Floods can happen anywhere and at any time, but the most common sources of flooding are from:
Rivers and Coastal Waters
River flooding occurs when a watercourse cannot cope with the run-off, due to rainfall or melting snow, from the surrounding catchment. The UK has experienced an increased awareness of the effects of river flooding in recent years.
Coastal flooding usually occurs during a combination of high tides and stormy conditions. Tidal surges can occur when low atmospheric pressure coincides with a high tide.
Surface Water Drainage
In simple terms this generally occurs after heavy rainfall; which exceeds the capacity of the drainage system. However as Surface Water flooding can be affected by many factors it can become a complex subject. The conveyance of surface water into the underground drainage system is often restricted by poorly maintained systems and inlets, e.g gullies and drainage channels.
Whilst any flooding is distressing Sewer flooding is arguably one of the most distressing types of flooding and it poses an increased risk to physical health. In 2007-08 over 7,500 properties in the UK were flooded internally from sewers.
Flooding can be caused by blockages within the sewer network or by heavy rainfall in areas utilising combined sewers. Whilst it is now uncommon to build new combined sewers there are a large number of systems dating back to Victorian times still in use. It is estimated that between the UK’s twelve water companies there are in excess of 20,000 combined sewer overflows which discharge into Rivers and the sea when the capacity of the drainage system is reached.
Flooding occurs when water levels in the ground rise above ground level. It is most likely to occur in areas above aquifers. Hydrogeology deals with the distribution and movement of groundwater and is an interdisciplinary subject that is quite complex. Groundwater does not always flow down-hill as flow is in relation to pressure meaning that flow can be upwards from deeper high pressure areas into shallower low pressure areas.
Some reservoirs hold large volumes of water above the surrounding natural ground level. Whilst in the UK the safety record for reservoirs is excellent, they still pose the risk of rapidly releasing a large quantity of water should they fail.