Flood risk maps and flooding zones

There is a fairly common missunderstanding as to what the EA flood risk maps show. The EA flood risk maps are intended to provide basic guidance not professional advice.

EA Flood risk maps are not…

The flood risk maps are not directly applicable for individual properties. The flood risk maps are only dealing with one of the elements of flooding and are based on hydraulic modelled zones. They are not comprehensive. Often individual features such as brick walls, houses,fences and even kerbs could result in actual flooding being different. Flood risk maps should be considered as an aid to general understanding of the risk of flooding.

EA Flood risk map descriptions:

1. Floodplain

A floodplain is the area that would naturally be affected by flooding if a river rises above its banks, or high tides and stormy seas cause flooding in coastal areas.

There are two different kinds of area shown on the EA Flood risk maps. They can be described as follows:

  • Dark blue  shows the area that could be affected by flooding, either from rivers or the sea, if there were no flood defences. This area could be flooded:

    • from the sea by a flood that has a 0.5 per cent (1 in 200) or greater chance of happening each year;
    • or from a river by a flood that has a 1 per cent (1 in 100) or greater chance of happening each year.

(For planning and development purposes, this is the same as Flood Zone 3, in England only.)

There is a difference between functional flood plains and hydrological flood plains.

  • Light blue  shows the additional extent of an extreme flood from rivers or the sea. These outlying areas are likely to be affected by a major flood, with up to a 0.1 per cent (1 in 1000) chance of occurring each year.

(For planning and development purposes, this is the same as Flood Zone 2, in England only.)

These two colours show the extent of the natural floodplain if there were no flood defences or certain other manmade structures and channel improvements.

  • Clear   shows the area where flooding from rivers and the sea is very unlikely. There is less than a 0.1 per cent (1 in 1000) chance of flooding occurring each year. The majority of England and Wales falls within this area.

(For planning and development purposes, this is the same as Flood Zone 1, in England only.)

2. Flood Defences

The purple line  shows all flood defences built in the last five years to protect against river floods with a 1per cent (1 in 100) chance of happening each year, or floods from the sea with a 0.5 per cent (1 in 200) chance of happening each year, together with some, but not all, older defences and defences which protect against smaller floods. Flood defences that are not yet shown and the areas that benefit from them, will be gradually added. The EA sometimes allocate such zones to try and secure funds from developments in the area towards such their schemes.
Hatched areas  benefit from the flood defences shown, in the event of a river flood with a 1 per cent (1 in 100) chance of happening each year, or a flood from the sea with a 0.5 per cent (1 in 200) chance of happening each year. If the defences were not there, these areas would be flooded. Flood defence zones are not consistantly shown. It can be considered to be a "work in progress".

Flood defences do not completely remove the chance of flooding and can be overtopped or fail in extreme weather conditions. Full information on during what event they would fail is not always available and is a "work in progress".

It is important to consider all the flood risks applicable to a household or proposed development. These include:

  • River and sea flooding
  • Stream, ditch and minor watercourse
  • Groundwater or vadose
  • Surface water
  • Reservoir/lake/pond breach
  • Foul, surface or water mains service burst.

The evidence to consider these is not located centrally anywhere in the UK.

Assessment of flood risks requires considerable hydraulic experience and should always be undertaken by employing a civil engineer with specialist experience in hydraulics, such as Wilsham Consulting. 

The EA flood risk maps and zones are therfore useful for considering some of the applicable flood risks is the Environment Agency website.

Your Local Authority should also have a stategic flood risk map. This should confirm which flood risk zone you site/house is located in. The quality of these plans is highly variable across the UK. Some Local Authorities invested in professional plans. Unfortuately many Local Authorities did not understand what the purpose was and some obtained relatively meaningless generic reports with barely a single plan showing flood zones and then at a scale which is practically useless.

Beware Flood Risk Report and Flood Risk Map scams

Beware of the multitiude of scam websites offering "professional flood risk assessments" for around £20. They are basic computer generated screening reports, largely offering information you could get for free from the EA. They are not compliant and don't consider all the risks. They have fine print excluding their responsibility and often work to a general post code which could be completely inappropriate for your site.

Says Wilsham Consulting Director – Stephen Gibson.

Beware also companies charging for flood risk maps and flood risk mapping. Check that the flood risk maps they provide are not already free from the EA.

A professional flood risk report should be…

A professional flood risk assessment, is a bespoke report produced by an experience professional for your house, site or development. Done correctly, it usually takes a number of days and that costs money.

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I want you to help, how do I move it forward?

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You can:

yes Speak to Wilsham Consulting Director – Stephen Gibson directly about your project on 01235 529646

mail Email your proposals directly to Stephen Gibson at stephen@wilsham.co.uk

Please forward the following:

  • Planning conditions
  • Any existing FRA report
  • Full site location details
  • Architects base layout
  • Site photos