A Study of Bakreshwar River Basin, West Bengal
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Journal of Indian Geomorphology Volume 4, 2016 ISSN 2320-0731 Indian Institute of Geomorphologists (IGI) Spatial Heterogeneity of Catchment Morphology and Channel Responses: A Study of Bakreshwar River Basin, West Bengal Krishna Gopal Ghosh Department of Geography, Visva-Bharati, Santiniketan-731235 E-mail: [email protected] Abstract: This paper synthesises the upshot of catchment topographic asymmetry on chan- nel morphological structures and functions for the 5th order un-gauged Bakreshwar wa- tershed. The study has revealed that, asymmetry in different physical attributes at a catchment scale explains a number of consequent channel hydro-geomorphic events. A little topographic tilting of the basin towards the north or left bank has acted as a domi- nant control on alignment of water flow paths, flood spreading etc. The spatially skewed nature of fluvio-geomorphic activities of the basin is manifested in (i) the tendency of rivers to migrate towards the north leaving a number of palaeochannels in the southern part of the basin, (ii) position of the thalweg closer to the left bank, (iii) concentration of erosion and stream instability in the left bank, and (iv) spread of floodwater more in the left side of the channel. However, the role of catchment geology in controlling the hetero- geneity of fluvial forms of the area is a key element, which needs investigation. Introduction meandering, and migration of channels Fluvial morphology results from the (Bloschl, 2005; Rodriguez-Iturbe and Valdes, complex interplay of geomorphic processes 1979; Horton, 1945; Strahler, 1964; that occur in a basin at different spatial and Zevenbergen and Thorne 1987; Patton, 1988; temporal scales (Schumm, 1988; Piegay and Leopold and Wolman, 1957). Hence, it is Schumm, 2003; Church, 2008; Schumm and rightly regarded that, streams are the Lichty, 1965; Lane and Richards, 1997). manifestation of the landscapes that they drain Among the variables that affect fluvial (Hynes, 1975). Channels, respective to the systems, catchment topography is one of the basin topography tend towards an equilibrium most important and have causal link to the state by means of changes in channel course, hydrological forces, the channel network, shape and profile through valley erosion and valley form, spreading of flood water, deposition of sediments (Montgomery and SPATIAL HETEROGENEITY OF BAKRESHWAR BASIN 47 Figure 1. Reference Map: (a) The Ganga catchment, (b) The Mayurakshi river basin, (c) The Bakreshwar river basin. 48 JOURNAL OF INDIAN GEOMORPHOLOGY: VOLUME 4, 2016 Buffington, 1998). Different authors have of Mayurakshi river system of eastern India. reviewed this influence of the drainage basin The present study area, Bakreshwar river on fluvial channels like, Anderson and Burt catchment (BRC) is a part of lateritic Rarh (1978), Amos and Burbank (2007); Begin plain region extending from 23º43′23.28″ N (1981), Beven, et al. (1988), Masoud (2013) to 23º56′31.16″ N latitude and 87º17′1.59" E etc. Brierley and Fryirs (2005) argued that, to 87º47′16.07″ E longitude. The drainage natural channels are ‘dynamically adjusted’ basin is covered by five Survey of India (SoI) and works within a range of ‘variability set by topographical sheets — 73M/5, 73M/9, the river style’ and the ‘catchment context’. 73M/10 and 73M/13, on 1:50,000 scale. The Drainage basins have long been described study area lies in between the river based on the main geomorphic processes of Mayurakshi in the North and Kuya in the erosion, transportation and sedimentation South. It is bounded by district boundary of (Schumm, 1977; Gregory and Walling, 1973). Birbhum in the west while the boundary of It is however, rather surprising that catchment Murshidabad district of West Bengal forms topographic influence on channel hydro- the eastern limit of the region. The river morphology has received relatively little originates from a large pond (23°55'22.13"N, attention. In most hydrological systems of 87°17'18.46"E) and then flows through the interest, this would however appear to be a hot springs of Bakreshwar (23.88°N 87.37°E) difficult task because; the response time of the and joins river Kopai at 23°47'06.3"N and geomorphological systems are much longer 87°48'26.09"E. than hydrological systems. Consequently, the channel hydro-morphology recorded today Regional settings may, in most catchments, reflect responses of The Bakreshwar river basin (basin area: catchment topography in a longer time span. 714.56 km2) is a fifth order basin having 3 This paper will concentrate on the fourth order, 13 third order, 54 second order influence of catchment topographical and 318 first order sub-basins, all of which are heterogeneity and asymmetry on channel rain fed with little perennial water. Total morphology of the 5th order un-gauged length of the river is 86.385 km. of which Bakreshwar watershed. In the present work about 13.9 km. is perennial. catchment topography has been considered as The catchment has dry, mild sub-humid one significant variable to regulate the and subtropical monsoon climate. As per 33 processes and the responses reflected in year meteorological data (1980–2013), channel hydro-morphological nature and rainfall varies with elevation from 828.8 mm functions. Few observations have been to 1917.1 mm (IMD, 2013). During monsoon incorporated in the present study, addressing (June to September) advective rainfall is the said links. However, there is yet an dominant. Presence of laterites indicates the obvious need for collecting new field data and existence of tropical wet-dry climate in this consider other variables that affect stream area, probably during Tertiary. Hard massive systems. basalt of Jurassic to Cretaceous age, soft and medium hard laterite of Cenozoic age and Geographic location of the study area china clay of late Pleistocene to early Eocene The Bakreshwar river is a 5th order age (GSI, 1985) are found in different parts of tributary of the river Kopai (Kuya) and a part the basin and at different depths (Saha, 1961 SPATIAL HETEROGENEITY OF BAKRESHWAR BASIN 49 Figure 2. Relief classes of Bakreshwar river basin (based on SRTM data of February 2000) and GSI, 1985). The area is covered mostly monsoon the average water level is near about by reddish, loose, friable and sterile laterite 6 m depth (November, 2009) (SWID, 2009). soil with ferruginous concretions, locally Upper catchment of the basin is covered with called kankar. Owing to extreme infertility, Sal (Shoria robusta) forest but a large part of the land is named Rarh or sterile land the forest is decaying rapidly. (O’Malley, 1910). The soil catena consists of plateau fringe and high plains with laterite soil Catchment morphological heterogeneity (ultisols), adjacent slopes with sandy and and asymmetry loamy soils and small valley floors with older Profile analysis of the basin alluvial soils (ulfisols) (NATMO, 2001). Profile indicates the nature of surface Moderate to maximum physical and chemical configuration and the degree of dissection of weathering, moderate to maximum hill slope terrain (Woldenberg, 1967). 7 Serial profiles processes including sheet wash, rain splash, are drawn across the Bakreshwar river basin rill and gully erosion and laterisation are some aligned North-South. (Fig. 3). One important major pedo-geomorphic processes operating aspect which can be ascertained from these in the region. The typical morphological profiles is that; the slope and relief character features in the upper part of the basin include: is not symmetric along two banks of the main rolling lateritic uplands with ferricrete- river. The N-S cross profiles indicate that the dominated duricrusts and deep red weathering slope and height in the left hand side of the zones, amd wide planation surfaces. Badlands catchment is relatively less, compared to the and low lying flats characterise the basin’s right hand side. This kind of pattern may lower part. It is found that in pre-monsoon the encourage river shifting and greater spread of average water level generally remains above flood water towards the left bank of the main 12 m depth (April, 2009) and in post- water course. 50 JOURNAL OF INDIAN GEOMORPHOLOGY: VOLUME 4, 2016 Figure 3. North–South profiles across Bakreshwar river basin from upstream to downstream at an interval of about 6.5 km. Note the pointer indicating the basin mid points (Based on ASTER DEM of February 2012). SPATIAL HETEROGENEITY OF BAKRESHWAR BASIN 51 Table 1. Indices used for basin asymmetry assessment. Basin asymmetry indices Formula/Derivations Reference Where, Al denotes one side of the main stream having less area and Areal Asymmetry Index (AAI) Am denotes one side of the main Das, P. et al.,2014 stream having more area. Stream Number Asymmetry Index Where, Nµl = One side of the main stream hav- Das, P. et al.,2013 (NAI) ing less total number of stream; Nµm =One side of the main stream having more total stream number. Stream Length Asymmetry Index Where, Lµl = One side of the main stream hav- Das, P. et al.,2013 (LAI) ing less total stream length; Lµm =Onesideof the main stream having more total stream length. Where, N = Number of parameters and PAI = Parameter Specific Asymmetry Index. Here, Composite Basin Asymmetry ∑PAI = (AAI+NAI+LAI)/3 Or Das, P. et al.,2013 Index (CAIB) PAI B values range from 0-1. ‘0’ means high asymmetry and ‘1’ means symmetry. TSF = Score has been obtained by dividing proportion of length in two sides of the principal Transverse Topographic Symme- Moglen, G.E. and drainage line when the drainage basin is divided try Factor (TSF) Bras, R.L., 1995. into sections by the straight line traversing the main river at 900 angle. Catchment Front Sinousity Index Bull, W.B., and (SIcf) Where, SIcf = Catchment Front Sinuosity Index McFadden, L.D., 1977 Lcf = straight line distance along a contour line, Ls = true distance along the same contour line.