Erosion D'une Vase De L'estuaire De La Loire Sous L'action Du Courant

摘要

from measurements of the hydraulic pressure loss. Finally, the validity of Eq. 5 has been confirmed by five calibration tests on the incipient of the movement of sands for which results are compared with Shields diagram in Fig. 2. The observed erosion mechanisms affecting cohesive sediments depend mainly on the value of the yield stress. For a fluid mud (τ_y less than 3 N m~(-2)), the bed shear stress produces at first a wavy motion on the bed surface with a progressive undulation. When the shear stress increases, resuspension produces a diluted sediment cloud which is entrained and dispersed by the flow. For a plastic mud (τ_y greater than 3 N m~(-2)), erosion occurs by a wrenching of aggregates which are transported near the bottom. Initially, the eroded aggregates measure several millimetres in size; but, once transported, aggregates break into very rigid and compact aggregates of maximum size close to 1 mm in all the cases. During erosion tests, erosion volumetric rates E_v have been evaluated under steady-state bed shear stress τ_o from the observed variation in time of suspended sediment concentration. A generalized erosion is assumed for E_v≥ E_(vo)= 3 × 10~(-7) m s~(-1) (that is the equivalent of 1 mm sediment layer eroded per hour). The generalized erosion occurs above a critical bed shear stress τ_(oe) which is linked to τ_y and mud density through Eq. 7. A phenomenological law defined by Eq. 8 is proposed to calculate the erosion rate as a function of yield stress and hydrodynamic shear stress at the bottom. For τ_o< τ_(oe), a small erosion is observed, which is described by a formulation given by Cerco et al. (Water quality model of Florida Bay. U.S. Army Engineer Research and Development Center, ERDC/EL TR-00-10, Vicksburg, USA, 260 p, 2000). Above τ_(oe), a small increase in τ_o produces an important erosion rate rise which is described by a formulation suggested by Mehta and Par-theniades (Resuspension of deposited cohesive sediment beds. In: Proceedings of the 18th coastal engineering conference, Cape Town, South Africa, 2:1569-1588, 1982). Figure 3 shows as a function of τ_o the observed values of E_v (discrete symbols) as well as the proposed model (in solid lines) for the six concentrations. For the theoretically maximum value of E_v, the hydrodynamic shear stress is very high with regard to the sediment cohesion; and then, the water flux produces an entrainment of underlying fluid mud layers. The asymptotic law obtained from the model of Kranenburg and Winterwerp (1997) plotted in dashed lines in Fig. 3, is expressed by E_v= 0.3 u_*. A sediment trap inserted in the experimental system allows a sample of eroded mud aggregates to be obtained. It is observed that the maximum value of the diameter D_M of the eroded aggregates depends on the density and yield stress of the initially deposited mud according to Eq. 10. In the same way, the density of the aggregates issued from plastic mud erosion is measured following an original experimental method and procedure (Table 3). The erosion of plastic muds with a concentration from 310 to 420 kg m~(3) produces aggregates with a concentration close to 400 kg m~(-3) and yield stress a little greater than 100 N m~(-2).%Le phénomène d'érosion des vases sous l'action d'un courant est abordé expérimentalement. Cette étude comporte six séries d'essais d'érosion réalisées en laboratoire sur une vase de Loire pour six concentrations différentes. Pour chaque concentration, trois essais successifs ont été effectués afin de pouvoir analyser les résultats statistiquement. La rigidité initiale et la concentration des vases sont mesurées en suivant un protocole d'essai. Des lois phénoménologiques sont proposées pour relier le taux d'érosion mesuré à la rigidité initiale du sédiment et à la contrainte hydrodynamique. Un piège à sédiments intégré dans le dispositif d'essai, permet d'obtenir un échantillon de matériau érodé. La taille des copeaux de vase érodés et leur densité sont mesurées grâce à une technique expérimentale originale. Une expression mathématique est ainsi avancée pour relier la taille des copeaux à la densité et à la rigidité initiale du dépôt dont ils proviennent.