River impoundment is one of the important anthropogenic processes
River impoundment is one of the important anthropogenic processes. River impoundment is an ancient idea. Egypt king Means impounded Nile river with clay and stone for prevent flood hazard and use floodwater for irrigation. The morphology and dynamics of rivers are not only related to each other but also dependent upon environmental variables such as climate, lithology, topography, landuse and vegetation. One of the most effective agents of change in a watershed is human. 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 Land use change in fluvial environment effect on channel morphometry hydrological properties and flow regimes of a natural river. About 10,000 year BP alteration of natural morphology/landscape started due to anthropogenic transformation. Agricultural practices, urbanization, industrialization effect on natural landscape. Grorgy Perkin Maroh wrote a book “Man and Nature” in 1864, the title of this book was change and new title was “The Earth as modified by Human Action: a new edition of Man and Nature”. Marsh point out how much erosion and sedimentation had to increase due to Anthropogenic action Main cause of landform transformation are deforestation, landuse change, urbanization, river impoundments. Changes in water and sediment input can result from a wide variety of land uses, including mining operations (Gilbert, 1917; Pickup and Warner, 1984; Higgins et al., 1987; Knighton, 1989; James, 1991), agriculture (Happ et al., 1940; Costa, 1975; Knox, 1972, 1977, 1987; Trimble and Lund, 1982; Jacobson and Coleman, 1986; Jacobson and Pugh, 1992; Jacobson, 1995; Knox and Hudson, 1995; Ruhlman and Nutter, 1999), urbanization (Wolman, 1967; Wolman and Schick, 1967; Hammer, 1972; Fox, 1974; Park, 1977; Ebisemiju, 1989), or mixtures of different land uses (Miller et al., 1993). Local landlords in the nineteenth century constructed embankment to protect their land and property from floods along the Lower Bengal River (Gastrell 1863; Bhattacharyya 1998; 1999–2000a). These embankments were intended to save the paddy crop, the main crop of Bengal, as well as to protect the towns and villages (Sengupta 1951), these embankments are 4,000 years old. (Kapil Bhattacharyya 1959). Before British period embankment were constructed by Zamindars. (O’Malley and Chakravarty 1909; Bhattacharyya 1998, 1999–2000a). In zamindary period embankment were mainly was very much sporadic in nature. With the growing need for protecting people’s properties, agricultural land and crop earthen embankment and flood control embankments is getting importance. However, in India embankment breaching is a big threat. Irrigation and Waterways Department, Government of West Bengal, has made extensive attempts to control flood of Lower Ajay Basin. Embankments can protect only from low intensity flood but it is neither possible to control the flood totally by embankment. River embankment has huge negative impact on floodplain as well as entire environment. Occasional breaching of the embankment is a common phenomenon in the lower reaches of the Ajay basin as far as the poor stability of the embankment is concerned. For the stability of the embankment, soil texture, bulk density, compressive strength and safety factor are most important geotechnical parameters. The balance among the geo-technical parameters are considered as the most important potential cause of the embankment breaching against the flood condition. The earthen embankments of River Ajay breached maximum during rainy seasons. Embankment breaching in lower reaches of Ajay river, highly affected the cultivable land and human habitation.
REGIONAL SETTING OF LOWER AJAY BASIN
Ajay is a perennial, tropical meandering river that flows for a length of 290km covering a catchment area of 6095sq km in the north-eastern part of India. At first it passes through the Archaean gneissic complex(biotite – gneiss, hornblende gneiss, pryroxene- graulite, basic intrusive, pegmatite & quartzite) for 32 km ,over the older alluvial (Bolpur sandstones, shales, laterites) & recent alluvium for the rest of the lower course. Elevation range varies from less than 10 meters to 20 meters. Pre-Cambrian rocks containing variations of granitoid and schistose structure occur to the western part of the study area. In the Archaean formations, sedimentaries also occur. These sedimentaries were originally deposited as sandy, clayey and calcareous sediments of more or less impure quality and subsequently these were consolidated as sandstones, shales and limestone by diverse composition (Banerjee et.al 1973). The studied segment of The Ajay river basin located from Illambazar (23°36’17.60″N & 87°32′ 05.40″E) to Katwa (23° 39′ N &88° 08’E)(fig 1). Embankment found both side of the river at lower reaches. Discontinuity of embankment creates several problems for the floodplain areas of Ajay. The area received an average rainfall of 150-200 mm. Flow regimes of Ajay river is diverse in nature. Mean Discharge rate varies from 278 mm to 525mm.