Bedform Amplitude Research Articles

Hyporheic Exchange in a Meandering Pool‐Riffle Stream

AbstractInteractions among stream characteristics, hyporheic exchange (HE), and groundwater flow are a crucial control for physicochemical and biological reactions in hyporheic zone (HZ). Many previous studies solely investigated the HE induced by pool‐riffle bedforms or meanders but few research studied the interactions between them despite this pair of morphology generally coexisting in natural streams. Adopting similar laboratory and numerical methods as Huang and Chui (2021), https://doi.org/10.1029/2020WR029182 whose pool‐riffle streams were straight, this study adequately studied the impacts of hydraulic and morphologic factors on the HE in meandering pool‐riffle streams and highlighted the mutual influence among various factors such as bedform amplitude and sinuosity. The results show that meanders significantly complicate the three‐dimensional hyporheic flow paths and increase the flowrate and scale of HE. The HE flowrate increases linearly with discharge, bed slope, and sinuosity. The total HZ volume and maximum lateral and vertical extents of HZ generally increase with discharge. They also increase with bed slope and sinuosity but only up to a particular threshold, and they remain nearly constant or slightly decrease after exceeding the threshold. All HE characteristics exponentially decrease with both losing and gaining groundwater flow, and losing conditions have less significant effects on the HE compared with gaining conditions. The HE in stream bank can be larger than that in streambed, and the former is generally more influenced by discharge and sinuosity than the latter. Effects of bedform amplitude on HE exhibit very complex patterns due to the mutual influence of key factors.

Impacts of Peak‐Flow Events on Hyporheic Denitrification Potential

AbstractSubsurface flows, particularly hyporheic exchange fluxes, driven by streambed topography, permeability, channel gradient and dynamic flow conditions provide prominent ecological services such as nitrate removal from streams and aquifers. Stream flow dynamics cause strongly nonlinear and often episodic contributions of nutrient concentrations in river‐aquifer systems. Using a fully coupled transient flow and reactive transport model, we investigated the denitrification potential of hyporheic zones during peak‐flow events. The effects of streambed permeability, channel gradient and bedform amplitude on the spatio‐temporal distribution of nitrate and dissolved organic carbon in streambeds and the associated denitrification potential were explored. Distinct peak‐flow events with different intensity, duration and hydrograph shape were selected to represent a wide range of peak‐flow scenarios. Our results indicated that the specific hydrodynamic characteristics of individual flow events largely determine the average positive or negative nitrate removal capacity of hyporheic zones, however the magnitude of this capacity is controlled by geomorphological settings (i.e., channel slope, streambed permeability and bedform amplitude). Specifically, events with longer duration and higher intensity were shown to promote higher nitrate removal efficiency with higher magnitude of removal efficiency in the scenarios with higher slope and permeability values. These results are essential for better assessment of the subsurface nitrate removal capacity under the influence of flow dynamics and particularly peak‐flow events in order to provide tailored solutions for effective restoration of interconnected river‐aquifer systems.

Tracking Microplastics Across the Streambed Interface: Using Laser‐Induced‐Fluorescence to Quantitatively Analyze Microplastic Transport in an Experimental Flume

AbstractRivers and streams are a primary transport vector for microplastics (MPs), connecting terrestrial sources to marine environments. While previous studies indicated that pore‐scale MPs can accumulate in streambed sediments, the specific MPs transport and retention mechanisms in fluvial systems remain poorly understood. As part of this technical note, we present a novel method for a quantitative analysis of the spatiotemporal transport and retention of pore‐scale MPs in an experimental flume. A continuous mass balance for MPs in surface water was achieved using two online fluorometers, while a laser‐induced Fluorescence‐Imaging‐System was developed to track and quantify the spatial migration of MPs through the streambed sediments. The detection limit was <1 μg/L for 1 μm polystyrene microbeads with the fluorometers and 3 μg/L for the Fluorescence‐Imaging‐System. The system was able to quantitatively track the advective transfer of MPs into the streambed sediments: a process that has yet not been observed experimentally. Results showed that MPs infiltrated into the streambed sediments up to a depth twice the bedform amplitude. This work provides a novel experimental method to quantitatively monitor MP transport through porous media and advective exchange of MP across the streambed interface.

Hyporheic Exchange in a Straight Pool‐Riffle Stream With Floodplain

AbstractHyporheic exchange (HE) is the exchange between surface water, including suspended and dissolved matter, and pore water in the sediment surrounding a stream. It is essential to physicochemical and biological processes in both the stream and the surrounding sediment. This study systematically investigated the effects of the stream characteristics (i.e., stream discharge, bedform amplitude and wavelength, stream bed slope and groundwater flow) on the characteristics of HE (i.e., flowrate, scale, and residence time) in a straight pool‐riffle stream with floodplain using laboratory experiments and numerical simulations. The HE in the stream bank and streambed and its controlling factors were evaluated using numerical models. The results show that the HE flowrate and volume in the stream banks (i.e., in the floodplain) and streambed are significant and comparable at a high discharge. The discharge, bedform amplitude and bed slope directly affect the flow state in the pool, and the flow state determines the patterns of the effects of these three factors on the HE flowrate. The HE flowrate shows a linear relationship with discharge and bed slope when the surface flow in the pool remains subcritical. If the flow state in the pool changes from subcritical to supercritical conditions as discharge or bed slope increase, the HE flowrate becomes non‐monotonic with discharge and it first increases and then decreases with increasing bed slope. The flowrate, scale, and flux‐weighted mean residence time of HE linearly increase with bedform wavelength and exponentially decrease with both gaining and losing groundwater flow, independent of discharge.

Alluvial Morphodynamics of Bedrock Reaches Transporting Mixed‐Size Sand. Laboratory Experiments

AbstractResearch on the morphodynamics of bedrock rivers has primarily focused on bedrock incision, and little is known about the alluvial morphodynamics of rivers with exposed bedrock surfaces. More specifically, there is a lack of information on the morphodynamics of low slope bedrock reaches due to the recent recognition of such systems. Here, we present the results of laboratory experiments specifically designed to gain novel insight into flow resistances, flow hydrodynamics, and sediment transport processes in equilibrium partially exposed bedrock reaches transporting nonuniform sand as bed material in low slope areas. The experiments show that (1) downstream of a stable alluvial‐bedrock transition flow depth decreases in the streamwise direction, (2) bedform amplitude may decrease in the streamwise direction, and (3) stable patterns of downstream fining may form. Given the bedrock geometry, the water surface elevation at downstream boundary and the characteristics of the bedform regime in an alluvial channel subject to the same flow rate and sediment supply at equilibrium control bedform characteristics and sediment sorting patterns in the bedrock reach. When this distance is significantly smaller than the alluvial equilibrium flow depth or when the alluvial equilibrium bedform regime is close to the dune‐antidune transition, bedforms in the bedrock reach are closer to the dune‐antidune transition than at alluvial equilibrium with a consequent reduction in bedform amplitude. If the distance between the water level at the downstream boundary and the bedrock surface is close to the alluvial equilibrium flow depth and the alluvial equilibrium bedforms are well in the dune regime, a stable pattern of downstream fining can be expected. The comparisons between experimental and modeled sediment transport rates and equilibrium grain size distributions of the sediment further show that surface‐based bedload transport models derived for alluvial systems reasonably predict equilibrium sediment transport rates and bed surface size distributions in bedrock reaches if the presence of exposed bedrock is accounted for in terms of alluvial cover fraction.

Topographic controls on pyroclastic density current dynamics: Insight from 18 May 1980 deposits at Mount St. Helens, Washington (USA)

Our ability to interpret the deposits of pyroclastic density currents (PDCs) is critical for understanding the transport and depositional processes that control PDC dynamics. This paper focuses on the influence of slope on flow dynamics and criticality as recorded in PDC deposits from the 18 May 1980 eruption of Mt. St. Helens (USA). PDC deposits are found along the steep flanks (10°–30°) and across the pumice plain (~5°) up to 8km north of the volcano. Granulometry, componentry and descriptions of depositional characteristics (e.g., bedform morphology) are recorded with distance from source. The pumice plain deposits are primarily thick (3–12m), massive and poorly-sorted, and represent deposition from a series of concentrated PDCs. By contrast, the steep flank deposits are stratified to cross-stratified, suggesting deposition from PDCs where turbulence strongly influenced transport and depositional processes. We propose that acceleration of the concentrated PDCs along the steep flanks resulted in thinning of the concentrated, basal region of the current(s). Enhanced entrainment of ambient air, and autofluidization from upward fluxes of air from substrate interstices and plunging breakers across rugged, irregular topography further inflated the currents to the point that the overriding turbulent region strongly influenced transport and depositional mechanisms. Acceleration in combination with partial confinement in slot canyons and high surface roughness would also increase basal shear stress, further promoting shear and traction transport in the basal region of the current. Conditions along the steep flank resulted in supercritical flow, as recorded by regressive bedforms, which gradually transitioned to subcritical flow downstream as the concentrated basal region thickness increased as a function of decreasing slope and flow energy. We also find that (1) PDCs were erosive into the underlying granular substrate along high slopes (>25°) where currents were partially confined in steep slot canyons, suggesting that basal shear stress is an important control on erosive capacity, and (2) bedform amplitude, wavelength and the presence of regressive bedforms increase with increasing slope and proximity to source along the steep flank, suggesting a link between bedform morphology, flow velocity, and flow criticality. While our results indicate that slope and irregular topography strongly influence PDC dynamics, criticality and erosive capacity, the influence of these conditions on ultimate flow runout distance is unclear. The work here also highlights the issue that relationships between the controls on bedform size and morphology in density stratified flows remain poorly constrained, limiting our ability to extract important information about the currents that produced them. These final two points warrant further exploration through the combination of field, experimental and numerical approaches.

Acoustic mode coupling due to subaqueous sand dunes in the South China Sea: Extension of the adiabatic criterion to waveguides with bedforms

The large subaqueous sand dunes on the upper continental slope of the South China Sea (SCS) create a range-dependent ocean acoustic waveguide within which acoustic energy is expected to couple between propagating normal modes. Here, the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms. The morphological features of the bedforms modeled in this theoretical and numerical investigation are based on echosounder observations of the SCS sand dune field during a research cruise in the spring of 2012 on the Taiwanese R/V Ocean Researcher 2 (OR2). Using the extended criterion for adiabatic invariance to examine mode propagation over these bedforms, results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios; increasing bedform amplitude enhances mode coupling. Physically, initially bottom-trapped mode 1 energy abruptly couples to higher adjacent modes, with most of the energy preferentially settling into a few select modes. The scattered energy fills the water column to near-surface depths downrange of the bedforms. Numerical simulations confirm the extended criterion parameterization. [This work was supported by National Science Council of Taiwan.]

Acoustic mode coupling due to subaqueous sand dunes in the South China Sea

The large subaqueous sand dunes on the upper continental slope of the South China Sea are expected to couple acoustic propagating normal modes. In this letter, the criterion of adiabatic invariance is extended to the case of a waveguide possessing bedforms. Using the extended criterion to examine mode propagation over the bedforms observed in the sand dune field in 2012, results demonstrate that bedforms increase mode coupling strength such that the criterion for adiabatic propagation is exceeded for waveguides with small bedform amplitude to water depth ratios; increasing bedform amplitude enhances mode coupling. Numerical simulations confirm the extended criterion parameterization.

Conceptual model for complex river responses using an expanded Lane's relation

In 1955, Lane introduced the qualitative expression Q w S α Q s D s to relate water discharge ( Q w ), channel slope ( S), sediment discharge or load ( Q s ), and a representative bed sediment size ( D s ) for a river reach under dynamic equilibrium conditions. For over 50 years, Lane's relation has provided a unique conceptual model useful to engineers, geomorphologists, and educators for visualizing and describing the concept of dynamic equilibrium in rivers. However, the ability of Lane's relation to describe river adjustments is inherently limited because the expression cannot account for the changes in cross-sectional, planform and bedform geometry that are typically associated with complex channel adjustments. By demonstrating that Q s is inversely proportional to the width-to-depth ratio ( W/d), we expand Lane's relation to include the width-to-depth ratio as a cross-sectional geometry term: Q w S α Q s D s ( W/d). We further demonstrate that the slope term ( S) is proportional to the total change in elevation along a channel (∆ z) and inversely proportional to sinuosity ( P) and bedform amplitude ( H ¯ a ): S α ∆ z/( P H ¯ a ). The introduction of the width-to-depth ratio and expressing channel slope in terms of channel geometry expands the original Lane's relation to explicitly include measures of cross-sectional, planform and bedform geometry as additional degrees of freedom. This significantly broadens the usefulness of Lane's original relation as a conceptual model and allows the visualization of complex river responses that commonly involve adjustments to cross-sectional, planform, and/or bedform geometry.

Riffle‐pool morphometry and stage‐dependant morphodynamics of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany)

ABSTRACTRiffle‐pool sequences are a common feature of gravel‐bed rivers. However, mechanisms of their generation and maintenance are still not fully understood. In this study a monitoring approach is employed that focuses on analysing cross‐sectional and longitudinal channel geometry of a large floodplain river (Vereinigte Mulde, Sachsen‐Anhalt, Germany) with a high temporal and spatial resolution, in order to conclude from stage‐dependant morphometric changes to riffle and pool maintaining processes.In accordance with previous authors, pool cross‐sections of the Mulde River are narrow and riffle cross‐sections are wide suggesting that they should rather be addressed as two general types of channel cross‐sections than solely as bedforms. At high flows, riffles and pools in the study reaches changed in length and height but not in position. Pools were scoured and riffles aggraded, a development which was reversed during receding flows below the threshold of 0·4Qbf (40% bankfull discharge). An index for the longitudinal amplitude of riffle‐pool sequences, the bed undulation intensity or bedform amplitude, is introduced and proved to be highly significant as a form parameter, its first derivative as a process parameter. The process of pool scour and riffle fill is addressed as bedform maintenance or bedform accentuation. It is indicated by increasing longitudinal bed amplitudes.According to the observed dynamics of bed amplitudes, maintenance of riffle‐pool sequences lags behind discharge peaks. Maximum bed amplitudes may be reached with a delay of several days after peak discharges. Increasing bed undulation intensity is interpreted to indicate bed mobility. Post‐flood decrease of the bed undulation intensity indicates a retrograde phase when transport from pools to riffles has ceased and bed mobility is restricted to riffle tails and heads of pools. This type of transport behaviour is referred to as disconnected mobility.The comparison of two river reaches, one with undisturbed sediment supply, the other with sediment deficit, suggests that high bed undulation intensity values at low flows indicate sediment deficit and potentially channel degrading conditions. It is more generally hypothesized that channel bed undulations constitute a major component of form roughness and that increased bed amplitudes are an important feature of channel bed adjustment to sediment deficit be it temporally during late floods or permanently due to a supply limitation of bedload. Copyright © 2011 John Wiley & Sons, Ltd.

Validation of Bed-Load Transport Measurements with Time-Sequenced Bathymetric Data

Advances in bathymetric data acquisition have made it possible to explore alternative methods for measuring bed-load transport in rivers. The method validated herein consists of measuring rates of bed scour by using time-sequenced bathymetric data. The validation is performed in a laboratory flume by comparing the measured rates of bed scour with direct measurements of bed-load transport. The bed forms in the flume are dunes traveling at nearly constant speed. The shape of the dunes remains nearly constant. No suspended load is present. The ranges for Froude and Rouse numbers are 0.24–0.50 and 4.6–10.4, respectively. The study shows that under the given conditions, bed-load transport determined from time-sequenced bathymetric data is equally accurate to that determined from measurements of bed-form amplitude and speed. Obtaining bed-load transport from time-sequenced bathymetric data is often more expedient than traditional methods.

Reach-scale channel geometry of mountain streams

The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength ( H/ L). Pool-riffle channels have greater width/depth ratios and relative grain submergence ( R/ D 84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence ( R/ H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.

Hyporheic exchange in gravel bed rivers with pool‐riffle morphology: Laboratory experiments and three‐dimensional modeling

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.

Analytical and numerical modeling of sandbanks dynamics

Linear and nonlinear behavior of large‐scale underwater bedform patterns like sandbanks are studied using linear stability analysis and numerical modeling. The model is based on depth‐integrated hydrodynamics equations with a quadratic bottom friction law and a bed load sediment transport model including a bottom slope effect. First, the linear stability analysis of a flat erodible bottom subject to a steady current is performed. The direction of the most amplified bedform is controlled by the current, whereas its wavelength is selected by the gravity driven sediment flow. The instability proved to be due to the velocity component parallel to the mean flow, whereas the transverse velocity and the bottom slope effect are damping processes. The growth rate is then related to the spatial phase‐lag between flow velocity and bathymetry. In order to validate the numerical model in this linear regime, the growth rate and the phase celerity of an infinitesimal bottom perturbation are computed as a function of its wavevector. A good agreement with linear stability results is found. Second, using the property that the growth rates for block‐function and steady current are the same, the nonlinear behavior of the instability is investigated for a simple tidal current. The saturation height of the theoretically most amplified mode is estimated using the numerical model. The analysis of the sediment fluxes gradients shows that the saturation is mostly due to the hydrodynamic processes, although the saturation height slightly depends on the value of the bottom slope effect coefficient. Third, a Landau equation, whose coefficients are computed from the previous results, is used to predict the temporal evolution of the bedform amplitude from the initial infinitesimal perturbation to the saturation. A comparison with the characteristics of continental shelf sandbanks shows that the model gives a reasonable estimation of the temporal dynamics of these large‐scale bedforms. However, the saturation height seems to be slightly overestimated. The limitations of the method are also discussed.

Flow resistance under conditions of intense gravel transport

A study of flow resistance was undertaken in a channelized reach of the North Fork Toutle River, downstream of Mount St. Helens, Washington. Hydraulic and sediment transport data were collected in flows with velocities up to 3 m/s and shear stresses up to 7 times the critical value needed for bed load transport. Details of the flow structure as revealed in vertical velocity profiles indicate that weak bed load transport over a plane gravel bed has little effect on flow resistance. The plane gravel bed persists up to stresses ∼3 times critical, at which point, irregular bed forms appear. Bed forms greatly increase flow resistance and cause velocity profiles to become distorted. The latter arises as an effect of flows becoming depth‐limited as bed form amplitude increases. At very high rates of bed load transport, an upper stage plane bed appeared. Velocity profiles measured in these flows match the law of the wall closely, with the equivalent roughness being well represented by ks = 3D84 of the bed load. The effects noted here will be important in very large floods or in rivers that are not free to widen, such as those cut into bedrock.