Flood

Definition of Flood
General Definition

 A flood is a temporary covering of land by water outside its normal confines

(Gouldby & Samuels, 2005).
 It refers to an overflow of water that submerges normally dry land, often
described as a deluge.

Causes of Flooding

 Overflow from rivers, lakes, or other water bodies when water escapes its
usual boundaries.
 Inflow of tides or accumulation of rainwater on already saturated ground.

EU Flood Directive Definition

The European Union (EU) Floods Directive defines flood as a covering of land not
normally covered by water.
It may occur due to:
 Overflow of rivers or lakes
 Rainwater accumulation on saturated ground

Hydrological Definition (UNESCO-WMO, 1974)

From a hydrological perspective, a flood is:

 A brief rise in water level in a stream to a peak, followed by a slower
recession.
 It involves a progressive and abnormal increase in the streamflow level that
reaches a maximum, then gradually returns to normal — all within a specific
time period.

Consequences When Flood Occurs

When riverbanks are overtopped:

 Water spreads over floodplains
 Leads to suffering for people, including:
 Damage to crops, vegetation, infrastructure
 Displacement of inhabitants
 Economic losses on a national scale

People Living with Flood in Bangladesh

 For thousands of years, people in Bengal have adapted to living with rivers and
floods.
 High lands were selected for building homes, while lowlands were used for
farming.
 In areas without high land, the traditional method called "dig-elevate-dwell" was
used:
 Land was dug out to create ponds or canals, and the soil was used to
elevate living spaces.
 This technique helps preserve and even enhance space for river overflow.
 Agricultural practices were also adapted to the deltaic environment.
 Special flood-resistant rice varieties were developed:
 Bona Aman can grow up to 20 feet tall, and as much as 1 foot in 24 hours.
 Ropa Aman and Boro varieties are also well-suited to flood-prone areas

Major Floods in Bangladesh

 The most devastating and impactful floods occurred in the years: 1954, 1987,
1988, 1998, 2004, and 2007
 These floods severely affected a large portion of the population and caused major
disruptions to: Social life & Economic activities

Causes of Floods
Natural Causes

1. Meteorological Factors:
 Heavy and prolonged rainfall
 Cyclones and tropical storms
 Storm surges
 Unusual high tides
 Rainfall over snow-covered land
2. Hydrological Factors:
 Melting of ice and snow
 Saturated ground unable to absorb more water
 Impermeable surfaces (rocky or paved land)
 Intense rainfall over small hilly catchments
 Tsunamis or natural dam breaks
3.Anthropogenic (Human-Induced) Causes
 Urbanization: Increases impermeable surfaces, reducing natural drainage
 Unplanned flood control measures: Poor infrastructure like faulty dams or
embankments
 Socio-economic development: Encroachment on floodplains and wetlands
 Climate change: Intensifies rainfall and storm events
 Deforestation: Reduces water retention and increases runoff
Principal Types of Flood
1) Riverine Flood: Riverine floods are caused by overflowing rivers and may
include flash floods resulting from intense thunderstorms or sudden releases of
water due to dam failure, landslides, or melting glaciers.
2) Estuarine Flood: Estuarine floods occur in estuaries and are typically caused by
tidal surges, which are triggered by storm-force winds during tropical or extra-
tropical cyclones.
3) Coastal Flood: Coastal floods are caused by severe sea storms, tsunamis, or
hurricanes and often involve storm surges generated by tropical or extra-tropical
cyclones.
4) Catastrophic Flood: Catastrophic floods result from sudden large-scale natural
or man-made disasters such as dam breakage, earthquakes, or volcanic eruptions,
leading to unexpected and massive inundation.
5) Muddy Flood: Muddy floods are caused by surface runoff on agricultural lands
and carry mud and sediments, resulting in muddy water flowing over the land.
6) Flash Flood: Flash floods are rapid and intense floods that affect low-lying areas
such as rivers, streams, and washes, typically caused by heavy rainfall from
thunderstorms, hurricanes, or tropical storms, and are extremely dangerous due to
their sudden onset.
Flood Types
1. Fluvial Flood: Fluvial flood refers to flooding caused by major river systems
overflowing their banks during periods of heavy rainfall or snowmelt.
2. Flash Flood: Flash flood occurs in hilly or mountainous regions due to intense
and sudden rainfall, leading to rapid water flow through narrow river channels.
3. Urban Flood: Urban flood takes place in built-up city areas where drainage
systems are often inadequate, causing streets, roads, and properties to be
submerged after heavy rain.
4. Tidal Flood: Tidal flood is caused by the regular rise of sea tides, which inundate
low-lying coastal areas, especially during high tide or spring tide events.
5. Fluvio-tidal Flood (Compound Flooding): Fluvio-tidal flood is a combination
of both fluvial flood from upstream rivers and tidal flood from the sea, resulting
in more severe flooding in deltaic regions.
6. Storm Surge Flood: Storm surge flood is caused by cyclones or hurricanes that
generate high sea waves and push seawater onto coastal lands, leading to
widespread inundation.
7. Flooding due to Monsoon Depression: This type of flood results from low-
pressure systems or depressions formed over the ocean during the monsoon
season, which bring prolonged and heavy rainfall to coastal and inland areas.
8. Pluvial Flood: Pluvial flood is caused directly by heavy rainfall when the ground
cannot absorb the water quickly enough, leading to surface water accumulation
and localized flooding.

Flood Types in Coastal Bangladesh

1) Fluvial Flood: Fluvial flood in coastal Bangladesh is caused by the flow of water
from upstream rivers during the monsoon season, leading to the overflow of
major river systems.
2) Fluvio-tidal Flood (Compound Flooding): Fluvio-tidal flood, also known as
compound flooding, occurs due to the combined effect of upstream river flows
(fluvial) and tidal inflow from the Bay of Bengal, making the flood impact more
severe.
3) Tidal Flood: Tidal flood is caused by regular tidal variations over different time
scales, which inundate unprotected low-lying coastal areas during high tide
events.
4) Flooding due to Monsoon Depression: This type of flooding is triggered by
low-pressure depressions formed over the ocean during the monsoon, which
bring continuous and heavy rainfall to the coastal region.
5) Storm Surge Flood: Storm surge flood occurs due to cyclones generated in the
Bay of Bengal, which push large volumes of seawater onto the coastal land,
causing severe inundation and damage.
6) Pluvial Flood due to Rainfall: Pluvial flood happens during the monsoon or
cyclone events when intense rainfall overwhelms the drainage capacity,
especially inside polder areas, leading to waterlogging and surface flooding.

Note: Coastal Bangladesh is particularly vulnerable to storm surge flooding, making it

a significant concern for disaster management in the region.
Urban Flood

Urban floods occur due to a combination of meteorological and hydrological extremes

such as intense rainfall and inadequate drainage. In urban environments, these natural
factors are often worsened by human-induced activities like unplanned development,
encroachment on water bodies, and poor stormwater management. As a result, flood
hazards in cities should be understood as the outcome of both natural processes and
man-made interventions, requiring integrated planning and sustainable urban
infrastructure to reduce the risks.
Major Effects of Floods

1. Primary Effects
 Physical damage includes destruction of infrastructure such as bridges,
buildings, sewer systems, roads, vehicles, and canals.
 Casualties occur as people and livestock drown during floods, and there is a
risk of outbreaks of epidemics and water-borne diseases.
2. Secondary Effects
 Water supply contamination makes clean drinking water scarce and unsafe.
 Disease spread increases due to unhygienic conditions and stagnant
floodwaters, leading to water-borne illnesses.
 Crop and food shortages result from the loss of entire harvests, threatening
food security in affected areas.
 Tree damage occurs when non-flood-tolerant species die from suffocation
due to prolonged submersion.
3. Tertiary / Long-term Effects
 Economic hardship is caused by a decline in tourism, high rebuilding costs,
and food shortages that lead to rising prices, affecting the overall economy
and livelihoods.

Elements of Flood Management

1. Structural Measures

 Protection against flood

 Modification of water flow
 Pollution reduction

2. Non-Structural Measures

 Emergency response measures

 Flood preparedness measures
 Environmental impact assessment
 Local and state legislation
3. Economic Measures

 Economic instruments for flood control

 Flood insurance
 Financial assistance

4. Flood Recovery Measures

 Shelter and health programs

 Rehabilitation of affected communities
 Financial support and long-term recovery planning

Effects of Flood

1. Primary Effects
 Physical Damage: Floods cause severe destruction to buildings, roads, bridges,
vehicles, sewer systems, canals, and public infrastructure.
 Casualties: People and livestock often lose their lives due to drowning, and
there is a high risk of injury and immediate health emergencies.
2.Secondary Effects
 Water Supplies: Flooding contaminates water sources, making clean drinking
water scarce and unsafe.
 Diseases: Unhygienic conditions promote the spread of water-borne diseases
such as cholera, diarrhea, and skin infections.
 Crops and Food Supplies: Entire harvests may be destroyed, causing food
shortages and disrupting local food security.
 Trees: Non-flood-tolerant tree species can die from prolonged waterlogging or
root suffocation.
3.Tertiary / Long-Term Effects
 Economic Hardship: Floods lead to long-term economic impacts including
loss of livelihood, increased food prices, tourism decline, and high rebuilding
costs.

Control of Floods

1. Planting vegetation helps retain excess rainwater and reduce runoff.

2. Terracing hillsides slows down the downhill flow of water and prevents rapid
surface runoff.
3. Construction of floodways, which are man-made channels, diverts excess
floodwater away from populated or vulnerable areas.
4. Building levees, dikes, dams, reservoirs, or retention ponds helps store extra
water and control flood levels during high rainfall periods.
5. Using emergency measures, such as sandbags and portable inflatable tubes,
provides temporary flood protection during critical situations.

Flood Control Measures

 Flooding is a natural phenomenon that cannot be entirely prevented but can be

managed through effective control strategies.
 Flood control measures and policies should focus on mitigating flood damage
rather than trying to stop floods completely.
 Low-cost indigenous solutions, such as modifying housing designs and altering
crop patterns, can significantly reduce flood-related damage.
 Real-time flood and drought warning systems are crucial for early preparedness
and response, helping reduce potential losses.
 An integrated approach that includes both:
i. Structural measures (physical infrastructure)
ii. Non-structural measures (planning and management strategies) should be
adopted for effective flood management.

1.Structural Measures
a) Construction of Embankments: Barriers built along rivers or coastlines to
prevent floodwater from entering inhabited areas.
b) Reservoirs: Large storage areas to collect and hold excess rainwater or river
water, releasing it slowly to prevent downstream flooding.
c) Diversion Channels: Artificial water channels constructed to divert excess
water away from vulnerable areas.
d) River Dredging: The process of removing sediment and debris from riverbeds
to increase their capacity and flow efficiency, reducing overflow risk.
2.Non-Structural Measures
Non-structural measures are an alternative strategy for flood hazard mitigation that
focus on social adjustment and planning rather than physical infrastructure. These
approaches aim to reduce the impact of floods through better preparedness, land use
practices, and community resilience.
 Dissemination of meteorological and flood forecasts to provide early warnings
and improve community preparedness.
 Speedy evacuation procedures to protect lives during flood emergencies.
  Land management strategies aimed at reducing surface runoff and enhancing
natural water absorption.
 Land use changes and enforcement of building codes to ensure safer
construction in flood-prone areas.
 Diversification of agricultural production to reduce dependence on flood-
sensitive crops and ensure food security.
 Floodplain zoning by regulating land use in high-risk areas to minimize
damage and control development.
 Development of restrictive regulations to limit unsafe construction and land
conversion in flood-prone zones.
 Disaster relief efforts during and after floods, which play a vital role in
recovery and rebuilding affected communities

Impact, Damage, and Loss Caused by Coastal Flooding

Impacts
 Inundation (land covered by floodwater)
 Waterlogging (long-term standing water)
 Sediment deposition on floodplains
 Soil becomes salty (soil salinization)
 Groundwater becomes salty (GW salinization)
 Loss of jobs, homes, and sources of income
Damage and Loss
 Loss of human lives and health problems
 Damage to infrastructure (roads, bridges, houses)
 Loss of agriculture and crops
 Damage to fisheries
 Destruction of mangrove forests
  Death or loss of livestock
Existing and Planned Adaptations Against Coastal Flooding
Structural Adaptations
Existing:
 139 polders (embankment systems) already built
Future Plans:
 Heightening of existing polders
 Building new polders in South-Central region
 Construction of sea walls
Non-Structural Adaptations
 Planting greenbelts (trees and vegetation along coasts)
 Installing early warning systems for floods
 Providing safe drinking water
 Changing cropping systems to flood-resilient types
 Promoting livestock suited to coastal areas
 Expanding fish culture (aquaculture)
 Ensuring social protection programs
 Supporting livelihood migration for vulnerable people