The coast of the Ile-de-Cayenne is the only rocky outcrop of an ancient shelf along 2000km between the Amazon and Orinoco deltas. The coast is made up of a succession of low sandy stretches (when the mud and the mangroves are absent) and rocky cliffs (insular shelf, magma rocks…). In the estuaries, the rivers are always edged by mangroves. The sandy coasts are all sensitive to sea erosion, but they develop differently depending on their position, their size and their morphology, while the rocky coasts correspond to the relief of the platform.
A peculiarity of certain parts of the Montravel coast and above all Mahury, is that they are made up of large rounded blocks, piled one on top of the other to depths that are apparently large but unknown. The coastal fringe of the study sector, as well as the whole Guiana coast, is an area with a very active sedimentary dynamic, marked by the alternation of rapid phases of sedimentation and erosion linked to a drift towards the northwest, under the influence of the Guianas current, of a trail of silt coming from the Amazon arranged in the form of banks tens of kilometres long. The local factors in the coastal dynamic are particularly complex and currently remain very little studied and little known. These hydrodynamic agents are: currents, impact of the local rivers, swell and tide.
This coast is essentially devoted to recreational and tourist uses. From the time when the beaches are “de-silted” (inter-bank period) and the mangrove has been destroyed, they show the required characteristics (sand, swell) for swimming or sporting uses (windsurfing, dinghies…). Around the Mahury mountain there are some precariously built houses on some small beaches occupied by Brazilian families living off traditional fishing. The estuaries, notably and above all that of the River Mahury in the southeast, have an important economic function, including the presence of fishing ports (River Cayenne) or commercial ports (River Mahury) sheltered at the back of the estuary.
Today, it is noticeable that erosion is continuing in “protected” areas. Structures have introduced such a discontinuity in the beach that it is impossible to predict the development of the sector in the short term. Considering the general dynamic affecting the coast (movement of silt banks), the causes which seem to have led to the considerable developments noticed in the medium-term past, and the importance of the Guianese coastal strip, the option sustained is to recover - with replenishments of sand - lengths of beach equivalent to those of the past and to protect the sectors currently under threat. The set of measures is meant to lead to creating a coastal space available to everyone and accessible to all, and to end economic disasters (organisation of local services for beach users).