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Hazard

Flood damage to property and infrastructure

Hazard Knowledge

Property damage caused by flooding can be significant. Short-term cleaning up costs can be high, and water can also cause longer term cosmetic damage, warp timbers, cause degradation of structural elements and lead to mould and damp. Scouring by fast-moving water may affect the stability of foundations or cause subsidence.

In addition to the damage caused by flowing water, lateral pressure can create significant forces. The greater the difference between the height of water on either side of an impermeable surface, the greater the pressure created. This pressure can cause structural damage to properties and potentially result in the collapse of temporary or unstable structures.

Structures may be built of partially permeable materials such as brick. The differences in pressure will cause water to move through semipermeable surfaces even if all apertures are closed. Over a certain height even a home with property level protection such as door barriers or sand bags will start to see some water ingress. The effect of lateral hydrostatic pressure should be considered when making a decision to pump out a structure, as the combination of scouring, water damage and the pressure differential created by lowering the internal water level by pumping may cause unnecessary structural damage. During flooding, it may be prudent to wait for water to subside rather than pumping properties out during flooding.

Secondary effects of flooding include disruption to infrastructure such as the supply of clean water, electricity and transport and can include damage to Critical National Infrastructure (CNI). Other effects of flooding may include failure of automatic telling machines, disruption to food and water supplies and health care facilities, potentially resulting in long-lasting damage to the economy of an area. The inability to access resources, provision of food and clean water, delays in responding and the effects on the lives of responders outside of their employment should be considered during planning for floods, particularly for extended out-of-area response.

Water treatment works

Water treatment works can be affected by flooding, preventing the production of clean water for drinking and bathing. Prolonged loss can increase the risk of ill health and lead to dehydration amongst vulnerable communities. Although the provision of clean water during an emergency will be considered as part of a response to flood, this supply will be limited and costly. Where possible, water treatment facilities should be protected rather than relying on temporary provision.

Transport networks

Flooding can make transport routes impassable and prevent access and egress to incidents. Safe routes may become congested as the public attempt to evacuate or find passable routes. Landslides linked to flooding may cause routes to become blocked and areas of road or rail may become so damaged by water or debris that they cannot be used after flood waters recede. People may become stranded either on the road network or at transport hubs and require evacuation.