Experimental study and modeling of critical shear stress for cohesive sediment erosion

Yet، the estimation of critical shear stress for cohesive sediment erosion as a function of degree of consolidation and elevation of exposed layer of the bed sediment is under consideration around the world. Sediment shear strength in the vicinity of the interface can be a good delegate of critical shear stress. Because one can relate this parameter to some parameters such as volumetric concentration and effective stress which are function of time and space and can be modeled analytically. Moreover one can measure the coefficients such as fractal Dimension، effective stress coefficient، Creep coefficient، lateral stress coefficient، internal friction angle and cohesion coefficient، using experimental techniques. Some of these coefficients are used to be measured by soil mechanical laboratorial experiment. However for measurinf of fractal dimension and effective stress coefficient، there is not any documented method. In this paper، an experimental method has been developed using a settling cylinder for bed sediments of Sefidrud Dam reservoir. Experimental results have shown that every fractal dimension has always been jointed with specific effective stress coefficient. Moreover the best value for fractal dimension is the mode of its frequency distribution. Therefore the best value for effective stress coefficient is its average between the pairs with the same selected fractal Dimension. In General، critical shear stress is very sensitive to fractal dimension and it has been shown that the less fractal dimension، the less rate of critical shear stress with respect to volumetric concentration.