Tectonic Zonation using Multi-Criteria Decision-Making (MCDM) Techniques: A case study of Kosi Fan, India Basement Structures Kosi flood plain overlies a highly uneven basement formed by the major tectonic features such as Purnea depression, Monghyr-Saharasa (M-S) ridge and Madhubani depression. Two basement ridges, namely the Bansihan uplift and Mongher-Saharsa ridge are present in the east and west of the graben respectively (Figure 2). Monghyr-Saharasa ridge is a prominent basement high overlain by a thin layer of sediments. This basement swell in the Eastern Gangetic Plain is also considered an extension of the Lesser Himalayas. M-S ridge is traversed by the basement faults such as NNW-SSE trending Kishanganj and Malda basement faults in the east and the Bhawanipur fault in the west (Rao, 1973). All these faults follow straight line with the NNE-SSW direction except Malda-Kishanganj fault. The arcuate shaped Malda-Kishanganj shows a westward bending near M-S ridge. East Patna basement fault forms a graben like structure (Dasgupta et al., 1987) and extends into the Madhubani depression. Madhubani depression has a thick accumulation of sediments with a maximum thickness of 6 km while Purnea depression is situated further east of the M-S ridge but at a shallower depth. Eastern Gangetic Plain is neotectonically active as evidenced by occurrence of several major (1883, 1934 and 1988) and minor earthquakes in this region. The great earthquake of 1934 (intensity 8.4 Richter scale) had its epicenter in the Madhubani area and is thought to have occurred due to movements along a fracture zone between Motihari and Purnea (Dunn et al., 1939). The maximum intensity reached in 1988 Bihar-Nepal earthquake was IX in Modified Meracalli (MM) scale over an area northeast of Madhubani, Bihar (G.S.I., 1933). The river basin in the region is subsiding at a rate of 1 m per 1000 years (Agarwal and Bhoj, 1992). The entire north Indian plate is subjected to NNE-SSW directed (approximately N-S) compression that resulted from post-collisional northward underthrusting of the Indian plate (Bilham, R. 1995).  Figure 2 Shifting courses of Kosi river (Gole and Chitale, 1966) Kosi Megafan Kosi megafan has a spatial extent of 180 km long and 150 km wide. The area comprises of Gangetic alluvial in the center bounded by Himalayas in the north. Gondwana sediments and Rajmahal traps occur in the west whereas the Archaen rocks of Peninsular India define southern margin. The synoptic view offered by satellite images at almost any scale helped in identifying the various regional features on the image. Kosi megafan area is marked in the landscape by a series abandoned channels aligned lakes and offset stream channels. Positions of the palaeochannels of Kosi river can be easily identified on these images. At small scale enough to display the entire megafan an interesting network of interlocking channels is the principal feature to notice. The whole area can be broadly divided into piedmont zone, alluvial fan zone, alluvial plain and flood plain of Kosi river. A combination of factors is influencing the complex behaviour of Kosi. These are hydrological factors, sedimentological factors, tectonic factors related to Himalaya as well as response of basement structures to plate movement, regional slope and geomorphology etc.. Alluvial fan morphology is an indicator of active tectonics because the fan form reflects varying rates of tectonic processes such as uplift of the catchment on mountains along a fault or tilting of the fan surface. The fanhead deposition associated with the Kosi megafan suggest that the rate of uplift of the mountain front is higher relative to rate of stream-channel downcutting in the mountain. The westward skewed shape of Kosi megafan suggests that alluvial fan area is being tilted westward due to subsidence in Madhubani depression and uplift of Himalayas. Saptkosi and other small streams in the Lesser Himalayas are offset several kilometers along the thrust. The directional relationship between Kosi river and Saptkosi gorge has been modified due to uplift and subsidence related to neotectonic movement in this belt. The arcuate geometry of Malda-Kishanganj fault bears the testimony of such tectonic deformations and consequent modification in the spatial relationship of Kosi floodPlain and upper reaches. It appears that direction of the Saptkosi gorge in Himalayas has been changed from NNW-SSE to N-S and finally NNE-SSW with the continued northward movement of the Indian plate. Huge amount of discharge running down to Kosi alluvial plain is significantly affected by the spatial position of the channel in the Himalayas. In fact, it follows the direction of Saptkosi river. As the direction of the Saptkosi channel in Himalayas gradually changed so the flow direction of water in the active channel of Kosi river also changed. The migration of Kosi took place in three major steps. In the first episode, it was under the influence of Malda-Kishanganj fault and Purnea depression. It was a tributary of Mahananda river with its confluence near Lava. A tectonic disturbance caused movement along M-S pair fault and upliftment of the block on the eastern margin of the M-S ridge with consequent change in the direction of Kosi channel. Kosi shifted its channel to attain a preferred N-S direction in this stage. Similarly, next tectonic disturbance changed its hydrologic preferences by modifying the regional slope and channel morphology and thus helped it to move in western direction. Its westward migration was further supported by E-W regional slope and presence of many Quaternary surface faults forming diversion point and flood breaches. It has maximum shifting tendency during this phase that took place in the twentieth century. One possible mechanism of Kosi shifting may be by sequential discharge diversion in various channels of Kosi group of rivers during its early phases of tectonic instability. The amount of discharge in its earlier active channel was reduced gradually due to shifting of the diversion points or river course in response to the uplift of M-S ridge. Later, its migration was supported by the hydrological factors, further shifting of the diversion points, topography and basement tectonics. Thus, it can be concluded that Kosi shifted its channel in much different way than the previously suggested mechanisms where whole Kosi river shifted its position from the very beginning due to silting and braiding. Tectonic Control on Kosi River A Quaternary fault system has been identified in the region. This is an echelon pattern of surface faults associated with Begusarai fault. Within this fault zone, various geomorphic features are found which have their origin in both the lateral and vertical movement of fault-bounded slices, as well as in the persistent strike-slip. These features include sag depressions and sag ponds, offset and deflected stream channels, flood breaches, linear ridges, scarps, fault-controlled drainage. Along its entire course, the fault zone exhibits peculiar, anomalous drainage patterns. In regions where tectonic activity is less pronounced, streams generally flow more or less perpendiculr to the adjacent highlands. Movement within the network of branching and anostomosing fault blocks controls the lateral movement of river course. The dominantly lateral slip across the fault zone and rate of slip, from 1 cm to a few centimetres per year, compel stream channels to offset right laterally. River course is offset along the trend of the smaller faults while maintaing the continuity of flow along Begusarai fault. In addition to the effect of lateral slip, streams are extremely sensitive to vertical slip on fault and warping of the land surface. Only a small vertical movement of a block or a change in the slope and aspect of the land surface on the downstream side of a fault crossing a stream may divert the stream either to the left or right of the main stream. Many diversion point and flood breaches all along its middle course have been mapped. All the diversion points and flood breaches occur on the intersection point of Begusarai fault with other echelon faults in its vicinity. Earliest diversion point possibly formed during the Late Pleistocene is located at the Himalayan foothills where channels present a radial pattern. This diversion point represents the intersection of Nahan, Malda, Bhawanipur and the Begusarai faults at the foothills of Himalayas. Kosi group of rivers includes many relatively smaller tributaries or its abandoned channels originating from the first diversion point. One of the branches of Kosi (on the eastern margin of the megafan) confluence with Ganga River near Manihari, 15 km south-west of Lava and probably has been influenced by Kishanganj fault. While other two located in the central and western part of the Kosi megafan meet present channel of Kosi River. These two rivers flow in a direction close to the trend of Bhawanipur and Begusarai fault respectively. These smaller rivers show characters similar to the present active channel of Kosi river but with a gradual decrease of discharge form east to west. Convergence point is fixed at a location near Kursela, which is close to the intersection of Bhagalpur and Bhawanipur faults. The rate of movement of Kosi river is highly variable irrespective of the discharge and sediment load indicating an obvious control of tectonics. There was a high rate (0.7 miles/yr, 1770-1825) when several earthquakes have occurred in adjacent Nepal Himalayas in 1720, 1764 and 1826. The rate of movement of Kosi River was relatively less when it was flowing in Purnea graben (prior to 1956 position). Its movement was further slowed by Bhawanipur fault marking the boundary between M-S ridge and the Purnea graben. Once Kosi river crossed this region its rate of movement gradually increased again. Kosi river behaviour has been different on either side of the north-south line (Chatra-Kursela) which is collinear with the Bhawanipur fault and the eastern margin of M-S ridge. Gole and Chitale (1966) have explained this change of behavior in terms of river’s preference for shortest route in the eastern side and the most favourable slope on the west of the megafan. There was sudden increase in the rate of movement (1.6 mile/yr) when it encountered few cross faults over the Begusarai fault. This stage (1922-1933) coincided with the phase of intense tectonic disturbances going on within Madhubani depression related to pre and post seismic movements of 1933 earthquake. This region is tectonically active and experienced many strong ground motions in the past. Such earthquakes and associated seismic motions resulted in the lowering of the basin in Madhubani depression and relative upliftment of M-S ridge area due to reactivation of pair faults forming M-S ridge. This relatively lowered surface in the west attracted Kosi river to deposit its sediments. Presence of surface faults and an arcuate shape of Malda-Kishanganj fault support this mechanism. The present rate of shifting has been calculated with the help of satellite images of year 1991 and 1998. Principal component 2 image has been found to enhance active channels very effectively. Principal component images have been used to classify the water bodies and then change detection of the active channel has been performed. River shifted around 1 mile in this period and 24 miles since 1950 position. Direct geologic evidence of Quaternary faulting and land subsidence has been observed at various places in Saharasa and Begusarai districts. These observations support the view that the faults/lineaments mapped in the region are tectonic in origin and form part of a Late Quaternary fault system located in an area showing active seismicity. These features indicate that the neotectonic movement of mountain uplift and basement tectonics have influenced the diversion point and flood breaches. A belt of small and moderate magnitude earthquakes occur along a narrow belt referred as the Himalayan Seismic Belt (Gahalaut, 2000). It coincides with the topographic front between Lesser and Higher Himalayas and zone of increased gradient of Himalayan rivers (Seeber, 1983). The palaeochannel development is characterized by a gradual westward shift of the diversion points along some preferred lines following the trend of echelon faults mapped in the region. The structural map, contour map of the basement and cross-sections showing Neogene-Quaternary formations suggest a regional tilt from east to west along the Malda-Kishanganj fault, which got accentuated in the recent past due to reactivation along the faults /lineaments resulting in westward migration of the river. Alluvial fans of different ages have developed sequentially. During the Late Pleistocene, the Kosi river has developed an alluvial fan radiating from the diversion point located near Tribeni. In the early Holocene, its diversion point moved downstream and at present it is located further downstream near Birpur, Bihar. As the rigid landmass was uplifted, the diversion point moved in northeastern direction relative to the foreland basin, flow of the discharge was redirected and diverged in other rivers of the Kosi group. Thus, older mountain front fan moved westward in response to Indian plate movement as evidenced from a general skewness in the shape of the fans. The great earthquake of 1934 (intensity 8.3, epicentre: Madhubani) was the manifestation of this ongoing activity in this region. The shifting rates are highest in period before and after the 1933 earthquakes. Similarly, other diversion points on the margin of the fan moved westwards in response to accelerated erosion due to increased sediment load and discharge. Reactivation of fault was accentuated by the increased tectonic activity within the Madhubani depression. The downstream movement of diversion point and flood breaches in response to neotectonic movement is the main cause behind the westwards shifting of Kosi river. All the rivers in the area including Kosi, Baghmati, Kamla-Balan and their numerous small tributaries are migrating westwards towards the most effective centre of the subsidence associated with the subsiding Madhubani depression on the west. Baghmati is also migrating westward, as the palaeochannels are mostly concentrated on its eastern side of the present channel. Basement topography favours separation of Baghmati river from Kosi and Kamal-Balan rivers in the form of Sitamarhi ridge in the north. Although, Baghmati is moving westwards but towards Gandak depression. Kamla-Balan river has least shifting tendency among the above mentioned rivers as evidenced by very few palaeochannels around them. This migration of rivers in this area will remain continue until they reach their centre of subsidence located near Darbhanga. These rivers will finally form convergence point over a suitable point or line near their center of subsidence. Once the Kosi river approaches this position, its westward shifting rate will decrease. There is a need to include the basement tectonics and buried sand body geometry in formulating the flood management plan for this region. It seems Kamla-Balan river system is nearest to this center of subsidence, although, detailed high resolution geophysical surveys are required to know its exact three-dimensional subsurface geometry and to resolve lateral and vertical changes in subsurface structure and stratigraphy on a scale comparable to the variations in structures observed at the surface. |