Numerical investigation on critical length of impermeable plate below underwater pipeline under steady current

This study proposes a new approach in which an impermeable plate is placed under the pipeline to prevent the local scour
around the pipeline. In order to understand the performance of this approach, the finite volume method (FVM) and volume of
fluid (VOF) method are adopted to simulate the flow field around the pipeline. The pressure distribution along the sandy bed
surface is obtained by considering the variation of water surface. Furthermore, the effects of water depth, unidirectional and
bidirectional impermeable plates on pressure difference are discussed. The seepage flow field of sandy bed near underwater
pipeline is numerically simulated using the laminar and porous media model. On this basis, the effect of the impermeable plate
length on hydraulic gradient is investigated and the critical length of impermeable plate is obtained. The simulated results show
that when the water depth is smaller than 5.00D (D is the diameter of pipeline), the effect of the water depth on the pressure
difference is remarkable. The pressure differences between two endpoints of both the unidirectional and bidirectional plates
decrease with the increase of the plate length. The variations of the pressure differences for both the unidirectional and bidirectional
plates are similar. With the increase of plate length, the hydraulic gradient decreases and the piping at the seepage exit is
avoided effectively as long as it reaches a certain length. Such a critical length of the plate decreases with the increase of the
water depth. When water depth is larger than 4.00D, the effect of the water depth on the critical length is small. For the same
water depth, the critical length of impermeable plate increases with the increase of the dimensionless flow parameter. Numerical
simulation results are in good agreement with the available experimental measurements.