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This paper examines the effect of river channel migration on agriculture production using remote sensing data, which has been little studied in Nepal. Assessment of historical changes in a river channel centerline, combined with changes in eroded agricultural land, has revealed how channel migration rate has affected the production of agricultural land.
In this study, migration patterns from 1990 to 2014 for upper, central and downstream reaches of the Kandra River was analyzed, using channel planform, satellite images and hydro-meteorological data. Impact on agriculture was seen significantly due to channel shifting. The majority of agricultural land was lost in the Kandra river area were due to bank cutting. Channel migration and its associated hazard such as erosion has significant effect in agricultural production. Although agricultural production of any area depends upon various factor such as irrigation, soil fertility, rainfall, but production in meandering zone river channel was largely affected by the channel migration process.
Meandering streams are more dynamic, and their tendency to shift location across the valley bottom increase with sinuosity, bed load, and slope. Meandering rivers, exhibiting progressive change in channel location as they migrate across their floodplains over time (Thorne 1992), is not only complex but has a role in floodplain development and landscape evolution (Leopold et al. 1964; Thorne 2002) and also has economic and social consequences of bank erosion and channel migration (Leopold and Wolman 1957; Randle 2004).
Meandering River channel offers a wide field of research on environmental domain at both spatial and temporal scale covering various concepts, approaches and methods. .Of many, a substantial f studies on were focused in modelling the RCM(Guneralp et al. 2012; Hasegawa 1989a; Hasegawa 1989b; Howard 1976; Ikeda et al. 1981; Larsen and Greco 2002; Nagata et al. 2000) Morphodynamics characteristics of bend migration (Aldesoky 1992; Hickin and Nanson 1975; Hickin and Nanson 1984; Klaassen and Masselink 1992; Marston et al. 1995; O’Connor et al. 2003; Sarkar et al. 2012) whereas with evolution of Geographical Information System and Remote sensing various study were focused on the application of GIS and RS to study the channel migration (Ercan and Younis 2008; Guneralp et al. 2012; Hudson and Kesel 2000; Sarkar et al. 2012; Sharma 2003.; Thorne 2002).
While some of the study addresses the effect riparian vegetation, forest area (MacDonald et al. 1991; Micheli and Kirchner 2002 ; Micheli and Kirchner 2002; Micheli et al. 2004 ) in river migration. Agricultural land and production has been effected largely by the process of channel shifting (Sarkar et al. 2012). As many hectares of agricultural land nearby river are eroded away by river by cutting and extensive inundation due to frequent change in the river courses, bank erosion and erosion in the river meanders (Poudel 2002) agricultural production nearby river has been largely affected..
All these problem associated with river enhances the phenomenon of river channel Migration (Randle 2004) is creating the problem to agriculture sector (Howard 1976). Also, western Himalayan river system had numerous tributaries which origins from highly fragile Siwalik hills which had high sediment transport rate as well as exhibit meander nature in its southern lowland of Indo-Gangetic plains. Thus this paper aims to quantify the effect of channel migration on agricultural production of floodplain area of Churiya originate Kandra river.
The study was conducted in along reaches of the Kandra River, a major tributary of the Mohana River, Kailai district of far western Nepal which originates from Churiya hills. Total length of river is 62 km up to Indo-Nepal border, which flow across Ram Shikar, Jhala, Darakha, Pahalmanpur, Khailad and Bhajani VDC of Kailai districts. Relief of study area ranges from 133m to 198m altitude. Tropical to sub-tropical climatic condition with maximum temperature of 40°C in summer and minimum of 6°C in winter prevails. On the basis of morphological characteristics, the entire studied river reach was divided into three zones viz. upper Terai near the Bhawar zone, middle Terai and inner Terai. In each zone, 2 km reach section was selected for study on the basis of Sinuosity of channel.
The study reaches were located in between Pahalmanpur Bridge (Mahendra Highway) and Indo-Nepal Border. Altogether three reaches were selected in between them on the basis of geomorphological division of Terai of Nepal. One reach was selected in middle terai region while two reaches were selected in lower terai with 2km distance. Beside first reach other two reaches was located near villages ( Khailad and Bhajini) with agricultural land covering most of river periphery. While first reach located near Basanta Corridor (International biological corridor between Nepal and India, where it was covered with forest and agricultural land.
Analysis of river migration rate using remote sensing and GIS include preparation of base map with the help of Survey of India topographical maps and satellite image (Micheli et al. 2004 ; Sarkar et al. 2012).
After preparation of NDWI image, Entire river channel from the Pahalmanpur Bridge to the confluence of river with Mohana River (62 km) were manually digitized in Arc GIS 10.1. Then the channel thalweg centerline were delineated for each year. Centerline were digitized through small mid channel bar.
Rates of lateral migration of three bends were measured yearly from 1990 to 2014. Centerline of two year were intersected to define the polygons that represents the areas of flood plain eroded over were taken to calculate average migration rate. Average migration rate for given Eroded area polygon were calculated by the method developed by Michelli (2000), average migration rate was calculated by dividing the polygon area by one half of its perimeter and number of year elapsed between centerline dates.
To assess the impact of river channel migration on agriculture production, multiple regression analysis was performed with the loss in agricultural production of various crops as dependent variables and migration rate, eroded land area and productivity of major crops as independent variables.
Agricultural land of each year was delineated from satellite images. Then the spatial change in agricultural land near Kandra River was assess through overlaying agricultural area of different years.
District Agricultural office has record of agricultural production of various crop produced in district. For this study productivity of major crops produced such as Paddy, Wheat and Cereals were used.
The channel migration rate of Kandra River was found to be 12.60 +7.88 m/year in Reach 1 i.e. near the Phalmanpur Bridge, while it was found to be 12.72 +4.89 m/year in Reach 2 i.e. in Khailad village and in reach 3 i.e. Bhajini channel migration rate was found to be 12.24+7.35m/year. River was significantly migrating annually (P<0.03), whereas there was no significant difference in Channel migration upstream and downstream (P>0.05). Maximum channel centerline migration was found to be in year 1990/91 for Reach1 (40.04 m/year), while for Reach 2 (21.92 m/year) and Reach 3 (21.18 m/year). Year 2010/11 had highest rate in Reach 2 and Reach 3. Figure below shows the decline in trend of migration rate in each segments of the Kandra River
Lateral migration rate of any channel is the function of sediment characteristics, flow parameters (Duan and Julien 2005; Ikeda et al. 1981) and plan form characteristics (Parker 1983). River channel with cohesive material migrates slowly than non-cohesive materials (Hudson and Kesel 2000). From analysis of satellite maps Kandra river was found migrating significantly annually with the average rate of 12.60 m/year (s.d. =7.88) in upstream, 12.72 m/year (s.d. =4.89) in midstream and 12.24 m/year (s.d. =7.35) in downstream segments.
In river and streams where bank height is less than root penetration of bank vegetation, resistance of the bank increases. From field observation, it was found that bank height was more than root penetration height in reach 1 and 2 which shows the high cohesion in surface but lack anchoring the toe of bank. This results in the toe cutting and hence bank collapses. That is why, although river segment in upstream and midstream consist of forested bank they faces migration rate similar to downstream segment which comprises the agricultural land. The channel migration rate can be said higher in contest of Meander River with cohesive materials such as gravel.
For example, lateral migration rate of Sacramento River with gravel bank was found to be 4.9 + 2.3 m/year (Constantine et al. 2009). River channel shifting for any meander channel is governed by flow parameter (Marston et al. 1995) such as velocity near bank, bank full condition of river and sediment characteristics such as sediment particles, particle density and dynamics. Due to large channel width and sandy clayey bank materials of river, which can be easily eroded by water, river channel is highly vulnerable for erosion and hence for channel migration.
Study by Michelli and Krigner, (2002) shows that the migration rate of river is high on the agricultural land than in forested land. Migration of the river was observed comparatively high in the agricultural land than in forested land in Kandra River. Trees in forest area acts as anchor for the bank sediment while due to lack of such anchoring vegetation agricultural land was effected mostly by the channel shifting process (Leopold et al. 1964). This is major reason why Reach 2 had more agricultural land loss than Reach 1.
Agricultural land near the river channel is vulnerable to the loss in production due to channel migration. The effect on agricultural can be observed by either Land lost due to bank cutting or by deposition of sand and silt over agricultural land due to inundation. There are various factor that determine the loss in agriculture production among them is the loss of agricultural land. From table 3 shows that there is high loss agricultural due to erosion activities associated with the channel migration process.
This is because as we move from upstream to downstream the resistance of channel also decrease with increase of shear stress of water as the agricultural land binding material which decreases the potentiality of resisting the shear stress. Impact on agriculture was seen significantly due to channel shifting. The majority of agricultural land were lost in the Kandra river area were due to bank cutting. From the multiple regression analysis of channel migration rate, land lost due to erosion, productivity and the rainfall we can say that, loss in agricultural production in the area near the meander river with active migration is the function of channel migration rate.
With increase in human activity near river channel such as road contruction practice of agriculture river movement had been effected. But, also the river had great impact on those community near river channel which basically observed as flooding, erosion and bank failure. Thus, with development of GIS and RS we can analyze the effect of channel migration on agricultural land by quantifying the loss of production it bears with certain rate of migration.
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