Effectiveness of Silt Screens

Dredging and reclamation works are known to generate significant amounts of turbidity. In most coastal zones around the world, the resulting clouds of suspended fine sediment bring along environmental risks. Dredging contractors take mitigating measures to prevent free spreading of turbidity, which is usually also demanded by their clients and the local authorities. One possibility is the application of hanging silt screens, which are flexible, virtually impermeable screens. They are intended to divert the sediment-laden current towards the opening between the screen's lower edge and the bottom, which is thought to result in rapid settling of the dredging spill. The effectiveness of hanging silt screens when subjected to cross flow is doubted by those parties experienced with their application, because sediment can still pass the screen by horizontal or vertical diversion (around the screen’s side edges and below the screen’s lower edge respectively). Hence this research aims to obtain insight into the most important processes determining transport of suspended sediment in the vicinity of hanging silt screens and to determine the effectiveness of hanging silt screens under varying circumstances. A large series of numerical simulations with varying flow velocity, screen height, screen width, settling velocity of the sediment, water depth and upstream sediment concentration profile is conducted. The process of vertical diversion is investigated in a 3D modeling approach using large eddy simulation (LES). Performance of this numerical model is validated by means of laboratory experiments. The process of horizontal diversion is assessed with a two-dimensional horizontal (2DH) modelling approach. It appears that the turbulent structures resulting from flow separation at the screen’s lower edge give rise to intense downstream mixing. This process prevents the screen from being effective when compared to a reference situation (without a screen) in terms of reducing the environmental threat posed by suspended fine sediment. The figure illustrates this finding. Furthermore, significant amounts of horizontal flow diversion occur when lateral restrictions are absent. This counteracts the intended use of silt screens as a vertical current deflector. Finally, flow contraction around the screen’s lower edge causes high near-bed flow velocities, which enhances erosion if erodible bed material is available. Efforts should be made to convince all parties involved in dredging practice of these findings, so that protection of the marine environment can make another step forward.Environmental Fluid DynamicsHydraulic EngineeringCivil Engineering and Geoscience