Model development and application on coupled hyporheic exchange flux in river-hyporheic zones
Restricted (Penn State Only)
- Author:
- Li, Bing
- Graduate Program:
- Civil Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 29, 2021
- Committee Members:
- Li Li, Major Field Member
Xiaofeng Liu, Chair & Dissertation Advisor
Ying Pan, Outside Unit & Field Member
Lauren McPhillips, Major Field Member
Patrick Fox, Program Head/Chair - Keywords:
- Hydrology
hyporheicFoam
Hyporheic exchange flux
OpenFOAM
Transient storage model
mass transfer coefficient
hyporheic zone
heterogeneous - Abstract:
- Quantifying hyporheic exchange is important for understanding and predicting flow and solute transport in the natural environment, and for accurate accounting of terrestrial carbon budget at a global scale. Hyporheic exchange flow can transport chemical reactants and products which will affect both instantaneous and ultimate distribution of solute within the intimately connected river and hyporheic zones (R-HZs). Many factors affect hyporheic exchange processes, e.g., the flow rate in the channel, the permeability of sediment, and water depth. Although the bedform-driven hyporheic exchange is vital to biogeochemical reaction, a full understanding of those natural processes is still lacking due to the complicated interactions between flow dynamics, chemical reactions, and biology. Investigations of solute transport in rivers have extensively used numerical models. However, modeling such a system is difficult as it involves turbulent flow in river channel, hyporheic flow in sediment, reactive transport of solutes within both domains, and above all the coupling among them. Many of the existing numerical models are incomplete or overly simplified. This thesis work tried to develop a fully coupled three-dimensional (3D) computational model to predictively understand the hyporheic exchange flux (HEF) in the R-HZs. One particular limitation of existing models is that most of them used a sequential, one-way coupling between two domains of the R-HZs, i.e., surface and subsurface. This one-way coupling only considers the effect of the surface domain on the subsurface domain and not vice versa. Thus, it misses the two-way coupling natural of the dynamic system and potentially introduces error and uncertainty in model results. To overcome this, an open-source model, hyporheicFoam, was developed to capture the coupled flow and multicomponent reactive transport processes within both surface and subsurface domains and across their interface. The model capability is illustrated through modeling of both conservative and reactive hyporheic flow and transport through dune bedforms. With the novel coupled model, it is now possible to quantify reactions wherein the reactants and products are constantly exchanging between domains and have feedbacks. hyporheicFoam can simulate large, three-dimensional domain owing to the computational flexibility and power offered by the code structure and parallel design of OpenFOAM, an open source computational physics platform based on which the hyporheicFoam model was built. Applying the fully coupled model to large-scale domains, we investigated the physical meaning of parameters used in a simple transient storage model (TSM). For convenience, solute transport in interconnected rivers and hyporheic zones are typically modeled through dual-domain models where first-order solute mass transfer between the two domains, $\Omega_{R}$ and $\Omega_{HZ}$, is represented by a coefficient alpha. In practice, alpha is determined by fitting the tails of solute tracer breakthrough curves using a TSM. This approach has led to ambiguity regarding alpha's physical meaning and transferability. Here, we established the physical basis for alpha and tested it with the fully coupled hyporheicFoam model for the $\Omega_{R}-\Omega_{HZ}$ system. hyporheicFoam explicitly simulated coupled flow and solute transport over a kilometer of reach length. Model results were analyzed to directly calculate alpha following its theoretical definition. When this calculated alpha is used within a TSM, the TSM is able to reproduce the true solute transport, thereby showing that there is and this is the real physical-based alpha. This thesis works further investigated the effect of sediment heterogeneity on HEFs. A new framework to construct realistic heterogeneous HZs and simulate the flow transport was developed based on Bedformsv4.0 and hyporheicFoam. In particular, the generation of heterogeneous structures starts from the configuration of the particle distribution. Following the hydraulic rules, several parameters of relevance, such as particle size, permeability, height of bed and flow velocity, can be quantified. In this paper, a series of reasonable heterogeneous river-hyporheic zone cases have been created. The sediment beds have the cross-bedding structure, which is common in natural environment. We used the fully coupled model, hyporheicFoam, to solve flow fields and calculate bi-directional HEFs in both heterogeneous and equivalent homogeneous cases. In many previous researches, an equivalent permeability is usually used to represent the heterogeneous bed. The results show that for the equivalent homogeneous cases, the HEF is 25% - 45% larger than that in the heterogeneous case. The upwelling and downwelling fluxes across the sediment-water interface (SWI) are influenced by the global permeability, the pattern of binary field within the HZs, and the local permeability near the SWI. Here, the proposed new tool can easily construct more reasonable small-scale, high-resolution heterogeneous HZs, which will be of great value for future studies. This thesis work was funded by U.S. Department of Energy (DOE), Office of Biological and Environmental Research (BER), as part of BER’s Subsurface Biogeochemical Research Program (SBR), under award DE-SC0018042. The findings and conclusions do not necessarily reflect the view of the funding agency.