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    A comparative analysis of coherent structures around a pile over rigid-bed and scoured-bottom
    (Pergamon-Elsevier Science Ltd, 2021) Aksel, Murat; Yagci, Oral; Kirca, V. S. Ozgur; Erdog, Eryilmaz; Heidari, Naghmeh
    Environmentally friendly piled coastal defence structures gained a particular significance in recent years, given these structures do not interrupt the longshore/cross-shore sediment transport. Flow around an emergent pile is known to generate certain typical coherent flow patterns depending on the flow and boundary conditions, as well as geometry. When the bed is erodible, the scouring process, which typically takes place around the pile, significantly alters these generated coherent structures. The primary objective of this study is to understand the influence of the presence of a scour-hole around the pile on the coherent flow structures, time-averaged kinematic characteristics (i.e. mean flow and turbulence) as well as dynamic pressure characteristics. To achieve this objective, a three-dimensional numerical model was utilized. The model that solves the flow around the emergent pile on rigid-bed was calibrated by using a spatially and temporally high-resolution experimental dataset. Later on, high-resolution DEM data of a scoured-bottom from flume experiments were imported into the calibrated model, and the model was run with identical hydraulic conditions with the experiments. Then, the outputs of the two models, with rigid-bed and with scoured-bottom conditions, were comparatively analysed. It was seen that as a consequence of the presence of a scour-hole, lee-wake vortices were dramatically weakened behind the cylinder. Instead of that, a couple of pronounced large-scale counter-rotating streamwise vortices (LSCSV) behind the cylinder dominate the flow domain to a great extent. Results suggest that the presence of a scour-hole on the bed renders the pile more streamlined against the flow.
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    A numerical study of the flow patterns around midchannel islands in lowland rivers and their possible biogeomorphological impacts
    (2023) Heıdarı, Naghmeh; Heidari, Naghmeh; Yagcı, Oral; Valyrakis, Manousos
    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.
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    Basin-scale factors in emergence of midchannel islands: a process-based morphometric analysis
    (Springer, 2023) Heidari, Naghmeh; Yagci, Oral; Aksel, Murat
    Midchannel islands were conventionally considered undesirable additional hydraulic resistance to flow for many years and tended to be removed from the channels as a measure against flood. Today, it is known that these geomorphic units provide key eco-geomorphological services in the active corridor of natural waterways. This study examined the dynamics behind the occurrence mechanism of midchannel islands considering basin-scale (i.e., land use, elevation, slope, form factor, and relief) and fluvial (i.e., sediment yield) variables. Eleven drainage basins were selected as samples from the different ecoregions on Earth. Morphometric and hydrological characteristics were determined using hydrological datasets and satellite-based images by means of ArcGIS. Sediment delivery magnitudes for each examined basin were calculated based on an empirical expression. In addition, the development process of isolated midchannel islands was investigated by quantifying their planform morphometric features. The basin-scale analysis showed that the channel slope, i.e., gravity-induced factors, controls the emergence location of MCIs to a great extent, and the basin sediment yield impacts the density (i.e., number of islands) of MCIs in a unit length of the river. Furthermore, the morphometric analysis of MCIs in the individual scale revealed that when MCIs develop from the preliminary stage to the advanced stage, the contraction effect becomes more prominent which increases the streamlining effect.
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    Midchannel islands in lowland river corridors and their impacts on flow structure and morphology:A numerical based conceptual analysis
    (Elsevier, 2021) Heidari, Naghmeh; Yagci, Oral; Aksel, Murat
    Midchannel islands (MCIs) are influential geomorphic units in regulating morphological and ecological dynamics in sand-dominated rivers. The primary objectives of the study were to understand a) the relationship between morphometric characteristics of the basin and the occurrence location of the MCIs, b) the role of MCIs on the hydraulic gradient and the other hydraulic variables along the reach, and c) the influence of island growth process on hydrodynamics in the wake and contraction regions. Spatial distributions of MCIs depending on the morphometric features of the three characteristic basins were considered. While the basin-scale analysis was conducted using GIS-based data, the reach and individual scale analyses were performed based on a Reynoldsaveraged Navier-Stokes (RANS) model. Uniform circular cylinder and island geometries were exposed to identical flow conditions. The expansion of these island geometries was mimicked both in rectangular and trapezoidal cross-sections. The variation of water surface slope and bottom shear stress variables, which greatly trigger the intensive bilateral interaction between hydrological-morphological-ecological elements in river corridors, depending on the growth of the island, was determined. The role of imposed island geometry on the secondary flow structure within the downstream cross-section of the island and streamwise velocity fluctuations in contraction regions were analyzed. The basin-scale analysis showed that with decreasing basin slopes, the family of islands emerged at a longer distance to the basin outlet. It was also seen that MCIs location is closer to the basin outlet forbasins with a lower aspect ratio. Besides, asymmetrical large-scale counter-rotating streamwise vortices were detected behind the MCIs based on numerical simulations. GIS-based data showed that this coherent flow structure brought about channel adjustments in downstream of the island. In addition to morphological consequences, it was hypothesized that this alteration in the flow structure due to MCI has potential impacts on riverine ecology. These impacts are increase in groundwater recharge, vertical exchange of surface-subsurface water, local heterogeneity in sediment characteristics (i.e., convenient fish spawning areas), and enlargement in the hyporheic zone.
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    Morphodynamic Controls on Thermal Plume Dispersion at River Mouths: Insights from Field Data and Numerical Modeling
    (Mdpi, 2025) Heidari, Naghmeh; Aksel, Murat; Yagci, Oral; Erbisim, Mehmet Yusuf; Cokgor, Sevket; Valyrakis, Manousos
    Thermal discharge from power plants causes significant concerns in aquatic environments. The purpose of this study is to evaluate how river mouth morphodynamics, particularly spit development and removal, influence the dispersion of thermal plumes. To achieve this, a case study was carried out at a coastal power plant in southwest T & uuml;rkiye, where thermal effluent is conveyed to the sea through a low-flow river. Field measurements combined with numerical modeling were used to analyze plume dynamics under varying spit configurations. Results revealed that the evolution of a spit on one side of the river mouth influences plume dispersion and redirects the mixing zone toward the opposite shoreline. Numerical simulations demonstrated that spit development reduces dispersion efficiency (by over 75%), while the physical removal of the spit significantly improves it, reducing temperature excess from 4-5 degrees C to 0-1 degrees C within the mixing zone, meeting safe environmental standards. The findings highlight the pivotal role of morphological changes in governing thermal discharge behavior and emphasize the importance of continuous monitoring and management strategies, such as periodic dredging, to ensure compliance with environmental regulations.