Large Braided River Research Articles

Evolution and morphodynamics of the large braided river Brahmaputra during the last four decades of the Quaternary epoch: a spatio-temporal assessment using geoinformatics

ABSTRACT Spatio-temporal assessment of the evolution and morphodynamics of the Brahmaputra River (BR) can contribute significantly in mitigating challenges offered by rapid urbanisation and climate change. The present remote sensing and geographic information system (RS and GIS) based analysis of the BR revealed an increased Braiding Intensity (BI) from 1976 to 2000, which declined in 2010, and re-emerged in 2020. The average maximum width of the river has generally widened over time and sandbar distribution has exhibited fluctuations. The overall active channel area increased by 1.5 times, and the sandbar area expanded by 1.2 times between 1976 and 2020. The interplay between braiding intensity and width reveals a positive relationship. The sandbar-water ratio reveals a significant prevalence of sandbars in the Brahmaputra River. Furthermore, the study highlights changes in the Bar Development Index (BDI), showing a significant increase in micro-bars and meso-bars but a decrease in macro-bars. With the approach of change detection, key factors observed within the river driving the changes are erosion, channel bifurcations, sandbar formation due to sediment deposition, and sandbar loss due to shifts in channel course. These characteristics are potentially influenced by high water and sediment influx, climate change, and anthropological activities.

Identifying the stability trajectory of a large braided Brahmaputra river using reach-scale process-based approach

The dynamic evolution of a large braided river system is characterized by sporadic operation, impacted by a diverse range of natural and anthropogenic factors. The hydro-geomorphological interaction within the river corridor and its surrounding ecosystem presents a complex process, posing challenges in comprehending its dynamics. This study investigated the spatiotemporal heterogeneity in the vegetation-flow regime of a large braided reach of mighty Brahmaputra river. A detailed statistical analysis for the last three decades (1990 to 2019) was performed to demonstrate the in-stream vegetation characteristics for the selected middle reach (240 km) of the river. Google earth engine platform and advanced geospatial techniques have been used in this study. A novel indicator called stability trajectory indicator was developed that utilized vegetation information to identify the stability state of braided reaches. The findings of this study indicate a substantial increase in moderate and dense vegetation cover compared to light vegetation cover over the past decade. These results highlight the pronounced influence of increased moderate and dense vegetation type on the stability criteria of the reach, surpassing the contribution of light vegetation. The approach outlined in this study holds potential for river engineers to develop an effective management strategy for monitoring and maintaining river health.

Petrology and geochemistry of sandstone from Lower Cretaceous Kikhla Formation, NW Libya: Implications for provenance and depositional setting.

This study aims to determine and investigate the provenance of the source rocks and the depositional setting of the Kikhla Formation, northwestern Libya. It is mainly composed of mature quartzitic sandstone alternating with conglomerate and clay. The study proved that the sandstone is yellow-white in color and commonly carries quartz granular and small fragments of silicified wood. The environmental deposition is under differing fluvial conditions and by a large braided river. Texturally, the Kikhla sandstone is poorly cemented, coarse to very coarse grained and strongly unimodal and is classified as litharenite and subarkose by the modal composition, which is supported by geochemical studies. According to petrography and geochemistry, the Kikhla sandstone was deposited in a passive continental margin basin, mainly from granitic sources. On the basis of major elements ratios, and petrographic interpretation, the source rocks are most likely recognized as Proterozoic granites that were exposed via rifting. The CIA values (73.28-93.97) indicate a high degree of chemical weathering, which could be attributed to the arid climate conditions in the source area and mainly controlled by the source-rock provenance, hydraulic sorting during transport and deposition, diagenesis and depositional environment. According to chemical investigations, sandstone exhibits significant concentrations of SiO2, Na2O> K2O, and Fe2O3, which is compatible with the modal data.

Developing process-based geomorphic indicators for understanding river dynamics of a highly braided system: Implications for designing resilience based management strategies

The present paper utilizes the Google Earth Engine (GEE) cloud computing platform and hydro-morphological dataset to understand the complex process-form relationships of the highly braided Brahmaputra River, India. To identify dynamic gradients between low and high risk of geomorphic adjustment/change, an analytical approach is developed to track and quantify the spatial patterns of geomorphic adjustment. In order to address the dominant geomorphological adjustments, two processed-based novel indicators are developed- the Normalised Threshold Exceedance (NTE) considering the excess energy theory, and the Normalised Process Gradient (NPG) utilizing the gradient between hierarchical thresholds. The results show that in a highly braided river system, the sand bar adjustments prevail over other forms of morphological changes and is closely dependent on the process gradient between the hierarchical thresholds. Further, the scale of planform evolution is computed from the Location Probability Index (LPI), and the geomorphic stationarity concept is introduced for a large braided river. Finally, a multi-faceted resilience-based freedom space management approach is designed based on system state trajectory, LPI variability, proximity to thresholds, and in-channel landform configuration.

North slope transition zone of Songnan-Baodao sag in Qiongdongnan Basin and its control on medium and large gas fields, South China Sea

Based on analysis of newly collected 3D seismic and drilled well data, the geological structure and fault system of Baodao sag have been systematically examined to figure out characteristics of the transition fault terrace belt and its control on the formation of natural gas reservoirs. The research results show that the Baodao sag has the northern fault terrace belt, central depression belt and southern slope belt developed, among them, the northern fault terrace belt consists of multiple transition fault terrace belts such as Baodao B, A and C from west to east which control the source rocks, traps, reservoirs, oil and gas migration and hydrocarbon enrichment in the Baodao sag. The activity of the main fault of the transition belt in the sedimentary period of Yacheng Formation in the Early Oligocene controlled the hydrocarbon generation kitchen and hydrocarbon generation potential. From west to east, getting closer to the provenance, the transition belt increased in activity strength, thickness of source rock and scale of delta, and had multiple hydrocarbon generation depressions developed. The main fault had local compression under the background of tension and torsion, giving rise to composite traps under the background of large nose structure, and the Baodao A and Baodao C traps to the east are larger than Baodao B trap. Multiple fault terraces controlled the material source input from the uplift area to form large delta sand bodies, and the synthetic transition belt of the west and middle sections and the gentle slope of the east section of the F12 fault in the Baodao A transition belt controlled the input of two major material sources, giving rise to a number of delta lobes in the west and east branches. The large structural ridge formed under the control of the main fault close to the hydrocarbon generation center allows efficient migration and accumulation of oil and gas. The combination mode and active time of the main faults matched well with the natural gas charging period, resulting in the hydrocarbon gas enrichment. Baodao A transition belt is adjacent to Baodao 27, 25 and 21 lows, where large braided river delta deposits supplied by Shenhu uplift provenance develop, and it is characterized by large structural ridges allowing high efficient hydrocarbon accumulation, parallel combination of main faults and early cessation of faulting activity, so it is a favorable area for hydrocarbon gas accumulation. Thick high-quality gas reservoirs have been revealed through drilling, leading to the discovery of the first large-scale gas field in Baodo 21-1 of Baodao sag. This discovery also confirms that the north transition zone of Songnan-Baodao sag has good reservoir forming conditions, and the transition fault terrace belt has great exploration potential eastward.

Understanding the entropy-based morphological variability and energy expenditure mechanism of a large braided river system

Understanding the morphological erraticism for large braided river systems is still a challenging issue in the present era of fluvial geomorphology. The present paper reports thorough spatio-temporal variability of sand bar dynamics (2000–2019) and their influence on the braided belt disorderness in the large braided Brahmaputra River. The sand bar area is detected using the modified normalized difference water index (MNDWI), and an entropy-based intensity disorder index (IDI) is further proposed to measure the spatio-temporal disorderness in the post-monsoon period. The IDI varies at a lowest value of less than 0.5 to a highest value of more than 0.9 for the selected study reach. Interestingly, IDI displays a declining trend for the first decade, followed by an increasing trend in the subsequent time slice, which signals the large-scale heterogeneity in process-form interactions for the Brahmaputra. An attempt was made to establish relationship between IDI, stream power derivatives (monsoon, peak flood, and post-monsoon), and bar area classes in order to facilitate a process-based understanding of the geomorphic adjustments. In comparison with other stream power derivatives, the peak-flood stream power showed a better correlation with the disorderness measure of the reach. Finally, this study has highlighted that smaller, highly immobile bars have significant control on the IDI variations and govern the yearly planform disorderness of the Brahmaputra. Further, the findings of this study can be used to construct a process-form-interaction framework for large braided river systems.

Application of remote sensing in the description of fluvial system in dryland: A case study of Golmud distributive fluvial system, Qaidam Basin, NW China

Distributive fluvial systems (DFS) are widespread in drylands in the northwestern China. Analyzing differences in fluvial morphology in drylands is beneficial for studying ancient rock records and the extra-terrestrial surface environment. The remote sensing image, characterized by real-time and possibility of repeated observations, is a vital tool for recording and comparing fluvial systems in drylands. Satellite remote sensing technology is a method of investigating fluvial morphologies. Due to the limited accuracy of satellite imagery, there are few reports on the detailed description of the fluvial system in drylands of NW China. We analyze the pattern of fluvial morphology changes in the Golmud distributive fluvial system (DFS) in the Qaidam Basin, northwestern China, using satellite remote sensing and unmanned aerial vehicles (UAV). Firstly, we use Google Earth real-time image data, historical image data, and radar digital elevation data to extract geomorphological information; then the UAV remote sensing image data were used to interpret fluvial network information; finally, we use the gray-scale differential vector method to describe the fluvial morphologies. Three zones have been identified in the Golmud DFS: the proximal, the medial, and the distal, by comparing the differences in topographic and geomorphic characteristics, fluvial morphologies, and sedimentary characteristics of the Golmud DFS. The proximal slope is higher than the other two zones, and the geomorphic features are mainly gravel gobi. The proximal fluvial morphologies are mainly large braided rivers, and sediments are more gravelly and less sandy. The medial slope is relatively small, and the geomorphic features are mostly oasis plains. The medial fluvial morphologies are mainly meandering rivers associating with braided rivers, and sediments are more sandy and less gravelly. The distal slope is the lowest, and the geomorphic features are mostly oasis plains, lakes, and marsh plains. The distal fluvial morphologies are mainly meandering rivers, and sediments are sandy and muddy. Comparison of the DFS from proximal to medial to distal in Golmud confirmed the potential of remote sensing image technology in identifying the fluvial morphologies and sedimentary facies distribution in dryland.

Sandstone characteristics and hydrocarbon exploration significance of the Middle-Lower Jurassic of the central-eastern part of the northern tectonic belt of the Kuqa Depression, Tarim Basin, China

The Jurassic at the northern tectonic belt of the Kuqa Depression, Tarim Basin, is the key frontier of replacement for oil and gas exploration in the 13th five-year plan of China. The 4th national resource evaluation of China demonstrates that this area possesses resources amounting to 565 × 106 t of oil equivalent, and yet the proven resource accounts for only 3.7% of the total. To determine the exploration target of the Mid-Lower Jurassic and identify the favorable zone and trap type, studies on sedimentary microfacies and sand body configuration have been carried out based on data from outcrops, drilling, and well logging. It is clarified that the braided river delta plain-front-shore-shallow lake sedimentary system deposited during lake transgression in the wide and gentle lake basin occurs in the Middle and Lower Jurassic. The Ahe Formation is developed with the ultra-thick sand body of the large braided river channel in the braided river upper delta plain. Meanwhile, the Yangxia and Kezilenuer formations are found with the mid-thin sand body of the medium and small fluvial channel of the braided river lower delta plain-front sedimentation. There are three sand bodybuilding models favorable for hydrocarbon accumulation that has been developed. Moreover, it is proposed that the sand body of the Ahe Formation, originating from vertical accretion of the braided river channel upper delta plain fluvial, is the exploration zone for structural reservoirs; the lateral imbricated sand body of the braided river channel lower delta plain fluvial of the Yangxia Formation, and the lenticular sand body resulting from lateral accretion of the delta front fluvial channel of the Kezilenuer Formation, are the exploration zone for structural-lithological and lithological reservoirs. The Kezilenuer and Yangxia formations at the eastern, northern, and western flanks of the Tugeerming anticline are the most favorable for structural-lithological reservoirs, and a favorable zone of 700 km2 at the flanks has been demarcated. The platform zone at the Dibei slope is the most preferable for structural reservoirs, where in the average porosity of the Ahe Formation reservoir rock is 5.6%, the average permeability is 0.75 × 10−3μm2, and the favorable zone covers 750 km2. Solid evidence has been provided for the determination of the favorable zone and trap type in the Middle and Lower Jurassic at the northern tectonic belt of the Kuqa Depression.

Confluence Dynamics in a Modelled Large Sand-Bed Braided River

Confluences are key morphological nodes in braided rivers where flow converges, creating complex flow patterns and rapid bed deformation. Field surveys and laboratory experiments have been carried out to investigate the morphodynamic features in individual confluences, but few have investigated the evolution processes of confluences in large braided rivers. In the current study, a physics-based numerical model was applied to simulate a large lowland braided river dominated by suspended sediment transport, and analyze the morphologic changes at confluences and their controlling factors. It was found that the confluences in large braided rivers exhibit some dynamic processes and geometric characteristics that are similar to those observed in individual confluences arising from two tributaries. However, they also show some unique characteristics that may result from the influence of the overall braided pattern and especially of their neighboring upstream channels.

Does spatial heterogeneity of hyporheic fauna vary similarly with natural and artificial changes in braided river width?

Heterogeneity of hyporheic fauna is associated with geomorphological features and related vertical water exchanges. Constrictions on river floodplain are known to induce groundwater inputs and increase stygobite fauna. Two floodplain constrictions were studied in a large braided river (the Drôme River): one linked to a natural process (valley narrowing), another to an artificial river regulation (early 20th embankment). Spatial distribution of hyporheic organisms were sampled upstream and downstream of the two constrained sections, at 9 stations, 3 positions (left and right sides, centre of the braided strip), 3 replication points and at a depth of 50 cm in the river sediment. The spatial heterogeneity in community composition was higher near the banks than at the centre of the braided strip, no matter the width of the strip. The artificial constriction induced a decrease in spatial heterogeneity of the benthic fraction of the hyporheic fauna, but no changes were detected for the stygofauna. The natural valley narrowing reduced width and thickness of the alluvium and induced an inflow of groundwater resulting in an increase in stygofauna abundance. Natural floodplain narrowing linked to geology thus control the distribution of stygobite species, while artificial constrictions only modify the spatial distribution of the benthic fraction of the hyporheic fauna.

An entropy based morphological variability assessment of a large braided river

AbstractThe present paper utilizes entropy to investigate variability in the surface areas of sandbars distribution of the Brahmaputra River, India. The intensity entropy disorder index (IDI) was computed for the dry and the post‐monsoon season for a selected reach of the river. The planform disorder index (PDI) was computed to represent the annual scale planform change due to seasonal flood. Finally, annual monsoon and peak flood stream power were evaluated and related to PDI. The results showed that the intensity disorder index followed a decreasing trend in dry and post‐monsoon seasons; tending towards lower deviation from a uniform distribution. It was noted that except for extreme flood years the PDI trend correlated well with monsoonal stream power. Distinctive trends in stream power were also observed in the present study. © 2018 John Wiley & Sons, Ltd.

Geomorphological factors influencing hysteresis patterns between suspended load and flow rate in Alpine rivers

Suspended sediment load represents a large part of total solid fluxes transported in most rivers. Thus, for hydropower plan management or for environmental issues, it is crucial to understand how these sediments are produced, stored and transported in a given catchment. Hysteresis loops in discharge-suspended load signals are commonly used to assess sediment sources and production processes but most of the time the shape of this relation is analyzed qualitatively on short time series or for few events. In this study we analyzed quantitatively 10 long time series of suspended sediment load of various alpine catchments. This method allows us to compare events and to assess to which extent fine sediments originate from hillslope erosion processes or from river bed remobilization. We found that watersheds with braided bed morphology are dominated by clockwise loops while those with narrower bed as step-pool morphology are dominated by counter-clockwise hysteresis or have no general trend. These results suggest that storage and remobilization of fine sediments within the bed could play a major role in suspended sediment transport in Alpine streams, especially in large braided rivers.

Experimental bed active-layer survey with active RFID scour chains: Example of two braided rivers (the Drac and the Vénéon) in the French Alps

An innovative scour chains device composed of active UHF RFID tags is proposed to survey the bed active layer. This device is tested on three cross-sections deployed along two large Alpine braided rivers: the Drac and the Vénéon. A specific field deployment procedure is developed, using a technique of drilling with a tube and a jack-hammer. After each flood, the device allows recording presence/absence of active tags inside a scour chain column. This provides the maximum scouring depth of the bed at the position of the column, and the topographic resurvey of the channel provides the net fill depth. Results show that bed active layer can reach up to 1.43 m during high flow conditions and remains <0.1 m during low and moderate flows. The main advantage of this technology against traditional scour chains is the rapidity and easiness with which the scouring depth is obtained after a flow event, notably under conditions of massive net deposition after the flow. This technology is therefore particularly suitable for large braided rivers, where traditional scour chains necessitate too much field efforts for their relocation, and can be rapidly lost under deep active layers.

Prediction of Short-Term Morphological Change in Large Braided River Using 2D Numerical Model

AbstractPrediction of annual changes in braided-river morphology is important to assess the performance of river-training works and channel navigability. A two-dimensional (2D) depth-averaged numerical model was used for a 12-km-long reach of the Brahmaputra River in India. Major issues addressed in the modeling included approximation of boundary conditions, design of a flood hydrograph, and evaluation of river morphology. Simulated flow depth and velocity were calibrated with the observed data under bankfull discharge conditions, and predicted bed levels were validated with observed bed levels during low-flow conditions. The model predictions of thalweg shifting for two consecutive flood seasons agreed well with observed changes obtained from satellite imagery. The calibrated model was subsequently used to assess morphological changes in different groyne fields. The best groyne field was determined based on overall performance in scouring, deposition, channel alignment, and dredging volume.

Network response to disturbances in large sand-bed braided rivers

Abstract. The reach-scale effects of human-induced disturbances on the channel network in large braided rivers are a challenge to understand and to predict. In this study, we simulated different types of disturbances in a large braided river to get insight into the propagation of disturbances through a braided channel network. The results showed that the disturbances initiate an instability that propagates in the downstream direction by means of alteration of water and sediment division at bifurcations. These adjustments of the bifurcations change the migration and shape of bars, with a feedback to the upstream bifurcation and alteration of the approaching flow to the downstream bifurcation. This way, the morphological effect of a disturbance amplifies in the downstream direction. Thus, the interplay of bifurcation instability and asymmetrical reshaping of bars was found to be essential for propagation of the effects of a disturbance. The study also demonstrated that the large-scale bar statistics are hardly affected.

Temporal transformations of large braided river channel (near Aldan reach of the Lena River as an example)

River channels characterized by the multi-thread patterns are most complex form points of view of morphology, structure and deformation regime. The paper presents an analysis of the largest river channel deformations at the adjoint braids with an example of the Lena River reach near its confluence with the Aldan River including the three consecutive braids – Haityallah, Omulagan and Arbyn. During the 20 th century and the first decade of the 21 st century the studied Lena River reach has experienced periodic shifts of the main arm, associated changes of discharge ratios between arms within braids and the resulting relocation of the navigable fairway track. Temporal patterns of discharge fluctuations in the Lena and Aldan Rivers do not coincide, causing alternations of the dominant influence of the main river flow onto its tributary or the opposite during the peak discharge periods. It is shown that the intermittent phase of temporal transformation of the adjoint braids channel pattern into the parallel arms channel pattern occurs between the longer stages of dominant development of left or right arms. Duration of that phase depends mainly upon the average water discharge relatively to its long-term fluctuations (increases during higher runoff periods and decreases during lower runoff periods), flow backing conditions caused by the inflowing tributary (the Aldan River) affecting the entire length of the studied Lena River reach and local channel deformation conditions. In general, such a transitional phase of the channel development (between the alternating stages of dominance of left or right arms within the adjoint braids chain) can last for significant period of 10-15 years or even more. Duration of the transitional phase is comparable to that of the alternating stages of dominance of left or right arms on large- or medium-sized rivers where parallel arms pattern of the multi-thread channel does not develop. In turn, channel deformations at the adjoint braids exert significant influence on development of main arms of the internal delta situated at the confluence of the Lena and Aldan Rivers, where the main control is associated with alternation of flow backing or no backing conditions related to different discharge ratios between the main river and its main tributary.

Tectono-geomorphic and environmental set-up deduced during deposition of Mio-Pleistocene sediments in NW Himalaya, India

Detailed pedofacies characterization along-with lithofacies investigations of the Mio-Pleistocene Siwalik sediments exposed in the Ramnagar sub-basin have been studied so as to elucidate variability in time and space of fluvial processes and the role of intra- and extra-basinal controls on fluvial sedimentation during the evolution of the Himalayan foreland basin (HFB).Dominance of multiple, moderately to strongly developed palaeosol assemblages during deposition of Lower Siwalik (~12–10.8Ma) sediments suggest that the HFB was marked by Upland set-up of Thomas et al. (2002). Activity of intra-basinal faults on the uplands and deposition of terminal fans at different times caused the development of multiple soils. Further, detailed pedofacies along-with lithofacies studies indicate prevalence of stable tectonic conditions and development of meandering streams with broad floodplains. However, the Middle Siwalik (~10.8–4.92Ma) sub-group is marked by multistoried sandstones and minor mudstone and mainly weakly developed palaeosols, indicating deposition by large braided rivers in the form of megafans in a Lowland set-up of Thomas et al. (2002). Significant change in nature and size of rivers from the Lower to Middle Siwalik at ~10Ma is found almost throughout of the basin from Kohat Plateau (Pakistan) to Nepal because the Himalayan orogeny witnessed its greatest tectonic upheaval at this time leading to attainment of great heights by the Himalaya, intensification of the monsoon, development of large rivers systems and a high rate of sedimentation, hereby a major change from the Upland set-up to the Lowland set-up over major parts of the HFB.An interesting geomorphic environmental set-up prevailed in the Ramnagar sub-basin during deposition of the studied Upper Siwalik (~4.92 to <1.68Ma) sediments as observed from the degree of pedogenesis and the type of palaeosols. In general, the Upper Siwalik sub-group in the Ramnagar sub-basin is subdivided from bottom to top into the Purmandal sandstone (4.92–4.49Ma), Nagrota (4.49–1.68Ma) and Boulder Conglomerate (<1.68Ma) formations on the basis of sedimentological characters and change in dominant lithology. Presence of mudstone, a few thin gravel beds and dominant sandstone lithology with weakly to moderately developed palaeosols in the Purmandal sandstone Fm. indicates deposition by shallow braided fluvial streams. The deposition of mudstone dominant Nagrota Fm. with moderately to some well developed palaeosols and a zone of gleyed palaeosols with laminated mudstones and thin sandstones took place in an environment marked by numerous small lakes, water-logged regions and small streams in an environment just south of the Piedmont zone, perhaps similar to what is happening presently in the Upland region/the Upper Gangetic plain. This area is locally called the ‘Trai region’ (Pascoe, 1964). Deposition of Boulder Conglomerate Fm. took place by gravelly braided river system close to the Himalayan Ranges. Activity along the Main Boundary Fault led to progradation of these environments distal-ward and led to development of on the whole a coarsening upward sequence.

Vegetation turnover in a braided river: frequency and effectiveness of floods of different magnitude

AbstractThis work addresses the temporal dynamics of riparian vegetation in large braided rivers, exploring the relationship between vegetation erosion and flood magnitude. In particular, it investigates the existence of a threshold discharge, or a range of discharges, above which erosion of vegetated patches within the channel occurs. The research was conducted on a 14 km long reach of the Tagliamento River, a braided river in north‐eastern Italy. Ten sets of aerial photographs were used to investigate vegetation dynamics in the period 1954–2011. By using different geographic information system (GIS) procedures, three aspects of geomorphic‐vegetation dynamics and interactions were addressed: (i) long‐term (1954–2011) channel evolution and vegetation dynamics; (ii) the relationship between vegetation erosion/establishment and flow regime; (iii) vegetation turnover, in the period 1986–2011. Results show that vegetation turnover is remarkably rapid in the study reach with 50% of in‐channel vegetation persisting for less than 5–6 years and only 10% of vegetation persisting for more than 18–19 years. The analysis shows that significant vegetation erosion is determined by relatively frequent floods, i.e. floods with a recurrence interval of c. 1–2.5 years, although some differences exist between sub‐reaches with different densities of vegetation cover. These findings suggest that the erosion of riparian vegetation in braided rivers may not be controlled solely by very large floods, as is the case for lower energy gravel‐bed rivers. Besides flow regime, other factors seem to play a significant role for in‐channel vegetation cover over long time spans. In particular, erosion of marginal vegetation, which supplies large wood elements to the channel, increased notably over the study period and was an important factor for in‐channel vegetation trends. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Physics‐based numerical modelling of large braided rivers dominated by suspended sediment

AbstractPhysics‐based models have been increasingly developed in recent years and applied to simulate the braiding process and evolution of channel units in braided rivers. However, limited attention is given to lowland braided rivers where the transport of suspended sediment plays a dominant role. In the present study, a numerical model based on the basic physics laws of hydrodynamics and sediment transport is used to simulate the evolution process of a braided river dominated by suspended load transport. The model employs a fractional method to simulate the transport of graded sediments and uses a multiple‐bed‐layer approach to represent the sediment sorting process. An idealized braided river has been produced, with the hydrodynamic, sediment transport and morphological processes being analysed. In particular, the formation process of local pool–bar units in the predicted river has been investigated. A sensitivity analysis has also been undertaken to investigate the effects of grid resolution and an upstream perturbation on the model prediction. A variety of methods are applied to analyse the geometrical and topographical properties of the modelled river. Self‐organizing characteristics related to river geometry and topography are analysed by state‐space plots, which indicate a close relationship with the periodical erosion and deposition cycles of braiding. Cross‐sectional topography and slope frequency display similar geometries to natural rivers. Scaling characteristics are found by correlation analysis of bar parameters. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Physics-based modeling of large braided sand-bed rivers: Bar pattern formation, dynamics, and sensitivity

[1] Braided rivers have complicated and dynamic bar patterns, which are challenging to fully understand and to predict both qualitatively and quantitatively. Linear theory ignores nonlinear processes that dominate fully developed bars, whereas natural river patterns are determined by the combined effects of boundary conditions, initial conditions such as planimetric forcing by fixed banks and the physical processes. Here we determine the capability of a state-of-the-art physics-based morphological model to reproduce morphology and dynamics characteristic of braided rivers and determine the model sensitivity to generally used constitutive relations for flow and sediment transport. We use the 2-D depth-averaged morphodynamic model Delft3D, which includes the necessary spiral flow and bed slope effects on morphology. We present idealized scenarios with the smallest possible number of enforced details in the planform and boundary conditions in order to allow free development of bars driven by the physical processes in the model. We analyze bar and channel shapes and dynamics quantified by a number of complementary metrics and compare these with imagery, field data captured in empirical relations, flume experiments, and predictions by linear analyses. The results show that the chosen set of boundary conditions and physics in the numerical model is sufficient to produce many morphological characteristics and dynamics of a braided river but insufficient for long-term modeling. Initially, braiding intensity with low-amplitude bars is high in agreement with linear analysis. In a second stage when bars merge, split, and increase amplitude up to the water surface, the shape, size, and dynamics of individual bars compare well to those in natural rivers. However, long-term modeling results in a reduction of bar and channel dynamics and formation of exaggerated bar height and length. This suggests that additional processes, such as physics-based bank erosion, or enforced fluctuations in boundary conditions, such as spatial-temporal discharge variation, are necessary for the simulation of a dynamic equilibrium river. The most important outcome is that the modeled pattern of bars and channels is highly sensitive to the constitutive relation for bed slope effects that is used in many morphological models. Regardless of this sensitivity and present model limitations of many models, this study shows that physics-based modeling of sand-bed braided improves our understanding and prediction of morphological patterns and dynamics in sand-bed braided rivers.