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COST STUDY of COASTAL PROTECTION Marc Igigabel1 And

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COST STUDY of COASTAL PROTECTION Marc Igigabel1 And COST STUDY OF COASTAL PROTECTION Marc Igigabel1 and Marissa Yates2 A better definition of the long term protection strategies against littoral hazards requires evaluating investment, management, and maintenance costs of various interventions on levees and other civil engineering structures, as well as on beaches and dunes. The cost study of coastal protections (Cerema 2018) aims at providing the assessment of the project global cost. Based on the study of a large number of projects completed in France, estimations were produced to enhance the reliability of these calculations at the scale of a site in the littoral zone or of an individual natural or artificial structure. For each structure type, the obtained results are essentially linear, surface, volume or mass costs (initial costs or annual average costs), as well as lifetime and intervention frequency. To obtain this information, the analysis mainly consisted of: • the creation of a typology which crosses « structure type » and « intervention type » which allows establishing homogeneous groups of characteristic operations; • the identification, for every group, of physical parameters influencing the associated costs, which allows the user of the manual to evaluate better the difference between the observed high and low values. This paper presents the context of the study, its methodological process, the progress of the investigations and the analyses, the obtained results, and finally the generic lessons learned. Keywords: cost, protection, defense, sea, flood, erosion, levee, dune, beach INTRODUCTION The traditional techniques of « sea defenses » are frequently used on the French coast, showing mixed results of their efficiency. Moreover, the use of these techniques is often controversial because of the artificialisation of the landscapes and the impacts on the environment. This has led to an evolution of public policies and to the emergence of new protection strategies. Coastal features vary significantly (beaches and dunes, polders and swamps, estuaries and deltas) but all have in common a dynamic mobility that causes their evolution in time. This instability strongly questions the sustainability of hard structures. In spite of the impression of robustness that materials such as armourstone, concrete and steel provide, these structures are fragile when subjected to storms. New evidence of this fragility was shown during the recent storms in France. On February 28th, 2010, the Xynthia storm highlighted the necessity of improving levee systems to protect against marine floods, and a series of storms in the winter 2013-2014 was a reminder of the sensitivity of our coasts to erosion. Natural features (consisting of mud, sand or pebbles), as well as certain types of vegetation, are often the best way to dissipate wave energy. The implementation of traditional protection structures modifies the complex sediment transport balances existing on coasts. In particular, foreshore erosion can be caused by: • the reflection of waves on concrete or steel seawalls; • the interruption of the sediment transport when groynes are placed along beach profiles. In spite of high construction, maintenance and repair costs, traditional works generate strong impacts on the environment and on landscapes, and may offer only limited protection against littoral hazards. Based on these limitations, alternative solutions have been developed that disrupt less the complex dynamics existing on the coast. If these solutions are already implemented at numerous sites, protection strategies against littoral hazards must still be questioned at an even greater number of sites, and in most cases, redefined. Yet, long-term project funding often represents the main barrier to this evolution. It was thus advisable to produce a methodological handbook assembling useful information and pertinent recommendations to compare in the long term the protection scenarios of projects employing these techniques. In particular, the goal is to calculate the global cost, summing the investment, management, and maintenance costs. This is the scope of the handbook produced by the Cerema (2018) on the basis of the observation of a wide variety of projects completed during the last fifteen years in France. The present article presents this study, focusing more particularly on information relevant not only in France, but on an international scale. The flowchart in Figure 1 gives an overview of the contents of the study and is a recommended outline to follow for similar studies. 1 Cerema, Technopôle Brest Iroise, 155 rue pierre Bouguer, BP5, 29280 Plouzané, France 2 Saint-Venant Hydraulics Laboratory & Cerema, 6 quai Watier, BP 49, 78401 Chatou, France 1 2 COASTAL ENGINEERING 2018 Figure 1. Flowchart of the study CONTEXT OF THE STUDY The study (Cerema 2018) is based on the observation of past projects. To appreciate the possibility to conduct this type of study, it is necessary: • first, to have a sufficient understanding of the protection strategies that these projects are fulfilling; • secondly, to make sure that the geographical perimeter of investigations and the nature of the concerned coast, in connection with the historic context of the studied period, guarantees favourable conditions to obtain sufficient data for the establishment of reliable estimations of unit costs and other researched data. Protection strategies against coastal hazards For certain sites, the main problem is the management of beaches and dunes in connection with the sediment transport. Other sites are more concerned by the protection against coastal flooding. To deal with these two problems, projects will inevitably refer to: • the shoreline position, which marks the limit between land and sea. This limit moves depending on erosion and accretion processes, under the influence of hydro-sedimentary and aerial processes, as well as changes in sea level. Generally, the main objective is to avoid that erosion causes shoreward movement of the shoreline position, ultimately threatening coastal infrastructure and populations; • levee systems, whose function is to protect against marine flooding. These systems consist of levees and hydraulic structures assuring the flow regulation between the various sections of the system and between the land and the sea. The position of levee system structures does not coincide a priori with the shoreline: setback levees, canals, or retaining basins are positioned land-side. Nevertheless, a variety of means to manage the shoreline position and to protect against marine flooding are common: in many cases, the preservation of the sedimentary stock, as well as structures positioned on the foreshore to absorb wave energy, are deemed necessary to reach these two objectives. COASTAL ENGINEERING 2018 3 Historical background and geographic scope of the study The study (Cerema 2018) was conducted in France. The coastline of metropolitan France extends nearly 6200 km, excluding estuaries and ports (MEEM 2015), and every coastal type is represented (IGN 2007): • 2200 km of sandy coast (35%); • 1500 km of marshes and tidal reservoirs (24%); • 2500 km of rocky coasts (41% with 13% cliffs). The evolution of public policies during the studied period 2002-2017 led to the implementation in France of a wide variety of protection strategies against coastal hazards. The most pronounced examples are the restorations of lidos (strips of land made of sand that close lagoons) in the Mediterranean Sea and the large-scale works on the Atlantic coast conducted after the Xynthia storm. These projects are defined in action plans that can be classified in three categories: 1. projects designed to protect against littoral hazards taking into account the hydro-sedimentary functioning of sites, but constrained by the strong public policy issues (which explains that the use of civil engineering structures is still frequent); 2. restoration of the coastal zone intended to return to a more natural functioning of the environment ; 3. management and maintenance on sites at which the natural character was widely protected (beaches and dunes, coastal marshes), or on the contrary, at sites marked by a strong human activity. In the first two categories, all in all 173 projects were taken into account in the study (Cerema 2018). The location of these projects is shown in Figure 2, but the figure does not include the maintenance interventions on natural sites of the third category (especially dunes), which were also considered on the basis of the annual activity reports. Figure 2. Location of main studied projects 4 COASTAL ENGINEERING 2018 To give a sense of scale of expenditures dedicated to these objectives in France: • over the period 2007-2013, on the West part of the Mediterranean coast (Occitanie), the investment over 7 years for the rehabilitation of the coastal zone represents 120 M€ (value updated in 2014); • over the period 2011-2016, on sectors affected by the storm Xynthia (center of the Atlantic facade), expenditures over 6 years amounted to 300 M€ (CGEDD 2013). CALCULATION OF THE GLOBAL COST AND COMPARISON OF PROTECTION SCENARIOS To define and estimate the cost of the projects protecting against littoral hazards requires having a relevant calculation method of the global cost. This method applies normally within the framework of the comparison of scenarios ending in the adoption of a strategy. Each scenario can concern several natural features or structures. Thus, the principle is, for each scenario, to calculate the global cost of each component. The
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