Flood in Nagaon district in central Assam in June, 2025.
Credit: DH Photo/Sumir Karmakar
Guwahati: The floods that Assam witnesses almost every year inundated between 50 per cent to 80 per cent areas of the state's 10 districts in the past 26 years, between 1998 and 2023, says the latest edition of the Flood Hazard Zonation Atlas.
The Atlas, which was prepared based on satellite datasets of floods in the past 26 years, found that cumulative flood inundation affected 70 per cent to 80 per cent of geographical areas of Bajali and Morigaon districts, while the same was found between 50 per cent to 70 per cent in Sivasagar, Nagaon, South Salmara, Lakhimpur, Nalbari, Dhemaji, Barpeta and Hojai.
Further, 19 districts witnessed flood impacts affecting 30 per cent to 50 per cent of their geographical area, while the remaining seven districts experienced less than 30 per cent being flood-affected, it said. Assam at present has 35 districts.
It said satellite-derived cumulative flood inundation data showed that the total area affected by flooding in the state were estimated at 27.08 lakh hectares. Of this, 3.26 lakh hectares had witnessed very high flood, while another 2.86 lakh hectares faced high floods, said the Atlas.
"The flood inundation frequency has been computed using a spatial resolution of 50m x 50m grids, derived from the historical data and temporal-spatial analysis for each pixel," It said.
The Atlas is designed to identify the areas most vulnerable to flooding and determining the frequency of inundation. It provides planners with a comprehensive understanding of the severity and frequency of flooding in various regions, which can aid in developing effective strategies to mitigate the problem and reduce the impact.
Home Minister Amit Shah released the Flood Hazard Zonation Atlas in New Delhi Monday. It has been prepared by the National Remote Sensing Centre, Indian Space Research Organisation in association with National Disaster Management Authority and Assam State Disaster Management Authority.
The inundation extent
The Atlas found that the extent of flood inundation varied significantly, ranging from 2.11 lakh hectares in 2009 to 12.85 lakh hectares in 2020, indicating the severity of the floods and their impact on the region over the years. These findings provide valuable insights for policymakers and stakeholders to take necessary measures to mitigate the effects of floods and protect the region from future inundation, it said.
Flood inundation of areas between 10 to 12.85 lakh hectares were observed during four years: 2002, 2017, 2020, and 2022. Areas between 8 to 10 lakh hectares were inundated in six years: 1998, 1999, 2003, 2004, 2007, and 2019. Further, flood inundation between 5 to 8 lakh hectares occurred in eight years: 2000, 2008, 2012, 2015, 2016, 2018, 2021, and 2023. Lastly, flood-affected areas ranging from 2–5 lakh ha were recorded in eight years: 2001, 2005, 2006, 2009, 2010, 2011, 2013, and 2014, it said.
Flood in Nagaon district in central Assam in June, 2025.
Credit: DH Photo/Sumir Karmakar
Population affected
The Atlas said population affected by floods from 2010 to 2020 ranged between seven to 70 lakh in different years. In 2017, 2019 and 2022, the affected population was more than 40 lakh annually, it said.
Annual problem
Flood is an annual problem in Assam and 40 per cent of its total geographical areas are flood-prone. The annual deluge cause deaths, destruction of crops and infrastructure. Landslides and river bank erosions triggered by flood have also become serious concerns over the years. This year, the flood and landslides have killed 28 people (22 in floods, six in landslides) and affected over six lakh people between May 30 and June 6.
Dam-induced flooding
The Atlas said that Assam also witnesses flash floods caused by the release of water from dams upstream in Bhutan and neighbouring states in the northeast.
Release of water from Doyang (Nagaland), Ranganadi (Arunachal Pradesh), and dams in Bhutan causes large-scale inundation in Assam. The Ranganadi dam has been the main cause of flash flooding in and around Lakhimpur district, adversely affecting the environment and socio-economic conditions of its people.
In July 2017, water was released from the Ranganadi dam without notice, catching the district administration and 300,000 people off-guard. Flood water damaged drainage channels under National Highway 15 between Lakhimpur and Dhemaji and washed away a 50-metre embankment stretch at Bogalijan in Lakhimpur.
"Dam-induced flooding is caused mainly because of human error. Timely communication by dam officials, early warning, and emergency evacuation, could save precious lives, and property," said the Atlas.
This year too, the water released from the Ranganadi dam inundated large areas in Lakhimpur and caused heavy devastation.