National Indicative Fluvial Mapping (NIFM) - User Guidance Notes

These maps are not the best achievable representation of projected flood extents, such as those that could be generated through detailed hydraulic modelling, and are only indicative of the predicted flood extent of any given probability at any particular location. They should not be used for local decision-making or any other purpose without verification and seeking the advice of a suitable professional.

Use of the National Indicative Fluvial Maps for the Purposes of Planning

The maps only provide an indication of areas that may be prone to flooding. They are not necessarily locally accurate, and should not be used as the sole basis for defining the Flood Zones nor for making decisions on planning applications.

The maps may be used in the Stage I Flood Risk Assessment (Flood Risk Identification) to identify areas where further assessment would be required if development is being considered within or adjacent to the flood extents shown on the maps. Similarly, the maps may be used to identify whether flood risk might be a relevant issue when considering a planning application, or when discussing a potential application at pre-planning stage.

Local site inspections, and / or making use of the knowledge of staff familiar with a particular area, are essential to determine if the maps for a given area are reasonable.

For the purposes of flood zoning, or making decisions on planning applications, it is strongly recommended that a Stage II Flood Risk Assessment (Initial Flood Risk Assessment), as set out in the Guidelines, is undertaken (where there are proposals for zoning or development, and where the area may be prone to flooding, as described above).

Understanding the National Indicative Fluvial Maps

These maps are ‘predictive’ flood maps showing indicative areas predicted to be inundated during a theoretical fluvial flood event with an estimated probability of occurrence, rather than information for actual floods that have occurred in the past, which is presented, where available, on the ‘past’ flood maps.

The maps refer to flood event probabilities in terms of a percentage Annual Exceedance Probability, or ‘AEP’. This represents the probability of an event of this severity occurring in any given year. They are also commonly referred to in terms of a return period (e.g. the 100-year flood), although this period is not the length of time that will elapse between two such events occurring, as, although unlikely, two very severe events may occur within a short space of time.

Table 1 sets out a range of flood event probabilities for which fluvial indicative flood maps are developed, expressed in terms of Annual Exceedance Probability (AEP), and identifies their parallels under other forms of expression.

Table 1 - Flood Event Probabilities:

Indicative flood maps have been produced for all watercourses that are on the EPA watercourse layers ‘WATER_RivNetRoutes’ and ‘WFD_LakeSegment’, have a catchment area greater than 5km 2, and for which flood maps were not produced under the National CFRAM Programme. Indicative fluvial maps have only been produced for catchments greater than 5km 2 as the chosen hydrological assessment methodology is not suitable for very small catchments. The National CFRAM Programme Flood Maps will be used for national scale risk assessments wherever these maps are available.

The watercourses for which indicative flood maps have been produced are mapped in the GIS layer ‘nifm_rcl_02’.

The NIFM datasets have been edited to remove overlaps with the datasets produced under the National CFRAM Programme and other flood studies. The NIFM datasets should be read in conjunction with the outputs of the National CFRAM Programme and other studies, as published on www.floodinfo.ie, at their downstream extremities.

Contents

There are a range of flood map types:

1. Flood Extent Maps

These maps indicate the estimated flood extents only from those river reaches that have been modelled. Flooding from other reaches of river may occur, but have not been mapped, and so areas that are not shown as being within a flood extent may therefore be at risk of flooding from un-modelled rivers (as well as from one of the other sources of flooding referred to below).

There are many other possible sources of flooding, such as tidal, surcharged urban drainage systems, ponding rainwater, groundwater or blockage of structures such as culverts. Flooding from these other sources has not been mapped, and so areas that are not shown as being within a flood extent may therefore be at risk from flooding from one or more of these other sources.

2. Flood Depth Maps

These indicate the maximum estimated depth of flooding at a given location, for a flood event of a particular probability. The flood depths are calculated by subtracting the ground levels from the predicted water level. Ground levels have been measured using Interferometric Synthetic Aperture Radar (IfSAR) to produce a Digital Terrain Model (DTM), see Technical Data below. The flood depths are mapped as constant depths over grid squares of 5m x 5m whereas in reality depths may vary within a given square.

Scenarios

Flood maps have been developed for the current scenario, and also for two potential future scenarios; the Mid-Range Future Scenario (MRFS) and the High-End Future Scenario (HEFS), taking into account the potential impacts of climate change.

The MRFS assumes a 20% increase in peak flow and the HEFS assumes a 30% increase in peak flows above the current scenario estimations.

Spatial Data Attribution

Vector spatial data is attributed as follows:

crs = Cordinate Reference System, a three digit code to represent the co-ordinate system and geoid:

• ing = Irish National Grid (EPSG: 29903) and geoid OSGM02
• itm = Irish Transverse Mercator (EPSG: 2157) and geoid OSGM02

mc = Model Code, a two digit code to represent the individual model that created an individual spatial entity. If only one model was used for the entire project, the number ‘01’ shall be entered into the relevant attribute field for all features.

ttt = Data Type, where one of the following codes shall be used as appropriate to define the Spatial Data:

• ext = Flood Extent
• dep = Flood Depth
• nod = Flood Nodes
• lev = Flood Levels

s = Source Type, one of the following codes shall be used as appropriate to define the source of flooding represented in the Spatial Data:

• f = Fluvial

c = Scenario, where one of the following codes shall be used as appropriate to define the scenario for the flooding represented in the Spatial Data:

• c = Current
• m = Mid-Range Future Scenario
• h = High End Future Scenario

r = Run Type, where one of the following codes shall be used as appropriate to define the nature of the run used to generate the flooding represented in the Spatial Data:

• c = Calibration Run
• v = Validation Run
• d = Design Run
• s = Sensitivity Run
• p = Pilot Run
• z = Other run type

pppp = Probability, return period is to be expressed as a four digit numeric character. Where the return period is not four digits, it must be represented as such by preceding the value with zero(s). Example:

a = Status, where one of the following codes shall be used to describe the status of the file:

• f = Final
• d = Internal Draft
• c = External Draft (for circulation for comment)

rn = Revision Number, This is a two digit revision number i.e. 01, 02 etc.

Accuracy

The National Indicative Fluvial Maps provide an indication of areas that may flood during a flood of an estimated probability of occurring. As detailed in the Technical Data, a number of assumptions have been made in order to produce a dataset suitable for national level flood risk assessments.

The National Indicative Fluvial Maps are not the best achievable representation of flood extents and they are not as accurate as the Flood Maps produced under the National Catchment Flood Risk Assessment and Management (CFRAM) Programme.

The maps should not be used to assess the flood risk associated with individual properties or point locations, or to replace a detailed site-specific flood risk assessment.

Date of Preparation

The indicative fluvial flood maps were finalised in December 2020.

Responsible authorities

The Office of Public Works (OPW), as the lead agency for flood risk management in Ireland, is the authority responsible for the publication of the indicative fluvial flood maps shown here.

Flood Mapping – Technical Data

Set out below is a summary of the technical process for developing the indicative fluvial flood maps.

Identification, Assessment or Calculation of Flooding probabilities or return periods

The maps refer to flood event probabilities in terms of a percentage Annual Exceedance Probability (AEP). This represents the probability of an event of this severity occurring in any given year. These probabilities are also commonly referred to in terms of a return period (e.g., the 100-year flood), although it should be understood that this period is not the length of time that will elapse between two such events occurring, as, although unlikely, two very severe events may occur within a short space of time.

The indicative fluvial flood maps are produced for the 0.1% AEP flood event, the 1% AEP flood event and the 5% AEP flood event.

Identification, Assessment or Calculation of Flooding extent

The flood extent maps indicate the estimated maximum extent of flooding (subject to limitations referred to herein) for a given flood event and flooding in some areas, such as near the edge of the flooded area, might be very shallow.

The indicative fluvial flood maps were developed using hydrodynamic modelling, based on calculated design river flows, Digital Terrain Models, and other relevant datasets (e.g. land use, data on past floods, etc.).

A summary of the process is provided below.

Hydrological analysis

1. Extract the relevant Physical Catchment Descriptor (PCD) data for each Flood Studies Update (FSU) node in the study area.
2. From these data, the index flood flow (Qmed) could be estimated at every location.
3. Growth curves were then derived so that the peak flow for the required AEP could be estimated.
4. Finally, the full shape of the flood hydrograph could be defined.

Step 1 Extraction of PCDs

All FSU nodes for watercourses covered by the National Indicative Fluvial Mapping were extracted from a nationally compiled GIS dataset. The nodes were then reconciled with the rivers to establish linear referencing between the two datasets, and to remove nodes that recorded an upstream catchment area of less than 5 km².

Step 2 Estimation of the index flood flow

The index flood Qmed was estimated from the PCD using the 7-variable equation as per Flood Studies Update Work Package 2.3, Equation 3.1:

Qmed = 1.237x10-5AREA0.937BFIsoil-0.922SAAR1.306FARL2.217DRAIND0.341S10850.185(1 + ARTDRAIN2)0.408

The ungauged estimation of Qmed was then adjusted using the pivotal site approach. Pivotal sites were selected as the nearest downstream gauge on the same river; in the circumstances where no downstream gauge exists, the nearest gauge has been used.

Step 3 Estimation of the flood growth curve

One set of growth curves were developed per Hydrometric Area (HA). The growth curves were derived from those used for the National CFRAM Programme and scaled to account for the additional years of hydrometric data available since its completion.

Step 4 Derivation of the flood hydrograph

The shape of the flood hydrographs were derived using FSU equations based on regression analysis of the available river discharge data and PCDs of the gauging stations.

Hydraulic modelling

Hydraulic modelling has been undertaken for all subject watercourses with an upstream catchment area of greater than 5 km². InfoWorks ICM has been used for the hydraulic modelling task.

The models are all 2D high resolution irregular mesh models for the river channel and floodplain.

Cross sectional surveys have not been used to define the dimensions of river channels and structures within the 2D model. Channels have been represented in the 2D model by assuming their channel capacity is equivalent to the estimation of Qmed. Where appropriate, based on channel or catchment characteristics, the relationship between Qmed and channel capacity has been modified as per Table 2.

Table 2 - Estimated channel capacity

An analysis of channel capacity versus Qmed at gauged sites in urban areas found that the median channel capacity for urban sites was 170% Qmed, however there is considerable scatter in the data. As the data did not demonstrate a consistent increase in capacity in urban areas, a conservative estimate of channel capacity equalling Qmed has been adopted. It is anticipated that the results for the results for the 5% AEP scenario would be most influenced, with less impact on the 0.1% AEP results.

Analysis was carried out to determine suitable assumptions to make regarding the capacity of various Arterial Drainage channels. This analysis included a review of:

• Arterial Drainage Scheme design drawings showing long section and cross-sections;
• Arterial Drainage design guidance describing the objectives, data required, design standards, and design process;
• Guidance from OPW Engineers;
• Price Water House Cooper (1999) Report on Measurement of Return on Investment, Arterial Drainage Maintenance Programme, Office of Public Work;
• LiDAR data in areas with Arterial Drainage channels

The resulting assumptions made regarding the capacity of Arterial Drainage channels for flow are set out in Table 3.

Table 3 Estimated capacity of Arterial Drainage channels by Hydrometric Area

Assumptions made regarding channel capacity for flow are solely made for the purpose of the NIFM and should not be applied for any other purposes.

Manning’s n roughness values have been estimated for each land use category in the CORINE land cover dataset. An increased manning’s n has been applied at known bridges to simulate the impact of afflux at this structures.

The downstream boundary of each model was based on data from one of three sources:

• Sea level at a coastal boundary
• Water level from a National CFRAM Programme hydraulic model
• Water level from the downstream NIFM model

Topographical Data

A Digital Terrain Model (DTM) is used to generate the maps. The DTM is derived from airborne survey techniques. Interferometric Synthetic Aperture Radar (IfSAR) data has been used to derive the DTM, which has a vertical and horizontal Root Mean Square Equivalent (RMSE) of typically less than 0.7m, and a grid scale of 5m.

The DTM is a ‘bare earth’ model of the ground surface with most man-made and natural landscape features such as vegetation, buildings and bridges digitally removed.

Pilot Study

The adopted hydrological and hydraulic methodology was developed and tested in two pilot Hydrometric Areas (HA), HA 18 and HA 26. Indicative flood maps were produced in the pilot study for areas previously mapped under the National CFRAM Programme. This allowed the indicative flood maps to be compared with the more accurate outputs from the National CFRAM Programme for the pilot study areas. An ‘F statistical’ measure was used to compare the two mapping outputs, where F = Number of cells that are wet in both the model AND the observed / number of cells that are wet in either the model OR the observed.

F = 100% is a perfect fit where all wet cells are the same in each dataset, F = 0% means none of the wet cells are the same. The outcome of this assessment for each pilot HA is as follows:

HA 18: F-statistic = 85.5% match between NIFM and CFRAM

HA 26: F-statistic = 73.3% match between NIFM and CFRAM

The NIFM mapping was also verified against historic flood events and ‘Benefitting Land’ maps.

Identification, Assessment or Calculation of Depth

The indicative flood depth maps indicate the estimated depth of flooding at a given location, for a flood event of a particular probability. The flood depths are calculated by subtracting the DTM ground levels from the predicted water level. The flood depths are mapped as constant depths over grid squares of 5m, whereas in reality depths may vary within a given square.