1,720,983 research outputs found
STIR-RST: A Software tool for reactive smart tracer studies
Full text linkThe 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
A Parametric Study on the Effects of Green Roofs, Green Walls and Trees on Air Quality, Temperature and Velocity
Full text linkThe rapid increase in urbanisation and population growth living in urban areas leads to major problems including increased rates of air pollution and global warming. Assessing the impact of buildings on wind flow, air temperature and pollution dispersion on people at the pedestrian level is, therefore, of crucial importance for urban design. In this study, the effect of different forms of urban vegetation including green roofs, green walls and trees on velocity, air temperature and air quality is assessed using computational fluid dynamics (CFD) for a selected area of the East Village. This study indicates that adding a building increases air temperature, pollution concentration and velocity at the pedestrian level. A parametric analysis is conducted to assess the impact of various key parameters on air temperature, pollution and velocity at the pedestrian level. The variables under consideration include wind speed, ranging from 4–8 m/s at a reference height of 10 m, and vegetation cooling intensity, ranging from 250–500 W·m−3. Three scenarios are tested in which the streets have no bottom heating, 2 °C bottom heating and 10 °C bottom heating. Pollution is simulated as a form of passive scalar with an emission rate of 100 ppb s−1, considering NO2 as the pollutant. In all cases, vegetation is found to reduce air velocity, pollutant concentration and temperature. However, the presence of vegetation in various forms alters the pattern of pollution dispersion differently. More specifically, the results indicate that planting trees (e.g., birch trees) close to the edge of buildings can decrease the air temperature by up to 2–3 °C at the pedestrian level. Increasing the cooling intensity of the vegetation from 250 to 500 W·m−3 results in significantly lower air temperature, whereas lower wind speeds result in a higher concentration of pollutants at the pedestrian level. A combination of green walls and trees is found to be the most effective strategy to improve the thermal environment and air quality
Solute transport in rivers with multiple storage zones: the STIR model
No full textSolute transport in rivers is controlled by surface hydrodynamics and by mass exchanges between the surface stream and distinct retention zones. This paper presents a residence time model for stream transport of solutes, Solute Transport in Rivers (STIR), that accounts for the effect of the stream-subsurface interactions on river mixing. A stochastic approach is used to derive a relation between the in-stream solute concentration and the residence time distributions (RTDs) in different retention domains. Particular forms of the RTD are suggested for the temporary storage within surface dead zones and for bed form–induced hyporheic exchange. This approach is advantageous for at least two reasons. The first advantage is that exchange parameters can generally be expressed as functions of physical quantities that can be reasonably estimated or directly measured. This gives the model predictive capabilities, and the results can be generalized to conditions different from those directly observed in field experiments. The second reason is that individual exchange processes are represented separately by appropriate residence time distributions, making the model flexible and modular, capable of incorporating the effects of a variety of exchange processes and chemical reactions in a detailed way. The capability of the model is illustrated with an example and with an application to a field case. Analogies and differences with other established models are also discussed
A Eulerian-Lagrangian model for simulating fish pathline distributions in vertical slot fishways
Full text linkRecovery of the river connectivity relies on the application of structural and non-structural solutions. Within the structural solutions, vertical slot fishways (VSF) are most popular, but achieving the desired efficiency remains a challenge. To this end, evaluation of both biological and hydraulic efficiency is imperative. A Eulerian Lagrangian Agent Based Model (ELAM) of fish upstream migration through VSFs is presented in this work that considers one passive and three active behaviors affecting fish swimming pathlines of bighead carps (Hypophthalmichthys nobilis) and silver carps (Hypophthalmichthys molitrix). The passive behavior consists in a 2-D random walk where no external stimuli are detected. The first active behavior represents fish attraction to a nearby flow region as a Gaussian distribution function of turbulent kinetic energy, velocity magnitude and velocity strain rate. The second active behavior represents wall collision avoidance, whereas the third considers the tendency of the fish to escape from high velocity zones. Stochasticity is considered in the model to represent the dispersion in the experimental pathlines. The ELAM is tested for two different VSF configurations and is found to well reproduce the experimental pathlines, particularly when the escaping response to high velocity zones is considered. Results show that the ELAM model can be used as an appropriate tool for modelers and designers to evaluate fish passage efficiency
A Method for Calibrating the Transient Storage Model from the Early and Late-Time Behavior of Breakthrough Curves
Full text linkSolute transport in rivers is controlled by mixing processes that occur over a wide spectrum of spatial and temporal scales. Deviations from the classic advection–dispersion model observed in tracer test studies are known to be generated by the temporary trapping of solutes in storage zones where velocities and mixing rates are relatively small. In this work, the relation between the early and late-time behavior of solute breakthrough curves (BTCs) and the key parameters of the Transient Storage Model (TSM) is analyzed using non-asymptotic approximations of the model equations. Two main slopes are identified corresponding to the rising and decreasing limbs of the BTCs which are linked by specific relationships to transport and storage parameters. The validity of the proposed approximations is demonstrated with both synthetic and experimental data. Consistent with the TSM assumptions, the range of validity of the proposed approximations represents the limit of separability between surface dispersion and transient storage and can be expressed as a function of a nondimensional parameter. The results of this work can help environmental scientists identify solute transport and transient storage parameters and support the design of enhanced field tracer experiments
Eulerian Lagrangian Agent Based Model (ELAM) for upstream fish migration in Vertical Slot Fishways
No full textEvaluating the effect of aquifer heterogeneity on multiobjective optimization of in-situ groundwater bioremediation
No full textIn-situ bioremediation is a cost-effective technique to eliminate petroleum hydrocarbons from groundwater. In previous studies, this has been mostly applied under the approximation of a homogeneous porous medium, even though in reality aquifers are often characterized by significant heterogeneities. Here, different heterogeneous and equivalent homogeneous hydraulic conductivity fields are considered to study the effect of aquifer heterogeneity on optimal in-situ bioremediation. For this, the multiobjective simulation-optimization (S/O) model referred to as BIOEFGM-NSGA II is proposed — which uses the element-free Galerkin method (EFGM) for discretization of the governing equations, and the non-dominated sorting genetic algorithm II (NSGA II) for multiobjective optimization. This model is then applied to different heterogeneous conductivity fields generated through a pseudo-random correlated field generator for different combinations of variance and correlation lengths with constant mean. Results show that the optimal pumping policy for an equivalent homogeneous conductivity field violates the constraint of maximum allowable concentration in all the studied heterogeneous fields. To satisfy this constraint, in-situ bioremediation cost increases by 8.47% to 56.83% to that of a homogeneous field. It shows the significance of aquifer heterogeneity in designing an optimized in-situ bioremediation system and hence, should be incorporated in the S/O model for in-situ groundwater bioremediation
Hyporheic Flows in Stratified Beds
No full textSurface-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
Variation in contaminant removal efficiency in free-water surface wetlands with heterogeneous vegetation density
No full textA design framework for considering spatial equity in sustainable urban drainage infrastructure
Full text linkThe 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
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