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Renewable energy system development and improved operation can mitigate climate change. In many regions, hydropower is called to counterbalance the temporal variability of intermittent renewables like solar and wind. However, using hydropower to integrate these renewables can affect aquatic ecosystems and increase cross-sectoral water conflicts. We develop and apply an artificial intelligence-assisted multisector design framework in Ghana, which shows how hydropower’s flexibility alone could enable expanding intermittent renewables by 38% but would increase sub-daily Volta River flow variability by up to 22 times compared to historical baseload hydropower operations. This would damage river ecosystems and reduce agricultural sector revenues by US$169 million per year. A diversified investment strategy identified using the proposed framework, including intermittent renewables, bioenergy, transmission lines and strategic hydropower re-operation could reduce sub-daily flow variability and enhance agricultural performance while meeting future national energy service goals and reducing CO2 emissions. The tool supports national climate planning instruments such as nationally determined contributions (NDCs) by steering towards diversified and efficient power systems and highlighting their sectoral and emission trade-offs and synergies.

Sustainable urban drainage systems are multi-functional nature-based solutions that can facilitate flood management in urban catchments while improving stormwater runoff quality. Traditionally, the evaluation of the performance of sustainable drainage infrastructure has been limited to a narrow set of design objectives to simplify their implementation and decision-making process. In this study, the spatial design of sustainable urban drainage systems is optimized considering five objective functions, including minimization of flood volume, flood duration, average peak runoff, total suspended solids, and capital cost. This allows selecting an ensemble of admissible portfolios that best trade-off capital costs and the other important urban drainage services. The impact of the average surface slope of the urban catchment on the optimal design solutions is discussed in terms of spatial distribution of sustainable drainage types. Results show that different subcatchment slopes result in non-uniform distributional designs of sustainable urban drainage systems, with higher capital costs and larger surface areas of green assets associated with steeper slopes. This has two implications. First, urban areas with different surface slopes should not have a one-size-fits-all design policy. Second, spatial equality must be taken into account when applying optimization models to urban subcatchments with different surface slopes to avoid unequal distribution of environmental and human health co-benefits associated with green drainage infrastructure.

In this paper, the meshless element-free Galerkin method (EFGM) based coupled model is developed to simulate the groundwater flow and multispecies reactive transport coupled with sequential decay reactions in unconfined aquifers. The use of moving least squares (MLS) approximation in approximating the head and concentration variables in EFGM tends to provide the solutions with better numerically stability than the finite difference (FDM) and finite element methods (FEM). Therefore, the proposed model is less affected by numerical dispersion unlike FDM/FEM based models. For the validation of the proposed model, it is applied to multispecies transport equations coupled with sequential reactions, reversible reactions, spatially or temporally varying transport velocities, time dependent decay rates and species concentration at the boundaries. EFGM solution is matching very well with the available analytical solutions for all the selected problems. However, FEM model fails to converge to the analytical solution for the selected problems with the time dependent transport coefficients. Finally, the developed EFGM coupled model is used to simulate groundwater flow and transport of chlorinated solvents in hypothetical and synthetic field unconfined aquifers. The EFGM solutions are compared with those from MODFLOW-RT3D simulations. The good comparison between both solutions indicates the applicability of the proposed model to multispecies transport.

The application of optimisation approaches to the spatial design of urban water systems can result in unequal distributions of infrastructure services among different communities unless spatial equity objectives are explicitly considered. Differences between urban communities can be exacerbated if the co-benefits of water infrastructure are not properly distributed over the urban catchment. This is especially important when designing sustainable urban drainage systems, which carry several co-benefits in addition to the attenuation of stormwater runoff, such as the improvement of urban landscape and mental health of the residents. In this work, we propose a new multi-objective optimisation framework that takes into account both cost-effectiveness and socio-economic aspects of urban drainage infrastructure design. The proposed framework considers the minimisation of capital costs, average flood damages, and total suspended solids as design objectives, as well as the minimisation of inequalities in the spatial distribution of flood damages and green infrastructure benefits quantified via appropriate equity metrics. A population-based multi-objective optimisation method is linked to a hydrologic-hydraulic model to determine a set of Pareto-optimal design portfolios associated with different trade-offs between traditional drainage design objectives and social goals. The proposed multi-criteria design approach was applied to a synthetic case study where sustainable urban drainage systems are used to expand the drainage capacity of an existing pipe network. The results demonstrate the applicability of the proposed methodology to the design of urban drainage systems and provide insights into the trade-offs between overall cost-effectiveness and social equity in urban infrastructure design decisions.

The design of urban drainage systems has traditionally aimed for cost and service objectives without considering broader socio-economic implications of design decisions. This can result in designs with unequal distributions of infrastructure services among urban communities. In this paper, we propose a design approach for multi-criteria design of urban drainage systems which combines traditional design objectives, such as reliability and cost-effectiveness, with social goals like reducing inequality in the spatial distribution of infrastructure benefits. A multi-dimensional search algorithm is linked to an urban rainfall-runoff simulation model to identify portfolios of efficient (Pareto-optimal) and spatially equitable drainage infrastructure developments. A measure of the difference in flood damage between different subregions is also considered as a decision-making aide along with other criteria. An illustrative case study shows how the design framework can help planners make trade-offs, for example between spatial equity and cost-effectiveness, when selecting future interventions in urban water systems. The results imply that traditional optimisation models can lead to inequity in spatial distribution of urban drainage infrastructure services and that equality in spatial distribution of green drainage assets does not necessarily entail fair spatial distribution of flood damage between different urban neighbourhoods.

Multi-objective optimization can help identify efficient and appealing designs of urban drainage systems. However, their application to large-scale problems is hindered by the computational cost of urban drainage simulation. We propose a novel disaggregation approach that allows simulating a portion of a drainage network while the remaining part is represented by a surrogate model that maps changes in the region of interest to hydraulic head time-series at synthetic nodes shared with the remaining part of the network. The proposed approach is demonstrated with an application to the many-objective optimization of sustainable urban drainage systems in two urban areas. The design problem’s decision variables include the types of sustainable drainage systems, their combination within a subcatchment, their surface areas and spatial distribution, whereas the objectives include the minimization of capital cost, flood volume, flood duration, and total suspended solids or average peak runoff. The results show that the proposed disaggregation-emulation approach can provide an accurate representation of the system dynamics while significantly reducing the computational time compared to a model that simulates the whole network dynamics. Two alternative surrogate models are considered based on multilayer perceptron (MLP) and generalized regression neural networks (GRNN). MLP is found to be more accurate compared to GRNN at the cost of a larger computational time for the training process.

Numerical simulation of groundwater flows in real aquifers has been commonly based on Finite Difference (FDM) and Finite Element Methods (FEM). These mesh-based methods require a computational grid, which can be costly to generate in case of complex geometries and can significantly increase the computational cost when adaptive remeshing is required. This work presents the Element-Free Galerkin Method (EFGM) for solution of the groundwater equation and demonstrates its application to a real unconfined sloping aquifer. EFGM uses the Moving Least Square (MLS) method for approximating the unknown head and computing the EFGM shape function, leading to increased accuracy, stability in function approximation and higher convergence rate than mesh-based methods. In the present study, EFGM is initially applied to one- and two-dimensional hypothetical problems, and validated with analytical solutions. Subsequently, EFGM is applied to the simulation of groundwater flow in the Blue Lake Aquifer (BLA) in North California, and the simulation results are compared with those obtained using MODFLOW. The results demonstrate the potential of EFGM for analyzing real aquifer problems.

The design of conventional and sustainable urban drainage systems is a complex task that requires consideration of several design objectives and decision variables. Simulation-based optimization models allow exploring the decision space and identify design options that best meet the design criteria. However, existing approaches generally require simulation of the system hydraulics for each function evaluation, which leads to prohibitive computational cost when applied to large drainage networks.In this work, a disaggregation-emulation approach is proposed which allows sequential optimization of multiple sub-catchments in an urban area without having to simulate the full system dynamics. This is achieved by using artificial neural networks (ANN) to represent the boundary condition at the interface between neighboring sub-catchments. The approach is demonstrated with an application to a many-objective optimization problem in which sustainable drainage systems are used to expand the capacity of an existing drainage network. The evaluation of the objective function using the emulation model is found to be 22 times faster than using the physically based model, resulting in a significant speed-up of the optimization process. Unlike previously proposed optimization approaches that rely on surrogate models to emulate the objective functions, the proposed approach remains physically based for the individual sub-catchments, thus reducing the chance of bias in the optimization results.

The use of optimization approaches for designing in-situ groundwater bioremediation systems has been demonstrated in a number of previous studies under the assumption of homogenous soil. However, in real applications the soil is typically heterogeneous and knowledge of the spatial conductivity distribution is, to some degree, uncertain. Here, a systematic attempt is made to quantify the effect of soil heterogeneity on the optimal design of in-situ bioremediation systems. To determine the optimal placement of injection and extraction wells within the computational domain, the meshless element-free Galerkin method (EFGM) was coupled with particle swarm optimization (PSO), resulting in a simulation-optimization (S/O) model which is referred to as BIOEFGM-PSO. A hypothetical case study is considered where the design of an in-situ bioremediation system is optimized considering different degrees of heterogeneity of the porous medium. Heterogeneous conductivity fields are generated using a pseudo-random field generator with same mean and varying variance and correlation lengths. The BIOEFGM-PSO model was then applied to the different soil scenarios, and the resulting bioremediation costs were compared. Results show that the optimal placement of injection and extraction wells depends on the soil properties and, on average, heterogeneous soils have higher in-situ bioremediation costs compared with a homogeneous soil with the same mean conductivity. This highlights the importance of considering soil heterogeneity in designing cost-effective in-situ bioremediation systems, and further demonstrates the general applicability of the BIOEFGM-PSO model.

Injection of free-phase supercritical CO2 into deep geological reservoirs is associated with risk of considerable return flows towards the land surface due to the buoyancy of CO2, which is lighter than the resident brine in the reservoir. Such upward movements can be avoided if CO2 is injected in the dissolved phase (CO2aq). In this work, injection of CO2-saturated brine in a subsurface carbonate reservoir was modelled. Physical and geochemical interactions of injected low-pH CO2-saturated brine with the carbonate minerals (calcite, dolomite and siderite) were investigated in the reactive transport modelling. CO2-saturated brine, being low in pH, showed high reactivity with the reservoir minerals, resulting in a significant mineral dissolution and CO2 conversion in reactions. Over the injection period of 10 yr, up to 16% of the injected CO2 was found consumed in geochemical reactions. Sorption included in the transport analysis resulted in additional quantities of CO2 mass stored. However, for the considered carbonate minerals, the consumption of injected CO2aq was found mainly in the form of ionic trapping.

Inverse modeling approaches based on tracer data are often used to characterize transport processes in streams and rivers. This generally involves the calibration of a one-dimensional transport model using concentrations measured in the surface water at one or multiple locations along a stream reach. A major concern is whether the calibrated model parameters are representative of the physical transport processes occurring in the water column and the underlying sediment bed. This study looks at the identifiability of the parameters of a physically based one-dimensional stream transport model that represents hyporheic exchange as a vertically attenuated mixing process in accordance with recent experimental evidence. It is shown that, if the average flow velocity and hydraulic radius are not predetermined, there are infinite sets of parameter values that generate the same space-time concentration distributions in the water column. The result implies that in-stream transport and hyporheic exchange parameters cannot be determined from sole measurements of solute breakthrough curves in the surface water unless stream discharge and average cross-sectional geometry can be independently estimated.

The stochasticity of fluid and sediment parameters has been identified as a source of diffusion, particularly anomalous diffusion at different temporal and spatial scales of bed load particle trajectories. Data from two sets of flume experiments are presented, one data set has gravel particle trajectories tracked over a limited area and was used in identifying the influence of different shear stress conditions on diffusive processes. A new experiment was performed using spherical particles moving as bed load in an annular flume in order to address concerns about censorship effects caused by the size of the detection window. An annular flume allowed collection of practically uncensored particle trajectories over longer time period than has been previously possible in the laboratory. Three diffusive regimes were observed at distinct stages of particle motion: (i) ballistic regime at the local range; (ii) Fickian diffusion at the intermediate range; (iii) subdiffusion at the global range. Characteristic time scales separate the regimes and correlate with the mean traveling and resting times of particles. Fickian diffusion in the intermediate range is first recognized as a result of the balance between intermittent weak transport and near-bed turbulence, as first predicted by Nikora et al. (2002, https://doi.org/10.1029/2001WR000513). In the global range, extreme values were observed in the distribution of particle resting times, suggesting that two types of distributions (related to surface motion and vertical mixing) were responsible for the subdiffusion at longer time scales. Diffusion was found to be anisotropic at all stages of particle motion.

© 2017 Elsevier B.V. The hydraulic efficiency of wetlands for wastewater treatment was investigated as a function of wetland shape and vegetation density using a 2D depth-averaged numerical model. First, the numerical model was calibrated and validated against field data and then was applied to 8 hypothetical wetlands of rectangular and elliptical shape and different aspect ratio (i.e. 1:1–4:1). The vegetation density was varied from 0 to 1000 stems/m2. The effect of inlet-outlet configuration was analyzed by simulating the hydraulic response of wetlands with different alignment of the flow inlet and outlet and wetlands with multiple inlets. The resulting Residence Time Distributions (RTDs) were derived from numerical simulations of the flow field and the temporal evolution of the outlet concentration of a passive tracer injected at the inlet. The simulated velocity field demonstrated that wetland shape can have significant impact on the size of dead zone areas, which is also reflected in the RTD. Efficiency metrics associated with detention time and degree of mixing improved for an elliptical shape compared to a rectangular shape. An ellipse shape improved the wetland performance by reducing the area of dead zones at the corners, and thereby increasing the effective wetland volume contributing to the treatment process. Configurations in which inlet and outlet were located at opposite corners of the wetland, and wetlands with multiple inlets produced smaller dead zones, which reduced the variance of the RTD. The simulation results also revealed an interesting threshold behavior with regard to stem density. For stem density above 300 stems/m2, which is typical of treatment wetlands, the model predictions were not sensitive to the exact value of stem density selected, which simplifies the parameterization of models. This quantitative analysis of the effect of wetland shape, inlet-outlet configuration and vegetation density can help engineers to achieve more efficient and cost-effective design solutions for wastewater treatment wetlands.

Climate-driven fluctuations in the runoff and potential energy of surface water are generally large in comparison to the capacity of hydropower regulation, particularly when hydropower is used to balance the electricity production from covarying renewable energy sources such as wind power. To define the bounds of reservoir storage capacity, we introduce a dedicated reservoir volume that aggregates the storage capacity of several reservoirs to handle runoff from specific watersheds. We show how the storage bounds can be related to a spectrum of the climate-driven modes of variability in water availability and to the co-variation between water and wind availability. A regional case study of the entire hydropower system in Sweden indicates that the longest regulation period possible to consider spans from a few days of individual sub-watersheds up to several years, with an average limit of a couple of months. Watershed damping of the runoff substantially increases the longest considered regulation period and capacity. The high covariance found between the potential energy of the surface water and wind energy significantly reduces the longest considered regulation period when hydropower is used to balance the fluctuating wind power.

Solute transport in rivers has been traditionally represented using one-dimensional models assuming advection and Fickian dispersion along the main flow direction and transient storage in surface and subsurface dead zones. Experimental evidence from several stream tracer studies has shown that the longitudinal scaling of the moments of the breakthrough curves (BTCs) is inconsistent with classic 1-D solute transport models. In this work, simulations of advection and diffusion in a 2-D and 3-D channel flow over a porous bed are presented assuming an exponentially attenuated profile of the transverse mixing coefficient in the porous medium, as suggested by recent experimental and numerical studies. It is shown that the longitudinal transport in the channel is superdiffusive, and the skewness of the concentration distributions can be almost constant over a broad temporal range, with no sign of approaching zero at large times. A sensitivity analysis shows that, at large times, longitudinal dispersion is controlled by the cross-sectional profile of the in-bed transverse mixing coefficient, and by the difference between the average velocity in the channel and in the porous bed. The normalized concentration distributions can be approximated by beta-distributions with time-dependent parameters, and relationships are derived between the scaling of the parameters under power-law approximation and the scaling of the spatial and temporal moments. The results provide new insights into the physical mechanisms that control the anomalous scaling of the moments observed in field tracer studies and opens new possibilities for predictive and inverse modeling of transport processes in rivers and their catchments.

Mixing in the hyporheic zone plays a key role in controlling the fate and transport of contaminants in streams and rivers. Consistently with recent experimental and numerical results, a physically based one‐dimensional solute transport model is presented that represents hyporheic mixing as a diffusion process exponentially attenuated with depth. When vertical diffusion in the sediment bed is not limited by an impermeable boundary, the moments of the BTCs generated by the model exhibit persistent non‐Fickian behavior and are shown to scale consistently with existing experimental evidence. The ability of the exponentially attenuated mixing model (EAMM) to represent experimental BTCs was tested using data from field and flume tracer experiments, and its performance was compared with that of a classic two‐storage zone model (TSZM). While both models provided an equally good approximation for the BTCs observed in field tracer tests, the EAMM provided better fits for the flume experiments. Analysis of the model equations and their solutions shows that, if the flow cross‐sectional area and wetted perimeter are not predetermined, there are infinite sets of parameter values that produce the same space‐time concentration distributions. The result implies that the physical parameters of hyporheic exchange cannot be determined by sole measurements of the solute BTCs in the surface water unless flow cross‐sectional area and average flow depth can be independently estimated.

The introduction of “smart” tracer techniques in recent years has provided new ways to investigate sediment-water interactions and microbial activity in stream corridors. In this study, the formulation of the STIR model (Marion et al., 2008) is extended to represent the transport and transformation of Resazurin-Resorufin smart tracers, and an object-oriented toolbox, STIR-RST, is presented for model evaluation and calibration. STIR-RST allows different storage processes to be represented by specific residence time distributions (RTDs), with two possible arrangements of the storage zones: nested (in-series) or competing (in-parallel). The application of STIR-RST to field tracer data is demonstrated assuming two storage zones with exponential RTD. Results show that the assumption of two storage zones provides a better approximation of the observed BTCs compared to that of a single storage zone, at the cost of higher parameter uncertainty. Similar fits are obtained for nested and competing zone arrangements.

Summary Vegetation plays an important role in controlling mixing and contaminant removal in wetlands. Recent studies have shown that the hydraulic performance of a wetland can be significantly affected by the presence of a main flow channel (MFC) where vegetation density is much lower than the average vegetation density in the wetland. The existence of a main flow channel induces short-circuiting, which reduces hydraulic and treatment efficiency. A numerical study was carried out to analyze the effect of channel sinuosity and vegetation density on the hydraulic performance of a channelized wetland. A rectangular wetland characterized by a meandering channel and later vegetated zones (LVZs) was considered, and numerical simulations were carried out using a 2-D depth-average hydrodynamic and solute transport model. The hydraulic performance was analyzed as a function of the average vegetation density, channel sinuosity and the ratio of vegetation densities in the LVZs and the MFC. Results show that increasing sinuosity of the main flow channel can promote mixing and improve hydraulic efficiency. Different performance metrics also indicate negligible impact of the average vegetation density on the hydraulic performance, especially when the width of the MFC is relatively large.

Increased reliance on variable and intermittent energy sources is likely to lead to a change in the production strategies of hydropower, thereby increasing the importance of accurate forecasting of production. For optimization models applied to water reservoirs, the computational cost increases with the number of reservoirs and future time-steps considered, often requiring simplification of the physical description of the flow dynamics. Here it is demonstrated that deficiency of the model of the flow dynamics on stream-reaches gives rise to errors in short-term planning, which leads to sub-optimal production. Here a simplified hydraulic model based on the kinematic-diffusion wave model was incorporated in the optimization of reservoir production planning. The time-lag distributions of the streams were evaluated for River Dalalven and implemented in a computationally efficient form of the kinematic-diffusion wave equation incorporated in a production optimization algorithm for a series of reservoirs. Compared to using a single time-lag for the water transfer on flow reaches between hydropower stations, the wave diffusion was found to affect the management as a deviation between the actual production and the planned production. The deviation was found to increase with increasing short-term regulation and decreasing Peclet number below about 10. For a sufficiently high Peclet number and long wavelength characterizing individual stream reaches, the distribution of time-lags become sufficiently narrow to motivate being replaced by a simpler description such as the constant time-lag.

An optimal design of in-situ bioremediation system for contaminated aquifer sites mainly depends on the injection/extraction well locations and their pumping rates. Previous studies have used discretization approaches such as finite difference method (FDM) or finite element method (FEM) to solve the equations of groundwater flow and contaminant transport (GFCT) that characterize the bioremediation processes. In these numerical models, well locations can only lie at the grid nodes and therefore, the locations other than the grid nodes remain unexplored for the optimal in-situ bioremediation. To explore such locations, in this study a meshless simulation model called BIOEFGM is developed using the element-free Galerkin method (EFGM) and coupled with the particle swarm optimization (PSO). BIOEFGM model provides flexibility in adding injection/extraction wells anywhere in the computational domain during the entire optimization procedure. However, FDM/FEM requires meshing and re-meshing of the computational domain for each set of wells and it increases the computational efforts significantly for solving the optimization problem. In this paper, the proposed BIOEFGM-PSO simulation-optimization (S/O) model is first applied to a well-known bioremediation problem and then to a field type large aquifer problem. The simulation results of BIOEFGM are verified with those from the RT3D simulations. The estimated bioremediation cost from the BIOEFGM-PSO model for the first problem is found to be lesser in comparison to the same calculated with different S/O models in the previous studies. The results of this problem also show that optimized in-situ bioremediation system designed by proposed S/O model takes less remediation time and also favor effective biodegradation of contaminants. For both the problems, BIOEFGM-PSO identified optimal well locations at positions other than that of discretized nodes and it leads to efficient bioremediation. It indicates the effectiveness of the BIOEFGM-PSO model and therefore, can be applied for designing the better optimized in-situ bioremediation systems for field problems.

The interaction between flow and vegetation in constructed wetlands plays a major role in determining wetland performance. In this study, a two-dimensional depth-averaged model was used to simulate flow, mass transport and contaminant removal in a conceptual free-water surface (FWS) wetland with heterogeneous vegetation patterns. The main objectives of this study were (1) to quantify the effectiveness of FWS wetlands with different vegetation patterns in reducing pollutant load, and (2) to identify optimal vegetation distributions that maximize contaminant removal. Simulations were performed for different random vegetation fields characterized by imposed mean, variance and correlation lengths of stem density. The wetland was assumed to receive water from a stream and to deliver it back to the same stream according to an imposed head drop. The simulations show that the concentration reduction efficiency increases monotonically with average stem density, whereas mass removal has a peak for an intermediate value of average stem density. The ensemble average of the total mass removal decreases for increasing stem density variance and correlation length, because the presence of vegetation patches with significantly different stem density promotes preferential flow paths. Preferential flow paths parallel to the mean flow direction were found to reduce the hydraulic efficiency of wetlands by producing short-circuiting, whereas, for the same mean stem density, alternating stripes of stem density perpendicular to the flow direction provided higher concentration and mass reduction. By providing a quantitative understanding of the impact of spatial vegetation heterogeneity, the results provide useful guidelines for design and maintenance of constructed wetlands for wastewater treatment.

The temporal retention in storage zones (SZs) has a strong influence on mass transport processes in natural streams. It has been shown that solute retention affects solute breakthrough curves (BTCs) by producing longer tails and thereby increasing their skewness. In terms of ecological effects, this retention increases the contact time of solute with aquatic interfaces and living species, which can lead to degradation of eco-systems when the transported substances are pollutants. An important question that arises is whether the currently available metrics can adequately represent complex retention processes. In this study, we examine the performance of two existing metrics: the hydrological retention factor (R H ) and the fraction of median travel time due to transient storage (F med ). The results presented are based on two conservative tracer tests. The tracer tests were performed in streams with distinct morphological, sediment composition, vegetation and hydraulic characteristics. The recorded concentration-time series were used to derive storage zone parameters such as storage zone area, exchange coefficient and mean residence time. The storage zone parameters were computed using a multiple storage zone model STIR with two separate exponential residence time models for transient storage, representing short timescale (STS) and long timescale storage (LTS) processes. The retention metrics were estimated separately for short and long timescale retention, and for the combined retention. The cross-correlation between the retention metrics and the storage parameters was analyzed using Pearson’s R- and significance p-values. In general, the results reveal a poor correlation between retention metrics and storage zone parameters, except for the exchange rate associated with long timescale storage, α 2. A strong cross-correlation is instead found between the retention metrics.

Constructed wetlands for wastewater treatment are increasingly recognized as a valid alternative to conventional water treatment methods with a high ecological value. Sediment transport and deposition processes play a key role in determining the treatment performance and the morphological evolution of a wetland, and must be carefully considered both in the design and the maintenance phase. This work presents a 2-D numerical study of the effect of vegetation density on sedimentation in wetlands. A depth-averaged hydrodynamic and mass transport model was applied to a rectangular wetland with uniform vegetation density and flat topography. Sediment settling and resuspension are represented in the model by a first-order source/sink term that depends on grain size and shear velocity. Results show that, for the same inflow discharge, the removal efficiency for relatively small grain sizes is lower in wetlands with higher vegetation density. This is a consequence of the more uniform flow distribution found in more densely vegetated wetlands. However, the condition of total removal of suspended sediment is achieved for higher grain sizes in more sparsely vegetated wetlands, meaning there is a range of relatively large grain sizes for which the removal efficiency is higher for higher vegetation densities.

A complete understanding of the role of grain-scale particle-flow interaction in sediment entrainment and transport has still not\ud been achieved in spite of recent technological advancement in measurement capabilities. In this study, the initial motion of natural sediment\ud particles in a gravel deposit was detected and combined with simultaneous local measurements of the velocities on a horizontal plane located\ud above the bed surface using a three-component stereoscopic PIV. A series of experimental tests with increasing low values of boundary shear\ud stress were conducted. The acquisition system allowed coupling between streamwise and vertical near-bed velocity and the entrainment of\ud more than 900 individual grains. Initial analysis agreed with previous observations on the predominance of sweeps (Quadrant IV), and to a\ud lesser extent, of outward interactions (Quadrant I) in entraining gravel particles. However, the latter were found to move sediments just as\ud efficiently as sweeps impacting on particles that had long periods of rest and so were exhibiting higher levels of stability. This behavior\ud suggests that sweep-induced lift based on Bernoulli’s principle does not entirely explain the generation of vertical forces on highly stable bed\ud particles. Closer inspection of the data revealed that many entrainments were correlated to occasions when stable bed grains interacted with\ud grains travelling in their close vicinity. Approximately 30% of the entrained population was observed to initiate motion in this type of\ud situation. For this subsample of entrainment events, outward interactions were found to be comparatively more effective than for the noninterference\ud case, whereas the relative contribution of sweeps exhibited an opposite trend.

This paper is an attempt to explain influence of vegetation and flow rate in natural stream (Epre, Germany) on mixing and transport processes. For this purpose, we conducted two tracer tests in Germany using rhodamine WT (RWT) as a fluorescence dye. Both tests were performed under different vegetation and flow rate conditions. The STIR (Solute Transport In Rivers) code was used for calibration of dispersion coefficients, exchange rates and residence times. We used the STIR model to separate short—and long—time retention. Our tracer test results confirm previous findings and also reveal a correlation between storage zone exchanges rate and reach lengths, strong influence of vegetation and flow rate on transport and mixing parameters, and the significance of the equipment on storage domain characterisation.

Hydraulic efficiency of wetlands, evaluated through retention time and mixing levels, was investigated as a function of wetland shape and vegetation density using a two-dimensional numerical model. The numerical model was applied to four different aspect ratios of a rectangular wetland (i.e. 1:1 to 1:4) with 1-ha area and vegetation density varying from 20 to 1500 stems/m2. The results, modeled velocity field and the simulated transport of a continuously injected tracer, were used to develop Residence Time Distribution graphs (RTDs). Analysis of RTDs showed that the efficiency measure related to retention time, e, and the measure of mixing, λ p , improved for denser vegetation before reaching to a constant value. It was also observed that narrow rectangular-shaped wetlands (higher aspect ratio) have better efficiency than square wetlands. The results from the study provide a quantitative understanding of hydraulic efficiency in connection with wetland vegetation and shape which may help engineers to design more efficient and cost-effective water systems.

In this paper a modelling approach is presented to predict local scour under time varying flow conditions. The approach is validated using experimental data of unsteady scour at bed sills. The mode ...

Solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges with distinct retention zones. Surface and hyporheic retention processes can be accounted for separately in solute transport models with multiple storage compartments. In the simplest two component model, short term storage can be associated to in-channel transient retention, e.g. produced by riparian vegetation or surface dead zones, and the long-term storage can be associated to hyporheic exchange. The STIR (Solute Transport In Rivers) multiple domain transport model is applied here to tracer test data from three very different Mediterranean streams with distinctive characteristics in terms of flow discharge, vegetation and substrate material. The model is used with an exponential residence time distribution (RTD) to represent surface storage processes and two distinct modeling closures are tested to simulate hyporheic retention: a second exponential RTD and a power-law distribution approximating a known solution for bedform-induced hyporheic exchange. Each stream shows distinct retention patterns characterized by different timescales of the storage time distribution. Both modeling closures lead to very good approximations of the observed breakthrough curves in the two rivers with permeable bed exposed to the flow, where hyporheic flows are expected to occur. In the one case where the occurrence of hyporheic flows is inhibited by bottom vegetation, only the two exponential RTD model is acceptable and the time scales of the two components are of the same magnitude. The significant finding of this work is the recognition of a strong signature of the river properties on tracer data and the evidence of the ability of multiple-component models to describe individual stream responses. This evidence may open a new perspective in river contamination studies, where rivers could possibly be classified based on their ability to trap and release pollutants.

Average water travel times through a stream network were determined as a function of stage (discharge) and stream network properties. Contrary to most previous studies on the topic, the present work allowed for streamflow velocities to vary spatially (for most of the analyses) as well as temporally. The results show that different stream network mechanisms and properties interact in a complex and stage-dependent manner, implying that the relative importance of the different hydraulic properties varies in space and over time. Theoretical reasoning, based on the central temporal moments derived from the kinematic-diffusive wave equation in a semi-2-D formulation including the effects of flooded cross sections, shows that the hydraulic properties in contrast to the geomorphological properties will become increasingly important as the discharge increases, stressing the importance of accurately describing the hydraulic mechanisms within stream networks. Using the physically based, stage-dependent response function as a parameterization basis for the streamflow routing routine (a linear reservoir) of a hydrological model, discharge predictions were shown to improve in two Swedish catchments, compared with a conventional, statistically based parameterization scheme. Predictions improved for a wide range of modeled scenarios, for the entire discharge series as well as for peak flow conditions. The foremost novelty of the study lies in that the physically based response function for a streamflow routing routine has successfully been determined independent of calibration, i.e., entirely through process-based hydraulic stream network modeling.

The fate of solute and pollutants is controlled by a broad number of different transport and storage mechanisms, ranging from simple processes (i.e. molecular diffusion, advection etc.) to more complex phenomena (i.e. evapotranspiration, groundwater flows, etc.). Different mathematical models, accounting for different exchange processes, have been developed and applied to specific experimental studies to assess transport and storage parameters. Experimental research focused on transport and retention processes induced by the transient storage in the dead zones, by the river bed topography and vegetation, by evapotranspiration. The analysis of these physical processes is generally conducted observing the behavior of solutes in field environments or in scaled laboratory models, using artificial or environmental tracers to track the fate of transported substances and assess transport and retention parameters. To improve the knowledge of pollutant exchange mechanism between a river and the surrounding environment, new experimental techniques focusing on long timescale retention and investigating the link between river biology and hydrodynamics are required. The development of new protocols for tracer tests design and the use of new specific tracers will open future research perspectives.

One concern regarding the underground storage of carbon dioxide (CO2) is its potential leakage from reservoirs. Over short period of time, the leakage risk is related mainly to CO2 as a separate supercritical fluid phase. However, over longer periods upon complete dissolution of injected CO2 in the fluid, the leakage risk is associated with dissolved phase CO2. Over the geological time scales, large-scale groundwater motion may cause displacement of brine containing dissolved CO2 along the conducting pathways. In this paper, we present a comprehensive modeling framework that describes the reactive transport of CO2-saturated brine along a fracture in the clay caprock based on the future, hypothetical leakage of the dissolved phase CO2. This study shows that the transport of leaked dissolved CO2 is significantly retarded by a combination of various physical and geochemical processes, such as mass exchange between conducting fracture and the neighboring rock matrix through molecular diffusion, sorption and calcite dissolution in the rock matrix. Mass stored in aqueous and adsorbed states in the rock matrix caused retention of dissolved CO2 along the leakage pathway. Calcite dissolution reaction in the rock matrix resulted in consumption of leaking dissolved CO2 and reduced its mass along the leakage pathway. Consumption and retention of dissolved CO2 along the leakage pathway have important implications for analyzing the potential reduction of CO2 fluxes from storage reservoirs over large periods and long travel pathways.

Intake free-surface vortices can cause efficiency losses, flow fluctuations and even structural damages. Experiments were performed to examine the effect of entrance shapes on the critical submergence. Seven entrance shapes were devised and tested, including a square-edged, a bell-mouthed, three symmetrical conical and two conical profiles with eccentricity. The focus of the study was on a range of Froude numbers from 0.25 to 0.65. The square-edged shape appeared to show the highest local head-loss compared to other shapes. Steady counter-clockwise vortices characterize all the intake profiles except in a narrow water tank. The experiments show both discrepancy and similarity between the intake profiles. The critical submergence of the bell-mouthed intake is lower when compared to the square-edged shape. For the other profiles, it is proportional to the Froude number. A closer sidewall may lead to larger critical submergence in the case of weak circulations. The results demonstrate that the intake...

Advancing our predictive capabilities of heat fluxes in streams and rivers is important because of the effects on ecology and the general use of heat fluxes as analogues for solute transport. Along ...

Fluvial sediment transport is caused by a complex interaction of interdependent grain and fluid processes many of which are stochastic in nature and cannot be adequately represented by deterministic equations. Random variable analysis has been used previously but limited data are available to describe the variability of grain resistance combined with particle arrangements, and thus validate such analysis. In this study low to medium bed load transport tests were carried out in a flume where sediment movement was monitored using a three-camera 3D PIV system. Simultaneous grain motion and flow velocity measurements were made on a plane located slightly above and parallel to the sediment bed. Detailed statistical velocity information was acquired to model the velocity distribution at the bed level. This was combined with the joint probabilistic distribution of particle exposures and grain resistance to motion, which were obtained from discrete particle modeling (DPM) simulations. DPM simulations were used to provide a stochastic mathematical description of the risk that a stationary particle is entrained by the flow. Predictions from the stochastic model equations replicated the observed pulsation in sediment transport. This demonstrates that it is possible to simulate sediment entrainment and transport at a high resolution by adequately modeling all the sub-processes. A number of flow patterns were identified that caused large fluctuations of the entrainment rate. These all exhibit high velocity flow structures, but they selectively cause the dislodgement of individual particles located at different positions. This selective behavior follows from the variability of the interaction between the near-bed flow and the particles having different exposure.

A characterization of hyporheic exchange for dry and wet season baseflow, as well as partially dewatered discharge, was done in Prieta Creek, a first-order cascade in northern Honduras. The cascade ...

This study analyzes the effect of advective pumping and pore scale dispersion on bed form-induced hyporheic exchange. Advection and dispersion play a competitive role in the exchange dynamics betwe ...

River engineering Transport and fate of pollutants in rivers

Microbial biofilms are the prime site of nutrient and contaminant removal in streams. It is therefore essential to understand how biofilms affect hydrodynamic exchange, solute transport, and retention in systems where geomorphology and induced hydrodynamics shape their growth and structure. We experimented with large-scale streamside flumes with streambed landscapes constructed from graded bedforms of constant height and wavelength. Each flume had a different bedform height and was covered with a layer of gravel as substratum for benthic microbial biofilms. Biofilms developed different biomass and physical structures in response to the hydrodynamic conditions induced by the streambed morphology. Step injections of conservative tracers were performed at different biofilm growth stages. The experimental breakthrough curves were analyzed with the STIR model, using a residence time approach to characterize the retention effects associated with biofilms. The retained mass of the solute increased with biofilm biomass and the biofilm-associated retention was furthermore related to bedform height. We tentatively relate this behavior to biofilm structural differentiation induced by bed morphology, which highlights the strong linkage between geomorphology, hydrodynamics, and biofilms in natural streams and provide important clues for stream restoration.

Surface-subsurface exchange fluxes are receiving increasing interest because of their importance in the fate of contaminants, nutrients, and other ecologically relevant substances in a variety of aquatic systems. Solutions have previously been developed for pore water flows induced by geometrical irregularities such as bed forms for the cases of homogeneous sediment beds and idealized heterogeneous beds, but these solutions have not accounted for the fact that streambed sediments are subject to sorting processes that often produce well-defined subsurface structures. Sediments at the streambed surface are often coarser than the underlying material because of size-selective sediment transport, producing relatively thin armor layers. Episodic erosional and depositional processes also create thick layers of different composition within the porous medium, forming stratified beds. A series of experiments were conducted to observe conservative solute transport in armored and stratified beds. An analytical solution was developed for advective exchange with stratified beds and provides appropriate scaling of the physical variables that control exchange flows. The results show that armor layers are too thin to significantly alter the advective pumping process but provide significant solute storage at short time scales. Stratified beds with layers of significant thickness favor development of horizontal flow paths within the bed and change the rate of solute transfer across the stream-subsurface interface compared to homogeneous beds.

Experiments were carried out using a mobile gravel bed placed in a tilting flume with a modified particle image velocimetry (PIV) system. Individual grain movements were surveyed using data from time series of images. Near-bed velocity flow field measurements were made simultaneously above the same area of the sediment surface by applying cross-correlation techniques to the collected plan view images. Statistics of grain motions were collected through a semiautomatic procedure. Significant changes in the flow field were observed in the proximity of the entrained or deposited particles. A strong correlation is shown between the changes in the local streamwise and lateral velocity and the movement of the grains. The theory of Grass is revisited and developed based on the experimental results. The probability distribution of individual grain resistance has been derived from the statistics of the near-bed velocity field and of the entrainment risk.