ABSTRACT
Three factors can be distinguished in Equation 1: geometry, scale and resistance. (1) The geometry factor depends on the ratio between thickness of the sand layer and seepage length, and can be determined using a numerical groundwater model (MSeep). (2) The scale factor determines how the seepage length influences the critical gradient. The critical gradient decreases with increasing seepage length, due to this length scale effect. The physics that causes this length scale effect are still not completely clear, but the effect has been confirmed by various experiments (Silvis 1991; van Beek et al. in prep.). A possible explanation can be the singularity that exists in the flow equation at the outflow point. In theory, this results in gradients that rise to infinity close to the outflow point. However, calculation of the average gradient over some grains results in a finite value of the gradient. The average gradient over a certain fixed length will be higher in case of a larger seepage length. This scale effect has a significant impact for scale model tests. It means that it is not possible to use a scaled model that will have the same critical gradient as can be expected in the field. (3) The resistance factor is related to the equilibrium of forces on the grains in the channel. The degree of resistance of grains against rolling can be adjusted by changing the bedding angle. Although, the relationship between bedding angle and sand characteristics has not yet been established, the value of the bedding angle has been calibrated in large-scale experiments (Silvis 1991).
