SEDIMENTATION ON THE KOSI ALLUVIAL MEGA-FAN, NORTH BIHAR, INDIA AND NEPAL

Abstract

The Kosi river is one of the largest beaided streams of the world and forms a mega-fan in North Bihar (India) and Nepal in the Indogaagetic molasse basin. The river carries an annual run-off .. of 52X109 m3 and shows a highly variable discharge-monsoonal discharge being 10 times the lean period discharge. The river also carries a very high concentration of suspension lP3d-2to5gral/liduring the monsoon months. The present investigations include morphological and lithofacies studies of the modern Kosi river bed and its fan. Photomorphological maps of the river bed and fah haoe ^been prepared. Vertical and lateral changes in lithofacies have been examined by making trenches and cleaning stream-cuts at more than 100 locations. Also, textural and petrographic analyses of these sediments have been attempted. The basic objectives have been to understand geomorphological and sedimentological processes acting on the river bed and fan. Channels of the Kosi river bed are classified as primary and secondary ones. Primary channels are large, deep and active throughout the year. Secondary channels are small and active during the high discharge periods only. Secondary channels of straight and meandering nature have been ident ified. F«ur topographic levels, viz. levels 1,2,3 and 4 have

been recognized in the Kosi river bed between Chatra and Koparia* Levels 1 and 2 are onty subject to modification by the lean period flows whereas level 3 is repeatedly sculptured by the monsoonal flows. Level 4 is affected by peak dis charges during monsoon periods. These levels are best developed in the braided zones of the river. Morphologically the Kosi river can be divided into four zones, viz. zones 1,2, 3 and 4. Zone 1 is drained by one primary channel v/hereas two or more primary channels' may be present in zone -2. Further downstream in zones 3 and 4, the Kosi flows through one primary channel. In zones 1 and 2, primary channels are braided, having numerous mid-channel bars. The primary channel in zone 3 is straight and the channel becomes meandering in zone 4. Zones 1 and 2 are marked by numerous mid-channel bars and side bars with level-3. Some islands and. banks with level 4 are present. Level 4 shows some splay deposits and is marked by some small, straight and meandering streams. Zone-1 differs from zone -2 in that gravels are present in the channels and on bars in the proximal area, whereas sands are dominant in zone 2, In zone- 3, side bars are the most common bed features. Floodplain with numerous swamps and ponds develops in this zone. In zone-4, point bars, natural levees and floodplain are the major features associated with the Kosi river. Natural levees and splays are found at the outer concave bends of the channel. Although the river has been confined by the construc tion of embankments in 1959, the channels migrate back and forth within the area confined by the embankments. The study of the topological parameters and braiding index of Brice (1964) suggest that changes in the intensity of braiding take place in the downstream direction and with time. Relative intensity of braiding in parts of the river bed is controlled by long term rates of aggradation and degra dation. Distinct changes in lithology of the river bed occur in the downstream direction. The zone-1 consists of gravels to coarse sands, zone-2 is comprised of fine to coarse sands, zone-3 is composed of fine to medium sands while very fine sands and silts dominate the zone-4. The dominant lithofacies in zone 1 include facies Gm, Sp and St v/ith minor Sh, Ss, Sr and Fl (See Miall, 1978 for definition of symbols used), in zone 2, trenching of levels 2 and 3 show similar fining up sequences of the type Sp *—>St—>Sb— •>Ss—>Sr and these result in two sJtacked fining up sequences in the river bed sediments. Cuts in level- 4 are marked by sequences similar to those in level -3 but with a number of cycles of Sr, Fl and Fm facies. The fining-up sequences of shallow secondary channels on level- 4 lie with an erosional base on other sequences. The sequences developed in zone 3 are - SI >Sp~f—>Sh- >Sr—>F1 >fm, vi The typical lithofacies sequences in zone* 4 are - SI—->Sp<—>St—>Sr—>Fl—>Fm, The sequences described here for zones 2, 3 and 4-»?.y be over lain with an erosional surface by thin fining-up sequences of small, straight or meandering streams. The Kosi fan has been divided into two physiographic units - the Young Alluvial Plain (YAP) and Old Alluvial Plain (OAP)• The YAP has been further divided into four sub-units - Abandoned Channels, Interchannel Areas, Lowlying Plain and Distal Fan Plain. Abandoned Channel areas represent the earlier Kosi courses. These are marked by thick (> 5m) sequence of sands with a very thin silt cover at places. The vertical litho facies sequence observed is St--—->Sh——>Ss~—>Sr. The upper few decimeters of the old bar surfaces at places have been modified by wind and sediments in these areas have been moulded into sand mounds. The internal structures observed in the mounds include faint to well developed horizontal laminations and at places herring -bone cross-bedding. Abandoned Channel areas are drained by meandering groundwater-fed streams of perennial character. They have dissected valleys within the abandoned channel courses. Point bars of these streams show two "tiers' corresponding to lean period and monsoonal discharges. These deposit up to 1.5 m thick fining-up sequences. These streams have reworked some of the earlier Kosi sediments. with tine, these streams-may evolve to >noro sinuous ones ihnd nay" be choked vii with fine sediments and hyacinth growth into swamps. Interchannel Areas are the largest geomorphic unit of the fan. These areas are highly disturbed due to intensive cultivation. Some abandoned courses of earlier channels and swamps are present at places. Thin silt layer constitutes the surficial sediment at many places. These are marked by thin (< 1.5 m) fining-up sequences of the type St—>Sp~>Sr >F1~>Fm. The Low-lying Plain remains water-logged for the most part of the year. This plain seems to be a part of the modern Kosi river isolated by the Eastern Embankment from the main river bed. The Distal Fan Plain occupies the southern most part of the fan. The groundwater-fed stream deposits fining-up sequences of about 1.5 m thickness along their valleys. The rest of the plain is covered by about 3.5 m thick sequence of Fl and Fm mainly contributed by overbank floods of the meandering Kosi at the foot of the fan. Old Alluvial Plain represents the Old Kosi plain which received little sedimentation during the recent sweep of the Kosi from east to west. They are drained by southerly flowing groundwater-fed meandering stream. Sediments of this plain consist of fine to medium sand which are yellow to red in colour. In most of the places sedimentary struc tures in these sediments cannot be identified in the field. These are overlain by < 30 cm of grey sands or < 2.5 m thick fining-up sequences of the groundwater-fed streams.

Examination of lithologs of tube-wells on the fan indicates that the top cycle of about 10 m thickness depo sited by the Kosi in recent times over most of the fan is a part of a larger (> 80 m) fining-up cycle. The bottom of the cycle is marked by gravels or gravelly sands over large areas on the fan. The development of the larger cycle is thought to be in response to climatic change in the area or due to tectonic rise of the provenance. Lithofacies of the fan and the zone 2 of the active Kosi river bed are similar except that the facies St domi nates the fan sediments, while both facies Sp and St are equally abundant in the present Kosi river bed sediments. Also, two stacked fining-up sequences are observed in the river bed, whereas the fan is marked by one such thick sequ ence in the abandoned Channel areas and two or more finingup sequences in the Interchannel Areas. Morphological mapping of the fan brings out that the Kosi river has changed its courses on the fan in discrete steps leaving out some areas undisturbed in addition to gradual lateral sweeping. Average grain size of the Kosi river bed decreases downstream. Also, an overall fining-up trend is discer nible in most of the vertical sequences observed both on the fan and river bed. Two saltation sub-populations are recognised in the Kosi sediments. Similar sub-populations hhv • been earlier described from beach deposits. Grain size statistical parameters are not useful in differentiating sediments from different sub-environments.

Petrographic analyses of the Kosi river bed and fan indicates that these are mainly litharenite and feldspathic litharenites according to Folk's (1968) classification. Inspite of the fact that medium to high grade metamorphic rocks cover about 33>< of the source area, sands of the Kosi reveal the abundance of undulose monocrystalline quartz, characteristic of low-grade metamorphic rocks. Morphological features and lithofacies of the Kosi river bed are strongly related to distinct moneoonal and lean period discharges. The river bed develops two sets of levels i.e. levels 1-2 and levels 3-4 and two stacked fining-up sequences in response to these flows. The river bed shows distinct channel pattern changes from Chatra to Koparia and is sculptured mainly by high and waning stage flows. Wind activity also modifies the river bed. The Kosi fan has responded to distinct discharges in a diff erent way than the river bed in that it has developed thicker sequences in the Abandoned Channel areas and thinner sequences in the Interchannel Areas. After shifting away of the river, the fan sediments are at places reworked by groundwater-fed streams and wind action.