The Impact of the Secondary Flow Structures on Shear Stress and Sediment Transport Under Open and Ice-coverage Conditions

Related to climate change, the total coverage of river ice has declined globally, and new river dynamics might cause the unalike morphologic transformation of channel beds in cold regions, especially during the meltdown. The impact of the ice layer on flow structures is still not well-known, even though it has drawn attention recently from various perspectives (e.g. hydrological, morphological).

Turbulent statistics and flow characteristics are monitored to scrutinize the fluctuations of hydrodynamic features under the ice layer. In this study, changes in the high-velocity core distribution related to the morphological conditions of the river bed will be elucidated on the field scale. Many previous researches, especially for ice-coverage cases, are conducted in a laboratory environment without a validation against the field-scale study. The main objective of this study is to clarify unknown fundamentals of natural streams underthe  ice layer by performing high Reynolds number cases with computational methods, and validating them against the field measurements.

Project Objectives:

The major objectives of this study are:

1. define the behavior of primary and secondary flow structures in sharp bends under both open surface and ice-coverage conditions,
2. identify the impact of flow structures on shear stress profiles, and
3. reveal the effect of ice-layer on sediment transport.

Progress:

Field Measurements

Monitoring the flow characteristics in nature is a challenging process due to the many environmental difficulties, such as access issues, extreme weather conditions, and strong currents. A portion of Red River near Lindenwood Park, Fargo, was selected as the study field in this research, and many reasons played a significant role in this preference. Process expediting man-made structures (i.e. dock and bridge) and the existence of a diuturnal ice layer are the main factors behind this selection, in addition to the advantage of easy access throughout the year. Acoustic Doppler Current Profiler (ADCP) is used to measure the velocity components and river morphology in both Winter and Fall seasons. Generally, two main techniques were used to collect the data: (1) stationary measurement for both seasons, (2) moving boat for Fall seasons. While the moving boat technique is aiming to collect bathymetry and cross-sectional data, the stationary technique collects the vertical profile of a certain location. Field measurements are carried out between Aug/2019 to Feb/2022.

Data Processing

1. Three-dimensional Model of the Study Field

Following the field surveys, generating the Digital Terrain Model (DTM) of the river bed with the surrounding area was the next step in this research. Accordingly, the study field was extracted from the LiDAR data of the entire city of Fargo, which was previously provided by the North Dakota Water Commission. Collected river bathymetry during the field expeditions is merged with LiDAR data because the technology of LiDAR is constrained within the land area. Inverse-distance interpolation is then performed to more than 3 million data points into a rectangular zone with approximately 2 km² area. This process is repeated following every measurement in Fall, in order to increase the reliability of the river morphology.

2. Velocity Distribution and Secondary Flow Patterns

Collected data sets were processed to investigate the velocity distribution under ice-covered condition. Time-averaged observations were plotted and compared with the conventional logarithmic law method as well as the recently developed quartic solution approach. Results show that the flow does not follow logarithmic law during the existence of ice cover; however, the quartic solution fits well with the observations if the flow is sufficiently deep. Secondary flow patterns are also calculated using Rozovskii’s rotation method and the cross-stream flow behavior near the banks is discussed to address the sediment transport.

Large-eddy Simulation (LES)

The in-house code (Virtual Flow Simulator) is employed to simulate the turbulent flow in the study area. Multi-physics / multi-phase flows are able to be simulated by the code with advanced turbulence models (LES) over complex topographies. This model is comprehensively validated by the laboratory experiments previously. Various flow rates under open surface and ice-coverage conditions are simulated by using the HPC. During the simulations, computational resources are used from The Center for Computationally Assisted Science and Technology (CCAST) of NDSU and Bridges of Pittsburgh Supercomputing Center (PSC).

Conference/Seminar Presentations:

     Koyuncu, B., and Akintug, B. 2019. Trend Analysis of Evapotranspiration in Northern Cyprus. To be presented at the 2019 European Geosciences Union (EGU) General Assembly, 7-12 April, Vienna, Austria.

     Koyuncu, B., and Le, T. 2020. Secondary Flow Structures in Natural Channels Under Open and Ice Coverage Condition. 2020 Division of Fluid Dynamics (DFD) of American Physical Society, November 22-24 (virtual).

     Koyuncu, B., and Le, T. 2020. Structures of Icy Flows in River Bends. 2020 American Geophysical Union (AGU) Fall Meeting, December 7-11 (virtual).

     Koyuncu, B., and Le, T. 2020. Large-eddy Simulation Under Ice Coverage. 2021 ESIP Winter Meeting, January 26-29 (virtual).

     Le, T. And Koyuncu, B. 2021. Large Eddy Simulation of ice-covered flows in a river bend. 2021 Division of Fluid Dynamics (DFD) of American Physical Society, November 21-23, Phoenix, AZ.

     Koyuncu, B. and Le, T. 2021. On the structures of the ice-covered flow in a bend. 2021 Division of Fluid Dynamics (DFD) of American Physical Society, November 21-23, Phoenix, AZ.

     Koyuncu, B. and Le, T. 2021. Impacts of ice coverage on velocity profiles in a bend. American Geophysical Union (AGU) Fall Meeting, December, 13-17, New Orleans, LA.

     Le, T., Koyuncu, B., Arumugam, D., and Yellavajjala, R. 2021. Modal analysis of surface flows in a river bend using autoencoder. American Geophysical Union (AGU) Fall Meeting, December, 13-17, New Orleans, LA.

     Koyuncu, B. and Le, T. 2022. Flow under ice coverage in the Red River. North Dakota Water Quality Monitoring Conference, March 21-23, Bismarck, ND.