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Rivers originating in the Himalayas are the lifeline for one-fifth of the global population. 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. Floods in the Himalayas are part of natural processes and are inevitable. However large Himalayan floods, like those in Leh (2010), Kedarnath (2013) and Rishiganga (2021), are becoming more frequent.
What are the reasons for an increase in the Himalayan floods?
Increase in population: During the past fifty years (1961 -2011), the number of people living in the Himalayan region has grown from 19.9 to 52.8 million. This has resulted in growing urban centres.
Intense rainfall events and Cloudbursts: The rise in surface temperature is increasing the availability of atmospheric energy and total precipitation. The reports of the Inter-governmental Panel on Climate Change, indicate an overall increase in the frequency of high-intensity rainfall events in the Himalayas.
Note: High-intensity rainfall events are extreme amounts of precipitation in a short period of time.
Glacial dammed Lake outbursts (GLOFs): Retreating glaciers, usually result in the formation of lakes at their tips. These lakes are called proglacial lakes. These proglacial lakes are often bound by sediments, boulders, and moraines. If the boundaries of these lakes are breached, then flooding will take place downstream of that glacial lake.
|Read more: Glacial Lake Outburst Flood (GLOF) in Uttarakhand -Explained
Shyok river in the Himalayan-Karakoram region frequently witnesses such GLOFs. The 1779 and 1932 events are well documented.
The 2013 Kedarnath incident in the Garhwal Himalayas, besides widespread rainfall, was compounded by a breach of a moraine-dammed lake in the Chaurabari glacial region.
Landslide dammed lake outbursts (LLOFs): These floods occur due to breach of dammed lakes which form as a result of obstruction of water flow of river by debris of landslides. Landslides may occur due to rainfall, seismic activity like earthquakes, etc.
Small channels may take a long time before causing floods. For instance, landslide dammed lake (Gohana Tal) of Birahi Ganga river, survived for 76 years and caused floods in 1970. It devastated the town of Srinagar (Garhwal), and significantly damaged the Ganga canal downstream in Haridwar.
Sutlej river valley (Himachal Himalayas) also witnessed massive devastation due to LLOFs in the years 2000 and 2005.
What are the challenges in forecasting the Himalayan floods?
Lack of flood data: India does not have flood level observation beyond hundred years. This is not enough to understand the long-term variability of floods and the forcing factors behind large events.
Challenges in determining the rate of rising in flood: Regional high-intensity rainfall is generally followed by gradual rise of waters which makes flood prediction reliable. On the other hand, the events like GLOFs and LLOFs will induce a faster rate of rise in water levels which makes it hard to predict sudden floods.
What can be done to mitigate the Himalayan floods?
Use of technology: The rate of rising of flood and flow velocity can be measured effectively by Differential Global Positioning Systems (DGPS), Artificial Intelligence (AI), and LiDAR (Light Detection and Ranging) technologies. So, the government has to install a dense network of flood gauging systems.
Similarly, state-of-the-art Internet of Things (loT) and radars can be used to quickly disseminate the data to remote locations and flood management centres.
|Read more: Govt launches LiDAR survey reports to augment water in forest areas
Create Slack Water Deposits (SWDs) for data: SWDs are couplets of sand and silt which represent earlier flood events. These are deposited at several locations along the river channel during a flood event. A study of SWDs can help long term trend of flood events (beyond 100 years) which can help understand long-term variability of floods and forcing factors behind large scale flood events. The government has to analyse these SWDs and create a repository of data.
Note: At present, a study is exploring SWDs in the Indus, the Sutlej, the Ganga, and the Brahmaputra river to understand the history of floods.
Damage Predictive Models for the Himalayas: Proper understanding of the orography of the Himalayas and the past flood events has to be used to prepare damage prediction models. Further, the model should include Landslide and glacial lake monitoring systems. This can help in deciding the focus, magnitude, and type of infrastructural development to be done in the Himalayas.