Scour Protection of Partially Embedded Pipelines Using Sloping Curtains

A geotextile mattress with sloping curtain (GMSC) was introduced to protect underwater pipelines from scour in steady currents. GMSC is a newly developed countermeasure against river and estuarine scour. To better understand the beneficial effects of GMSC on seepage flow under pipelines, dye was injected to determine the hydraulic gradients across the pipeline in a series of verification tests. Hydraulic gradients across the pipeline with and without a GMSC were quantified for different curtain angles, curtain heights of the GMSC, and distances between the pipeline and the GMSC, and installing GMSCs with or without sand-pass openings, which are openings to allow bottom flow with a high concentration of sediment to pass. The results of the experiments show that a GMSC is capable of protecting a pipeline against scour in a steady current by effectively reducing the hydraulic gradient in the bed soil below the pipeline. The hydraulic gradient across the pipeline decreases nonlinearly with the increase of curtain height and angle of the GMSC. The effect of sand-pass openings on the hydraulic gradient across the pipeline is not significant. The hydraulic gradient decreases nonlinearly with decreasing distance between the pipeline and the GMSC. When the distance decreases below a critical point, the seepage flow under the pipeline reverses. The GMSC may provide an optimum protection effect on the pipeline when the distance between the pipeline and the GMSC is about 6 times the obstacle height of the GMSC ($H_c\sin \alpha$) by reducing the flow velocity approaching the pipeline close to zero.