Development of scour in non-cohesive sediment under a poorly erodible top layer

The rivers in the South-West of The Netherlands flow over an erosion-resistant bed, consisting of peat and clay, overlying non-cohesive sediments. As they are experiencing general incision, the erosion resistant layer becomes thinner and sometimes locally breaks, exposing the more easily eroded non-cohesive sediments. Consequently sudden scour holes may be formed that can pose risks to the stability of nearby flood defences. Mobile bed laboratory experiments were carried out to study the corresponding scour process, using a local opening in steel plates to represent a broken erosion-resistant top layer. I found that once the scour hole extends to the edge of the top layer, this layer is undermined. As a result, the sediment is only transported in suspension and although the turbulence within the scour hole is increasing, the rate of scour decreases. With the stiff steel plates at a fixed location the scour hole approaches an equilibrium depth. However, undermining in the field, will eventually lead to failure of the top layer and consequent growth of the horizontal extent of the scour hole. But not all scour holes in the field are growing, indicating other processes. This raised the assumption that the poorly erodible layer can also fail by deformation and covers the slopes of the scour hole; thus functions as a falling apron. Numerical computations with FINEL3D showed the shear stresses at bed of the scour hole. High shear stresses were observed at the edges and inner slopes of the scour hole. These shear stresses exceeded the critical shear stress of sand, which indicated that the slope are protected by the poorly erodible layer. Field research has to be carried out to validate the models and the proposition of the behaviour of the poorly erodible top layer.

Subject

Scour
Laboratoy experiments
Poorly erodible layer
Undermining
Suspended transport
Shear stress
Turbulence models
FINEL3D

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Student theses

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master thesis

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