Floods and flooding are natural processes.  However, if they affect built-up areas or encroach on essential infrastructure (e.g. power stations, water supply facilities, communication systems, transportation links) or important public services (e.g. hospitals, medical centres, schools, fire/police/ambulance stations, care homes) then they pose a great physical, social and economic threat to the people, property and services present.

Sources of Flooding

The most common forms of floods are:

Fluvial Flooding

That occurs when a watercourse cannot cope with the water draining into it from the surrounding land.  This can happen, for example, when heavy rain falls on an already waterlogged catchment.

Tidal Flooding

That results from a combination of high tides and stormy conditions.  If low atmospheric pressure coincides with a high tide, a tidal surge may happen which can cause serious flooding.

Surface Water Flooding

Which occurs when heavy rainfall overwhelms the drainage capacity of the local area.  It is difficult to predict and pinpoint, much more so than river or coastal flooding. This type of flooding is also known as ‘pluvial’ flooding.

Sewer Flooding

That occurs when sewers are overwhelmed by heavy rainfall or when they become blocked.  The likelihood of flooding depends on the capacity of the local sewerage system.  Land and property can be flooded with water contaminated with raw sewage as a result. Rivers can also become polluted by sewer overflows.

Groundwater Flooding

That occurs when water levels in the ground rise above surface levels. It is most likely to occur in areas underlain by permeable rocks, called aquifers.  These can be extensive, regional aquifers, such as chalk or sandstone, or may be more local sand or river gravels in valley bottoms underlain by less permeable rocks.

Reservoir Flooding

That occurs when the dam structure fails releasing a large volume of water very quickly. Some reservoirs hold large volumes of water above ground level, contained by walls, or 'dams'. Although the safety record for reservoirs in the UK is excellent, it is still possible that a dam could fail.

Sources of Flooding