Contenu du sommaire : Aménagement des littoraux et conséquences géomorphologiques / Les littoraux sableux et dunaires

The Rio de La Plata has been for centuries the main way for water transport. Large areas of economical activities as well as huge metropolises such as Buenos Aires and Montevideo are concentrated on its banks. The riverside populations that account for 15 millions of people have found there a privileged setting for their leisure activities. The organisation of these coastal areas devoted to recreational activities results from a complex combination of physical, historical, economical and social factors, sometimes shared in common by the two banks of the estuary, sometimes specific to only one of the waterside countries.

Since the middle of the twentieth century, an ever-increasing number of people has settled on the metropolitan shore of the Channel and the Atlantic. Around the seaside resorts, certain "natural" coastal sites have received a lot of visitors over the past few decades. Since the middle of the 1970s, the latter has been protected by regulations and has been bought up by land-owning trusts whose mission is to protect them from urbanization. The managers of these sites are confronted with the success that they meet to the point that management of access to them seems unavoidable. This should consist in a greater degree of control of the flow of visitors and of the numerous recreational activities which take place, but also of types of urban behaviour, certain of which can be harmful for these very delicate "natural" sites.

This paper will mainly focus on the relations between geographical analysis and law. In France, space planning has largely and traditionnaly been state-driven and regulations-based. For the past two decades, this process has revealed itself as highly debatable as far as coastal zones are concerned. The new paradigm of environmental issues highlights the limits of such practices : bottom-up actions, in contrast with top-down regulation habits, make accurate geographical analysis even more needed. In such a new way, integrated coastal zone managements necessitate a closer dialogue between geography and law.

This study aims at explaining different kinds of hazards in Ivory Coast. We first discuss of society risk e.g. mainly links to occurrence of natural hazards threatening social dwelling. We find in this case coastal recession, submarine landslides, floods of coastal plains. We discuss on the other hand of environmental risks linked to social hazards against natural stakes : we can find in this way impact of coastal defencies, quarries of beach sand, sanitary conditions, overgrow of aquatics plants, sedimentary fill of lagoons. Each kind of risks is highlighted with maps from a coastal area databasis.

CETMEF is a state technical institute devoted to marine and fluvial engineering. Coastal zone management is one of its most important activities. Its principal mission is to elaborate and diffuse technical recommendations to state services. To improve its competences, CETMEF implies itself in research and engineering projects. Because of its very specialised know-how, CETMEF is in charge of public purchasing for specific material and of environmental monitoring.

The impact of jetties on sediment movement, especially in port areas, has long been accurately reported and assessed. This article presents a new example of the influence of such constructions on local hydro-sedimentary conditions. Where the present work is original is in the appearance of a huge and fairly recent accumulation of sand (within the last 10 years) in the Dieppe harbour fairway, whereas this granulometric fracture was until now considered rare on the infratidal area of the Pays de Caux. By using a naturalist approach and a diachronic analysis of the bathymetric port surveys, this article attempts to show the relationship between the port site configuration evolution and the hydro-sedimentary effects that appear. The latter stems from a combination of several factors and processes which alternate in time and space. However, in the absence of new constructions, the recent formation of huge sand accumulation in fairways of other ports in the Haute-Normandy region would suggest an evolution of infratidal hydro-sedimentary conditions of the sector studied.

Two technical operations concerning the mechanical opening of the channel rivers of the Yar and the Roscoat in the Saint-Michel-en-Grève bay were carried out between 1998 and 1999. These operations were undertaken in order to eliminate the confluence of the two rivers on the shore and to allow the fill up of a large depression which was located on the upper part of the beach, just behind the confluence. The fill up of the depression brought important morphodynamic changes inducing feedback phenomenon such as dune erosion located on each side of the rivers. A monitoring based on topographic measurements allowed to analyse and quantify these processes.

Since 1949, efforts done to protect the coastline from erosion result in the development of increasingly artificial shoreline in Brittany. Using data collected in a coastal georef database, a spatial and temporal analyse of the shore protection is done in relationship with the location of coastal damages and meteorological variability. The aim of these analyses is to show that coastal management trends in Brittany answer locally to erosional crisis without taking into account coastal dynamics on the long term and at a global scale.

Enhancing the resilience of coastal systems has been suggested as an appropriate proactive adaptative response to reduce their increasing vulnerability. However, the concept includes two components, morpho-ecological and socio-economic, each of which possesses its own adaptative capacity to perturbations. Human activities often reduce the natural coastal resilience, but although much has been made in recent decades of the use of "soft" engineering in place of the more traditional "hard" structures, the latter are privileged by the public opinion. The socio-economic vulnerability is determined not only by the society 's technical proficiency, but also by its cultural ability to prevent or cope with potential impacts.

On many coasts large amounts of sediments were drifted shoreward from the sea floor during the world-wide Early Holocène transgression. As still-stand approached around 5 or 6 000 years ago, extended beaches developed and eventually dune fields formed on coastal plains. When progradation ceased because the sediment supply from the offshore zone was depleted, erosion started and became prevalent except where large amounts of sediments were carried down to the coast by rivers, in particular in the deltaic areas.

This paper presents a review of three conceptual models published recently in order to explain the formation and the evolution of established foredunes. In 1982 Short and Hesp published a wave, beach and dune interaction model for the southeast coast of Australia. Psuty (1988, 1992) and Sherman and Bauer (1993) proposed a foredune sediment budget model. Hesp (1988) published a morpho-ecologic classification of southeast Australian established foredunes. A model of established foredune morphology, dynamics and evolutionary trends on stable, accreting and eroding coasts has been recently proposed by this author (Hesp, 2002). From this review, the question of the applicability of these models along coastlines suffering anthropogenic pressure is discussed.

The sandy beaches and associated coastal dunes of the côte d'Opale (North of France) belong to macrotidal coastal systems. In this context, the instantaneous shoreline periodically migrates across the beach profile according to the "tidal translation rate". It determines where and how long the different nearshore processes act upon the beach profile. These processes depend upon the marine dynamics (linked with the dissipation of incident wave energy in the shoaling, surf and swash zones) and wind dynamics. During a lunar tidal cycle, and all along the year, the respective duration of both dynamics and their spatial impact vary considerably upon the beach profile. Swash processes affect the upper shoreface during only a relatively short time. It explains that storms are really efficient upon the upper beach profile only when they occur during a high spring tide. The combined marine-aeolian dynamics is registered in the morphology of the upper shoreface and foredune profile. Landforms, which formed in different dynamics environments can survive and lay side by side, despite contrasted morphodynamics significance. The jumbling juxtaposition of marine or aeolian process-induced landforms has to be considered when sensing aerial photographs or topographic profiles. These data have no real significance if they are not spatially and timely related to the regional context. Although spectacular, the storm morphological effects may rapidly disappear in case of abundant sediment supply.

The field study of the fitting of military remains gives us quickly relevant information concerning the evolving landscape, particularly in dunes. This original method, which should be named "bunker archaeology" , is based on the fact that German bunkers are outstanding géomorphologic benchmarks. During World War Two, they are the basic elements of a defensive system : The Atlantic Wall (1942-1944). Each type of bunker is designed for a specific mission and settled in an accurate environment in order to reach this goal. Half a century after the end of the war, the typing of the bunkers quickly reveals eventual landscape evolvements with the assistance of this correlation between military purpose and original settlement.

Shoreline progradation is not common along the Côte d'Opale (North of France). The gently sloping 1500 m-wide beach of "Fort Vert" (east of Calais) is an exception. Abundant sand supply, hydrodynamics favouring accumulation and efficient aeolian deflation explain the development of "insular incipient dunes" on the upper beach. These dunes form below the highest sea-water level. Although periodically submerged, the largest ones survive from year to year, thanks to a very resistant dense rhizome framework due to Elymus farctus. This type of incipient foredunes is associated with long term accretion and shoreline progradation.

Several experiments were carried out on the macrotidal beaches of Côte d'Opale (Northern France) to elucidate the effect of ridge and runnel morphology on aeolian transport. These experiments involved deployments of anemometers, weather stations, saltiphones for counting grain impacts, and sand traps. Beach profiles and surface moisture content were systematically measured and surface roughness was noted. The results show a control of ridge and runnel morphology, moisture content and surface roughness on aeolian transport : the fetch notion has to be reconsidered on this beach morphology. In spite of a potentially large wind fetch given the beach width, the effective fetch is generally limited to the beach backshore and the inner ridge of the upper beach ; it is generally segmented by the runnels, which may act as efficient sand traps. The typical ridge and runnel morphology of the Côte d'Opale beaches seems to be a limiting factor for the beach-dune sand exchange.

Recent erosional phenomenon affected the dunes of Saint-Michel- en-Grève bay. Therefore, a study based on dune morphosedimentary dynamics of this sector was undertaken by the local council of the Côtes d'Armor in September 2002. This article presents part of this work focusing on the evolution of Saint-Michel- en- Grève and Tréduder dunes between 1990 and 2003. After a first phase characterised by rapid progradation (between 1990 and 1999), erosion takes place. The retreat of the Saint-Michel-en-Grève goes on to 2001, than a post-erosive adjustment phase appears. Inversely, erosion is still active on Tréduder dunes. Since 1999, dune retreat reaches more than 60 m inducing threat for coastal infrastructures of the road (D786) which is now exposed to wave and surge.

The western coast of Cotentin (Normandy) stretches approximately 125 kilometres on a north-south axis. More than 100 kilometres of this shoreline are occupied by sand bars and dune, not forested. This raises an interesting question as many other dunes situated to the north or to the south are covered with forests. The absence of forestation does not exclude an important ligneous dynamics, especially over the last twenty years. This dynamics indicating the spatial strategies of shrubby and arboreous species also illustrates the general dune field evolution.