Flood Defense Methods and Flood Management Strategies

Posted on Mar 29, 2025 in Geology

Hard Flood Defense Structures

Hard solutions are engineered structures built to defend against floodwater.

How Hard Solutions Work

• Build flood banks: Walls are constructed on either side of the river to prevent water from overflowing.
• Increase river channel size: The river channel is widened and deepened so it can hold more water.
• Divert the river: The river’s course is moved away from vulnerable areas like city centers, making it surround the area instead of flowing through it.
• Increase drain size: Drains that carry rainwater to the river are enlarged.
• Increase maintenance budget: More funding is allocated for cleaning and maintaining drainage systems.

Soft Flood Management Solutions

Soft solutions involve adapting to flood risks and allowing natural processes to manage rainwater.

How Soft Solutions Work

Flood Abatement

This involves changing land use upstream.

• Advantages: It is relatively cheap and can create a more pleasant environment.
• Disadvantages: The soil may become very humid and wet, potentially limiting access to the area.

Flood Proofing

Designing new buildings or altering existing ones to reduce flood risk.

• Advantages: Allows more people to live safely in an area.
• Disadvantages: Can be expensive. People might still be hesitant to live there due to the underlying flood risk.

Flood Plain Zoning

Refusing planning permission for development in areas where flood risk is high.

• Advantages: Less damage occurs during floods as fewer properties are exposed.
• Disadvantages: Restricts development in risk areas, potentially impacting property values and limiting urban expansion.

Flood Prediction and Warning

The Environment Agency monitors rivers and uses forecasts from the Met Office to issue warnings.

• Advantages: Damage can be reduced, and fewer people are affected as they have time to prepare or evacuate.
• Disadvantages: These systems require funding for operation and maintenance. They warn about floods but do not prevent them.

Understanding Storm Hydrographs

A storm hydrograph is a graph showing how a river’s discharge changes over time in response to a rainfall event.

Sheffield Flood Case Study: Impacts

The effects of flooding in Sheffield included:

Short-Term Impacts

• Social: Many people were caught unawares and had to be evacuated from flooded buildings.

Medium-Term Impacts

• Social: There were health risks associated with contaminated water. Trains and buses were cancelled, preventing people from getting home. Displaced people suffered stress.
• Economic: Some industries were badly affected.

Long-Term Impacts

• Social: People had to wait a long time until they could return to their homes.
• Economic: Hillsborough football stadium was flooded up to 8 meters deep. Repairing the damage cost several million pounds.
• Environmental: There were severe health risks from raw sewage escaping into the floodwaters.

The Process of Flooding

From Rain to Soil

Leaves and branches of plants trap much of the falling rain; this is known as the interception zone. Some intercepted water evaporates into the atmosphere. The rest drips from leaves to the soil and soaks in – this is called infiltration. Eventually, the soil becomes saturated and cannot absorb any more water. Any additional rain flows over the ground, called surface runoff. How quickly this happens depends on three factors:

1. How much rain has fallen recently – known as antecedent rainfall.
2. How permeable the soil is. Sandy soils are permeable (they absorb water easily, and surface runoff rarely occurs). Clay soils are more closely packed, making them impermeable (water cannot infiltrate easily).
3. How heavily the rain falls.

From Soil to River

Once water enters the soil:

• Some is taken up by plants and transpired through leaves into the atmosphere.
• Some seeps into the river through soil air spaces – known as throughflow.
• Some continues down into solid rock, saturating it. The upper limit of this saturated rock is known as the water table. From here, water seeps slowly towards the river as groundwater flow, which keeps a river flowing even when there has been no recent rain.

Comparing Flood Solution Scales

Small-Scale Solutions: Pros and Cons

• Benefits: They are cheap. People in developing countries can often use and repair them. They can be replicated easily. They are often more sustainable than large-scale projects.
• Problems: They might be inefficient for large floods. In areas where many people suffer from illnesses like HIV/AIDS (e.g., parts of Africa), individuals may be too ill to operate or maintain them. They typically only help a small number of people.

Large-Scale Solutions: Pros and Cons

• Benefits: Can control floods over large areas. Dams can attract industry and provide recreational opportunities (scenic assets). Can create habitats for water birds. Useful for managing water resources for big regions. Provides easier access to water supplies.
• Problems: Can lead to the loss of farmland and villages due to reservoir creation. Can reduce water quality downstream. Can cause ‘clear water’ erosion below dams. Require a well-trained workforce for construction and maintenance. Require significant funding to set up. Can lead to sedimentation build-up in reservoirs/lakes.