Etude de l'erodabilite des sediments superficiels du Fjord du Saguenay (Quebec, Canada) (French and English text)

A flash flood that occurred in July 1996 in the Saguenay area (Québec, Canada), deposited a bed of cohesive sediments, on average 10–60 cm thick (but locally up to 5 m thick in the proximal delta environment) over an area of 65 km2 of the deep fjord basin of the upper reaches of the Saguenay Fjord. Because this turbidite covered contaminated sediments, the resuspension potential of surficial sediments is an important parameter in order to determine efficiency of the 1996 layer, to act as a capping layer. Importance of the resuspension of surficial sediments lie in the hypothesis that contaminants may diffuse through the 1996 layer from underlying sediments or that new contaminants settle.; The resuspension potential of surficial sediments in Saguenay Fjord was investigated over a three-year period at fifteen sites in Baie des Ha! Ha! and the North Arm. For the first time at such depths, a circular benthic flume, Miniflume, was used to measure the erodability of cohesive sediments. The critical shear stress (τ_c) varies between 0.07 and 0.48 Pa, i.e. critical velocity between 6 and 15 cm.s−1 . The variability in the results is linked to the great spatial variability of benthic sediments rather than to temporal variability. Geotechnical data, particle sizes analysis, polychaete tubes distribution, water content of surficial sediments vary greatly on small and large scales. Parchure and Mehta (1985) erosion law applied to the whole data has a determination coefficient ( r2) of 0.6. This relation links erosion rate E (kg.m−2.s−1) with τ _c and the bed shear stress τ_b (Pa). Erosion depths (ze, mm) were calculated using Miniflume and axial tomodensitometer data. Tomodensitometer data provide variation of sediment density at an interval of 0.13 mm. General relationship between ze and τ_b was derived with a r2 of 0.82. Bottom current speeds recorded in the North Arm and in Baie des Ha! Ha! from May to August 2000 vary between 0 and 27 cm.s−1. For current speeds between 10 and 25 cm.s−1 erosion depths vary between 0.01 and 3.51 mm. For benthic current speeds recorded in Saguenay Fjord, resuspension of a thin sediments layer can be an agent for the remobilisation of contaminants since pollutants are often adsorbed onto and desorbed from fine-grained suspended sediments (<20 μm).