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Tectonic Zonation using Multi-Criteria Decision-Making (MCDM) Techniques: A case study of Kosi Fan, India

Ajay Srivastava
Department of Remote Sensing
Birla Institute of Technology
Mesra, Ranchi, India
Email: ajaybit@yahoo.com
Ph.: 91-0651-276054 (R), 91-0651-276003 (O)

River Kosi once called the “Sorrow of Bihar” exhibits unique character. During the past few hundred years (since 1731) Kosi has shifted its course from east to west to a distance of 210 miles (Gole and Chitale, 1966), leaving behind vast tracts of uncultivable land. The magnitude of the shifting of Kosi river is comparable to only Yellow river also known as “Sorrow of China”, which shifted 375 miles north from its 1852 position. Kosi is an antecedent river older than the mighty Himalayas and finds place in many Hindu mythological texts as a very agile river called “Kausiki”. Kausiki was a mermaid goddess worshiped by the citizens of “Matsya Pradesh” as described in the “Vishnu Purana”.

Kosi and its tributaries originating in the northern Tethyan Himalayan zone cut across the Great Himalayan and Lesser Himalayan Ranges in a number of deep gorges and ultimately flow into the great alluvial flood plains of the Indo-Gangetic Plain. The Indo-Gangetic Plain is a deep crustal trough filled with Quaternary sediments. Its origin and structure are closely related with the rise of the Himalayas. Changes are still taking place at the bottom of this trough giving rise to occasional earthquakes in the north Indian plains. The Indo-Gangetic Plain is divided into four shelf areas separated from one another by three major transverse ‘highs’ in the basement. The highs are known from west to east, as Delhi-Hardwar Ridge, Faizabad Ridge and Monghyr-Saharsa Ridge. Many smaller sub-swells and sub-depressions namely: Sarda depression, Gandak depression, Muzaffarpur uplift, Sitamarhi ridge, Madhubani depression, Purnea depression have been recorded during geophysical surveys conducted for petroleum explorations in the last few decades. These high or basement upwarps can act as water divide or in other words, the basement depressions can act as centre of attraction for the rivers and ground water. Neotectonic movements affect regional slope by sinking or uplifting a particular block of the crust. A change of gradient, even if very slow, affects the direction and rapidity of surface run-off and river discharge.

Kosi and its Himalayan tributaries
Kosi river originates in Tibetan Himalayas at a height of 18,000 feet and drains a catchment of 22988 sq. miles in mountainous terrain with a length of only 450 miles. While Ganga, the longest river of India has a length of 1600 miles with its source at Gangotri glacier situated at a height of 13,000 feet. Size of catchment area (7,330 sq miles) makes it only third biggest Himalayan river in India after Indus and Brahmaputra rivers. It has seven tributaries: Arun Kosi, Sun Kosi, Tamur, Indrawati, Bhotia, Doodhi and Tamba (Figure 1). Its three major tributaries, Sun Kosi, Arun Kosi and Tamur join at Tribeni (Nepal) to form Saptkosi. Saptkosi cuts it path through a 6 miles long deep gorge in the Central Himalayan range and debauches into the plains near Chatra in Bihar where it is called Kosi. Lower down the Chatra, Kosi travels for about 198 miles in an alluvial plain to meet the river Ganges near Kursela, Bihar. During April and May sporadic thunderstorms start in the hills. The bulk of the rain, about 75-80 percent of the total fall occurs during the south-west monsoon period. The maximum rainfall occurs during monsoon in July and August. The highest discharge in the Saptkosi River, so far observed is 8,55,237 cusecs in the August 1954 whereas 1948 recorded the lowest discharge of 9,106 cusecs. The average annual runoff of Saptkosi is 40.4 million-acre feet and approximately half of which is contributed by Sunkosi. The normal flood discharge of Kosi usually remains from 1.5 to 2.0 lakh cusecs. About 75 to 84 percent of the total run-off occurs in the monsoon months of June to October. The annual run-off of Saptkosi has ranged from 34,32 to 49,24 million-acre feet. The annual total sediment load in the Kosi has varied from 47.9 thousand-acre feet (1961) to 229.8 thousand-acre feet (1954). The average annual sediment load for 1948 to 1964 comes to about 80 thousand-acre feet. The average concentration of coarse, medium and fine grained sediments during a year are 15.96, 28.22 and 55.80 percent respectively, and on an average total sediments are 0.20 percent of the total run-off. About 95 percent silt load comes down the river during the monsoon floods and only 05 percent of the sediments come down in the remaining non-monsoon months. The total run-off during the non-monsoon months, however is on an average about 19 percent of the total annual run-off.

Figure 1 Tectonic map of Malda-Purnea graben (agrawal and Bhoj, 1992)

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