ABSTRACT
This study proposes a scour equation for abutments that is based on a combination of Newton’s second Law and a turbulence approach. A control volume of the scour hole is considered on which the sum of forces equals nil, i.e., when the changes of the bed are negligible. The effect of abutments on local scour is usually modelled by a empirical K-factor. For this factor still no physical description is available. Here, we deduce a relation between abutment shape and scour by using engineering tools from turbulence science. We pay attention to the scour effects of several types of abutments. The unknowns in the proposed scour equation are calibrated and validated with several scour tests on a small and medium scale.
Scour is a natural phenomenon caused by the erosive action of flow on alluvial beds. Different scour processes can be distinguished, viz. overall degradation, constriction scour, bend scour, confluence scour and local scour. This study investigates the sum of constriction scour and local scour at abutments.
At abutments (e.g. at groins or spur dikes) in rivers, scour is generally concerned to be the main cause of failure of these hydraulic structures. The main scour mechanism is that the presence of this structure induces horse shoe vortices and wake vortices. Horse-shoe vortices and associated down flow can lead to 3–4 times higher bed shear stresses near the nose of a bridge abutment. The scour depth, which determines the risk of failure to a large extent, is determined by complex three-dimensional flow and sediment transport. Numerous empirical and semi-empirical relations have been determined to predict the local scour depth at abutments, often without taking into account the level of turbulence.
To determine scour at abutments a part of the scour hole is considered where the sum of forces equals zero in the equilibrium phase (application of Newton’s second law). The proposed equation for local scour includes a term that predicts the maximum turbulence intensity as function of the abutment shape, which we both calibrated and validated by several flume experiments. This limits the application of the correction factors to standard constructions solely. A more physical based relation would be widely applicable.
