A numerical study on scaling relations for field-scale scour development at pile foundations

Local erosion around foundation elements, generally referred to as ’scour’, is of major concern when designing hydraulic structures. When placed in natural open water systems, these structures influence the local dynamics of the flow and bed, which could result in lowering of the embedment level, with potentially severe consequences. The majority of knowledge on scour originates from laboratory experiments, which are inevitably associated with scaling conflicts. The absence of full-scale scour development data might be complemented with results from numerical modelling on typical field scale dimensions. In this study, a numerical model was developed making use of Large Eddy Simulation to compute the complex three-dimensional flow field around the foundation structure and its interaction with the erodible bed. The model was able to predict scour time development from experimental data within 10% accuracy. Furthermore, the model results show consistency in scour depth behaviour as a function of flow intensity, ratio between pile size and median grain diameter and pile based Froude number. Scour time development curves for simulations on laboratory scale and on field scale dimensions are compared, showing a reduction in relative scour depth on prototype scales. The results demonstrate that these differences can be attributed to differences in pile-based Froude number and the ratio between pile size and grain diameter, although a full separation of individual parameter effects is difficult due to their interrelation.