Abstract
Convential models for the erosion of non-cohesive sediments overestimate the erosion rate induced by high flow velocities. These high flow velocities occur, for instance, in breaching of embankments or dunes (flow velocities up to 10 m/s) or in jetting sand with a trailing suction hopper dredger (30 to 60 m/s). At these very 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 of Van Rhee (2007, 2010) 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 in the Netherlands (Visser, 1998) are analyzed to evaluate the formula.
Highlights
In the Netherlands the safety level of a dike is expressed in terms of risk
The flow velocity in a breach can range up to approximately 10 m/s. Due to these large flow velocities, the application of conventional sediment erosion formulae, like that of Van Rijn (1993), in breach models leads to significant overestimations of the breach growth
Just before high tide a small pilot channel was made in the crest of the sand-dike to ensure breaching near the middle of the Zwin Channel
Summary
Risk is defined as the product of the probability of inundation (failure of the dike) and the expected damage caused by the inundation (Visser, 1998). The process of polder inundation depends heavily on the flow velocity through the breach and the development of the breach width in time. The flow velocity in a breach can range up to approximately 10 m/s. Due to these large flow velocities, the application of conventional sediment erosion formulae, like that of Van Rijn (1993), in breach models leads to significant overestimations of the breach growth. A suction head is trailed over the sea bed and the granular sediment is loosened using water jets. As in the breaching process, the conventional erosion functions overestimate the erosion rate during jetting
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