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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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    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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    Evaluation of pre- and post-fire flood risk by analytical hierarchy process method: a case study for the 2021 wildfires in Bodrum, Turkey
    (Springer Japan Kk, 2023) Yilmaz, Osman Salih; Akyuz, Dilek Eren; Aksel, Murat; Dikici, Mehmet; Akgul, Mehmet Adil; Yagci, Oral; Balik Sanli, Fusun
    Wildfires are regarded as one of the devastating natural disturbances to natural ecosystems, and threatening the lives of many species. In July 2021, a wildfire took place in the Mediterranean region of Turkey in multiple areas. In Bodrum, a town with high touristic value and attraction, approximately 17,600 hectares of forest have been affected by the wildfire. In this study, the fire-affected areas were determined using an analytical hierarchy process (AHP) and geographical information system (GIS). Rainfall, slope, distance from the stream, pre- and post-fire land use and land cover, elevation, curvature, topographic wetness index, and lithology were selected as the governing variables for the AHP model. The contribution of each variable was determined from the literature. Based on the model, it was found that the area with a very high flood risk increased from 8.6 to 18.4%, implying flood risk in a particular region doubled following the wildfire. Immediately after the forest fire, floods occurred in Mazikoy in the region and its surroundings. The model accuracy was tested by using randomly selected 61 points in and around the flooded area. The model accuracy was quantified by the receiver operating characteristic (ROC) curves method. Pre- and post-fire areas under curve (AUC) values were found 0.925 and 0.933, respectively, which implies that the prediction ability of the model is acceptably accurate. The study revealed that the model could quantify the increased flood risk for vulnerable areas after a forest fire. Such knowledge may aid local authorities in determining the priorities of the precautions that need to be taken after a forest fire.
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    Flow Structures around a Sphere Attached to the Bottom of a Prismatic Sloshing Tank: Problem-oriented basic research
    (Nan Yang Academy of Sciences Pte. Ltd, 2024) Aksel, Murat; Yagci, Oral; Valyrakis, Manousos; Kirca, V. S.Ozgur
    This study aimed to investigate the spatiotemporal variation of hydrodynamic variables around a sphere rigidly fixed to the bottom of a sloshing tank. The experimental measurement of the variations of dynamic variables around a body in a sloshing tank requires non-intrusive measurements that are usually expensive and sometimes inapplicable. Therefore, the numerical model could serve as a cost-effective tool for such problems. A two-stage analysis was con-ducted. In the first stage, an experimental study was carried out in a testing system comprising a water tank with uniaxial freedom of movement constructed on a monorail operated by a computer-controlled step motor. The primary objective of the experiments was to generate reliable data for calibrating the numerical model. During the experiments, the tank’s movements were recorded using an accelerometer and ultrasonic sensors with a sampling frequency of 200 Hz for each. The accelerometer and ultrasonic sensor data were used to impose the motion of the sloshing tank into a Reynolds-Averaged Navier-Stokes (RANS)-based numerical model. The video recordings, which comprised temporal fluctuations of the water surface, were used to calibrate the Model 1. Once the first numerical model was calibrated based on water surface level records using image processing methods, the second numerical model was constructed to accommodate a rigid spherical body with a 17 mm diameter connected to the bottom of the sloshing tank. The initial and boundary conditions used in the second numerical model were identical to those used in the first model to measure the spatiotemporal fluctuations of the surrounding spherical body’s kinematic and dynamic variables, respectively. The findings revealed that sloshing motion exerts a significant impact on the boundary layer separation process around the sphere. It was also witnessed that the stage of the sloshing motion controls the temporal lag between the pressure, velocity and water surface level. © 2024 by the author(s). Published by Nan Yang Academy of Sciences Pte. Ltd.
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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.
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    Scour around tripod/tripile foundations used in offshore wind turbines: use of a finite array of cylinders as tripod piles
    (Springernature, 2025) Yagci, Oral; Telci, Sefa; Celik, Mehmet Furkan; Turker, Umut; Aksel, Murat
    Scour around pier foundations for coastal structures is an inevitable process when these structures are mounted on an erodible bed. Recent findings have indicated that a group of permeable piles offers certain advantages as a supporting structure in marine structures. In line with this vision, in this study we present an experimental investigation of the characteristics of clear water scour at solid cylinder and porous arrays arranged in the form of a tripod. Based on an experimental perspective, the study is motivated by the influence of different solid volume fractions of cylindrical arrays on the scour depth, width, volume, and area. To accomplish this, experiments are conducted using a uniform sand bed, two different tripod configurations, and three different distances for the sides of the tripod forms. The channel in which the experiments were conducted is 26 m long, 1 m wide and 0.85 m deep. The experimental observations indicated that in favorable hydrodynamic form (FHD) layouts, the local scour depth, width, and volume resulting from the isolated upstream cylinder were consistently less than those induced by the downstream side-by-side cylinders. The highest levels of local scour values were seen near the adjacent pairs of cylinders positioned in the downstream direction, in all FHD configurations. Additionally, it has been shown that how many sediment mounds that develop behind the tripod is directly dictated by the number of tripod piles positioned downstream. It was also seen that the formation of local scour holes for each individual pile of the tripod remains autonomous.