Flood risk information and its use

What do the outlines for floodplains and inundations areas show?

Floodplains are the relatively flat areas of land adjacent to rivers and sea that are estimated to be subject to periodic coverage by flood water. The extents of the floodplains have been estimated using predictive computer modelling techniques. Each of the floodplains highlighted in the maps are associated with a flood event which has a particular chance of occurring. For example the area highlighted within the 1% AEP floodplain outline for rivers has a 1% or greater chance of flooding in any year. Similarly the area highlighted within the 10% AEP floodplain has a 10% or greater chance of flooding in any year.

It is important to note that even within an illustrated floodplain some areas are more likely to flood than others. For example, an area that is located at the extreme outer edge of the 10% AEP river floodplain is estimated to have a 10% chance of flooding in any year but other areas that are situated closer to the river may have a greater than 10% chance of flooding in any year.

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. Surface water flooding occurs when the ground is unable to absorb the rainwater, causing it to flow over the surface and fill depressions and low spots in the landscape where local natural and engineered drainage systems are overwhelmed.

The Surface Water Flood Maps illustrate the low lying areas and hollows that are estimated to be prone to flooding from rainfall events with specific return periods. As small changes to the height of ground surfaces can markedly change the direction of surface water flows, it is difficult to predict with certainty the exact route along which surface water will flow and pond. Therefore the inundation areas illustrated within the Surface Water Maps should not be viewed as exact or precise.

I know that my property has never flooded - why is it shown within the flood plain?

A one-hundred-year flood is a flood event that has a 1% probability of occurring, or being exceeded in any given year.

The 100-year flood is also referred to as the 1% flood, since its annual exceedance probability is 1%. However, the flood may not occur within a 200 year period and yet conversely 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.

The tables below indicate, for 1% and 10% AEP floods, what the chances are that one or more floods will exceed the magnitude of flood over a set number of years.

100 year return period - 1% AEP*

10 year return period - 10% AEP

*AEP - Annual Exceedence Probability

*Probability that one or more flood will exceed the AEP flood over a set number of years.

Therefore, it follows that over a 40 year period, the chances of an individual witnessing the 100 year (or 1% AEP flood) is 33% or 1 in 3.

And using the same logic, over a 40 year period, the chances of an individual witnessing the 10 year flood is 99% or almost certain.

Incorrect floodplain information

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

Flood Map (NI) Contact Details

Telephone: 028 8676 8342
Email: rivers.registry@infrastructure-ni.gov.uk

Will other government agencies use Flood Maps (NI)?

Yes, other government agencies and key stakeholders, with an interest or role in the management of flood risk, will have access to the floods maps and these, along with other relevant flood risk information, will help in their flood alleviation, land use planning and emergency planning work.

Does Flood Maps (NI) differ from the flood maps which cover Scotland, England and Wales?

All of the other regional flood authorities have provided maps illustrating the areas of land which are estimated to be at risk of flooding from rivers, the sea and surface water. All of these maps have been created using a broadly similar approach and have been derived from predictive flood models. However, differences between the maps do exist as they have been developed independently.

For example, Flood Maps (NI) contains climate change flood maps which take account of the best available climate change predictions and illustrates flood plains for the year 2030. Climate change maps have not been published for other UK regions. Also, NI has produced a suite of detailed flood hazard maps at a scale of 1 in 5000, whereas some other authorities have restricted the scale of their maps to 1 in 25,000.

Why am I shown as being at risk when there are flood defences in my town?

It is important to recognise that whilst a flood defence system is designed to reduce the risk of flooding, it does not prevent it completely. Each flood defence provides a specific area with a standard of protection which can be estimated through the use of predictive flood models. For example, a flood wall which is estimated to provide a 1 in 100 year standard of protection would be expected to protect the defended area behind the defence from all floods up to and including the 1 in 100 year (1% AEP) flood event.

However if a flood defence has been determined to provide a 1 in 75 year standard of protection it would be overtopped by the 1 in 100 year (1% AEP) flood event and this would result in flooding behind the defence. The flood maps identify the locations of existing flood defence systems. These defences are colour coded to show the level of protection which they provide.

Will DfI Rivers be carrying out works to protect properties within a flood plain from flooding?

In the past, when communities or infrastructure have been affected by flooding, DfI Rivers has undertaken works to construct flood and sea defences to alleviate this risk where this was economically viable. With the introduction of the flood maps, which uses predictive modelling techniques to identify all of the areas throughout Northern Ireland that are prone to flooding, the DfI Rivers 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.

DfI Rivers is developing Flood Risk Management Plans (FRMPs) for every area that is determined to be at potential significant flood risk. The FRMPs highlight the flood hazard and flood risk in these areas, and set out objectives and measures for managing the risk. It is important to recognise that flood defence schemes can only be undertaken where 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.

For further information click: Flood Risk Management Plans.