MORPHOLOGY AND FACIES ANALYSIS OF SEDIMENTS OF THE KOSI MEGAFAN, INDIA AND NEPAL

Abstract

The river Kosi, with an annual discharge of 52xl0'm3 forms a megafan with a radius of about 60 km in north Bihar (India) and some parts of Nepal. In the present investigations, some morphological aspects and facies analysis of sediments of the Kosi megafan have been studied. Morphological studies include development of a mathematical model to represent changes in time and space in the morphology of the Kosi river-bed along the longitudinal profile of 124 km length based on data for the period of 1960-1979. In the present study, mainly Fourier sine series has been fitted to the river cross-sectional bed level elevations over the studied reach for differernt years. Results indicate that the minimum bed level decays exponentially with lonitudinal distance for a particular year and for a particular cross-section it oscillates sinusoidally with time having a time period of about 7 years. A typical variation of width of the Kosi river bed shows that it decreases in the upper reach, attains a lowest value, then it increases and attains a highest value in the middle reach and further decreases monotonica11y in the lower reach. The river bed width along the longitudinal profile varies exponentially for a particular cross-section and is a function of distance and has sinusoidal oscillations with time period of about 4 years. The maximum river depth varies as a sinusoidal function of distance having a wavelength of about 30 km and it is an involved function of time. The study suggests that a stream cross-section can be modelled satisfactorily as a sum of Fourier sine series having two terms to represent its general trend. Facies analysis of sediments of the Kosi megafan was carried out by studying the sediments by trenching and cleaning river-cut faces up to 4m depth at more than 100 places. Also 62 shallow ill wells mainly up to 10 m depth have been core-drilled and tube-wells lithologs up to 80 m depth were examined for Iithofacies/1ithologies variation. Distinct changes channel pattern of the Kosi are observed in the plain areas. Based on bed load size and bed features, Kosi river-bed can be divided into four zones 1-4. In zone 1, the Kosi has gravelly sandy braided character. In zone 2, it is sandy braided in nature. It changes to straight pattern in zone 3 and further downstream it has a meandering character in zone 4. Also four topographic levels are recoginsed. Level 1 represents the active channel course. Level 2 is 0.5 - 0.9 m higher than the water surface on level 1, during lean period it represents surface of low bars. Level 3 is 1 m higher than level 2 and gets submerged commonly during high flood season. Level 4 with elevations of 0.5 - 0.9 m higher than level 3 is submerged during high floods. Field investigations of lithofacies and architectural element analysis of different types of bars on the Kosi river-bed area was undertaken in Chatra, Belka, Raniganj, Bhaptiahi, Supaul, Nirmali and Mahesi in zones 1 to 3. Major features of zones 1 and 2 of the Kosi river-bed are sand flats (mid channel or side bars with level 3 and islands with level 4), and different types of linguoid bars (simple bars, compound bars, compound-compound bars) occurring at shallow depths. Linguoid bars with superposed dunes are also significant features of shallow depths. Dunes are present also at moderate

depths. Probably the lower part of the sequence was deposited in deeper parts of the channel by longitudinal bars consisting of gravels in zone 1 and large sand waves similar to those described from the Brahamaputra river by Coleman (1969) in zone 1. Process of lateral accretion of sandy bars is a significant process in both zone 1 and 2. i In zone 3 deposition on the side bars takes place also by lateral accretion similar to point bars. Linguoid bars may migrate up the side bars or move across them parallel to the main current direction of the channel. Examination of the core-drilled wells sediments shows that the maximum thickness of the youngest sequence deposited by the Kosi on its megafan is 8-10 m. This formation is comprised typically of sands and muds. It may consist of one or two storeys in the braided zones 1 and 2. Lithological variation at deeper levels (about 80 m) indicates that the Kosi megafan is marked by at least two major formations A and B. The lower formation A with a thickness of > 60 m is thicker of the two and is composed of gravelly sand in the proximal regions, coarse to medium sand in the western middle and central distal region, gravelly sand to fine sand in the southeastern distal region, the lower formation is considered to have been deposited in the Holocene Period, when due to melting of glaciers, huge amounts of coarse material stored in the drainage basin during the last glaciation was brought into the plains and spread over large areas. The Kosi river maintained braided character over most of its megafan during its deposition. Formation B almost coincides with the youngest sequence on the megafan. This formation is mainly sandy in the northern half and and is comprised of sand and mud in the southern half of the megafan. This formation has been deposited by braided Kosi in the northern half and straight Kosi river in the southern half. Also, the groundwater-fed meandering stream deposits overlying braided stream deposits of the Kosi on its megafan are characteristic feature of humid Kosi megafan. A preliminary petrographic study of light mineral size-dependent modal trends of sands of the Kosi and its tributaries shows that these are classified as litharenite and few samples as feldspathic litharenite according to Folk's (1968) classification of sandsones. Changes in light mineral composition of sands of the main Kosi river and with additions from i tributaries is strongly related to relatve abundance of major constituents in sands of the tributaries and relative abundance of bed load from the contributing tributaries.