensed, this is an important short-term goal of the observatory. Articles in journals and magazines and the organisation of conferences are also initiatives that are already underway. It is imperative that the public is fully involved in the initiatives of the observatory through partnerships such as with local associations and through participative grassroots management ventures as well as through local governance of the coast. Participative actions and awareness initiatives regarding conservation of the environmental legacy constitute, in 10

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this regard, a necessary preamble to efficient operational mobilisation, especially in island contexts, where actors are likely to be more durably engaged in territorial projects (David et al., 2010; Colado et al., 2011). In time, the scientific data acquired by the observatory should contribute to a better understanding of the effects of climate change on sensitive tropical coastal and insular environments, especially regarding potential wind and wave regime modifications, and sealevel rise. Similar observatories are also being set up since 2003 on several French tropical islands based on a the research programme with the acronym of ALERT (Dolique et al., 2007): Tahiti and Moorea (French Polynesia), Martinique, Guadeloupe, Saint-Martin and Saint-Barthélemy (Lesser Antilles), as well as in the Scattered Islands (Mozambique Channel). This initiative is similar, in its coastal vulnerability assessment dimension, to that being implemented by the Pacific Islands Applied Geoscience Commission (SOPAC), an inter-governmental organisation dedicated to providing services to promote sustainable development in the member countries, notably South Pacific islands. Although the preliminary results of the Mayotte observatory are promising, the structure needs to prove that it can be immediately operational in the case of an extreme event. One of the main objectives of the project, that of characterising morphosedimentary resilience following an extreme event still needs to be implemented. The level of natural resilience of the coastal morphotypes on the island needs to be characterised in order to determine the reconstruction potential of the sediment stock following a major cyclone, for instance.

Acknowledgements The authors acknowledge aid from ULCO, BRGM, the French Ministry of Overseas Territories, IRD of Reunion and French Guiana, and local services in Mayotte involved in the management of the coastal and lagoon systems, especially the DEAL. Thorough reviews by Norb Psuty, Samuel Etienne and an anonymous reviewer are acknowledged. References Ahamada S, Bijoux J, Cauvin B, Hagan A, Harris A, Koonjul M, Meunier S, Quod J.-P (2008) Status of the Coral Reefs of the South West Indian Ocean Island States. In: Wilkinson C (ed) Status of coral reefs of the world: 2008. Global Coral Reef Monitoring Network and Reef and Rainforest Research Center, Townsville, Australia, pp 105-118 Anthony EJ (1989) Chenier plain development in northern Sierra Leone, West Africa. Mar Geol 90:297-309 Anthony EJ, Gardel A, Gratiot N, Proisy C, Allison MA, Dolique F, Fromard F (2010) The Amazon-influenced muddy coast of South America: A review of mud bank-shoreline interactions. Earth-Sci Rev 103:99-129. Audru J-C, Bitri A, Desprats J-F, Dominique P, Eucher G, Hachim S, Jossot O, Mathon C, Nédellec J-L, Sabourault P, Sedan O, Stollsteiner P, Terrier-Sedan M (2010) Major

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natural hazards in a tropical volcanic island: A review for Mayotte Island, Comoros archipelago, Indian Ocean. Eng Geol 114:364-381 Augustinus PGEF, Hazelhoff L, Kroon A (1989) The chenier coast of Suriname: modern and geological development. Mar Geol 90:145-151 Boruff BJ, Emrich C, Cutter SL (2005) Erosion hazard vulnerability of US coastal counties, J Coast Res 21:932-942 Calado H, Borges P, Phillips M, Ng K, Alves F (2011) The Azores archipelago, Portugal: improved understanding of small island coastal hazards and mitigation measures. Nat Hazards 58:427-444 Cazenave A, Llovel W (2010) Contemporary Sea Level Rise. Annu Rev Mar Sci 2:145-173 Cicin-Sain B, Knecht R (1998) Integrated coastal and ocean management: Concepts and practices. Island Press Church JA, White NJ (2011) Sea-level rise from the late 19th to the early 21st century. Surv Geophys. DOI 10.1007/s10712-011-9119-1 Church JA, White NJ, Hunter JR (2006) Sea-level rise at tropical Pacific and Indian Ocean islands. Global Planet Change 53:155-168 David G, Leopold M, Dumas PS, Ferraris J, Herrenschmidt JB, Fontenelle G (2010) Integrated coastal zone management perspectives to ensure the sustainability of coral reefs in New Caledonia. Mar Pollut Bull 61: 323-334 Dolique F, Jeanson M, Besson J (2007) A monitoring network for assessing the impact of extreme marine meteorological events on tropical beaches. J Coast Res SI 50:77-81 Doney SC, Fabry VJ, Feely RA, Keypas JA (2009) Ocean acidification: the other CO2 problem. Annu Rev Mar Sci 1:169-192 Forbes D, James T, Sutherland M, Nichols S (2013) Physical basis of coastal adaptation on tropical small islands. Sustain Sci 8:327–344 Gilman EL, Ellison J, Duke NC, Field C (2008) Threats to mangroves from climate change and adaptation options. Aquat Bot 89:237-250 Hay J (2013) Small island developing states: coastal systems, global change and sustainability. Sustain Sci 8:309-326 Hoegh-Guldberg O, Bruno JF (2010) The impact of climate change on the world’s marine ecosystems. Science 328:1523-1528 Hoegh-Gulberg O, Munby PJ, Hooten AJ, Steneck RS, Greenfield P, Gomez E, Harvell CD, Sale PF, Edwards AJ, Caldeira K, Knowlton N, Eakin CM, Iglesias-Prieto R, Muthiga N, Bradbury RH, Dubi A, Hatziolos ME (2007) Coral reefs under rapid climate change and ocean acidification. Science 318:1737-1742 Hughes TP, Baird AH, Bellwood DR, Card M, Connolly SR, Folke C, Grosberg R, HoeghGuldberg O, Jackson JBC, Kleypas J, Marshall P, Nyström M, Palumbi SR, Pandolfi JM, Rosen B, Roughgarden J (2003) Climate change, human impacts and the resilience of coral reefs. Science 301:929-933 INSEE (2012) Recensement général de la population à Mayotte. Insee infos 61 Jeanson M (2009) Morphodynamique du littoral de Mayotte - Des processus au réseau de surveillance. PhD dissertation, Université du Littoral Côte d’Opale

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Jeanson M, Anthony EJ, Dolique F, Aubry, A (2013) Wave characteristics and morphological variations of pocket beaches in a coral reef-lagoon setting, Mayotte Island, Indian Ocean. Geomorphology 182:190-209. Laulan P, Robbe C, M’Changama M, Ali Sifari B, Barthelat F, Rolland R (2006) Atlas des mangroves de Mayotte, Service Environnement, Direction de l’Agriculture et de la Forêt Lebigre J-M (1997) Problèmes d’érosion dans le marais à mangrove de Mayotte (archipel des Comores). Trav Lab Geog Phys Appl 15:45-48 McLeod E, Salm RV (2006) Managing Mangroves for Resilience to Climate Change. UICN, Gland, Suisse Mimura N, Nurse L, McLean RF, Agard J, Briguglio L, Lefale P, Payet R, Sem G (2007) Small islands. In: Parry ML, Canziani OF, Palutikof JP, Van Der Linden PJ, Hanson CE (Eds) Climate Change 2007: Impacts, Adaptation and Vulnerability, Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, pp 687-716. Moore LJ (2000) Shoreline mapping techniques. J Coast Res 15:111–124 Mori N, Yasusa T, Mase H, Tom T, Oku Y (2010) Projection of extreme wave climate change under global warming. Hydrol Res Lett 4:15-19 Nicet JB, Jamon A, Simian G, Chabanet P, Bissery C, Guigou A, Aboutoihi L, Bigot L, Quod JP (2012) ORC8 – Suivi 2011 de l’état de santé des récifs coralliens de Mayotte - Suivi benthique et ichtyologique, et impact du blanchissement de 2010. Survey report for the DEAL Mayotte Nicholls RJ, Wong PP, Burkett VR, Codignotto JO, Hay JE, McLean RF, Ragoonaden S, Woodroffe CD (2007) Coastal systems and low-lying areas. In: Parry ML, Canziani OF, Palutikof JP, Van Der Linden PJ, Hanson CE (Eds) Climate Change 2007: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, UK, pp 315-356 Pelling M, Uitto JI (2001) Small island developing states: natural disaster vulnerability and global change. Environ Hazards 3:49-62 Romieu E, Welle T, Schneiderbauer S, Pelling M, Vinchon C (2010) Vulnerability assessment within climate change and natural hazard contexts: revealing gaps and synergies through coastal applications. Sustain Sci 5:159-170 Sheppard C, Dixon DJ, Gourlay M, Sheppard A, Payet TR (2005) Coral mortality increases wave energy reaching shore protecting by reef flat: Examples from Seychelles. Estuar Coast Shelf S 64:223-234 Solomon S, Qin D, Manning M, Chen Z, Marquis M, Averyt KB, Tignor M, Miller HL (eds) (2007) Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA Storlazzi CD, Elias E, Field ME, Presto MK (2011) Numerical modeling of the impact of sealevel rise on fringing coral reef hydrodynamics and sediment transport. Coral Reefs 30:83-96. DOI 10.1007/s00338-011-0723-9 13

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Thomassin BA, Garcia F, Sarrazin L, Schembri T, Wafo E, Lagadec V, Risoul V, Wickel J (2011) Coastal seawater pollutants in the coral reef lagoon of a small tropical island in development: the Mayotte example (N Mozambique Channel, SW Indian Ocean). In: Ceccaldi HJ, Dekeyser I, Girault M, Stora G (Eds.) Global Change: Mankind-Marine Environment Interactions. Springer, Netherlands, pp 401-407 Titus JG (2005) Greenhouse effect and global warming. In: Schwartz M, Encyclopedia of coastal science. Springer ed, pp 494-502 UNESCO (2003) Monitoring beach changes as an integral component of coastal management. Final report of the project on: Institutional strengthening of beach management capabilities in the Organisation of Eastern Caribbean States and the Turks and Caicos Islands. CSI info 15, UNESCO, Paris Vinchon C, Aubie S, Balouin Y, Closset L, Garcin M, Idier D, Mallet C (2009) Anticipate response of climate change risks at regional scale in Aquitaine and Languedoc Roussillon (France). Ocean Coast Manage 52:47-56 Wickel J, Thomassin BA (2005) Les récifs coralliens frangeants de l’île de Mayotte (Grande Terre) : bilan de l’état de santé en 2004 et évolution depuis 1989. Survey report for the DAF Mayotte Wilkinson C (2008) Status of coral reefs of the world: 2008. Global Coral Reef Monitoring Network and Reef and Rainforest Research Center, Australia, Townsville Woodroffe CD, Grime D (1999) Storm impact and evolution of a mangrove-fringed chenier plain, Shoal Bay, Darwin, Australia. Mar Geol 159:303-321 Woodworth PL, Gregory JM, Nicholls RJ (2004) Long term sea-level changes and their impacts. In: Robinson AR, Brink K (eds) The Sea, Volume 13, The Global Coastal Ocean, Harvard, USA, Harvard University Press, pp 715-753

List of figures Figure 1. (a) Location of Mayotte Island in the Indian Ocean (the collective term of ‘island’ is used throughout the text to designate the two main islands and numerous islets of the territory), (b) simplified map of the island, (c) wind rose (1951-2007 data at Pamadzi station from Météo France). Figure 2. Trajectories of cyclones that came as close as within 300 km of Mayotte over the 1967-2010 (Regional Specialized Meteorological Centre, La Reunion). Figure 3. (a) Evolution of the population of Mayotte since 1958 (data from the French Demographic and Statistical Studies Institute, INSEE, 2012); (b) photograph of a hotel on the south coast, an example of increasing development and tourist pressures on the coast; (c) aerial photograph showing the extension of the town of Mamoudzou along the coast. Figure 4. Chart depicting the goals, structure and organisation of the coastal observatory.

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Figure 5. Locations of field observation, measurement and monitoring sites of the surveillance network. Asterisks indicate the initial 10 selected ‘sensitive’ sites. Figure 6. Instruments and methods used in the surveillance network: (a) installation of topographic reference points using DGPS; (b) Total electronic station for conducting topographic profile surveys; (c) deployment of a current meter on the coral reef; (d) example of mangrove area analysis using sequences of aerial photographs. Figure 7. Example of results acquired on topographic changes on Mtsanga Gouela beach: (a) location of topographic profiles and photograph of the beach; (b) topographic monitoring, (c) beach volume variations (from Jeanson et al., 2013). Figure 8. Screen shots of the interactive map application of the Manguiers web portal, http://manguiers.teledetection.fr/, and of synthetic files of results from the surveillance network.

15

Fig1

b

10°S

Mo zam biq ue

No

Madagascar Mauritius

20°S

ea

12°40'

Indian Ocean Mayotte

Mozambique Channel

rth

a

Comoro Islands

st b

ar rie r

Reunion

re e

f( Gr an

Mtsamboro

d

40°E

50°E

Ré

cif

du

No

rd -

es

Grande Terre

12°45'

t)

Koungou

Mamoudzou

Pamandzi

Petite Terre Weather station

12°50'

N

c NW

Dembéni

Sada

NE

Mont Bénara (660 m)

W

5%

E

10%

15%

Bandrélé 12°55' SW

SE

Kani Kéli

4 - 8 >8 - 12

>12

Altitude (m) Settlements

600 500

Coral reefs

13°S

400 300 200

Mangroves

100 0

0 45°E

45°05'

South barrier reef (Récif du Sud)

45'10'

5 45°15'

10 km 45°20'

Fig2

Fig3

250

Population (103)

a

200 150 100 50 0 1950

b

1960

1970

1980

1990

c

2000

2010

2020

Fig5

Beach Mangrove

Topographic survey

Cliff Artificial coastline

Diachronic analysis of aerial photographs Hydrodynamic measurements Settlements

Hamjago Mtsamboro

Coral reefs Bandraboua Kangani

Trévani*

Acoua* Dzoumogné Mliha

Miangani Longoni

Majicavo Koropa

12°45'

Majicavo Lamir

Soulou* Kawéni* Mtsangamouji Tsingoni

Mtsapéré* Mgonbani Baobab

Tzoundzou Passamainti*

Chiconi

Pamandzi*

Ironi Bé

12°50' Dembéni

Sada*

Hajangoua Iloni Hajangoua Sud

Tahiti Plage

Mtsanga Sakouli

Bandrélé

Mtsanga Gouéla

Bouéni Bay

Turtles Bay

12°55' Musicale Plage

Mzouazia Mbouéanatsa

Mounyambani

Kani Kéli Dapani* Ngouja*

Mronabéja Mbouini

13°S

0 45°E

45°05'

45°10'

5 45°15'

10 km 45°20'

Fig6

b

c

d

Mangrove limits: 1949 1969 1989 1997 2003 2008

Mangrove area (ha)

11

10.49

50 10.03

40

10

30

9

20

8.11

10

8 7.07 7

0 -10

6.35

- 4.39

5.63

6 - 22.69

5

-30 - 32.60 - 39.47

4 1949

1969

1989

-20

1997

2003

- 46.33

-40 -50

2008

Mangrove change since 1949 (%)

a

0

100

Meters

200

Fig7 a.

Aerial photograph: Google Earth

P300 Beach

Reef flat Lagoon

P200

P100 0

b.

50

Profile evolution - Mtsanga Gouela P100

5

Elevation (m IGN 1950)

100 m

4 3 2

MHWS

1 MHWN

0

MSL MLWN

-1 -2



5 4 3 2

MHWS

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MSL MLWN

-1 -2



5 4 3 2

MHWS

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MSL MLWN

-1 -2 0

10

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30

Distance (m)

February 2005 September 2005 February 2006

40

50

February 2007 June 2007 February 2008

60

Currentmeter

c. Volume change (m3/m)

60

Feb. 2005 to Sept. 2005

Sept. 2005 to Feb. 2006

Feb. 2006 to Feb. 2007

Feb. 2007 to June 2007

June 2007 to Feb. 2008

40 20 0

-20 -40 -60

P100

P200

P300

Net change (-16.79 m3/m)

Fig8

Fig4

Primary data

Management issues

- Previous studies (BRGM) - Bibliography

Consultations with local representatives of state environmental organisms (Conservatoire du littoral, DEAL, BRGM...)

Definition of vulnerable coastal landforms and resilience

Field data

External data

- Offshore wave climate observations - Coastal topographic surveys - Coastal hydrodynamic measurements - Mangrove surface area evolution

- Topographic maps - Hydrographic maps - Coral reef maps - Aerial photography

GIS Administration - Data collection - Data conversion - GIS design

Geographic Information System (GIS) - Data bank - Synthesis and integration - Characterisation of coastal changes - Data display and download

OUTPUT Enhanced understanding Information and communication - coastal dynamics - definition of response strategies

- Decision-makers - Researchers - General public