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•Sediment dynamics of a large tropical river in the Ganga basin.•Annual deposition of ∼20–50 MT of sediments in different reaches.•High sediment flux linked to channel instability and ...frequent flooding.•Human interventions have affected sediment dynamics significantly.•Sediment dynamics is the most important river management issue in the Kosi basin.
Hydrological and sediment transport characteristics for the Kosi basin, which covers parts of Nepal and India, were analysed to understand the spatiotemporal variability of the hydrology and sediment dynamics of the Kosi basin and its implications for flood hazard and sediment dynamics. The study revealed that ∼56% of the discharge at Chatara (where all major tributaries of the Kosi meet) is contributed from the western part of the basin even though this constitutes only 34% of the total basin area. In contrast, the central and eastern parts of the basin constitute 57% and 8% of the basin area but contribute ∼38% and ∼16% of the discharge at Chatara, respectively. The contribution of sediment load at Chatara from the different tributaries of the Kosi River also shows a similar pattern. Of a total of ∼100 million tonnes of the annual sediment load at Chatara, ∼56% is transported from four tributaries: the Indrawati, Bhote Kosi, Tama Kosi (all draining from the west), and Tamor. The remaining ∼44% is transported by other tributaries upstream of Chatara, the most important being the Arun, Dudh Kosi, and Sun Kosi. Sediment budgeting in this study, based on annual sediment load data, suggested that ∼20 million tonnes of sediments are deposited between Chatara and Birpur annually. This study also found that ∼53 million tonnes of sediments are being accommodated between Birpur and Baltara annually. Sediment dynamics in the Kosi basin emerges as the most important river management issue, and this is closely linked to channel instability and frequent flooding in the alluvial plains.
It has been proposed that the Kosi River continuously migrated >113 km westward across the surface of the megafan over the last two centuries. Examination of a number of old maps published between ...1760 and 1960 shows that during most of this period the Kosi River occupied a position slightly east of the megafan axis. The apparent channel movement shown in these maps is oscillating in nature and not unidirectional. Instead of encountering deposits left behind by a sweeping braided Kosi-like stream, a preliminary study of the uppermost 2–5 m succession in the north-central part of the megafan reveals overwhelming dominance of meandering stream deposits. Assuming the existing notion of Kosi River migration, the rate of deposition averaged over ∼100–200 years for the uppermost ∼5–10 m of megafan deposits, works out to be unusually high (>50 mm/y). All these observations question the soundness of the hypothesis of rapid westward migration of the Kosi River over the last two centuries. The existing facies model for the uppermost 8–10 m of the megafan deposits also appears untenable.
The three-dimensional geometry of the Kosi megafan is similar to those of typical alluvial fans, but with much gentler gradient (0.05°–0.01°) and with larger area (>10,000 km
2). Based mainly on the patterns of paleo and modern channels recognised in the satellite images, three major accretionary lobes can be identified on the Kosi megafan. Relative age of the lobes determined from the truncating relationship of the paleochannels indicates a random shift of the trunk channel forming these lobes. Similar multilobate form and evidences of random switching of the loci of lobe aggradation are also found to be common in the Tista and Taquari megafans.
The factors known to favour avulsion and the results of the recent simulation studies of alluvial deposits are inconsistent with the notion of unidirectional shift of the channels for more than 100 km across the entire megafan surface. This study suggests that the relocation of the Kosi River in the past was through random nodal avulsion rather than systematic unidirectional shift. The recent avulsion of the Kosi channel by a large distance to the east follows this expected pattern. Further study and age dating is required for a comprehensive understanding of the depositional dynamics of the megafan and the pattern of channel movement on it. Future flood predictions and disaster management plans should be based on such comprehensive understanding.
•A novel method to derive distribution of stream power (SP) utilising a physically-based hydrological model (SWAT).•SWAT derived hydrological variability was used to derive the basin scale specific ...stream power (SSP) distribution pattern.•Stream power distribution along tributaries shows multiple peaks which relate to erosion rates and geomorphic processes.•Monsoonal rainfall determines SP and SSP peaks which are 100–1500 times greater in monsoon than in non-monsoon periods.
River processes in a drainage basin are strongly governed by the spatio-temporal variability in stream power. Recently, various approaches have been developed to derive the spatial variability of stream power in a river basin; however, the temporal variability in stream power due to hydrological changes remains poorly understood. An assessment of the temporal variability of stream power is critical for river studies in monsoonal regimes because significant geomorphic work is performed during this period, particularly in the Himalayan rivers. This paper presents a novel methodology to derive the stream power distribution at a basin-scale using a physically-based-hydrological model, the Soil and Water Assessment Tool (SWAT). Hydrological changes in the upper Kosi (also called Koshi) basin in the Central Himalaya were analysed and integrated with slope variability and channel width to obtain the basin-scale distribution of total stream power and specific stream power. Most of the tributaries of the Kosi River are characterized by a multi-peak distribution of stream power. Stream power distribution in the basin shows no systematic downstream increase or decrease of either total stream power or specific stream power. Total and specific stream power peaks during the monsoon period are 100–1500 times higher compared to the non-monsoon period and exceed the threshold for erosion and sediment transport processes. The SWAT model also helped to assess controls on the stream power distribution in the basin. The downstream distribution of stream power between confluence points is influenced by slope variability, while major increases at confluence points are due to discharge variability. Geological characteristics control the spatial distribution of stream power while the annual rainfall distribution influences the seasonal variability of stream power.
•Flood risk assessment by estimating flood hazard and vulnerability separately in Kosi megafan region, India.•Use of multi-decision making criteria for flood risk assessment in a GIS framework.•About ...67 % of the Kosi megafan is prone to flood hazard out of which 62 % falls in high risk zone.•Advocates soft engineering solutions for flood management and flood risk reduction.
Floods are one of the most devastating natural events that lead to enormous and recurring loss of life, properties, and resources around the globe. In India, floods remain a major hazard during the monsoon season in many parts of the country but most seriously in the flat and monotonous Gangetic plains in the Himalayan foreland. One of the most flood-prone rivers in India is the Kosi in north Bihar, eastern India, which also forms the largest active megafan in the world. The Kosi River is known as the “sorrow of Bihar” because of its anomalous behavior and the damage it causes due to frequent floods in this region. This study evaluates the flood hazard and flood vulnerability as separate entities and combines them to assess the flood risk in the Kosi megafan region. Geomorphological, hydrological, and socio-economic data have been integrated in a GIS framework using a multi-criteria decision tool called the Analytical Hierarchy Process (AHP) to generate a process-based flood risk map for 105 blocks (administrative units) of both Nepal and India (Bihar). Such maps should encourage soft engineering solutions for flood mitigation aimed at minimizing the consequences of flooding and reducing flood risks. This study has significant implications for developing measures and plans that will help government and relief agencies in the identification of flood-prone areas and for planning emergency management strategies in this region.
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, 12km upstream of the Kosi barrage in Nepal, provided an opportunity to study a large-scale avulsion (~120km) 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.
•Documents one of the largest avulsions (Kosi River) affecting ~3million people•Proposes an avulsion threshold index integrating planform dynamics in GIS•Identifies excessive sediment flux and human interventions as major causal factors•Identifies future avulsion sites and suggests monitoring of critical reaches
The Kosi megafan region of eastern Bihar, India, comprising of eight districts, is regularly afflicted by large floods that cause extensive damage. Mapping the possible inundation susceptible zones ...in the region accurately is, therefore, paramount for land resource conservation and livelihood preservation. This paper compares the relative efficiency of two flood zonation methods, the frequency ratio and fuzzy logic models, for flood susceptibility evaluation and delineation. Flooded extents from two past events were combined to create the training dataset, which was then compared individually with respective maps of 12 well-documented causal factors of inundation to gauge their relative influence on the flood spread. These were merged using the two models to generate the respective flood susceptibility maps, which were subsequently validated using the Receiver Operating Characteristics Curve and the Seed Cell Area Index methods. Results revealed that an enhanced fuzzy logic model (prepared by assigning relative weights to the respective causal factors as obtained from their analysis in the frequency ratio model) was more accurate and robust in demarcating flood susceptibility zones, as was further ascertained by comparison with a recent flood event, thereby providing a novel way to merge these methods. The western and southern tracts of the region were found to be more inundation prone, with the greatest risk posed along the narrow interfluve between the Kosi and Ganga rivers prior to their confluence further east. The districts most likely to be inundated were identified by computing their respective proportionate areas under the very highly susceptible flood class.
•Additional nutrient supplements with recommended dose of macro nutrients helps to improve the growth and yields attributes.•Application of zinc sulphate and iron sulphate leads to higher response ...for per unit land yields, quality and economics.•Use of supplementary nutrients gave higher net income and benefit-cost ratio.•Overall improve the livelihood of menthol mint growers.
Supplementation of nutrients plays a vital role in the growth yields and quality attributes of the crops. An experiment and demo trials were conducted in CSIR-Central Institute of Medicinal and Aromatic Plants (CIMAP) research farm and at farmers’ field(s) simultaneously, to study the adequacy of additional nutrients application in menthol mint (Mentha arvensis L. cv. Kosi and Kranti) for improving productivity and profitability. Results of both trials clearly indicated that NPK (recommended dose) and micronutrients based fertilizers eventually leads to accrue the net income. Commercially grown menthol mint farmers benefitted by consequently improvising fresh herbage, oil content, oil yield, gross returns, net income and benefit-cost ratio (B:C ratio) (50.86%, 3.45%, 55.04%, 55.04%, 207.80%, and 50.75%, respectively in cv. Kosi) in treatment T4 NPK (100:60:40 kg ha−1) + ZnSO4.7H2O @ 25 kg ha−1) concerning the recommended dose of NPK fertilizers T1 NPK (100:60:40 kg ha−1). Simultaneously, for the second demo trial, conducted in the farmer’s fields, forty two farmers were selected for the cultivation of menthol mint. From the results, it was revealed that recommended dose of NPK fertilizers with the addition of zinc sulphate found significantly maximum yield and net returns in the plots; an increase of oil yields, gross returns, net income, and B:C ratio was 37.05%, 37.05%, 111.18%, and 32.88%, respectively in T2 NPK (100:60:40 kg ha−1) + ZnSO4.7H2O @ 25 kg ha−1). Data attained from soil analysis distinctly showed that moreover increasing the yields and return of menthol mint cultivation, the integration of vermicompost, macro-micro nutrients, and bio inoculants with the recommended dose of NPK remarkably upgrades the soil fertility status in terms of organic carbon, available N, P, and K.
Present study was related to the study of landslide hazards of Lower Kosi Watershed, Central Himalaya. The study was aimed at identifying the relation of natural and man-induced factors to the ...incidences of landslides in the Himalayan territory. The computation of landslide frequency ratio (
FR
) and density ratio (
DR
) was accomplished through numerical examination. 1 is a standard value of
FR
and
DR
. Therefore, ratio values above 1 and below 1 denote a strong and poor relationship between landslides and the given factors, respectively. The variables which were strongly associated with landslide occurrences were observed in Bhatwari–Barakot Formation; ‘steep’ slope (>40°); ‘high hills’ (geomorphic unit); ‘high’ lineament density (>2.5 km/km
2
); ‘very high’ drainage density (>6 km stream length/km
2
); ‘high’ stream frequency (12–16 stream/km
2
); ‘very high’ dissection index (>0.29); ‘very high’ relative relief zones (>600 m); slope aspect – ‘east facing’ on one hand and the land use category – ‘fallow land’ on the other. The outcomes of the research study provide scientific data on natural and man-induced factors characterizing landslide occurrences in the watershed. Present method based on the relationship between natural factors and man-induced factors with landslide occurrences may contribute to intensive landslide hazards mapping and further land use planning of the region.
Research highlights
The study divulges the contribution of natural as well as man-induced factors in determining landslide occurrences in mountainous terrain of Lower Kosi watershed.
The natural factors, viz., Nagthat, Chandpur, Bhatwari–Barkot and Blaini formations; steep and very steep slopes; denuded hills, moderate dissected hills and high hills; high and very high lineament density zones; moderate, high and very high drainage density zones; low, moderate and high stream frequency zones; high and very high dissection index zones; moderate, high and very high relative relief zones; east, south-west, north-east and north facing slope aspect establish strong association with landslide occurrences.
Under man-induced factors, the land use/cover, viz., barren/rocky land, fallow land, open forest and agricultural land are strongly associated with landslide occurrences.
This paper reports a hydrogeological characterization of a groundwater-dependent lakes system, the Lakes Kosi Bay catchment located in the north-eastern coast of South Africa. Conventional ...hydrogeological techniques including water balance analysis, hydrogeochemical and environmental isotope methodologies were applied in the study. Three distinct aquifer systems are recognized in the area, namely the unconfined Holocene cover sands, the Kosi Bay and Port Durnford Formations, and the Umkhwelane and Uloa Formations. Groundwater recharge to the topmost aquifer, which is connected to the lakes, is estimated using the chloride mass balance method to be 12 % of the mean annual precipitation (MAP = 939 mm/a). Evaporation rate from the lakes and evapotranspiration from the catchment are 1310 and 1135 mm/a, respectively. The mean annual water balance components indicate a positive balance between inputs and outputs. Groundwater flows from the west through the lakes (flow-through lakes) to the Indian Ocean. Groundwater in the shallow Holocene aquifer and streams has similar hydrochemical and isotopic signature indicating strong interconnection. These groundwater and streams have a hydrochemistry dominated mainly by a Na–Cl–HCO
3
water type, a very low salinity (ranging from 88 to 400 µS/cm) and a negative isotopic signature (negative δD and δ
18
O values). The lakes are characterized by Na–Cl hydrochemical water type, have salinity that ranges from 1014 to 25,300 µS/cm, and positive stable isotopic signature (positive δD and δ
18
O signature) indicating strong evaporation effects. The study further reveals a salinity and isotopic series among the lakes from south to north, towards the estuary.
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.
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