Hindered erosion of granular sediments
Type:
Presented during:
CEDA Dredging Days 2009 - Dredging Tools for the Future, Rotterdam
Authors:
Bisschop F - ARCADIS/Delft University of Technology; Visser PJ and van Rhee C - Delft University of Technology; The Netherlands
Abstract: In case of flooding of a polder, the inundation rate is one of the determining factors in predicting the resulting damage (casualties, damage infrastructure, economic damage, etc.). The inundation rate depends heavily on the flow velocity through the breach in the dike and the development of the breach in time, so on the process of erosion of the dike material. Existing sediment erosion formulae, like the Van Rijn formula (Van Rijn, 1993), overestimate the erosion rate of the breach in case of non-cohesive dike material. The same difference is encountered in dredging practice. Trailing suction hopper dredgers extract sand from the sea bed for reclamation purposes. As in the breaching process, the conventional erosion functions also overestimate the erosion rate during jetting.
Sediment erosion formulae like that of Van Rijn are basically pick-up functions of single particles in case of relative low flow velocities (0.5 tot 2.0 m/s). Flow velocities in dike breaches can increase to approximately 10 m/s. Typical flow velocities during jetting sand using a trailing suction hopper dredger are around 30 to 60 m/s. At these large flow velocities the erosion process is significantly influenced by the properties of the soil mass (non-cohesive particles). Governing parameters at higher flow velocities are dilatancy, permeability and the (un)drained shear strength of the soil. The sediment concentration in the water also influences the erosion process, especially in case of higher erosion rates.
Based on the concept presented by Van Rhee (Van Rhee, 2007) a simple analytical formula is derived that gives a clear insight into the parameters influencing hindered erosion. The concept of hindered erosion is explained by two properties of granular soils: dilatancy and permeability. This implicates that the erosion behaviour of granular soils cannot be described by the behaviour of single particles alone. The properties of the whole soil mass should be considered in predicting erosion at higher flow velocities. Results of a large-scale breach experiment performed in 1994 in the Zwin Channel (the Netherlands) are analyzed to evaluate the formula (Visser, 1998).
Keywords: erosion, granular sediments, breaching, jetting