The Hidden Flood Risk - Surface Water flooding (‘Pluvial Flooding’)

Surface water flooding is when there is an overflow of water from the drains and sewers when they are no longer able to cope with the level or frequency of water draining into them, usually during prolonged , heavy rainfall. Such flooding can happen rapidly as a result of a sudden, exceptional downpour or after several days or weeks of heavy rain due to the fact that the surrounding ground becomes saturated and is unable to absorb further water which then flows into the drains and sewers and flooding occurs.

Flooding heightened by urbanization

Surface water flooding has become an increasing issue as more and more land is built on with, for example, concrete and tarmac surfaces making it less permeable and reducing the space available for the surface water to flow or ‘run-off’. Other factors in the increase in urban flooding are cited as being the increase in the number of houses of multiple occupation, pressure on parking (which causes more and more people to turn their front gardens into hard standing), and the deregulation of planning for many home extensions. Another much cited cause of the increase in urban flooding is the lack of investment over many years in the sewerage infrastructure which has left many towns and cities with antiquated Victorian sewers having to cope with a demand many times larger than that envisaged when they were first constructed.

Hidden Cause of Urban flooding

The risk of surface water flooding can be said to be ‘hidden’ as it is not taken into account in the Environment Agency flood risk warnings – despite the fact that the Environment Agency themselves estimated, in 2009, that around 3.8 million properties in England are susceptible to surface water flooding compared to 2.4 million properties that are at risk of flooding from rivers (‘Fluvial flooding’). RICS state in their publication ‘A clear guide to Flooding For Property Owners’ that around 50% of UK flooding is due to surface water or groundwater flooding.

The Environment Agency has recently introduced flood maps for surface water flooding on their website, however this is very limited in its application as surface water flooding is often localised and may occur to just one or two properties in isolation as well as whole streets and estates.

Properties that have never been affected by flooding and are not considered to be in a flood risk area , may fall victim to surface water flooding often due to issues out of the control of the owner of the Property. For example, if a new estate is built locally and the developers use the common method of raising the ground level to reduce the chance of the estate being affected by flooding this may have the effect of making the surface water flow downhill and cause flooding in the lower surrounding properties. This is a common side effect where new developments are built on flood plains due to the fact that the flood plains can no longer accommodate the excess water – the water has to go somewhere and is thus spreads causing flooding in the previously flood free neighbouring properties.

The Risk of Surface Water Flooding and Conveyancing

When acting for a purchaser it is becoming increasingly important for a Conveyancer to address the issue of flooding. This is particularly the case with newly built properties in light of the fact that all properties built after 2009 are excluded from the Governments recent agreement with the Buildings Insurance companies over insuring properties in areas prone to flooding.

When dealing with new builds it is important to:
  • Read the Planning Consents carefully as they will often contain conditions relating to flood risk assessments and measures to mitigate the risk of flooding. It is important to get evidence that these have been complied with.
  • The environmental report (if provided) or the results of the environmental search carried out as part of the conveyancing processes.
  • Look at the detail of the results of the environmental search in relation to flooding- often the front summary will say that the Property is not in a flood risk area however further in, it will reveal a risk from surface water flooding.
  • Ask what measures have been taken in the construction of the Property to mitigate the risk of flooding.
  • Ask how surface water drainage is being dealt with – be sure to highlight to the client if the surface water drainage is into the main sewer as this may pose a greater risk of flooding especially if there is a large number of properties being built on the development.
  • If the method used for surface water drainage is through ‘soak aways’ - ask for the detail, again this method is okay on its own for a few properties but may not be suitable for an estate of several hundred properties.
When dealing with properties generally, all or most of the above enquiries need to be raised and in addition, of course, the Conveyancer needs to ask not only if the Property has flooded but whether or not there has been flooding in neighbouring properties. Lastly, it needs to be noted that the insurance premium and/or availability of flood insurance can be decided by postcode. If the postcode in which the Property is situated has a history of flooding, the building insurers will be more reluctant to insure even if the Property itself has never been the victim of flooding so if the environmental search indicates that the area is at risk of flooding the question always has to be raised as to whether or not the Seller has had difficulty getting insurance or if they have a higher premium or excess and not just whether or not the Property itself has previously experienced flooding.


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