Should the inner shore of the Arcachon Bay (Gironde, France) be depolderised .... counting the number of walkers, making an inventory of hunting grounds, data ...
How to establish operational recommendations to manage sea flooding risk on a reclaimed coast? Should the inner shore of the Arcachon Bay (Gironde, France) be depolderised or should the dykes be maintained ? Goeldner-‐Gianella Lydie1, Bertrand Frédéric2 1. Géographie, UMR 8586 PRODIG, Université Paris 1. 191 rue Saint Jacques 75 005 PARIS. Lydie.Goeldner@univ-‐paris1.fr 2. Géographie, UMR 8586 PRODIG, Université Paris 4. 191 rue Saint Jacques 75 005 PARIS. frederic.bertrand@paris-‐sorbonne.fr ABSTRACT. The BARCASUB program dealt with coastal flooding risk in the Arcachon Bay (France) and with the
possible management of this risk through a soft defense measure : "de-‐polderisation". The article doesn't describe the whole aspects of the program. It only presents the operational methodology developed to establish proposals of flooding risk management, such as de-‐polderisation in particular, compared to doing nothing, conserving the dikes or reinforcing them. We detail the steps that we have followed : establishing firstly a gradient of eligibility for depolderisation between the management units of the studied polders (2) and doing then, for the most eligible sectors, a cost-‐benefit analysis of each possible method of flooding risk management (3), in order to obtain distinguished management recommendations for each polder (4). This methodology seems reproducible, provided a strong interdisciplinary work is conducted to obtain both physical and social data or quantitative and qualitative data, all needed for the evaluation. RESUME. Le programme BARCASUB a porté sur le risque de submersion marine à l'intérieur du bassin d'Arcachon (France) et sur la gestion potentielle de ce risque par une mesure souple de défense : la "dépoldérisation". La communication n'évoque pas la totalité des aspects traités dans le programme, mais expose la méthode opérationnelle que nous avons développée pour établir des préconisations de gestion du risque de submersion passant en particulier par la dépoldérisation, que nous avons comparée au laisser-‐faire, à la conservation des digues ou à leur rehaussement. Nous détaillons les étapes que nous avons suivies : établissement d'un gradient d'éligibilité à la dépoldérisation entre les casiers des polders étudiés (2) et analyse coûts/avantages de chaque mode possible de gestion du risque pour les casiers les plus éligibles (3), afin d'aboutir à des préconisations différenciées de gestion selon les polders étudiés (4). Cette méthodologie paraît reproductible, à condition de travailler en forte interdisciplinarité pour obtenir des informations tant physiques et sociales que quantitatives et qualitatives, toutes nécessaires à l'évaluation. KEYWORDS : risk management, marine flooding, de-‐polderisation, assessment, issues, exposure, costs. MOTS-‐CLÉS : gestion du risque, submersion marine, dépoldérisation, évaluation, enjeux, exposition, coûts.
1. Introduction The BARCASUB Research Programme (2010-‐13) dealt with the risk of sea-‐flooding in the Arcachon Bay (France) and potential management of this risk through use of the "depolderisation" technique (Bertrand and Goeldner-‐Gianella, 2013). This project brought five teams together – the research laboratories PRODIG, LGP, EPOC, the BRGM and the EA PRISM – and four local managers or stakeholders. Traditionally in France, the risk management techniques and sea-‐flooding prevention have consisted of heavy defences taking the form of embankments or reinforcement of the dykes that protect the polders. There is another, more flexible, method of risk management consisting in reopening the polders to the sea, by using locks or creating breaches in the dykes, or even dismantling a dyke. This depolderisation generally translates in the rapid restoration of saltmarsh, through the effects of sedimentation. In North America, as well as in north-‐western Europe, such natural environments are perceived ever more frequently as infrastructure in terms of defence against sea incursion (French, 2006) and of adaptation to climate change. Increased attention is thus being paid to these saltmarshes, their extent and plant density being positive related to the attenuation of the energy of the swell and height of the waves, as well as with the stabilisation of the coastline (Shepard et al., 2011). It should therefore be possible, where the saltmarsh is sufficiently extensive, to limit the build-‐up of dykes. This solution should be of great economic interest in the context of climate change, as the British have well understood (Goeldner-‐Gianella, 2013). If depolderisation is not mentioned or implemented to any great extent in France, the current scientific and political context would seem to be increasingly favourable to its implementation. Consequently, BARCASUB was created following a consultation launched nationally by the Conservatoire du littoral at the beginning of the twenty-‐first century concerning the adaptation of its properties to climate change. It was storm Xynthia that highlighted the dangers of sea flooding in France and this translated into the publication of a national coastline management strategy (2012) that evoked the mobility of the coastline and more flexible management techniques, such as the restoration of the ecosystems. This strategy is also the basis for experimentation with strategic withdrawal at pilot sites. Finally, these events promoted the development of an operational debate about depolderisation, especially along the coastline of the reclaimed Bas-‐Champs of Picardy (Bawedin, 2013). At regional level, the authorities in Aquitaine (such as the "GIP Littoral aquitain") who are very aware of the coastal risks, launched a regional management strategy for the coastal strip and are participating in the national strategic withdrawal experiment. Finally, on a local level, the Arcachon Bay has been hit in recent years by numerous storms, resulting of accidental depolderisations. The owner of the sites affected, the Conservatoire du littoral, is interested in this innovative management method since it finds itself having to deal with the increasing costs of dyke maintenance (Clus-‐Auby et al., 2006). Finally the current context of having to establish the first local sea-‐flooding risk prevention plans around the Arcachon Bay has proved very sensitive. The BARCASUB project team wondered whether it were physically possible to use depolderisation as a risk management method to prevent sea flooding in Arcachon Bay and whether this would be considered socially acceptable and economically advantageous. Our paper will not discuss all of the aspects dealt with under this programme – such as an assessment of the local sea-‐flooding hazard and the inland expansion of submersions, the role played by a depolderised marsh ecosystem in attenuating the risk of sea flooding and social protest against sea-‐flooding and depolderisation (see Goeldner-‐Gianella et al., 2013). We shall discuss here the operational method we developed to make recommendations for managing the risk of flooding from the sea, in combination with possible depolderisation. The method was devised for three polders on the south-‐eastern shore of the
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Arcachon Bay (Certes, Graveyron, Malprat) which were originally built in the eighteenth century for fish-‐farming purposes. The Conservatoire du Littoral acquired these polders starting from 1984, to preserve their natural and cultural heritage and open them to the public. The cost of repairing the dykes as a result of the storms and flooding in 1999, 2009 (Klaus) and 2010 (Xynthia) exceeded €700,000 (Bertrand and Goeldner-‐Gianella, 2013) and could further increase in the future due to increased submersion of these polders (Anselme et al., 2008). Consequently, the Conservatoire has contemplated whether to allow the sea to flood some of the polders, or at least to stop maintaining some of the dykes. Thus, it recently abandoned to the sea the "Pointes" of the polders of Graveyron and Malprat that had accidentally been breached. The method presented here concerns the three above-‐mentioned polders, which are broken down into internal "basins" ringed by low dykes or channels and edged by dykes of soil widely seeded with ripraps and sometimes separated from the sea by a saltmarsh dotted with hunting lakes (figures 1 and 2). 2. Establishing a scale of eligibility for depolderisation in view of the risk of sea-‐flooding In order to establish management recommendations for the three polders in question, we initially constructed a scale of eligibility for depolderisation in view of the flooding risk to the internal “management units” for these polders (units determined in the polder management plans). A dual assessment – that of the factors involved in exposure to sea-‐flooding hazard (2.1) and that of the issues constituted by each polder (2.2.) – made it possible to create this scale (2.3). 2.1. Assessing the factors of exposure to sea-‐flooding hazard and the degree of exposure of management units In assessing the factors of exposure to sea-‐flooding hazard that would involve damaging the polders and their management units, we took into account the natural sea defence infrastructure, i.e., the saltmarshes alongside the dykes, as well as the condition of the artificial defence infrastructure, i.e. the condition of the dykes themselves, and finally the topography of the basins in relation to their degree of submergibility. Based on very carefully spatialised information scaled to the size of the management units or sections of dykes, we assessed the respective importance of the mentioned exposure factors: factor absent or only faintly present (marked 0), with an average presence (marked 1) or strongly present (marked 2). The first damage factor, by order of intervention in the course of flooding, concerns the condition of the saltmarsh in view of the generalised morphological disintegration, largely due to the creation of the hunting lakes. Two indicators made it possible to define the condition of the saltmarsh and the degree of anthropic pressure exercised on this environment, despite the regulation of hunting in the marine public domain: the relative area of the hunting-‐lakes in proportion to the total area of the saltmarsh – reduced to the linear measurement of the frontal dyke for each unit – and the speed at which the edge of the saltmarsh retracted between 2000 and 2009 also compared with the adjacent management unit. The second factor involved in the progress of flooding – through overflowing, rising higher than or breaching the dykes – was their state of repair. Irregular maintenance of sea dykes in fact explains why the crest of the dykes are all built to different heights, as is shown by topographical elevations and LIDAR data. Such irregularities can produce increased effect of overflow at the lowest points of each crest, thus favouring the acceleration of waves crashing against the inner side of the dyke and the tearing out of entire sections of dyke, which in itself is liable to breaching, especially in sections that are susceptible to external erosion. The state of the dykes was thus
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differentiated on the basis of the average height of the sections of each unit. The last factor to be considered is the topography of the management units on which flooding is propagated. The conversion to MNT of the LIDAR data makes it possible to determine the height reached by the potential flooding, resulting from the propagation of the tidal wave that moves closer and closer to the inside of the polders as the water levels reach 4.69 m IGN -‐ an extreme water level combined with wave and wind conditions of storm Klaus and a tide with a coefficient of 95 taking account of a sea level of +0.25 cm based on the worst case scenario adopted by the ONERC in 2030. The addition of the three exposure factors provides, for each management unit, a degree of exposure to submersion, whether it be slight (marked between 0 and 2), average (3 or 4) or high (5 or 6). The sectors the most exposed to submersion can thus be found in the parts downstream of the polders, especially at their Pointes corresponding to Branne and Brèche units (fig. 1).
Figure 1. Factors and degrees of exposure to flooding hazard in the polders of Certes and Graveyron 2.2. Assessment of the issues present in the polders and their extent per management unit In a second stage, an assessment of the issues present in the three polders was operated on the basis of distribution into major categories, namely, the production factors (inland fishing and cattle-‐ rearing), recreational facilities (hunting from a hide, rambling on the dykes) and heritage sites (flora and bird life, sites of outstanding natural beauty). In the case of Malprat, there is also an environmental health factor involving the control of mosquitoes that can be added to the previous three factors. This assessment is based on different sources, such as interviews with the operators, counting the number of walkers, making an inventory of hunting grounds, data for the management plans, etc.). Certain factors are absent from the polders, but are present on the dykes or in their
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vicinity and have been associated with adjacent management unit. For instance, hunting and walking have been associated with the neighbouring units since these activities can rely or affect the dykes. Based on carefully spatialised information in relation to the management units or sections of dyke, we assessed the respective importance of each issue, using the same notation principles as previously (from 0 to 2). After adding up these marks, each management unit achieved a set of factors considered to be low (a mark of < 4), average (a mark of 4 to 6) or high (a mark higher than or equal to 7). Here again, the strongest factors were concentrated in the parts below the polders, especially the Branne and Brèche Pointes (figure 2).
Figure 2. Types and extent of the issues exposed to a sea-‐flooding risk at Certes and Graveyron 2.3. Establishing an eligibility scale for depolderisation to deal with the risk of sea-‐flooding The eligibility scale for depolderisation was obtained by cross-‐matching the results from this dual assessment, applying an algebraic sum and subtracting for each management unit the total value allocated to the issues in the total value assigned to the exposure to hazard, in such a way as to obtain a scale of "eligibility for depolderisation" (marked from -‐17 to +4). We consider a management unit to be more eligible for depolderisation if its exposure to hazard exceeds the value of the issues present. Positive values or those slightly less than 0 thus translate into high eligibility, since they represent greater risks or lesser issues. The Brèche unit at Graveyron is a case in point, since the exposure is considerable and the risk issues are low due to an accidental opening to the sea that has been there for many years. The Grandes Plaines unit at Certes, on the other hand, illustrates a situation in which the exposure is less than the scope of the issues (which are heritage-‐based and above all recreational) and thus the unit would appear on the face of it to be less eligible. This
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eligibility scale for depolderisation has helped us to find basins which deserve a cost-‐benefit analysis that is more detailed of all the possible risk management scenarios. 3. Creating a cost-‐benefit analysis for management units eligible for depolderisation In the case of the management units that are most eligible for depolderisation, a cost-‐benefit analysis was performed, taking account of the costs of all the possible risk management methods for these units (3.1.), the way in which the issues developed depending on the method chosen (3.2.) and the potential for reconstituting a saltmarsh if the site were to be depolderised (3.3.). 3.1. Costs of the various risk management methods with or without depolderisation An assessment of the cost of the various risk management methods has been difficult to conduct, due particularly to the fact that there is extreme variability in the cost of ripraps which depends on both their availability (changing according to the weather or economic circumstances) and their quality. The results obtained for Arcachon Bay circa. 2010 are therefore hard to use for generalisation purposes. The dykes should be built up by one metre at Malprat and by 1.30 m at Certes and Graveyron to reach a height of 4.70 m which would be only slightly higher than the highest water level of flooding by 2030 as defined above. The cost of dyke reinforcement would be less than that of heightening them, not necessarily in linear metres but due to the generalisation of the extension process which would also involve changing the level of thirty or so tidal gates. Depolderisation of a management unit – which at first sight might appear to be less costly – would only really be cheaper on condition that it would not be necessary to build an additional rear protective dyke for the other management units. Furthermore, depolderisation using gates would be more costly than depolderisation through creating a breach without riprap. The costs need to be estimated case by case however, depending on the configuration of the defence system chosen and the length of the front and rear dykes to be repaired, heightened or built. 3.2. Changes to the issues involved, especially in cases of depolderisation The changes in issues based on the management methods chosen also deserve to be examined. Certain activities appeared to us to be better suited to depolderisation than others, such as walking, for which one can imagine maintaining footbridges installed over the breaches, or hunting for which the cabins and the lakes could be moved – an option currently studied in the Bas-‐Champs of Picardy. While certain factors will inevitably change in the case of depolderisation (decline of inland fishing and heritage factors), animal husbandry would be transformable without great loss of value. To move from cattle grazing on wet meadows to sheep rearing on saltmarshes would be conceivable and viable as far as the farmers of today are concerned as long as a herd of 60 cows and a flock of about 100 ewes could be kept. At high tide, the sheep would shelter on the higher ground of the saltmarsh and in the case of flooding they would be sheltered by the polders. 3.3. Assessment of the potential for the reconstitution of a saltmarsh in a case of depolderisation The choice of a submersion management method also depends on the effective potential for reconstituting the saltmarsh. In order to establish this potential for the recolonisation by vegetation of a depolderised area, F. Bertrand used as the basis, the follow-‐up performed since 2003 in the sectors that were accidentally depolderised, in such a way as to determine the current ecological
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gradient of the expanding saltmarsh. The lower limit adopted is that of the continuous plant ground cover in which cover with vegetation exceeds 75%, corresponding to the lower limit of a saltmarsh that is firmly established. The upper limit is that of the limit of the plant growth in the highest part of the saltmarsh. When these values – which are different for each polder – are known, it will be possible to determine the extent of the area that could be recovered by a saltmarsh in a case of depolderisation. The results thus show a strong potential for a saltmarsh reconstitution of virtually the entire areas of these three polders. 4. Proposals for Arcachon Bay and conditions for reproduction The taking account of the financial (3.1), socio-‐economic (3.2) and bio-‐geographical (3.3) criteria has made it possible to arrive at the creation of ten operational datasheets combining the results for management units that would initially appear to be eligible for depolderisation, and resulting in recommendations. BARCASUB will deliver its various recommendations fir each polder (4.1). We also consider this operational methodology to be reproducible under certain conditions (4.2). 4.1. Differentiated recommendations for the three polders At the conclusion of this analysis, we should like to discuss the best management options proposed for each polder. At Certes, due to the cost, which would be lower than the current cost of maintaining the dykes, depolderisation by creating an unstabilised breach at Branne Pointe, with the construction of a short rear dyke, would seem to be the most desirable solution. This would have little impact overall on the current issues (hunting, walking, animal husbandry) and would deal with the high exposure to sea-‐flooding hazard.
Figure 3. Example of the operational datasheet proposed for the western part of the Certes polder
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While depolderisation by means of a tidal gate, being considered for the Grandes Plaines unit, seems expensive to us in view of the complete absence of impact due to hazard exposure (fig. 1), it could nevertheless be contemplated in the context of the extension of the recent reopening of the Cabanot unit, which in this case involves objectives that are not defensive but solely ecological. At Graveyron, due to the high cost of depolderisation and the extent of the issues to be preserved on and at the foot of the dykes (walking and hunting), we merely propose retaining the current breach (which equates with accepting the accidental depolderisation of the Brèche unit), without additional reinforcement of the rear dyke but with preservation of the ruined dykes and the high plants present in this recolonised area (Tamaris, Baccharis). At Malprat, considering the average level of issues other than those related to hunting and the proven potential for plant recolonisation, depolderisation by an accidental break of the two rear units would appear to be a desirable extension of the accidental depolderisation that has already occurred for the Pointe. 4.2. Scientific and operational conditions of reproducibility of the assessment This operational methodology, described in greater detail in the final report of the BARCASUB project (Bertrand and Goeldner-‐Gianella, 2013) would appear to be replicable on condition that the information was necessary for performing successive assessments relative to the levels of submersion and their modelling, the state of the natural and artificial infrastructures, the local cost of dyke construction and maintenance, knowledge of the current issues and the potential changes to the restored saltmarshes. This information was first acquired by BARCASUB through fruitful cooperation between scientists and managers, as well as between exact sciences and humanities. Acknowledgements: We thank our partners for their financial support (MEDDE (Liteau), Conseil Gé-‐ néral de Gironde, Conseil Régional d'Aquitaine) as well as the Conservatoire du littoral and the SIBA. References Anselme B., Durand P., Goeldner-‐Gianella L., Bertrand F. (2008). Impacts de l’élévation du niveau marin sur l’évolution future d’un marais maritime endigué (bassin d’Arcachon, France). VertigO, vol. 8/1254. Bawedin V. (2013). L'acceptation de l'élément marin dans la gestion du trait de côte : une nouvelle gouvernance face au risque de submersion ? Annales de géographie, vol. 692, p. 422-‐444. Bertrand F., Goeldner-‐Gianella L. (2013). BARCASUB. La submersion marine et ses impacts environnementaux et sociaux dans le Bassin d’Arcachon (France) : est-‐il possible, acceptable et avantageux de gérer ce risque par la dépoldérisation ? Rapport final pour le programme Liteau, juillet 2013, 198 p. Clus-‐Auby C., Paskoff R., Verger F. (2006). Le patrimoine foncier du Conservatoire du littoral et le climatique: scénarios d'évolution par érosion et submersion. Annales de géographie, vol. 648, p. 115-‐132. French P.W. (2006). Managed realignment – the developing story of a comparatively new approach to soft engineering. Estuarine, Coastal and Shelf Science, n° 67, p. 409-‐423. Goeldner-‐Gianella L. (2013). Dépoldériser en Europe occidentale. Pour une géographie et une gestion intégrées du littoral, Publications de la Sorbonne, Paris, 350 p. Goeldner-‐Gianella L., Bertrand F., Pratlong F., Gauthier-‐Gaillard S. (2013). Submersion marine et dépoldérisation: le poids des représentations sociales et des pratiques locales dans la gestion du risque littoral. Espace, Populations, Sociétés, vol. 1-‐2, p. 193-‐209. Shepard C.C., Crain C.M. & Beck M.W. (2011). The Protective Role of Coastal Marshes: A Systematic Review and Meta-‐analysis, http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0027374.
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