International Journal of Geology, Earth & Environmental Sciences ISSN: 2277-2081 (Online) An Online International Journal Available at http://www.cibtech.org/jgee.htm 2013 Vol.3 (3) September-December, pp.1-8/Tamang Research Article 1 HUMAN INDUCED BANK FAILURES ALONG THE LOWER BALASON RIVER IN DARJEELING DISTRICT, WEST BENGAL * Tamang L. Department of Geography, St. Joseph’s College, P.O. North Point, Dist. Darjeeling – 734 104, West Bengal *Author for Correspondence ABSTRACT Balason River with a total length of 46.40 km is the most important right bank tributary of Mahananda River having its origin from Lepchajagat (2361 m). Failure of the banks along the lower Balason river has been noticed as the river undergoes lateral adjustment due to high fluctuating discharge with frequent flow diversions, cohesiveness of the bank materials and anthropogenic effects in the form of human occupancy on the bank and extensive bed material extraction. Mostly, extraction sites close to the bank are preferred as it reduces both labour and transportation costs. The effect of such near-bank extraction is the ultimate lowering of the bed and in many sites such extraction process has created scours which results into diversion of channel towards the bank during high discharge, hence increasing the bank failures. The retreat of side banks are favoring the widening of the river, possessing threat to local inhabitants. The failure of exposed banks are also acting as local sediment sources for the river and due to continuous bank failure at few stretches, the finer sediments are being carried away by the river during monsoon flows. Such suspended loads are being deposited in the lower segments causing siltation near its confluence. Although the concerned authorities are constantly considering the bank retreat through construction of embankments and other necessary actions after 1970’s, the gradual increase in encroachment of the flood plains by the growing worker population engaged in bed material extraction should also be restricted. The adverse consequence of increased stream bank erosion results not only in accelerated sediment yields, but also to changes in stream channel instability and associated stream type changes. Key Words: Bank Failures, Bed Material Extraction, Human Occupancy INTRODUCTION The adjustments of the channel width by mass-wasting and related processes can be represented as an important mechanism of channel response and energy dissipation in alluvial channels (Simon et al., 1999). Under such lateral adjustments, the channel initiates bank failure depending upon the flow energy and shear strength, composition and environmental setting of the banks. Failure of the banks along the lower Balason River (figure 1) has been noticed as the river undergoes lateral adjustment due to high fluctuating discharge with frequent flow diversions, cohesiveness of the bank materials and anthropogenic effects in the form of human occupancy on the bank and extensive bed material extraction. During monsoon high peak flood flows, sudden outburst of water in the channel increases its shear stress causing failure of the exposed banks annually (Tamang, 2012). The bank erosion rates were observed following the monsoon season and the recession or further erosion after monsoon flush were compared with the pre monsoon bank conditions with reference to selected permanent structures (houses, roads, pillars, trees, etc) along the bank line. Bank line survey using GPS are superimposed with past bank conditions from available satellite imageries which indicate that the bank failure has been prevalent and is amongst the important modifications by the lower Balason River under increasing human interferences (figure 2). The Study Area The Funnel shaped Balason basin has its source from a place named Lepchajagat, located on the Ghum- Simana ridge at an altitude of 2361 m and with latitude of 27˚03΄55˝N and Longitude of 88˚14΄12˝E. It is the major right bank tributary of Mahananda River covering an area of 367.42 km 2 . The Balason being a