Pools And Riffles Research Articles

An algorithm to generate 2D bathymetry of an Alpine river for habitat suitability assessment.

Here we present an original approach to generate 2D high detail riverbed based on a drone photogrammetric survey, and RTK bathymetry measurements for Mera river in the Italian Alps. The aim is to better represent macro-roughness and riverbed structure of the river, also extending it to an ungauged area. Specifically, we apply a step-by-step approach. I) Depth and average slope of the riverbed were calculated from bathymetry data. II) Thus, a trapezoidal channel with constant slope and variable width was defined using the drone images. III) Riffle-pool sequence was assessed as a function of river width and applied to the generated channel. IV) Finally, the semi-random Perlin Noise was added to recreate riverbed irregularities in the natural stream. HEC-RAS 2D hydraulic software was then implemented to assess spatialized water depth and velocity. The proposed methodology could be quite relevant in river hydraulics to decouple roughness coefficient from water submergence, and in Physical Habitat Simulation Model (PHABSIM), where the dependency of the output is not linear with hydraulic parameters (i.e. water depth and velocity). Indeed, we apply PHABSIM for a case study of a stretch of the river and results are compared with a previous environmental study for Mera river.

Quantification of bedrock structural controls of longitudinal sediment connectivity using the probability of connectivity and sediment continuity model

The structural controls of sediment connectivity are poorly understood in low-gradient watersheds and stream networks. We developed a field- and modeling-based framework to understand the timescales and thresholds associated with the structural controls of sediment connectivity and continuity in the Inner Bluegrass region of Kentucky, USA. Field reconnaissance was carried out to assess stream morphology and sediment connectivity, which was leveraged to derive instream sediment connectivity indices for 181 pool-riffle sequences in the low-gradient bedrock stream network. Our model framework coupled a probability-based connectivity formulation with the sediment continuity formula to assess sediment transport and connectivity in the stream network. Results indicated that the bedrock system reflects an end-member in which lithology controls longitudinal stream morphology. Specifically, we found that regional morphology controls pool-riffle length scales via bedrock outcrops, which therein controls the timescales of sediment connectivity over multiple hydrologic regimes. The pool to riffle length ratio was found to be 4.6, an order of magnitude greater than the ratio for alluvial systems. The new formulation of the sediment continuity formula coupled with the probability of connectivity framework adequately captured both short-term and long-term continuous dynamics of sediment transport, as indicated by evaluation metrics with measured sediment flux and field reconnaissance. Based on the analysis of 362 riffles and pools, we found that sediment in pools upstream of bedrock outcrop barriers are disconnected 93 % of the time whereas riffles are connected and allow full continuity 99 % of the time. Longitudinal sediment connectivity within the stream network was found to be controlled by both functional connectivity and structural connectivity, which contrasts recent findings describing the controls of sediment connectivity over lateral pathways on hillslopes.

Flow dynamics in rivers with riffle-pool morphology: a dataset from case studies and field experiments

Bars and pools, common for natural riverbeds, form sequential patterns due to interactions between river flow, alluvium and vegetation. While the morphodynamics of bar-pool units are relatively well understood, far less is known about associated riffle-pool hydrodynamics because of a lack of high-resolution data collected in rivers and problems attaining natural scaling in laboratory studies. Here we present a dataset on turbulent flow structure in riffle-pool sequences of a natural river. Two case studies and two field-based experiments were carried out in a side branch of the braided gravel-bed Tagliamento River in Italy. Our case studies deliver detailed information about the three-dimensional structure of mean and turbulent flows in natural riffle-pool/run and pool-riffle/glide transitions. Field-based experiments completed with the in-stream flume models of a riffle-pool transition and a shallow jet model provide a methodological bridge for linking simplified hydrodynamic theories of shallow jets to complex flow structure documented by our case studies. Therefore, this dataset enables examination of scaling effects and can be widely used for validation of numerical models.

Interplay of flow and bed morphology caused by pool-riffle sequence formation without upstream sediment supply in gravel-bed rivers

Pool-riffle sequences (PRSs) are widely distributed in gravel-bed rivers, but their formation regimes under conditions with no sediment supply have not yet been explored. This study selected one field section of PRSs in the East River and measured the bed surface and bedload longitudinally and used these observations to design scaled, variable width flume experiments where we measured feedback of flow and bed morphology under different flows and slopes without a sediment supply. Experimental results show that there exists the critical value of width-depth ratio during PRS formation, corresponding to the aggregated velocity distribution. Meanwhile, the peak value of turbulent kinetic energy easily occurs in the middle riffle under flood discharge conditions, promoting the removal of fine sediment and its transport downstream. The formation and destruction of the periodic armoring layer in the PRS become common without sediment supply. In fact, downstream sorting effect of the armoring layer is the dominant factor to lead the formation-destruction of bed surface, and causes discontinuous surface GSD (grain size distribution) longitudinally in the field. The increase in the flow and slope can effectively promote PRS development, expanding the variation in the lateral sedimentation but shortening the length of pools and riffles. Furthermore, self-maintenance of adjacent riffles and pools response to the lack of sediment supply is faster and more frequent than that with sediment supply, and higher flow can promote incipient sediment and bedload transport on the bed surface. The PRS with variable-width largely increases the influence of sediment supply on the bedload transport, e.g., the increase of bed load transport rate with incoming sediment supply of the PRS is much higher than that of constant-width channel.

Rivers under Ice: Evaluating Simulated Morphodynamics through a Riffle-Pool Sequence

Modeling in ice-covered rivers is limited due to added computational complexity, specifically challenges with the collection of field calibration data. Using River2D, a 2-dimensional hydrodynamic modeling software, this study simulates depth-averaged velocity and shear stress distributions under ice cover and in open-water conditions during varying flow conditions in a small, shallow riffle-pool sequence. The results demonstrated differences in velocity distribution throughout the channel and increases in discharge were found to impact the velocity magnitude under ice cover, while the spatial distribution remained consistent. A recirculating eddy found along the pool’s left bank was exacerbated under ice cover, with potential implications for silver shiner habitat suitability. Bed shear stress magnitude did not vary significantly between ice and open water, although the spatial distribution differed notably. Model validation demonstrated success in simulating water depth and velocities, and the shear stress was estimated within a reasonable margin. Using hydrodynamic models provides valuable insight into seasonal changes in velocities and shear stress when ice is present.

Turbulence Characteristics in Mild and Steep Entrance Slopes of Pool-Riffle Sequences

This study investigates the distributions of velocity and Reynolds stress in an artificial pool-riffle sequence for two entrance slopes of 5° and 20° in a laboratory and compares the results with a similar pool-riffle sequence in a gravel-bed river in central Iran. Quadrant analysis is applied to find out the contributions of different events on turbulent flow structures. At an entrance slope of 5°, the velocity distribution follows an exponential trend without flow separation, but for the entrance slope of 20°, the velocity distribution indicates a S-shaped pattern with flow separation. Variation of entrance slope does not influence the convex form of Reynolds stress (RS) distribution, but it influences the location of the maximum value of RS and the flow separation zone length. The results reveal that outward and ejection are the dominant events for both field and laboratory settings. The sweep displays a decreasing trend from near the bed toward the water surface, however, the outward contribution indicates an increasing trend from the bed toward the water surface. The agreement between laboratory and field results in velocity and Reynolds stress will help the river engineers to better manage complex fluvial processes. Bursting process events depend on the aspect ratio for the same entrance pool slope. The agreement in the results of velocity and Reynolds stress distributions and the dominant events of bursting process in the laboratory and field are encouraging for better restoration of rivers and decreasing the cost of projects.

Coupling between downstream variations of channel width and local pool–riffle bed topography

Abstract. A potential control of downstream channel width variations on the structure and planform of pool–riffle sequence local bed topography is a key to the dynamics of gravel bed rivers. How established pool–riffle sequences respond to time-varying changes in channel width at specific locations, however, is largely unexplored and challenging to address with field-based study. Here, we report results of a flume experiment aimed at building understanding of how statistically steady pool–riffle sequence profiles adjust to spatially prescribed channel width changes. We find that local bed slopes near steady-state conditions inversely correlate with local downstream width gradients when the upstream sediment supply approximates the estimated transport capacity. This result constrains conditions prior to and following the imposed local width changes. Furthermore, this relationship between local channel bed slope and downstream width gradient is consistent with expectations from scaling theory and a broad set of field-based, numerical, and experimental studies (n=88). However, upstream disruptions to coarse sediment supply through actions such as dam removal can result in a transient flipping of the expected inverse correlation between bed slope and width gradient, collectively highlighting that understanding local conditions is critical before typically implemented spatial averaging schemes can be reliably applied.

Microhabitat use of larval fish in a South Carolina Piedmont stream

Understanding habitat use and nursery areas of larval fish is a key component to managing and conserving riverine fishes. Yet, freshwater researchers often focus only on adult fishes, resulting in a limited understanding of the habitat requirements for the early life stages of freshwater fishes. The goal of this study was to quantify the larval fish microhabitat use of three fish families in Twelvemile Creek, a fifth-order tributary of Lake Hartwell (Savannah River basin) in the Piedmont ecoregion of South Carolina, USA. We used handheld dipnets to sample larval fishes along 20 equidistant transects spaced 10 m apart weekly from May to July 2021 along a 200 m stream reach. We also collected microhabitat data at each larval fish capture location. Most captured individuals were in the metalarval stage and were identified to the family level. A partial distance-based redundancy analysis indicated that water velocity contributed to changes in larval fish assemblage structure. Larval fishes occupied a subset of the available habitat that was characterized by low water velocity, non-Podostemum substrate, and shallow habitats close to the shore or bed rock structure. We also detected temporal patterns in larval fish counts, with peak Percidae and Leuciscidae counts in late July and the highest Catostomidae counts in late May–early June. Our results suggest that larval fishes select habitats with low water velocity and shallow habitats close to shore microhabitat characteristics, and that riffle-pool sequences may serve as a nursery habitat for Percidae, Catostomidae and Leuciscidae metalarvae.

Effect of vegetative bed on flow structure through a pool-riffle morphology

Natural habitats are created and developed through pool-riffle sequences in rivers, while vegetation cover could play a critical role in the sediment transfer and its quantity and quality. In this study, the effect of vegetation cover on the flow structure in a pool-riffle sequence is investigated in a laboratory flume under bed formation to compare with non-vegetated cover. In this context, instantaneous point velocities were measured by ADV to determine averaged velocity, shear velocity, root mean square velocity, friction factor, Reynolds shear stress and turbulence intensities. Results showed that the vegetation cover increases the thickness of the wall law. Meanwhile, the length of the flow separation zone in the vegetated bedform is more than in the non-vegetated bedform. Variation in roughness coefficients may cause a new boundary layer in which local flow velocities decrease. In both cases (vegetated and non-vegetated bedforms), the momentum is mostly transferred by ejection and sweep phenomena between flow and bedform.

Flow structure investigation over a pool-riffle sequence in a variable width river

Flow structure investigation over a pool-riffle sequence in a variable width river

Quantitative analysis of ecological suitability and stability of meandering rivers.

As the most widely distributed river form in the world, meandering river is of great significance for stabilizing the physical structure of the river and maintaining ecosystem. To quantitatively study the positive effects of meandering rivers, the Chishui River, a natural tributary of the Yangtze River in Southwest China, is selected as the research area, and two typical river sections with different meandering degrees are selected as the research objects. Based on the field survey data, the local endemic fish Procypris rabaudi (Tchang) is considered the object fish, and a hydrodynamic model was used to simulate the distribution of water depth and flow velocity in certain river reaches at different flows. By introducing the weighted usable area (WUA) and hydraulic unit diversity index, combined with the suitability curves of the study species, the hydraulic characteristics and habitat suitability changes of two river reaches under different flows are summarized and analyzed, and the hydrogeomorphological process of the studied river section is generalized. With the change in discharge, a positive correlation is observed between the maximum velocity and depth of the meandering river and the discharge, whereas the WUA and hydraulic unit index of the meandering river have relatively small changes. Under low discharge, the distribution of pool-riffle sequences can be seen in the meandering reach, which is essential to improve the ecological suitability and stability of the river. This study provides scientific sustentation for river restoration and fish conservation.

Interplay of Flow and Bed Morphology Caused by Pool-Riffle Sequence Formation Without Upstream Sediment Supply in Gravel-Bed Rivers

Interplay of Flow and Bed Morphology Caused by Pool-Riffle Sequence Formation Without Upstream Sediment Supply in Gravel-Bed Rivers

Changes in the hydrological regime and channel morphology as the effects of dams and bridges in the Barakar River, India

The anthropogenic activities affect the river channel as well as the whole system in different magnitudes and dimensions. Barakar River, the main tributary to the Damodar River in eastern India, is modified by several engineering structures. Hydrological parameters, such as monthly discharge, peak flow discharge and geomorphological factors, such as gradient, width–depth ratio, grain size, braid–channel ratio, sinuosity ratio, riffle-pool sequence, and stream power are taken into consideration to highlight the significant alterations of the river due to dam and bridge construction. The alterations are assessed with the help of hydrological data, satellite images, and digital elevation data along with field survey. The downstream section of the dams, the river is characterised by high braiding, sinuous, total and unit stream power along with the presence of a box-shaped bedrock channel, high gradient, bed coarsening and armouring due to the release of high-velocity sediment-free ‘hungry water’. In the upstream reach, the grain size decreases towards the dam, and it increases suddenly with poor sorting at the immediate downstream regime of the dam. The effects of bridges on the Barakar river morphology include an increase of gradient, width and depth of the river channel at the downstream of the bridges. The construction of bridges influences riffle-pool sequences. Thereby, the pool depth spacing is greater than the riffle crest spacing. However, the integration of natural as well as human-induced factors can be the best approach to understand the anthropogenic alteration of the river. Moreover, construction of some check dams at the upper section of the tributaries of the Barakar River can be very effective for morphological stability.

Hyporheic exchanges due to channel bed and width undulations

Riffle-pool sequences are fundamental, ubiquitous morphological features of alluvial rivers that are thoroughly studied in general and commonly incorporated into river restoration projects. Most previous investigations on the effect of riffle-pool sequences on hyporheic exchanges focused on solely bed undulation, because that is widely thought to be the defining feature of riffle-pool sequences. However, riffle- pool sequences also have significant width undulations that are vital to riffle-pool sequences morphodynamics, yet relatively few studies exist on the effect of such undulations on hyporheic exchanges. Thus, in this study based on laboratory experiments and numerical simulations, we investigate the effect of bed and width undulations on hyporheic zone characteristics for various ratio of width amplitude to bed amplitude. The variation of hyporheic exchange characteristics (i.e. hyporheic exchange flux and residence time) for different riffle-pool designs were also assessed by creating prescribed topography of synthetic river valleys. The results showed that due to pressure variation along width undulations, the upwelling and downwelling patterns can also be observed for only width undulations in rivers, in the absence of bed undulations, and as width undulation amplitude decreases, the normalized hyporheic exchanges (Q*HZ) increase and normalized median residence time (RT*) decreases. Also, simultaneous channel bed and width undulations result in higher Q*HZ especially when a pool is located in an expansion (aka “oversized” cross-section) and a riffle in a constriction (“nozzle”). Our results suggest that river restoration project that artificially construct wide pools and constricted riffles will achieve maximum Q*HZ and RT*, though they will not be geomorphically self-sustainable.

Laboratory study on fish behavioral response to meandering flow and riffle-pool sequence driven by deflectors in straight concrete flood channels

Sectional channelization of natural rivers for flood mitigation causes discontinuity of the riverine corridor, leading to unfavorable conditions for fish movement in the longitudinal direction. To minimize the negative effects, hydraulic structures (e.g. deflectors) are proposed as potential restoration measures in concrete flood channels. In this study, indoor flume experiments were carried out to assess the variation of flow field (velocity magnitude, turbulent kinetic energy, and Reynolds stress) and bed morphology with the inclusion of deflectors. The flow and sediment regimes were then compared in the presence or absence of deflectors. The experimental results indicated that deflectors triggered a non-uniform flow pattern and stimulated riffle-pool sequence formation through local scouring and subsequent deposition. To better understand the response of predaceous chub (Parazacco spilurus) and mud carp (Cirrhinus molitorella) to deflectors, fish trajectories, distribution, and number of transits were analyzed using a video monitoring system. The results suggested that fish responded differently to the baseline (without deflector) and deflector scenarios: (1) fish struggled against the oncoming current when crossing the physical barrier in the channelized section; (2) fish moved and rested in the diverse habitats created by deflectors. Additionally, the presence of sediments in the channel bed was essential in attracting fish to the channelized section. Therefore, it was concluded that using deflectors at appropriate intervals and allowing the existence of sediments in flood channel sections can benefit fish movement and enhance channel naturalization.

Changes in the vertical distribution of hyporheic and benthic fauna associated with low flow conditions in the headwaters of the Tafna river (northwest Algeria)

This study investigated temporal changes in the vertical distribution of invertebrates during periods of low flows in the Tafna wadi, a semi-arid Mediterranean river characterised by a general decline in average flows. The hyporheos was sampled along a 130-m stretch at depths of 0.3, 0.6 and 1 m in a pool-riffle sequence and a lateral gravel bar. Benthic fauna was collected on each sampling occasion from different locations in the studied sections. Sampling was conducted in two consecutive years (2014 and 2015) at the end of the spring (high waters) and over a 2-month period during the low summer flow. The results indicated that high surface water temperatures may initiate the movement of some taxa into the interstitial habitat, thus supporting the hyporheic refuge hypothesis. In the hyporheic zone, the low oxygen rates recorded deeper in the sediment and in the last low-flow months caused significant declines in taxonomic richness, and lower pH values resulted in a reduction in invertebrate abundances. In the benthic zone, high flow velocities increased richness but decreased invertebrate abundances. A large number of taxa were found in the gravel bar section reflecting the more suitable living conditions in this habitat compared with the river reach (high DO rates).

Automatic identification of alternating morphological units in river channels using wavelet analysis and ridge extraction

Abstract. The accuracy of hydraulic models depends on the quality of the bathymetric data they are based on, whatever the scale at which they are applied. The along-stream (longitudinal) and cross-sectional geometry of natural rivers is known to vary at the scale of the hydrographic network (e.g., generally decreasing slope, increasing width in the downstream direction), allowing parameterizations of main cross-sectional parameters with large-scale proxies such as drainage area or bankfull discharge (an approach coined downstream hydraulic geometry, DHG). However, higher-frequency morphological variability (i.e., at river reach scale) is known to occur for many stream types, associated with varying flow conditions along a given reach, such as the alternate bars or the pool–riffle sequences and meanders. To consider this high-frequency variability of the geometry in the hydraulic models, a first step is to design robust methods to characterize the scales at which it occurs. In this paper, we introduce new wavelet analysis tools in the field of geomorphic analysis (namely, wavelet ridge extraction) to identify the pseudo-periodicity of alternating morphological units from a general point of view (focusing on pool–riffle sequences) for six small French rivers. This analysis can be performed on a single variable (univariate case) but also on multiple variables (multivariate case). In this study, we choose a set of four variables describing the flow degrees of freedom: velocity, hydraulic radius, bed shear stress, and a planform descriptor that quantifies the local deviation of the channel from its mean direction. Finally, this method is compared with the bedform differencing technique (BDT), by computing the mean, median, and standard deviation of their longitudinal spacings. The two methods show agreement in the estimation of the wavelength in all reaches except one. The method aims to extract a pseudo-periodicity of the alternating bedforms that allow objective identification of morphological units in a continuous approach with the maintenance of correlations between variables (i.e., at many station hydraulic geometry, AMHG) without the need to define a prior threshold for each variable to characterize the transition from one unit to another.

The Influence of Pool-Riffle Morphological Features on River Mixing

Accurate prediction of pollutant concentrations in a river course is of great importance in environmental management. Mathematical dispersion models are often used to predict the spatial distribution of substances to help achieve these objectives. In practice, these models use a dispersion coefficient as a calibration parameter that is calculated through either expensive field tracer experiments or through empirical equations available in the scientific literature. The latter are based on reach-averaged values obtained from laboratory flumes or simple river reaches, which often show great variability when applied to natural streams. These equations cannot directly account for mixing that relates specifically to spatial fluctuations of channel geometry and complex bed morphology. This study isolated the influence of mixing related to bed morphology and presented a means of calculating a predictive longitudinal mixing equation that directly accounted for pool-riffle sequences. As an example, a predictive equation was developed by means of a three-dimensional numerical model based on synthetically generated pool-riffle bathymetries. The predictive equation was validated with numerical experiments and field tracer studies. The resulting equation was shown to more accurately represent mixing across complex morphology than those relations selected from the literature.

Recent adjustments of pool-riffle distribution along the channels in the Pearl River Delta, China

The adjustments of pool-riffle distribution along the deltaic channels in the Pearl River Delta (PRD) under the combined effects of human interventions and natural evolution are examined in the three periods, namely 1952–1977 period, 1977–1999 period, and 1999–2014 period, corresponding to the different dominant factors. The residual depths along the channel thalweg and their spatial autocorrelation are introduced to quantify the pool-riffle distribution. The results show that the average pool depths of the deltaic channels increased significantly and the percentages of longitudinal length occupied by riffles decreased significantly by large-scale sand excavation during the 1977–1999 period and by sediment input decrease during the 1999–2014 period. It is highlighted sand excavation has a greater impact on the pool-riffle distribution of the North River channel and sediment input decrease has a greater impact on the pool-riffle distribution of the West River channel. The changes in spatial autocorrelation of pool-riffle sequence reveal a dramatic decrease of the average pool spacing after sand excavation, reflecting a rearrangement in the pattern of pool-riffle sequence. Moreover, the rearrangement process of the North River channel is ahead of that of the West River channel. In addition, the inhomogeneity indexes of deltaic channels are decreasing during the study period, indicating a deterioration of aquatic habitats diversity. It is anticipated that these results are useful for river management and utilization in the PRD and other river deltas worldwide that are subject to large-scale human interventions.

Channel width, bedform length and turbulence: numerical investigation of flow dynamics over laboratory-scale pool–riffle sequences

Spatial dimensions of bedforms relative to the flow depth are of great interest for both engineers and geoscientists, and continue to be an active area of research. These morphological features are of significant consequence for critical hydrodynamic parameters, which in turn has an impact on sediment and solute transport through the river system. In this study, we present results from three-dimensional large eddy simulation of flows over such bedforms in a straight channel. Rigid three-dimensional pool–riffle structures occupying the full span of the channel were considered as the macro-bedforms. The presence of pool–riffle affects the spatial heterogeneity of the flow parameters such as lateral flow concentration. We observed that the lateral flow concentration increases in convective deceleration (CDF) zone with peak values at the central plane in channels with narrower span. The peak values shift from the central plane towards sidewall in channels with larger width. Flow recovery is found to be faster in narrower channels in the conducted simulations. High-magnitude counter-rotating vortices (HCRVs) in CDF and pool transport the momentum of the flow towards the centre of the channel. Larger channel width ensures persistent presence of HCRV in CDF and pool. Longer riffle length in the streamwise direction is observed to further alter the lateral flow concentration compared to the shorter riffles. Instantaneous bed shear stress mapping reveals the presence of coherent high- and low-velocity streaks on riffles. The coherence of these structures is lost in CDF and pool accompanied with an increase in turbulence represented by an increase in turbulent kinetic energy and Reynolds stress.