Article

A laboratory method for the visualization and quantification of hyporheic flow paths and velocities

Canadian Science Publishing
Canadian Journal of Civil Engineering
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Abstract

Hyporheic flow, the flow of water through the permeable material immediately surrounding a river, is important for nutrient cycling, dissolved oxygen transport, and contaminant transport. In addition, there is recent concern regarding the role of hyporheic flow on the contamination of rivers following oil spills. To better understand hyporheic flow paths and velocities, it is important to measure hyporheic flow at high spatial and temporal resolution. A practical method to measure hyporheic flow in a laboratory flume based on dye injection, digital images, and moment analysis was developed. An experiment conducted using a single gravel bar demonstrated good agreement between observations and estimates based on image processing. The measured hyporheic flow field showed upstream and downstream flow that discharged downstream of the bar top, the presence of a flow divide and flow stagnation, and hyporheic flow velocities indicative of turbulent flow for which Darcy’s law is not applicable.

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... The distance between each injection point is 10 cm. No hyporheic path was observed at points 15, 16, 17, 18, 19, 20 and 21, which was also observed by (Fruetel 2016;Fruetel et al. 2019). The interesting point is that the zone between points 15 to 21 is where the flow separation zone was observed by Kabiri et al. (2022a, b). ...
... subsurface flow occurrence in the direction of surface flow (from upstream to downstream) while the latter is formed downstream of the crest with the difference that subsurface flow occurs in the opposite direction of the surface flow. The point at which the formers and the latter converge may lead to a hypothetical line, which is called the flow divide line (Fruetel et al. 2019). Hyporheic flows consist of downwelling flows supplying dissolved oxygen and organic matter to micro-invertebrates inhabiting the subsurface region underneath river beds while upwelling flows supply nutrients to stream organisms (Boulton et al. 1998;Hakenkamp and Palmer 2000). ...
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The hyporheic zone represents the interface between surface and subsurface flow. An experimental investigation was conducted in a laboratory flume featuring a maximum flow rate of 50 L per second, utilizing artificial grass completely submerged in water. Two sets of experiments were carried out, one with vegetation cover and the other without. The findings revealed that vegetation cover led to a reduction in hyporheic velocity, whereas the absence of vegetation increased hyporheic velocity. The study also noted that the absence of a hyporheic zone in vegetation, compared to gravel, could be attributed to the formation of a separation zone. Additionally, it was observed that vegetation cover facilitated the supply of more nutrients around the divide line, owing to upwelling flows from both upstream and downstream directions. Given the limited dataset, Soft Computing (SC) techniques, namely Wavelet-GEP (WGEP) and Gene Expression Programming (GEP), were employed to formulate mathematical equations for estimating hyporheic velocity under steady-state conditions for a given discharge. These equations, incorporating hydraulic and geomorphic variables, proved effective in estimating hyporheic velocity under stable bed conditions during steady flow. The models were trained and tested using 70% and 30% of the collected data, respectively. The performance of the models was assessed using statistical criteria. The results indicated a strong correlation between noise reduction in data and improved performance of the WGEP model compared to the GEP model in estimating velocity.
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1] In-channel stream restoration structures readjust surface water hydraulics, streambed pressure, and subsurface hyporheic exchange characteristics. In this study, we conducted flume experiments (pool-riffle amplitude of 0.03 m and wavelengths of 0.5 m) and computational fluid dynamic (CFD) simulations to quantify how restoration structures impacted hyporheic penetration depth, D p , and hyporheic vertical discharge rate, Q v . Restoration structures were channel-spanning vanes with subsurface footers placed in the gravel bed at each riffle. Hyporheic vertical discharge rate was estimated by analyzing solute concentration decay data, and maximum hyporheic penetration depth was measured as the interface between hyporheic water and groundwater using dye tracing experiments. The CFD was verified with literature-based flume hydraulic data and with D p and Q z observations, and the CFD was then used to document how D p and Q z varied with flume discharge, Q, ranging from 1 to 15 L/s (3E þ 03 < Re < 5E þ 04). Flume experiments and CFD simulations showed that restoration structures increased Q z and decreased D p , creating a shallower but higher flux hyporheic zone. Q z had a positive linear relationship with Q, while D p initially grew as Q increased, but then shrunk when a hydraulic jump with low streambed pressured formed downstream of the structure. The restoration structures created counter-acting forces of increased downwelling head due to backwater effects, and increased upwelling due to low streambed pressure and standing waves downstream of the structure. Citation: Zhou, T., and T. A. Endreny (2013), Reshaping of the hyporheic zone beneath river restoration structures: Flume and hydrodynamic experiments, Water Resour. Res., 49, doi:10.1002/wrcr.20384.
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Stream-subsurface exchange processes are important because of their role in controlling the transport of contaminants and ecologically relevant substances in streams. Laboratory flume experiments were conducted to examine solute exchange with gravel streambeds. Two morphologies were studied: flat beds and beds covered by dune-shaped bedforms. High rates of exchange were observed with flat beds under a wide range of stream flow conditions, indicating that there was considerable turbulent coupling of stream and pore water flows. The presence of bedforms produced additional exchange under all flow conditions. The exchange with bedforms could be represented well by considering solute flux caused by bedform-induced advective pumping. Pumping exchange was enhanced by inertial effects, including non-Darcy flow and turbulent diffusion. For the flat bed case, dye injections showed that exchange also occurred by a combination of advective pore water flow and turbulent diffusion near the stream-subsurface interface. The relative effects of advective and diffusive transport processes could not be separated due to the complex nature of the induced flows in the gravel bed. However, exchange was found to scale with the square of the stream Reynolds number in all cases. Comparison of these results with those obtained with coarser and finer sediments demonstrated that the exchange rate is also proportional to the square of the characteristic bed sediment size. These scaling relationships can be used to improve interpretation of solute transport observed in natural rivers.
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A general method for the formulation of flow characteristics which are functions of the Reynolds number of the system is presented. It is assumed that the flow characteristics exhibit a strong variation with the Reynolds number when the Reynolds number is "small," and that they become independent of it when the Reynolds number is "large." The method is illustrated by finding mathematical expressions for the experimentally determined "roughness" function curve and for the sediment transport initiation curve (Shields' curve), which are relevant for the analysis of flow and sediment transport in pipes and open channels. The two expressions thus obtained can be used in practice for computational purposes.Key words: Reynolds number functions, mathematical expression, roughness function, Shields' curve.
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1] We report the first laboratory simulations of hyporheic exchange in gravel pool-riffle channels, which are characterized by coarse sediment, steep slopes, and three-dimensional bed forms that strongly influence surface flow. These channels are particularly important habitat for salmonids, many of which are currently at risk worldwide and which incubate their offspring within the hyporheic zone. Here we perform a set of laboratory experiments examining the effects of discharge and bed form amplitude on hyporheic exchange, with surface-subsurface mixing measured directly from the concentration decay of a conservative tracer (fluorescein) injected into the surface flow. Near-bed pressure measurements were also used to predict hyporheic exchange from a three-dimensional pumping transport model. Comparison of the predicted and observed hyporheic exchange shows good agreement, indicating that the major mechanism for exchange is bed form–induced advection. However, the effect of bed forms is modulated by discharge and the degree of topographic submergence. We also tested the performance of the hydrostatic pressure as a proxy for the observed near-bed pressure in driving hyporheic exchange, which would facilitate field measurement and analysis of hyporheic flow in natural rivers. We found agreement with measured hyporheic exchange only for low bed form amplitudes and high flows. Citation: Tonina, D., and J. M. Buffington (2007), Hyporheic exchange in gravel bed rivers with pool-riffle morphology: Laboratory experiments and three-dimensional modeling, Water Resour. Res., 43, W01421, doi:10.1029/2005WR004328.
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1. This review focuses on the connectivity between river and groundwater ecosystems, viewing them as linked components of a hydrological continuum. Ecological processes that maintain the integrity of both systems and those that are mediated by their ecotones are evaluated. 2. The hyporheic zone, as the connecting ecotone, shows diverse gradients. Thus it can be characterized by hydrological, chemical, zoological and metabolic criteria. However, the characteristics of the hyporheic zone tend to vary widely in space and time as well as from system to system. The exact limits are difficult to designate and the construction of static concepts is inadequate for the representation of ecological processes. The hyporheic interstices are functionally a part of both the fluvial and groundwater ecosystems. 3. The permeability of the ecotone depends on the hydraulic conductivity of the sediment layers which, because of their heterogeneity, form many flowpath connections between the stream and the catchment, from the small scale of a single microhabitat to the large scale of an entire alluvial aquifer. Local up- and downwellings are determined by geomorphologic features such as streambed topography, whereas large-scale exchange processes are determined mainly by the geological properties of the catchment. Colmation—clogging of the top layer of the channel sediments—includes all processes leading to a reduction of pore volume, consolidation of the sediment matrix, and decreased permeability of the stream bed. Consequently, colmation can hinder exchange processes between surface water and groundwater. 4. Physicochemical gradients in the interstices result from several processes: (i) hyporheic flow pattern and the different properties of surface and groundwaters; (ii) retention, caused by the filtering effect of pore size and lithologic sorption as well as the transient storage of solutes caused by diminished water velocities; (iii) biogeochemical transformations in conjunction with local residence time. Each physicochemical parameter may develop its own vertical dynamics laterally from the active channel into the banks as well as longitudinally because of geomorphologic changes. 5. The river–groundwater interface can act as a source or sink for dissolved organic matter, depending on the volume and direction of flow, dissolved organic carbon concentrations and biotic activity. Interstitial storage of particulate organic matter is influenced mainly by grain size distribution and by spates involving bedload movement that may import or release matter, depending on the season. After initial transient and abiotic storage, hyporheic organic matter is mobilized and transformed by the biota. Micro-organisms account for over 90% of the community respiration. In subterranean waters most bacteria are attached to surfaces and remain in a biofilm. 6. Hyporheic interstices are functionally significant for phreatic and riverine metazoans because they act as a refuge against adverse conditions. The net flow direction exerts a dominant influence on interstitial colonization, but many other factors also seem to be important in structuring the hyporheos. 7. The hyporheic corridor concept emphasizes connectivity and interactions between subterranean and surface flow on an ecosystem level for floodplain rivers. It is a complementary concept to others which focus on surficial processes in the lateral and longitudinal dimensions. 8. The ecological integrity of groundwater and fluvial systems is often threatened by human activities: (i) by reducing connectivity; (ii) by altering exchange processes; and (iii) by toxic or organic contamination.
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We combined flume experiments and numerical simulations to examine solute transport processes in a streambed with periodic bedforms. Solute originating from the stream was subjected to advective transport driven by pore water circulation due to current-bedform interactions as well as hydrodynamic dispersion in the porous bed. The experimental and numerical results showed that advection played a dominant role at the early stage of solute transport, which took place in the hyporheic zone. Downward solute transfer to the deep ambient flow zone was controlled by transverse dispersion at the later stage when the elapsed time exceeded the advective transport characteristic time. The advection-based pumping exchange model was found to predict reasonably well solute transfer between the overlying water and streambed at the early stage but its performance deteriorated at the later stage. With dispersion neglected, the pumping exchange model underestimated the long-term rate and total mass of solute transfer from the overlying water to the bed. Therefore both advective and dispersive transport components are essential for quantification of hyporheic exchange processes.
Conference Paper
This paper originates from a study to evaluate the long term effect of river sediment contamination by oil spills, recently initiated at Queen’s University. The paper presents results of the first phase of the study, dedi-cated to exploring a suitable computational framework for the simulation of hyporheic flow through gravel bars. Results of a fully coupled solution of the Reynolds-Averaged Navier-Stokes (RANS) equations for free surface flow and the Darcy-Forchheimer equation, implemented with the CFD package OpenFOAM, are compared against the results of a laboratory experiment carried out by the authors. The agreement is found to be good, as all features of both the free surface and hyporheic flows are accurately captured by the model.
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Freshwater Science published a special series of papers on groundwater-surface-water (GW-SW) interactions in this issue (2015), marking the anniversary of an earlier special series of papers on the hyporheic zone published in 1993. In this concluding paper, I compare the 2 special series of papers and use this comparison to examine the development of the science in the years between 1993 and 2015. The 1993 papers marked the beginning of a period of exponential growth in the study of, and publication of, papers on GW-SW interactions. The 1993 papers tended to be forward looking, proposing conceptual models of GW-SW interactions across stream networks and identifying critical gaps. The 2015 special series of papers contrasts sharply with that of 1993. Broad issue papers are mostly lacking from the current special series. Instead, the special series is dominated by papers focusing on process-based or descriptive studies using empirical approaches. This difference probably stems from major methodological advancements over the past 2 decades that make it possible to study GW-SW interactions with ever greater detail and, thus, allows more complete understanding of specific processes. In contrast, the authors of the 1993 special series were acutely aware of the paucity of GW-SW studies and, thus, posed their conceptual models as hypotheses. Surprisingly, these hypotheses have not been rigorously tested in the decades since their publication. Perhaps it is time to re-examine such broad conceptual models. There remains a critical need for a holistic understanding of how GW-SW interactions vary among streams types and sizes and with changes in discharge among seasons or over storm events and how these GW-SW interactions influence stream ecosystem processes.
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Increased experimentation is essential if we are to understand the biotically rich hyporheic zone and how it influences processes in other parts of lotic systems. Unique challenges face experimenters because the hyporheic zone is both extremely complex hydrologically and relatively inaccessible and difficult to manipulate. I discuss five major obstacles that impede experimental progress in hyporheic research and I suggest corollary experiments or technical developments that are critical to our future research agenda. The first obstacle is that a conceptualization of the hyporheic zone, in which there are clearly definable boundaries that apply across all streams, is unlikely because of significant variations in geology and hydrology between drainage basins. How this zone is delineated may depend on the experimental question and we should "exploit" cross-system differences to motivate experiments. Second, developing methods of obtaining deep streambed samples in an undisturbed fashion, from defined positions within the streambed, and evaluating the effect of our experimental intervention on the process under study requires serious effort. Third, evaluating the impact of bed movement on hyporheic processes and quantifying or controlling for this movement in experiments will require new or modified techniques. Fourth, developing methods for quantifying and experimentally manipulating subsurface flows should be an essential part of research to determine how these flows influence system and community-level processes. This will require adaptation of existing techniques for measuring subsurface flows, so that they are useable at a variety of spatial scales, and simulation of subsurface flows in artificial streams and in-stream chambers. Fifth, the nature and extent to which a hyporheic component or ecological process is influenced by surface vs. groundwater inputs must be quantified. This will require the ability to separate surface from groundwater effects by means of experimental manipulation or design.
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Fifty years of hyporheic zone research have shown the important role played by the hyporheic zone as an interface between groundwater and surface waters. However, it is only in the last two decades that what began as an empirical science has become a mechanistic science devoted to modeling studies of the complex fluid dynamical and biogeochemical mechanisms occurring in the hyporheic zone. These efforts have led to the picture of surface-subsurface water interactions as regulators of the form and function of fluvial ecosystems. Rather than being isolated systems, surface water bodies continuously interact with the subsurface. Exploration of hyporheic zone processes has led to a new appreciation of their wide reaching consequences for water quality and stream ecology. Modern research aims toward a unified approach, in which processes occurring in the hyporheic zone are key elements for the appreciation, management, and restoration of the whole river environment. In this unifying context, this review summarizes results from modeling studies and field observations about flow and transport processes in the hyporheic zone and describes the theories proposed in hydrology and fluid dynamics developed to quantitatively model and predict the hyporheic transport of water, heat, and dissolved and suspended compounds from sediment grain scale up to the watershed scale. The implications of these processes for stream biogeochemistry and ecology are also discussed.
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The hyporheic exchange process is often analyzed considering a homogeneous hyporheic zone. In reality, the streambed sediments are typically heterogeneous, which may result in complex hyporheic exchange patterns and spatially variable interfacial fluxes. We performed salt and dye injection experiments to study hyporheic exchange under different flow and sedimentary conditions in a recirculating laboratory flume packed with a heterogeneous sediment bed, designed as a correlated random hydraulic conductivity field. We analyzed the experimental results using a numerical hyporheic exchange model developed on the basis of pressure-driven advective pore water flow (pumping) and explicit representation of the detailed sediment structure. We also compared these results with an analytical solution for pore water flow. Both analytical and numerical exchange models reasonably predicted the bulk hyporheic exchange. However, the numerical model gave a better prediction of the faster exchange during early times, and predicted the heterogeneity-induced complex hyporheic pathways with good accuracy. The numerical model also explained the effect of heterogeneity on interfacial water flux, solute penetration depth and hyporheic residence time. In general, heterogeneity caused greater average water flux through the bed surface, but decreased both the solute penetration depth and residence time compared to an equivalent homogeneous bed.
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Experimental results of measurements characterising the pressure and velocity above and within a porous gravel layer are presented. The goal of this study is to give a better understanding of the flow in the hyporheic interstitial under the influence of turbulence in the main flow.Latest developments in measuring techniques were applied: miniaturised piezoelectric pressure sensors (MPPS) were used to measure turbulent pressure fluctuations inside the gravel layer. Velocity measurements were carried out by a 3D-particle tracking velocimetry system (3D-PTV) using miniaturised endoscopic stereo setups within artificial gravel pores. Additionally, in the main flow a 1D-acoustic doppler current profiler (1D-ADCP) was used. Within the main flow, alternating faster and slower fluid packets with a size scaling with the flow depth were observed. Pressure fluctuations rms(p) as well as velocity fluctuations rms(u, v, w) decrease exponentially with increasing gravel depth, mainly within the first two layers of gravel grains.
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Experiments were performed to determine the mechanisms and rates of solute exchange between a flowing stream and porous streambed. Mass balance, flow visualisation, and depth profiles of concentration were used to investigate the exchange of an inert tracer in a laboratory flume. The measured mass transfer was compared to the exchange predicted by models of exchange related to bed forms, which are presented in a companion paper. For the initial stages with stationary bed forms or slowly moving bed forms the net exchange was predicted satisfactorily and was dominated by bed form-induced interstitial flows (pumping). In the initial stages with rapidly moving bed forms the exchange was dominated by scour/deposition as bed forms propagated (turnover). At later times the models of bed form-related exchange significantly underpredicted the measured exchange. The additional exchange, and the exchange to a flat bed, may be related to bed inhomogeneity or irregular variations in pressure at the bed surface which are not related to bed forms. Such effects are likely to be more pronounced in the field situation.
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Local pressure variations of the order of 100-1000 N/m2 can be observed between the upstream and downstream faces of the typically triangular-shaped dunelike sediment structures that form at the sediment-water interface of rivers. Laboratory experiments were conducted examining the influence of this localized pressure variation on contaminant transport processes within the sediment. Numerical modeling of the in-bed flow via boundary element methods was also undertaken in order to predict convective transport under typical field conditions. The laboratory experiments and numerical simulation of the in-bed flow in several rivers verified that the pressure distribution observed on the sediment surface and the resulting interstitial fluid convection can control transport of chemically inert, nonsorbed contaminants in stable sediments. In-bed Peclet numbers were of the order of 100-1000, indicating the negligible influence of diffusion under the conditions examined.
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Convective flow within bed sediment is an important mechanism enhancing the mobility of chemicals, both natural and anthropogenic, and thermal energy in this region of aquatic environments1,2. Experimental observations indicated that significant in-bed convection currents can be generated by water flowing over small obstructions on the surface of a porous bed. Significant porewater flow is induced by imbalances in pressure over distance, generated by differences in temperature, density and hydrostatic head3. We demonstrate here by laboratory simulation and a vignette model that flow over bedforms induces additional pressure imbalances which generate significant and complex convection currents within porous bed sediment. A model is proposed for estimating Peclet numbers for this effect/The results have particular application to chemical transport in the upper sediment layer that is often the recipient of high levels of chemical contamination. Although our analysis reflects river conditions, the results may have wider applications and include submarine currents moving over dune-like mega ripples on the ocean floor.
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Developing and testing models for solute transport in the field requires experimental data on the spreading of solutes in the soil. Obtaining such data is costly, and a substantial part of the total costs is in the preparation and chemical analysis of the tracing compounds in the gathered samples. We developed a cheap method to quantify the concentration of the mobile dye tracer Brilliant Blue FCF from digitized photographs of stained soil profiles, and we have tested it in the field. Soil sampling and chemical analyses were necessary only to establish a calibration relation between the dye content and the colour of the soil. The digital images were corrected for geometrical distortions, varying background brightness, and colour tinges, and then they were analysed to determine the soil colour at sampling points in the profiles. The resident concentration of the dye was modelled by polynomial regression with the primary colours red, green, blue and the soil depth as explanatory variables. Concentration maps of Brilliant Blue were then computed from the digitized images with a spatial resolution of 1 mm. Validation of the technique with independent data showed that the method predicted the concentration of the dye well, provided the corrected images contained only the colours included in the calibration.
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The hyporheic zone is an active ecotone between the surface stream and ground-water. Exchanges of water, nutrients, and organic matter occur in response to variations in discharge and bed topography and porosity. Upwelling subsurface water supplies stream organisms with nutrients while downwelling stream water provides dissolved oxygen and organic matter to microbes and invertebrates in the hyporheic zone. Dynamic gradients exist at all scales and vary temporally. At the microscale, gradients in redox potential control chemical and microbially medi-ated nutrient transformations occurring on particle surfaces. At the stream-reach scale, hydrological exchange and water residence time are reflected in gradients in hyporheic faunal composition, uptake of dissolved organic carbon, and nitri-fication. The hyporheic corridor concept describes gradients at the catchment scale, extending to alluvial aquifers kilometers from the main channel. Across all scales, the functional significance of the hyporheic zone relates to its activity and connection with the surface stream.
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1] The conceptual model of hyporheic exchange below river steps may oversimplify exchange flow paths if it depicts a uniform pattern of downstream-directed upwelling. This research used nonmobile, porous bed flume experiments and hydrodynamic simulation (CFD) to characterize hyporheic flow paths below a river step with a hydraulic jump. Bed slope was 1%, step height was 4 cm, downstream flow depth was 4 cm, substrate was 1 cm median diameter gravel, and hydraulic jump length was 25 cm in the flume and CFD experiments. With the hydraulic jump, flow paths changed to include downwelling beneath the water plunging into the pool and upstream-directed upwelling at the base of the step and beneath the length of jump. Failure to represent the influence of static and dynamic pressures associated with hydraulic jumps leads to erroneous prediction of subsurface flow paths in 75% of the streambed beneath the jump. A refined conceptual model for hyporheic flow paths below a step with a hydraulic jump includes reversed hyporheic circulation cells, in which downwelling water moves upstream and then upwells, and flow reversals, in which the larger flow net of downstream-directed upwelling encounters a nested flow path of upstream-directed upwelling. Heterogeneity in hyporheic flow paths at hydraulic jumps has the potential to explain field-observed mosaics in streambed redox patterns and expand structure-function relationships used in river management and restoration. Citation: Endreny, T., L. Lautz, and D. I. Siegel (2011), Hyporheic flow path response to hydraulic jumps at river steps: Flume and hydrodynamic models, Water Resour. Res., 47, W02517, doi:10.1029/2009WR008631.
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) spawning habitat. Information exists on the microhabitat characteristics that define suitable salmon spawning habitat. However, traditional spawning habitat models that use these characteristics to predict available spawning habitat are restricted because they can not account for the heterogeneous nature of rivers. We present a conceptual spawning habitat model for fall chinook salmon that describes how geomorphic features of river channels create hydraulic processes, including hyporheic flows, that influence where salmon spawn in unconstrained reaches of large mainstem alluvial rivers. Two case studies based on empirical data from fall chinook salmon spawning areas in the Hanford Reach of the Columbia River are presented to illustrate important aspects of our conceptual model. We suggest that traditional habitat models and our conceptual model be combined to predict the limits of suitable fall chinook salmon spawning habitat. This approach can incorporate quantitative measures of river channel morphology, including general descriptors of geomorphic features at different spatial scales, in order to understand the processes influencing redd site selection and spawning habitat use. This information is needed in order to protect existing salmon spawning habitat in large rivers, as well as to recover habitat already lost.
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The hydrodynamics of a system where there is a coupled flow above and below a sediment–water interface (SWI) are not completely understood. We numerically simulate mean two-dimensional, unidirectional, steady, viscous flow in these systems using a sequentially coupled formulation. Simulations were conducted to determine fundamental relationships between bedform geometry, Reynolds number for the water-column flow (Re), interfacial exchange zone depth (dz) in the sediments, and flux through the SWI (qint); the latter two parameters play a significant role in biogeochemical and aquatic-life processes across the SWI. dz and Re are functionally related through an asymptotic growth-curve model while qint and Re follow a power function. These relationships are dynamically explained by the manner in which pressure gradients along the SWI develop due to current–bedform interactions at different Res and by Darcy’s Law. We found that the coupling between water column and exchange zone flow is controlled by the behavior of the water-column eddy. The eddy detaches at or near the point of minimum pressure along the interface, and reattaches near the point of maximum pressure. These two critical points determine the pressure gradient along the bed surface that controls the exchange zone flow field. Moreover, the reattachment point corresponds to flow divides within the sediments. Lastly, pore-water velocities drop with depth below the SWI, and are larger below the bedform crests than below the troughs.
Article
We investigated the role of increasingly well-constrained geologic structures in the subsurface (i.e., subsurface architecture) in predicting streambed flux and hyporheic residence time distribution (RTD) for a headwater stream. Five subsurface realizations with increasingly resolved lithological boundaries were simulated in which model geometries were based on increasing information about flow and transport using soil and geologic maps, surface observations, probing to depth to refusal, seismic refraction, electrical resistivity (ER) imaging of subsurface architecture, and time-lapse ER imaging during a solute tracer study. Particle tracking was used to generate RTDs for each model run. We demonstrate how improved characterization of complex lithological boundaries and calibration of porosity and hydraulic conductivity affect model prediction of hyporheic flow and transport. Models using hydraulic conductivity calibrated using transient ER data yield estimates of streambed flux that are three orders of magnitude larger than uncalibrated models using estimated values for hydraulic conductivity based on values published for nearby hillslopes (10(-4) vs. 10(-7) m(2) /s, respectively). Median residence times for uncalibrated and calibrated models are 10(3) and 10(0) h, respectively. Increasingly well-resolved subsurface architectures yield wider hyporheic RTDs, indicative of more complex hyporheic flowpath networks and potentially important to biogeochemical cycling. The use of ER imaging to monitor solute tracers informs subsurface structure not apparent from other techniques, and helps to define transport properties of the subsurface (i.e., hydraulic conductivity). Results of this study demonstrate the value of geophysical measurements to more realistically simulate flow and transport along hyporheic flowpaths.
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Contaminant hydrogeological processes occurring in porous media are typically not amenable to direct observation. As a result, indirect measurements (e.g., contaminant breakthrough at a fixed location) are often used to infer processes occurring at different scales, locations, or times. To overcome this limitation, non-invasive imaging methods are increasingly being used in contaminant hydrogeology research. Four of the most common methods, and the subjects of this review, are optical imaging using UV or visible light, dual-energy gamma radiation, X-ray microtomography, and magnetic resonance imaging (MRI). Non-invasive imaging techniques have provided valuable insights into a variety of complex systems and processes, including porous media characterization, multiphase fluid distribution, fluid flow, solute transport and mixing, colloidal transport and deposition, and reactions. In this paper we review the theory underlying these methods, applications of these methods to contaminant hydrogeology research, and methods' advantages and disadvantages. As expected, there is no perfect method or tool for non-invasive imaging. However, optical methods generally present the least expensive and easiest options for imaging fluid distribution, solute and fluid flow, colloid transport, and reactions in artificial two-dimensional (2D) porous media. Gamma radiation methods present the best opportunity for characterization of fluid distributions in 2D at the Darcy scale. X-ray methods present the highest resolution and flexibility for three-dimensional (3D) natural porous media characterization, and 3D characterization of fluid distributions in natural porous media. And MRI presents the best option for 3D characterization of fluid distribution, fluid flow, colloid transport, and reaction in artificial porous media. Obvious deficiencies ripe for method development are the ability to image transient processes such as fluid flow and colloid transport in natural porous media in three dimensions, the ability to image many reactions of environmental interest in artificial and natural porous media, and the ability to image selected processes over a range of scales in artificial and natural porous media.
Article
The development and evaluation of a 2-dimensional physical model, which is designed to assist in the characterisation of complex solute transport problems in porous media, is described. The laboratory model is a transparent 2-dimensional porous media of nominal thickness and uses a non-invasive imaging technique in conjunction with a fluorescent dye tracer (sodium fluorescein) to monitor solute movements. Under ultraviolet (UV) illumination the dye emits visible light which is imaged by a CCD (Charge Coupled Device) camera. The image is processed to estimate the 2-dimensional distribution of tracer concentrations. The system can successfully model a simple contaminant plume within a homogenous porous matrix constructed from glass beads (60-100 microm). Experimental results show that transverse dispersion coefficient was 3.9 x 10(-10) m2/s when sodium fluorescein transported in porous matrix with a walter velocity of 5.71 x 10(-6) m/s. The low transverse dispersion coefficient suggests that the molecular diffusion of solute cannot be neglected under low velocity of the water. The advantages of using UV rather than an ordinary light system are a reduction in noise and experimental errors. Errors due to light dispersion within the model are shown to be negligible for the current model. Since contaminant with aromatic rings are usually fluorescent and biological samples can be labelled by fluorescent dye, this imaging technique using UV excited fluorescent dye will be used to investigate biodegradation process in porous media.
Article
Vertical transverse mixing is known to be a controlling factor in natural attenuation of extended biodegradable plumes originating from continuously emitting sources. We perform conservative and reactive tracer tests in a quasi two-dimensional 14 m long sand box in order to quantify vertical mixing in heterogeneous media. The filling mimics natural sediments including a distribution of different hydro-facies, made of different sand mixtures, and micro-structures within the sand lenses. We quantify the concentration distribution of the conservative tracer by the analysis of digital images taken at steady state during the tracer-dye experiment. Heterogeneity causes plume meandering, leading to distorted concentration profiles. Without knowledge about the velocity distribution, it is not possible to determine meaningful vertical dispersion coefficients from the concentration profiles. Using the stream-line pattern resulting from an inverse model of previous experiments in the sand box, we can correct for the plume meandering. The resulting vertical dispersion coefficient is approximately approximately 4 x 10(-)(9) m(2)/s. We observe no distinct increase in the vertical dispersion coefficient with increasing travel distance, indicating that heterogeneity has hardly any impact on vertical transverse mixing. In the reactive tracer test, we continuously inject an alkaline solution over a certain height into the domain that is occupied otherwise by an acidic solution. The outline of the alkaline plume is visualized by adding a pH indicator into both solutions. From the height and length of the reactive plume, we estimate a transverse dispersion coefficient of approximately 3 x 10(-)(9) m(2)/s. Overall, the vertical transverse dispersion coefficients are less than an order of magnitude larger than pore diffusion coefficients and hardly increase due to heterogeneity. Thus, we conclude for the assessment of natural attenuation that reactive plumes might become very large if they are controlled by vertical dispersive mixing.
Analysis of hyporheic flow induced by a bar in a gravel stream: an experimental study
  • C J Fruetel
Fruetel, C.J. 2016. Analysis of hyporheic flow induced by a bar in a gravel stream: an experimental study. M.A.Sc. thesis, Department of Civil Engineering, Queen's University, Kingston, Ontario.
Hydrodynamics of coupled flow above and below a sediment-water interface with triangular bedforms
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  • J L Silva
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Cardenas, M.B., and Wilson, J.L. 2007. Hydrodynamics of coupled flow above and below a sediment-water interface with triangular bedforms. Advances in Water Resources, 30(3): 301-313. doi:10.1016/j.advwatres.2006.06.009. da Silva, A.M.F., and Bolisetti, T. 2000. A method for the formulation of Reynolds number functions. Canadian Journal of Civil Engineering, 27(4): 829-833. doi:10.1139/l00-002.
Expert Panel Report on the Behaviour and Environmental Impacts of Crude Oil Released into Aqueous Environments
  • K Lee
  • M Boufadel
  • B Chen
  • J Foght
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  • A Venosa
Lee, K., Boufadel, M., Chen, B., Foght, J., Hodson, P., Swanson, S., and Venosa, A. 2015. Expert Panel Report on the Behaviour and Environmental Impacts of Crude Oil Released into Aqueous Environments. Royal Society of Canada, Ottawa, Ont. [ISBN: 978-1-928140-02-3.]