How Flood Maps (NI) was produced

Flood Maps (NI) has been developed to provide a general overview of the flood risk in Northern Ireland. Its main aim is to increase awareness among the general public, local authorities and other organisations, of the likelihood of flooding and to encourage them to take appropriate action to manage the risk.

What is Flood Maps (NI)?

Flood Maps (NI) contains a suite of detailed flood hazard maps that have been produced in accordance with the requirements of the EU Floods Directive. These maps have been prepared for areas that have been determined by government to be at significant risk of flooding and are an important step that will lead to the development of flood risk management plans for these areas.  DfI Rivers is actively engaging with all stakeholders in the development of these catchment-based plans which are programmed for completion by the end of 2015. The shared challenge will be to produce plans that contain appropriate, affordable solutions that reduce flood risk and protect and enhance the environment.

Flood Maps (NI) is used by DOE to inform the planning process in terms of controlling development and in the preparation of development plans. DOE will also use other sources of information to inform planning decisions such as historical evidence and where required, detailed site specific flood risk assessments.

The publication of Flood Maps (NI) is consistent with the approach that is taken to the communication of flood risk in Scotland, England and Wales where similar flood maps are available through the Environment Agency and SEPA websites.

How have the flood maps been created?

In the case of the historical flood map, the flooded area outlines are based on records (ie maps, sketches, photographs, flood height measurements) taken or obtained by DfI Rivers at the time of actual flood events.  In contrast, the flood outlines in all of the other map layers have been produced using predictive flood models. Broad scale strategic flood models, that cover the whole of the province, have been developed for each of the flood sources (rivers, sea and surface water).  In addition, detailed flood models for rivers and the sea have been produced for areas that are determined to be at significant risk of flooding.  The maps produced from the detailed flood models are always displayed in favour of those produced using the strategic models.

Technical information about the methodologies used to create the flood maps

Fluvial hazard map

Rivers have been hydro-dynamically modelled using latest  (1D/2D) modelling software. The data in the models include topographical surveys of the river channels, bridges, culverts, weirs and flood defences.

The Digital Terrain Model (DTM) is derived from Light Detection and Ranging Radar (LIDAR) that provided ground height data. Hydrology is based on the Floods Estimation Handbook (FEH) methodology and applied as inflow hydrographs. Models are sensitivity tested and validated against hydrometric stations and/or known historical data where available.

Models are run for the high, medium and low probability. The mapping produced takes into account flood defences and the extent portrayed is the maximum extent of inundation over the model run.

All fluvial hazard mapping is based on river modeling with the appropriate downstream and upstream boundary condition.

Coastal downstream boundary models eg River Lagan, Newry River and River Foyle, the fluvial model output is based on a median (Q2) tidal curve downstream boundary combined with the low, medium or high probability fluvial model run.

For Lough Neagh boundary models eg River Blackwater, Sixmilewater and River Maine, the fluvial model output is based on a 13.0 m OD downstream boundary combined with the low, medium or high probability fluvial model run.

In urban areas DfI Rivers culvert and open watercourse network has been modelled and integrated with the main river models. The mapping outputs from the culvert modelling are based on the assumption that inlet grilles are clear and the culverts are unobstructed and free flowing.

Coastal hazard map

Detailed modelling of the propagation of surge events has been carried out for Belfast Lough, Carlingford Lough and Lough Foyle.

Coastal mapping for the estuarine zones in Belfast, Newry and Londonderry are based on modelling in 1D/2D using the high (Q10), medium (Q200), low (Q1000) probability tidal curves as the downstream boundary condition with the respective base fluvial flow on the River Lagan, Newry River and River Foyle.

The mapping produced takes into account flood defences and the extent portrayed is the maximum extent of inundation over the model run.
Coastal mapping of non estuarine urban areas are based on extreme tide levels derived for the strategic flood map. The mapping of non estuarine areas is based on a horizontal projection method, this GIS modelling approach identifies all land at a lower elevation than the relevant design sea level as being at risk of flooding.

Surface water map

The surface water flood outlines have been generated using a relatively new and largely untested strategic surface water modelling technique.

Although the surface water model used is currently the best available for a broad scale strategic assessment and similar to the models used by the other flood authorities within the UK and Ireland, the results should be treated with a degree of caution due to uncertainties in the methodology.

For example, there is some difficulty in establishing an appropriate extreme rainfall event storm profile for specific return periods; there are deficiencies in the topographical data which can markedly affect the flood routing within the model and broad assumptions have been made about the capacity of existing drainage systems which affect the estimates of net surface water flows, particularly in urban areas.

As a consequence the flooded area maps derived from the model should be treated with caution and viewed as indicative rather than accurate or precise.

How accurate are the flood maps?

Within the indicative flood maps each of the flood maps for the various sources are based, at least in part, on strategic flood models which use fairly coarse modelling techniques and data.  As there is a moderate degree of uncertainty in regard to the extents of the flood plains derived from these strategic models, these maps should be used only to identify general areas that may be prone to flooding.  The indicative flood maps should not be viewed as accurate and cannot be used to predict the flood risk to a particular property or specific point location.

There is always some uncertainty in regard to the accuracy of predictive flood models. However, the detailed models used to produce the flood hazard maps for rivers and the sea have been developed using the best available modelling techniques, tools and data and as a result there is a high degree of confidence in their accuracy.  As a result, the hazard maps for rivers and sea are considered suitable for assessing the likelihood of flooding to individual properties.  

Although there are ‘detailed’ flood hazard maps available for surface water these have been produced using a strategic model and consequently, these should be viewed as ‘indicative maps’ and used only to identify general areas that may be prone to surface water flooding.

Does Floods Map (NI) take into account knowledge from previous floods?

With Flood Maps (NI) there is an historical flood map which highlights the areas that have been inundated during actual flood events in the past. This map has been generated using data that was collected by DfI Rivers at the time of the flood event and from aerial photographs. The historical flood map will continue to be updated as new information on future floods becomes available.  Although this information is useful insofar as it can serve as a reminder that the risk of flooding is very real, it does not of itself, provide clear insight to the likelihood of flooding now or in the future. The likelihood of flooding can only be established, with any degree of confidence, using predictive modelling techniques.  However, the historical flood data does have an important role to play in the validation of the predictive models by ensuring that their outputs are credible.

When and how often will Flood Maps (NI) be updated?

DfI Rivers is committed to providing the best available flood risk information to as wide an audience as possible and therefore updates and revisions to Flood Maps (NI) will be provided as appropriate. For example, DfI Rivers shall continue to collect data on actual flood events as they occur and, if these are considered to be significant, the Historical flood map layer shall be updated. Similarly, as we develop more detailed flood models and flood maps to meet the requirements of our capital works programme, the content of the viewer will be updated to reflect this new information.

Does Flood Maps (NI) cover small streams and burns?

Flood Maps (NI) does not show the floodplain for any watercourse where the catchment area is less than 3km2. As the map was developed to provide a general overview of flooding from rivers, it would not have been practical or appropriate to undertake the hydraulic modelling of every sheugh or ditch.

Does Flood Maps (NI) take the effects of climate change into account?

Within the indicative flood maps there are two distinct series of maps which are titled ‘Present Day’ and ‘Climate Change’. The present day map layer was established through modelling techniques which use the meteorological, river flow and sea level data which is relevant at this point in time and to that extent this has taken account of any changes to our climate that have already occurred.

It is however widely accepted that our climate is changing and that with the passage of time sea levels around our coastline will rise and the extreme rainfall events may become more frequent and intense. Therefore with this in mind, the climate change map layer has been produced to reflect the flood plain outlines for the year 2030 and this is based on the best available predictions for the meteorological conditions and sea levels at that time.

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