Gaeilge

General Map User Guidance Notes

Understanding the Flood Maps

These maps are ‘predictive’ flood maps showing areas predicted to be inundated during a theoretical or ‘design’ 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, or greater, severity occurring in any given year. These probabilities may also be expressed as odds (e.g. 100 to 1) of the event 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 and coastal flood maps are typically developed, expressed in terms of Annual Exceedance Probability (AEP), and identifies their parallels under other forms of expression.

Table 1 - Flood Event Probabilities:

Annual Exceedance Probability (%) Odds of Occurrence in an Given Year Return Period (Years)
10 (High Probability) 10 : 1 10
1 (Medium Probability –Fluvial/River Flood Maps) 100 : 1 100
0.5 (Medium Probability –Coastal Flood Map) 200 : 1 200
0.1 (Low probability) 1000 : 1 1000

Maps have been produced for the 'Areas of Further Assessment' (AFAs), as required by the EU 'Floods' Directive [2007/60/EC] and designated under the Preliminary Flood Risk Assessment 1 , and also for other reaches between the AFAs and down to the sea that are referred to as 'Medium Priority Watercourses' (MPWs).

Contents

There are a range of flood map types:

1. Flood Extent Maps

These maps indicate the estimated extents, peak water levels and flows associated with flooding from only those river reaches, estuaries and coastlines that have been modelled. River reaches that have been modelled are indicated on the PDF flood maps with a thick orange line along the centre - line of the river. Flooding from other reaches of river may occur, but has not been mapped, and so areas that are not shown as being within a flood extent may therefore be at risk of flooding from unmodelled rivers (as well as from one of the other sources referred to below).

There are many other possible sources of flooding, such as from surcharged urban drainage systems, ponding rainwater, groundwater or blockage of structures such as culverts. Flooding from these other sources has typically 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. The flood depths are mapped as constant depths over grid squares (of 5-10m for the AFA flood maps), whereas in reality depths may vary within a given square.

3. Flood Risk Maps

These maps show:

  • The indicative number of inhabitants potentially affected by floods, which provides an indication of risks to human health and communities.
  • The types of economic activity potentially affected by the flooding.
  • Protected areas of environmental value and potential sources of pollution (IED sites) that may be prone to flooding.

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 and other possible future changes. These scenarios include for changes as set out in Table 2.

Table 2: Allowances in Flood Parameters for the Mid-Range and High-End Future Scenarios

Parameter MRFS HEFS
Extreme Rainfall Depths + 20% + 30%
Peak Flood Flows + 20% + 30%
Mean Sea Level Rise + 500 mm + 1000 mm
Land Movement - 0.5 mm / year1 - 0.5 mm / year1
Urbanisation No General Allowance – Reviewed on Case-by-Case Basis No General Allowance – Reviewed on Case-by-Case Basis
Forestation - 1/6 Tp2 - 1/3 Tp2
+ 10% SPR3
Note 1: Applicable to the southern part of the country only (Dublin – Galway and south of this)
Note 2: Reduction in the time to peak (Tp) to allow for potential accelerated runoff that may arise as a result of drainage of afforested land
Note 3: Add 10% to the Standard Percentage Runoff (SPR) rate: This allows for temporary increased runoff rates that may arise following felling of forestry.

Legends

The datasets described below are shown and appear on the legend of the PDF flood extent and depth maps (please refer to the legends on the risk maps for descriptions as to what is shown on those maps). A legend is provided on the web-viewer for the interactive maps.

Model Nodes

Nodes at which estimates of maximum design event flood flows and maximum flood levels are reported on the maps.

Modelled River Centreline

An indicator of the channels that have been included in the river network model and from which the resultant fluvial flood extents are derived.

Flood Extents

The areas that are estimated to be inundated at some point during a flood with the respective Annual Exceedance Probabilities (AEPs). Three extents are typically shown on the PDF flood extent maps – Low Probability (0.1% AEP); Medium Probability (1% AEP fluvial or 0.5% AEP coastal); and, where appropriate, High Probability (10% AEP).

Area for Further Assessment (AFA)

The outer bounds of the AFA where, based on the Preliminary Flood Risk Assessment (OPW, 2012), the risks associated with flooding are potentially significant, and where further, more detailed assessment has been undertaken to determine the degree of flood risk.

Defended Areas

Hatched polygons on the flood extent mapsshow the defended areas that benefit from existing flood defences. The Standard of Protection (SoP) for the defended area is also noted on the flood defence on the PDF flood map, e.g. a 1% AEP SoP describes a defence that has been designed to protect an area in the event of a 1:100 annual exceedance probability flood.

Flood Depth (in metres or “m”)

The maximum depth estimated to occur at some point during a flood with the respective Annual Exceedance Probability (AEP) at the mapped location.

Scale

The PDF versions of the flood maps are produced at between 1:5 000 and 1:10 000 scale at A3 size within the AFA, as shown on the map. This scale has been selected to permit users to view individual properties, streets, infrastructure assets, etc., and as it is compatible with the scale of the cadastral background mapping.

For maps showing the General Environmental Risks for each AFA, the PDF map scale is 1:50,000 at A3 size.

Maps showing the Specific Types of Economic Activity across Units of Management vary in scale depending on the size of each Unit of Management.

The scale for the maps viewed directly on the web-viewer are user-defined and variable.

Accuracy

For fluvial flood levels, calibration and verification of the models make use of the best available data including hydrometric records, photographs, videos, press articles and anecdotal information. Subject to the availability of suitable calibration data, models are verified in so far as possible to target vertical water level accuracies of approximately +/-0.2m for areas within the AFAs, and approximately +/-0.4m along the MPWs.

All fluvial models are run, and maps produced, assuming clear flow through culverts and bridges, and the models and flood maps do not account for blockage of such structures.

For coastal flood levels, the accuracy of the predicted annual exceedance probability (AEP) of combined tide and surge levels depends on the accuracy of the various components used in deriving these levels i.e. accuracy of the tidal and surge model, the accuracy of the statistical data and the accuracy for the conversion from marine datum to land levelling datum. The output of the water level modelling, combined with the extreme value analysis undertaken as detailed above is generally within +/-0.2m for confidence limits of 95% at the 0.1% AEP. Higher probability (lower return period) events are expected to have tighter confidence limits.

Date of Preparation

The date the maps were prepared is indicated in the title box of the PDF maps.

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 flood maps shown here.

Local Authorities provide support to the OPW as partners on the CFRAM Programme and the capital flood risk management programme, contributing to, or in some instances commissioning, the development of the flood maps.

Flood Mapping – Technical Data

Set out below is a summary of the typical technical process for developing the flood maps. The process may vary for particular areas or maps. Technical Hydrology and Hydraulics Reports set out full technical details on the derivation of the flood maps. Users of the maps should familiarise themselves fully with the contents of these reports in advance of the use of the 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, or greater, 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.

These probabilities were selected to cover a wide range of event probabilities that can cause flooding and provide the range of information necessary to fully assess the flood risk for each AFA and hence develop appropriate flood risk management measures; up to a nominal extreme event (the 0.1% AEP flood event - equivalent to an event with an average return period of 1000-years) that is significantly beyond the range of reliable statistical event projections.

The PDF flood risk maps are produced for a Low Probability (the 0.1% AEP flood event), a Medium Probability (the 1% (fluvial &pluvial) or 0.5% (coastal) AEP flood events) and, where appropriate, a High Probability (the 10% AEP flood event), although risk analysis has been undertaken for other flood events to provide detailed flood risk data.

Identification, Assessment or Calculation of Flooding extent

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

Fluvial and coastal flood maps are developed using hydrodynamic modelling, based on calculated design river flows and extreme sea levels, surveyed channel cross-sections, in-bank / bank-side / coastal structures, Digital Terrain Models, and other relevant datasets (e.g. land use, data on past floods for model calibration, etc.).

A summary of the process is provided below.

Key stages:

  • Hydrological analysis: Estimation of the flood flows (cubic metres of water per second: m3/s) and tidal levels for the design flood events.
  • Hydraulic modelling: Estimation of the flood levels at intervals along a river; or for locations on a floodplain, based on the design flood flows (river flooding) and local physical and hydraulic conditions.
  • Analysis of flooding: Estimation of how flooding would propagate from the river, estuary or tidal area over the land, and the associated flood extents, depths, velocities, etc.

Hydrological Analysis

Fluvial flood flows have typically been calculated based on analysis of gauged data, and the use of the national methods for determining flows in ungauged catchments, and for calculating statistical peak flow growth curves and hydrograph shapes; namely the Flood Studies Update (see http://opw.hydronet.com/ ).

Coastal extreme sea levels have been determined from 2-D modelling under the Irish Coastal Protection Strategy Study. Extreme value analysis (EVA) was undertaken by fitting theoretical probability distributions to the water level values extracted from the results of the tidal surge model simulations. A partial duration series (peak over threshold model) was used to select the largest events which occurred within the dataset (details available from reports available under the OPW website – see here ).

Hydraulic modelling

For fluvial flood mapping, the hydrodynamic modelling software packages used are typically the ISIS and MIKE suites and Infoworks ICM (c), according to context, need and preference of the modeller. The fluvial models make use of ground-based survey of channel cross-sections and of in-bank / bankside structures. Channel cross-section spacing is typically 50-100m through the AFA, and typically 500m in rural areas outside of the AFAs.

For coastal flood mapping, the extreme levels are propagated inland using 2-D flow models.

For the purposes of the production of the flood maps, structures, such as culverts and bridges, have been modelled as surveyed, with no blockage assumed.

Topographical Data

A Digital Terrain Model (DTM) is used to generate the maps. The DTM is derived from airborne survey techniques. The majority of this data in AFAs is Light Detection and Ranging (LiDAR) data, which has a vertical and horizontal RMSE of typically less than 0.2m, and a typical grid scale of 5 or 10m. Where LiDAR data was not available, which would typically be rural areas outside of the AFAs, Interferometric Synthetic Aperture Radar (IfSAR) datahas been used to derive the DTM, which has a vertical and horizontal 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. In addition, ‘cleansing’ is undertaken during flood map production, which involves various processes such as the removal of very small areas of flooding that are remote and isolated, the removal of very small islands (areas modelled as not flooding) within the flooded area, etc. Therefore, the maps should not be used to assess the flood risk associated with individual properties or point locations, or to replace a detailed local flood risk assessment.

Buildings and other infrastructure (e.g., bridges, embankments) are reintroduced to the modelling process in an appropriate manner (see hydraulic modelling reports for details) and so are considered in the hydraulic analysis and preparation of the flood maps.

The approach taken to determining the standard of protection (SoP) for flood defences is based on the crest level of the defence relative to the flood levels for the range of event probabilities (where the SoP is taken as the lowest annual exceedance probability that does not overtop). The condition, fragility and likelihood of failure of the defence are not considered for the purpose of mapping the areas defended or determining the SoP.

The maps were produced based on survey data captured prior to, and during the early part of the CFRAM programme. They do not account for changes in development, infrastructure or topography that occurred after the date of survey data capture.

Identification, Assessment or Calculation of Depth

The 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 5- 10m for the AFA flood maps, whereas in reality depths may vary within a given square.

Identification, Assessment or Calculation of Depth

The 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 5- 10m for the AFA flood maps, whereas in reality depths may vary within a given square.

Models used, datasets, uncertainties

Flood levels, depths and velocities are derived from the hydrodynamic models for the various event probabilities and scenarios. The models have been calibrated to information on past floods events where available.

Uncertainty in flood levels can arise due to uncertainties in topographic, bathymetric and other survey data and in meteorological, rainfall and flow data, assumptions and / or approximations in the hydraulic / hydrodynamic models and parameters in representing physical reality, and datum conversions, etc. Uncertainty in flood extents can arise due to uncertainties in flood levels, topographic and other survey data, assumptions and / or approximations in the way that flooding spreads over a floodplain, etc.

Map User Guidance Notes: Raphoe Pluvial Flooding

Understanding the Pluvial Flood Maps

These maps are ‘predictive’ flood maps showing areas predicted to be inundated during a theoretical or ‘design’ flood event with an estimated probability of occurrence, rather than information for actual floods that have occurred in the past, which is presented on ‘historic’ 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, or greater, severity occurring in any given year. These probabilities may also be expressed as odds (e.g. 100 to 1) of the event 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 very short space of time. Table 1 sets out a range of flood event probabilities for which Pluvial flood hazard 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:

Annual Exceedance Probability (%) Odds of Occurrence in an Given Year Return Period (Years)
10 (High Probability) 10 : 1 10
1 (Medium Probability) 100 : 1 100
0.1 (Low probability) 1000 : 1 1000

Contents

There are a series of three map types:

1. Flood Extent Maps

These maps indicate the extents associated with pluvial flooding.

2. Flood Depth Maps

These 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 ground levels, from the predicted water level. The flood depths are mapped as constant depths over grid squares (of 25m or 12.5m resolution), whereas in reality depths may vary within a given square.

3. Flood Risk Maps

These maps show:

  • The indicative number of people potentially affected by floods, which provides an indication of risks to human health and communities.
  • The types of economic activity potentially affected by the flooding.
  • Protected areas of environmental value and potential sources of pollution (IED sites) that may be prone to flooding.

Legends

The following datasets are shown and appear on the legend of the flood maps:

Flood Extents –The areas that are estimated to be inundated at some point during a flood with the respective Annual Exceedance Probabilities (AEPs). Three extents are shown on the flood extent maps – High Probability (10% AEP); Medium Probability (1% AEP); and Low Probability (0.5% AEP).

Flood Depth (in metres or “m”) – The maximum depth estimated to occur at some point during a flood with the respective Annual Exceedance Probability (AEP) at the mapped location.

Scale

The PDF versions of the flood hazard maps are produced at 1:10,000 scale at A3 size.

The PDF versions of the flood risk maps are produced at 1:8,000 scale at A3 size.

Accuracy

Drainage survey drawings identified that the internal geometry of the urban drainage, including some large culverts, is highly complex comprising numerous sections of varying shape, size, and length. Attempting to replicate this complexity within the model results in instabilities within the InfoWorks RS model therefore the representation used is a simplified, yet representative, geometry informed by engineering judgement.

For the shorter of the two main culverts, a narrow ‘dummy’ 1D channel has been placed on top of the culvert. This was linked to the 2D domain and therefore allows water to pass from one side of the culvert to the other.

It is worth noting that in the case of shallower depths internal property flooding may not occur, i.e. surface water is confined to roads and other areas around properties.

Date of Preparation

The date the maps were prepared is indicated in the title box of the maps.

Responsible authorities

The Office of Public Worksis the responsible authority for the pluvial flood maps for Raphoe.

Flood Mapping Technical Data: Raphoe Pluvial Flooding

Identification, Assessment or Calculation of Flooding probabilities or return periods

Design rainfall profiles are the main hydrological inputs and these have been derived using the most up-to-date DDF rainfall data based on observed Irish rainfall data. It is generally accepted that 0.1% AEP flood maps contain the greatest uncertainty anyway since observed data is not available upon which to base the analysis.

Modelling Approach

Design rainfall profiles have been derived as the main hydraulic model inputs so that the surface - water run off flood mechanism could be simulated for the aforementioned range of AEP flood events. Thesedesign rainfall profiles were distributed uniformly over a 2D domain within the hydraulic model which allowed routing of flow paths across the grid as controlled by topography as depicted by LiDAR data. Fluvial flood risk is also simulated applying a design fluvial hydrograph within the 1D portion of the integrated model which represents the town watercourse.

Model Verification

Raphoe town has experienced flooding in the past from a combination of pluvial and fluvial flooding, most notably in September 2006 and June 2007. The associated rainstorm events that occurred have been derived as hydraulic model input files so that the events could be simulated and the model calibrated. Rainfall data was taken from the Raphoe Tops daily gauged located within the town, with temporal profiles taken from rainfall radar data recorded by Castor Bay Station near Lough Neagh in Northern Ireland. The station does cover the Raphoe area and was deemed to be the best available data after alternative sources of sub-daily rainfall profiles were screened out as un-useable. Castlederg Hourly rainfall station and the nearest NRA rain gauges recorded almost no rainfall during both the 2006 and 2007 events, and Malin Head station is too far away.

Calibration of the model to the September 2006 event was undertaken by visual qualitative comparison of modelled flood outlines with observed flow paths and properties affected. This demonstrated a good fit between modelled and observed data. This was also the case for the June 2007 event. However additional quantitative comparison was also undertaken for this event since recorded flood depths were available. The model achieved vertical accuracies of less than 0.2m in three of the four locations for which flood depths were recorded, and a ve rtical accuracy within 0.05m at two of the locations. The model is therefore considered to be adequately calibrated for the purposes of the Study.

Topographical Data

A Digital Terrain Model (DTM) is used to generate the maps. The DTM is derived from airborne survey data. This is Light Detection and Ranging (LiDAR) data, which has a vertical and horizontal RMSE of typically less than 0.2m, and a typical grid scale of 2m.

Key Assumptions and Limitations

Model grid resolution

The model accuracy is a function of the base DTM (LiDAR data) and the resolution of the model grid. The model uses a triangular flexible mesh with a maximum resolution of 6m2 over an area of 4.41km2.

Rainfall infiltration

After investigation of the model results and comparison with observed events it was evident that some allowance for storm drainage was required. Insufficient information was available to explicitly incorporate the drainage network into the model, so an assumption as to the efficiency of the existing drainage was made. For design runs it was assumed that the drainage network within the town had the capacity to convey the 50% AEP event for rain falling with the town limits. An additional rainfall boundary was therefore incorporated based on the town development boundary with the associated rainfall profiles amended to reduce the rainfall intensity by the 50% AEP values at each model time-step.

Identification, Assessment or Calculation of Depth

The 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 the flexible mesh squares of maximum resolution of 6m2 for the AFA flood maps, whereas in reality depths may vary within a given square.

Further Information

Full details on the methodology used for the preparation of the pluvial flood maps are available from: https://www.FloodInfo.ie

Map User Guidance Notes: Dublin City Pluvial

Understanding the Pluvial Flood Maps

These maps are ‘predictive’ flood maps showing areas predicted to be inundated during a theoretical or ‘design’ flood event with an estimated probability of occurrence, rather than information for actual floods that have occurred in the past, which is presented on ‘historic’ 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, or greater, severity occurring in any given year. These probabilities may also be expressed as odds (e.g. 100 to 1) of the event 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 very short space of time. Table 1 sets out a range of flood event probabilities for which Pluvial flood hazard 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:

Annual Exceedance Probability (%) Odds of Occurrence in an Given Year Return Period (Years)
10 (High Probability) 10 : 1 10
1 (Medium Probability 100 : 1 100
0.5 (Low probability) 200 : 1 200

Contents

There are a series of three map types:

1. Flood Extent Maps

These maps indicate the extents associated with pluvial flooding.

2. Flood Depth Maps

These 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 ground levels, from the predicted water level. The flood depths are mapped as constant depths over grid squares (of 25m or 12.5m resolution), whereas in reality depths may vary within a given square.

3. Flood Risk Maps

These maps show:

  • The indicative number of people potentially affected by floods, which provides an indication of risks to human health and communities.
  • The types of economic activity potentially affected by the flooding.
  • Protected areas of environmental value and potential sources of pollution (IED sites) that may be prone to flooding.

Legends

The following datasets are shown and appear on the legend of the flood maps:

Flood Extents –The areas that are estimated to be inundated at some point during a flood with the respective Annual Exceedance Probabilities (AEPs). Three extents are shown on the flood extent maps – High Probability (10% AEP); Medium Probability (1% AEP); and Low Probability (0.5% AEP).

Flood Depth (in metres or “m”) – The maximum depth estimated to occur at some point during a flood with the respective Annual Exceedance Probability (AEP) at the mapped location.

Model Boundary - This shows the outer extent of the mapping of pluvial flooding. Pluvial flooding may occur outside of this boundary but this has not been assessed.

Scale

The PDF versions of the flood maps are produced at 1:50,000 scale at A3 size.

Accuracy

Type 1 flood mapping predictions show flooding across the whole of the Dublin city area. Some broad observations can be made where there is some correlation between clusters of incident locations (either in groups or along linear features) and depths. There are also a number of locations at which pluvial flooding is predicted by the Type 1 model, but do not have flood incidents recorded in the database. This may be attributed to under-reporting in some instances.

It is worth noting that in the case of shallower depths internal property flooding may not occur, i.e. surface water is confined to roads and other areas around properties.

Date of Preparation

The mapping and final report were published in October 2012.

Responsible authorities

Dublin City Council is the responsible authority for the pluvial flood maps for Dublin City.

Flood Mapping Technical Data: Dublin City Pluvial Flooding

Identification, Assessment or Calculation of Flooding probabilities or return periods.

City-wide (Type 1) Modelling’ aims to provide an overall assessment of Dublin City’s vulnerability to pluvial flood hazard and risk. In delivering this objective, the following three step process has been undertaken:

1. Type 1 ‘Dry’ GIS Mapping

Potential pluvial hotspots have been initially identified through a preliminary screening process using GIS techniques, which has been supported by site inspections and an indicative risk assessment to better understand pluvial flooding processes and gain an initial overview of significant risk factors specific to Dublin City. This was used to inform the subsequent ‘wet’ modelling and mapping stages. The outputs are displayed in Final Report Appendices V2-A and V2-B.

2. Type 1 ‘Wet’ Hydraulic Modelling

Six two dimensional (2D) models based on a regular grid of 25m cell size covering the surface water catchments within the Dublin City Council administrative boundary have been constructed using TUFLOW modelling software.

The models simulate pluvial flooding for a range of rainfall events of various severities (in duration and intensity). Flood depth, velocity and hazard rating (a function of both depth and velocity) as model outputs have been subsequently processed to produce pluvial flood maps.

3. Type 1 Pluvial Flood Mapping

Using information derived from the previous steps, and following the development of pluvial flood depth maps, Pluvial Flood Risk Maps with the following receptor groups appraised have been produced:

  • The indicative number of inhabitants potentially affected by floods, which provides an indication of risk to human health and the community
  • The types of economic activity potentially affected by the flooding
  • Protected areas of environmental value and potential sources of pollution (IED sites) that may be prone to flooding

Model Verification

The Type 1 hydraulic model outputs were assessed against two historical flood events that occurred in August 2008 and July 2009. During both events, a large number of flood incidents occurred within Dublin City directly due to either rainfall runoff or local surcharging of the drainage system.

Topographical Data

A Digital Terrain Model (DTM) is used to generate the maps. The DTM is derived from airborne survey data. This is Light Detection and Ranging (LiDAR) data, which has a vertical and horizontal RMSE of typically less than 0.2m, and a typical grid scale of 20m. This is tied into local benchmarks to increase accuracy.

Key Assumptions and Limitations

Model grid resolution

The model accuracy is a function of the base DTM (LiDAR data) and the resolution of the model grid. The model uses a regular 25m cell size with breaklines to represent key hydraulic features which may have a significant impact on the propagation of rainfall runoff across the study area. However, smaller features at sub-cell scale (e.g. narrow streets, kerbs, and building basements), which may also influence shallow surface water flow, are not explicitly represented.

Rainfall infiltration

The Type 1 model schematisations account for a limited representation of the urban drainage systems that essentially rely on an estimation of the drainage network capacity translated into a loss factor applied to the rainfall. Whilst this assumption is deemed appropriate for the level of detail required by this study, the current model schematisation does not represent the complex interaction that might occur between the overland flow and the drainage network, or the spatial and temporal responses of the drainage system to events of different magnitudes.

Identification, Assessment or Calculation of Depth

The 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 5-10m for the Dublin City pluvial flood maps, whereas in reality depths may vary within a given square.

Further Information

Full details on the methodology used for the preparation of the pluvial flood maps are available here

While these pluvial flood maps are as accurate as possible given the resources and data available: a large number of uncertainties remain in their compilation and Dublin City Council or their agents will not carry any liability for inaccuracies in or associated with them. Changes in topography since their compilation may also lead to significant changes in local floodplains.

Updated: 3rd May 2018

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