Hyporheic Exchange Research Articles

Characterization of Hyporheic Exchange Drivers and Patterns within a Low-Gradient, First-Order, River Confluence during Low and High Flow

Confluences are nodes in riverine networks characterized by complex three-dimensional changes in flow hydrodynamics and riverbed morphology, and are valued for important ecological functions. This physical complexity is often investigated within the water column or riverbed, while few studies have focused on hyporheic fluxes, which is the mixing of surface water and groundwater across the riverbed. This study aims to understand how hyporheic flux across the riverbed is organized by confluence physical drivers. Field investigations were carried out at a low gradient, headwater confluence between Baltimore Brook and Cold Brook in Marcellus, New York, USA. The study measured channel bathymetry, hydraulic permeability, and vertical temperature profiles, as indicators of the hyporheic exchange due to temperature gradients. Confluence geometry, hydrodynamics, and morphodynamics were found to significantly affect hyporheic exchange rate and patterns. Local scale bed morphology, such as the confluence scour hole and minor topographic irregularities, influenced the distribution of bed pressure head and the related patterns of downwelling/upwelling. Furthermore, classical back-to-back bend planform and the related secondary circulation probably affected hyporheic exchange patterns around the confluence shear layer. Finally, even variations in the hydrological conditions played a role on hyporheic fluxes modifying confluence planform, and, in turn, flow circulation patterns.

Effect of Hyporheic Exchange on Macroinvertebrate Community in the Weihe River Basin, China

The effect of hyporheic exchange on macroinvertebrates is a significant topic in ecohydraulics. A field study was conducted during May and June 2017 to investigate the impacts of magnitude and patterns of hyporheic exchange on the sediment macroinvertebrate community in the Weihe River basin. The results demonstrate that upwelling flows cause resuspension of riverbed sediment, increasing the proportion of swimmer groups (such as Baetidae) in the macroinvertebrate community. However, large resuspension of river bed sediment results in a reduced abundance of macroinvertebrates. By controlling the transport processes of dissolved oxygen (DO), dissolved organic carbon (DOC), nutrients, temperature, and different patterns of hyporheic exchange strongly influence the structure of macroinvertebrate communities. Downwelling is more likely to produce rich invertebrate communities than upwelling. The magnitude for the hyporheic flux of 150–200 mm/d was optimal for the macroinvertebrate community in the Weihe River Basin. Above or below this rate results in a decline in community abundance and diversity. We suggest that research is conducted to better understand the effects of hyporheic exchange across bedforms on macroinvertebrate communities. The study supports any activities to preserve the ecological functions and health of rivers dominated by fine-grained sediments.

Filling the Void: The Effect of Stream Bank Soil Pipes on Transient Hyporheic Exchange During a Peak Flow Event

AbstractThe hyporheic zone is the ecotone between stream and river channel flow and groundwater that can process nutrients and improve water quality. Transient hyporheic zones occur in the riparian zone (bank storage or “lung model” exchange) during channel stage fluctuations. Recent studies show that soil pipes are widespread in stream banks and beneath floodplains, creating highly preferential flow between channel and riparian groundwater such that the traditional Darcy model of flow does not apply. We used MODFLOW with the conduit flow package to model a series of stream bank soil pipes and examined soil pipe density (number per m), length, diameter, height above baseflow water surface, connectivity, and matrix hydraulic conductivity on transient particle flow paths and total hyporheic exchange volume (i.e., bank storage) over the course of a peak flow (e.g., storm) event. We found that adding five soil pipes per meter more than doubled hyporheic volume. Soil pipe length was the most important control; adding one 1.5‐m‐long soil pipe caused a 73.4% increase in hyporheic volume. The effect of increasing soil pipe diameter on hyporheic volume leveled off at ~1 cm, as flow limitation switched from pipe flow to pipe‐matrix exchange. To validate our approach, we used the model to successfully reproduce trends from field studies. Our results highlight the need to consider soil pipes when modeling, monitoring, or managing bank storage, floodplain connectivity, or hyporheic exchange.

Fine Sediment Deposition and Filtration Under Losing and Gaining Flow Conditions: A Particle Tracking Model Approach

AbstractFine particle deposition within riverbeds plays a major role in riverine ecology and biogeochemistry by altering hyporheic exchange flux. Moreover, it is ubiquitous within streams and rivers across all flow stages. However, the dynamics of fine particle deposition are still not completely understood in rivers, and continuum models like the advection dispersion equation require modifications to represent the processes accurately. To enhance understanding of fine particle dynamics, we developed a novel numerical particle tracking model that simulates fine particle deposition as a stochastic process under losing, neutral, and gaining streamflow conditions. These flow conditions generate three different velocity profiles by combining the free surface and groundwater flows. In addition, a novel aspect of our model is the storage of filtered particles to estimate concentration fields within the bed. Our simulated results are qualitatively compared with previous laboratory flume experimental results of kaolinite deposition under similar conditions. The model indicates that fine particle deposition patterns and residence time functions depend heavily on the exchange flux between stream and groundwater, as well as bed filtration properties as the deposition of particles occurs at greater depths in the losing stream condition than in the neutral and gaining cases. Therefore, the spatial pattern of particle deposition is a direct result of pore water velocity profiles, while the concentration depends on filtration dynamics within the bed.

DTSGUI: A Python Program to Process and Visualize Fiber-Optic Distributed Temperature Sensing Data.

Fiber-optic distributed temperature sensing (FO-DTS) has proven to be a transformative technology for the hydrologic sciences, with application to diverse problems including hyporheic exchange, groundwater/surface-water interaction, fractured-rock characterization, and cold regions hydrology. FO-DTS produces large, complex, and information-rich datasets. Despite the potential of FO-DTS, adoption of the technology has been impeded by lack of tools for data processing, analysis, and visualization. New tools are needed to efficiently and fully capitalize on the information content of FO-DTS datasets. To this end, we present DTSGUI, a public-domain Python-based software package for editing, parsing, processing, statistical analysis, georeferencing, and visualization of FO-DTS data.

A model to integrate urban river thermal cooling in river restoration

River water quality and habitats are degraded by thermal pollution from urban areas caused by warm surface runoff, lack of riparian forests, and impervious channels that transfer heat and block cool subsurface flows. This study updates the i-Tree Cool River model to simulate restoration of these processes to reverse the urban river syndrome, while using the HEC-RAS model water surface profiles needed for flood hazard analysis in restoration planning. The new model was tested in a mountain river within the New York City drinking water supply area (Sawmill, SM, Creek), and then used for base case and restoration scenarios on the 17.5 km reach of the Los Angeles (LA) River where a multi-million dollar riverine restoration project is planned. The model simulated the LA River average temperature in the base case decreased from 29.5 °C by 0.3 °C when warm surface inflows were converted to cooler groundwater inflows by terrestrial green infrastructure; by 0.7 °C when subsurface hyporheic exchange was increased by removal of armoring and installation of riffle-pool bedforms; by 3.6 °C when riparian forests shaded the river; and by 6.4 °C when floodplain forests were added to riparian forests to cool surface reservoirs and local air temperatures. Applying all four restoration treatments lowered river temperature by 7.2 °C. The simulated decreases in river temperature lead to increased saturated dissolved oxygen levels, reaching 8.7 mg/L, up from the 7.6 mg/L in the base case scenario, providing improved fish habitat and reducing eutrophication and hypoxic zones. This study evaluating the performance of environmental management scenarios could help managers control the thermal pollution in rivers.

Modeling and comparative analysis of a flow and heat coupling model of the riparian zone based on thermal conductivity empirical models

Recently, the use of heat as a tracer to evaluate the process of hyporheic exchange in riparian zones has attracted wide attention. A more accurate flow-heat coupling model of riparian zones could help us understand the patterns of water flow and heat transfer in riparian zones and provide a scientific basis for the comprehensive management and efficient utilization of these zones. In this paper, a flow-heat coupling model of a riparian zone based on thermal conductivity empirical models (TCEMs) was built by customizing partial differential equations (PDEs) based on the simulation of soil water flow using porous media and the subsurface flow module in COMSOL. Combined with the data collected from the field heat tracer test in the riparian zone, the flow-heat coupling model of the riparian zone under 12 types of TCEMs was validated and compared. The results show that the use of the PDE module instead of the heat transfer in porous media (HTPM) module is very effective for modeling; the PDE module could basically replace the HTPM module in the heat transfer calculation. The performance of the flow-heat coupling model varies under different types of TCEMs, and the Chung and Horton (1987) model shows better simulation effects, with a root mean square error (RMSE), coefficient of determination (R2), mean absolute error (MAE), and mean relative error (MRE) ranging from 1.37 to 2.48 ℃, 0.73–0.94, 1.06–2.08 ℃, and 11.94–15.79%, respectively. Therefore, this model could better reflect the dynamic temperature variations in the riparian zone. The sensitivity analysis results illustrate that the hydraulic conductivity (Ks), van Genuchten parameter (β), volumetric heat capacity of dry solids (Cs), and porosity (n) of the flow-heat coupling model greatly influence riparian zone temperature variations, and the parameters β, Ks, and residual water content (θr) significantly affect the lateral hyporheic exchange rate in the riparian zone, of which β has the largest effect.

Stream and floodplain restoration impacts riparian zone hydrology of agricultural streams.

While the influence of stream restoration on vertical and lateral hyporheic exchange has been documented, impacts on broader riparian zone hydrology have not been thoroughly investigated. We quantified riparian water table dynamics, hydraulic gradient, and groundwater flow paths and fluxes across a range of hydrologic conditions following natural channel design restoration (riparian regrading, in-channel cross-vane structure installation). Water table measurements were collected at least once per season for 2.5 years to capture water levels during baseflow conditions from networks of wells and piezometers at sites with different stream morphology (created riffle-cross-vane-scour pool complexes versus natural pools and riffles), restoration status (agricultural restored, unrestored, forested reference), and riparian characteristics (slope, soils, topography) in North Carolina, USA. The regraded riparian zone had higher near-stream water tables (< 0.5 m below ground surface) than the unrestored site. Riffle-cross-vane complexes induced a zone of low hydraulic gradient that spanned 30-40% of the riparian area, similar to groundwater dynamics near beaver dams. This effect persisted regardless of hydrologic condition (wet or dry) or scour pool status (functioning or filled-in). Riffles also promoted a lower near-stream hydraulic gradient at the forested site. Conversely, the influence of stream features on riparian groundwater was minimal at the unrestored site, where groundwater fluxes were controlled by hillslope inputs and riparian geomorphology. Overall, restoration enhanced stream-riparian zone hydrologic interaction beyond the immediate hyporheic zone. Our work stresses that cross-vanes, even when partially buried by sediments post-restoration, impact whole floodplain hydrology in a more significant way than shown by prior stream restoration studies.

Impact of Riparian and Stream Restoration on Denitrification in Geomorphic Features of Agricultural Streams

HighlightsDenitrification enzyme activity (DEA) was measured in stream sediments of restored and unrestored agricultural streams.Nitrate, sediment characteristics, riparian vegetation, and geomorphology influenced DEA.Pools at restored sites had lower organic carbon, coarser sediment textures, and lower denitrification potential.Restoration strategies should increase organic carbon and residence times through complex flowpaths.Abstract. Agricultural land use, channel modifications, and riparian vegetation composition can affect instream denitrification by altering geomorphic features, such as sediment texture, organic matter, retention time, and hyporheic exchange. Stream and riparian restoration is widely implemented in agricultural watersheds to mitigate excess nutrient export to sensitive downstream waters; however, the cumulative impact of channel reconstruction and altered channel and near-stream morphology on nitrogen dynamics remains poorly understood. We measured denitrification enzyme activity (DEA) and environmental variables (e.g., water chemistry, sediment texture, and organic matter) in different geomorphic features in agriculturally influenced streams in North Carolina with varied channel and riparian zone characteristics. Our results indicate that denitrification is primarily influenced by increased transport of nitrate (NO3-) to the streams in wetter months. Secondarily, structural factors, including riparian vegetation and stream geomorphology, impact denitrification by controlling the distribution of sediment texture and organic carbon. In the newly restored stream, we observed coarser streambed sediments and low sediment organic carbon, especially in scour pools constructed downstream from cross-vanes. Lower DEA was observed in restored pools (39.1 ng N g-1 dry mass h-1) compared to naturally occurring pools (70.7 to 278.1 ng N g-1 dry mass h-1). These results highlight the need for restoration strategies to be directed at increasing organic carbon and residence times through complex flowpaths (e.g., meanders, root wads, artificial woody debris dams). Keywords: Denitrification, Freshwater, Nitrogen, Restoration, Riparian, Stream, Water quality.

I-Tree cool river: An open source, freeware tool to simulate river water temperature coupled with HEC-RAS

This method paper explains the i-Tree Cool River model algorithms for simulating the response of river water temperature to urban greening. The model captures the warming and cooling impacts of urban development and restoration through a water and energy budget. The water budget includes river inflows from urban storm sewers and reservoirs, and the associated water temperatures. The energy budget adjusts radiation fluxes due to riparian shading and evapotranspiration, and propagates temperature downstream. Restorative cooling of the river can be simulated through algorithms for cool groundwater, either as direct inflows or by river water replacement called hyporheic exchange. Novel features in the model include diurnal variation in riparian shading, use of the Army Corps of Engineers HEC-RAS model predicted river depths and velocities, and periodic boundary conditions to rapidly extend restoration scenarios.•Freely available code in C++ for Visual Studio, with a detailed manual and sample inputs and outputs at http://www.itreetools.org/research_suite/coolriver.•Useful for simulating river warming or cooling due to urban development or greening.•Well documented source code compatible with requirements of other modeling groups.

Multiscale Feature‐feature Interactions Control Patterns of Hyporheic Exchange in a Simulated Headwater Mountain Stream

AbstractRecent predictions of hyporheic exchange at the basin‐scale assume individual features control exchange independently of each other, which has been demonstrated in relatively uniform, low‐gradient rivers. However, this assumption may not hold in steep catchments where both the type and size of individual features may vary over short distances, leading to irregular patterns of feature dominance on hyporheic exchange flows. Also, steep longitudinal gradients support substantial downvalley flows in the subsurface, which may create feedbacks between adjacent features. In this study, we test the extent to which features interact with one another and whether they can be aggregated to make reach‐scale predictions in a headwater mountain stream. Using systematic manipulations of a 2‐D stream centerline model and spectral analyses, we test for the presence of both feature‐feature and multiscale interactions. Our results show that changing the height of individual step‐pool features can alter hyporheic flow fields in neighboring, and sometimes distant, features. Spectral analyses revealed two scales of streambed topography—a local scale of single features and an intermediate scale that spanned multiple local‐scale features. All features produced hyporheic exchange, but turnover of deeper hyporheic water only occurred at a few key locations where local‐ and intermediate‐scale features amplified each other. Further, shallow bedrock increases the ratio of local‐ and intermediate‐scale flowpaths to regional‐scale flowpaths. Conceptual models portraying hyporheic exchange as a series of nested flowpaths should include the interactions among streambed topographic features in mountain streams. These results have implications for upscaling, field experiments, and stream restoration in steep catchments.

Fine‐Particle Deposition, Retention, and Resuspension Within a Sand‐Bedded Stream Are Determined by Streambed Morphodynamics

AbstractFine‐particle dynamics occupy an integral role in nutrient, contaminant, and pathogen transport in rivers. Due to low settling velocities, fine particles are often treated as if they pass through the river without interacting significantly with bed sediments. However, fine particles are advected toward and into the bed by turbulence and hyporheic exchange, leading to deposition by filtration. Extensive remobilization of fine particles is commonly observed during high flow conditions, but these dynamics are not well understood. Here we analyze the remobilization of fine tracer particles within the context of sediment morphodynamics during an experimental flood. Field data collected within Clear Run, a sand‐bedded stream, by Harvey et al. (2012, https://doi.org/10.1029/2012JG002043) represent subsurface and water column time series of solute and fine particulate tracers. Overhead time lapse photography of the motion and growth of bed forms during the flood are used to extract and quantify morphodynamics, which we relate to the resuspension of fine particles by transforming bed form wavelength statistics into scour depth through a set of geometric relations derived from laboratory experiments. Particles deposited near the bed surface such that scour from larger bed forms remobilized the majority of particles within the streambed (&gt;72%) accounting for the increase in resuspended fine particles during the flood. This analysis demonstrates that the inclusion of bed form dynamics and scour is necessary for understanding the remobilization of fine particles in Clear Run and that interpreting fine‐particle dynamics during and beyond the event scale requires coupling with bed morphodynamics.

The effect of unsteady streamflow and stream-groundwater interactions on oxygen consumption in a sandy streambed

Streamflow dynamics are often ignored when studying biogeochemical processes in the hyporheic zone. We explored the interactive effects of unsteady streamflow and groundwater fluxes on the delivery and consumption of oxygen within the hyporheic zone using a recirculating flume packed with natural sandy sediments. The flume was equipped with a programmable streamflow control and drainage system that was used to impose losing and gaining fluxes. Tracer tests were used to measure hyporheic exchange flux and a planar optode was used to measure subsurface oxygen concentration patterns. It was found that the volume of the oxic zone decreased when the losing flux declined, and was drastically decreased when gaining conditions were applied. It was also found that unsteady streamflow led to a slight increase in the average volume of the oxic zone, compared to the average volume of the oxic zone under steady streamflow. However, the average oxygen consumption rates were significantly higher under unsteady streamflow compared to steady streamflow under all groundwater conditions with the exception of the highest losing flux. The present study provides the first insight into the interactions between streamflow unsteadiness and losing/gaining fluxes and improve understanding of their impact on microbial metabolism in the hyporheic zone.

Dynamics of heat transport across sediment deposited hyporheic zone inside reservoirs following hydropower production

Heat transport through hyporheic zones is critical for many temperature-sensitive biogeochemical processes and survival of aquatic organisms. Water storing and releasing in the reservoir are often made to modify hydropower production, which may cause hydrological variation and alter heat transport regime across the hyporheic zone inside the reservoir. In this study, we investigated heat transport across the hyporheic zone within a sediment deposited bar inside a hydropower reservoir on the mainstream of the upper Mekong River. Hydrothermal dynamics in the hyporheic zone following hydropower production were analyzed by monitoring changes of water level and temperature using automatic monitoring wells. Results showed that hydropower production induced water level fluctuations, enhanced hyporheic exchanges and thereby accelerated heat transport across the bar. The hyporheic zone was a heat source for the reservoir water regardless of the time of the year. In summer, heat transport from the bar hyporheic zone to the reservoir was strong (0.50 × 108–10.28 × 108 J m−1 d−1) and mainly occurred through near-surface sediment layer (0–0.5 m), which was attributed to active hyporheic exchanges and strong solar radiation. In winter, heat transport was weak (0.01 × 108–1.01 × 108 J m−1 d−1) and mainly occurred in sediment bottom layer (< 1.0 m) with limited energy inputs from microbial activities. This study adds to our understanding of hydrothermal dynamics in hyporheic zones under flow regulations by dams.

Enhanced riparian denitrification in reservoirs following hydropower production

It is commonly assumed that dams retain nutrients and cause eutrophication in reservoirs after impoundment. In this study, we explored denitrification in a riparian zone at the land–water interface in a hydropower reservoir of the upper Mekong River. Results demonstrated that hydropower production intensified water level fluctuations in the reservoir and significantly accelerated riparian denitrification process at the land–water interface; the denitrification rate exhibited a strong positive relationship with the period of water level rising-falling cycle (r2 = 0.98, P < 0.05). Intensified water level fluctuations created frequently alternate wetting–drying cycles and enhanced hyporheic exchanges in the riparian zone, which accelerated organic carbon mineralization and thereby increased carbon source supply to fuel denitrification. In addition, river impoundment after dam construction appreciably enlarged the hot spots for riparian denitrification. The accelerated denitrification as well as the enlarged hot spots area jointly enhanced nitrogen removal from impounded rivers. These findings could advance fundamental knowledge on the environmental impacts of hydropower development on rivers.

Modeling the Effect of Hyporheic Mixing on Stream Solute Transport

AbstractMixing 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 breakthrough curves (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 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. While both models provided an equally good approximation for the BTCs observed in field tracer tests, the exponentially attenuated mixing model 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.

A review of methods for measuring groundwater–surface water exchange in braided rivers

Abstract. Braided rivers, while uncommon internationally, are significant in terms of their unique ecosystems and as vital freshwater resources at locations where they occur. With an increasing awareness of the connected nature of surface water and groundwater, there have been many studies examining groundwater–surface water exchange in various types of waterbodies, but significantly less research has been conducted in braided rivers. Thus, there is currently limited understanding of how characteristics unique to braided rivers, such as channel shifting, expanding and narrowing margins, and a high degree of heterogeneity affect groundwater–surface water flow paths. This article provides an overview of characteristics specific to braided rivers, including a map showing the regions where braided rivers are mainly found at the global scale: Alaska, Canada, the Japanese and European Alps, the Himalayas, Russia, and New Zealand. To the authors' knowledge, this is the first map of its kind. This is followed by a review of prior studies that have investigated groundwater–surface water interactions in braided rivers and their associated aquifers. The various methods used to characterise these processes are discussed with emphasis on their effectiveness in achieving the studies' objectives and their applicability in braided rivers. We also discuss additional methods that appear promising to apply in braided river settings. The aim is to provide guidance on methodologies most suitable for future work in braided rivers. In many cases, previous studies found a multi-method approach useful to produce more robust results and compare data collected at various scales. Given the challenges of working directly in braided rivers, there is considerable scope for the increased use of remote sensing techniques. There is also opportunity for new approaches to modelling braided rivers using integrated techniques that incorporate the complex river bed terrain and geomorphology of braided rivers explicitly. We also identify a critical need to improve the conceptual understanding of hyporheic exchange in braided rivers, rates of recharge to and from braided rivers, and historical patterns of dry and low-flow periods in these rivers.

Occurence of microplastics in the hyporheic zone of rivers

Although recent studies indicate that fluvial systems can be accumulation areas for microplastics (MPs), the common perception still treats rivers and streams primarily as pure transport vectors for MPs. In this study we investigate the occurrence of MPs in a yet unnoticed but essential compartment of fluvial ecosystems - the hyporheic zone (HZ). Larger MP particles (500–5,000 µm) were detected using attenuated total reflectance (ATR) - Fourier-transform infrared (FTIR) spectroscopy. Our analysis of MPs (500–5,000 µm) in five freeze cores extracted for the Roter Main River sediments (Germany) showed that MPs were detectable down to a depth of 0.6 m below the streambed in low abundances (≪1 particle per kg dry weight). Additionally, one core was analyzed as an example for smaller MPs (20–500 µm) with focal plane array (FPA)- based µFTIR spectroscopy. Highest MP abundances (~30,000 particles per kg dry weight) were measured for pore scale particles (20–50 µm). The detected high abundances indicate that the HZ can be a significant accumulation area for pore scale MPs (20–50 µm), a size fraction that yet is not considered in literature. As the HZ is known as an important habitat for invertebrates representing the base of riverine food webs, aquatic food webs can potentially be threatened by the presence of MPs in the HZ. Hyporheic exchange is discussed as a potential mechanism leading to a transfer of pore scale MPs from surface flow into streambed sediments and as a potential vector for small MPs to enter the local aquifer. MPs in the HZ therefore may be a potential risk for drinking water supplies, particularly during drinking water production via river bank filtration.

Identification of floodplain and riverbed sediment heterogeneity in a meandering UK lowland stream by ground penetrating radar

Complex spatial heterogeneity in riverbed and floodplain sediments control the spatio-temporal exchange of groundwater and surface water in the hyporheic zone, inducing hot spots of microbial activity and biogeochemical cycling. However, the characterization of hyporheic exchange dynamics has thus far failed to adequately account for the complex subsurface heterogeneity of river bed sediments in a spatial explicitly manner, in particular for highly complex lowland river bed sediments. Here we demonstrate the ability of ground penetrating radar (GPR) to efficiently map floodplain and river bed sediment structures within a lowland meandering river. The aim of this study was to delineate the type and spatial extent of complex, texturally heterogeneous facies of high and low conductive streambed materials. GPR surveys in this study involved not only state-of-the-art terrestrial applications but also an aquatic survey conducted from a floating rig. The surveys revealed substantial sub-surface heterogeneity of depositional materials in the streambed and riparian zone. Eight characteristic radar facies were identified through the floodplain and ground-truthed against core samples and exposures of bank deposits. The majority of GPR profiles were dominated by trough-shaped depositional elements with erosional, curved, concave upward bounding surfaces, indicative of abandoned and chute stream channel structures. The identified abandoned channel structure was found to extend into the riverbed and to be filled by suspension fall-out fine-grained deposits (mud with organic matter and interbedded clay as indicated by observed signal attenuations). GPR proved to be a successful method to identify the spatial patterns of low conductivity peat and clay structures in the streambed and riparian zone of the investigated meander bend, highlighting its potential for larger scale analysis of these structures that have shown to control the exchange flow patterns between groundwater and surface water in lowland rivers.

Integral Flow Modelling Approach for Surface Water-Groundwater Interactions along a Rippled Streambed

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