Multi-Scale Surface Water-Groundwater Interaction
Implications for GroundwaterDischarge Patterns
Time: Fri 2021-11-26 13.00
Subject area: Civil and Architectural Engineering, Hydraulic and Hydrologic Engineering
Doctoral student: Babak Brian Mojarrad , Resurser, energi och infrastruktur
Opponent: Professor Jan Fleckenstein, Helmholz Centre for Environmental Research, Leipzig
Supervisor: Professor Anders Wörman, Resurser, energi och infrastruktur; Universitetslektor Joakim Riml, Resurser, energi och infrastruktur
Rivers and aquifers are continuously exchanging water, driven by processes that occur on various temporal and spatial scales, ranging from small streambed features to large geological structures. The interaction between these two components occurs in permeable sediments below the stream channel, called the hyporheic zone. This zone is an important ecotone in which water, energy, and solutes originating from groundwater and stream water mix. The exchange fluxes through the hyporheic zone are controlled by a distribution of hierarchically nested flow cells of different sizes that are generated by a spectrum of spatial scales of the hydraulic head condition. Thus, a multiscale mathematical approach is required to reach a comprehensive understanding of the hyporheic exchange processes. Therefore, this thesis investigates the roles of regional groundwater flow and hyporheic fluxes in a nested flow system within the streambed sediment. Next, the study assesses the importance of regional and local parameters in generalizing the surface water and groundwater interaction. This division of the top-boundary condition in two scale-intervals of the sub-surface flow is arbitrary but facilitates the analytical procedure. The regional groundwater flow field is evaluated using numerical modeling, accounting for the site-specific landscape morphology and geological heterogeneity of a Swedish boreal catchment. An exact spectral solution is applied to the hyporheic flow with account taken to local streambed topography fluctuation. Combinatorial sampling of the modeled flow data and a Monte Carlo simulation are used in a sensitivity analysis to address the uncertainty in hydrostatic and dynamic head contributions to the hyporheic flow field. Then, the impact of the regional groundwater and the hyporheic flows on the nested flow system in aquatic sediment are studied through superpositioning of the flow fields. This is an efficient approach to analyze the nested flow system because the impact on individual scale intervals can be evaluated separately. Additionally, the impacts of streamflow discharge intensity on hyporheic exchange flow fields are investigated through field investigation. In this study, the hyporheic fluxes velocity at the streambed interface were generally at least one order of magnitude higher than groundwater flow velocity. This reflects the domination of hyporheic fluxes at the streambed interface, leading to significant impacts on the discharge of deeper groundwater through the hyporheic zone. Significant effects were found in flow travel time, direction and discharge areas at the streambed sediment. Thus, the upward groundwater flow contracted near the streambed surface and discharged in a fragmented pinhole pattern at the sediment–water interface. The results also indicated that the magnitude of groundwater flow and the heterogeneity of the subsurface sediment (i.e., the depth decaying hydraulic conductivity of streambed sediment) controlled the depth of hyporheic exchange flow in aquatic sediment. Furthermore, the increased stream flow intensity led to a wide range of hyporheic flow residence times in which temperature was used to evaluate stream segments with gaining and losing conditions.