Flood Risk Information and its Uses FAQs

   Return to Strategic Flood Map Frequently Asked Questions

1. What do the highlighted flood plain outlines in the Strategic Flood Map mean?

Flood plains are the relatively flat areas of land adjacent to rivers and sea that are estimated to be subject to periodic coverage by flood water. For the purposes of this strategic assessment, the extents of the flood plains have been estimated using predictive computer modelling techniques. The flood plains highlighted in the maps illustrate those areas that in any year have a 1 in 100 or greater chance (1% AEP) of flooding from a river or a 1 in 200 or greater chance (0.5% AEP) of flooding from the sea.

It is important to note that even within the flood plain some areas are more likely to flood than others.  For example, an area that is located at the extreme outer edge of a river flood plain will have a chance of flooding of around 1 in 100 in any year but other areas that are situated closer to the river may have a greater than 1 in 100 chance flooding in any year. However, the methodology used to generate the flood plains for the Strategic Flood Map are not sufficiently sophisticated to determine the precise likelihood or chance of flooding for specific areas within the flood plain.

Surface water does not have a ‘natural’ floodplain as such, particularly in the urban environment where the flow of the surface water is tightly constrained by the built environment.  The areas illustrated within the Surface Water Map are estimated to be prone to inundation from surface water as a consequence of a 1 in 200 rainfall event.  The inundation areas highlighted are those in which the depth of flooding is estimated to exceed 100mm.

2. Can you explain what is meant by a 1 in 100 chance of flooding in any year?

If an area that is located next to a river has a 1 in 100 chance of flooding in any year a simple way to describe this chance may be as follows:
Place 99 white balls and 1 black ball in a bag and shake the bag. Invite someone wearing a blindfold to draw a ball from the bag. If the selected ball is black the area will flood in the coming year, if the selected ball is white (which is 99 times more likely) the area will not flood in the coming year. The selected ball is returned to the bag and on the same day the following year the procedure is repeated. Likewise, if a coastal area has a 1 in 200 chance of flooding in any year the chance can be explained in a similar manner except that there will be 199 white balls and 1 black ball in the bag.

3. I live near a river and my property is located in an area within the flood plain illustrated in the Strategic Flood Map.  How likely am I to be flooded?

In itself, the Strategic Flood Map is not suitable for determining the actual likelihood of flooding to a particular property or specific point location, as this can be affected by many factors such as the building design, the local ground levels, flood defences and the effectiveness of local drainage systems. However, the flood plain outline for rivers, illustrates areas that are subject to periodic coverage by water and which could be inundated by a flood from the river that has a 1 in 100 or greater chance of happening in any year. Even within the extents of the illustrated flood plain all areas do not have an equal chance of flooding. For example, those areas immediately adjacent to a river may flood more frequently than those located on the fringes of the illustrated flood plain. Therefore it is not possible to give a more precise indication of the actual likelihood of flooding to specific locations without conducting a detailed local study.

4. I’ve lived in my area for 50 years and I know that my property has never flooded.  Why is it shown to lie within the flood plain of the local river?

A flood with a 1 in 100 chance of happening in any one year may not occur within the space of 50 years. It may not even occur within a 200 year period and yet conversely it could occur twice over the next 10 years. The only way that the accuracy of an estimated flood prediction of this nature can be judged is over a very long time period. Therefore it would be reasonable to expect that an area with a 1 in 100 chance of flooding in any year may experience flooding around 10 times over the next 1000 years however, when these events are likely to occur is impossible to predict.

5. I don’t agree with the flood plain information shown for my area. Can I challenge it?

If you believe that an area has been incorrectly highlighted as prone to flooding in the Strategic Flood Map, or that you have relevant information that we may not have taken into account, please contact us. Rivers Agency shall consider your comments and advise you of our findings. However, we are unlikely to consider an amendment to the Strategic Flood Map that is based on anecdotal evidence alone.

6. Will other government agencies use the Strategic Flood Map?

Yes, other government agencies will have access to the Strategic Flood Map along with other flood risk information they may have. The information will help in their flood alleviation, land use planning and emergency planning work.

7. Does the Strategic Flood Map (NI) differ from the flood maps that are available on the internet to cover Scotland, England and Wales?

In general terms they are very similar – all the maps provide an indication of areas of land which might be at risk of flooding from either rivers, the sea or surface water and have been created using similar approaches. However, differences between the maps do exist as they have been developed independently. For example, the Strategic Flood Map (NI) contains climate change maps which take account of the best available climate change predictions and illustrates flood plains for the year 2030.

8. I am not at risk because we have flood defences in my town. Why am I shown as being at risk?

In line with government’s Planning Policy Statement 15 (PPS 15) – Planning and Flood Risk, the flood plains illustrated in the Strategic Flood Map include those areas which would flood if it was not for the presence of an existing flood defence. However, by viewing the Flood Defence/ Defended Area Map users can identify the location of existing flood defences and the areas which benefit from a reduction in the likelihood of flooding as a consequence of their presence. It is important to recognise that whilst flood defences are designed to reduce the risk of flooding they do not prevent it completely.

9. My property is shown to be within the flood plain.  Will Rivers Agency be carrying out works to protect it from flooding?

In the past, when communities or infrastructure have been affected by flooding, Rivers Agency has undertaken works to construct flood and sea defences to alleviate this risk. With the introduction of the Strategic Flood Map, which uses predictive modelling techniques to identify all of the areas throughout Northern Ireland that are prone to flooding, the Agency now has the information it needs to develop plans to pro-actively manage the flood risk for the whole region rather than waiting for a flood event to occur and reacting to it.  The Agency has recently embarked on a process that will lead to the development of flood risk management plans for every area at potential significant flood risk throughout Northern Ireland. This enormous task will take a number of years to complete and the flood risk management plans will include proposals to provide additional defences where these are appropriate. It is important to recognise that schemes can only be undertaken were there is a robust business case to justify public funding and that potential schemes must be prioritised to secure the maximum benefit from this investment.  Therefore, whilst it is possible that flood defences may be constructed to alleviate the risk of flooding to your area, the Agency is unlikely to be able to advise if, or when this may happen, until the flood risk management plan for your area is completed. Rivers Agency shall be embarking upon a wide-ranging consultation exercise with the public and other stakeholders throughout the development of each of the flood risk management plans and shall welcome your input to the plan at that time.