Subject: Urban floods

Why in the News

In the third week of July, several south bank rivers of Upper Assam rose with extreme speed, swallowed paddy fields and grazing lands, and swept through villages and towns in districts that had not previously faced catastrophic floods. The event exposes a conflict between two explanations of the disaster, one that treats the flood as a hydrological event to be excluded by engineering, and one that treats it as the outcome of a landscape whose capacity to absorb rain has been dismantled over two centuries.

What is embankment based flood control?

1. About: An embankment is an earthen barrier raised along a river to confine its flow within the channel and shield the settled floodplain from inundation.
2. When it was adopted in Assam: A techno bureaucratic campaign in the mid 20th century set out to shield settled floodplains from floods that had until then been treated as predictable and nourishing for agriculture.
3. The design assumption: The approach treats the flood as an external event to be kept out, rather than as the process that builds the plain it inundates.
4. The sediment consequence: These barriers interrupted the flow of sediment onto the floodplain, confining silt to the channel instead of spreading it across the fields.
5. The coverage limit: By 1988, even after thousands of kilometres of embankments had been built, two thirds of the valley still lay open to flooding.

What are the south bank tributaries of Upper Assam?

1. About: The south bank tributaries are the rivers that rise in the hills south and east of the Brahmaputra valley and join the main river from its right bank, including the Buridihing, Disang, Dikhow, Jhanji and Dhansiri.
2. Why they matter here: They are fed by rainfall over the Naga Hills and eastern Arunachal Pradesh, so their flood peaks are set by rain falling outside Assam’s own boundaries.

What is riverbed aggradation?

1. About: Aggradation is the raising of a riverbed by deposition of sediment that the river can no longer carry downstream.
2. Why it worsens flooding: A raised bed reduces the channel’s carrying capacity, so the same discharge overtops the banks at a lower volume than before.

What is a flash flood?

1. About: A flash flood is a rapid rise in water level within hours of intense rainfall, typical of steep catchments where runoff reaches the channel before it can infiltrate the soil.
2. The determining factor: The severity depends on how quickly the catchment sheds water, which is a function of forest cover and soil condition rather than rainfall volume alone.

What made the July flood different from a routine Brahmaputra flood?

1. Damage before the main river peaked: Upper Assam felt the brunt of the disaster before the Brahmaputra swelled to its highest, which rules out the main channel as the primary cause.
2. Rain fell outside Assam: Relentless rain pounded the Naga Hills and Arunachal Pradesh, soaked the slopes and unleashed sudden torrents into the southern tributaries.
3. New districts affected: Several districts once strangers to such catastrophic floods found themselves engulfed.
4. Extraction accelerated the runoff: Stone and boulder extraction from riverbeds and hillsides, common in both highlands and lowlands, was identified as a factor that hastened the rainfall’s journey downstream.
5. The regional setting: Assam is cushioned between the Eastern Himalaya to the north, the Patkai and Barail ranges to the east and the Bay of Bengal to the south, so the southwest monsoon links highlands, floodplains, billions of tonnes of sediment and the shifting channels of hundreds of rivers into a single interdependent system.

What has changed in the highland catchments that feed Upper Assam?

1. Shorter cultivation cycles: Growing populations in the highlands of Nagaland and eastern Arunachal Pradesh have made cultivation cycles shorter and more intense, cutting the fallow period that allowed soil to recover.
2. Small scale coal mining: Mining woven into local economies now competes with the highland agrarian economy and operates through complex networks of speculators.
3. Relentless logging: Continued removal of tree cover strips the canopy and root systems that slow rainfall reaching the ground.
4. Expanding infrastructure: New construction seals and compacts surfaces, adding to runoff.
5. The combined effect: This pressure on the uplands erodes the land’s ability to absorb rainfall, so a given storm now delivers more water, faster, to the tributaries below.

How did the south bank lose its historic resilience?

1. The earlier condition: The south bank districts of Upper Assam were once among the Brahmaputra valley’s most resilient regions, defined by thick forests, scattered settlements and a safe distance from the Brahmaputra’s main channel.
2. The colonial turn: The calm began to unravel in the mid 19th century, when these areas drew the attention of colonial tea planters.
3. The land use conversion: Land where rain once vanished quietly into the forest floor was transformed into plantations, sites of mineral extraction and farms.
4. The hydrological result: The converted land could no longer hold back the water, which shifted the flood response of the whole south bank.

What did the embankment campaign change?

1. The reversal of the flood’s meaning: Floods that had been seen as predictable and nourishing for agriculture were reclassified as a hazard to be excluded.
2. The sediment interruption: The barriers cut off the annual deposition of silt that had renewed floodplain fertility.
3. The incomplete shield: By 1988, thousands of kilometres of embankments still left two thirds of the valley exposed.
4. The failure mode in July: Embankments along the southern tributaries gave way before the main river crested, producing sudden breaches.
5. Why a breach is worse than a flood: The breach released a fall of water onto an unembanked plain, concentrating the discharge instead of spreading it.

How is the monsoon itself changing?

1. Greater unpredictability: India’s rainy season has become more unpredictable, marked by sudden downpours separated by long dry spells rather than simply more rain overall.
2. Corroboration for the Northeast: Studies focused on Northeast India echo these findings.
3. The explanatory shift: The gap between intense rainfall and a weakened landscape explains the flash floods more fully than the quantity of rain alone.
4. Why the distinction matters: A landscape that once absorbed a heavy monsoon now converts the same rainfall into a peak discharge, so historical rainfall thresholds no longer predict damage.

Do embankments protect the floodplain or deepen its exposure?

1. The protection is real but partial: Embankments shielded settled floodplains and made permanent cultivation and settlement possible on land that had flooded annually.
2. The cost is the sediment: The same barriers interrupted sediment flow, denying the plain the silt that renewed it and confining deposition to the channel bed.
3. Protection invites exposure: A shielded plain attracts denser settlement, which raises the population and assets at risk when a breach occurs.
4. Failure is concentrated, not gradual: An unembanked plain floods slowly and predictably, while an embanked one stays dry until the barrier gives way and then receives the full discharge at once.
5. The measure of the approach: After thousands of kilometres of construction, two thirds of the valley remained open to flooding, which shows the strategy could not be completed at the scale it assumed.
6. The deeper limitation: The state’s ecology has been treated as a puzzle for engineers, with each crisis examined in isolation, so the cumulative loss of catchment capacity is never entered into the calculation.

Challenges to flood management in Assam

1. Ageing embankments past their design life: Most of Assam’s embankment network was built decades ago and now fails at multiple points each season. e.g. the breach of the Bethukandi embankment on the Barak in June 2022, which submerged Silchar town for days.
2. Bank erosion and permanent land loss: The braided Brahmaputra shifts its channels and consumes cultivated land and villages every year. e.g. Majuli, the large river island in Assam, which has lost a substantial part of its area to erosion since the 1950s.
3. A catchment that lies outside the State’s jurisdiction: The rainfall that determines Assam’s flood peak falls in Arunachal Pradesh, Nagaland and beyond the international border. e.g. the July flood peak on the south bank tributaries generated by rain over the Naga Hills.
4. Sudden releases from upstream hydropower projects: Reservoir operation upstream can add a flood wave to an already rising river. e.g. water released from the Ranganadi project in Arunachal Pradesh flooding parts of Lakhimpur district.
5. Loss of wetlands that once absorbed flood water: The valley’s beels have been filled for construction and encroached upon. e.g. shrinkage of Deepor Beel, the Ramsar site adjoining Guwahati, which has intensified urban flooding in the city.
6. Relief centred rather than mitigation centred spending: Public expenditure concentrates on camps, compensation and post flood repair rather than catchment restoration. e.g. annual embankment repair works sanctioned after each season’s breaches rather than a basin wide restoration programme.
7. Riverbed and hillside extraction: Removal of stone, boulders and sand strips the roughness that slows runoff and destabilises slopes. e.g. boulder extraction from riverbeds in the foothills feeding the south bank tributaries.

Conclusion

The July flooding in Upper Assam is the outcome of vulnerabilities built over two centuries, not a seasonal misfortune produced by a river’s behaviour. Plantation conversion from the mid 19th century, embankment construction from the mid 20th century and current highland extraction have each reduced the landscape’s capacity to absorb rain, while the monsoon has shifted towards sudden concentrated downpours. Assam’s environment is approaching a tipping point and is not yet beyond repair. What remains unresolved is the framing itself, since the connections between highland and lowland, water and land, and forests and floodplains cannot be rebuilt by engineering alone.

“[2017, GS1, 15 marks] In what way can floods be converted into a sustainable source of irrigation and all-weather inland navigation in India?”