Channel Morphology Research Articles

Groundwater–surface water interactions in a river estuary and the importance of geomorphology: Insights from hydraulic, thermal and geophysical observations

AbstractThis study presents the groundwater flow and salinity dynamics along a river estuary, the Werribee River in Victoria, Australia, at local and regional scales. Along a single reach, salinity across a transverse section of the channel (~80 m long) with a point bar was monitored using time‐lapse electrical resistivity (ER) through a tidal cycle. Groundwater fluxes were concurrently estimated by monitoring groundwater levels and temperature profiles. Regional porewater salinity distribution was mapped using 6‐km long longitudinal ER surveys during summer and winter. The time‐lapse ER across the channel revealed a static electrically resistive zone on the side of the channel with a pronounced cut bank. Upward groundwater flux and steep vertical temperature gradients with colder temperatures deeper within the sediment suggested a stable zone of fresh groundwater discharge along this cut bank area. Generally, less resistive zones were observed at the shallow portion of the inner meander bank and at the channel center. Subsurface temperatures close to surface water values, vertical head gradients indicating both upward and downward groundwater flux, and higher porewater salinity closer to that of estuary water suggest strong hyporheic circulation in these zones. The longitudinal surveys revealed higher ER values along deep and sinuous segments and low ER values in shallow and straighter reaches in both summer and winter; these patterns are consistent with the local channel‐scale observations. This study highlights the interacting effects of channel morphology, broad groundwater–surface water interaction, and hyporheic exchange on porewater salinity dynamics underneath and adjacent to a river estuary.

Linkages between bedload displacements and topographic change

SummaryChanges in bed topography that build and maintain channel morphology are driven by the displacements of individual particles, either though their entrainment or deposition. However, the linkages between these topographic changes and individual grain displacements have not been comprehensively addressed, as many historical tracer studies have not included coincident topographic data. In this study, we compare the movements of bedload tracers to the differences in repeat topographic surveys across four gravel‐bed river reaches. To do this, we apply a 1‐D Bayesian survival process model to the starting and ending locations of tracers. This model estimates downstream trapping probabilities, which represent the likelihood that a given segment of channel will “trap” an entrained particle. We then adapt this model to estimate downstream trapping probabilities using digital elevation models of difference and compare the results. The estimates from the tracer and topographic trapping models showed general alignment, meaning that tracers were preferentially trapped in segments that experienced deposition along the channel. Thus, tracers in this study were able to identify downstream differences in bedload transport. The comparison also highlighted that tracer‐estimated trapping probabilities were larger than topographically estimated ones. This supports previous observations that sediment travel distances estimated using tracers are shorter than those estimated using morphological methods. We find that the differences between these two estimates vary systematically across study environments. These variations are attributable to either study design (i.e., tracers being larger than the median size of the sediment that deforms the bed) or differences in compensating scour and fill. We explore potential causes for differences in compensating scour and fill, including hydrograph shape, sediment delivery regime, channel deformation style, and channel width, highlighting that morphodynamics needs to be considered in designing bedload tracer studies.

Sediment transport at the network scale and its link to channel morphology in the braided Vjosa River system

AbstractIn this article we apply the CASCADE network‐scale sediment connectivity model to the Vjosa River in Albania. The Vjosa is one of the last unimpaired braided rivers in Europe and, at the same time, a data scarce environment, which limits our ability to model how this pristine river might respond to future human disturbance. To initialize the model, we use remotely sensed data and modeled hydrology from a regional model. We perform a reach‐by‐reach optimization of surface grain size distribution (GSD) and bedload transport capacity to ensure equilibrium conditions throughout the network. In order to account for the various sources of uncertainty in the calculation of transport capacity, we performed a global sensitivity analysis. The modeled GSD distributions generated by the sensitivity analysis generally match the six GSDs measured at different locations within the network. The modeled bedload sediment fluxes increase systematically downstream, and annual fluxes at the outlet of the Vjosa are well within an order of magnitude of fluxes derived from previous estimates of the annual suspended sediment load.We then use the modeled sediment fluxes as input to a set of theoretically derived functions that successfully discriminate between multi‐thread and single‐thread channel patterns. This finding provides additional validation of the model results by showing a clear connection between modeled sediment concentrations and observed river morphology. Finally, we observe that a reduction in sediment flux of about 50% (e.g., due to dams) would likely cause existing braided reaches to shift toward single thread morphology. The proposed method is widely applicable and opens a new avenue for application of network‐scale sediment models that aid in the exploration of river stability to changes in water and sediment fluxes.

Palaeoflood hydrology of the fluvial continental records of western India: A synthesis

Palaeoflood hydrology has emerged as an important tool to infer quantitative and qualitative aspects of ungauged floods based on their physical evidence. Palaeoflood studies in India have largely been undertaken in the rivers of Peninsular India, western India, Ganga plains and the Himalayas to determine the magnitude and age of extreme floods and their connection to variations in the monsoon intensity. Usually, the alluvial domains are unfavourable for the occurrence and preservation of flood deposits and related discharge estimation. However, the alluvial rivers of western India owing to their semi–confined banks comprising late Pleistocene sediments provide an opportunity for investigating both, the high magnitude flood events as well as average flow conditions. In this synthesis we concisely review the recent palaeohydrological studies in western India in terms of flood magnitude, occurrence of extreme events and its relation to the southwest monsoon variability over various time scales. Based on palaeo–fluvial reconstructions, the sedimentation pattern during late Pleistocene appears to be related to changes in channel gradient and the water surface width rather than to discharge variability. On the other hand, the aggradation in channels during early Holocene was largely controlled by the huge sediment influx and the incision that followed was in response to the increase in the discharge and competence of the river flow. The slackwater records from the bedrock channels have revealed that the large magnitude flood events occurred during wet climate phases during the last two millennia. A clustering of high magnitude events at climatic transitions and arid periods during mid–late Holocene has been surmised. Further the flood associated deposits delimited within Quaternary fluvial landforms and channel morphology are vital as these allow quantification of past flood discharges, velocities and stage levels and thus improve the future flood predictions.

Depositional patterns of slowly plugging neck cutoffs from core analysis and estimates of bedload transport, White River, Arkansas

AbstractMeander cutoffs and oxbow lakes are very common features of fluvial landscapes that add complexity and diversity to floodplain alluvial architecture and riverine habitats. Following initial cutoff, sediment accumulates within the entrance and exit of the original bend forming plugs that eventually disconnect the abandoned bend from the main channel. While studies have examined the sedimentology of these plugs once they have fully disconnected from the abandoned bend, fewer studies have detailed the sedimentological processes occurring during the early stages following cutoff initiation. Furthermore, recent studies have highlighted the importance that planform geometry plays in the evolution of neck cutoffs. This study examines the spatial depositional patterns of two neck cutoffs on the White River in central Arkansas, USA, that remain hydrologically connected to the main channel, providing a unique opportunity to research sedimentological processes before disconnection of the abandoned bend. Sediment cores were collected in key locations of each cutoff, including the entrance and exit of the abandoned bends, abandoned bend apices, and newly developed cutoff bars in the downstream channel. The cores were logged and interpreted and grain‐size analyses were performed. In addition to sediment cores, repeat high‐resolution multibeam echo sounding surveys were conducted roughly four hours apart to estimate bedload transport rates and patterns of bedload routing through each cutoff. Results from this research are different from previous studies. Sediment core results show a pattern of deposition typically associated with lower diversion angle chute cutoffs instead of higher diversion angle neck cutoffs. Previous research has indicated that plugging of abandoned bends drives disconnection from the active channel; however, this research shows that disconnection is more associated with the prevention of sediment from being delivered to the abandoned bends due to flow being pulled away from the abandoned bends and the evolving channel morphology.

Evaluation of a geomorphic instream flow tool for conducting hydraulic‐habitat modelling

AbstractConventional hydraulic‐habitat modelling methods are time‐consuming to implement. In response to repeated calls for more efficient and practical approaches, researchers have developed a geomorphic instream‐flow tool (GIFT) that combines a method to simulate reach‐averaged hydraulics at flows less than bankfull and depth and velocity frequency distributions to develop streamflow‐fish habitat relationships. This approach requires fewer resources to implement than conventional methods, but it has not been widely adopted because it has been subject to minimal testing and validation. This study evaluates the performance of GIFT by comparing its outputs to empirical measurements and conventional model outputs from eight rivers in western North America. The results of this comparison indicate that the root mean square errors for average depth and velocity were 0.078 m and 0.047 m/s, respectively, and the fit of modelled depth and velocity frequency distributions was satisfactory (index of agreement >0.9) for 11 of 15 surveys for depth and 12 of 15 surveys for velocity. GIFT‐derived fish habitat‐streamflow relationships peaked at lower flows than benchmark relationships in smaller streams (mean annual discharge [MAD] < 0.15 m3/s) and are markedly differed from the benchmark in the largest river (MAD of 87 m3/s). GIFT was also paired with a geomorphic regime model to predict the direction of changes in channel morphology and fish habitat following forest harvesting in one watershed. GIFT provides an alternative to conventional modelling approaches for single‐thread, gravel‐bed rivers with a MAD of around 15 m3/s or less. Application of this technique outside of these bounds, or in other regions should proceed with caution, as these scenarios have not been tested.

Channel and inter-channel morphology resulting from the long-term interplay of alongslope and downslope processes, NE Rockall Trough, NE Atlantic

With this contribution we use a pair of overlapping 3D seismic surveys and two exploration wells to document the response of long-lived slope channels to the onset of bottom currents sweeping the lower slope in the NE Rockall Basin, offshore Ireland. Downslope gravity current activity, linked to a phase of uplift, prevailed throughout the Eocene and led to the formation of multiple channels, most notably a large-scale sinuous channel complex (Channel 4 Complex) tied to a persistent sediment entry point on the margin. Channels fed lobes on the floor of the basin, with increased axial tilting forcing gravity currents to flow parallel to the base of slope. A phase of margin-wide differential subsidence and basin deepening in the Late Eocene then activated bottom current circulation across the basin. Northward-flowing bottom currents first erosionally refashioned the lower slope creating a prominent unconformity with contourites that then initially forming infill drifts. Bottom currents swept along and obliquely upslope, building plastered drifts that straddled the lower sections of the still active channels. The drifts modified and amplified the spurs separating the active channels, with interaction between along and downslope processes accreting sediment that built and maintained channel relief and allowed the channel mouths to extend further basinward over and across the earlier lobes. Contourite-forced channel extension on account of lower slope depositional re-profiling represents another manifestation of the interplay between slope channels and bottom currents. In the case of NE Rockall, the accreted base of slope wedge and mounded geometry resembles similar features elsewhere along the eastern Rockall margin. Those have previously been ascribed to mass-transport and base-up channel initiation, however, the similarities are so striking that we propose that these also are the result of lower slope re-profiling by bottom currents.

The effects of dams on river channel morphology: a case study at the historical Çarşamba Bridge on the Yeşilırmak River in northern Turkey

The effects of dams on river channel morphology: a case study at the historical Çarşamba Bridge on the Yeşilırmak River in northern Turkey

Toward natural approaches in restoration: Experiments of co‐evolving physical and biological structures in a self‐organizing step‐pool channel

AbstractAlthough the science and practice of river restoration has progressed in recent years toward more natural approaches, few example projects of headwater step‐pool channels exist in the literature. This study examined a restoration design in the headwaters of Wildcat Creek (Berkeley, California, USA) that allowed the creek to form its own stable morphology, instead of utilizing an engineered form. The creation of an experimental reach—where natural flows redistributed randomly placed clasts into step‐pool sequences after construction of an initial plane‐bed channel—afforded opportunity to test hypotheses regarding a co‐evolution of self‐organizing geomorphic and ecological responses over time. During three seasons following the initial restoration, we documented how the streambed self‐organized into step‐pool sequences by constructing plan maps and surveying changes in channel morphology. Concurrently, we sampled benthic macroinvertebrates in step, pool, and intermediate run habitats in the experimental reach and in an upstream reference reach. Results showed how natural flows created a characteristic step‐pool pattern quickly following the second post‐construction storm season. Taxonomic and functional metrics revealed that benthic macroinvertebrate communities assembled at a similarly rapid pace, becoming more distinct and diverse among habitat types as the step‐pool morphology developed. Multivariate ordination revealed that the benthic macroinvertebrate community in the experimental reach became more similar to that in the reference reach over time. Findings suggest a rapid morphologic and ecological co‐organization in the development of a complex stable step‐pool channel, giving promise for natural approaches in future restoration designs of “letting the river do the work,” to the extent afforded by local socioeconomic factors.

Morphometric analysis of the proximal thoracic pedicles in Lenke II and IV adolescent idiopathic scoliosis: an evaluation of the feasibility for pedicle screw insertion.

Pedicles on the concave side of the proximal thoracic (PT) curve in adolescent idiopathic scoliosis (AIS) patients with Lenke II and IV deformities tend to be narrow and dysplastic, making pedicle screw (PS) insertion challenging. The aim of this study was to evaluate the feasibility for PS placement in these patients using pedicle chord length, diameter, and channel morphology. In this retrospective study, 56 consecutive AIS patients with Lenke II or IV curves who underwent instrumented posterior spinal fusion (PSF) were studied. The mean age at surgery was 14.8years and the mean PT curve measured 45°. Two independent investigators evaluated all visible pedicles from T1 to T6 vertebral levels using axial images from intraoperative computed tomography-guided navigation recording the pedicle: (1) maximum transverse diameter 'd' at the isthmus, (2) maximum chord length 'l', and (3) qualitative assessment of the channel morphology (types A-D). Two hundred and sixty-eight concave and 264 convex pedicles were measured. The mean 'd' of the concave pedicles at T3 and T4 was < 3.0mm, compared to > 5.0mm for the convex counterparts (p < 0.001). Of all concave pedicle channels, 48% had morphology characteristics that were riskier for PS cannulation (type C or D) compared to 2% of all convex pedicle channels (type A or B) (p < 0.001). Almost half of all concave pedicles have morphologic characteristics that make them too small to accommodate a PS. Though PSs could be inserted using an in-out-in technique in these patients, alternative fixation anchors may improve strength and safety.

Hydromorphological rehabilitation improves channel morphology, instream biotopes, and macroinvertebrate communities, and thus enhances the conservation of an urban river

Abstract Many river rehabilitation projects have been criticized for failing to meet their goals or for being insufficiently monitored. There is, therefore, an urgent need to develop robust approaches for assessing treatment efficacy and thus guide the increasing investment in rehabilitation. Instream biotopes (formerly called ‘functional habitats’ or ‘mesohabitats’ by different authors) and their macroinvertebrate assemblages were used to assess the effectiveness of entire‐channel hydromorphological rehabilitation of a 1.8‐km stretch of a lowland stream through the town of Market Harborough, Leicestershire, UK. The project successfully enhanced the physical diversity, measured in the rehabilitated reach as the coefficients of variability for channel water depth and width, wet surface area, number of instream biotopes, and biotope diversity. The project also enhanced the biodiversity conservation value, measured as macroinvertebrate total density, total biomass, richness, diversity, Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness, EPT diversity, EPT count%, and EPT biomass%, all of which significantly increased following rehabilitation. Chironomidae count% and biomass% significantly decreased following rehabilitation. Rehabilitation was also successful in significantly increasing macroinvertebrate shredder, scraper, and filter‐feeder group density. Changes in the macroinvertebrate community metrics in the rehabilitated reach were related significantly to changes in the percentages of cover of instream biotopes and increases in biotope diversity in the rehabilitated reach. Macroinvertebrate structural and functional metrics can provide quantitative data for assessing reach‐level rehabilitation outcomes, if samples are collected in a pre‐defined sampling protocol stratified at the instream biotope level. The practical implications of this work are that the design of rehabilitation projects, if based upon the recreation of biotope heterogeneity, will succeed in improving biological value and restoring the area to near naturalness if a suitable upstream source of macroinvertebrates for natural recolonization is available. The study shows that the concept of biotopes has an important role to play in river conservation management.

Sinuous bar fingers of digitate shallow‐water deltas: Insights into their formative processes and deposits from integrating morphological and sedimentological studies with mathematical modelling

AbstractThis paper helps to address the growing need to resolve the severe loss of deltaic lands by providing a new understanding for shallow‐water digitate delta growth. The integration of satellite image analysis of modern deltas, field studies of the Ganjiang Delta in the Poyang Lake and ‘Delft3D’ simulations further results in improved facies models for shallow‐water digitate deltas. These analyses show that shallow‐water digitate delta bar fingers are sinuous in contrast to the straight deep‐water digitate delta bar fingers. These differences are assigned to the effect of water depth on outflow hydraulics, where friction‐dominated shallow‐water delta effluents promote mouth bar deposition that then divert flow around the mouth bar, resulting in the formation of sinuous bar fingers. These effects are further strengthened by the meandering of the shallow‐water jet that increases lateral sediment transport, and by the higher flow resistance and lower gradient of the shallow‐water outflows. Our data and analyses further show differences in the morphology and deposits of the shallow‐water sinuous bar fingers, where some bar fingers develop sinuous and others meandering (with point bars) distributary channels. Lateral channel migration and point bar formation (meandering) occur as a secondary process that does not change the shape or width of the bar fingers themselves, and is suggested to be a function of slight initial differences in channel sinuosity. These differences in distributary channel morphology have a strong effect on bar finger facies distribution. Sediment cohesion is another important control on bar finger bending processes, because high cohesion promotes formation of enclosed bays, where their bank strength exceeds the centrifugal force of water flow. Lower sediment cohesion results in sinuous bar fingers without formation of enclosed bays. This work provides insights into natural and artificial shallow‐water digitate delta growth and provides new quantitative facies models for shallow‐water digitate deltas.

Spatio-temporal study of morpho-dynamics of the Brahmaputra River along its Majuli Island reach

The River Brahmaputra is one of the largest rivers, ranking seventh in terms of mean annual discharge (19, 800 m3s-1). The river has a multi-channel, braided/anastomosed planform that changes frequently and exhibits rapid transformations in channel morphology. This research was set in the context of the River Brahmaputra near Majuli Island, Assam, India. Morphologically, this part of the river is very volatile; increased braiding intensity of the river is believed by many to be responsible for severe erosion in the southern part of Majuli Island. In 2016, Newton-Bhaba funding facilitated the authors to study the existing braid parameters for developing new ones to quantitatively define changes in the braided channel. The authors then employed these new parameters to evaluate historical and current trends of morpho-dynamic changes in the Brahmaputra River along Majuli Island. Our quantitative analysis establishes on a rational foundation the extent to which braiding intensity has increased. Trends in our new parameters indicate that the frequencies with which braid bars form, deform and erode are increasing, accelerating the rate at which channel morphology evolves near Majuli Island. Alarmingly, our analysis brings out that the deepest channel of the Brahmaputra (i.e., ‘thalweg’) is moving northwards – bringing it ever closer to Majuli Island. Engineered structures were constructed in 2004 to provide protection to the southern bank of Majuli Island and its flood embankments from erosion. To scientifically establish the effect of these river training measures on braiding forms and processes, we applied our newly developed braid indexes to the study reach before and after 2004. We could find that river channel braiding was relatively less intense, and the river was less unstable prior to 2004. However, following the construction of bank protection and river training works at Majuli Island in 2004, braiding intensity registered a sharp increase and the river waterway widened. Also, the deepest sub-channel started to migrate northwards, and through time it has become deeply entrenched along the southern bank line of Majuli Island around Ahatguri posing a major streambank erosion threat to it.

A practical gas permeability equation for tight and ultra-tight rocks

Gas rarefaction phenomenon during gas flows in tight pore spaces makes the gas permeability of tight and ultra-tight rocks higher than the intrinsic permeability. Gas permeability equations in the literature usually assume a pre-defined pore morphology (or geometry) which may not reflect the real pore structure of rocks. This study aims to develop a practical gas permeability equation that considers the pore morphology variance in different tight/ultra-tight rocks. We build an analytical equation for describing the full-Knudsen-number-ranged rarefied gas flows through single channels. By approximating this equation within a Knudsen number range of (0, 1), we develop a second-order equation of channel permeability vs. Knudsen number. In the literature, most of the second-order equations of channel permeability vs. Knudsen number were built for a 2D channel. Herein, we establish such second-order relation with different sets of parameter values valid for the rarefied gas flows in a cylinder, in a triangular prism, and in a duct, respectively. After checking the relation between a Knudsen number and real gas properties, we further transform the second-order equation in terms of Knudsen number into a second-order equation in terms of the reciprocal of gas pressure. We then quantify how a channel morphology influences the parameter values in the permeability equation. Accordingly, a constraint on the parameters of the permeability equation is proposed by considering the most observed pore morphologies in tight/ultra-tight rocks. The second-order permeability equation with the constraint is utilized to well fit Lattice Boltzmann simulations on reconstructed rock samples and multiple sets of permeability data that are measured in the literature. When being used to fit the measured permeability data of gas flows at pressures of above 0.5 MPa in the rocks with an intrinsic permeability of below 0.1 μD, the proposed second-order permeability equation demonstrates superiority to the permeability equations selected from the literature. The regressed values of the parameters in the proposed permeability equation suggest that the dominant pores in tight and ultra-tight rocks would normally have a rectangular cross-section, a circular cross-section, or a triangular cross-section. The proposed equation with the constraint offers a simpler quantitative description of the interplay among pore morphology, gas properties, and tight/ultra-tight rock permeability than the permeability equations in the literature.

Patent Foramen Ovale Channel Morphometric Characteristics Associated with Cryptogenic Stroke: The MorPFO Score

It is still disputable whether the specific morphologic properties of patent foramen ovale (PFO) may contribute to the occurrence of stroke. The aim of this study was to evaluate the differences in the morphometric and functional features of the PFO channel in patients with cryptogenic stroke and those without stroke. PFO channel morphology in 106 consecutive patients with cryptogenic stroke and 93 control patients without stroke with diagnosed PFO (by transesophageal echocardiography) was analyzed using transesophageal echocardiography. A validation cohort was established that consisted of 31 patients with cryptogenic stroke and 30 without stroke. Multivariable regression logistic analyses indicated PFO channel length change (odds ratio [OR], 2.50; 95% confidence interval [CI], 1.75-3.55; P<.001), PFO length/height ratio during the Valsalva maneuver (OR, 0.75; 95% CI, 0.60-0.95; P=.015), septum primum thickness (OR, 0.34; 95% CI, 0.14-0.80; P=.013), septum secundum height (OR, 0.91; 95% CI, 0.84-0.98; P=.013), the presence of an atrial septal aneurysm (OR, 3.38; 95% CI, 1.27-8.97; P=.014), and large shunt (OR, 2.49; 95% CI, 1.13-5.46; P=.022) as PFO-related stroke factors. The Morphologic Stroke Factors of PFO (MorPFO) score was developed, in which six factors were included: PFO channel length reduction (≥21%; 7 points), short septum secundum (<8.6mm; 5 points), thin septum primum (<1.6mm; 3 points), large right-to-left shunt (3 points), low PFO channel length/height ratio during the Valsalva maneuver (≤2.1; 2 points), and atrial septal aneurysm presence (1 point). Patients with scores of 0 to 7 points have low-risk PFO channels, those with scores of 8 to 11 points have intermediate-risk PFO channels, and those with scores of 12 to 21 points have high-risk PFO channels. External validation showed good MorPFO score performance (C index=0.90). Transesophageal echocardiography can be used to differentiate pathogenic from incidental PFO channels on the basis of their morphologic characteristics. The MorPFO score may help identify high-stroke-risk PFO channels.

Research on the Influence of Typical Soil Parameters on Critical Breakdown Field Strength and Residual Resistivity Based on Discharge Topography

Partial discharge of soil occurs when a lightning current enters the ground, and the strength of partial discharge is closely related to the magnitude of its critical breakdown field strength. Therefore, how to accurately obtain the variation law of the typical soil critical breakdown field strength and residual resistivity is the key to realizing the safe operation of the grounding devices and cables in the ground. This paper first selects a variety of typical soils to study the influence of various factors on the morphology of the discharge channel, and then studies the calculation methods of the soil critical breakdown field strength and residual resistivity under the introduction of different discharge channel morphologies and structures, and further discusses the reason why typical soil media factors have a small impact on the critical breakdown field. The experimental results show that under the same conditions, the critical breakdown field strengths of different soils from small to large are sand soil, loam soil and Yellow cinnamon soil. The largest ratio of residual resistivity to initial resistivity of the three soils is sand soil.

Impacts of dams and land-use changes on hydromorphology of braided channels in the Lhasa River of the Qinghai-Tibet Plateau, China

Among braided rivers developed on the Qinghai-Tibet Plateau of China at very high elevations (>3,500 m), the middle and lower reaches of the Lhasa River have been affected by comprehensive human activities mainly involving dam construction, urbanization, farming, afforestation, and mining. In the current study, the impacts of these human activities on hydrology and morphology of the four braided reaches downstream of a cascaded of two dams are investigated. The study period was divided into 1985–2006 (P1), 2006–2013 (P2), and 2013–2019 (P3), representing the natural and changed flow regimes by dams. Using available daily discharge data at two stations within the four braided reaches, dam-induced hydrological alteration was analyzed based on the indicators of hydrologic alteration and range of variability approach and key discharge proxies were calculated. Remotely sensed images also were selected in the three periods and morphological metrics extracted from them were compared for the four reaches among these periods. Attenuated hydrological regimes were found for only two reaches. The total channel width (Wc) and braiding intensity (BIt) followed different temporal trends among the four reaches. Annual average shift rates of the main channel in the four reaches were higher in the short (P2–P3) than in the long (P1–P2 and P1–P3) periods. The longitudinal changes of Wc and the number of channels did not have any identifiable trend among the four reaches. By linking the morphological changes to quantified spatial and temporal patterns of various human activities, it was found that (1) the two dams had insignificant impact on channel morphology, suggesting that the studied braided river might have a short relaxation time and (2) the evolutional trajectories of morphological changes in most of the four reaches were similar, suggesting that temporal trends of morphological changes due to complex human activities are not affected by the different physiographic settings of the reaches. Continuous exploitation of the valley area requires comprehensive river management strategies for coordinating various human activities.

Morphodynamics of an erodible channel under varying discharge

AbstractAlluvial channels arise through the interaction between morphology, hydraulics, and sediment transport, known as the ‘fluvial trinity’. Over relatively short timescales where climate and geology are fixed but discharge and sediment supply may vary, this process facilitates adjustments towards steady state, where the system oscillates around a mean condition. The relationship between changes in conditions and geomorphic response may be highly complex and nonlinear, especially in systems with multiple modes of adjustment. This study examines the adjustment of an erodible channel with fixed banks and a widely graded sediment mixture to successive increases in discharge. With each increase in discharge, components of the fluvial trinity adjusted towards a steady state. Particularly at relatively low discharges, adjustments were controlled by intrinsic thresholds and highlighted important morphodynamic processes. Notably, there was a strong interplay between channel morphology and sediment transport, and an effect whereby larger‐than‐average grains controlled channel deformation. These two processes occurred at the bar scale and were highly spatialised, which has two important implications: (1) reach‐averaged representations of process provide only partial insight into morphodynamics; and (2) models of rivers that suppress these process feedbacks and size‐dependent transport may not replicate morphodynamics that typically occur in field conditions. The experiments provide quantitative evidence for conceptual models describing exponential approaches towards steady state and the potential for transiency if disturbance frequency exceeds the recovery time. They also highlight how in natural rivers, particularly those with greater degrees of freedom for adjustment (notably, lateral adjustment and meandering), continuous changes in discharge may lead to nonlinear rather than steady‐state behaviour. In these settings, more holistic analytical frameworks that embrace different aspects of the system are critical in understanding the direction, magnitude and timing of channel adjustments.

Intraseasonal‐to‐Interannual Analysis of Discharge and Suspended Sediment Concentration Time‐Series of the Upper Changjiang (Yangtze River)

AbstractSediment transport is the main driver of the channel morphology and landscape evolution, with implications for chemical and biological river processes, and human‐related activities. Understanding the processes governing the relations between discharge and suspended sediments is essential for the management of river catchments and river networks. Here, we use the method of wavelet transformation to identify the time‐scale dependency of suspended sediment patterns concerning the temporally and spatially uneven transient processes of sediment production, accumulation, and transport. We analyze the temporal variation of concurrent discharge and suspended sediment fluxes for the Upper Changjiang (Yangtze River, China) at Pingshan station by using a long‐term database collected for over 50 years. Furthermore, we bridge the limitations of pure predictive models to learn from temporal data structures with the main purpose of identifying the mechanisms underpinning the suspended sediment patterns (e.g., climatic forces). Intraseasonal‐to‐seasonal, annual, and inter‐annual dominant time‐scales are thus identified. The short time‐scales are driven by the bi‐modal seasonal precipitation pattern specific to the climate of the region and provide a continuous supply of sediments to the river. The large time‐scales, controlled by high magnitude flow events and within‐reach sediment storage, display alternating periods of increasing and decreasing sediment fluxes; ultimately, they maintain the river channel within balance or within a moderate positive sediment accumulation process. This analysis and methodology help to understand temporal sediment dynamics, and ultimately to manage river catchments.

Reconstruction of the Upper Slope Conditions of an Extraordinary Hydro-Meteorological Event Along the Jamapa Glacier Drainage System, Citlaltépetl (Pico de Orizaba) Volcano, Mexico

A singular precipitation event on the summit glacial slopes of Mexico’s highest volcanic peak, Citlatépetl (also known as Pico de Orizaba), associated with the passage of Hurricane Ernesto across the southern Mexico mainland in August 2012, resulted in a debris flow at altitudes above 4,400 m asl, culminating in a hyperconcentrated flow downstream that had major impacts to a river valley’s channel morphology as well as to communities along a 25 km runout. The lahar originated at the terminal moraine and proglacial ramp of the Little Ice Age (LIA) extent of Citlaltépetl’s Jamapa glacier. Precipitation amounts were estimated based on nearby CONAGUA stations, but also on TRMM satellite images leading to an estimated 106 mm for a 3 day total, with 85 mm (80% of the total) falling on August 9th, the date when the lahar event occurred. The initial debris flow removed a minimum estimated 60,000 m3 of material from the proglacial ramp. A possible causative scenario is that the precipitation event overpressured the groundwater hydrology of an already unstable glacial-melt-saturated moraine. We demonstrate a methodology for the recreation of a pre-event landscape and the environmental conditions at the onset of the lahar, utilizing satellite products, in-situ geomorphological and geological evidence, and UAS technology.