Field and laboratory measurements of sediment erodibility: A comparison

Abstract

The erosion of fine-grained cohesive sediments is a ubiquitous phenomenon in marine and estuarine environments. Critical shear stress and erosion rates are predominant variables in transport mechanics. This study presents a comparison between in situ and laboratory measurements of sediment erodibility. The main goal was to assess the reliability of critical shear stress of erosion determined in a laboratory flume. The laboratory experiment allowed us to define the erodibility of sediments sampled at a reference station in the Gulf of Fos. This site is frequently influenced by Rhône river inflows. Two different values of critical shear stress of erosion were determined, the lowest (τ c1 = 0.02 ± 0.002 N m − 2 ) related to the erosion of the fluff layer, and the highest (τ c2 = 0.05 ± 0.005 N m − 2 ) to a more consolidated layer or to coarser particles. These values of critical shear stress were very close to commonly in situ based estimates of fluff critical shear stresses, which ranged between 0.02 and 0.05 N m − 2 . The comparison with acoustic backscatter data recorded during a field cruise in autumn attested in a qualitative way to the reliability of the critical shear stress of erosion determined in our laboratory flume. The erosion coefficients determined during this study, varying between 3.4 10 − 6 and 7.5 10 − 6 kg m − 2 s − 1 , were used to calculate in situ erosion rates. Between 6 and 9 October, the mean erosion rate at the reference station was 20 g m − 2 h − 1 and maximum values reached 65 g m − 2 h − 1 . The calculated erosion rates were used to predict in situ variations in suspended sediment concentration (SPM). The predicted increase in SPM was 14 mg l − 1 , close to the measured increase in SPM (10 mg l − 1 ). The results from this initial attempt to compare in situ and laboratory erosion measurements are encouraging but further investigation is needed. Potential organic matter fluxes determined in our flume were around 1.7 mmol PON m − 2 h − 1 and 115 μmol POP m − 2 h − 1 . An estimation based on these results showed that the organic material introduced into the water column by erosion amounted to between 12 and 200% of the daily pelagic primary production. This is considerable and should be considered in biogeochemical studies of coastal systems.