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Climate change and analysis of flooding in the Kosi Basin

Climate change and analysis of flooding in the Kosi Basin. Vikrant Jain Associate Professor (Earth Sciences) IIT Gandhinagar. River basin – basic unit. Climate Change Rainfall variation  Runoff generation. catchment processes & sediment transport. ? connectivity.

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Climate change and analysis of flooding in the Kosi Basin

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  1. Climate change and analysis of flooding in the Kosi Basin Vikrant Jain Associate Professor (Earth Sciences) IIT Gandhinagar

  2. River basin – basic unit Climate Change Rainfall variation  Runoff generation catchment processes & sediment transport ? connectivity Qs, Q at different reaches channel processes ? Threshold Channel shape & morphological changes habitats  river ecology Hazards due to flux variation Analysing rivers in climate change scenario river health / hazards (Jain et al., 2012)

  3. River basin – basic unit River Basin

  4. Meteorology Rainfall duration Rainfall intensity Hydrology Discharge Sediment load Hydraulics w/d ratio hydraulic radius Rating curves Glaciology Glacial cover Melting dynamics Groundwater GW-surface water relationship Geology Rock/soil type Active tectonics Ecology Biodiversity Hillslope Geomorphology Sediment supply Catchment erosion Landslides Water quality River Science Climate change Fluvial Geomorphology Drainage basin Channel morphology Sediment transport River dynamics Connectivity Anthropogenic Livelihood Population River engineering Religious rituals Geochemistry Proxy for sediment dynamics Landuse & Landcover, urbanization Soil data Remote Sensing & GIS Satellite data DEM data Statistical & Mathematical modeling Time series analysis Flood frequency analysis Process modelling Policy & Governance (Sinha, Jain & Tandon, 2012)

  5. Driving force Resisting force River system Stream power  f{Q.s} Lane, 1955; Source: Brierley & Fryirs, 2005

  6. Threshold of geomorphic change Stream Power  Amount and size of bedload  Geomorphic change and importance of threshold Threshold : the condition of significant landform change (Schumm, 1979) Ferguson, 1987

  7. Geomorphic threshold and Flooding Hazards in the Kosi Basin

  8. 18 August, 2008 ~22km wide channel 120 km The furious Kosi!! Kosi Megafan Until 18 August, 2008 • Unprecedented migration (‘avulsion’) and floods • ‘Trapped’ between the embankments • Large-scale aggradation of river bed and floodplain • Human disaster?

  9. Fluvial dynamics of the Kosi Kosi barrage (1963) Western afflux bund (1955-56) Paleochannels Eastern afflux bund (1955-56) General trend of migration (1731-2008) Causes:Aggradation, active tectonics

  10. Sa Convergence point Se Divergence point No Convergence Channel avulsion Channel Avulsion Sudden movement around a nodal point (divergence point) Occurs when an event of sufficient magnitude (usually a flood) occurs along a river that is at or near avulsion threshold Increase in Sa/Se Sa -- Slope of potential channel Se -- Slope of existing channel

  11. How and why does Avulsion occur? Avulsion ‘triggers’ • Sa/Se (or Scv/Sdv) increases (critical ratio ~3-5; ‘gradient advantage’) • Tectonic uplift (Se decreases) • Lateral tilting (Sa increases) No Avulsion Avulsion ‘Gradient advantage’ Balance between stream power and sediment supply Other causes Stream Power • Morphological changes • Base level changes • Alluvial ridge growth • River capture • Fan/delta growth • Lead to a threshold condition • Avulsion triggered by floods Sediment supply  = QS/w Wm-2 = Qs/A Sa – potential avulsion slope Se – Existing channel slope Sdv – Down valley slope Scv- cross valley slope

  12. Classification map of the Kosi River channel based on avulsion threshold index Post-avulsion Kosi Pre-avulsion Kosi (Sinha et al., 2014)

  13. Field Data

  14. Bar Surface CS4 Embankment Floodplain CS7 Embankment Bar Surface Floodplain Field survey data Elevation (m) Distance (m)

  15. Kosi Avulsion threshold index values

  16. Connectivity (hydrological) mapping (Sinha et al., 2014)

  17. Next Stage • Model the dynamics of sediment dynamics (Geomorphic connectivity) • Hillslope processes (erosion) – sediment generation • Sediment transport in downstream reaches - Stream power based modelling of channel transporting capacity

  18. D H Similar Example: Baghmati River • High sediment load • Hazard - Channel silting & flood hazard • Morphology – hyper-avulsive anabranching nature (Jain & Sinha, 2004) • Most sediments from upstream catchment (Jain & Sinha, 2004) Total Suspended Load Dheng (u/s)–10.4 MT/yr Hayaghat (d/s)–7.1 MT/yr

  19. Summary 1924 1989 2000 Dynamics of the Baghmati River Baghmati River (Jain and Sinha, 2003a, b, 2004)

  20. Avulsion Process: Reconstruction and causes 1924 1959 1986 1989 2000 B A SF Increase in Sa/Se (Jones & Schumm, 1999) Sa -- Slope of potential channel (SE flowing) Se -- Slope of existing channel (South flowing) (Jain & Sinha, 2004)

  21. Flood Hazard – Indian Scenario • Second most flood affected country in the world • total money spend = Rs. 2509 crore (1954-1989) • Total length of embankments constructed - 14,500 km. Population affected (millions) Area affected by flood Total damage (Rs. in crore) (Agarwal & Narain, 1996)

  22. Climate Change analysis and its components Hydrological Models (Spatio/temporal variation Geomorphic Model (Sensitivity/Threshold/Connectivity) Climate Models Rare events Average variability Ecological model (Habitat-ecosystem resilence)

  23. Various components are responsible for any hazards • Major challenge  Interaction of different components • Geomorphic model needs to be develop to understand the effect of climate change on earth surface • Quantitative understanding of geomorphic threshold and connectivity is required in building such geomorphic model Final message

  24. Thank You!

  25. Connectivity The way in which different landscape compartments fit together in the catchment (Brierley, et al., 2005) Why is it important? • Movement of biophysical fluxes in the dispersal system • Connectivity shapes the operation of geomorphic processes over a range of spatial and temporal scale • To predict future landscape trajectories

  26. Himalayan Hinterland WGP Himalayan Hinterland Himalayan Hinterland EGP Cratonic Hinterland LGP WGP EGP LGP Tandon et al., 2008 Cratonic Hinterland Bay of Bengal Cratonic Hinterland (Dis)Connectivity Inter-Class ConnectivityLongitudinal Connectivity Eg. Residence Time of Sediments •  106 yrs  Him-Bay of Bengal based on sediment diffusion model (Metevier & Gaudemer, 1999) • Him- Bay of Bengal  connected >100 % increase in sed rate in delta & Monsoon intensification at 11-7 ka (Goodbred & Kuehl, 2000) • 100 ka  MF-Confluence, EGP (Mt-fed rivers - Ghaghara) 160-250 ka  MF-Confluence, EGP (Foothill-fed rivers- Rapti) Based on 238U-234U-230Th disequilibria analysis (Chabaux et al., 2006) Jain & Tandon, 2010; Geomorphology

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