Bed material extraction and its effects on the forms and processes of the lower Balason River in the Darjeeling Himalayas, India

Abstract

High intensity rains bring about landslides and high surface run -off in the upper catchment of the Balason River. Huge quantities of weathered materials as sediment load are being deposited in the river's lower course. The extent and intensity of this bed material extraction have taken such a toll that immediate attention is necessary to reduce the associated effects on the forms and processes of the Balason River's lower course. In this paper, an attempt has been made to closely analyse the prevailing effects of human interference noticed by the authors during this study. The methodology adopted during this study is field based. The changing cross sections and fluvial characteristic of the river were continuously observed. The extraction of bed materials was done directly from the riverbed and from adjacent flood plains and also from terraces. The extractions are responsible for the prevailing bed erosion and channel diversions because extraction pits are what are left behind. In the long term, if not controlled, the effects would be detrimental to the entire fluvial characteristics of the river.

Socioenvironmental issues of river bed material extraction in the Himalayan piedmont (India)

“…Along with the development of the town of Siliguri as a major transport and trade hub, with its population growing from 33,000 in 1951 to 513,000 in 2011 (Census of India 1951, 2011, the clearance of forest and expansion of settlements to cater for the increasing population has led to increased utilisation of (i.e., pressure on) river ecosystem services. In this process, the water resources from the local rivers have supported the basic needs of the region.…”

“…Besides that, with the growth of settlement and expansion of Siliguri, the demand for construction materials has increased manifold, which has stimulated the extraction of construction materials directly from the river bed and also from the adjoining floodplains and river terraces. River bed material is extracted from almost all of the Himalayan rivers of the region, but the extent of this extraction in the Balason river is one of the largest (Supplementary material Photo 1) (Tamang and Mandal 2015).…”

“…During the monsoon season, mostly larger boulders and gravel brought down by the flow are extracted near the banks. Instream extraction is limited in this period as the river becomes inaccessible because of the increased channel flow (Tamang and Mandal 2015).…”

“…The study of the cross-sections of the Balason river channel presented by Tamang and Mandal (2015) shows that the supply of material by surges during the monsoon season did not always compensate for the losses resulting from massscale extraction of river bed material during the dry season. The changes of the river bottom level are clearly observable through changes of the mean bed elevations estimated as an average value of all bed levels across the section recorded during the geodetic survey.…”

Human Impact on the River Channels Morphology in the Himalayas and Its Piedmont (India)

“…The geomorphological effects of such form of anthropopressure in the above mentioned region have been presented in other scientific studies and the Balason river is one of the best studied rivers in this aspect in the North India, e.g. [27], [28]. Surveys conducted among the local community involved in extracting bed material from the Balason river (Photo 3) showed a significant dependence of the existence of the studied population on the work in the extraction of material, and the extraction will be continued [19].…”