Kosi megafan: Historical records, geomorphology and the recent avulsion of the Kosi River

Abstract

It has been proposed that the Kosi River continuously migrated >113 km westward across the surface of the megafan over the last two centuries. Examination of a number of old maps published between 1760 and 1960 shows that during most of this period the Kosi River occupied a position slightly east of the megafan axis. The apparent channel movement shown in these maps is oscillating in nature and not unidirectional. Instead of encountering deposits left behind by a sweeping braided Kosi-like stream, a preliminary study of the uppermost 2–5 m succession in the north-central part of the megafan reveals overwhelming dominance of meandering stream deposits. Assuming the existing notion of Kosi River migration, the rate of deposition averaged over ∼100–200 years for the uppermost ∼5–10 m of megafan deposits, works out to be unusually high (>50 mm/y). All these observations question the soundness of the hypothesis of rapid westward migration of the Kosi River over the last two centuries. The existing facies model for the uppermost 8–10 m of the megafan deposits also appears untenable.

The three-dimensional geometry of the Kosi megafan is similar to those of typical alluvial fans, but with much gentler gradient (0.05°–0.01°) and with larger area (>10,000 km²). Based mainly on the patterns of paleo and modern channels recognised in the satellite images, three major accretionary lobes can be identified on the Kosi megafan. Relative age of the lobes determined from the truncating relationship of the paleochannels indicates a random shift of the trunk channel forming these lobes. Similar multilobate form and evidences of random switching of the loci of lobe aggradation are also found to be common in the Tista and Taquari megafans.

The factors known to favour avulsion and the results of the recent simulation studies of alluvial deposits are inconsistent with the notion of unidirectional shift of the channels for more than 100 km across the entire megafan surface. This study suggests that the relocation of the Kosi River in the past was through random nodal avulsion rather than systematic unidirectional shift. The recent avulsion of the Kosi channel by a large distance to the east follows this expected pattern. Further study and age dating is required for a comprehensive understanding of the depositional dynamics of the megafan and the pattern of channel movement on it. Future flood predictions and disaster management plans should be based on such comprehensive understanding.

Introduction

Kosi megafan is one of the important examples of megafans occurring in the Ganga plain (Fig. 1; Geddes, 1960, Gohain and Prakash, 1990, Singh et al., 1993, Collinson, 1996). It has been mentioned in many textbooks and a number of research publications that the Kosi River has shifted about 113 km from the eastern margin to its present position at the western extremity of the megafan over the last two centuries (Mookerjea, 1961, Mookerjea and Aich, 1963, Gole and Chitale, 1966, Wells and Dorr, 1987, Gohain and Prakash, 1990, Duff, 1992, Singh et al., 1993, Mackey and Bridge, 1995, Collinson, 1996, Decelles and Cavazza, 1999, Bridge, 2003, Assine, 2005). The suggested shift was continuous or in discrete steps but always towards west, sweeping across the entire megafan surface at an unusually fast rate. In August 2008, an avulsion relocated the Kosi River by about 60 km to the east (measured in the central part of the megafan) and a major flood water channel, ∼20 km wide, started flowing along the axial part of the megafan (Fig. 1), marooning hundreds of villages, destroying croplands and bringing disaster to thousands of people living in these areas (Fig. 2a and b). This region was not considered flood-prone as it was located far off the existing channel belt of the Kosi River. The administration and the people of this area were, therefore, less prepared to face the calamity, aggravating the damage done by the sudden change of the river course. In contrast to overtopping of the banks, an annual phenomenon known to the people living close to the present-day course of the Kosi, this was a major change in the course of the river (Sinha, 2009). The avulsion exposed the inadequacy of the understanding of the behaviour of the Kosi River. A better understanding of the underlying factors that control river course changes are essential for better flood prediction and for adopting more effective strategies for disaster management (Sinha, 2008, Sinha, 2009).

Behaviour of the Kosi River is inextricably interlinked with the geomorphology of the Kosi megafan and its formative mechanism. A preliminary examination of the published data gives rise to the following questions:

(a) What factor(s) forced the Kosi River to sweep continuously to west for >113 km during the last two centuries, and at times (eastern half of the megafan) up the slope? (b) How reliable and precise are the dates, and a dozen positions of the paleo-Kosi, compiled from the eighteenth or early nineteenth century maps? (c) Can the sandy fining-upwards successions described from the shallow boreholes from the north-central part of the megafan (Singh et al., 1993) be correlated with the characteristic deposit of a large mountain-fed braided river comparable to that of the modern Kosi River? If not, then on what basis is the uppermost 8–10 m of the megafan deposit correlated with the sweeping Kosi River? (d) During the flood of August 2008, the Kosi avulsed to the east (Fig. 1). Why did it stop moving further west and or to the extreme east near Purnea (as predicted by Shillingfield, 1893 quoted in Gole and Chitale, 1966)?

Thus, there is a need to re-examine and critically evaluate the data from the Kosi megafan in the light of the present day understanding of the Himalayan foreland basin and the megafan sedimentation models. As a part of an effort to understand the Kosi megafan-river system, the geomorphology of the Kosi megafan was studied initially using satellite images and processed SRTM data. Following this, preliminary fieldwork in the north-central part of the megafan studied the uppermost megafan deposits exposed in river bank sections (Fig. 3). Also studied were 28 historical maps, published between 1760 and 1960, to re-evaluate the pattern of historical channel changes on this megafan (Fig. 4a). Finally, the observations in this study were compared with observations on some of the other megafans in order to reflect on the general pattern of growth of the megafans and behaviour of the trunk channels.