Résumé
The French Guiana marine window, including rivers-estuaries-slope-shelf-deep sea basin, cov-ers an area of 126,000 km2and is largely influenced by water and sediments (mud and sands)from the Amazon River, making it unique in terms of its dynamics and exceptional dimension.Within this essentially muddy environment, sandy beaches in French Guiana are rare (15% ofthe coastline) and are sensitive as sandy sedimentary stocks are limited in volume and distribu-tion. These beaches have important economic (human settlements, roads...) asset and a uniqueprotective and ecological role (turtle nesting...). They are vulnerable with rapidly changing dy-namics, and subject to major phases of erosion and accretion, even if no influence of climatechange have been demonstrated up to now. In recent decades, erosion has been aggravated bysand extractions directly on the beaches and the classical sedimentology approach and coastlineinvestigations were ineffective to understand the origins and movement of sand. The ambitionof the project (GUIABEACH) is thus to constrain the sand transfers along the Guiana coast inconnection with the sediment supply from the Guyana Shield watersheds and external to theAmazon.There is also a general lack of faith amongst a large proportion of the public in the knowledgeand thus in the governance of decision-making. These main challenges to reliable knowledge-decision making can be related to the specific objectives of the project that aim to (1) identify themechanisms of coastal accretion and erosion associated with sand dynamics in strategic sectorsof this shoreline (coastal urbanization...) ; (2) define the relationships between rivers and thecoastal zone ; (3) elaborate a better understanding of the forcing mechanisms (marine forcing,river flooding events, rising sea level...) and rate variability of sand migration on the coast; (4)design a comprehensive model of coastal sands transfer and ensure its appropriation by localauthorities and professionals involved in the economic development of the region. This includessocial and human sciences approaches as well as communication actions. Local representatives∗Intervenant40will be involved from the beginning of the project and investigations will be organized to studythe social perceptions of the project results.This idea can be expanded to the Brazilian window, in the frame of a larger project to bebuilt, considering the vulnerability of the coast due to storm wave attack and associated risk tothe urbanized areas. Thus a question is posed about the stability of the shoreline consideringthe evidences of erosion as either a general trend of a backward shift of the coastline or only aresponse to extreme events with subsequent recovery of the shoreline position.To achieve the ambitious objectives, the project will need to combine the application of innova-tive geochemical tools and more classical sedimentologic approaches. It is intended to trace thesource areas of sands, by coupling several chemical and isotopic tracers on heavy minerals at thegrain scale, while up to know the study of the distribution of heavy minerals on beaches andbeach ridges sands was not sufficiently relevant to identify the sand sources. This must be com-bined with the quantification of the sedimentary inputs from the rivers in order to evaluate theimpact on the sand transport dynamics from catchments to shelf. In addition, a special interestneeds to be given to the study of the morphodynamics of beaches and cheniers along the coastand to identify fine-scale mechanisms involved in morphodynamic shoreline adjustments and seg-regation of sand. All the results could therefore be aggregated in an integrative model of sandbehavior from inland to the continental shelf. The use and social perceptions of beaches couldbe investigated through an analysis of pressures on the sandy resources and their societal andstructural reasons. Obviously, the results of the project should be transferred and disseminatedtowards regional authorities and stakeholders involved in management planning and economic development of the areas.