1 Erosion of the upper layer of cohesive sediments: characterization of some properties Ouafae El Ganaoui 1 , Estelle Schaaff 2 , Patrick Boyer 1 , Muriel Amielh 3 , Fabien Anselmet 3 and Christian Grenz 2 ABSTRACT: A recent paper (El Ganaoui et al. 2004) reported an experimental protocol used to analyze sediment properties. This protocol identified for both freshwater and marine sediments a surface layer with specific dynamic properties (critical erosion shear stresses in the range 0.025 - 0.05 N m -2 ) and a second layer with critical erosion shear stresses about ten times larger. The present work compares these former results with the work of Pilotti and Menduni (2001) who extended the applicability domain of the Shields diagram to very fine particles. The surface layer is shown to consist in fine and unconsolidated sediments that behave like non-cohesive material while the second layer is characterized as being cohesive. This is similar to the two-layer concept of the sediment interface already discussed in Thomsen and Gust (2000) but for aggregates. It is shown that the surface layer is mainly representative of recent deposits of suspended particles. This points out the existence of a fluffy layer of fine sized particles resting near the bed, with specific erosion characteristics, which has to be considered separately when studying sediment properties. CE Database subject heading: erosion, cohesive sediments, settling velocity INTRODUCTION In both marine and freshwater systems, sediment dynamics play a major role for water quality. Consequently, the modeling of water quality requires the description of sediment bed properties. In practice, these properties are inferred from direct in-situ measurements (Amos et al. 1992; Black et al. 2002) or using laboratory flume experiments (Schaaff et al. 2002). As natural water flows contain a wide and continuous range of granulometric sizes of particles of different types, it is also useful to identify these classes of particles and their specific dynamic behaviour. At this end, a companion paper (El Ganaoui et al. 2004) presents an analytical method based on a multi-class model to determine the main classes with their associated erosion rate, critical erosion stress and settling velocity. This work revealed the existence of two classes of particles (Table 1). A first class was associated to a surface layer of fine particles with mechanical properties close to those of a “fluff layer”. The critical erosion shear stresses in the range 0.025 - 0.05 N m -2 were very similar to those already reported for the resuspension of benthic fluff layers by tidal currents in deep stratified waters (Jago et al. 2002). The second-class had properties similar to cohesive sediments (i.e. critical erosion shear stresses of about 0.2 N m -2 ). The present paper reports a refined analysis of these results aimed at determining the origin and the nature of these layers. EXPERIMENTS The experimental device is a 3.6 m long PVC recirculating flume described by Schaaff et al. (2002). The main channel has a width and height of 40 cm, and the velocity in the channel can be varied between 5 and 40 cm s -1 . During the resuspension experiments, the velocity was continuously monitored with an electro-acoustic currentmeter (ME – MeeresElectronik Gmbh) operating at a frequency of 5 Hz (accuracy 0.2 cm s -1 ). The test section, in the bed of which four PVC rings of 10 cm height containing sediment core samples are inserted, is located 2.1 m downstream of the main entry. 1 Institut de Radioprotection et de Sûreté Nucléaire ; CE Cadarache, Bat 159 ; 13108 St-Paul Lez Durance, France 2 Univ de la Méditerranée, Laboratoire d'Océanographie et de Biogéochimie, 13007 Marseille, France 3 IRPHE-CNRS, Technopôle de Château-Gombert, 49 rue Joliot-Curie, B.P.146, 13384 Marseille Cedex 13, France. fabien.anselmet@irphe.univ-mrs.fr Sediment cores were sampled at three different sites. Rhône1 is at 3 m from the bank in a calm area of the Rhône River. Rhône2 is a more turbulent zone, 10 m away from the bank, at the entrance of the previously quoted calm area. SOFI is a coastal station located on the continental shelf break (43°04N, 5°07E) where cores were sampled at a depth of 160 m. For each of these sites, 8 sediment cores with an internal diameter of 15 cm were collected with a multitube corer (Minicorer Mark VI). The top 9 cm sediment layers of four of these cores were transferred into the test section using a PVC ring, the sediment-water interface being continuously in contact with overlying water. The four other cores were used to analyze water content vertical profiles and grain size distributions. Resuspension was monitored with a turbidity sensor (AANDERAA 3712) located at 30 cm downstream of the test section and 5 cm above the bed. The turbidity was continuously recorded on a computer at 0.2 Hz and related to the water load through suspended matter concentration determined from filtration of water samples collected every 3 minutes. RESULTS Granulometry of the uppermost layer The grain size distribution of the topmost sediment layer was determined by a laser granulometer Malvern Mastersizer. These distributions (Figure 1) point out a difference between Rhône2 and the two other stations (Rhône1 and SOFI). These latter sites are characterized by a decay of the contributions from the finest to the largest grains while Rhône 2 displays an opposite behavior. The φ 50 value is between 10 and 20 μm for Rhône1 and SOFI, and between 40 and 70 μm for Rhône2. These differences are explained by the locations of the sampling areas. Rhône1 and SOFI correspond to calm areas where the deposition of the finest particles is possible. Rhône2 is less calm and therefore less subject to fine particle sedimentation. Water content profiles Several sub samples of sediments were taken, from the top to a depth of 4 cm, from the cores that were not used for the resuspension experiments. Assuming a particle density ρ SS of 2650 kg m -3 , vertical profiles of the water content (p) were calculated from weight loss after freeze-drying. Figure 2 shows that this quantity is larger for Rhône1 than for the two other sites, Rhône2 and SOFI, which are equivalent.