A numerical study of the flow patterns around midchannel islands in lowland rivers and their possible biogeomorphological impacts

Abstract

Midchannel islands (MCIs) are instream geomorphic units generally emerge in lowland rivers. Despite their significant ecomorphological services in the river ecosystem, the flow patterns around these self-forming riparian landforms and their impacts on aquatic life are not fully understood yet. Understanding the flow pattern around these formations enables practitioners to produce cost-effective, sustainable, and eco-friendly river management projects and strategies, forming the motivation of this study. Herein, the secondary flow pattern around MCIs was analyzed by employing a RANS-based numerical model. Flow around the simplified bodies was simulated to give a more precise analysis regarding flow-island interactions. Once the numerical validation process was completed for the cylinder using an experimental dataset, the validated model was implemented for islands (streamlined island, vertically sloped island (VSI), and realistically sloped island (RSI)). Analysis of the model results revealed the following key findings: 1) the RSI acted like a streamlined object and produced weaker lee-wake vortices with a longer recovery distance compared to the streamlined island and the VSI, 2) the RSI gained a better-streamlined form near the bed than near the water surface due to enhanced elongation, 3) this situation in the RSI case generated highly variable flow patterns along the depth behind the MCI, and 4) due to the three-dimensional geometry of the RSI, the generated large-scale vortices propagated asymmetrically towards the sides of the channel rather than remaining around the centerline.