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Flood Maps - Frequently asked questions

What are flood maps?

Indicative flood maps - River & Tidal flood risk

These maps provide an indication of flood risk from rivers and the sea for the Isle of Man. Although they give a good indication of the areas of land at risk of flooding, they do not provide detail of the risk to individual properties.

Flood risk areas are areas around the Island which could be affected in the event of flooding from rivers and the sea. 

  • in the case of river flooding, the flood maps indicate the extent of a flood with a 1% (1 in 100) chance of happening in any year

  • in the case of flooding from the sea, the flood maps indicate the extent of a flood with a 0.5% (1 in 200) chance of happening in any year

The flood maps also take into account climate change for both river and coastal flooding

Indicative flood maps - Surface water flood risk

Manx Utilities have developed island-wide indicative flood maps for surface water or pluvial sources.

Surface water risk is flooding from intense rainfall. It is when a downpour of rain saturates the urban drainage system and the excess water cannot be absorbed. Ponding on roads can be referred to as surface water flooding.

The following events have been mapped:

  • Extent of a flood with a 3% (1 in 30) chance of happening in any year

  • Extent of a flood with a 1% (1 in 100) chance of happening in any year

  • Extent of a flood with a <1% (1 in 1000) chance of happening in any year

The 1 in 100-year outline has been incorporated into the interactive maps developed as part of the National Strategy on Sea Defences, Flooding and Coastal Erosion. They do not show the level of risk to individual properties. You can view the maps here.

How are flood maps developed?

How are flood maps developed?

The maps are developed using a combination of:

  • Detailed survey data from the relevant river and floodplain areas;

  • Hydrology assessments of rainfall profiles, tidal conditions and catchment conditions; and

  • Computer modelling to enable the estimation of water levels and determine the likely extent of flooding.

Will the flood maps be updated and could they change?

The national flood risk maps were first developed in 2010. Since then the maps have been reviewed and updated several times. Manx Utilities plan to review the maps as and when new data becomes available or there is an improved understanding of flood risk. The indicative flood maps are not definitive boundaries, they are, and always will be subject to change.

Advances in science and technology means that modelling processes and tools continue to evolve and improve over time and ultimately improvements can be seen in the outputs produced i.e. flood maps.

Some of the models available for the Isle of Man have undergone improvements as a result of new data and advances in modelling tools. The changes have allowed for a better understanding of channel capacity, better representation of interactions between channel and floodplain, as well as more explicit account of floodplain topography and possible floodplain flood routes and storage areas. Because of the improved modelling, our understanding of the degree of flood risk has changed in some locations and these have been reflected in new revisions of the flood maps.

Data collection is an ongoing process that will continue to feed into the development and improvements of modelling, mapping, and our understanding of flood risk on the island.

What if I disagree with the flood maps?

The information provided is not at a resolution appropriate for localised flood risk assessments and should not be assumed as definitive at the property scale. Individual properties may, therefore, not always be at the same risk of flooding as surrounding

areas/properties. If you believe that a particular property has a different risk of flooding to that shown on the Flood Map and/or if you have information you believe may not have taken into account when preparing the Flood Map please contact us on (01624) 687687. If this relates to a planning application, it is likely that you will be required to carry out further flood risk assessments at property level.

If my property is in a flood map, what does that mean?

The flood maps are core to the Planning Policy Statement and allow for an assessment of where development should generally be avoided. Should any planning applications be submitted to make alterations or additions to your property it is likely further flood risk assessments at property level would be required. 

The purpose of the maps is also to raise awareness of the potential risk in a rare and high magnitude flood event. Please see our be flood prepared page for information.

The insurance industry set criteria based on their own information and Manx Utilities have no involvement in this.

 

How accurate are the flood maps?

The flood maps have been produced in line with UK standard practice and the latest techniques, using the best information we currently have, and they should give a good indication of the areas at risk of flooding. 

However, flood mapping is a complex, detailed and extensive process which involves an approximation of natural features and processes.

 

Why are there no flood maps for my area?

Manx Utilities have developed flood maps for all designated main watercourses and some other non-designated watercourses where there is a known history of flooding. The extent of flood mapping is limited to the data available. Manx Utilities will continue to review the areas covered.

Flood map disclaimer

Flood Maps displaying the indicative 1 in 100-year river and 1 in 200-year tidal flood outlines:

Please note the flood map only shows the predicted likelihood of flooding from rivers or the sea for defined areas and is not sufficiently detailed to account for individual addresses. Individual properties may, therefore, not always be at the same risk of flooding as surrounding areas/properties. If you believe that a particular property has a different risk of flooding to that shown on the flood map and/or if you have information you believe we may not have taken into account when preparing the flood map please contact us.

Climate change is accounted for in the river flood by increasing the 1 in 100 year peak flow estimate by 20%. For the tidal flood, an annual climate change allowance has been added for 60 years.

Whilst flood defences [are/may be] indicated on the flood map, given that they do not completely remove the risk of flooding and can be breached or fail in extreme weather conditions the flood map does not take into account flood defences and/or the effect any such defences (in place now or in the future) may have on the risk of flooding.

As and when additional relevant information becomes available to us the flood map will be improved and updated. The flood map was produced on behalf of Manx Utilities and all or any intellectual property rights in and to the flood map vest in Manx Utilities.

We believe that the flood map and information contained therein accords with the information currently available to it, but no representations or warranties (expressed or implied) are given with regard to the accuracy of the flood map. To the fullest extent permitted by law Manx Utilities will not accept liability for any losses resulting in connection with the flood map.

Mapping is based upon the Isle of Man survey map with permission of the Department of Infrastructure.

© Crown Copyright Department of Infrastructure, Isle of Man

Unauthorised reproduction infringes copyright and may lead to prosecution or civil proceedings.

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