Flooding

What are the causes of flooding?

A flood occurs when water accumulates on typically dry land. It happens when a river exceeds its banks, causing water to spill over and spread across the nearby floodplain.

The main causes of floods are typically climatic forces, which have become more erratic and severe with climate change. Other contributing factors are specific to drainage basins and have been increasingly influenced by human activities.

Natural Causes of Flooding

Heavy Precipitation

In the UK, floods are often linked to deep depressions (low-pressure systems) during autumn and winter. They typically last long periods and cover large areas.

Prolonged Precipitation 

Extended periods of continuous rain can saturate the soil, leading to increased surface runoff. This causes river levels to rise quickly, potentially overflowing their banks.

Seasonal Variations in Discharge 

In countries like India, up to 70% of annual rainfall can occur in a hundred summer days during the southwest monsoon. Additionally, melting snow from mountains like the Himalayas, Rockies, and Alps can lead to widespread flooding during spring.

Relief 

Steep slopes can prompt swift run-off. Flood damage potential multiplies with velocity, and speeds exceeding 3m per second can be highly destructive.

High Tides

High tides can prevent rivers from draining into the sea, forcing water back up the river channel and potentially causing flooding.

Other natural factors

Impermeable rocks, high drainage density, sparse vegetation, and soils with low infiltration capacity all contribute to faster runoff, increasing the risk of flooding.

Human Causes of Flooding

Urbanisation and urban growth contribute to flooding: 

• Impermeable Surfaces: Urbanisation has created impermeable surfaces, such as concrete and asphalt, which reduce the amount of water that can infiltrate into the ground. This increases surface runoff, contributing to higher river levels.
• Dense Drainage Networks: Urban areas often have extensive networks of drains and sewers that rapidly transfer runoff into rivers, increasing drainage density and accelerating water flow into river channels.
• Sewer System Overload: Many sewer systems cannot handle the increased storm runoff, leading to overflow during peak rainfall.
• Constrictions in River Channels: Human infrastructure such as bridge supports and riverside developments can narrow natural river channels, reducing their capacity to carry water and increasing the likelihood of flooding.
• Floodplain Development: The expansion of housing and economic activities on floodplains, which often involves replacing natural land with impermeable surfaces, has increased the risk of flooding for many communities. Deforestation and poor land management also contribute to reduced interception and increased runoff.
• Human-Induced Climate Change: A warming climate produces a more moisture-rich atmosphere, leading to heavier rainfall. Higher temperatures can also cause more frequent “rain-on-snow” events, accelerating snowmelt. Additionally, climate change is expected to increase the frequency of hurricanes and other extreme weather events.
• Groundwater Flooding: Groundwater flooding occurs when the water table rises to the surface, particularly in low-lying areas, after prolonged or intense rainfall. This flooding can damage agriculture, causing significantly higher losses than river flooding. Mapping the risk of groundwater flooding is more complex than river flood risk. Still, it is an increasing concern in regions with high precipitation and a naturally high water table, such as many parts of Europe.

The prediction and prevention of flooding

Prediction

Flood forecasting and warning accuracy have improved over recent years, particularly in developed nations. As per the US Geological Survey (USGS), flood prediction needs data on rainfall, storm type, river/channel network discharge rate changes, and drainage basin characteristics. Swift communication about impending flood threats and their severity is crucial. For instance, the UK government website enables users to assess their flood risks and provides flood preparedness, response, and recovery guidelines.

Prevention

Hard Engineering

Historically, flood management has relied on “hard engineering” techniques aimed at directly controlling water flow. These methods include the construction of dams, reservoirs, levees, channel straightening, and flood relief channels. While effective at reducing flood risk in specific areas, these approaches can have unintended consequences elsewhere in the river basin. Negative impacts may include increased sediment buildup, erosion of riverbanks and beds, degraded water quality, and habitat loss. One of the most common hard engineering solutions is the use of artificial levees, such as the extensive levee system along over 4,500 km of the Mississippi River in the United States.

Soft Engineering

More recently, “soft engineering” methods have gained popularity, focusing on working with natural processes rather than attempting to control them. These techniques include:

• Catchment flood management plans: Developing strategies to manage flood risks sustainably across an entire river basin.
• Land-use zoning: Restricting certain types of land use on floodplains to minimize flood damage.
• River restoration: Returning engineered rivers to more natural states to reduce flood risks downstream.
• Wetland conservation: Preserving wetlands to enhance water storage and decrease river discharge.

Flood Abatement

Flood abatement aims to reduce the volume of runoff entering a drainage basin. This can be achieved through various measures, including:

• Reforestation or afforestation: Planting trees to increase water storage and intercept rainfall.
• Reseeding: Replanting vegetation in sparsely covered areas.
• Slope treatment: Using techniques like contour ploughing or terracing to manage water flow.
• Stream maintenance: Removing debris and sediment from streams to prevent blockages.
• Preserving natural water-storage areas: Protecting lakes and wetlands that act as natural reservoirs.

Land-Use Zoning

Land-use zoning involves designating specific areas on floodplains for different types of development. This strategy, dating back to the mid-20th century, helps reduce the number of people and properties at risk by controlling where development occurs.

There are four levels of adjustments to building and site design in flood risk areas identified by the Environment Agency:

• Flood Avoidance: This involves designing buildings and selecting sites to prevent exposure to flooding. It includes measures like building in areas outside of flood-prone zones, elevating structures above known flood levels, and using landscape features to divert floodwaters away from the property.
• Flood Resistance: Flood resistance focuses on preventing floodwater from entering buildings. This includes using impermeable materials and barriers, such as flood doors, walls, or shields, and sealing entry points like doors and windows to keep water out during flood events.
• Flood Resilience: This approach accepts that some water may enter the building but focuses on minimizing the damage it can cause. It includes using water-resistant materials inside buildings, raising electrical outlets, and designing interiors to withstand and recover quickly from floods.
• Flood Repairable: In this level, the building is designed so that if flood damage occurs, repairs can be carried out quickly and cost-effectively. This includes easily replaceable finishes and fixtures, along with construction techniques that facilitate quick drying and cleaning.

Hazard-Resistant Building Design

Flood-resistant building design, or flood-proofing, involves modifying buildings and their contents to minimise flood damage. This includes architectural adjustments that can withstand or divert floodwaters, reducing potential losses.

Sustainable Drainage Systems (SuDS)

Sustainable drainage systems (SuDS) offer a more natural approach to managing surface water in urban areas. They aim to slow water flow into rivers and streams, increase water storage capacity, and improve water quality by reducing pollutants. SuDS strategies include:

1. Source control: Methods like green roofs capture runoff for reuse or allow it to evaporate, reducing the volume of water entering drainage systems.
2. Pre-treatment: Using features like vegetated swales and filter trenches to remove pollutants from surface water before they reach rivers.
3. Retention systems: Ponds, retention basins, and wetlands that store water and delay its release into waterways.
4. Infiltration systems: Features like infiltration basins and soakaways allow water to soak into the ground instead of flowing directly into rivers.