Characterization of fine sediment dynamics in small rivers of French Guiana induced by the natural and artificial erosion of watersheds

Résumé

The climate of French Guiana is representative of an Amazonian environment with a strong rainfall regime, and significant differences between the wet and dry seasons. Precipitation during the rainy season is very high, with up to 4.5 m of water per year in certain areas, resulting in the formation of an important hydrological network. Most of the actual hydrological network is in a natural state, but several anthropogenic factors influence the network, such as deforestation, urbanization, agricultural and forestry activities, and legal and illegal mining. French Guiana is marked by red soils as well as more drained, clayey-sandy soils, which are very sensitive to erosion. Therefore, infiltration of fine sediments and clogging of the coarser matrix is an important issue in terms of global change. The objective of this project is to evaluate the potential impact of anthropogenic activities on the clogging of small rivers. Ten hydro-sedimentary stations were installed in the Cacao agricultural region, which also includes the site of Boulanger, Guiana's oldest legal gold mining area. Each station consists of a water level probe, a turbidity probe connected to a data logger, that allow us to measure both discharge and fine sediment concentration continuously. First results show that a significant correlation exists between rainfall, discharge and suspended sediment concentrations (SSC), indicating that the studied watersheds are very reactive. SSC are negligible in the absence of rain, but can increase up to 1 g/L during a flood. Some occasional increases in SSC were also observed without having any variation of the discharge. These events can be explained by local anthropogenic factors of resuspension of fine particles such as large-scale works nearby or the passage of motorized vehicles in the watercourse. It is nevertheless interesting to note that first results already showed differences in term of fines fluxes between natural catchments and catchments impacted by anthropogenic factors. Higher SSC are measured for the latter ones both for the low water regime and during events. On the other hand, all sites present a large clogging compared to what is presented in the literature. For sites strongly impacted by anthropogenic factors, clogging is even more significant, sometimes making impossible to implement infiltration tests.

Auteurs, date et publication :

Auteurs M Bianchi , S Lyonnet , M Gallay , B Camenen

Date : 2024