The work evolved between 2015-2020 at the Chair of Water Resources Management and Modeling of Hydrosystems, Institute of Civil Engineering, School VI Plannung Building Environment, Technische Universität Berlin.
Day of scientific discussion: 08 December 2020
While former research studies mainly considered groundwater and surface water separately, the importance of their interactions is nowadays widely acknowledged. Especially the hyporheic zone, which is the zone where stream and shallow groundwater exchange, is addressed in many investigations. This zone is recognized for retention, transformation and attenuation of solutes and enables to improve water quality significantly, while it additionally serves as refuge and habitat for many aquatic organisms.
But even though the importance of groundwater and surface water interactions is nowadays recognized to a large extent, in numerical models both resources are still investigated separately in most cases due to different temporal dimensions. For investigations at the hyporheic zone, flow and transport processes are commonly determined using coupled numerical models. A surface water model and a groundwater model are executed successively, often with no feedback from groundwater to surface water. In contrast to previous research with coupled models, in the prevailing work, processes at the groundwater-surface water interface are investigated with an integral numerical model. Since high computational effort is needed for the application of the integral solver, processes on a small-scale close to the interface of surface water and porous media are focused on. In a first step, the two-phase solver interFoam is extended for the investigation of tracer retention and free surface flow at rippled streambeds. The Navier-Stokes equations are solved in combination with an implemented advection-diffusion equation. The transport of tracer pulses from surface water to dead zones between ripples at the streambed with varying morphologies and different surface hydraulics are examined.
Similar as for the coupled approaches, pressure gradients at the streambed are used to account for hyporheic exchange, assuming surface water moving from high to low pressure zones. It was found out that flow velocities, ripple sizes and spaces between the ripples show to significantly effect pressure gradients at the streambed. Parts of the injected tracer mass are temporarily retained between the ripples due to low velocities and recirculation. In a further step the sediment is included and the same ripple geometries and surface water velocities are assumed as in the previous step. The porousInter solver that extends the interFoam solver for the application in the subsurface is used to determine exchange processes of groundwater and surface water at a small-scale with high resolution. PorousInter solves an extended version of the Navier-Stokes equations and includes porosities as well as grain size diameters within an additional drag term. The validity for groundwater-surface water interactions is first demonstrated using analytical examples. In contrast to the one-way coupled models, the integral model shows the advantage to account for feedback from surface water to the sediment and vice versa and is also applicable in non-Darcy flow areas. In- and outflowing fluxes at the interface of groundwater and surface water are determined for various hydrological and morphological factors. For all investigated cases with sand, non-Darcy-flow occurred in the upper part of the ripple, while for the cases with gravel non-Darcy-flow is observed over several decimetres in depth. Also, a feedback from the sediment to surface water flow is recognized.
Finally, the integral solver is further extended to determine transport processes at the interface. Observations of a conservative dye tracer that was injected into surface water and spread into rippled streambeds inside a flume are compared with modelling results gained with the integral solver. Neutral conditions as well as conditions with up- and downwelling groundwater flow are considered. The results gained with the integral solver show a good agreement with laboratory observations and provide additional information of prevailing flow processes at the interface. For downwelling groundwater flow the highest velocities within the sediment were found, which leads to shorter residence times compared to neutral conditions or upwelling groundwater, while under neutral conditions the hyporheic exchange was the largest.
The main outcome of this thesis is the description, validation and extension of a new integral solver for flow and transport processes at the interface of groundwater and surface water. Simulation results at various rippled streambeds show effects of small-scale topologies, groundwater and surface water velocities and grain sizes on flow and transport processes at the interface. The integral solver can be used for water management practices, e.g. engineering hyporheic zones, but is also applicable for further surface water-porous media interactions as simultaneous flow over and through dikes or breakwaters.