Floods: A Natural Hazard - Clarity Desk Hub IAS

Let’s begin with a simple definition.

A flood occurs when water overflows its normal limits — whether it’s a river, lake, or sea — and inundates adjoining land areas.

In simple terms, a flood is when too much water appears at the wrong place and time.

India is highly vulnerable to floods.
According to Rashtriya Barh Ayog (RBA), over 40 million hectares — that’s nearly 12% of India’s land area — is flood-prone.

⚙️ Causes of Floods

Floods can be caused by natural as well as human (anthropogenic) factors.

Let’s study both👇

A. Natural Causes

1️⃣ Heavy Rainfall

The most common reason — when rainfall in a river’s catchment area exceeds its capacity.
Rivers overflow their banks, flooding nearby land.
In the Himalayas, sudden cloudbursts worsen this situation.

💡 Example: Uttarakhand 2013 flash floods (Kedarnath).

2️⃣ Glacial Lake Outburst Flood (GLOF)

Sometimes, glaciers form lakes dammed by ice or debris.
When these natural dams break suddenly due to melting or collapse, they release large volumes of water abruptly.
This causes catastrophic flash floods downstream.

💡 Example: Sikkim GLOF, 2023.

3️⃣ Sediment Deposition

Rivers carry silt and sand.
Over time, this reduces their depth and carrying capacity, leading to overflow during heavy rains.

4️⃣ Cyclones

Cyclones push sea water inland, creating storm surges and coastal flooding.

💡 Example: 1999 Odisha Super Cyclone — major flooding in coastal districts.

5️⃣ Change in River Course

Natural meandering, erosion of banks, or landslides can divert rivers, flooding new areas.

6️⃣ Tsunami

A tsunami can submerge large coastal areas due to the sudden rise in sea level.

7️⃣ Disappearance of Natural Lakes/Wetlands

Lakes and wetlands act as natural sponges — they store excess rainwater.
Shrinking or encroaching upon them increases flood frequency.

B. Anthropogenic (Human) Causes

1️⃣ Deforestation

Trees absorb and slow down surface runoff.
When forests are cleared, rainwater flows rapidly into rivers → sudden increase in discharge → floods.

2️⃣ Unplanned Urbanization

Cities replace soil with concrete and asphalt, which block percolation.
Old drainage systems can’t handle intense rain → urban flooding.

💡 Example: Mumbai floods (2005), Chennai floods (2015).

3️⃣ Encroachment on Natural Drainage Channels

Construction over natural drains, riverbeds, and floodplains disrupts water flow, causing stagnation and flooding.

4️⃣ Poor Water and Sewage Management

Old, inadequate drainage and sewage systems collapse during monsoon — seen across major Indian cities.

5️⃣ International Dimension

Rivers originating in Nepal, Bhutan, and China (like Kosi, Gandak, Brahmaputra) bring floods to U.P., Bihar, Assam, and W.B.
So, transboundary flood management cooperation is essential.

6️⃣ Population Pressure and Over-Exploitation

Overgrazing, land encroachment, soil erosion, and excessive cultivation increase runoff and sedimentation — worsening floods.

🇮🇳 Flood Risks in India

Floods occur in almost all major river basins.
India receives ~1200 mm annual rainfall, and 85% of it comes during June–September.
Rivers exceed their capacity, leading to mild or severe flood situations.

🗺️ Regional Distribution Pattern of Flood-Prone Areas

Let’s look at the major river regions affected by floods 👇

1️⃣ Brahmaputra River Region

States: Assam, Arunachal Pradesh, Meghalaya, Mizoram, Tripura, Nagaland, Sikkim, North Bengal.

Causes:

Heavy monsoon rainfall in catchments.
High silt load from fragile Himalayan slopes.
Frequent earthquakes → landslides → river blockages.
Cloudbursts and flash floods are common.

💡 Result: Assam and Arunachal face annual floods with huge displacement.

2️⃣ Ganga River Region

States: Uttarakhand, U.P., Bihar, Jharkhand, W.B., Punjab, Haryana, Himachal Pradesh, Rajasthan, M.P., Delhi.

Causes:

Floods mainly on northern tributaries (Gandak, Ghaghra, Kosi, etc.).
Problem increases from west to east.
Massive encroachment on floodplains for settlements and agriculture.

💡 Note: The Kosi River is called “The Sorrow of Bihar.”

3️⃣ North-West River Region

Rivers: Indus, Jhelum, Chenab, Ravi, Beas, Sutlej.
States: J&K, Punjab, parts of Haryana, Himachal, Rajasthan.

Causes:

Less rainfall, but problem of poor drainage and waterlogging due to irrigation canals.
Rivers often change course leaving sandy wastelands.

4️⃣ Central & Deccan Region

Rivers: Narmada, Tapi, Mahanadi, Godavari, Krishna, Cauvery.

Features:

Generally stable courses and well-defined banks.
Floods confined mostly to deltaic and coastal areas, especially in Odisha (Mahanadi, Subarnarekha, Brahmani).
Cyclones worsen coastal flooding.

⚠️ Consequences of Floods

1️⃣ Economic Losses

Crops, houses, roads, bridges, and railways damaged.

2️⃣ Displacement and Casualties

Millions rendered homeless; livestock lost.

3️⃣ Health Hazards

Spread of water-borne diseases (cholera, hepatitis, gastroenteritis).

4️⃣ Environmental Impact

Soil erosion, siltation, and habitat loss.

5️⃣ Positive Impact (Yes!)

Floods deposit nutrient-rich alluvium, which rejuvenates agricultural soil fertility.

🧱 NDMA Guidelines on Flood Management

The National Disaster Management Authority (NDMA) recommends a comprehensive flood management framework that combines both structural and non-structural measures to mitigate flood risks and minimize losses.

1️⃣ Structural Measures

These are engineering and physical interventions designed to control or regulate floodwaters.

a) Reservoirs, Dams, and Water Storage

Constructed to store excess runoff and regulate downstream flow.
However, large projects like the Damodar Valley Project show that structural measures alone cannot completely prevent floods.

b) Embankments, Levees, and Flood Walls

Barriers built along rivers to contain rising water within the channel.
Example: Yamuna embankments near Delhi have helped protect adjoining settlements.

c) Drainage Improvement

Restoring and maintaining natural drainage systems obstructed by infrastructure (roads, canals, railways) to enable smooth runoff.

d) Channel Improvement / Desilting / Dredging

Deepening and widening river channels to increase their carrying capacity.
Particularly effective near river confluences and outfalls.

e) Diversion of Floodwaters

Redirecting excess water through spill channels or alternate routes.
Examples: Flood Spill Channel around Srinagar, Supplementary Drain in Delhi.

f) Catchment Area Treatment and Afforestation

Watershed management through soil conservation, tree plantation, contour bunding, and check dams.
Reduces runoff velocity and flood peaks.

g) Flood Peak Control (Detention Basins and Dams)

Use of dams, reservoirs, marshlands, and abandoned quarries as detention basins to temporarily store floodwater and release it gradually.

h) Channel Management

Stream channelization through canal systems to direct and distribute water.
Bank stabilization using dykes, spurs, and deep-rooted vegetation to prevent erosion.
🩺 Think of this as unclogging the river’s arteries to prevent a “flood heart attack.”

i) Flood Diversion and Utilization

Diverting overflow into low-lying depressions, wetlands, or agricultural fields for temporary storage.
Example: Ghaggar Reversion Scheme uses flood diversion for controlled inundation.

2️⃣ Non-Structural Measures

These involve policies, planning, forecasting, and community-based initiatives to reduce vulnerability without physically altering the river course.

a) Floodplain Zoning

Regulates land use in flood-prone areas through scientific mapping.
Only low-risk activities (like farming, recreation) should occur in floodplains, while settlements and critical infrastructure remain outside danger zones.

b) Flood-Proofing

Raising buildings and utilities above maximum flood level.
Constructing multi-storey shelters for humans and livestock.
Elevating drinking water sources to prevent contamination.

c) Flood Management Plans (FMPs)

Each department — Irrigation, Transport, Health, Power, etc. — must prepare its flood-specific plan outlining preparedness, mitigation, and response roles.

d) Integrated Water Resources Management (IWRM)

Adopts a basin-level approach for managing floods, irrigation, and conservation collectively — ensuring harmony between development and safety.

e) Flood Forecasting and Early Warning

Real-time monitoring by:
- Central Water Commission (CWC) — for river discharge and flood forecasting.
- India Meteorological Department (IMD) — for rainfall prediction and cyclone tracking.
Timely alerts help prevent loss of life and property.

f) Reducing Surface Runoff (Infiltration-Based Methods)

Afforestation: Trees slow surface flow and aid groundwater recharge.
Recharge Wells: Facilitate percolation of rainwater.
Biopore Infiltration Method:
- Small vertical holes filled with biodegradable waste.
- Microbial action creates natural infiltration tunnels.
- Useful in hard-soil areas (e.g. Northeast India).
  💡 Think of it as the Earth “digesting” water slowly like our body digests food.

🚨 Aapda Mitra Scheme

A Centrally Sponsored Scheme under NDMA.
Objective: Train community volunteers in the 30 most flood-prone districts across 25 states.
Aim: Equip locals to perform search, rescue, and relief operations before formal agencies arrive.

💡 Tagline essence: “Community is the first responder.”

Real-world Examples

Let’s look at some case studies to understand how both nature and human folly worsen floods.

Kerala Floods (2018)

Worst in 100 years
Excess rainfall + human errors
- Unchecked quarrying, illegal construction, deforestation
- Encroachment in dam catchments (e.g., Idukki dam)
- Poor reservoir management before monsoon
- Absence of a dedicated State Disaster Response Force

Other Examples

Bihar & Surat (2006)

Drainage failure and poor dam management led to prolonged inundation.

Uttarakhand (2013)

375% excess rainfall, glacier lake outburst, fragile construction in hilly terrain, and ignored scientific warnings.

Chennai (2015)

Torrential rains + dam mismanagement.
CAG report blamed lack of flood forecasting and reservoir safety violations.

Mumbai (Recurring)

Encroached Mithi River, airport built on reclaimed land, distorted river flow, rising sea levels, and Bandra-Worli Sea Link altering river mouth flow – all contributing to severe urban flood vulnerability.

Recent Major Floods

Assam Floods (2022 & 2023)

Millions affected across districts like Cachar, Darrang, and Nagaon.
River Brahmaputra breached danger levels.
Major causes: Embankment breaches, intense monsoon rains, deforestation in catchment areas, and poor drainage in urban clusters.

Delhi Floods (2023)

Yamuna river crossed danger mark after record rains in Delhi and heavy releases from Hathnikund Barrage.
Areas near ISBT, ITO, and Rajghat submerged.
Urban encroachment on Yamuna floodplain + inadequate drainage infrastructure worsened the crisis.

Himachal Pradesh Floods (2023)

Cloudbursts and landslides caused massive damage across Kullu, Mandi, and Shimla.
Infrastructure collapse (roads, bridges) due to unchecked hillside construction.
Loss of lives, crops, and tourism infrastructure.

Sikkim Glacial Lake Outburst Flood (GLOF), 2023

South Lhonak Lake burst caused flash floods along Teesta River.
Chungthang Dam was overwhelmed, submerging entire stretches downstream.
Highlights climate change impact on Himalayan glacial systems + inadequate risk mapping.

Urban Floods

What is an urban flood?

Urban flooding is not merely “a flood that happens in a town.”
It is flooding caused by excessive runoff in urban areas because the drainage system is overburdened and because of unregulated construction that ignores natural drainage and topography. In short: cities produce water faster and trap it worse.

How is urban flooding different from riverine flooding?

Ask yourself: how quickly does water rise in a city vs in a rural floodplain?

Faster onset. Urbanisation makes catchments “developed” (impervious surfaces), increasing flood peaks 1.8 to 8 times and flood volumes up to 6 times. So floods occur in minutes, not hours.
Higher vulnerability. Dense population + critical infrastructure (transport, power, markets) = larger human & economic exposure.
Secondary risks (disease, interruption of services) are greater and spread faster.

Causes of urban floods

A. Meteorological factors

Southwest Monsoon: localized, heavy rainfall in 3–4 months (e.g., Mumbai 2005).
Northeast Monsoon: Oct–Dec cyclones and heavy rains (e.g., Chennai 2015).
Depressions & Cyclones: bring heavy rain + storm surge (coastal urban flooding).
Thunderstorms / Cloudbursts: intense, highly-localized rainfall — common in hills.
Western disturbances: cause extra-tropical rain in north/west.

B. Anthropogenic (human) factors

Surface sealing (roads, pavements, roofs) — reduces infiltration and speeds runoff.
Occupation of floodplains and blocking natural channels — obstructs flow.
Improper solid waste disposal / dumping of construction debris — clogs drains, increases hydraulic roughness and local waterlogging.
Urban Heat Island effect — can intensify localized convection and rainfall.
Unregulated release from upstream reservoirs — sudden discharges can spike city flood peaks.
Large gatherings / concretization (e.g., poorly managed religious events) that alter river channels.

C. Hydrological factors

Synchronization of runoff from different subcatchments (simultaneous peaks).
Presence of impervious cover and prior soil moisture / groundwater levels.
High tide that impedes drainage of stormwater into the sea.
Channel cross section & roughness — determine conveyance capacity.

Urban flood risks in India

Cities like Mumbai (2005, 2017), Chennai (2015), Surat (2006), Hyderabad (2020) and Delhi (2023) exemplify growing urban flood vulnerability. Reasons: explosive urban growth, aging drains, informal settlements on low land, and climate variability.

NDMA guidance

1. Early Warning & Communication

Integrate National Hydro-meteorological Network and Doppler Weather Radars with city planning.
Use automated rain gauges and nowcasting (ultra short-term forecast: 5–30 min) to generate actionable lead time (typically 3–6 hours).
Once forecasts trigger thresholds, execute flood management plans and communicate via official channels.

2. Vulnerability Analysis & Risk Assessment

Hazard Risk Zoning of urban areas: map risks, classify structures by function (hospitals, transport hubs, schools), estimate losses — feed this into evacuation and continuity planning.

3. Strengthening Governance & Decision Support

Establish a Decision Support System (DSS) at ULB level for real-time hazard mapping and warnings.
Official warnings must be channelled via government authorities to avoid panic/misinformation.

4. Design & Management of Urban Drainage (technical backbone)

Key actions for drainage design:

Use catchment boundaries (watershed delineation) rather than administrative lines.
Maintain accurate contour and elevation data for flow directions.
Compute design flow (peak flow rates) for sizing drains and detention facilities.
Maintain drain inlet connectivity — road drains must connect to roadside drains properly.
Remove solid waste rigorously — prevent drain blockages.
Consider nature-based solutions like rain gardens (porous beds that capture, pond and infiltrate runoff).

5. Institutional mechanism — Urban Flooding Cell (UFC)

A dedicated UFC within MoUD (and local units in ULBs) to coordinate Urban Flood Disaster Management (UFDM).

6. Response & Recovery

Emergency Operation Centres, Incident Response System, flood shelters, search & rescue, emergency logistics.
Special focus on sanitation and public health (prevent malaria, dengue, cholera).

7. Investment in Resilience

Capacity development, community training, documentation, participatory planning with citizens and stakeholders.

Practical non-structural measures

Floodplain zoning and strict land-use regulation.
Flood-proofing: raised platforms, elevated utilities, double-storey living with ground floor as sacrificial space.
Integrated Water Resources Management (IWRM) at watershed scale.
Regular drills and community volunteer training (Aapda Mitra-like models).

Key implementation challenges

Poor or no comprehensive urban flood risk assessments.
Lack of city-specific mapping and integration into masterplans.
Fragmented institutional responsibilities — poor coordination among agencies.
Data gaps and poor information sharing.
Ad hoc investment decisions without stakeholder consultation.
Weak enforcement of drainage protection and anti-encroachment rules.