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Flooding and Ground Water

Flooding

There are three main types of flooding:

• Coastal flooding
  Coastal flooding is caused by the failure of sea defences, either because of local sea- level rise or as a result of their being breached during storm tides. Once sea defences are overwhelmed there can be prolonged flooding of low-lying areas behind them. With sea levels rising and storminess set to increase, coastal flooding is likely to become a growing problem.

  Defences can be built to protect certain areas from rising sea levels, but only at great cost and an increased risk to neighbouring coastline. In some cases Defra will choose ‘managed realignment’ – a process by which the sea is allowed to claim some areas of low lying land to protect others.
   

• River flooding

  Rivers carrying excessive flow can breach their banks, and in the worst cases may overwhelm flood protection. With increases in winter rainfall, river flooding may become more common.

• Run-off flooding (flash flooding)

  Heavy rain in a short space of time can overcome the capacity of local drainage and cause brief but sudden flooding. This is most likely when the rain is falling onto water-resistant surfaces -dry soil or paving- and so is a particular problem in urban areas.
   

Increases in summer drought followed by heavier rainfall will make flash flooding an increasing problem, especially in cities.

It seems certain that one of the effects of climate change in England will be flooding on a larger scale in most winters than we have been used to. Recent floods in South Yorkshire, Gloucestershire and Oxfordshire have shown how much damage and disruption this can cause.

Hotter drier summers tend to dry out clay soils. This increases the risk of run-off flooding when the winter rainfalls start. Meanwhile sea levels are predicted to continue to rise . In South East England this could be by as much 26 to 86cm above current levels by the 2080s.

In England, Defra has overall policy responsibility for flood and coastal erosion risk with the Environment Agency responsible for inland and coastal flood defence.
 

There are a number of actions a householder can take to prevent or at least minimise damage from flooding. These are explained in detail in the English Heritage technical advice note.

Ground water problems

Heavier winter rainfalls, higher temperatures, and drier summers are all likely to lead to increased subsidence. Water tables may drop if the demand on aquifers increases during drier summers, and shrinking and drying of clay soil may also increase subsidence. This is likely to be worst in the South East where the soils are clay-rich, summer rainfall seems likely to decrease and farming already makes extensive use of ground water. As the soil dries, trees send out longer and deeper roots, which may cause problems with foundations and drains.

The problem has been exacerbated by the extensive paving-over of open ground for roads, footpaths, driveways, and even paved gardens.  This prevents rainwater from soaking into the earth and replenishing the aquifers.The shallow foundations of traditional buildings can be vulnerable to the natural expansion and contraction of the ground particularly in areas where highly plastic clay dominates. Traditional materials such as lime mortar can absorb a good deal of movement. If subsidence is suspected and the movement is progressive and is threatening the safety of the building then a structural engineer familiar with traditional buildings should be consulted.
 

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