BACKGROUND PAPER NO. 4 OCTOBER 2006 RIVER FLOODING AND EROSION IN NORTHEAST INDIA BY NORTHWEST HYDRAULICS CONSULTANTS (NHC)* EDMONTON, ALBERTA, CANADA This paper was commissioned as an input to the study “Development and Growth in Northeast India: The Natural Resources, Water, and Environment Nexus” * Contact Person: Herb Wiebe Table of contents Glossary......................................................................................................................................................iv Executive summary ...................................................................................................................................1 I. Introduction .....................................................................................................................................1 II. The Brahmaputra River in Assam...............................................................................................1 III. The Barak River in Assam...........................................................................................................2 IV. Flooding and related costs ..........................................................................................................3 V. Flood management strategy considerations..............................................................................4 1. Background..........................................................................................................................................7 1.1 Context...........................................................................................................................................7 1.2 Study purpose...............................................................................................................................9 1.3 Study limitations ..........................................................................................................................9 1.4 Document organization...............................................................................................................9 2. The Brahmaputra River in Assam..................................................................................................10 2.1 Introduction.................................................................................................................................10 2.2 Overall river features .................................................................................................................10 2.3 Geology, seismicity, and geomorphology ..............................................................................11 2.4 River gradients and dimensions...............................................................................................11 2.5 River flows and sediment transport ........................................................................................13 2.6 Channel shifting and bank erosion..........................................................................................14 2.7 Key implications for control of flooding and erosion ...........................................................15 3. The Barak River in Assam ...............................................................................................................17 3.1 Introduction.................................................................................................................................17 3.2 Physiographic and morphologic features...............................................................................17 3.3 Hydrology and hydraulics........................................................................................................19 3.4 Sediment transport.....................................................................................................................21 3.5 Flooding.......................................................................................................................................21 4. Flooding and related costs...............................................................................................................24 4.1 Significance and causes of flooding.........................................................................................24 4.2 Flood damage..............................................................................................................................26 4.3 Flood relief...................................................................................................................................27 5. Flood management...........................................................................................................................29 5.1 History .........................................................................................................................................29 ii 5.2 Institutions...................................................................................................................................30 5.3 Flood management assets .........................................................................................................32 5.4 Flood management issues .........................................................................................................34 6. Future development considerations ..............................................................................................37 6.1 Existing plans..............................................................................................................................37 6.2 Strategy considerations..............................................................................................................40 6.3 Discussion of key issues ............................................................................................................42 Reference list.............................................................................................................................................44 Appendices ...............................................................................................................................................45 Tables Table A. Flood damage trends in the Brahmaputra valley of Assam.................................................3 Table 1. States of the Northeast: Selected population and natural resource indicators...................8 Table 2. Flood damage trends in the Brahmaputra valley of Assam................................................26 Table 3. Justifiable capital expenditure for corresponding % reduction in average annual flood damage..............................................................................................................................................27 Table 4. Central flood assistance sought and received and area affected........................................28 Table 5. Northeastern Region flood management infrastructure by state.......................................30 Table 6. Quantity and estimated value of existing flood management infrastructure and related assets .................................................................................................................................................33 Figures Figure 1. Flood-affected crop area as percentage of total cropped area...........................................25 Figure 2. Flood-affected area as percentage of total area ...................................................................25 Boxes Box 1. The flood control controversy ....................................................................................................35 Box 2. Indus river basin In Pakistan: Control of flooding and erosion ............................................37 Appendices Figure A-1. Brahmaputra Plan View…………………………………………………………………..46 Figure A-2. Satellite Image of Upper Reach of Brahmaputra in Assam…………………………...47 Figure A-3. Satellite Image of Lower Brahmaputra in Assam……………………………………....48 Figure A-4. Long Section of the Brahmaputra River………………………………………………....49 Figure A-5. Selected Historical Imagery for bank line assessment………………………………....50 iii Glossary Accretion. Growth of floodplain land by deposition of river-borne sediment, often on the insides of bends (point bars) or on midchannel bars and islands. Alluvial (river or channel). Formed or flowing in mainly noncohesive deposits laid down by river processes. Amplitude (of meanders). Average distance between outer points of a meander pattern or belt. Bankfull. Condition where a river is flowing up to average top-of-bank level with negligible overbank flow. Bedload. That portion of the total sediment load that moves in contact with the bed. Bed sediment. Comprises those sizes of sediment that are normally found on or in the riverbed under low-flow conditions. Bifurcation. Division of a river from a single upstream channel into two downstream channels. Clay. Cohesive deposit of mineral origin, composed essentially of grain sizes finer than 0.002 millimeters; or its disaggregated form suspended in water. Cutoff. New channel formed naturally or artificially, cutting across the narrow neck of an enlarged or contorted meander loop. Gradient (river). Fall in elevation per unit length, as measured along the main channel or course; usually refers to a water surface corresponding to a consistent flow condition all along the river. Morphological. Relating to geometry and shape; with respect to a river, to its appearance in plan, its cross-sections, and its longitudinal profile. Node or nodal point. Location where a river channel is constrained on both sides, either