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Sources of flood risk

There are several different types of flood risk. Communities here in Yorkshire could be affected by any one of the following:

River flooding

Occurs when a stream or river (watercourse) cannot cope with the amount of water that is flowing into it from the surrounding land. This can occur with rivers, usually the larger ones in the county as well as some smaller streams that are important for drainage, or ordinary watercourses, including other smaller rivers, streams or ditches.

River flooding

Coastal flooding

Caused by a combination of high tides and stormy conditions, which causes sea levels to rise above coastal defences. If low atmospheric pressure coincides with a high tide, a tidal surge can occur, which can cause serious flooding.

Tidal flooding

Groundwater flooding

Occurs when water levels in the ground rise above the surface. It is common after long periods of heavy rainfall and is more likely to happen in low-lying areas, areas with natural springs and areas built over permeable rocks or aquifers, such as chalk.

Image courtesy of East Riding of Yorkshire Council

Surface water flooding

Occurs when rainwater does not soak into the land or enter a drain or river so flows or ‘ponds’ over the surface. It usually happens during periods of heavy rainfall (over 30 millimetres per hour) and is worsened by blocked ditches and drains, dry and hard soils after periods of drought and compacted or saturated soil. This is the most common cause of flooding in the UK.

Image courtesy of City of York Council

Sewer flooding

Occurs when pipes in the sewer network are blocked or when the sewers cannot accommodate the amount of water entering them because they are not designed to cope with heavy, prolonged rainfall. Land and property can be flooded with water contaminated by raw sewage as a result, and rivers can be polluted by sewer overflows.