There are a number of different sources of flooding including:
Rivers and Streams
Excessive rainfall, snow or hail, or a combination of high river levels and high tides can cause river flooding. Flooding occurs when surface water run-off from the surrounding area exceeds the flow capacity of the river or stream. Saturation of surface soils due to wet weather can lead to greater run-off rates and higher flooding levels.
Human activity has increased the risk of flooding from rivers and streams in many areas. Development has reduced the natural capacity of floodplains and increased the rate of surface water run-off.
Most areas are protected against river flooding by man-made flood defences. While these defences reduce the likelihood of flooding they cannot eliminate risk altogether. Defences are designed to withstand specific flood heights but they can be breached or overtopped should more extreme events occur.
Flooding from the sea can be caused by high tides, storm surges, waves overtopping or breaching sea defences, or a combination of these factors.
Flooding from groundwater is most likely to occur in areas of chalk, limestone or other aquifers. This type of flooding generally affects older buildings that back onto hillsides, buildings close to winterbourne streams or houses with basements that are particularly prone to groundwater flooding.
Depending on the local geology, groundwater flooding can take a long time to recede. Properties can still be underwater many months after the heavy rains that caused the flooding have passed.
Overland flows can be caused by heavy rainfall falling on saturated ground, where groundwater levels are already high, or on paved areas of tarmac or concrete with inadequate drainage. Properties can be flooded by overland flows if they are located in areas where floodwater can accumulate. Paved areas, such as roads, can act as channels for overland flows.
Blocked or overloaded drainage systems
Localized flash flooding from blocked or overloaded drainage systems can occur at times of heavy rainfall. This type of flooding is unpredictable and often occurs in unexpected locations depending on the location and intensity of rainfall. Such drainage systems include open drainage ditches & culverts and buried drains and sewers.
Where flooding occurs from foul sewers the floodwater will often be contaminated with sewage. In some cases, contaminated floodwater can flow back though foul sewers causing flooding inside buildings. Flooding can also be caused by blocked or overloaded roof drainage systems.
Broken water mains
Localized flooding can be caused by burst water mains, although this rarely leads to property flooding above ground level. Basements can be flooded if a water main bursts close by.
(China.org.cn September 12, 2008)