Introduction to coastal flooding

Coastal flooding occurs when seawater inundates low-lying areas along the coast, often leading to significant risks for human settlements and ecosystems. This can happen rapidly through events like storm surges or more gradually due to rising sea levels. Understanding the factors that heighten flood risk is essential, as coasts vary in their vulnerability based on physical characteristics and human influences.

Key types of coastal flooding

• Quick-onset flooding - Triggered by extreme weather events, such as storm surges during tropical cyclones, which cause sudden rises in sea level.
• Slow-onset flooding - Resulting from gradual processes like sea level rise, which progressively increases the area exposed to inundation.

Coasts with high rates of recession (the retreat of the coastline due to erosion) do not always face the highest flood risk, as flooding depends more on elevation and protective features.

Factors influencing the risk of coastal flooding

Certain physical and human-related factors make some coasts more prone to flooding than others. Low elevation plays a major role, as it allows water to spread inland more easily during high sea levels or storms.

Physical factors increasing flood risk

• Low-lying landforms - Areas like estuaries (where rivers meet the sea) and deltas (landforms built from river sediment deposits) are particularly vulnerable because storm waves and elevated sea levels can flood them readily. Low-lying islands face the risk of complete submergence.
• Subsidence in deltas - Some deltas are sinking due to increased river flows that wash away sediment, combined with the weight of growing settlements. This process expands the extent of low-lying land, heightening susceptibility to flooding.
• Coastal shape and seabed features - If the coastline or river mouth funnels water into a narrower area, it can amplify sea level rises. Similarly, a gently sloping seabed near the coast promotes wave shoaling, where waves build up height and force as they approach shallower water, increasing flood potential.

Human factors increasing flood risk

• Vegetation removal - Clearing coastal plants like mangroves reduces natural protection. Mangroves (tropical trees that grow in coastal swamps) promote sediment deposition and land stabilisation, which helps resist flooding. They also absorb wave energy, reducing erosion and the impact of storm surges from tropical storms (intense low-pressure systems with strong winds and heavy rain).
• Land use changes - Building on unstable sediments or draining areas for agriculture can destabilise coasts, making them more prone to erosion and flooding.

For example, if sea levels rise by 0.5 metres, most of low-lying regions like the Maldives (a group of coral atolls with a maximum elevation of 2.4 metres) could be submerged.

Case study: Flood risk in Bangladesh

Bangladesh provides a clear example of how physical geography and human actions combine to elevate coastal flood risk. This low-lying country in South Asia is crisscrossed by major rivers and faces the Bay of Bengal, making it highly vulnerable.

Physical features contributing to vulnerability

• Elevation and landforms - About 80% of Bangladesh lies at 10 metres above sea level or less, with around 20 million people in areas below 1 metre. Much of this is coastal delta land formed by the Brahmaputra, Ganges, and Meghna rivers, which deposit sediment into the Bay of Bengal.
• Tropical cyclones and monsoons - These weather events generate storm surges and heavy rains, adding to floodwater volumes.
• Unstable islands:
  • Hundreds of small islands in the Bay of Bengal are made from unconsolidated (loose) sediment from river deposits, making them prone to flooding and shape changes during monsoons (seasonal winds bringing heavy rain).
  • Rising sea levels could submerge 8,000 square kilometres if levels increase by 0.3 metres.
  • The islands' shapes often shift annually with the monsoon.

Human actions exacerbating risk

• Mangrove clearance - Population growth has led to deforestation of mangroves for farmland, shifting parts of the delta from net deposition (sediment buildup) to net erosion (sediment loss). This, combined with low elevation, increases flooding from sea level rise.
• Adaptation strategies - Some farmers use floating beds (raised platforms on water for crops) in delta districts like Pirojpur to cope with flooding.

Causes and mechanisms of storm surges

Storm surges are a primary cause of quick-onset coastal flooding during extreme weather events. These occur when atmospheric conditions temporarily raise sea levels, pushing water onshore with destructive force.

How storm surges form

1. Pressure and wind effects - A storm surge is a temporary sea level rise driven by high winds from a low-pressure system pushing water towards the coast. For every 1 millibar drop in air pressure, sea level rises by about 1 centimetre. Tropical cyclones can cause pressure drops of around 100 millibars, raising sea levels by up to a metre.
2. Wind dynamics - Air flows from high to low pressure areas, generating strong winds over the sea that create surges in localised zones.
3. Wave shoaling - As waves approach the shore, water builds up behind their crests, making them taller and more forceful. This effect intensifies if the surge coincides with high tide or if the coastline funnels water.

Example of a storm surge event

Typhoon Haiyan in 2013 devastated Southeast Asia with a massive storm surge.

Impacts of climate change on coastal flood risk

Climate change is expected to heighten the risk of coastal flooding through several interconnected processes. While uncertainties exist about the pace and extent, the overall trend points to increased vulnerability.

Key climate change effects

• Intensified tropical cyclones - Warmer sea temperatures may not increase cyclone frequency but could boost their magnitude (strength), leading to larger storm surges and greater flood risk.
• Sea level rise - Global warming causes thermal expansion (water volume increase due to heat) in oceans and adds water from melting ice sheets and glaciers.
• Stronger waves and erosion - Higher ocean temperatures strengthen winds, producing more powerful waves that erode coasts and damage protective structures like embankments (raised barriers to prevent flooding).

Scientists predict more people will be affected in coming decades, though exact outcomes depend on human responses and potential negative feedback mechanisms (self-regulating processes that might limit extreme changes, such as wind speed caps).

Economic and social impacts of coastal flooding

Coastal flooding, especially from storm surges, has far-reaching effects that persist long after the water recedes. These impacts vary between developed and developing countries but often involve both economic losses and social disruptions.

Economic impacts

• Agricultural damage - Saltwater intrusion (seawater seeping into freshwater sources) contaminates irrigation water, harming farmland and reducing export revenues in agriculture-dependent nations.
• Infrastructure disruption - Ports and trade facilities can be damaged, requiring costly investments in resilient replacements. Developing countries may lack funds to rebuild effectively between events.
• Recovery challenges - Frequent flooding strains resources, preventing long-term improvements.

Social impacts

• Health and isolation - Floodwater contaminates drinking supplies, and standing water hinders movement, isolating communities. Developed areas reliant on imported food and fuel may suffer more from supply disruptions.
• Housing and displacement - Buildings weaken or collapse due to water damage (e.g., timber expansion), leaving people homeless. Sediment deposits can block roads and drains, requiring cleanup efforts.

The rise of environmental refugees

As coastal flooding intensifies, more people are displaced, becoming environmental refugees (individuals forced to migrate due to environmental changes like submersion or resource loss).

Causes of displacement

• Sea level rise and saltwater intrusion - Rising waters submerge land and contaminate soil and water with salt, making areas uninhabitable or unproductive.
• Vulnerability of small islands - Small Island Developing States (SIDS), such as Tuvalu (a Pacific atoll nation with low elevation and high population density), are particularly at risk. Around 2,400 Tuvaluans have already relocated to larger islands or countries like New Zealand due to rising seas.