THE HIMALAYAN FLOODS

INTRODUCTION

• Rivers originating in the Himalayas are the lifeline of one-fifth of the global population.
• Large floods that are becoming more frequent in this region, due to rising population more frequent in this region, due to rising Population and urbanisation, are increasingly disastrous.

HIMALAYAS EXTEND AND RIVERS

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

URBANISATION

• During the last fifty years (1961-2011), the number of people living in the Himalayan region has grown from 19.9 million to 52.8 million.
• If the population keeps growing at the above same rate, it is expected to touch the mark of 260 million by 2061 (Apollo, 2017)

EXAMPLE OF FLOODS:-

• The 2010 floods of Leh
• 2013 Kedarnath floods
• 2021 Rishiganga floods

IPCC-2019 reports

• The reports of inter-government panel on climate change (IPCC-2019) indicate an overall increase in the frequency of high-intensity rainfall events in the Himalayas and this requires a careful understanding of extreme hydrological events as they interact through the variable orography and geology of the Himalayas.

Availability of Flood Data Monitoring

• The available data of flood monitoring hardly extends to one hundred years which is not enough for flood mapping and reconstruction of longer flood records in different climatic settings of the Himalayas.

FUTURE HIMALAYAN FLOOD PREDICTION

• During the upcoming 30 years, a surface warming trend of Himalaya is predicted, where it is suggested that regions above 2000m asl (above sea level) will witness a higher rate of surface warming.

This temperature rise will increase the available atmospheric energy and total precipitation, which in combination with mountain fragility and the growing urban centres is a perfect condition for disasters.

HIMALAYAN MOUNTAIN BELT

• The Himalayan Mountain Belt, tectonically, is divisible into from north and South as follows:

1. The Indus suture Zone (ISZ) of LADAKH
2. The Tethyan Himalaya
3. The Higher Himalayan Crystalline Zone,
4. The Lesser Himalaya
5. The Siwalik of outer HImalaya

(i)  The Indus Suture zone:-

• The ISZ lies in the rain shadow zone of the Indian Summer Monsoon (ISM), above an elevation of almost 3000m asl where the scanty rainfall (~100mm/years) occurs mostly under the influence of westeries.
• The flood in these rivers are normally induced by the interplay of ISM and upper atmospheric interactions.

(ii) The Tethyan Himalayan Belt : The Tethyan 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.

(iii) The Higher Himalayan Crystalline zone lies above an elevation of ~1800m asl and receives full spectrum of ISM rainfall.

(iv) The Lesser Himalaya and outer siwalik Himalaya are gentler and also receive a high amount of ISM rainfall.

CAUSES OF FLOODS IN THE HIMALAYAS

• The large floods in the Himalayas are caused by:

(i)  Intense rainfall events

(ii) Landslide dammed lake outbursts (LLOFs)

(iii) Glacial dammed Lake outbursts(GLOFs)

(iv) Cloud bursts

Intense rainfall events : During warmer and strengthened  monsoon years, the southern front of the Himalayas receives longer spell of precipitation that cause large floods that may last for almost full peak in the monsoon season.

Glacial Lake Outburst Floods (GLOFs): GLOFs are generated by the breach of water bodies that are formed due to damming of streams by surging or advancing glaciers or by impounding hill-slope runoff and snowmelt between the two moraine ridges (called moraine-dammed lake)

Example:

• Shoyk river in the Himalayan-Karakoram region (1779 and 1932)
• 2013 Kedarnath Incident in Garhwal Himalayas
• Landslide Lake outburst floods (LLOFs): LLOFs are analogous, with the dams being formed by landslides. Landslide activity that generally occurs during monsoon or an earthquake may potentially dam small channels for a longer duration.

EFFECT OF OROGRAPHY AND GEOLOGY OF THE HIMALAYAS

• The orography and pattern of rainfall distribution decide the damage pattern and hot spots of erosion in the event of large floods.
• Another Important thing to remember is that the headwaters of the rivers like Indus, the Ganga, and the rivers draining the central Himalayan range lie in rain deficient arid zones where extreme rainfall events can potentially create glacial/moraine-dammed lake outbursts, and massive snowmelt leading to a flood.

SUMMARY OF CAUSES OF FLOOD

• The Floods in the Himalayas are common and are caused by a combination of natural surface processes and rainfall distribution.
• The magnitude of flood is a function of overall:
• Geology
• Orography
• Catchment wide distribution of lakes
• Landslide zone
• Rainfall

FLOOD MAPPING

• Mapping of floods has four elements:

(i)  Vertical rise in river level

(ii) Rate of rising of flood

(iii) Flow velocity

(iv) Lateral inundation of floods plain

• This rate of rise in flood is a function of how the drainage network efficiently delivers surface runoff to the channel
• This will factor in parameters like:-
• Infiltration
• Drainage density
• Vegetation cover

MEASUREMENT EQUIPMENT:-

1. Vertical rise in river level :

• It is normally done at river gauzing stations.
• Gauzing stations can now be equipped with state-of-the-art Internet of Things (IoT) and radars.

2. Rate of rising of flood :

• It can be precisely measured using Differential Global Positioning System (DGPS)

3. Flow Velocity :

• The Flood Velocities are generally measured by using current meters, acoustic Doppler current profilers, tracers and floaters . The most precise would be Doppler current profiler as it can measure flow velocity at different depths and can average out the turbulence.

4. Lateral inundation of flood plains:

• It can be mapped using satellite images and LiDAR (Light Detection and Ranging) data.

5. Sediment Load of flood waters :

• It can be measured by sediment monitoring gauges or sensors equipped with laser in-situ Scattering and Tranmissometry.

REDUCING FLOOD VULNERABILITY IN THE HIMALAYAS

• The floods are natural processes and are inevitable, though the damage done by these events can be minimsied if attempted using scientifically sound databases and models.
• Proper understanding of the orography of the Himalayas and how past flood events have interacted with it and what were the damage patterns can help in preparing the damage predictive models of the Himalayas.
• The above model can help in deciding the focus, magnitude, and type of infrastructural development to be done in the Himalayas
• The foremost towards this is monitoring at various levels like:

(i)   Installing a dense network of flood gauging systems and Radar.

(ii)  Channelling all the real time data to flood management centers using IoT.

(iii) Longer time series of data archival using large monitoring networks, historical and geological archieves ought to be prepared.

(iv) Landslide and glacial Lake monitoring systems should be in place.

(v)  The Combination of data on flood levels, flood hydrographs, and lateral inundation can be used to manage the floods and minimise the destruction.