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Sources of Flooding

There are multiple sources of flooding, and it should be noted that an area can suffer from more than one single source of flooding simultaneously.  Sources of flooding include groundwater, surface water, river, coastal, reservoir and sewer flooding (please scroll down this page for further information).  Whilst the terminology can appear confusing, the net result is usually the same: unwanted flood waters enter your property and cause significant and long lasting damage, not to mention the trauma associated with seeing your precious possessions consigned to the skip, standing by helplessly whilst dirty, contaminated water wrecks the fabric of the building and its contents. Though property owners and tenants in traditionally high flood risk areas may well be aware of their local flood risk, we are increasingly seeing areas of the UK under water following a prolonged period of intense rainfall, despite being nowhere near a river or water course.   It is this need to educate and raise awareness that drives the Flood Advisory Service in its work of community outreach and information sharing.

Click on following link for further information on up to date flood risk in your area:



Check your flood risk

 The following map is a useful indicator of flood risk from river and coastal flooding, however it does not address the risk of surface water, ground water, run off or sewer flooding.  There are a few specialist organisations operating in the ground-breaking area of flood risk mapping and modelling, and these, in addition to the Environment Agency will provide the cutting edge technology that we need to help manage flood risk at a property level.

My Flood Risk

Groundwater Flooding

Groundwater is a general term that refers to any water found beneath the surface that fills pores or cracks in the underlying soil and rocks.

When rain falls on the surface, some of the water may run off directly over the surface into rivers, streams and drains, some may evaporate and some may be taken up by plants. However, a significant proportion of the rain is absorbed by the soil, and subsequently seeps into deeper layers of soil and rock below the surface.

At a certain depth below the ground, the soils and rock will become saturated with the water that has seeped through from above. The boundary between this deeper, permanently saturated ground and the shallower, drier ground above is known as the water table. 

Click here to view an online booklet produced by the Environment Agency on the impacts of groundwater flooding.

Groundwater flooding occurs when water tables are higher than usual, and during prolonged periods of high rainfall, the level of groundwater rises to the surface, beyond its normal range, which causes flooding.  Groundwater flooding is significant as it is the number 1 risk to infrastructure and property assets as climate change accelerates; estimated to cost £530 million in average economic loss in England alone.


Surface Water Flooding

Surface water flooding occurs when intense rainfall creates a flood event independent of an overflowing watercourse.   

One of the most common misconceptions about flood risk is that one must be located near a body of water to be at risk. Surface water flooding debunks that myth, as it can happen in any urban area - even higher elevation areas that lie above coastal and river floodplains.

There are two common types of surface water flooding:

  • Intense rain saturates an urban drainage system. The system becomes overwhelmed and water flows out into streets and nearby structures.
  • Run-off or flowing water from rain falling on hillsides that are unable to absorb the water. Hillsides with recent forest fires are notorious sources of surface water floods, as are suburban communities on hillsides.

This can occur during any of our seasons, despite common perceptions of flooding occurring during the winter months.  Some of the worst flooding the UK has ever seen has happened in June/July/August, for example the catastrophic floods of June and July 2007 which cost the country approximately £3.2bn.  

Click here for further information on surface water flooding.


River (Fluvial) Flooding

River flooding occurs when excessive rainfall over an extended period of time causes a river to exceed its capacity.

It can also be caused by heavy snow melt. The damage from a river flood can be widespread as the overflow affects smaller rivers downstream, inundating lower lying areas.

There are two main types of river flooding:

  • Overtopping flooding occurs when water rises overflows over the edges of a river or stream. This is the most common and can occur in any size channel — from small streams to huge rivers.
  • Flash flooding is characterised by an intense, high velocity torrent of water that occurs in an existing river channel with little to no notice. Flash floods are very dangerous and destructive not only because of the force of the water, but also the hurtling debris that is often swept up in the flow.

Coastal Flooding

Coastal flooding occurs when normally dry, low-lying land is flooded by seawater.

The extent of coastal flooding is a function of the elevation inland flood waters penetrate which is controlled by the topography of the coastal land exposed to flooding.  Coastal flooding happens because of a combination of high tides, storm surges and waves. A storm surge is a temporary large-scale rise in sea level caused by strong winds pushing water towards the coast where it ‘piles up’, and by low pressure at the centre of storms – this ‘pulls’ the sea surface up by about 1cm for every millibar that air pressure drops. Often, the worst coastal flooding occurs when the peak storm surge coincides with high spring tide. Storms can also produce large waves, which can overtop coastal defences and cause erosion.

The worst natural disaster to affect the UK in modern times was the ‘Big Flood’ of early 1953. 307 people were killed in South-east England and 24,000 fled their homes, while almost 2,000 lives were lost in the Netherlands and Belgium. This was the driving force for the creation of the Thames Storm Surge Barrier and other flood defence schemes around the country. It also led to the establishment of the UK Coastal Monitoring and Forecasting Service. Today, this provides warnings of impending high sea levels, helping people prepare for flooding emergencies.

Over the winter of 2013/14, these flood defence and forecasting services were tested on a national scale when storms and floods relentlessly hit the UK coast, triggering intense media coverage and public attention. On several occasions the government assembled the Cobra crisis committee.

Particular events stand out. First, the storm on 5-6 December 2013 that generated what was widely referred to as ‘the biggest storm surge for 60 years’ and flooded 2,800 homes and 1,000 businesses. Second, the storm in early February that destroyed the Dawlish railway in Devon. Third, the dramatic ‘Valentine’s Day Storm’, which placed the south coast under severe flood alert. The fact that the damage was so limited during these storms, compared to the tragedy of 1953, is thanks to significant government investment in coastal defences, flood forecasting, sea-level monitoring and improved communications. However, 2.5 million people and £150 billion of assets are still at risk from coastal flooding in the UK today.


Reservoir Flooding

Reservoir flooding can occur as a result of dam failure. 

The likelihood of reservoir flooding is much lower than other forms of flooding. Current reservoir regulation, which has been further enhanced by the Flood and Water Management Act, aims to make sure that all reservoirs are properly maintained and monitored in order to detect and repair any problem.  In England and Wales, the Environment Agency has a regulatory role for reservoir safety, under the Reservoirs Act 1975. It ensures that reservoirs are regularly inspected and essential safety works are carried out.


Sewer Flooding

Most sewage flooding incidents are the result of overloaded sewers following heavy rainfall or blockages caused by misuse of the sewerage system.  

Another cause of sewer flooding is blocked pipes: pipes can block with fatty build ups, root growth and non-flushable objects like wet wipes.


The table below shows who is responsible for each type of flooding

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The information contained in this website is for general information purposes only. The information is provided by the Flood Advisory Service and while we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the website or the information, products, services, or related graphics contained on the website for any purpose. Any reliance you place on such information is therefore strictly at your own risk. In no event will the Flood Advisory Service be liable for any damages including, without limitation, indirect or consequential damages, or any damages whatsoever arising from the use or in connection with such use or loss of use of the site, whether in contract or in negligence.