Abstract
The Koshi Barrage was constructed in the Nepalese territory as per the Kosi Agreement signed between Nepal and India in 1954 and amended in 1966. Two irrigation systems, viz. the Koshi ...Distributary System and the Koshi Pump Lift Irrigation System, starting off from the Koshi Western Main Canal in India, are irrigating 11,300 and 13,180 ha of land in the Saptari District of Nepal, respectively. The average annual amount of water available in the Koshi Pump System and Koshi Distributary System is found to be 60.28 and 136.97 million cubic meters (MCM), respectively. The existing cropping intensity of these two systems is 170 and 190%, respectively. The sustainability of these irrigation systems was assessed using the Multi Criteria Analysis (MCA). The Koshi Pump Irrigation System is found to be a Sustained but At-Risk Project, whereas the Koshi Distributary Irrigation System is found to be a Not Sustained Project. Furthermore, the study concluded that these irrigation systems have low crop productivity and the conditions of the existing infrastructures are poor.
Floodplain wetlands are important features in alluvial settings across the world because of their ecological and hydrological significance. Understanding the geomorphic evolution of such wetlands is ...therefore not only interesting from an academic viewpoint but also necessary for their conservation and protection. Using a hierarchical system of geomorphological ‘elements’ and ‘units’, detailed geomorphic investigations have been carried out in the Kosi-Ganga interfluve, Eastern Ganga Plain (EGP) in the Himalayan foreland to understand the distribution and diversity of the floodplain wetlands in this region. The major geomorphic elements in this region consisting of lentic (wetlands, lakes) and lotic (streams) water bodies were mapped using multispectral satellite images. The distribution of the wetlands analysed through chord diagrams suggests that these are highly localised and associated with the regional geomorphic setting and the fluvial dynamicity. The morphometric characterisation of these wetlands using their count, % area, average area, and total area shows significant variability among the geomorphic units. The equal-area circle (EAC) such as the shoreline development index (DL) was found to be inadequate to assess the geomorphic maturity of the lentic water bodies and hence a new maturity index MCH, has been designed in this work using the convexity and area of the water bodies. Further, the space-for-time substitution approach has been used to develop the evolutionary pathway for the wetlands and the wetland-complexes in this interfluve over time scale of 10–102 years. We suggest that the evolution of the wetlands in a dynamic fluvial system such as the interfluve region of the EGP is governed by two different processes. Firstly, fluvial scouring and channel abandonment provide negative relief at a regional scale, and secondly, erosional processes (cut and fill) acting on the local scale transform the narrow and deep fluvial scours into large and shallow wetland systems.
•Evolutionary pathway of large floodplain-wetland complex in a dynamic alluvial setting•Fluvial scouring and channel abandonment initiate the process.•Cut and fill processes transform them into large wetland systems.•Convex hull-based index proposed to ascertain the maturity level of wetlands.•Implications for developing long terms strategies for wetland management
The Kosi and the Gandak are two major Himalayan tributaries of the Ganga River in the north Bihar plains India. With a large hinterland in the Nepal Himalaya, both these rivers have generated ...megafans in the plains over the Quaternary time scale. Both these rivers are known to be highly dynamic and sediment-charged. A few conceptual models and limited field data suggested that these megafans have produced thick sand sheets over Late Quaternary period but these ideas have remained speculative and there is no data on the size and dimension of these sand bodies. This paper attempts to reconstruct the subsurface stratigraphy and alluvial architecture for the upper ~100m of the megafans based on electrical resistivity soundings, borehole data and drill cores. Alluvial architecture of the Kosi megafan shows significant variability from proximal to medial parts of the fan in terms of sediment grain size and layer thicknesses. While the medial part shows ~20–30m thick medium to coarse sand sheets which are laterally stacked, the proximal part of the fan has a dominantly gravel unit below ~15m depth that is underlain and overlain by medium to coarse sand units. Further, the medial fan also shows significant vertical and lateral variability in alluvial stratigraphy. The near-surface (<20m depth) deposits from the Kosi megafan have pockets of clay and silt within large amalgamated sand bodies whereas the shallow sub-surface (50–100m depth) sediments are largely sandy and devoid of clay and silt pockets. Alluvial architecture of the Gandak megafan shows two major lithounits; the upper fan succession has a higher stacking density of smaller sand bodies perhaps reflecting the migratory behavior of the river whereas the lower succession shows narrow but thick sand fills reflecting incised channels. The western part of the Gandak megafan has more abundant sand bodies compared to the eastern side of the river along both transects. There are no significant differences between proximal and medial transects across the Gandak megafan. The absence of gravel deposits in the shallow subsurface of the Gandak megafan may be attributed to the presence of a prominent intermontane valley in the hinterland of the Gandak river which has acted as a ‘sediment filter’ thereby trapping most of the coarser fraction. On the other hand, the Kosi river exits directly through the mountain front, and therefore, has been able to transport gravels into the plains in pre-historic times. Our study thus suggests significant variability in subsurface stratigraphy of the Kosi and the Gandak megafans even though they are located in similar geographic region. Such differences are attributed to the geomorphic diversity of the mountain exits of these megafans and their sediment transport history.
•Documents alluvial stratigraphy of one of the largest active megafans in the world•Integrated approach of borehole logs, resistivity surveys and drill cores•Significant proximal to medial variability — a function of mountain front setting•Alluvial stratigraphy suggests temporal variability in hydrological and sediment flux.•Validates the available stratigraphic models from ancient records
The overall precipitation in the state of Bihar, India is showing a decreasing trend both annually and seasonally, and yet extreme flood events are on the rise. The Kosi river embankments built to ...safeguard communities against flood risk are a product of socio-political and historical events in the past, but have resulted in differential impacts on those living inside and outside these embankments. The geomorphology of the river Kosi also makes it highly susceptible to recurring floods because it forms one of the largest inland deltas in North Bihar. Flood protection structures such as embankments exacerbate the magnitude of floods by jacketing the heavy sediment load and thus raising the riverbeds and exacerbating the intensity and duration of floods. Our paper employs an interdisciplinary approach to analysing both the biophysical and socio-institutional causalities of increasing flood events. From the quantitative analysis of rainfall data, we find that the daily, as well as monthly rainfall alone are not responsible for extreme flood events. The extreme rainfall events in the summer monsoon also do not increase the odds of flooding. Therefore, we conclude that precipitation alone is not the main factor affecting community's vulnerabilities but, a combination of socio-institutional factors including spatial location with respect to the embankment, class and caste of these communities. Our statistical analysis correlating daily and monthly gridded rainfall to the occurrences of flooding at the district level suggest that there are fewer flood events in the presence of the embankments across all years. However, primary data from household interviews and field observations confirm that the frequency and intensity of floods have increased in the post-embankment period. We found that the breaching of the river embankments is one of the major factors responsible for floods outside of the embankments. Kosi's marginalized communities perceive that they have become more vulnerable to flood risk in the post-embankment period with a declining standard of living in the Kosi villages caused by lack of proper roads, economic opportunities, educational institutions, public utilities and healthcare facilities, especially in areas with embankments.
Movement of biophysical fluxes and resultant processes are governed by among other things, the (dis)connectivity structure of a landform. Hence, the quantification of connectivity structure of a ...landform is important in order to analyze water and sediment fluxes over a surface. Two dimensional connectivity structure through analysis of lateral and longitudinal connectivity for water and sediment flux has been quantitatively defined for the well-known Kosi megafan in north India. The avulsive behavior of the Kosi River has resulted in various paleochannels over the megafan, and they guide the flux transfer over the surface and also control the local topography of the megafan. As (dis)connectivity structure of landform is governed by its physical characteristics and also affected by anthropogenic disturbances, both these factors have been considered to quantitatively analyze the connectivity structure of the Kosi megafan for sediment and water fluxes. Megafan surface characteristics have been defined through local slope variability, land use-land cover map and flow length. These surface parameters have been used to map ‘buffers’ in the area. The distribution pattern of ‘buffers’ on the megafan surface has been used to define the ‘natural’ (dis)connectivity structure. Further, the megafan surface has also been affected by progressive development of the rail-road transport network, which is mostly east-west aligned and intersects the south flowing paleochannels. These rail-road network acts as an anthropogenic ‘barrier’ for water and sediment fluxes across the megafan surface. A detailed mapping of rail and road network in different years (1955, 1983 and 2010) has been used to characterize anthropogenic disturbance on the connectivity structure. The spatio-temporal variation in connectivity structure is attributed to density of the transport network. Finally, natural and anthropogenic disturbances on connectivity structure have also been integrated to quantitatively define the present day connectivity structure of the Kosi megafan for water as well as for sediment flux. Further, the (dis)connectivity structure has been used to explain the spatial variability of waterlogging over the megafan surface, which is presently a serious hazard in the region.
•Anthropogenic impacts over the Kosi megafan analysed in a GIS framework.•2-D connectivity structure defined by longitudinal and lateral connectivity.•Rail/road network significantly impacted the connectivity structure&sediment flux.•Disconnectivity over the megafan resulted indrainage congestion and waterlogging.
This study examines both the inlet dynamics, recorded in the back-barrier stratigraphy of Kosi Bay, and the competing effects of waterbody segmentation on along-strike changes in the character and ...nature of an incised valley fill at Kosi Bay, South Africa. The Kosi Bay system comprises four interconnected lakes (Lake Makhawulani, Lake Mpungwini, Lake Nhlange and Lake Amanzimnyama) subject to varying degrees of marine influence. Formerly, the system's only connection to the ocean was through the Bhanga Nek palaeo-inlet. The location of this system behind a continuous coastal barrier predisposes it to excellent archives of environmental change related to changing sea-level and sediment supply. In this context, two cores were extracted from the system (KB2 and KB4), together with over 150 km of seismic reflection profiling, as well as AMS radiocarbon analyses and legacy data in order to present a new stratigraphic evolution model for this back-barrier system. A total of 5 seismic units were imaged (Units A-E). A major unconformity surface (SB), characterised by broad u-shaped incised valleys, incises the underlying unconsolidated sandy acoustic basement (Unit A). Unit B forms as thickly developed central basin deposits, partially infilling the incised valleys. Two forms of prograding spits are evident -Unit C1 progrades from the palaeo-highs of the valley interfluves into the nearest available accommodation space and Unit C2 progrades from the margins of the system into the basin. These are separated by an erosional surface (T-RS) formed by migrating tidal channels as sea-level rose and marine waters entered the system during the Holocene. Unit C2 spits are in turn associated with slump deposits formed on the steepest part of the margins (Unit D) that were dated to 2900 ± 165 cal. BP. The unusual depth of Surface T-RS is attributed to (1) limited sediment supply during sea-level rise, (2) the presence of easily erodible fine sediments in which this surface erodes and (3) a larger tidal prism when the Bhanga Nek inlet was still open and system segmentation by Units C2 and D had not yet occurred. Unit E caps the stratigraphy and is interpreted as lacustrine fines with a thin layer of acoustically transparent material (gyttja) on top. The deposition of Unit E signifies the closure of the system to the ocean and the overall shift from an estuarine/lagoon environment to a lacustrine one. Recalibrated AMS dates place the system closure to between 3160 and 2900 cal. BP, coeval with the formation of the prograding marginal spits and waterbody segmentation (Unit C2). Along-strike variation in the timing of the basin infilling, attributed to changes to the inlet functioning coupled to the segmentation of the waterbody, is characteristic of this area. At 1450 cal. BP, a new tidal inlet formed to the north of Lake Makhawulani, where impounded waters were diverted via a low point in the back-barrier coastal dune cordon. The formation of this inlet suggests a prolonged period of back-barrier flooding until a low point in the barrier was breached during the Holocene sea-level highstand of 1.5 m. Such inlet development and along-strike back flooding of incised valleys is an unusual attribute of coastal waterbodies of South Africa.
•LGM-age incision into unconsolidated Kosi Bay Formation sediments.•Limited sediment supplied to the area during transgressing sea levels.•Post-LGM homogenous central basin fills only partially fill incised valleys.•Development of wind-driven spits signifying early stages of lagoonal segmentation.•Along-strike variation in the timing of the basin infilling.
The neodymium and hafnium isotopic compositions of most crustal and mantle rocks correlate to form the “Terrestrial Array”. However, it is now well established that whereas coarse detrital sediments ...follow this trend, fine-grained oceanic sediments have high Hf ratios relative to their Nd isotopic ratios. It remains uncertain whether this “decoupling” of the two isotopic systems only occurs in the oceanic environment or if it is induced by sedimentary processes in continental settings. In this study, the hafnium and neodymium isotopic compositions of sediments in large rivers is expressly used to constrain the behavior of the two isotopic systems during erosion and sediment transport from continent to ocean.
We report major and trace element concentrations together with Nd and Hf isotopic compositions of bedloads, suspended loads and river banks from the Ganges River and its tributaries draining the Himalayan Range i.e. the Karnali, the Narayani, the Kosi and the Marsyandi Rivers. The sample set includes sediments sampled within the Himalayan Range in Nepal, at the Himalayan mountain front, and also downstream on the floodplain and at the outflow of the Ganges in Bangladesh. Results show that hydrodynamic sorting of minerals explains the entire Hf isotopic range, i.e. more than 10εHf units, observed in the river sediments but does not affect the Nd isotopic composition. Bedloads and bank sediments have systematically lower εHf values than suspended loads sampled at the same location. Coarse-grained sediments lie below or on the Terrestrial Array in an εHf vs. εNd diagram. In contrast, fine-grained sediments, including most of the suspended loads, deviate from the Terrestrial Array toward higher εHf relative to their εNd, as is the case for oceanic terrigenous clays. The observed Nd–Hf decoupling is explained by mineralogical sorting processes that enrich bottom sediments in coarse and dense minerals, including unradiogenic zircons, while surface sediments are enriched in fine material with radiogenic Hf signatures. The data also show that Nd–Hf isotopic decoupling increases with sediment transport in the floodplain to reach its maximum at the river mouth. This implies that the Nd–Hf isotopic decoupling observed in worldwide oceanic clays and river sediments is likely to have the same origin. Finally, we estimated the Nd–Hf isotopic composition of the present-day mantle if oceanic sediments had never been subducted and conclude that the addition of oceanic sediments with their anomalous Nd–Hf isotopic compositions has slowly shifted the composition of the Earth’s mantle towards more radiogenic Hf values through time.
The Ganga River is one of the largest river systems in the world that has built extensive alluvial plains in northern India. The stretch of the Lower Ganga River is vulnerable to siltation because ...of: (a) the naturally low slope in the alluvial stretch; (b) the confluence of several highly sediment‐charged rivers such as the Ghaghra, Gandak, and Kosi; and (c) the reduction in non‐monsoon flows because of upstream ions of both surface and groundwater. Additionally, the Farakka barrage has impacted the morphology of the Ganga River significantly, both upstream and downstream of the barrage. Large‐scale siltation in several reaches has reduced the channel capacity, leading to catastrophic floods in this region even at low discharges. This work has utilized historical remote sensing data and UAV surveys to reconstruct channel morphodynamics and compute sediment volumes accumulated in the channel belt along the Lower Ganga River between Buxar and Farakka. The work was carried out by dividing the total length of the river into four continuous stretches: (a) Buxar–Gandhighat (GW1, 160 km); (b) Gandhighat–Hathidah (GW2, 106 km); (c) Hathidah–Azmabad (GW3, 182 km); and (d) Azmabad–Farakka (GW4, 132 km). We document that major ‘hotspots’ of siltation have developed in several reaches of the Lower Ganga during the last four to five decades. Sediment budgeting using planform maps provides estimates of ‘extractable’ volumes of sediment in GW1, GW2, GW3, and GW4 as 656 ± 48, 706 ± 52, 876 ± 71, and 200 ± 85 Mm3, respectively. These estimates are considerably lower than those computed from the hydrological approach using observed suspended sediment load data, which assumes uniform sedimentation between two stations. Further, our approach provides reach‐scale hotspots of aggradation and estimates of extractable sediment volumes, and this can be very useful for river managers to develop a strategic sediment management plan for the given stretch of the Ganga River.
This work has utilized historical remote sensing images, UAV surveys, and hydrological data sets to reconstruct channel morphodynamics and to develop a first‐order sediment budget for the Lower Ganga River. Major ‘hotspots’ of siltation in several reaches of the Lower Ganga have developed in the last 4‐5 decades. Total volume of ‘extractable’ sediments in this stretch computed from planform maps is 2437 ± 256 Mm3. This work has strong implications for developing strategic sediment management plan for the Ganga River.
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•Morphometric analysis integrated with sediment connectivity and stream power index.•Variable sediment flux in upper Ganga and upper Kosi basins linked to morphometric ...characteristics.•Variable hillslope-channel and catchment connectivity contributes to sediment flux variations.•Slope variations manifested in stream power index also influence sediment transport.•Morphometric characterization has significant implications for sediment management.
The Indo-Gangetic basin exhibits highly diverse hydro-geomorphic settings that influence the hydrology, sediment production, and transport rates of the rivers draining this region, and this, in turn, is manifested in morphometric diversity of river systems. The rivers draining the western Gangetic plains (WGP) show incised channels attributable to high stream power and low sediment yield. In contrast, the rivers draining the Eastern Gangetic Plains (EGP) have formed aggradational landforms such as megafans due to low stream power and high sediment yield. For a better process understanding of the causal factors of such spatial inhomogeneities in a river basin, a morphometric characterization of its hinterland is necessary. In this work, a set of morphometric parameters such as stream network, longitudinal profile, and hypsometric curve have been integrated with data on sediment connectivity and stream power index to establish the form-process relationships in two contrasting basins, the upper Ganga basin (UGB) and the upper Kosi basin (UKB). The results of this study suggest that high sediment flux in the UKB is attributed to high rainfall, steep topography, unstable channel longitudinal profiles, and high sediment connectivity. A large part of these sediments gets deposited within the channel belt in the alluvial reaches and is remobilized during high flow events. However, the UGB produces fewer sediments due to low rainfall, mild topography, stable channel longitudinal profile, and low sediment connectivity. Also, the presence of a major dam at Tehri further reduces the functional sediment connectivity in the Upper Ganga Basin.
