Modeling of Flow and Sediment Transport at a River Confluence with the EnSed2D model

摘要

This paper summarized the results of computational modeling study for the confluence of the Kankakee and the Iroquois Rivers. This project aims to study the effectiveness of engineering alternatives on reducing sedimentation at the confluence. The hydraulics and sediment transport patterns of three management scenarios, which are keeping natural situation without engineering structures, constructing three short dikes at the left banks of the Kankakee River, and constructing three longer dikes at the left banks are studied by applying the EnSed2D model. rnThe sediment transported in the Kankakee and the Iroquois Rivers are primarily suspended sediment. Channel bed has a thin layer of bed material, and occasionally bed rocks are exposed. Therefore, this study focused on the simulation of suspended sediment transport in the system. Two methods were applied to simulating suspended sediment deposition and erosion processes. One method assumes that bed material layer is too thin to allow suspended sediment concentration reach equilibrium at the bottom that only deposition occurs, the other method assumes there is a sufficient amount of sediment that can be entrained from channel bed so that the change of bed elevation is the difference between the rate of deposition and entrainment. rnThe simulated results showed that if there is no entrainment, there is no scour in front of the dikes, while if there is an entrainment, the scouring in front of dikes are very apparent. In case of no construction methods, the deposition at the confluence will spread all over the confluence as well as its immediate downstream. The construction of three short dikes will reduce the deposition of suspended sediment at the confluence and facilitate the passage of suspended sediment from the Iroquois River to the Kankakee River. But, the increasing of dike lengths will potentially block flow from the Iroquois River to the Kankakee River, and worsen deposition at the confluence. Therefore, the results of this study recommended that dikes with a reasonable length could be the most cost-effective alternative to reduce sedimentation at the confluence. The locations, alignments, and dimensions of these dikes should be determined through another detailed computational modeling study. To insure the mechanical stability and minimize the negative environmental effect of these dikes, flow hydrodynamics and sediment transport at the near dike region should be investigated by applying an advanced computational model or conducting physical laboratory experiment.