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This study presents a novel high-resolution simulation of free-surface flow and tracer retention over a streambed with ripples based on varying ripple morphologies, surface hydraulics and the transport of a tracer pulse from surface water to surface dead zone. For the simulations, the computational fluid dynamics (CFD) model OpenFOAM was used to solve the three-dimensional Navier-Stokes equations in combination with an implemented transport equation. Pressure gradients at the streambed were used to account for hyporheic exchange, assuming water flow from high pressure zones to low pressure zones. Flow velocities, ripple sizes and spacing showed to significantly affect these pressure gradients, but also the transport of a passive tracer at the streambed, which was not investigated so far. Due to the velocity field, large parts of the tracer mass were transported alongside the main stream above the ripples. Tracer mass reaching the space between the ripples was temporarily retained due to low velocities and recirculations. It was shown that the retention is depending on the ripple size and space between the ripples as well as on the flow velocity. Decreasing ripple sizes and higher flow velocities lead to a smaller tracer retention. Furthermore we showed that the ripple length to height ratio controls the generation of recirculation zones which affect the residence time of the tracer significantly. Ripple spacing leads to temporarily higher tracer concentration at the streambed, but smaller tracer retention. We conclude that the impact of the streambed morphology on the hydraulics in combination with tracer retention should be addressed for a comprehensive understanding of compound movement, exchange and transformation within the hyporheic zone.

Tube dwelling of chironomids often dominates benthic communities in freshwater ecosystems with high population density and pumping rates. This strongly enhances exchange across the sediment-water interface and impacts biogeochemical processes. Such processes are investigated by tracking the flow initiated by chironomid's pumping through and around burrows using laboratory and computer models. We used modeling and experimental results of other authors considering U-shaped burrows embedded in the sediment to improve process-understanding and prove the plausibility of an integral modeling approach. In contrast to coupled models of pipe (burrow), surface water (overlying water column) and groundwater flow (surrounding sediment), we present a novel high-resolution integral formulation for the porous medium-surface water domain (called porousInter as part of OpenFOAM (Open Field Operation and Manipulation)). This approach solves the extended version of the Navier-Stokes equations allowing simultaneous flow simulation in the burrow, the overlying water column and the surrounding sediment to better account for feedback effects between the sediment and surface water. Using similar model setup as of a coupled approach, we performed scenarios of flow through burrow and sediment triggered by pumping in the center of the burrow. Plausible agreement of our integral model with results of a coupled model and experimental results was obtained when comparing flow patterns around the burrows, between two burrow branches and at burrow inlet and outlet. [ABSTRACT FROM AUTHOR]

An integral approach which can simultaneously model turbulent flow and transport at the sediment–water interface has been recently developed and validated for homogeneous sediment which was achieved by comparing numerical results to flume experiments on flow and transport over a rippled streambed and through the sediment for neutral, gaining, and losing conditions. In the present study, we validated the approach for heterogeneous conditions by comparing numerical simulations of flow and transport in heterogeneous sediment to analytical solutions as well as flume experiments on flow and transport through rippled streambed consisting of heterogeneous sediment. For this complex setup, simulation and experimental results agree well showing that flow and tracer transport prefer paths through areas with bigger grain diameters and higher porosities. The effect of flow redirections under losing and gaining conditions on hyporheic flow and residence times is discussed. Article impact statement: An integral modeling approach is further validated to account for transport processes at the heterogenous sediment–surface water interface. [ABSTRACT FROM AUTHOR]

For the past 70 years, researchers have dealt with the investigation of odour in sewer systems caused by hydrogen sulphide formations and the development of approaches to describe it. The state-of-the-art models are one-dimensional. At the same time, flow and transport phenomena in sewers can be three-dimensional, for example the air flow velocities in circular pipes or flow velocities of water and air in the reach of drop structures. Within the past years, increasing computational capabilities enabled the development of more complex models. This paper uses a three-dimensional two-phase computational fluid dynamics model to describe mass transfer phenomena between the two phases: water and air. The solver has been extended to be capable of accounting account for temperature dependency, the influence of pH value and a conversion to describe simulated air phase concentrations as partial pressure. Its capabilities are being explored in different application examples and its advantages compared to existing models are demonstrated in a highly complex three-dimensional test case. The resulting interH2SFoam solver is a significant step in the direction of describing and analysing H2S emissions in sewers. [ABSTRACT FROM AUTHOR]

The hyporheic zone is the transitional zone between aquifer (groundwater) and river (surface water). It is a band of permeable sediments surrounding a river, where surface water and groundwater mix including river beds, river banks, sediments under dry bars, riparian and floodplain areas. Due to interaction of groundwater and surface water, steep physical and biogeochemical gradients occur in this zone. Many modelling efforts are conducted to address such interactions and quantify fluxes, conservative and reactive transport, residence times of substances and purification effects. In most cases, these are based on using coupling schemes of groundwater flow (using Darcy or Forchheimer law, etc) and surface water flow (shallow water or Navier-Stokes equations). More recently integral approaches have been developed which apply the same model concept (Navier-Stokes equations with turbulence models) for surface water and groundwater flow, however different porosities and resistance terms.This contribution compares an integral solver to a coupled modelling approach focusing on small-scale high resolution hydraulic exchange processes along a rippled streambed. Surface water is first modelled with a Navier-Stokes model (InterFoam solver of OpenFOAM) and the computed pressure head distributions along the rippled streambed serve as boundary conditions for a groundwater flow model (PCSiWaPro using Darcy law). This coupled approach is then compared with an integral approach implemented as porousInter solver in OpenFOAM using the same mesh.We compare velocities, pressure distributions and their hotspots in different vertical sections and along the streambeds. We also check how well the integral solver can resolve small scale recirculation zones in between ripples. Moreover, we discuss computational performance for both approaches. We expect an agreement of both approaches for a certain range of porosities and grain-size diameters, deviations far away from the interface. Further we expect to compute more realistic results with the integral approach in the domain close to the interface, especially where the groundwater flow is out of the range of the Darcy law. [ABSTRACT FROM AUTHOR]

For the past 70 years, researchers have dealt with the investigation of odour in sewer systems caused by hydrogen sulphide formations and the development of approaches to describe it. The state-of-the-art models are one-dimensional. At the same time, flow and transport phenomena in sewers can be three-dimensional, for example the air flow velocities in circular pipes or flow velocities of water and air in the reach of drop structures. Within the past years, increasing computational capabilities enabled the development of more complex models. This paper uses a three-dimensional two-phase computational fluid dynamics model to describe mass transfer phenomena between the two phases: water and air. The solver has been extended to be capable of accounting account for temperature dependency, the influence of pH value and a conversion to describe simulated air phase concentrations as partial pressure. Its capabilities are being explored in different application examples and its advantages compared to existing models are demonstrated in a highly complex three-dimensional test case. The resulting interH2SFoam solver is a significant step in the direction of describing and analysing H 2 S emissions in sewers.