Nature’s Wrath - 2022 Silchar Floods, Assam

Exploring the event with Sentinel Hub EO Browser and open EO data

A guest blog post by Pawan Muddu

Foreword by Sentinel Hub

This post is part of a series of guest blog posts written by authors talking about their entries to the Sentinel Hub Custom Script Contest. The author is one of the winners of our special edition of the Contest —Climate Change— which ran from May to September 2022. The award for the second best written story went to Pawan Muddu. His winning story is available here.

Motivation

As I’m currently working for the GREd Foundation, most of my colleagues have physically faced the impact of these floods. One of our GREd Foundation students, Anup Kumar Bhattacharya, is a field officer for the revenue department of State Disaster Management in Kokrajhar, Assam, and narrated the experience of how settlements were inundated with river water. According to him and many other local experts, the monsoon set in too early and before the floods in Silchar, and many settlements were vulnerable to landslides and flooding due to the terrain in the state.

This motivated me to use the Sentinel Hub EO Browser to visualize the impact of humanitarian organizations and political institutions providing aid when it matters to the marginalized community in that part of the world.

How is Sentinel Hub EO Browser Useful

One of the best features of the EO Browser is the timelapse animation. This animation helped me to visualize impacts over time and create a visual change detection for readers to imagine the real-time implications.

The EO Browser also allows you to choose thematically suitable satellite imagery. There are many themes to filter. The theme I have shown is Floods and Droughts.

Besides, you can also visualize, compare, or create pin stories and select suitable time ranges, as shown in the image above.

Nature Wrath: 2022 Floods in Assam (Silchar and Other Areas of Assam)

Introduction

The 2022 Silchar floods occurred due to the breach of a dyke on the river Barak; the catastrophic flooding happened on 19th June 2002. It was caused due to the breach of dyke of the Barak River. The flood created havoc and affected nearly 5.4 million people across 32 districts. It also caused in deaths of over 200 people across the affected districts.

Although the Chief Minister of Assam, Himanta Biswa Sarma, claimed that it was an artificial calamity, there was no denying that looking into the precipitation levels, the role of climate change is a significant concern in this part of the world.

Geographical Extent of the Region

The Image above is the Satellite Image, an actual color composite of the Silchar town on the banks of the river Barak. Source: Sentinel Hub EO Browser

Etymologically the town comes from the Bengali word ‘Shil’ and ‘Char’, meaning “rock” and “shoreline,” respectively.

It is located between longitudes 92º24’ E and 93º15’ E and latitudes 24º22’N and 25º8’N East and is 35 meters above mean sea level.

The satellite image above, taken from EO Browser, shows the extent of the Barak River, which U-shaped meandering, also known as Dahr’s in the local language. The city is in a flat alluvial plain with swamps, streams, and isolated small hills (locally known as tilla) marking its landscape. Apart from the Barak River, the other major river is the Ghagra river. Source: Sentinel Hub EO Browser

A Note of the Barak River

The Barak flows 900 kilometers through the states of Manipur, Nagaland, Mizoram, and Assam in India, Further, it flows into Bangladesh and is known by the name of Surma and later called Meghna before receiving a combined flow of Ganga (Ganges) and the Brahmaputra. This is a U-Shaped Meandering River, as shown in the image above. An official of the Water Resources Department noted that the Brahmaputra basin is among the most flood-prone areas in the world, followed closely by the Barak. With nearly 100 tributaries and sub-tributaries feeding them, the two rivers make 40 percent of Assam vulnerable to the deluge.

This is a False Color Urban time series of the town of Silchar that shows how settlements have grown near the river’s embankments. Source: Sentinel Hub EO Browser

The Climate of Silchar

According to Koppen’s classification, Silchar has a tropical Monsoon Climate. During the cool season, the weather is generally warm and dry with cool to mild mornings; however, the “wet” season begins early as the monsoon moves into the region during April, with the result that for seven months of the year, Silchar has very hot and humid weather with heavy thunderstorms almost every afternoon until the middle of October when there is usually a brief period of hot and relatively dry weather before the cool season sets in during November.

Taken from the Indian Meteorological Department, the Average Rainfall is much higher in Silchar compared to other regions in India, as shown in the statistics above: Source: IMD | Home

What has caused the floods and their relation to climate change?

In the Northeastern Part of India, torrential rainfall set in as early as March, a month earlier than expected. Rainfall, vital for the agrarian economies of India, has been shifting since the 1950s, with longer dry spells interspersed with heavy rain; according to the Indian Institute of Tropical Meteorology, extreme rainfall events are expected to increase.

Is the Indian Ocean Dipole the problem?

The Indian Ocean Dipole is often called the Indian Nino because of its Pacific equivalent El Nino which refers to differences in sea surface temperature.

Temperatures in the eastern part of the ocean oscillate between warm and cold compared with the western part, cycling through phases referred to as “positive,” “neutral,” and “negative.”

In this image, the Negative Indian Ocean Dipole has seen stronger than the average temperature within the Bay of Bengal; Given the situation of heatwaves within South Asia, these systems are driving strong, southwesterly winds over the Bay of Bengal (Source).

The Effect of Flood Damage

During the negative phase of the Indian Ocean Dipole, warmer-than-normal sea surface conditions west of Indonesia and cooler-than-normal conditions in the western Indian Ocean alter the atmospheric circulation in the Indian Ocean region. The Northeastern Parts of India, Southeastern Asia, and Australia are wetter than normal.

The Extent of Floods and Damage it Has Caused

Villagers use a boat to move to a safer spot in Assam amid heavy floods (Source).

An aerial view shows flooded areas in Silchar in the northeastern state of Assam. Source: REUTERS

As shown in the SWIR Time series as major embankments were breached water Inundated into the Alluvial plains which affected major farmlands and Households in the adjoining villages near the town, Large-scale erosion has been reported in Baksa, Barpeta, Biswanath, Bongaigaon, Kokrajhar, Lakhimpur, Morigaon, Nagaon, Nalbari, Sonitpur, South Salmara, Tamulpur and Tinsukia districts, while Cachar and Tinsukia witnessed urban flooding. Source: Sentinel Hub EO Browser

Areas near the Barak River which is inundated from May-August due to Incessant rains. Source: Sentinel Hub EO Browser

In this time series or Urban Classified, the Area in blue depicts the intrusion of river water into settlements as shown in white dot patches. A total of 548 houses have been fully damaged, and 1,034 have been partially damaged due to the floods. Source: Sentinel Hub EO Browser

A woman uses a temporary bamboo bridge to walk to fetch water at a flood-hit village, in the Nagaon district of Assam

The previous month before the Silchar Floods affected The Districts of Nagaon and Tezpur. Five dams were breached because of these floods during May, June, and July Due to incessant rains causing floods near the Brahmaputra basin in the same state of Assam; shown here is the extent of the inundation using Sentinel 1 — SAR Satellite Orthorectified Time series Images. Source: Sentinel Hub EO Browser

Villagers use a makeshift boat to move to a safer place from a flood-affected area following heavy rainfall in the Goalpara district Source: PTI

The Effect of Floods on Nuggets

• As many as 2,675 villages in 79 revenue circles have been affected across the state, with 312,085 people have taken shelter in 569 relief camps.
• Cachar is the worst affected, with 1,430,000 people suffering, followed by Barpeta (549,000) and Nagaon (519,000).
• Five barriers have been breached, while 177 roads and five bridges have been damaged.
• Large-scale erosion has been reported in Baksa, Barpeta, Biswanath, Bongaigaon, Kokrajhar, Lakhimpur, Morigaon, Nagaon, Nalbari, Sonitpur, South Salmara, Tamulpur, and Tinsukia districts, while Cachar and Tinsukia witnessed urban flooding.
• 548 houses have been fully damaged, and 1,034 have been partially damaged due to the floods.

The Use of the NDWI Index to Determine Floods

The Normalized Difference Water Index (NDWI) highlights open water features in a satellite image, allowing a water body to “stand out” against the soil and vegetation.

McPheeters proposed the NDWI index. Its primary use today is to detect and monitor slight changes in the water content of the water bodies.

To calculate NDWI, the equation looks like this:

NDWI = (Green — NIR)/ (Green + NIR)

The visible green wavelengths maximize the typical reflectance of the water surface. The near-infrared wavelengths maximize the high reflectance of terrestrial vegetation and soil features while minimizing the low reflectance of water features.

As seen in the Image, the darker blue areas show high water content due to the flooding period in the months after May in the Silchar Area. Source: Sentinel Hub EO Browser

Visualizing NDWI: Data extracted from a satellite image using the NDWI index is visualized as maps utilizing a color palette and graphs to show how values change over time as a curve. On a map, the higher values approaching +1 usually appear blue and correspond to either high water content or a water surface.

The Image on the Left is the Image taken on 8th May 2022, and the Image on the right was taken on 26 th June 2022 post the 18th of June Floods; the image on the right shows an NDWI value of 0.8 in most of the areas affected by the floods. Source: Sentinel Hub EO Browser

Donation for a Cause

For readers of this story, it would be extended kindness if you could humbly donate to the children of the flood-affected victims in Silchar Floods through the link Help children and families affected by the Assam floods | UNICEF Global. It would be a bliss towards humanity.

I also want to thank my NGO Organization, GREd Foundation in Assam, for trying to empower students with affordable education in GIS and Earth Observation for students who cannot afford expensive geospatial training. This is our organization’s link Home — GREd Foundation.

About the Author

I am Pawan Muddu, a national of India living in the city of Hyderabad; I am currently working for GREd foundation, Guwahati, as a Senior GIS Faculty (Online), having completed my MA in Geography from SP College, Pune, and a certificate program in NRM from ITC, Netherlands. I am pursuing a dual degree program (Online) called IGEON, having worked for prominent companies like EXL, Tomtom, and CSS Corp (Apple Maps).

The Sentinel Hub team would like to thank Pawan for his participation in the Sentinel Hub Custom Script Contest.

We recommend the Sentinel Hub Educational page and the Custom Scripts webinar to learn more about satellite imagery and custom scripts. You can also visit a dedicated topic in the Sentinel Hub Forum for more information. We’d also like to invite you to take a look at the other entries submitted to the Sentinel Hub Custom Script Contests, which can be found here.

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