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Flood And Sediment Management In Kosi River

Congress: 2015
Author(s): DIlip Kumar (Patna, India)


Keyword(s): Sub-theme 10: Management of water resources,
Article:
Abstract1.1 INTRODUCTION Bihar is one of India's most flood-prone states with large population in the north living under the recurring threat of flood devastation. Recurrent floods are devastating to Bihar's economy and undermine poverty alleviation efforts. The entire area north of Bihar falls in the flood plain of the rivers originating from Himalaya and joins River Ganga Traditional efforts at flood management have focused structural measures like isolating rivers by constructing a system of embankments. Despite the largely structural embankment solutions that have been the focus of flood management in the past decades, threat of floods remains as high as ever to the economy and livelihoods in Bihar. Being flood - prone not only affects existing investments, but also is a disincentive for additional investments in Bihar. There is a need to develop a long-term flood management strategy for Kosi Basin based on analysis and stakeholder inputs that builds upon the knowledge of the numerous studies preferred and documentation that currently exists on the problem. 2.1 KOSI RIVER BASIN The Kosi river basin is shared by three countries, i.e., Tibet, Nepal and India. The hilly upper catchment lies in Tibet and Nepal, whereas, the river's flood plain lies in Nepal and mostly in Bihar, India. The Kosi basin is shared by districts of Bihar (India), Nepal and Tibet. Kosi River is one of the biggest tributary of River Ganga originating from Tibet and joins the Ganges in Bihar state (India) via Nepal. Total drainage area of the Kosi River is 74,500 km2. Majority of the area comes from Tibet and Nepal (80%), and only 20% drainage area is in Indian Territory. The upper catchment (80% drainage) area is responsible for the morphological activities in the downstream reaches as it receives an average rainfall of 1,456 mm. It carries huge amount of sediment with its flow every year causing change in morphological behaviour of the river. The problem of the flooding is implicitly governed by river sedimentation. River sedimentation has gradually reduced the capacity of the river to carry extreme flows which cause flooding in the plains along the banks. 2.2 KOSI RIVER SHIFTING Kosi river has an exceptionally high sediment yield of 0.43 million tonnes/year/km2 which is accommodated in a very narrow alluvial plains almost one-fifth of the upland area. This has resulted in predominantly aggrading channels of the Kosi and building up of a very large positive topography called 'Megafan'. Most analysts agree that the confinement of the Kosi within the embankment further worsened the situation and has caused significant aggradation within the channel belt. Kosi River shows a large braided network of streams of various orders of magnitudes. Its (i) large dimension, (ii) multi-threads morphology, (iii) sandy bed, and (iv) avulsive behaviour make it very important to understand the sediment dynamics and propose a management strategy. Kosi River shifts laterally over the Himalaya foreland plain by continual minor cut-offs, bank cutting and by episodic major shifts across watersheds. Migration is unidirectional because after a channel is filled to instability, floodwater will drain preferentially into a new adjacent low area rather than across it to the next watershed or back to the last abandoned channel. Records show that between year 1736 and 1953 the river moved through a distance of about 112 Km. westward till its course was confined through jacketing by constructing embankments on both the banks. 3.1 ANALYSIS OF WATER AND SEDIMENT FLOWS The following are the outputs from model analysis of the water and sediment flows in the main river channel, the fan and the flood plains. 3.1.1 Main River Channel 1) Water levels and discharges along the river course 2) Long term aggradation of bed level, and possible future equilibrium condition 3) Scour depths at vulnerable locations 4) The velocity near the banks for protection measures 3.1.2 Fan and Flood Plains 1) Flood plain inundation for reaches without embankment 2) Inundation of the fan from breaches in the river embankment 3.1.3 Mitigation Measures The following mitigation measures have been tested with the mathematical models: 1) Effectiveness of protection measures at vulnerable locations 2) Impact of embanking unprotected reaches 3) Potential for diversion and storage of water and sediment from the main river over the fan 4) Monitoring, flood forecasting and warning system 5) Impact of possible future scenarios: storage dam upstream Chatra, climate change and soil conservation measures upstream. 4.1 RECOMMENDATIONS Watershed management measures in the upstream catchment and the construction of a large storage dam at Chatra will have a major beneficial impact on sedimentation in the lower catchment. Actual implementation is not seen to be imminent, and measures to mitigate disastrous flooding and sedimentation in the lower basin are required urgently. Progressive basin-wide implementation would normally suggest relatively high capital investment, with low annual maintenance costs. Implementation of upstream watershed management would render much of the high initial capital investment redundant, and reduce maintenance costs. 1. Journal of Rural Infrastructure & Development, Society of Engineers for Rural Development, Nepal, Vol.3, 2012 pp 124-132 2. Flood Disasters and Management -- Indian Scenario -- by Bikash Raymashay and Rajiv Sinha- Indian Institute of Technology Kanpur - 2012 3. Flood Hazards of North Bihar Rivers. Indo-Gangetic Plains -- by Rajiv Sinha and Vikrant Jain, Memoir Geological Survey of India, No.41, 1998 4. Flood Risk Analysis in the Kosi River Basin, North Bihar using Multi Parametric Approach of Analytical Hierarchy Process (AHP) -- R.Sinha, G.V.Bapalu, L.K.Singh, B.Rath (2008), Journal of Indian Society of Remote Sensing, December, Vol.36 pp 293-307 5. Guidelines for Preparation of Project Reports of Irrigation and Multipurpose Projects -- by Ministry of Water Resources, Government of India, 2010 6. Bihar Kosi Flood ( 2008) , Need Assessment Report, Prepared by GOB, World Bank, 2010. 7. Bihar Kosi Flood Recovery Project -- by Environmental and Social Management Framework -- June 2010 8. Catchment Area Treatment Plan (extracted from Environmental Management Plan for Parbati Hydro-electric Power Project Stage III, Kulu, Himachal Pradesh -- by NEERI) 9. Compendium of Guidelines in the field of Flood Management - by Ganga Flood Control Commission 2004 10. Comprehensive Plan for Flood Control for the Kosi River System, GFCC, 1986
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