Neck Cutoffs Research Articles

Morphometry of Tidal Meander Cutoffs Indicates Similarity to Fluvial Morphodynamics

AbstractSinuous channels wandering through coastal wetlands have been thought to lack lateral‐migration features like meander cutoffs and oxbows, spurring the broad interpretation that tidal and fluvial meanders differ morphodynamically. Motivated by recent work showing similarities in planform dynamics between tidal and fluvial meandering channels, we analyzed meander neck cutoffs from diverse tidal and fluvial environments worldwide, and show that tidal cutoffs are widespread. Their perceived paucity stems from pronounced channel density and hydrological connectivity in coastal wetlands, comparatively small size of most tidal channels, and typically dense vegetation cover. Although these factors do not efface tidal meander cutoffs, they collectively inhibit oxbow formation and make tidal cutoffs ephemeral features that can escape detection. We argue that similar morphodynamic processes drive cutoff formation in tidal and fluvial landscapes, with differences arising only during post‐cutoff evolution. Such process similarity has important implications for understanding coastal wetland ecomorphodynamics and predicting their long‐term evolution.

Exploring meandering river cutoffs

Abstract Cutoffs, which include neck and chute cutoffs, are the results of the fluvial processes that fundamentally influence evolution of meandering rivers. A neck cutoff happens when the two limbs of a highly sinuous bend touch, whereas a chute cutoff refers to the formation of a shortcut channel passing through a meander bend. In this review, we begin by distinguishing the morphological and hydrological conditions of the two cutoff types. Mechanisms driving the development of a neck cutoff are embodied in a variety of kinematical and hydrodynamic models simulating processes governing the long-term evolution of meandering rivers. These models adopt a morphological threshold for judging the occurrence of a neck cutoff, b = αw , where α is a constant ranging between 0 and 1, b is the bend neck width and w is the mean channel width. The potential underestimation of the evolutionary period during the late stage of bend evolution toward a neck cutoff when using this morphological threshold and the uncertainties in quantifying the migration–curvature relationship limit the abilities of existing models to predict the occurrence of neck cutoffs. We then suggest three possible directions for future research on meander neck cutoffs. Mechanisms controlling chute cutoffs are relevant to six key factors representing the meander hydrological regime, planform morphology, bed topography and floodplain characteristics. The combination of these factors gives rise to four distinguishable triggering mechanisms: headward-erosion, embayment, mid-channel bar and scroll–slough chute cutoffs, though the initiation of the chute cutoff may be caused by some combination of these. However, the hydraulic and morphological characteristics of meander bends under these triggering mechanisms are so complex that they are often site-specific, making it extremely challenging to generalize the known morphodynamic and hydrodynamic processes driving the formation of chute cutoffs in individual meander bends. We close the review by recommending three possible research directions on chute cutoffs for tackling the existing challenges in the future.

High-resolution modeling of meander neck cutoffs: laboratory and field scales

Meandering rivers are distinguished by their characteristic sinuosity, which is subject to modulation through channel cutoff, resulting in the formation of oxbow lakes within the abandoned meander loops. Throughout the evolutionary course of a river, these cutoffs establish a connection between the channel and floodplain systems, both crucial to maintaining the dynamic equilibrium of the river system. Nonetheless, the interactive dynamic between the channel and floodplain and its influence on the transient behavior of the channel’s morphodynamics during a cutoff event are frequently reduced to simplistic representations in computational models. This study introduces a comprehensive numerical model that elucidates the adaptive processes of bed and planform during and subsequent to the inception of cutoff and oxbow lakes. The model is assessed through its application to a laboratory scale cutoff, before being employed to a real-world meandering river, specifically the Ucayali River in Peru, in order to gain understanding into channel development and the intricate patterns of planform dynamics following cutoff events. The model is able to capture the main modes of planform migration, translation and expansion for the case of the bend in the Ucayali River. During the neck cutoff, the model simulates the progression of erosional and depositional waves traveling in upstream and downstream directions respectively, underscoring the importance of incorporating both hydrodynamic and morphodynamic factors in characterizing the river dynamics associated with meander cutoffs.

Fluvial-style migration controls autogenic aggradation in submarine channels: Joshua Channel, eastern Gulf of Mexico

Abstract There has been debate over the processes acting on deep-water channels, with comparisons made to the evolution of meandering fluvial systems. We characterized a three-dimensional seismic-reflection dataset of the Joshua deep-water channel–levee system located in the eastern Gulf of Mexico and interpreted 13 horizons showing its kinematic evolution over a 25 km reach. Over this reach, we documented channel migration through systematic bend expansion and downstream translation, which was sustained through channel aggradation as sinuosity increased from 1.25 to 2.3 at abandonment. An abrupt decrease in sinuosity was associated with a neck cutoff, which changed the subsequent migration direction of the channel in that locality. These processes are analogous to the evolution of meandering fluvial systems. We show that increasing channel sinuosity correlates to a reduction in channel slope and hypothesize that this promoted increasingly depositional turbidity currents that led to channel aggradation. Using a simple forward stratigraphic model in which vertical movements of the channel are governed by a stream power law, we show how aggradation can be driven autogenically. Trends in sinuosity, aggradation and slope are in broad agreement between the Joshua Channel and the model. This highlights the potential importance of intrinsic channel processes as a control on system evolution.

Experimental Study on Neck Cutoff in Meandering River under Variable Discharges

Neck cutoff is a sudden phenomenon that occurs during the evolution of meandering rivers, but its process and mechanism remain unclear. In this investigation, the neck-narrowing process before cutoff, the formation of a new river, and the appropriate conditions for cutoff under constant and variable discharges were studied using experimental methods in a laboratory flume. The results indicated that bank erosion upstream and downstream of the neck was the main factor that triggered cutoff, regardless of whether the discharge changed. The sinuosity of the model river was approximately 7, the ratio of the neck width to the average river width reached approximately 0.43, and the experimental material was nonuniform natural sand. Under the conditions that the unit width discharge ranged from 0.0039 to 0.0069 m2/s and the longitudinal gradient was 1‰, neck cutoff could occur after sufficient bank erosion. When neck cutoff was imminent, the sinuosity of the channel increased and the channel rotated downstream. After neck cutoff, the formation of a new river experienced three stages, namely, the short-term longitudinal scouring stage, the long-term horizontal widening stage, and the long-term transverse widening and normal slow development stage. The discharge resulting in neck scouring was the effective discharge contributing to the cutoff, and this discharge directly affected the cutoff time and position. We attempted to explain the factors leading to neck cutoff from an energy consumption perspective. Neck cutoff was a sudden phenomenon that occurred when the energy consumption of a meandering river increased to its maximum value upon entry to the recession stage from the relative equilibrium stage. To a certain extent, this study is significant in understanding river evolution and neck cutoff.

Dynamics of Meander Geometry and Formation of Neck Cutoff of River Raidak-I within Himalayan Foreland Basin, India

Abstract Tectonic activities and catastrophic flood events significantly assist to the process of channel avulsion and formation of river cutoffs on alluvial floodplain. The principal objectives of this research are (i) to examine the mechanism and causes of neck cutoff formation of river Raidak-1, India applying geospatial techniques, old maps and bank facies (1955-2019) and (ii) to investigate the dynamics of meander channel geometry using fluvio-hydrological techniques. Geospatial techniques and field based bank facies analysis have been used in this study. The result showed that the river channel has been significantly shifted (during the time span 1955-2019) owing to lateral channel migration resulting into formation of several neck cutoffs on the floodplain. Meander geometry study revealed that radius of curvature, meander wavelength, meander amplitude and meander belt width have been fluctuated during the entire study periods. The threshold value (less than 5°) of apex angle of two tangents at meander loop and neck distance (less than 1 km) helps to form regular neck cutoffs. The results show that high sinuosity, threshold value of meander loops; major flood events, high discharge and non-cohesive bank materials are responsible for the development of neck cutoffs. Moreover, probable sites of future cutoff formation have also been identified. Therefore, this study would give proper insights for river management approach particularly on dynamic Himalayan foreland basin, India.

Delayed neck cutoff in the meandering Black River of the Qinghai–Tibet plateau

AbstractNeck cutoffs in meandering rivers have long been thought to occur when the neck width (b) approximates the mean width of the parent channel (W). Empirical evidence for this paradigm is scarce, however, because tracking late‐stage evolution of meander bends prior to cutoff at sufficient temporal resolutions is difficult in natural rivers. In this study, we captured field and photogrammetric data depicting temporal changes of the banklines forming the neck of one meander bend, and values of the narrowest neck width for 14 highly sinuous bends, in the Black River of the Qinghai–Tibet Plateau (China) over nearly four decades. Results show that the duration of bend evolution from the classic morphological threshold (b ≈ W) to the occurrence of neck cutoff could range from 52 to 161 years. This long period has hitherto been ignored by fluvial geomorphologists, and needs to be included in forthcoming kinematic and hydrodynamic models of meander evolution. Further analyses indicate that the neck narrowing process is autogenous at the local bend scale, neither controlled by the maximum bend sinuosity nor by the annual peak discharge.

Modelling of cantilever bank failure for peat-type meander bends in the source region of the Yellow River

Bank retreat involving a combination of fluvial erosion and bank collapse has been found to be a major contributor to sediment transport, lateral migration, and planform evolution of meandering rivers. Previous studies have largely examined the general mechanism of cantilever bank failure. However, the composite process of beam (toppling) failure caused by shear failure of the lower part composed of non-cohesive soil remains poorly understood. The current paper investigates the diversity and coupling of this collapse process, which is the dominant pattern in the peat-type meandering rivers of the Zoige River basin in the source region of the Yellow River. For this purpose, morphologic and hydraulic measurements were done in the summers of 2016 and 2017. Moreover, a combined method of a theoretical model and numerical simulation was developed to be applied for a continuous meandering channel in the Zoige basin, focusing in particular on five typical cross sections of a highly convoluted bend approaching neck cutoff. Specifically, the theoretical model evaluating the stability of overhanging riverbank was based on static equilibrium theory considering the interaction between composite soil layers. In addition, the Bank Stability and Toe Erosion Model was used to simulate the retreat process of the silt layer based on field measurements including bank morphology, soil composition, and hydrological parameters. Thus, a calculation method for the occurrence of cantilever beam failure in the flood period is proposed, and temporal change and spatial variability in bank profiles caused by three consecutive failures is estimated at the five sections. Satisfactory results showed a relatively close agreement between the calculations and field measurements in terms of the width of the overhanging arms and the dimensions of the slump blocks.

Paleochannel of the Yellow River within the Zoige Basin and its environmental significance on the NE Tibetan Plateau

Paleochannel sedimentary sequences can provide abundant information on regional environmental changes. A typical paleochannel (paleo-oxbow lake type) section of the Yellow River was identified within the Zoige Basin on the NE Tibetan Plateau. A multi-index approach was used to accurately identify sediments of different genetic types, such as riverbed deposits of the Yellow River, paleo-oxbow lake deposits, and overbank flood deposits (OFD) in the section. Based on optically stimulated luminescence (OSL) and AMS 14C dates, we examined the environmental evolution recorded by the section. The results show that: (1) The section is a record of environmental change since 4.17 ± 0.49 ka. During 4.17 ± 0.49 to 3.24 ± 0.26 ka, the ancient Yellow River occupied the channel. At 3.24 ± 0.26 ka, the paleochannel experienced a neck cutoff, and the fluvial environment began to change into the oxbow lake environment. After 2.45 ± 0.11 ka, the paleo-oxbow lake gradually disappeared. Subalpine meadow soil has developed at this site since 1.31 ± 0.05 ka. (2) Paleoenvironmental proxies indicate that the Zoige Basin was warmer and wetter before ~3.00 ka, and became drier and colder after ~3.00 ka, which may be mainly related to the weakening of the East Asian summer monsoon (EASM) and the strengthening of the Westerlies. (3) Two episodes of extreme overbank flooding occurred at 2.96 ± 0.24 to 2.87 ± 0.27 ka and 1.84 ± 0.20 to 1.70 ± 0.16 ka, correlated with climate shift period from the mid-Holocene climatic optimum to the late Holocene and the Dark Age Cold Period (DACP), respectively. Due to the relatively cold and dry climate in these periods, glaciers generally advanced on the Tibetan Plateau, and the contribution of snow and ice meltwater weakened. Therefore, the strong rainfall caused by the abnormal atmospheric circulation may be the main cause of these extreme overbank flooding.

Analyzing Neck Circumference as a Tool for Evaluating Overweight and Obesity in Chinese Adolescents.

Overweight and obesity at an early age are important criteria for predicting chronic diseases. Each anthropometric method available to assess obesity has its limitations. Recently, neck circumference (NC) has received greater attention as a new evaluation index. This study aimed to investigate the relationship between NC and overweight in Chinese Yi adolescents. A total of 647 Chinese Yi male and female students, aged 13–18 years, were randomly selected from a junior high school and a senior middle school in Leshan, Sichuan Province of China. The measurement indexes included height, weight, waist circumference (WC), and NC, and clinical information was collected by trained physicians. The neck cutoff values were determined through the receiver operating characteristic (ROC) curve and the area under the curve (AUC). The correlation among NC, body mass index (BMI), and WC was determined by Pearson's correlation coefficient. The ROC analysis revealed that the AUC values were 0.79–0.95 for boys and 0.83–0.91 for girls. The correlation among NC, BMI, and WC in obese boys and girls was higher than 0.70 in both genders. In addition, the NC cutoff values of high BMI ranged from 31.0 cm to 36.1 cm for boys and 31.2 cm to 34.5 cm for girls, respectively. There was a significant positive correlation between NC and obesity in Chinese Yi adolescents. The NC can be used as an additional index to predict the obesity of Chinese Yi adolescents.

Processes of neck cutoff and channel adjustment affected by seeding herbaceous vegetation and variable discharges

The goal of this study is to quantify morphodynamic roles of riparian vegetation and variable discharges in the process of neck cutoff, which is difficult to determine in natural meandering rivers due to the prolonged process and unpredictable occurrence of neck cutoffs. We achieved the goal in a highly sinuous flume channel (25 m × 6 m × 0.4 m) that has a mobile bed and includes seven bends with the narrowest neck of 0.22 m. Its banks and floodplain were covered by dense herbaceous vegetation (i.e., Festuca elata) seeded 10 days before each of three experiments that had different vegetation density and discharge arrangements. They are the first set of experiments of achieving neck cutoffs in a laboratory flume channel with vegetation. We examined temporal changes of the narrowest neck width and planform of the bend that had neck cutoff, measured temporal changes of the mean channel width and its final width/depth ratio, and tracked the temporal changes of their mean slopes and width/depth ratio. Our results revealed that (1) herbaceous vegetation can significantly extend the period of neck narrowing process such that neck cutoff may still take a long time even after neck width is about 0.4 of the mean channel width; (2) higher variable discharges only have limited impact on shortening this period; (3) neck cutoff is triggered by seepage flow that is incapable of generating sediment pulses and thus the morphological adjustment of the upstream and downstream reaches are mainly caused by changes of channel hydraulics rather than sediment deposition as in the case of chute cutoff. Our experiments show a new way of replicating neck cutoff in flume experiments and our findings provide new insight into understanding processes of neck cutoff in natural highly sinuous meandering rivers.

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.

PyRiverBed: A Python framework to generate synthetic riverbed topography for constant-width meandering rivers

Meandering is one of the unique processes in Earth surface dynamics. Since the 1960s, fluvial geomorphologists have generalized numerous widely-acknowledged empirical or semi-empirical formulae to describe meandering rivers' characteristics. The most frequently applied one is the Kinoshita high-sinuosity curve to describe meander channel planform geometry. Combining the Kinoshita curve with the Beck equations to describe the riverbed topography at equilibrium state, a prototype of a synthetic riverbed topography generating model is made for idealized meandering rivers. Such a method can be readily extended and applied to arbitrary meandering rivers in the real world, resulting in the synthetic riverbed topography model, pyRiverBed, presented herein. A meander migration and neck cutoff submodel is also embedded in pyRiverBed. Unlike existing linear and non-linear bend theory-based models, pyRiverbed aims towards generating the riverbed topography for each snapshot during the migration process. The present model can facilitate meandering river researchers to interpolate their field-measured bathymetric data using the synthetic bed, design their non-flat bed laboratory flumes for experiments, and initialize their hydrodynamic and morphodynamic numerical models. It can also provide guidance in stream restoration projects on designing a channel with a morphodynamic equilibrium bed. The quality of generated synthetic bed topography is evaluated through the comparison against both laboratory experiment data and field-measured data. The meander migration submodel is validated using a real-world channel migration and neck cutoff event. The validation results prove that the synthetic riverbed's accuracy is reasonably good, and the meander migration submodel can successfully predict meander migration and neck cutoff.

Neck circumference cut-offs for overweight and obesity in a group of Mexican adolescents.

Neck circumference (NC) has been used for screening overweight/obesity for its high correlation with age, weight, body mass index (BMI), and waist circumference (WC). Cut-offs should be population specific. The aim of the study was to obtain cut-offs of NC for overweight/obesity in Mexican adolescents and to correlate them with anthropometric indicators. Weight, height, BMI, NC, WC, and mid-upper arm circumference (MUAC) of 465 12-14-year-old Mexican-mestizo students attending a public school in Mexico City were assessed. Using receiver operating characteristics analyses, neck cut-offs were obtained according to the WHO age and sex criteria for overweight/obesity. NC cut-offs with the highest sensitivity and specificity for males and females, respectively, were as follows: 30.0 and 29.3 cm for 12-year-olds; 31.9 and 30.4 cm for 13-year-olds; and, 33.5 and 30.7 cm for 14-year-old adolescents. Overweight/obesity was identified with NC cut-offs in 80% of males and 86% of females, and showed significant correlations (p < 0.01) in males and females with weight (r = 0.821 and r = 0.840, respectively), BMI (r = 0.649 and r = 0.819, respectively), WC (r = 0.710 and r = 0.813, respectively) and MUAC (r = 0.736 and r = 0.815, respectively). Neck circumference may be used as a first-stage screening tool or jointly with BMI to identify overweight/obesity in 12-14-year-old Mexican adolescents.

Past, present and future of a meandering river in the Bolivian Amazon basin

AbstractField observations on small rivers of the Amazon basin are less common due to their remote location and difficult accessibility. Here we show, through remote sensing analysis and field works, the planform evolution and riverbed topography of a small river located in the upper foreland Amazon basin, the Ichilo River. By tracking planform changes over 30 years, we identified the factors that control meander migration rates in the Ichilo River: cutoffs, climate and human interventions. The data suggest that neck cutoffs are the main controls in the Ichilo River, with an annual density of 0.022 cutoffs/km. In addition, climate controls have been identified in the form of high‐precipitation events that may have promoted cutoffs, an increase in meander migration rate and channel widening. The width distribution of the Ichilo River is well represented by general extreme value and inverse Gaussian distributions. The spatiotemporal variability of meandering migration rates in the Ichilo River is analysed in two locations where neck cutoffs are expected. Analysing the distance across the neck in these two points, we predict the occurrence of a new cutoff. The combined methodology of bathymetric surveys and structure from motion photogrammetry shows us the Ichilo riverbed topography and banks at high resolution, where two scour holes were identified. Finally, we discuss the impact of planform changes of the Ichilo River on communities that are established along its riverbanks.

Numerical Modeling of Two‐Dimensional Hydrodynamics in a Highly Curving and Actively Evolving Neck Cutoff Under Different Hydrologic Conditions

AbstractNeck cutoffs are a common morphologic feature of meandering rivers though their occurrence on natural rivers is intermittent. Morphologic adjustments following cutoff initiation are typically rapid making detailed field investigations of the interactions between flow and form challenging. However, numerical models can offer insights into the hydrodynamic characteristics within the early stages of cutoff evolution. Field measurements of channel bathymetry and three‐dimensional velocities were collected during the first two years of morphologic change for a neck cutoff on the White River in Arkansas, USA. These data were used to calibrate a two‐dimensional numerical model to investigate the influence of hydrologic variability on the hydrodynamic characteristics of neck cutoffs during the early stages morphologic adjustment. The modeling results reveal complex patterns of the two‐dimensional flow field, including multiple regions of flow separation and recirculation related to strong redirection of the flow nearly 180 degrees through the cutoff. These regions of recirculation are also shown to vary depending on discharge conditions and affect hydrologic connectivity between the main channel and abandoned bend. Model results for the initial cutoff morphology show a substantial portion of discharge still conveyed by the original bend for all three discharge scenarios, resulting in a shear layer within the downstream limb of the site that limits advection of high‐momentum fluid against the bank opposite of the cutoff. The findings from this investigation are among the first detailed flow structure studies during the initiation of a neck cutoff and are used to improve conceptual models for cutoff dynamics.

Channel morphologic processes of a highly sinuous bend approaching neck cutoff by bank erosion in the middle Yangtze River

The distal reach of the Lower Jingjiang River (LJR) in the middle of the Yangtze River consists of five adjacent bends, among which the Qigongling Bend is a Ω-shaped meander with a mean sinuosity of 2.2 and the narrowest neck 525 m in width. This bend is slowly approaching neck cutoff owing to progressive bank erosion. An abnormal phenomenon has occurred in this bend since the Three Gorges Reservoir (TGR) began to operate in 2003 which is erosion in the inner bank zone and deposition in the outer bank zone. This problem has not been fully understood because of the interplay of changes in water-sediment, bank erosion, and artificial bank revetment. In this study, aerial and remote sensing images, hydrological data, channel topography, and an existing bank erosion model are used to reveal channel morphodynamics of this bend and the trend of the potential neck cutoff induced by bank erosion. The study results show that the clear water released from the TGR has provided by forcefully eroded the point bar of inner bank but failed to scour the outer bank due to the protection of bank revetment since the 1990s. Thus far, the outer bank zone near the bend apex has increasingly widened in conjunction with the formation of 2 emerging sand bars. Consequently, the thalweg of the main channel has laterally shifted toward the inner bank by roughly 800 m. More severely, the rate of bank retreat on the upstream side of the bend neck was about 4.5 m/yr in 2010–2019, but the downstream side of this neck was experienced slight deposition. Bank erosion could be accelerated by progressively increasing erosion and eventually trigger the occurrence of neck cutoff in the next few decades, thereby significantly altering the quasi-equilibrium regime of channel morphodynamics in the LJR.

Formation Mechanism and Sedimentary Pattern of Abandoned Channels

AbstractAccurately identifying and quantitatively describing abandoned channels in meandering rivers are of great significance for improving hydrocarbon recovery. By using modern deposition analogy, field outcrop analysis, a dense well spacing, core observations and a review of the literature, this paper studied the formation process and space–time amalgamation of abandoned channels in meandering river. The results reveal that formation mechanisms of abandoned channels include chute cutoff patterns (shoal‐cutting, ditch‐scouring and embayment‐eroding patterns) and neck cutoff patterns. The chute cutoff pattern forms a gradually abandoned channel, while the neck cutoff pattern forms a suddenly abandoned channel. From upstream to downstream, the sedimentary pattern of the abandoned channel transforms from a chute cutoff pattern to a neck cutoff pattern, where the main controlling factors transition from the grain size and gradient to the flow and vegetation. An abandoned channel formed by a chute cutoff pattern consists mainly of siltstone, fine sandstone and thin gravel layers, which form a lithological‐physical barrier. The abandoned channel formed by a neck cutoff pattern consists mainly of mudstone and argillaceous siltstone, forming a lithological barrier. Based on the amalgamation and structure of the reservoir architectural elements, the abandoned channel can be divided into three planar sedimentary patterns (crescent, semilune and horseshoe) for a single channel and five vertical sedimentary patterns for composite channels.

Integrated UAS and LiDAR reveals the importance of land cover and flood magnitude on the formation of incipient chute holes and chute cutoff development

AbstractMeandering river sinuosity increases until the channel erodes into itself (neck cutoff) or forms a new channel over the floodplain (chute cutoff) and sinuosity is reduced. Unlike neck cutoff, which can be measured or modelled without considering overbank processes, chute cutoff must be at least partially controlled by channel‐forming processes on the floodplain. Even though chute cutoff controls meandering river form, the processes that cause chute cutoff are not well understood. This study analyses the morphology of two incipient chute cutoffs along the East Fork White River, Indiana, USA, using high temporal and spatial resolution UAS‐based LiDAR and aerial photography. LiDAR and aerial imagery obtained between 1998 and 2019 reveals that large scour holes formed in the centre of both chutes sometime after chute channel initiation. A larger analysis within the study watershed reveals that scour holes within incipient chutes can be stable or unstable, and tend to stabilize when the chute is colonized by native vegetation and forest. When the scour holes form in farmed floodplain, they enlarge rapidly after initial formation and contribute to complete chute cutoff. In addition, this study shows that the formation of scour holes can occur in response to common, relatively low‐magnitude floods and that the amount of incipient chute erosion does not depend on peak flood magnitude. The role of scour holes in enlarging chute channels could be an important mechanism for chute channel evolution in meandering rivers. This study also confirms that understanding the relationships among flow, land cover, and cutoff morphology is substantially improved with on‐demand remote sensing techniques like integrated UAS and LiDAR. © 2020 John Wiley &amp; Sons, Ltd.

Morphologic adjustments of actively evolving highly curved neck cutoffs

ABSTRACTNeck cutoffs and their resultant oxbow lakes are important and prominent features of riverine landscapes. Detailed field‐based research focusing on the morphologic evolution of neck cutoffs is currently insufficient to fully characterize cutoff evolution. High‐resolution bathymetric data were collected over 3 years for the purpose of determining channel morphology and morphologic change on three actively evolving neck cutoffs. Results indicate the following general trends in morphologic adjustment: (1) a longitudinal bar in the upstream meander limb that develops near the entrance to the abandoned bend; (2) a deep scour hole in the downstream meander limb immediately downstream of the cutoff channel; (3) erosion of the bank opposite the cutoff in the downstream meander limb; (4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb; and (5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. The results presented herein were used to develop a conceptual model that depicts the morphologic evolution of highly curving neck cutoffs. The findings of this research are combined with recent analyses of the three‐dimensional flow structure through neck cutoffs to provide a mechanistic explanation for the morphodynamics of neck cutoffs. © 2019 John Wiley &amp; Sons, Ltd.