FLOODS, URBAN FLOODS (FLASH FLOOD) AND GLOF

Published on 23 Feb 2025

Flood is the overflow of water onto normally dry land, often due to heavy rainfall, storm surges, or river overflow, causing inundation. According to NDMA about 12% (over 40 million hectares) of India’s land is prone to floods and river erosion. 

Flash floods and Glacial Lake Outburst Floods (GLOF) are two special types of floods.

• Sudden and intense rainfall, often occurring within a short period, can result in flash floods. These floods are particularly dangerous due to their rapid onset and high flow rates. 

• GLOF occurs when a glacial lake's dam or moraine (a ridge of glacial debris) holding the lake's water breaks or collapses.

Vulnerability of India

• Monsoon Climate: India's heavy dependence on the monsoon season for water supply and agriculture exposes the country to the risk of excessive rainfall and flooding during the monsoon months

• High Population Density: Many people live in flood-prone areas, which increases the potential impact of floods on lives, property, and infrastructure.

• Unplanned urbanization: Rapid urbanization often leads to the construction of buildings, roads, and other infrastructure on natural floodplains and wetlands. This reduces the land's ability to absorb water, leading to urban flooding.

• Example: Bengaluru flood, 2022.

• Climate Change: Rising global temperatures and changing precipitation patterns are contributing to more frequent and intense rainfall events, which can lead to flash floods and river flooding. Also melting of Himalayan glaciers would trigger more GLOFs.

• Deforestation: Deforestation and poor land management practices can lead to soil erosion, which increases the sediment load in rivers and reduces their capacity to carry water. This can result in altered river courses and increased risk of flooding.

• Encroachment on Riverbanks: Settlements and agriculture near riverbanks are at risk of river flooding, especially during periods of heavy rainfall or when dams release excess water.

• Example: Kerala floods, 2018 due to encroachment of banks of Periyar.

• Poor Infrastructure Maintenance: Dams, embankments, and levees can deteriorate over time due to poor maintenance, increasing the likelihood of failures during floods.

• Coastal Vulnerability: Low-lying coastal areas are at risk of storm surges and tidal flooding during cyclones and heavy rainfall, leading to coastal inundation.

Causes of floods

• Heavy rainfall: Excessive monsoon rainfall can lead to floods in the low-lying area.

• Example:  Floods across many North Indian states in 2023 due to heavy rainfall.

• Snowmelt: Rapid melting of snow in mountainous regions, especially during warm periods or due to rain-on-snow events, can lead to increased water runoff and contribute to river and downstream flooding.

• River Overflow: Excessive rainfall, snowmelt, or the combination of both can cause rivers to overflow their banks, flooding nearby areas.

• Example: Breach of Yamuna River in 2023.

• Dam or Levee Failures: The breach or failure of dams, levees, or other water control structures can result from factors such as poor maintenance, improper design, or unexpected water levels, leading to localized or widespread flooding.

• Example: 1979 Machchhu dam failure.

• Coastal Storms: Tropical storms, hurricanes, and cyclones can bring heavy rainfall and storm surges, causing coastal flooding and inundating coastal areas.

• Example: Flooding of eastern coast due to cyclone Amphan in 2020.

• Tidal Surges: High tides, often exacerbated by storm surges from coastal storms, can push seawater inland and cause coastal flooding.

• Glacial Lake Outburst: Glacial meltwater can accumulate behind natural or human-made dams. If these dams breach, they can release a sudden rush of water downstream, causing flooding.

• Example: Chamoli flood, 2021(Uttarakhand).

• Blockage of Drainage Systems: Accumulated debris, trash, and other materials can clog drainage systems, preventing efficient water flow and causing localized flooding.

Effects

• Socio-economic effects

• Health Impact: Floods can lead to the loss of human lives and cause injuries, as well as increase the risk of waterborne diseases due to contaminated water sources.

• Displacement: Floods often force people to evacuate their homes, leading to temporary or even long-term displacement. This can result in homelessness, overcrowded shelters, and a need for emergency housing.

• Infrastructure Damage: Floods can damage roads, bridges, buildings, and other critical infrastructure, disrupting transportation, communication, and essential services.

• Agricultural Losses: Floods can submerge farmland, destroying crops, livestock, and agricultural infrastructure. This can lead to food shortages, loss of income for farmers, and increased food prices.

• Environmental effects

• Soil erosion: Floodwaters can erode soil, riverbanks, and coastal areas, leading to loss of fertile land and habitat destruction.

• Water Contamination: Floodwaters can carry pollutants, sewage, and chemicals, contaminating water sources and posing risks to human and ecosystem health.

• Habitat Destruction: Floods can damage or destroy natural habitats, affecting wildlife and biodiversity.

• Siltation: Floods can deposit sediment in rivers, reservoirs, and water bodies, reducing their storage capacity and affecting water management.

• Saltwater Intrusion: Coastal floods, especially due to storm surges, can result in the intrusion of saltwater into freshwater sources, affecting agricultural land and drinking water supplies.

Challenges in mitigation

• Diverse Causes and Sources: Floods can be triggered by various factors, including heavy rainfall, snowmelt, storm surges, and dam failures. Mitigation efforts must consider these different causes and their specific characteristics.

• Lacking Data and Forecasting: Accurate and timely flood forecasting is crucial for effective mitigation. However, many areas lack sufficient data, monitoring systems, and advanced forecasting tools.

• Climate Change: Changing climate patterns can lead to more intense and unpredictable rainfall events, increasing the frequency and severity of floods. 

• Limited Resources: Adequate funding and resources are required to implement effective flood mitigation measures, but many regions, especially those with vulnerable populations, lack the necessary resources.

• Infrastructure Aging and Maintenance: Aging of existing infrastructure, such as dams, and drainage systems and inadequate maintenance can undermine their effectiveness.

• Coordination and Governance: Lack of coordination among various government agencies, local authorities, and stakeholders and unclear governance structures can hinder mitigation efforts.

NDMA guidelines

• Flood and flash floods

• Structural measures: Embankments/flood walls/flood levees have to be constructed for prevention of flooding after carrying out detailed hydrological and morphological studies regarding their favorable and adverse effects.

• Desilting rivers: The CWC and the state governments have to study the problem of rise in riverbeds in a scientific manner and explore the techno-economic viability of desilting/dredging as a remedial measure to mitigate the effects of rise in the riverbeds.

• Drainage Improvement: The state governments/SDMAs to review the adequacy of existing sluices and drainage channels in areas suffering from drainage congestion.

• Watershed management measures: Measures such as afforestation, check dams, detention basins etc., have to be implemented in the catchment of rivers to prevent soil erosion, enhance water conservation, and minimize water and sediment runoff.

• Sea walls: It must be planned and executed by the respective coastal states/port authorities, keeping in view the complexity of sea behavior and other environmental aspects.

• Flash floods forecasting and warning systems: using Doppler radars to be installed by the India Meteorological Department (IMD).

• Public Awareness: Campaigns and people participation drills need to be conducted regularly.

• GLOF

• Identify and Mapping Dangerous Lakes: Potentially dangerous lakes need to be identified with the help of field observations, past events, geomorphologic and geotechnical characteristics, etc.

• Use of Technology: It is recommended to utilize Synthetic-Aperture Radar imagery. It will automatically detect changes in water bodies, including new lake formations.

• Structural Measures: Recommends measures such as pumping or siphoning out water and making a tunnel through the moraine barrier or under an ice dam.

• Land Use Planning: In downstream areas, Infrastructure development should be monitored before, during, and after the construction.

• Empowered Local Manpower: Apart from specialized forces like NDRF, ITBP, and the military, there is a requirement for trained local manpower.

• Emergency response team: Quick Reaction Medical Teams, mobile field hospitals, Accident Relief Medical Vans, and heli-ambulances should be made available in areas inaccessible by roads.

India’s current preparedness

• Structural measures

• Embankment and Floodwalls: Constructing embankments and vertical barriers (floodwalls) along riverbanks and coastlines to prevent floodwater from inundating populated areas.

• Dams and Reservoirs: Building dams to regulate river flow, store excess water during heavy rainfall, and release it gradually to prevent downstream flooding.

• Flood Diversion Channels: Constructing channels that divert floodwaters away from populated areas to less vulnerable locations.

• Elevated Structures: Designing buildings and infrastructure on elevated platforms to prevent floodwater infiltration and minimize damage.

• Non-structural measures

• Early Warning Systems: Developing and implementing effective flood forecasting and early warning systems to provide timely alerts to communities.

• Floodplain Management Policies: Implementing policies that promote sustainable land use practices and discourage construction in flood-prone areas.

• Community Education and Awareness: Raising public awareness about flood risks, preparedness, evacuation plans, and response strategies.

• Emergency Response Planning: Developing and regularly updating comprehensive emergency response plans that outline actions to be taken before, during, and after a flood.

• National Floodplains Zoning Policy: It aims at demarcating zones or areas likely to be affected by floods of different magnitude or frequencies and probability levels and specify the types of permissible developments in these zones, so that the damage can be mitigated.

• National Flood Risk Mitigation Project (NFRMP): It aims at ensuring that arrangements are in place to mobilize the resources and capability for relief, rehabilitation, reconstruction, and recovery from disasters besides creating awareness among vulnerable communities.

• Dam Rehabilitation and Improvement Project (DRIP): To improve the safety and operational performance of selected existing dams.

Best practices

• IFLOWS-Mumbai: It is a monitoring and flood warning system that will be able to relay alerts of possible flood-prone areas anywhere between six to 72 hours in advance.

• Room for the River project (Netherlands): As much of the country lies below sea level, this project helped to mitigate recurring floods. This approach involves creating more space for rivers to safely accommodate high water levels during periods of heavy rainfall or flooding.

• United Kingdom - Thames Barrier: The Thames Barrier in London protects the city from storm surges and tidal flooding. It consists of movable gates that can be raised to block high tides and storm surges from entering the Thames Estuary.

• Vietnam - Polder Systems: Vietnam has implemented polder systems in the Mekong Delta, which involve enclosing low-lying areas with dikes and using pumps to manage water levels. This approach has protected agricultural land and communities from floods and saltwater intrusion.

Way Forward

• River basin approach: It is a holistic and integrated strategy for flood mitigation that considers the entire watershed or river basin as a management unit.

• Natural Flood Defenses: Protect and restore natural flood defenses like wetlands, floodplains, and mangroves, which can absorb excess water and reduce flood intensity.

• Community Engagement: Involve local communities in decision-making, planning, and implementation, ensuring their needs, knowledge, and perspectives are integrated into flood management efforts.

• Climate Resilience: Incorporate climate change projections into flood management strategies to anticipate changing rainfall patterns, sea levels, and other climate-related factors.

Tags:
Disaster Management

Keywords:
FLOODS URBAN FLOODS FLASH FLOOD GLOF disaster disaster management flood Vulnerability of India Causes of floods Effects of flood Challenges in mitigation NDMA guidelines for flood flash flood glof India’s current preparedness Best practices in

Syllabus:
General Studies Paper 3

Topics:
Disaster and Disaster Management