The physics and sediment containment phenomenon of a silt screen.
Vu, Thu Trang.
Date of Issue2013
School of Civil and Environmental Engineering
Silt screens have been considered as the “best management practice” to contain or control the dispersal of sediment during dredging and reclamation. Although silt screens have been widely deployed, the effectiveness of silt screens in containing sediment is variable and uncertain, especially when the screen is deployed in open configuration. In this study, the working mechanism of silt screens - i.e. how sediment is redistributed or otherwise contained around a silt screen - was investigated. The hydraulics and trajectories of flow and suspended sediment in the vicinity of a silt screen show that the movement of sediment about a silt screen is driven by three specific hydraulics characteristics. In the upstream retarded zone and downstream eddy zone, the dispersal of sediment is delayed. With the submerged jet-like flow formed in the lower water layer in the lee of the screen, entrainment and transport of sediment might take place. The characteristics of these flow patterns - the principle driver of sediment distribution - are governed by the incident flow velocity, screen penetration ratio, screen types, the vertical configuration of the silt screen and screen deflection angle. Based on the findings of this study, it was deduced that for a silt screen to be effective in sediment containment, the desirable flow conditions must be established. These flow conditions include the following: (1) the upstream flow should be such that settlement takes place readily and (2) as the flow travels through the gap below the screen, the bed materials would not be entrained and transported downstream. Based on the findings of the simulation study on the flow and the trajectories of the tracer particles around the silt screens (using Discrete Particles Module in ANSYS FLUENT (version 11)), four parameters are found to be relevant in the design of a silt screen. They are: (1) the incident flow approaching the silt screen, (2) screen penetration ratio, (3) the length of the screen in the transverse direction, and (4) the distance between the silt screen and the sediment source. The criteria of sediment containment in the screened area have also been established. The findings also revealed several limitations in the prevailing design practice, notable of which is the lack of consideration of important design parameters that influence flow distribution and sediment distribution around a silt screen. With respect to the performance of a silt screen, this study found that with the prevailing design practice, detention of sediment is limited. The study also found that there is a threshold incident flow velocity above which positive containment efficiency could not be achieved
DRNTU::Engineering::Environmental engineering::Environmental protection