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[Yojana Archive] Himalayan Floods

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October 2021

Background

  • Over the last few decades, urbanization has led to a dramatic increase in the population living in the Himalayan Mountain belt.

About the Himalayas

  • The Himalayas extend for about 2400 km from west to east with width varying between 200-400 km.
  • The two syntaxes of this mountain are drained by the rivers Indus (west) and the Brahmaputra (east).
  • The Ganga River system largely drains the central part of the Himalayas.

Recent flood events

  1. 2010, floods of Leh
  2. 2013, Kedarnath floods
  3. 2021, Rishiganga floods

The reports of the intergovernmental panel on climate change (IPCC-2019) indicate an overall increase in the frequency of high-intensity rainfall events in the Himalayas.

What makes Himalayas more prone to disasters?

  • Rise in atmospheric temperature further increases the available atmospheric energy and total precipitation.
  • This in combination with mountain fragility and the growing urban centers is a perfect condition for disasters

Himalayas & Flood

The Himalayan Mountain belt, tectonically, is divisible into from north and south in the following zones

  1. Indus Suture Zone (ISZ) of Ladakh
  2. Tethyan Himalaya
  3. Higher Himalayan Crystalline zone
  4. Lesser Himalaya
  5. Outer Siwalik

(1) Indus Suture Zone (ISZ) of Ladakh

  • The ISZ lies in the rain shadow zone of the Indian Summer Monsoon (ISM), where the scanty rainfall occurs mostly under the influence of westerlies.
  • The area is devoid of vegetation and due to extreme temperatures, physical weathering of rocks occurs that forms a thick debris mantle on hill slopes.
  • This debris mantle during excessive snow melting and rainstorm events, fail and block small and large streams that breach subsequently to create floods.

(2) Tethyan Himalaya

  • This belt also behaves in a similar manner even though it lies at the northern fringe of ISM and receives a slightly higher amount of rainfall as compared to ISZ.

(3) Higher Himalayan Crystalline zone

  • The higher Himalayan Crystalline zone receives full spectrum of ISM rainfall.
  • This zone is characterized by steep hill slopes and deep gorges with high gradient drainage systems.

(4, 5) Lesser Himalaya and Outer Siwalik

  • The Lesser Himalaya and outer Siwalik Himalaya are gentler and also receive a high amount of ISM rainfall.

Differential rainfall patterns

  • The headwaters of the rivers like Indus, the Ganga, and the rivers draining the central Himalayan ranges lie in rain deficient arid zones.
  • Here extreme rainfall events can potentially create glacial/moraine-dammed lake outbursts, and massive snowmelt, leading to a flood.
  • However, the headwater of the Brahmaputra, due to different orography and elevation receives less rainfall and downstream catchment receives higher rainfall.
  • These constraining characteristics between the two systems create floods that have discharges of different magnitude.

Factors influencing

In general, the large floods in the Himalayas are caused by:

  1. Climate change: Intense rainfall events, landslide dammed lake outbursts, glacial dammed Lake outbursts, cloud bursts
  2. Topography: The magnitude of flood is a function of overall geology, orography, catchment-wide distribution of lakes, landslide zone, and rainfall

Example:

  • 2013 Kedarnath incident in the Garhwal Himalayas: Widespread rainfall in combination of a breach of a moraine-dammed lake in the Chaurabari glacial region was responsible.
  • Breach of Gohna Tal of Birahi Ganga in 1970: Landslide activity that generally occurs during monsoon or an earthquake may potentially dam small channels for a longer duration. These dams may breach and cause floods in the downstream regions.

Causes of Floods in the Himalayas

In general, the large floods in the Himalayas are caused by

  1. Intense rainfall events
  2. Landslide dammed lake outbursts (LLOFs)
  3. Glacial dammed Lake outbursts (GLOFs)
  4. Cloud bursts

Flood Mapping

Mapping of floods has four elements:

  1. Vertical rise in river level
  2. Rate of rising of flood
  3. Flow velocity
  4. Lateral inundation of flood plains
  • Flood mapping is normally done at river gauging stations which are equipped with state-of-the-art Internet of Things (IoT) and radars.
  • Radars can help in tacking the locations of intense rainfalls and the temporal evolution of the storm.
  • The lateral extent of flood inundation can be mapped using satellite images and LiDAR (Light Detection and Ranging) data.
  • The flood velocities are generally measured using current meters, acoustic Doppler current profilers, tracers, and floaters.
  • Sediment load of floodwaters can be measured by sediment monitoring gauges or sensors equipped with Laser In-Situ Scattering or by physically sampling during the time of the flood.

Reducing Flood Vulnerability in the Himalayas

  • Proper understanding of the orography of the Himalayas, past flood events and the damage patterns can help in preparing the damage predictive models of the Himalayas.
  • The foremost way towards this is monitoring at various levels.
  • Landslide and glacial lake monitoring systems should be in place and be linked to flood management centres via IoT.
  •  The combination of data on flood levels, flood hydrographs, and lateral inundation can be used to manage the floods and minimise the destruction.
  • The inundation maps combined with maps of social infrastructure, should be analysed on GIS platforms and AI using long time series of datasets.
  • It will provide predictive models of flood events and damage patterns.
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