Braiding process and bank erosion in the Brahmaputra River
TL;DR: In this paper, an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure.
Abstract: The present work explores relations between stream power, braiding intensities and bank erosion in certain stretches of the Brahmaputra River. In this paper, an objective approach is presented to enable quantitative assessment of spatio-temporal behaviour of channel braiding process of the Brahmaputra River by using the Plan Form Index and corresponding estimation of stream power to establish a behavioural pattern of variability of potential energy expenditure. The braiding index is compared for discrete years to understand the morphological behaviour. Subsequently, a real time estimation of stream power for certain stretches of Brahmaputra River is done in order to analyse its variability in braiding intensity and bank erosion. The paper presents the dynamic behaviour of the channel pattern of the Brahmaputra River System in Assam valley of India over a time span of 18 years. The procedure addresses the selection of input parameters from digital satellite images, comprising scenes for the years 1990, 1997 and 2007 with specific dates, from Dhubri near Indo-Bangladesh Border to Upper Assam. Deployment of GIS technique has been made to extract the required parameters to derive Plan Form Indices for the entire study reach. Stream power estimation is done for corresponding latest floods and for corresponding dates of image scenes. The study indicated that due to consistent aggradation of riverbed inducing temporal declination of stream power, there is an occurrence of wide spread braiding. This in turn incurs substantial yearly land loss due to bank erosion, caused by flow concentrations due to temporal evolution of multiple channels in the Brahmaputra River.
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TL;DR: In this paper, the relationship between bar length and width can be predicted by a simple best-fit power function relating to self-similarity characteristics, and the results indicate that a small increase in channel width likely increased rapidly the shift cycle.
Abstract: This paper presents flume and field observations of a bank-confined braided river. Morphological features, including plan form configuration, channel width, and main channel migration, were examined by a series of experiments. Repeated measurements of channel morphology, provided a basis to estimate the relationship between noncumulative frequency of bars and bar area. Additional results from the Dajia River, located in Central Western Taiwan, were presented to provide a reference data set for comparing the laboratory and field data. The results indicate that the relationship between bar length and width can be predicted by a simple best-fit power function relating to self-similarity characteristics. The Hurst index by Walsh and Hicks (2002) provides acceptable predictions of the bar length and width observed in the experiments and confirmed by the field investigations. Eexperimental and field results both show that large river width yields a uniform distribution of bar areas with the similar discharge, leading to a large value of exponent (β) in the model. The river width is confirmed to be a critical parameter in the main channel shift. A small increase in channel width likely increased rapidly the shift cycle.
28 citations
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TL;DR: In this paper, the authors simulated different types of disturbances in a large braided river to get insight into the propagation of disturbances through a braided channel network, and found that the disturbances initiate an instability that propagates in the downstream direction by means of alteration of water and sediment division at bifurcations.
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.
22 citations
Cites background from "Braiding process and bank erosion i..."
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TL;DR: In this paper, a 2D depth-averaged numerical model was used for a 12 km-long reach of the Brahmaputra River in India to assess morphological changes in different groyne fields.
Abstract: Prediction 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.
18 citations
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TL;DR: The recent trends of channel changes suggest that the river planform has lost it’s naturally condition and it may, therefore, be predicted that increasing nature of channel width likely to continue in the immediate future.
Abstract: This paper deals with the morphological changes of the lower Brahmaputra-Jamuna River (BJR) in Bangladesh Within few decades, the planform of the river has been changed abruptly by the combined effect of natural process and human interventions Morphological features observing in this study were river planform, channel width, bankline migration and channel bed elevation Eighteen sets of remote sensing data series from 1973 to 2011were analyzed using ERDAS/Imagine and GIS to document the variation of geomorphic elements of the lower BJR GIS analysis of remote sensing data showed that the changes of channel planform were quite significant over the past 40 years, occurring two major phases of channel development The changing patterns in the first phase (ie, between 1973 and 1992) were quite irregular However, the second phase ranging between 1992 and 2011 was unidirectional (mostly eastward) In general, the studied river reach was widened and the average rate of migration was 225 m y–1 that was three–folds the values of the first phase The height and slope of sand bars were gradually increasing, showing the highest value around the Jamuna Multipurpose Bridge (JMB) section The planform characteristics of BJR at the downstream of JMB showed that the river reach was gradually widening and shifting eastwards However, the reach at the upstream of the JMB showed westward migration The recent trends of channel changes suggest that the river planform has lost it’s naturally condition and it may, therefore, be predicted that increasing nature of channel width likely to continue in the immediate future
16 citations
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TL;DR: Adding the process of shoal margin collapses increases the channel‐shoal interactions and that in intensively dredged estuaries shoal margins oversteepen, amplifying the number of collapses, but because of dredging the natural morphological response is interrupted.
Abstract: Shoal margin collapses of several Mm3 have occurred in the Western Scheldt estuary, the Netherlands, on average five times a year over the last decades. While these collapses involve significant volumes of material, their effect on the channel‐shoal morphology is unknown. We hypothesize that collapses dynamicise the channel‐shoal interactions, which could impact the ecological functioning, flood safety and navigation in the estuary. The objective is to investigate how locations, probability, type, and volume of shoal margin collapse affect the channel‐shoal dynamics. We implemented an empirically‐validated parameterization for shoal margin collapses and tested its effect on simulated estuary morphological development in a Delft3D schematization of the Western Scheldt. Three sets of scenarios were analyzed for near‐field and far‐field effects on flow pattern and channel‐shoal morphology: 1) an observed shoal margin collapse of 2014, 2) initial large collapses on 10 locations, and 3) continuous collapses predicted by our novel probabilistic model over a time span of decades. Results show that a single shoal margin collapse only affects the local dynamics in the longitudinal flow direction and dampen out within a year for typical volumes, whereas larger disturbances that reach the seaward or landward sill at tidal channel junctions grow. The direction of the strongest tidally‐averaged flow determined the redistribution of the collapsed sediment. We conclude that adding the process of shoal margin collapses increases the channel‐shoal interactions and that in intensively dredged estuaries shoal margins oversteepen, amplifying the number of collapses, but because of dredging the natural morphological response is interrupted.
13 citations
References
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TL;DR: In this paper, a series of experiments were performed in a large flume to determine the effect of slope and sediment load on channel patterns, and the results indicated that landforms may not always respond progressively to altered conditions and dramatic morphologic changes can occur abruptly when critical erosional and (or) depositional threshold values are exceeded.
Abstract: A series of experiments was performed in a large flume to determine the effect of slope and sediment load on channel patterns. Sediment loads and slopes were closely related, and as slope and sediment loads increased, threshold values of these variables were encountered, at which channel patterns altered significantly. At a very low slope and sediment load, the channels remained straight, but at a discharge of 0.15 cfs, a meandering-thalweg channel formed at slopes greater than 0.002. With increased slope and sediment loads, thalweg sinuosity increased to a maximum of 1.25. At slopes greater than 0.016, a braided channel formed. The model channels responded to increased sediment loads by maintaining steeper gradients and by major channel pattern changes, but at very gentle slopes and at steep slopes, the channel could not be forced to develop a meandering thalweg.
These experiments suggest that landforms may not always respond progressively to altered conditions. Rather, dramatic morphologic changes can occur abruptly when critical erosional and (or) depositional threshold values are exceeded.
The meandering-thalweg channel was not a meandering channel. A truly meandering channel with a sinuosity of 1.3 formed when a suspended-sediment load (3 percent concentrations of kaolinite) was introduced into the flow. The clay stabilized the alternate bars, and scour and deepening of the thalweg resulted. This in turn lowered the water level at constant discharge, and the alternate bars emerged o t form point bars. A meandering-thalweg channel was thus converted to a meandering channel by the type of sediment load change that has accompanied climatic and hydrologic changes of the recent geologic past.
724 citations
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TL;DR: In this paper, a stability analysis of meandering and braiding perturbations in a model alluvial river is described, and a perturbation technique involving a small parameter representing the ratio of sediment transport to water transport is used to obtain the following results.
Abstract: A stability analysis of meandering and braiding perturbations in a model alluvial river is described. A perturbation technique, involving a small parameter representing the ratio of sediment transport to water transport, is used to obtain the following results.Under appropriate conditions, the existence of sediment transport and friction are necessary conditions for the occurrence of instability in the flow and on the bed; thus instability is not inherent in the flow alone. An Anderson-type scale relation for longitudinal instability is obtained for meandering. A relation estimating the number of braids and differentiating between meandering and braided regimes is derived. These relations are independent of sediment transport.
454 citations
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TL;DR: In this article, the authors derived and applied new equations to consideration of longitudinal profiles of stable rivers and canals; nature of relationships were determined both for fixed and live bed conditions.
Abstract: Summary of aspects of stability with reference to E.W.Lane's tractive force theory, Strickler's formula, and G.Lacey's regime equations; new equations are derived and applied to consideration of longitudinal profiles of stable rivers and canals; nature of relationships is determined both for fixed and live bed conditions.
143 citations
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TL;DR: In this paper, a computer program is used to generate different combinations of velocity, slope, depth, and unit stream power for a given sediment load, which is then used to determine the minimum amount of power required to transport a given load.
Abstract: Fluvial hydraulics is complex because the velocity, slope, depth, and channel roughness are all subject to change. The unit stream power, defined as the rate of potential energy expenditure per unit weight of water, required to transport a given sediment load is sensitive to water depth when the sediment concentration is low. The interdependence between unit stream power and water depth decreases as sediment concentration increases. A computer program is used to generate different combinations of velocity, slope, depth, and unit stream power for a given sediment load. The generated unit stream power in the lower flow regime either has a clear minimum value at a particular depth or approaches a minimum value for a given sediment load. These agreements suggest that an alluvial channel in the lower flow regime adjusts its velocity, slope, depth, and roughness in such a manner that a given sediment load can be transported with the minimum amount of unit stream power.
132 citations
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