- 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
- 2010, floods of Leh
- 2013, Kedarnath floods
- 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
- Indus Suture Zone (ISZ) of Ladakh
- Tethyan Himalaya
- Higher Himalayan Crystalline zone
- Lesser Himalaya
- 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.
In general, the large floods in the Himalayas are caused by:
- Climate change: Intense rainfall events, landslide dammed lake outbursts, glacial dammed Lake outbursts, cloud bursts
- Topography: The magnitude of flood is a function of overall geology, orography, catchment-wide distribution of lakes, landslide zone, and rainfall
- 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
- Intense rainfall events
- Landslide dammed lake outbursts (LLOFs)
- Glacial dammed Lake outbursts (GLOFs)
- Cloud bursts
Mapping of floods has four elements:
- Vertical rise in river level
- Rate of rising of flood
- Flow velocity
- 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.