3.3 Floods: types, causes, and mitigation

Floods are a powerful force of nature that can devastate communities and reshape landscapes. From flash floods to coastal surges, these events come in various forms, each with unique causes and impacts. Understanding flood types is crucial for effective mitigation and preparedness.

Flood mitigation involves both structural and non-structural measures. While dams and levees can control water flow, land use planning and early warning systems are equally important. Balancing these approaches is key to reducing flood risks and protecting lives and property.

Flood Types and Causes

Categories and Underlying Factors

Top images from around the web for Categories and Underlying Factors

• 

  Frontiers | An Efficient Method for Approximately Simulating Drainage Capability for Urban Flood View original

  Is this image relevant?

• 

  HESS - Urban surface water flood modelling – a comprehensive review of current models and future ... View original

  Is this image relevant?

• 

  Frontiers | Nature-Based Engineering: A Review on Reducing Coastal Flood Risk With Mangroves View original

  Is this image relevant?

• 

  Frontiers | An Efficient Method for Approximately Simulating Drainage Capability for Urban Flood View original

  Is this image relevant?

• 

  HESS - Urban surface water flood modelling – a comprehensive review of current models and future ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Categories and Underlying Factors

• 

  Frontiers | An Efficient Method for Approximately Simulating Drainage Capability for Urban Flood View original

  Is this image relevant?

• 

  HESS - Urban surface water flood modelling – a comprehensive review of current models and future ... View original

  Is this image relevant?

• 

  Frontiers | Nature-Based Engineering: A Review on Reducing Coastal Flood Risk With Mangroves View original

  Is this image relevant?

• 

  Frontiers | An Efficient Method for Approximately Simulating Drainage Capability for Urban Flood View original

  Is this image relevant?

• 

  HESS - Urban surface water flood modelling – a comprehensive review of current models and future ... View original

  Is this image relevant?

1 of 3

• Floods can be categorized into different types based on their underlying causes, location, and duration (riverine floods, coastal floods, flash floods, urban floods)
• Riverine floods occur when water levels in rivers, streams, or other channels exceed the capacity of the channel causing water to overflow onto the surrounding floodplains
  • Often caused by prolonged or heavy rainfall, snowmelt, or a combination of both
• Coastal floods happen when storm surges, high tides, or tsunamis cause seawater to inundate low-lying coastal areas
  • Often associated with tropical cyclones, hurricanes, or other severe weather events
• Flash floods are characterized by their rapid onset and high velocity, often occurring within minutes or hours of heavy rainfall or dam failures
  • Common in areas with steep terrain, impermeable surfaces, or narrow channels
• Urban floods occur when the drainage capacity of a city's infrastructure is overwhelmed by heavy rainfall or snowmelt leading to the accumulation of water in streets, buildings, and other low-lying areas
  • Urbanization and the increase in impervious surfaces contribute to the frequency and severity

Examples of Flood Events

• The Mississippi River Flood of 1993 was a major riverine flood that affected nine states in the central United States causing billions of dollars in damage and displacing tens of thousands of people
• Hurricane Katrina in 2005 caused severe coastal flooding in New Orleans and other areas along the Gulf Coast resulting in over 1,800 deaths and widespread destruction
• The 2010 Pakistan floods were a combination of riverine and flash flooding triggered by heavy monsoon rains affecting over 20 million people and causing extensive damage to infrastructure and agricultural land
• The 2012 Beijing floods were a prime example of urban flooding with heavy rainfall overwhelming the city's drainage system leading to widespread inundation of streets and buildings

Flood Factors and Magnitude

Hydrological Factors

• Precipitation is a key hydrological factor influencing flood occurrence
  • The intensity, duration, and spatial distribution of rainfall or snowmelt determine the amount of water available for runoff and the potential for flooding
• Soil moisture conditions prior to a rainfall event can significantly affect flood magnitude
  • Saturated soils have a reduced capacity to absorb water leading to increased runoff and higher flood risk
• Antecedent conditions, such as the presence of snow cover or the occurrence of previous rainfall events, can affect the hydrological response of a watershed and the likelihood of flooding

Geomorphological Factors

• Land use and land cover changes, such as deforestation, urbanization, and agricultural practices, can alter the hydrological response of a watershed
  • These changes can increase surface runoff, reduce infiltration, and accelerate the timing of peak flows thereby increasing flood risk
• Topography and geomorphology play a crucial role in flood occurrence and magnitude
  • Steep slopes, narrow valleys, and confined channels can contribute to rapid runoff and high flow velocities increasing the potential for flash floods
  • The shape and size of a watershed, as well as the presence of natural or artificial drainage networks, can influence the timing and magnitude of flood peaks
• Examples of geomorphological factors affecting flood magnitude include:
  • The steep terrain and narrow canyons of the Colorado Front Range contribute to the high frequency of flash floods in the region
  • The extensive levee system along the Mississippi River has altered the natural floodplain dynamics and can exacerbate flooding downstream

Flood Mitigation Effectiveness

Structural Measures

• Structural measures involve the construction of physical infrastructure designed to control, divert, or store floodwaters (levees, dams, floodwalls, retention basins)
  • Levees and floodwalls can provide protection to specific areas but may increase flood risk downstream if not properly designed or maintained
  • Dams and retention basins can help regulate river flows and reduce peak flood levels but may have environmental and social impacts
• Examples of structural flood mitigation measures include:
  • The Thames Barrier in London, a series of movable gates that can be closed to protect the city from storm surges and high tides
  • The Three Gorges Dam in China, which provides flood control, hydroelectric power generation, and navigation benefits along the Yangtze River

Non-Structural Measures

• Non-structural measures focus on reducing flood vulnerability through land use planning, building codes, early warning systems, and public awareness campaigns
  • Floodplain zoning and development restrictions can limit exposure to flood hazards by guiding development away from high-risk areas
  • Building codes and flood-proofing techniques can reduce the vulnerability of structures to flood damage
  • Early warning systems and evacuation plans can help communities prepare for and respond to flood events minimizing loss of life and property
• Nature-based solutions, such as wetland restoration and green infrastructure, are gaining recognition as sustainable and cost-effective approaches to flood mitigation
  • These measures can provide multiple benefits including flood risk reduction, water quality improvement, and habitat enhancement
• Examples of non-structural flood mitigation measures include:
  • The National Flood Insurance Program in the United States, which provides flood insurance to property owners and encourages communities to adopt floodplain management regulations
  • The Room for the River program in the Netherlands, which focuses on giving rivers more space to overflow naturally by relocating dikes, creating floodplains, and restoring wetlands

Flood Impacts on Communities and Ecosystems

Socio-Economic Consequences

• The direct economic impacts of floods include damage to buildings, infrastructure, and agricultural land, as well as disruption of transportation, commerce, and public services
  • Indirect economic impacts may include loss of productivity, reduced tourism, and increased insurance premiums
• Floods can lead to loss of life, injuries, and displacement of affected populations
  • The physical and mental health impacts of floods can persist long after the event particularly for vulnerable groups such as the elderly, children, and low-income communities
• Floods can disrupt social networks and community cohesion as displaced individuals and families may face challenges in accessing housing, education, and healthcare services
• Examples of socio-economic consequences of floods include:
  • The 2011 Bangkok floods in Thailand caused an estimated $45 billion in economic losses and affected over 13 million people
  • The 2005 Hurricane Katrina led to the displacement of over 1 million people in the United States with many struggling to rebuild their lives and communities years after the event

Environmental Consequences

• The environmental consequences of floods can be both positive and negative
  • Floods can replenish wetlands, recharge groundwater aquifers, and provide nutrients to floodplain ecosystems
  • However, they can also cause soil erosion, water pollution, and damage to natural habitats
• Floods can introduce pollutants, such as sewage, chemicals, and debris, into water bodies affecting water quality and aquatic ecosystems
  • The deposition of sediment and debris can alter river morphology and habitat structure impacting aquatic species and riparian vegetation
• The long-term impacts of floods on communities and ecosystems depend on factors such as the frequency and magnitude of flood events, the resilience of affected populations, and the effectiveness of recovery and adaptation measures
• Examples of environmental consequences of floods include:
  • The 2011 Mississippi River floods deposited large amounts of sediment and nutrients in the Gulf of Mexico contributing to the formation of a massive hypoxic zone or "dead zone" with low oxygen levels
  • The 2019 Townsville floods in Australia caused extensive damage to the Great Barrier Reef with freshwater plumes and sediment runoff smothering coral reefs and seagrass beds