The Art of Screening: Effectiveness of Silt Screens

One of the potential environmental issues associated with dredging in the marine environment is the increase of suspended sediment concentrations (SSC) by generation and dispersion of sediment plumes. This can be mitigated by source control or by the installation of containment barriers like silt screens. This article focusses on hanging silt screens and describes: 1. The decision process for deployment of hanging silt screens: Decisions on the necessity of environmental mitigation measures and subsequently on the viability of silt screen deployment should be made using a receptor-based approach. This starts with the identification of (ecological) receptors and related impact levels, understanding the local environment, checking compatibility with work methods and determining cost and schedule impacts. 2. Effectiveness of hanging silt screens: Local hydrodynamic and morphological circumstances determine the effectiveness of silt screens. Results of extensive numerical modeling tests (3D and 2DH) supported by hands-on experiences from dredging projects are used to describe the effect of hanging silt screens on the distribution of SSC in the water under different conditions. Results show that when deploying silt screens it is important to realise that silt screens are flexible curtains; they do not block the flow. Therefore suspended sediments generated at a dredging project will always pass the hanging screen vertically and/or horizontally. Silt screens can only reduce the distribution of SSC by settling if local hydrodynamic conditions are favourable. 3. Adaptive management strategies as an alternative for silt screens: When local conditions are not optimal for deployment of silt screens, alternative mitigation measure, such as adaptive monitoring strategies, can be used to manage SSC around dredging projects. Mr Radermacher gratefully acknowledges his co-authors Fokko van der Goot, Project Engineer and Daan Rijks, Senior Project Engineer, both at Royal Boskalis Westminster and Lynyrd de Wit, Project Engineer, Svasek Hydraulics for their collaboration on this study. This article appeared in the Proceedings of WODCON XX, Brussels, Belgium, June 2013 and is published here in a slightly adapted version with the permission of the World Organization of Dredging Associations (WODA). THE ART OF SCREENING: EFFECTIVENESS OF SILT SCREENS M. RADERMACHER, F. VAN DER GOOT, D.C. RIJKS AND L. DE WIT Above: A silt screen being assembled at a reclamation site in Abu Dhabi. To minimise possible impacts of suspended sediments on sensitive receptors, a silt screen – a flexible barrier that diverts current flows containing increased suspended sediment concentrations – was installed. The Art of Screening: Effectiveness of Silt Screens 3 Nomenclature C = Suspended sediment concentration [kg/m] C max = Maximum C in upstream domain [kg/m] C * = Dimensionless C [-] E in = Inflow effectiveness [%] E ref = Reference effectiveness [%] Fr = Froude number [-] P = Environmental impact potential [-] Q rel = Relative discharge [-] U = Depth-averaged flow velocity [m/s] W s = Silt screen width [m] W * = Dimensionless silt screen width [-] g = Gravitational acceleration [m/s] h = Water depth [m] h rel = Relative silt screen height [-] h s = Silt screen height [m] w s = Settling velocity [m/s] z * = Dimensionless z-coordinate [-] θ = Velocity ratio [-] 4 Terra et Aqua | Number 132 | September 2013