Exchange processes of surface and groundwater are important for the management of water quantity and quality as well as for the ecological functioning. In contrast to most numerical simulations using coupled models to investigate these processes, we present a novel integral formulation for the sediment-water-interface. The computational fluid dynamics (CFD) model OpenFOAM was used to solve an extended version of the three-dimensional Navier–Stokes equations which is also applicable in non-Darcy-flow layers. Simulations were conducted to determine the influence of ripple morphologies and surface hydraulics on the flow processes within the hyporheic zone for a sandy and for a gravel sediment. In- and outflowing exchange fluxes along a ripple were determined for each case. The results indicate that larger grain size diameters, as well as ripple distances, increased hyporheic exchange fluxes significantly. For higher ripple dimensions, no clear relationship to hyporheic exchange was found. Larger ripple lengths decreased the hyporheic exchange fluxes due to less turbulence between the ripples. For all cases with sand, non-Darcy-flow was observed at an upper layer of the ripple, whereas for gravel non-Darcy-flow was recognized nearly down to the bottom boundary. Moreover, the sediment grain sizes influenced also the surface water flow significantly.

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Computational fluid dynamics (CFD) is gaining an increasing importance in the field of hydraulic engineering. This publication presents different application examples of a two-phase approach as implemented in the open source software OpenFOAM. The chosen approach is based on the volume of fluid method focusing on the simulation of flow in closed conduits.Three examples are presented: single-phase flow over a ground sill and free surface flow over a hill as well as complex free surface flow in a sewer model. The first example compares the results of different RANS turbulence models with experimental results. The results of the second example are compared with an analytical solution. In the last example the behaviour of the free surface flow is compared with the results of a model test and existing simulations using a simplified, open channel geometry for the closed conduit. For the examples analysed, the two-phase approach provides stable and reliable results.

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 threedimensional 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.

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.

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