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.

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.

Although both are crucial parts of the hydrological cycle, groundwater and surface water had traditionally been addressed separately. In recent decades, considering them as a single hydrological continuum in light of their continuous interaction has become well established in the scientific community through the development of numerous measurement and experimental techniques. Nevertheless, numerical models, as necessary tools to study a wide range of scenarios and future event predictions, are still based on outdated concepts that consider groundwater and surface water separately. This study compares these “coupled models”, which result from the successive execution of a surface water model and a groundwater model, to a recently developed “integral model”. The integral model uses a single set of equations to model both groundwater and surface water simultaneously, and can account for the continuous interaction at their interface. For comparison, we investigated small-scale flow across a rippled porous streambed. Although we applied identical model domain details and flow conditions, which resulted in very similar water tables and pressure distributions, comparing the integral and coupled models yielded very dissimilar velocity values across the groundwater–surface water interface. These differences highlight the impact of continuous exchange across the interface in the integral model, which imitates such flow processes more realistically than the coupled model. A few decimeters away from the interface, modeled velocity fields are very similar. Since the integral model and the surface water component of the coupled model are both CFD-based (computational fluid dynamics), they require very similar computational resources, namely access to cluster computers. Unfortunately, replacing the surface water component of the coupled model with the widely used shallow water equations model, which indeed would reduce the computational resources required, produces inaccuracy.

Transport processes that lead to exchange of mass between surface water and groundwater play a significant role for the ecological functioning of aquatic systems, for hydrological processes and for biogeochemical transformations. In this study, we present a novel integral modeling approach for flow and transport at the sediment–water interface. The model allows us to simultaneously simulate turbulent surface and subsurface flow and transport with the same conceptual approach. For this purpose, a conservative transport equation was implemented to an existing approach that uses an extended version of the Navier–Stokes equations. Based on previous flume studies which investigated the spreading of a dye tracer under neutral, losing and gaining flow conditions the new solver is validated. Tracer distributions of the experiments are in close agreement with the simulations. The simulated flow paths are significantly affected by in‐ and outflowing groundwater flow. The highest velocities within the sediment are found for losing condition, which leads to shorter residence times compared to neutral and gaining conditions. The largest extent of the hyporheic exchange flow is observed under neutral condition. The new solver can be used for further examinations of cases that are not suitable for the conventional coupled models, for example, if Reynolds numbers are larger than 10. Moreover, results gained with the integral solver provide high‐resolution information on pressure and velocity distributions at the rippled streambed, which can be used to improve flow predictions. This includes the extent of hyporheic exchange under varying ambient groundwater flow conditions.

The hyporheic zone as the transition zone between surface water in streams and groundwater is a key area for the ecological functioning of running waters. Quantities of hyporheic exchange fluxes can help to better understand hydrological, chemical and biological processes at the stream-aquifer interface. This paper presents hyporheic exchange fluxes based on a novel integral modelling approach for the hyporheic zone using the computational fluid dynamics (CFD) model OpenFOAM. Most applications are based on one ‐ way sequential coupled schemes of groundwater and surface water flow. The integral model uses an extended version of the Navier-Stokes equations for the whole domain and accounts for feedbacks from subsurface flow into the surface water domain and vice versa. For the validation of the integral solver, analytical and numerical solutions for seepages through two dams with varying geometries and varying ambient water levels were compared with numerical simulations using the integral flow model. A good agreement was achieved for both test cases. Hyporheic exchange fluxes through immobile ripples with varying morphologies were investigated for different surface hydraulics with high resolution simulations. Turbulent two-phase flow (water and air) was considered to depict water level fluctuations, which influence pressure distributions and consequently can have a high impact on hyporheic exchange fluxes. Simulation results indicated that higher flowrates in surface water as well as distances between the ripples increase hyporheic exchange fluxes. Decreasing ripple sizes reduced hyporheic exchange fluxes and bigger grain sizes enlarged the exchange fluxes drastically.

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.

Rivers are important ecosystems under continuous anthropogenic stresses. The hyporheic zone is a ubiquitous, reactive interface between the main channel and its surrounding sediments along the river network. We elaborate on the main physical, biological, and biogeochemical drivers and processes within the hyporheic zone that have been studied by multiple scientific disciplines for almost half a century. These previous efforts have shown that the hyporheic zone is a modulator for most metabolic stream processes and serves as a refuge and habitat for a diverse range of aquatic organisms. It also exerts a major control on river water quality by increasing the contact time with reactive environments, which in turn results in retention and transformation of nutrients, trace organic compounds, fine suspended particles, and microplastics, among others. The paper showcases the critical importance of hyporheic zones, both from a scientific and an applied perspective, and their role in ecosystem services to answer the question of the manuscript title. It identifies major research gaps in our understanding of hyporheic processes. In conclusion, we highlight the potential of hyporheic restoration to efficiently manage and reactivate ecosystem functions and services in river corridors.

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.

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.

This paper presents three computational fluid dynamics applications regarding multiphase modeling of hydro systems with the open source software OpenFOAM. The first model investigates flow processes of groundwater and surface water using an integral approach which solves the three-dimensional Navier–Stokes equations, extended by the consideration of porosities. For the validation, seepages through homogeneous dams with impervious foundations were compared with analytical and numerical solutions. A further application examines the water–air interface in sewer systems.The focus of the model lies on the description of in-sewer water–air flow and transformation processes, reaeration and hydrogen sulfide emission which highly depend on the three-dimensionality of the hydraulic behavior in the closed duct. A test case analyzing the hydraulic behavior in a sewer stretch showed a good agreement of the numerical results with measured water levels. In the third model, fluid–structure interaction is investigated applying FOAM Extend Project. Calculations of the fluid phase are linked with the solid phase via a coupling algorithm to achieve an equilibrium state. To describe the time-varying position of the fluid boundary, caused by the structural response, dynamic meshes are considered. A technical case, consisting of the air flow around a thin tower as well as a natural case, describing the water flow around aquatic vegetation and its response, were examined.

This paper presents different computational fluid dynamics applications using the multiphase solver interFoam which is implemented in the open source software OpenFOAM. The solver uses the volume of fluid approach. When modelling tracer transport in the proximity of the interface between two phases, the problem of non-physical tracer spreading across the interface has to be overcome. In this paper, two ways are presented to model such systems successfully. First, tracer transport around concrete probes in the headspace of a sewer pilot plant is considered. In this case a two-phase (water-air) system is assumed by describing an idealized water surface as a boundary condition and a passive tracer is applied. Second, flow in a complex sewer stretch containing a hydraulic jump is simulated and a tracer is applied in the water phase. A multiphase transport approach based on the Henry coefficient is used in this case and plausible results are obtained.

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.