Channel Avulsion in Jiadhol River of Brahmaputra Basin

The emergence of the study of fluvial dynamics of the alluvial monsoon rivers especially in India is being enriched through the application of remote sensing and GIS techniques. To study fluvial dynamics over time, channel avulsion no doubt is one of the most considerable phenomena. The study section (*75 km) in between the two confluences of the Bhagirathi River i.e., Ajay (Katwa) and Jalangi River (Nabadwip) confluence in Lower Ganga Plain of West Bengal is highly oscillatory in nature. Several satellite images and toposheets of different time periods (1955, 1973, 1990 and 2011) have been used in order to analyse the avulsive nature in this section of the river. These fluvial features (cut offs, ox-bow lakes, point bars etc.) form as a result of channel avulsion and have been identified with the help of Landsat satellite images. The decadal differences in sinuosity ratio in different parts of the studied section, channel migration rate and avulsion model explain the dynamic nature and crucial portions of high bank erosion rate of the channel in the mentioned time period. For both the reaches (R1 and R2), meander loop cut offs are dominant rather than lateral avulsion in the context of aggregate channel migration that also reflects the instability and erosional vulnerability at different meander bends.

http://eprints.soton.ac.uk/id/eprint/347113, 2012

The Landsat programme, which started in 1972, initiated an era of space-based Earth observation relevant to the study of large river systems through the provision of spatially continuous, synoptic and temporally repetitive multi-spectral data. Free access to the Landsat archive from mid-2008 has enabled the scientific community to reconstruct the Earth’s changing surface and, in particular, to reconstruct the planform dynamics of the world’s largest rivers. The present research reconstructs the planform dynamics in the lower reaches of one of the Asian mega-rivers, the River Ganga-Padma (Ganges), from 1972-2010 using the Landsat archive. The research based on sequential river planform maps generated from the time series revealed a periodic pattern of evolution of the river system over the study period which began by means of meandering at four locations. The meander bends increased in sinuosity until chute cut-offs were triggered, returning the river to a state similar to that at the beginning of the sequence. This periodic pattern is constrained by natural and artificial hard points, and by the Farakka Barrage, meaning that the observed cyclic pulsing is likely to continue into the future. The characteristics and dynamics of meandering rivers have been the subject of extensive research, though the mechanisms involved are still not completely understood. Presently, availability of archival satellite sensor data at regular and frequent intervals for almost four decades presents a great potential for increasing our understanding of the natural processes of meander growth. Though early research indicates that meander growth can be explained by instability of alternate bars in a straight channel, but research based on field data and simulation models have shown that instability of river meanders is an inherent property and the meanders reach a critical value of sinuosity when cut-offs occur and then the complex system undergoes an self-adjusting process. The meander dynamics of the lower reaches of the Ganga-Padma system has been studied in the context of threshold response of a complex system. A conceptual model was developed based on spatial information from the sensor data and quantitative information on river metrics to explain the behaviour of the river system including evidence for self-organising criticality and the attempts of the river to reach dynamic equilibrium. The meandering channel pattern with a tendency of braiding of the river Ganga-Padma were explained based on existing empirical models as well as models based on mobility number and channel stabilization criterion. The threshold for chute cut-off was explored and subsequently the conditions for soft avulsion / branching were studied which showed that the condition for chute cut-off in the Ganga-Padma system is not due to bankfull flow velocity and the super elevation of flow at the centreline of channel but may be due to lack of vegetation stabilization on the Ganga-Padma floodplain. The effect of tectonics and meandering in the moderately paced avulsion of the Ganga-Bhagirathi system to the present Ganga-Padma system was modelled in the present research. It was found that gradient advantage and bend upstream of bifurcation does not result in modelled avulsion as observed in small and medium rivers and large rivers in tectonically inactive regions. A tectonic uplift results in a modelled avulsion period consistent with historical observations. It was found that backwater effect and high sediment mobility keep both bifurcated channels active to attain an anabranching pattern. The backwater effect was found to play an important role for sustaining the anabranch planform of many of the largest rivers of the world.

Geomorphology, 2014

Models for river avulsions have identified the ratio between down-valley and cross-valley slopes of channels as the triggering factors for the sudden channel shift but have remained untested in the field. The August 2008 avulsion of the Kosi River at Kusaha, 12 km upstream of the Kosi barrage in Nepal, provided an opportunity to study a large-scale avulsion (~120 km) for its causal factors and driving mechanisms. We used the SRTM-based digital elevation model and remotely sensed data coupled with field topographic mapping with a kinematic GPS and a Total Station to characterise a~50-km-long stretch of the Kosi River. We have computed reach-scale avulsion threshold index (ATI) integrating SRTM-derived slopes and planform dynamics on a GIS platform. We show that several reaches along the Kosi River are avulsion-prone, including the Kusaha point that is consistent with the August 2008 avulsion. We suggest that apart from cross-valley and down-valley slopes, planform dynamics such as thalweg shift, sinuosity variation, and channel multiplicity significantly influence the avulsion threshold in alluvial reaches of the rivers such as the Kosi.

Geomorphology, 2014

One-dimensional modeling of a recent Ganga avulsion: assessing the potential effect of tectonic subsidence on a large river, Geomorphology (2014),

Journal of the Indian Society of Remote Sensing, 2018

The Alaknanda River is the most significant parental river of Ganga and forms an 11.5 km long and 2.5 km wide valley, locally known as the Srinagar Valley. The purpose of the present study is to highlight the recent landform changes in the Alaknanda channel course after the Kedarnath disaster, 2013. The Kedarnath flood completely changed the channel morphology of the Alaknanda river. The river changed its course at Srikot, SSB and Sriyantra Tapu with lower terraces being silted by sands at Ranihat, SSB, Bhaktiyana and Sriyantra Tapu. A new depositional terrace also formed opposite to Sriyantra Tapu. New lateral channel bars, braided channels, back swamp, rapids, pools and river souls were identified in the channel course of the river. Shifting of the channel course at Chauras still remains a serious problem for the Garhwal University Chauras Campus. About 2-5 m silt was deposited on the lower terrace at SSB, and ITI. The Srikot river bed was appended to 4.60 m. Shifting of channel course remains a serious threat to the Srinagar valley. Urbanization, sand and boulders mining, construction of dam, hydrological canal, road and settlements are the prominent example of anthropogenic activities which affect the shifting channel.

Earth Systems and Environment, 2020

Rapid infrastructural development, triggered by the economic policies implemented in 1991, subsequently modified the river channel planform of the Terai region of northern West Bengal, India, through perceptible channel fragmentation, excessive in-channel sediment mining and rampant landuse alterations of the floodplains. In this context, this study has attempted to assess the impacts of anthropogenic interventions on fluvial regime of the lower reaches of Balason and Mahananda River of the sub-Himalayan region over the last 30 years by adopting a combined methodological framework of remote sensing and field survey. Results showed that initially the naturally vegetated areas had converted into crop land and grass land, tea plantation or bare land, and afterwards, the majority of it had been converted into built-ups. Besides, a noticeable amount of channel narrowing was directly proportionate to unrestricted sediment mining and embanking of the river channels, evidenced between 1987 and 2017. Comparison of measured multi-temporal channel width along with cross-profiles showed remarkable channel narrowing (18.8 m/year) as well as significant bed lowering (3.15 m). Studies reveal that the rivers have started showing signs of losing their existing equilibrium condition and if that happens then that will lead to the sinking of the ground water table, decreasing flood occurrence interval, destabilization of existing infrastructures associated with rivers, and destruction of river ecology. In order to restore the state of socio-hydrological as well as eco-hydrological amenities of these rivers, certain recommendations have also been made towards promoting the wise use of riverine resources by the local communities and policy makers.

2012

Humans have utilized water resources for millennia by modifying natural river courses and such interventions have greatly influenced not only river flows and sediment fluxes, but also the overall river morphology. Situated in the Nepal's eastern Ganges region, the braided Koshi River is unique among the other rivers, because of the high frequency of channel avulsion and other morphological changes, such as: channel migration, channel width adjustment. This study examines effect of the Koshi barrage and related embankments on flooding and channel avulsion in Koshi River. In particular, it tries to explain the avulsion that occurred in 2008, studying the role of the Koshi barrage, and related embankments, constructed near the border between Nepal and India in 1963. Series of satellite images and historical maps show overall sedimentation, especially in the western side of the river channel, which lead to the shifting of the Koshi River towards the east (almost 6 km) during around 40 years period, since the construction of the barrage, although before, shifting toward the west. The barrage was constructed at eastern side of the river channel within two embankments, leading to sedimentation in western side, since river is flowing short courses. Sediment deposition upstream of the barrage brought to the conditions that lead to dike breaching in 2008. During the 2008 flooding event, huge amounts of previously deposited sediment were eroded from the river bed upstream of the barrage. The resulting bed lowering means a gain of time to prevent a similar event in the future, since new space for sediment inside the embankments system has been created. So, this time, estimated in 40-50 years, can be used to take proper river engineering measures.

Geomorphology, 1999

The sequential changes in the position of banklines as a result of bank erosion, as well as various changes in the channel have been studied in the Subansiri river in Assam, India. The study was carried out from available topographic maps of 1920, 1970 and satellite imagery of 1990. The types of changes taking place in the Subansiri river channel as observed from Ž. the above-mentioned maps and imagery are grouped into four categories: 1 alteration of the direction of flow due to neck Ž. Ž. Ž. cutoff , 2 widening of a channel in response to bar development, 3 development and subsequent abandonment of Ž. anabranches, and 4 progressive shifting of meander bends. The river channel has become substantially wider during the period of 1970-1990. There has also been a progressive increase in the number and area of mid-channel bars with a corresponding decrease in the number and area of point bars during 1920-1990. These periodic and progressive changes are responsible for a change in channel pattern of the river. A study made on nature and amount of change in the banklines of Ž. the Subansiri river by dividing the channel 84 km in 1970 into 10 equal segments reveals that the types of change are varied, and in some cases chaotic in nature. Study of the bankline shift of the Subansiri determined separately for the periods 1920-1970, 1970-1990 and 1920-1990 at 20 evenly spaced transverse sections reveals that there has been a substantial westward shift of both the banklines, up to a maximum of 6 km, in the three upstream sections during 1920-1970. However, shifting of banklines in both directions and the unequal amount of shifting in the same direction have caused a marked increase in channel width during 1970-1990. It is found that due to bankline migration, the total amount of area eroded along the entire course under study during 1920-1970 and 1970-1990 are 107.90 and 57.50 km 2 , respectively. The overall sinuosity which was 1.51 in 1920 has decreased to 1.30 in 1970, while the corresponding braiding intensity has increased from 0.79 to 1.32. The river channel seems to make a remarkable change from a meandering pattern in 1920 towards a braided pattern by 1990 as seen from gradual increase in overall braiding intensity.

Geology, 2013

The August 2008 avulsion of the Kosi River, northern India, resulted in a maximum eastward shift of >100 km and created an avulsion belt of 2722 km 2 . Based on A.D. 2000 Shuttle Radar Topography Mission data and on 2005 Landsat Thematic Mapper satellite imagederived channel network (pre-avulsion), we use a topography-driven connectivity model to simulate the avulsion pathway, which corresponds, to a large extent, to that observed in the post-avulsion period. We then use this model to postulate the avulsive course of the river from another upstream point based on avulsion threshold analysis.

Fluvial Sedimentology VII, 2005

The Baghmati River, a foothills-fed system in the Himalayan foreland basin of north Bihar, has an anabranching mid-stream reach and floodplains that aggraded rapidly during the late Holocene. The river is characterized by variable discharge, frequent and widespread overbank flooding, and high sediment load. Changes in river course on a decadal time-scale have resulted in temporarily abandoned reaches that are periodically reoccupied. Chute and neck cutoffs, and crevasse splays are also prominent. Borehole logs show that the anabranching reach is underlain by sandy channel bodies up to 25 m thick, separated by mudstone units up to 30 m thick. Extrapolation of floodplain accumulation rates to the mudstones suggests that channels were stably positioned for thousands to tens of thousands of years, allowing thick muds to accumulate. Repeated reoccupation of pre-existing drainage lines may have promoted the creation of thick, narrow channel bodies. Stacked overbank deposits probably form the bulk of the floodplain sediments, but channels that avulse into floodplain lakes (tals) may generate associated avulsion deposits. The Baghmati River sediments are a modern analogue for the deposits of rapidly subsiding extensional and foreland basins in the ancient record.