VOLUME 4 Detailed Pluvial Flood Risk Assessment of Pilot Areas

EU Interreg IVB FloodResilienCity Project

Volume Four

Detailed Pluvial Flood Risk Assessment of Pilot Areas

Document control sheet BPP 04 F8

Client: Dublin City Council Project: EU Interreg IVB FloodResilienCity

Project Job No: 32102500

Document Title: Volume Four Detailed Pluvial Flood Risk Assessment of Pilot Areas

Originator Checked by Reviewed by Approved by NAME NAME NAME NAME FINAL Mathieu Valois / Lewis Maani

Alastair Davis / Kelly Kasperczyk Ronnie Falconer Kelly Kasperczyk

DATE SIGNATURE SIGNATURE SIGNATURE SIGNATURE

October 2012

Document Status: Issue to Dublin City Council

Copyright Copyright Dublin City Council. All rights reserved. No part of this report may be copied or reproduced by any means without prior written permission from Dublin City Council. If you have received this report in error, please destroy all copies in your possession or control and notify Dublin City Council. Legal Disclaimer This report has been prepared for the exclusive use of the commissioning party (Dublin City Council) and unless otherwise agreed in writing by Jacobs Engineering Ireland Limited, no other party may use, make use of or rely on the contents of this report. Neither this report nor the services provided by Jacobs Engineering Ireland Limited are intended for the express or implied benefit of any third party. The commissioning party shall indemnify and hold Jacobs Engineering Ireland Limited harmless from any third party claims arising out of any use or reliance on the contents of this report. No liability is accepted by Jacobs Engineering Ireland Limited for any use of this report, other than for the purposes for which it was originally prepared and provided. Opinions and information provided in the report are on the basis of Jacobs Engineering Ireland Limited using due skill, care and diligence in the preparation of the same and no warranty is provided as to their accuracy. It should be noted and it is expressly stated that no independent verification of any of the documents or information supplied to Jacobs Engineering Ireland Limited has been made.

CONTENTS

PREFACE i

GLOSSARY iv

ABBREVIATIONS x

SECTION 1 INTRODUCTION 1 1.1 Objectives and Scope 1 1.2 Dublin City Pilot Areas 3

SECTION 2 DETAILED PLUVIAL MODELLING FOR THE PILOT AREAS 6 2.1 Available Data Used for the Type 2 Hydraulic Modelling (Step 1) 6 2.2 Detailed Modelling Approach – Type 2 Hydraulic Model (Step 2) 7 2.3 Verification of the Type 2 Hydraulic Models (Step 2) 13 2.4 Modelled Scenarios (Steps 2, 3, 6) 15 2.5 Type 2 Model Outputs (Steps 2, 3, 6) 16 2.6 Key Model Assumptions and Limitations (Step 2) 17

SECTION 3 BASELINE SCENARIO DETAILED PLUVIAL FLOOD HAZARD AND RISK MAPPING 19

3.1 Pluvial Flood Depth and Hazard Rating Mapping (Step 3) 19 3.2 Flood Mechanisms (Step 3) 20 3.3 ‘Existing’ Pluvial Flood Risk Mapping (Step 4) 21

SECTION 4 MODELLING APPRAISAL OF CORRECTIVE MEASURES 25 4.1 Basis for Identifying Mitigation Measures (Step 5) 25 4.2 Mitigation Measures under Consideration (Step 5) 26 4.3 Initial Screening of Measures for Trialling in Pilot Areas (Step 5) 28

SECTION 5 OPTION TECHNICAL AND ECONOMIC APPRAISAL 34 5.1 Flood Risk Management Options (Step 6) 34 5.2 Technical Assessment (Step 6) 35 5.3 Economic Assessment (Step 6) 36 5.4 With Scheme Pluvial Flood Maps (Steps 7, 8) 41

SECTION 6 SUMMARY OF EXISTING PROBLEM AREAS AND THE PREFERRED FRM OPTIONS 42

6.1 Dublin South East Pilot Area – Georges Quay & South Inner City 42 6.2 Dublin North Central Pilot Area – Marino and Fairview 43 6.3 Dublin South Central Pilot Area – Carrickfoyle Terrace 44 6.4 Dublin North West Pilot Area – Kippure Park 44 6.5 Dublin Central Pilot Area – East Wall 45 6.6 Conclusions and Recommendations 46

Volume Four: Detailed Pluvial Flood Risk Assessment of Pilot Are as

EU IVB FloodResilienCity Project Final Report – Dublin i

PREFACE

Dublin FloodResilienCity (FRC) Technical Report Vol ume Structure

This technical report, ‘Volume Four: Detailed Pluvial Flood Risk Assessment of Pilot Areas’ is one of five Volumes which accompany the Dublin FRC Project Non Technical Summary. The Non Technical Summary provides the background to the Dublin FRC Project and a summary of each of the technical report Volumes. These Volumes comprise:

Volume One Rainfall and Forecasting

Volume Two City-Wide Pluvial Flood Risk Assessment

Volume Three Pluvial Flood Risk Management

Volume Four Detailed Pluvial Flood Risk Assessment of Pilot A reas

Volume Five Pluvial Flood Alerting and Warning System Integration

Each Volume should be read in conjunction with the other Volumes as sections within each may be cross-referenced to other Volumes. Figure P1 illustrates both the one-way and two-way relationships between the report Volumes, with arrow size indicating relationship scale. The extent of the Dublin FRC Project study area is shown in Figure P2 which also indicates the five administrative areas within Dublin City.

Figure P1: Dublin FRC Technical Report Volume Rela tionships

Figure P2: Dublin FloodResilienCity Project Study Area


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Volume Four Structure

Section 1. This Section provides an Introduction to the detailed hydraulic modelling for the five Pilot Areas identified in Volume Two - City-Wide Pluvial Flood Risk Assessment. It sets out the objectives of the detailed modelling that will lead ultimately to the identification of measures to manage pluvial flood risk in each of the Pilot Areas. Section 2. This Section describes the Detailed Pluvial Modelling for the Pilot Areas based on a combined overground-underground hydraulic model. It describes the model set-up, the model verification and the model limitations. Section 3. This Section describes how the Baseline Scenario Detailed Pluvial Flood Hazard and Risk maps are used to identify the characteristics of each Pilot Areas, such as the existing flood mechanisms and flooding ‘hotspots’ or ‘problem areas’ where significant flood depths are predicted. Section 4. This Section describes the Modelling Appraisal of Corrective Measures for each of the five Pilot Areas, by considering a wide range of measures as identified in Code of Practice 2: Flood Resilience and Adaptation Measures (Volume Three, Appendix V3-B). Section 5. This Section describes the Option Technical and Economic Appraisal process undertaken to assess the various flood risk management options and identify the ‘preferred option’ for each of the five Pilot Areas. Section 6 . This Section provides a summary of the key pluvial flood-related characteristics of each Pilot Area as well as a summary of the preferred pluvial flood risk management option identified for each. Appendices. Appendices V4-A, V4-B, V4-C, V4-D, and V4-E describe the model outputs specific to a Pilot Area. The main flood mechanisms and flooding ‘hotspots’ or ‘problem areas’ are detailed, and the outputs of the modelling are illustrated using pluvial flood depth, velocity, hazard rating and risk mapping. Details of the flood risk management options assessed are outlined as well as a number of recommendations for the mitigation of flood risk in each Pilot Area. Volume Four informs the following Dublin FRC technical report Volume:

Volume Three – Pluvial Flood Risk Management:

• The detailed modelling results for the five Pilot Areas presented in Volume Four can inform the implementation of existing and/or proposed elements of policies, objectives, guiding principles and standards relating to spatial planning and building control as outlined in Volume Three.

• This detailed modelling can also be used to assist Dublin City Council in identifying which of the measures described in Volume Three are appropriate to specific areas of Dublin City, as well as assessing their effectiveness.


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Sustainable Pluvial Flood Risk Management

As with the Codes of Practice presented in Volume Three, the Detailed Pluvial Flood Risk Assessment outlined here in Volume Four supports Dublin City Council’s established approach to sustainable flood risk management through adoption of the 4As Model. The Detailed Pluvial Flood Risk Assessment outlined in this Volume can help Dublin City Council develop their ‘Beyond the 4As’ initiative which incorporates other specific elements in the overall sustainable flood risk management process – the ‘7As’:

• Awareness raising (politicians and policy makers, professionals, and the public) at strategic level; and at process level;

• Assessment of existing and emerging hazards;

• Analysis of the resulting risks;

• Avoidance of the known risks where possible;

• Alleviation of the unavoidable risk where practical;

• Action in response to the residual risk; and

• Assistance in recovery from the impacts.

Volume Three– Pluvial Flood Risk Management

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GLOSSARY

Aggregated Micro-Storage A corrective/mitigation measure used for storing surface water in hard standing areas in built

up areas (car parks, roof areas, sports facilities). This measure has the potential to maximize benefit from many relatively small storage areas and optimize the control of this storage in real time.

Annual Exceedance Probability (AEP)

This is the technical term used to express the likelihood, or chance, of a particular event (e.g. flood or rainfall) being equaled or exceeded in any one year. It is usually expressed as a percentage or a ratio i.e. the 10% AEP event or 1 in 10 AEP. In technical terms the rarity of an event is sometimes also referred to as a return period i.e. the 10% AEP event is equivalent to an event having a 10-year return period, but the use of return periods can be confusing to the wider public.

Attenuate Providing temporary storage or other measures designed to reduce the volume of surface runoff which could cause flooding. A particular focus of attenuation is on reducing peak flows through an area.

Blue Roof A form of roof which is designed to capture water, most typically rainfall.

Breakline Two dimensional geographical features (railway lines, rivers, roads and canals) which are represented in the modeling software as lines that may have a significant impact on the propagation of the rainfall runoff.

Catchment A catchment area or drainage basin is an extent or an area of land where surface water or fluvial flow converges to a single point; usually the exit of the basin, where the waters join another water body, such as a river, lake, reservoir, estuary, wetland, sea, or open sea.

Climate Change Long term variations in global temperature and weather patterns caused by natural and human actions.

Climate Fluctuation Variations in global temperature and weather patterns.

Coastal Flooding Coastal flooding that results from a combination of high tides and stormy conditions. If low atmospheric pressure coincides with a high tide, a tidal surge may happen which can cause serious flooding.

Contour Polygon Screening (CPS)

A GIS based technique for assessing topographical data and identifying hazardous depressions with regard to potential flooding.

Convective Available Potential Energy (CAPE)

A measure of the amount of energy available for convection (which can lead to intense rainfall).

Convective Rainfall Convective rainfall originates from convective clouds and falls with rapidly changing intensity over a small area for a relatively short period of time.

Conveyance Flow This is essentially the carrying capacity of a surface or culverted watercourse or a below-ground sewer or drainage system. It is significantly influenced by the roughness of the river or stream bed, or the piped system. Debris carried along in the flow and/or obstructions can reduce conveyance flow.

In relation to sewer design capacity the conveyance capacity of urban drainage networks is usually such that they will flow full in a 1 in 5 AEP (20%) rainfall event. In a more extreme event they will usually surcharge up to road level and no more flow will enter through road gullies. In a 1 in 10 AEP (10%) rainfall event and events greater than this severe road flooding and property flooding may result.

Critical Infrastructure Infrastructure (assets) essential for the functioning of society and the economy related to electrical generation, telecommunication and public health (i.e. hospitals, power stations, treatment works).


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Culvert A channel or pipe that carries a watercourse below the level of the ground.

Dam extreme operation / failure flooding

Some reservoirs hold large volumes of water above ground level. Although the safety record for reservoirs is excellent, it is not impossible that a dam could fail. This would result in a large volume of water being released very quickly.

Debris Factor A variable used to quantify hazard which represents the fact that deep, fast flowing flood waters might mobilize loose objects and move them along flow paths thus increasing flood hazard.

Department for Environment, Food and Rural Affairs (DEFRA)

DEFRA is the UK government department responsible for policy and regulations on the environment, food and rural affairs

Depth Duration Frequency (DDF)

Rainfall depth-duration-frequency (DDF) curves describe rainfall depth as a function of duration for given rainfall probabilities.

Depth Gauge A gauge used to measure the depth of accumulated rainfall.

Digital Elevation Model (DEM)

A digital elevation model which shows topographic information including buildings and vegetation.

Digital Terrain Model (DTM)

A digital terrain model which show topographic information excluding buildings and vegetation.

Direct Rainfall approach A hydraulic modeling approach which involves the application of rainfall hyetographs representative of storm events to active model cells within a two dimensional domain.

Drainage Infiltration and Exfiltration

In relation to sewer and drainage systems these terms are often used to describe seepage into or out of a drainage system through joints and cracks in the pipework. However for the purposes of this Pluvial Flood Study they are used to describe that portion of surface water flow that is carried into the below-ground system (infiltration) mainly via the roadside gullies, and the portion of flow which floods out from the below-ground system when capacity is exceeded (exfiltration).

Dry Mapping Digital mapping of potential pluvial hotspots using the Rolling Ball and Contour Polygon Screening Techniques. This mapping is based on topography (LiDAR) and does not include any hydraulic or hydrological assessments. It is usually applied for preliminary assessments.

Exceedance Flow This is normally used to describe the flow which exceeds the capacity of the below-ground sewerage or drainage system to carry stormwater flows. The Exceedance Flow is the portion that surcharges and floods at the ground surface and flows along the surface, often together with direct runoff from pluvial flooding.

External Resistance Measures

Measures designed to keep flood water out of properties and businesses (i.e. flood guards). Resistance measures can be fitted to prevent surface water entering buildings. Measures can be fitted to new properties or retrofitted to existing properties

External Stakeholder Stakeholders considered to be involved or affected but not as directly integrated into existing arrangements for flood risk management and risk identification.

Extreme Rainfall Defined within this study as rainfall that leads to (or is likely to lead to) pluvial flooding. Whilst all rainfall is, by definition, pluvial, it is only intense rainfall events that give rise to pluvial flooding

Federated Emergency Response Plan (FERP)

A FERP is designed to harmonize federal emergency response efforts with those of the provinces/territorial governments, non-governmental organizations, and the private sector.

Flood The temporary covering by water of land not normally covered with water


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Flood Alert Dissemination to interested parties of an early indication that a flood event exceeding a critical threshold is possible and a warning may be given.

Flood Defence Infrastructure used to protect an area against floods such as floodwalls and embankments; they are usually designed to a specific standard of protection (design standard).

Flood Depth Estimation System (FDES)

The Flood Depth Estimation System (FDES) is a GIS based tool (which Jacobs has developed) which allows for the calculation of flood damages based on the depth outputs from the TUFLOW modeling software.

Flood Forecast The prediction of a flood event through the application of measured and/or modeled scenarios.

Flood Hazard The potential for a flood to cause damage or harm – usually shown as the extent of flooding for a flood with a specific probability or likelihood. A flood hazard does not necessarily lead to harm unless there is a ‘receptor’ such as people or property that could be harmed or damaged.

Flood Information and Warning System (FLIWAS)

The Flood Information and Warning System is a web based GIS orientated application for the monitoring of forecasts and aiding the implementation of Emergency Plans and evacuation plans.

Flood Risk Flood Risk in flood risk management is defined as a product of the probability or likelihood of a flood occurring and the consequence of the flood, for example damage to property or harm to people.

Flood Studies Report (FSR)

The Flood Studies Report, published in 1975, is used in relation to rainfall events in the United Kingdom. It has since been replaced by the Flood Estimation Handbook.

Flood Warning The resultant dissemination of a forecast to a body of interested parties in order that they may prepare for the flood event with the aim of reducing its impact. Usually given once a critical threshold has been reached and involves taking action.

Flow Paths Surface water flow paths with supporting gradient and accumulation information.

Fluvial Flooding Flooding resulting from water levels exceeding the bank level of a river. Also known as river flooding, this occurs when a watercourse cannot accommodate the volume of water draining into it from the surrounding land. It is generally infrequent, but flooding can occur rapidly or over a long duration depending on the nature of the upstream catchment. Watercourses are more likely to be overwhelmed when rainwater cannot be absorbed into the land onto which it falls. It might be very steep, water logged, or built over. Rapid melting of snow also leads to river flooding in some cases. Also, obstructions such as collapsed buildings/walls can exacerbate flooding. Flooding from small urban watercourses can be a particular problem in urban areas even though the catchment area may be small. Impermeable ‘sealed’ surfaces in built up areas can result in increased and more rapid runoff to these small watercourses such that flows in the watercourse can build up rapidly and result in flash flooding (an extreme form of fluvial flooding). Urban watercourses are often culverted over long sections and the entrances to these culverts can often be flooding ‘hotspots’. These watercourses are also often constricted in places resulting in bottlenecks which can make flooding worse. Debris, both natural and man-made also often accumulates in urban watercourses which not only constricts the watercourse but can accumulate at culvert screens and even block these screens in extreme cases.

Food and Agriculture Organization of the United Nations (FAO)

The Food and Agriculture Organization of the United Nations (FAO) is a specialized agency of the United Nations that leads international efforts to defeat hunger, serving both developed and developing countries.

GeoDirectory A property database showing locations of properties in Ireland. Attribute information includes data such as property number, street name and coordinate information.

Geographic Information Systems (GIS)

A geographic information system integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information.


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Greater Dublin Strategic Drainage Study (GDSDS)

The GDSDS was a study commissioned in June 2001 to carry out a strategic analysis of the existing foul and surface water systems in the local authority areas of Dublin City, Fingal, South Dublin, Dun Laoghaire- Rathdown and the adjacent catchments in Counties Meath, Kildare and Wicklow

Groundwater Flooding Groundwater flooding occurs when water levels in the ground rise above the ground surface. It is most likely to occur in areas underlain by permeable rocks, or alluvial/coastal deposits. These can be extensive, regional aquifers, such as chalk or sandstone, or may be locally confined deposits such as sand or river gravels in valley bottoms underlain by less permeable rocks.

Gulley An artificial hole, cavity or pit in a gutter which is covered with a grating and normally conveys surface water to a drainage system.

Gulley Monitor A monitor used for measuring water levels within gullies.

Hydraulic Modelling Computer software based method of modeling the flow of water in rivers and drainage systems.

Hydraulic Roughness A means of accounting for the effect on the resistance to flow of surface materials, irregularities, obstructions and vegetation.

Hydro-meteorological Monitoring

A method for monitoring/forecasting conditions associated with flooding.

InfoWorks CS InfoWorks CS is a modeling software package which is used to undertake hydrological modeling of the urban water cycle. Other applications include urban flooding and pollution prediction and the modeling of water quality and sediment transport throughout a network.

Internal Resilience Measures

Measures designed to reduce the impact of water that enters property and businesses. This can involve ensuring that the walls, floors, and fixtures are less damaged by water (or not at all), and also re-organising the house so that valuable and costly items (including service meters and the boiler) are above the level of the flood.

Internal Stakeholder Stakeholders currently participating in the risk management and risk identification processes.

Interreg (IVB) Community initiative that aims to stimulate interregional cooperation in the European Union. It is a financial instrument of the European Union's Cohesion Policy. It funds projects which support transnational cooperation. The aim is to find innovative ways to make the most of territorial assets and tackle shared problems of Member States, regions and other authorities.

Isohyets A line joining points of equal precipitation on a map.

Light detection and Ranging (LiDAR)

A high Resolution digital terrain model showing elevation/topographic information. Can be supplied in either "filtered" (buildings and vegetation filtered out) or "unfiltered" (buildings and vegetation have not been stripped out).

Mass-balance Equation In analysing stormwater events it is convenient to consider the ‘mass balance relationship’ which can be expressed in the form:

Total Rain = Exceedance Flow (surface – as defined above)

+ Conveyance Flow (below-ground – as defined above)

+ Ground Infiltration (rainfall infiltration into sub-soils, gravels and bedrock)

+ Detained Infiltration (rainfall infiltration detained in storage systems)

Natural Infiltration Precipitation that soaks into subsurface soil and strata naturally.

No Flow Condition A parameter used in hydraulic models to stop flow from passing through an area or node. For example where flood defenses are present, a "No Flow” condition can be applied to a model.


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NOAH An Interreg IIIB organization for research in to the rapid transfer of data with respect to flood prevention. The NOAH partnership aims to strengthen the transfer of information between relevant EU funded projects, experts and public institutions, improve the knowledge base and transfer between water management authorities in order to strengthen transnational cooperation on these issues.

Numerical Weather Prediction

A form of weather prediction which utilises mathematical models of the ocean and atmosphere.

Office of Public Works (OPW)

The OPW is a service organization. Its clients include Government, other Departments, Offices and Agencies and the public. Core services provided by the OPW are property maintenance, property management, architectural and engineering services, heritage services, project management and procurement services, and flood risk management.

Ordnance Survey Ireland (OSI)

Ordnance Survey Ireland is the national mapping agency of the Republic of Ireland. It provides digital/hardcopy products and mapping services. It provides a range of urban, rural, tourist and leisure maps at a variety of scales. They also provide other products such as aerial photography and digital terrain models.

Pluvial flooding

Pluvial flooding is defined as flooding which results from rainfall-generated overland flow and ponding before runoff enters a watercourse or sewer or when it cannot enter because the drainage system is already full to capacity. It is also known as surface water flooding. The capacity of local drainage (both natural and man-made) is overwhelmed and surface ponding occurs sometimes to a significant depth. Such ponding, often in low spots in the ground surface topography can occur rapidly and be a particular risk to basements other below-ground facilities. Where slopes are steep, resulting high flood velocities along roads and streets can also be a hazard to pedestrians and traffic.

Ponding An area where runoff collects in a depression and cannot drain.

Preliminary Flood Risk Assessment (PFRA)

The Preliminary Flood Risk Assessment (PFRA) is a requirement of the EU ‘Floods’ Directive. The objective of the PFRA is to identify areas where the risks associated with flooding might be significant. These areas (referred to as Areas for Further Assessment or ‘AFAs’) are where more detailed assessment is required to more accurately assess the extent and degree of flood risk. The more detailed assessment that will focus on the AFAs are being undertaken through Catchment-based Flood Risk Assessment and Management (‘CFRAM’) Studies.

Principal Stakeholder Person, group, or organization that has direct (key) stake in an organisation (or project) because it can affect or be affected by the organisation's projects, actions, objectives, and policies.

Radio Detection and Ranging (RADAR)

Radio Detection and Ranging (RADAR) is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects.

Raingauge A gauge used to measure the depth of accumulated rainfall.

Rainfall Duration The length of time a rainfall event lasts.

Rainfall Hyetographs A graphical representation of rainfall distribution over time.

Rainfall infiltration Precipitation that enters drainage systems or below-ground strata.

Rainfall Intensity A measure of the amount of precipitation over time.

Rainfall Pattern Variations in precipitation frequency, duration and intensity averaged over time for particular areas.

Rapid Flood Spreading Model (RFSM)

The Rapid Flood Spreading Model (RFSM) is a modeling approach which is used in pluvial studies and surface water management plans to represent overland flow at a high level (large scale). It is topography based and provides and an indication of ponding areas and the potential depths of flooding within these areas. It is usually applicable for national or regional studies.

Receptor Different sensitive receptors, that could possibly be affected by flood events (i.e. human health, critical infrastructure, environmental and cultural heritage and economy).


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Return Period A return period, also known as a recurrence interval, is an estimate of the interval of time between flood events or river discharge flow of a certain intensity or size. It is a statistical measurement denoting the average recurrence interval over an extended period of time, and is often used for risk analysis (i.e. whether a project should be allowed to go forward in a zone of a certain risk) and also to dimension structures so that they are capable of withstanding a flood event of a certain return period.

Risk In flood risk management, risk is defined as a product of the probability or likelihood of a flood occurring, and the consequence of the flood.

Rolling Ball Technique A GIS based form of analysis used to predict pathways of preferential flow direction based on terrain slope.

Sewer flooding Flooding caused by a blockage or overflowing due to heavy rainfall in a sewer or urban drainage system.

Sewer flooding can occur when ‘combined’ sewers (which carry both foul sewage and stormwater) are overwhelmed by heavy rainfall or when they become blocked, or can be attributed to infrastructure failure (e.g. pumping station failure). The likelihood of flooding depends on the capacity of the local sewerage system. Land and property can be flooded with water contaminated with raw sewage as a result. Rivers can also become polluted by sewer overflows. In urban areas, pluvial flooding and sewer flooding often combine, polluting the floodwater. It should be noted that in some newer developments foul sewage and stormwater is conveyed in ‘separate’ systems. In such cases flooding due to heavy rainfall is usually associated with the stormwater system.

Stakeholder A person or organization affected by the problem or solution, or interested in the problem or solution. They can be individuals or organisations, and include the public and communities.

Street as Streams/Roads as Rivers (SaS/RaR)

This specific type of measure is used to manage surface and overland flow. It involves the identification of designated surface and overland flow pathways along streets and roads through the urban environment most likely to designated storage areas.

Surface Water Rainwater (including snow and other precipitation) which is on the surface of the ground (whether or not it is moving), and has not entered is not being conveyed by a watercourse, drainage system or public sewer. Surface Water Flooding is the term often used to describe the combined surface flooding from multiple sources and can include pluvial flooding, sewer flooding, groundwater flooding at the surface and flooding from small urban watercourses.

Sustainability Sustainability is the long-term maintenance of responsibility, which has environmental, economic, and social dimensions. It is a term used to define an approach (relating to the implementation of measures or a plan) which does not compromise the interconnected needs of the economy, society and environment in the future.

Sustainable Urban Drainage System (SuDs)

Methods of management practices and control structures that are designed to drain surface water in a more sustainable manner than some conventional techniques.

Tidal Flooding Flooding resulting from sea levels exceeding high tide levels, or coastal flood defences. This type of flooding occurs in coastal areas and places where tidal influence may affect water levels (i.e. estuaries, coastal inlets)

Tipping Bucket Raingauge A tipping bucket raingauge is a meteorological device that can measure rainfall intensity as well as the total amount of precipitation that has fallen.

TUFLOW Modeling software that simulates pluvial flooding for a range of rainfall events of various severities (in duration and intensity)

Wet Mapping Pluvial flood maps which have been produced as an outputs from the TUFLOW model, which demonstrate the possible flood depth, velocities and hazard.


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ABBREVIATIONS

1D/2D One dimensional/Two dimensional

AEP Annual Exceedance Probability

ARF Areal reduction factor

CAPE Convective Available Potential Energy

CFRAM Catchment Flood Risk Assessment and Management Study

CoP Code of Practice CPS Contour Polygon Screening

DCC Dublin City Council

DDF Depth-duration frequency

DEFRA Department for Environment, Food and Rural Affairs, UK

DEM Digital Elevation Model

DTM Digital Terrain Model

EPA Environmental Protection Agency

EWA European Water Association FAO Food and Agriculture Organization

FDES Flood Depth Estimation System (software developed by Jacobs)

FEH Flood Estimation Handbook (UK)

FERP Federated Emergency Response Plan

FLIWAS Flood Information and Warning System

FRC FloodResilienCity

FRM Flood Risk Management

FSR Flood Studies Report

FSU Flood Studies Update (Ireland)

GDSDS Greater Dublin Strategic Drainage Study

GIS Geographic Information Systems

GPRS General Packet Radio Service

GSM Global System for Mobile Communications

IZ Impact Zones

LiDAR Light Detecting and Ranging

MEM Major Emergency Management

NOAH An Interreg IIIB organization for research in to the rapid transfer of data with respect to flood prevention.

NRA National Roads Authority

OPW Office of Public Works

OSi Ordnance Survey Ireland

PFRA Preliminary Flood Risk Review

PRA Principal Response Agency

PSTN Public Switched Telephone Network

Radar Radio Detection and Ranging

RBD River Basin District

RDBMS Relational Database Management Systems

RFSM Rapid Flood Spreading Model

SAFER Strategies and Actions for Flood Emergency Risk management SaS/RaR Street as Streams/Roads as Rivers

SuDs Sustainable Urban Drainage

TBR Tipping Bucket Raingauge

WMO World Meteorological Organization

WPG Weighing Principle Gauge


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SECTION 1 INTRODUCTION

1.1 Objectives and Scope The ‘City-wide (Type 1) Modelling’1 aimed to identify Dublin city’s vulnerability to pluvial flood risk using a two dimensional (2D) hydraulic model to produce pluvial flood depth, hazard rating (which is a function of depth and velocity with a debris factor included) and risk maps appropriate to the scale of the Dublin City Council administrative area. This was based on flood routing calculation at 25m resolution. The City–wide Model provided a high level assessment of flood risk across Dublin and based on outcomes of that investigation, five ‘Pilot Areas’ were identified to be taken forward for detailed (Type 2) modelling. For each of the five ‘Pilot Areas’, the objectives of this element of the project, ‘Detailed Pluvial Flood Risk Assessment of Pilot Areas ’, were as follows:

• Development of a fine scale hydraulic model (5m grid resolution, including drainage system) to understand flood mechanisms and assess flood hazard and risk associated with pluvial flooding in the Pilot Area (‘Type 2’ Models), i.e. the ‘baseline’ scenario;

• Use of the hydraulic model to identify and appraise a series of ‘corrective’ (flood resilience and adaptation2) measures to mitigate pluvial flooding;

• Assessment of damages likely to be sustained during pluvial flooding to inform the likely benefits of proposed corrective options, along with assessment against other criteria (through a Multi-Criteria Analysis) to inform the overall viability of options;

• Selection of preferred corrective measures using technical feasibility and cost benefit criteria;

• Development of conceptual designs of feasible options;

• Identification of the preferred corrective option for each Pilot Area; and

• Provision of ‘corrective’ pluvial flood hazard and flood risk maps to indicate the benefits of the preferred corrective option (the ‘with-scheme’ scenario).

Site specific results of this detailed flood risk assessment for each Pilot Area are fully detailed in the accompanying Appendices to this report (Appendices A, B, C, D and E). Figure 1.1 depicts the stages of the detailed pluvial flood risk assessment which were applied to each of the Pilot Areas. A brief description relating to the purpose of each step is included with a reference to the relevant report sections which further describe the step process.

1 As documented in Dublin FRC Technical Report Volume Two: City-wide Pluvial Flood Risk Assessment 2 As documented in Dublin FRC Technical Report Volume Three: Pluvial Flood Risk Management



Figure 1.1: Detailed Pluvial Flood Risk Assessment Process Applied to the Pilot Areas



1.2 Dublin City Pilot Areas

City-wide ‘Type 1’ model results and maps were appraised with Dublin City Council representatives to identify Pilot Areas suitable for the further detailed investigation of potential pluvial flood risk, i.e. ‘Type 2’ modelling. To identify the Pilot Areas, the following characteristics were considered:

• Which areas indicate a high level of potential pluvial flood risk?

• Are the characteristics of an area appropriate to facilitate the assessment of pluvial flood risk management measures, such as the practicality of trialling mitigation measures (including land ownership)?

• Is there a good spread of Pilot Areas across the City? Table 1.1 lists the Pilot Areas under investigation. These are also shown in Figure 1.2. Table 1.1 – Pilot Areas Selected for Type 2 Detaile d Modelling

Pilot Area Location Model Name

Dublin South East Pearse Square & Georges Quay and South Inner City

Type 2 Georges Quay and South Inner City

Dublin North Central

Clonliffe Road; Ballybough Road, Kings Avenue, Bayview Avenue; Annesley Bridge Road; Clontarf Road; St. Aidan's Park & Fairview Avenue Upper

Type 2 Dublin North Central

Dublin South Central Carrickfoyle Terrace, Kilmanhaim Type 2 Dublin South Central

Dublin North West Kippure Park Type 2 Kippure Park

Dublin Central East Wall Type 2 East Wall

It should be noted that there are other areas within Dublin City Council’s administrative boundaries which are also likely to justify more detailed investigation and modelling. Those Pilot Areas tabulated above were selected to identify and trial pluvial flood risk assessment and management techniques for development and use in future programmes of work.



Figure 1.2: Pilot Areas and Type 2 Model Locations



Key learning outcomes from Section 1, which provides the Introduction to the detailed pluvial flood risk assessment of pilot areas, are as follows:

• The background to the detailed hydraulic modelling for the five Pilot Areas identified in Volume Two is provided. It sets out the objectives of the detailed modelling that will lead ultimately to the identification of appropriate measures which can assist with the management of pluvial flood risk in each of the Pilot Areas.

• The five Pilot Areas are summarised, noting that the selection of these Pilot Areas included consideration of the following:

o Which areas indicate the highest level of potential pluvial flood risk?

o Are the characteristics of an area appropriate to facilitate the assessment of pluvial flood risk management measures?

o Is there a good spread of Pilot Areas across the City



SECTION 2 DETAILED PLUVIAL MODELLING FOR THE PILOT AREAS

2.1 Available Data Used for the Type 2 Hydraulic Mo delling (Step 1)

The following section outlines the first step of the detailed pluvial flood risk assessment process outlined in Figure 1.1 above; collection of the relevant data required to build the detailed (Type 2) models for the Pilot Areas. This information is used to inform the model and represent the current hydraulic and baseline data for each of the Pilot Areas (Step 2).

Figure 2.1: Initial Stages of the Type 2 modelling; Step 1 – Step 2 link.

Table 2.1 summarises the datasets used to construct the five ‘Type 2’ models covering the Pilot Areas. Table 2.1: Data used for Type 2 Hydraulic Models

Data Description

Type 1 Model outputs

Rainfall runoff inflows (time series) used to provide boundary conditions to the Type 2 hydraulic models (see Section 2.2.4).

Filtered LiDAR

- Light Detecting And Ranging (LiDAR) digital terrain model generated from aerial imagery. A filtering process is applied to the raw dataset to remove building and vegetation elevations. - Used to inform the Type 2 model grids with accurate ground elevation (see Section 2.2.2)

OSi - NTF

- Ordnance Survey Ireland small scale (1 in 1000) background mapping. - Used to inform the Type 2 model grids with land use information (see Section 2.2.3)

Historical flood records

- Historic flooding datasets available for the August 2008 and July 2009 pluvial flood events. These datasets included information on the location (address) and type/extent of flooding. - Used to help verify the Type 2 model results (see Section 2.3).

Surface water drainage network

Greater Dublin Strategic Drainage Study3 (GDSDS) GIS data of the storm water drainage network used to construct the 1D components of the Type 2 models (see Section 2.2.5).

3 GDSDS Study, NDDS Drainage Area, Phase 2 – Model Preparation, Verification and System Performance Assessment Reports, August 2006



Data Description

GDSDS Infoworks CS models

GDSDS Infoworks CS models were run to inform the 1D component of the Type 2 models with boundary conditions (see Section 2.2.5).

Road Gulley network

Road Gulley GIS dataset used in the model to allow rainfall infiltration into the storm water drainage system (see Section 2.2.6).

Site Visits

Information collated during site visits of the Pilot Areas and also via a letter-drop/questionnaire exercise provided a better understanding of local flood risk issues and associated flooding mechanisms. This valuable information was used to help verify the Type 2 model results (see Section 2.3).

2.2 Detailed Modelling Approach – Type 2 Hydraulic Model (Step 2)

The following sections describe how the data and information collected (via Step 1) is used to construct and verify the detailed models for each of the Pilot Areas (Step 2).

2.2.1 Modelling Process

The Type 2 hydraulic models were constructed using the TUFLOW modelling software (20011-09-AF-iDP) to simulate pluvial flooding over the Pilot Areas. Each model comprised a two dimensional representation of approximately 1 to 3 km2 area (2D domain) in the form of a regular grid of square cells of 5m side dynamically linked to a one dimensional representation of the underlying storm drainage network (1D domain). Exceptions to this representation of the drainage network were applied to the Dublin South East (Georges Quay and South Inner City) and Type 2 Dublin North Central (Marino and Fairview) models for which representation of the ‘combined’ sewer network was included due to the historical development of the City Centre drainage system when sewers were constructed to collect foul and storm flows, for discharge to the River Liffey. Both the 2D and 1D model domains have been carefully selected to encompass sufficient coverage of the pluvial and drainage catchments to ensure correct representation of the flooding mechanisms within each Pilot Area. To simulate pluvial flooding across the study area, the hydraulic models used the same Direct Rainfall approach4 as described in Volume Two City-wide Pluvial Flood Risk Assessment, which consists of applying a rainfall hyetograph (a graphical representation of rainfall distribution over time) representative of a storm event to every active cell within the 2D domain. During the course of a simulated event, the hydraulic model computes the rainfall that would be absorbed through natural infiltration into the ground, the rainfall that would be routed overland by gravity and also the runoff volume that would drain into and be conveyed through the storm drainage network. This process is illustrated on Figure 2.2.

4 Direct Rainfall approach is defined in the Glossary as a hydraulic modelling approach which involves the application of rainfall hyetographs representative of storm events to active model cells within a two dimensional domain.



Figure 2.2: Overview of the Type 2 Modelling Proces s The overland flow routed through the built environment and the flow conveyed through the drainage system is dynamically linked via an approximate representation of the road gulley network at each manhole. This approach is fully detailed in Section 2.2.5.

2.2.2 2D Grid Schematisation

Each Type 2 model 5m resolution grid was populated with ground elevation using ‘filtered’ LiDAR data (i.e. buildings and vegetation are filtered out). Similar to the City-wide ‘Type 1’ approach described in Section 3.2 of Volume Two, wherever appropriate, breaklines were used in the 2D grid to accurately represent any geographical features (e.g. railway embankments, flood defence walls etc) that may have a significant impact on the propagation of the rainfall runoff across the modelled area, i.e. a ‘breakline’ may be needed to inform the model that e.g. water cannot cross a railway embankment. These lines are particularly useful where the TUFLOW fixed grid resolution (in this case 5m) does not guarantee that the crest along, for example, a narrow wall, is picked up from the LiDAR Digital Terrain Model (DTM). Where rivers cross the modelled areas, these were included in the model schematisations but only as topography driven flow paths. The purpose of the Type 2 models was not to simulate fluvial flooding; therefore no fluvial flows or fluvial water levels of any form were incorporated into the models. Structures such as underpasses under major roads or railway embankments required modification to the model grids or the use of 1D elements to ensure rainfall runoff flow path continuity through these structures. A 1D Element is the representation of a structure in the 2D model area using the 1D approach (e.g. culverts, subways)



2.2.3 Hydraulic Friction and Natural Rainfall Infil tration (2D Domain)

Hydraulic roughness, represented by Manning’s roughness coefficient ‘n’ in the hydraulic model, is a means of accounting for the effect on the resistance to flow of surface materials, irregularities, obstructions and vegetation (refer to Section 3.2.3 of Volume Two for further details relating to Manning’s roughness coefficient). To represent friction in the 2D domain of the Type 2 models, geographical regions of different land use such as roads, urban areas, gardens, buildings and green spaces were defined using Ordnance Survey Ireland vector data. The land use regions were input into the 2D model grids so that each 2D grid cell carries a land use reference number corresponding to its land use. As shown in Table 2.2, Manning’s ‘n’ values were assigned to each of the land use reference numbers. It should be noted that the use of filtered LiDAR data to populate the 2D model grids means that buildings were not physically represented in the models. Given the fact that any building is an obstruction to the flow and would have a major impact on the overland flow routes, a very high roughness value has been attributed to each building/house to model the effect of the obstruction. The use of a high Manning’s n value for a building effectively makes it ‘very difficult’ for water to enter / flow through a building, but it does not make it impossible, in contrast to representing a building as a solid ‘block’ through which no water can flow. Table 2.2 also shows the values chosen to represent the natural infiltration of the rainfall into the ground. Rainfall infiltration consists of two components; the first is an initial loss which corresponds to the amount of rainfall (in mm) that is initially lost to the model (wetting the surface and surface storage) and the second is the continuing loss which is a loss rate in mm/h representing continuing infiltration. Losses through infiltration were applied to the model grid cells on a land use basis. Adopting a conservative approach, only permeable land use regions allowed infiltration. Table 2.3 extracted from the Food and Agriculture Organisation of the United Nations (FAO)6 gives a generalised guide to basic infiltration rates. Infiltration rates used in this study are selected to represent typical ground conditions with regard to the soils and geology in Dublin City. Given that much of Dublin’s surface geology is Boulder Clay, which has highly variable permeability by nature, values of 10mm and 10mm/hr as initial and continuous loss (for permeable surfaces only) were selected for the Type 2 models.

6 http://www.fao.org/docrep/s8684e/s8684e0a.htm



Table 2.2: Hydraulic Friction and Infiltration Loss es in the 2D Type 2 model

Land use Reference

Manning’s ‘n’ Value

Infiltration Initial loss

(mm)

Infiltration Continuing

loss (mm/hr)

Land use Description

550,551,552,553,556, 557,547

0.025 0.5 0

Motorway, national primary road, national secondary road, regional roads, third

category road, fourth category roads

600, 601, 589 0.500 0 0 Buildings, solid structures

526, 527, 528, 529 0.12 10 10 Trees, forest, dense

vegetation

574,618,564 0.055 10 10 Open space, general urban, railway track

575,583 0.050 10 10 Sport grounds, cemetery

531, 532 0.035 0 0 Main rivers, watercourses

533,534,536,537,538, 539,582

0.020 0 0 Lakes, ponds, drains, canal, reservoir

Table 2.3: Basic Infiltration Rate According to the Soil Type (source: FAO)

2.2.4 Boundary Conditions (2D Domain)

Hydrological Inflows (2D Domain) Hydrological inflows to the 2D domain element of the Type 2 models i.e. the overland flow component consisted of:

• Runoff flowing into the modelled areas from outside the modelled boundary i.e. from upstream; and

• Direct rainfall falling over the modelled areas. The upstream runoff was accounted for by flow hydrographs (a plot of flow against time - QT time series) derived from using the Type 1 model results (flow) and set along the boundaries of the Type 2 models. This process required extracting flow and water level data from the Type 1 models and analysis of the Type 1 model flow results to determine which sections of the Type 2 model boundaries were to account for the upstream runoff flowing into the Type 2 model domain. To represent this in the model,

Soil type Basic infiltration rate (mm/hour)

Sand less than 30 Sandy loam 20 - 30

Loam 10 - 20 Clay loam 5 - 10

Clay 1 - 5



Plot Output (PO) lines are drawn in TUFLOW across the 2D domain (as illustrated in Figure 2.3). The rainfall falling over the Type 2 model areas was represented using the same design rainfall profiles common to the Type 1 model area associated with the Type 2 model. Section 3.2.4 in the Volume Two City-wide Pluvial Flood Risk Assessment Report provides further explanation on how the rainfall profiles have been derived. Figure 2.3 shows schematically how the two overland flow components – inflow from outside the model boundary and direct rainfall on to the catchment – are included in the Type 2 model. Downstream Conditions (2D Domain) Along the Type 2 model boundaries, where surface runoff would in reality flow out of the Type 2 modelled areas, free flow conditions were assumed. The schematic in Figure 2.3 illustrates the settings of 2D boundary conditions in the Type 2 models.

Figure 2.3: Settings of 2D Boundary Conditions in t he Type 2 Models

2.2.5 Representation of the Drainage Systems (1D Do main)

1D Domain Schematisation For each Pilot Area, the underlying storm drainage network has been modelled with ESTRY (a 1D scheme included in TUFLOW). The foul and combined (foul and storm drainage) sewer networks have not been represented in the models except for the Type 2 Dublin South East and Type 2 Dublin North Central models. As previously explained (see Section 2.2.1), the latter models include a representation of both the combined and storm drainage networks using ESTRY. Manhole and pipe data were readily extracted from the existing Infoworks CS models of the drainage systems developed for the GDSDS (2006). These were assembled together to form a one dimensional representation of the storm drainage network within each Type 2 model.



It was assumed that the pipe data (e.g. dimensions, invert levels, gradients etc.) was correct. No verification of these datasets has been undertaken. The same assumption applies for the hydraulic roughness coefficient set to each pipe within the networks using the Infoworks CS model information. Manholes were represented in the Type 2 model (1D domain) at pipework junctions to account for the energy dissipation associated with:

• Expansion/contraction of flow lines within the manhole chambers and outlet pipes;

• Change in direction of the pipes (i.e. bends); and

• Change in height and/or invert level of adjoining pipes. It should be noted that energy loss coefficients were calculated by the model using equations based on the work of Engelhund7. Boundary Conditions (1D Domain) Boundary conditions to the 1D domain were set up at the upstream extents of the pipes entering the modelled areas and at the downstream ends of the pipes leaving the modelled areas. For the former, inflow hydrographs were derived using the GDSDS Infoworks CS model results (flow time series) at appropriate locations8. For the latter, two types of downstream conditions were applied, either in the form of a rating curve based on the capacity of the exiting pipe, or a level hydrograph in the case of an outfall into a river.

2.2.6 Rainfall Runoff Infiltration into the Drainag e System (1D and 2D Domains) To represent the inflow to the storm water drainage network, 1D/2D links were set up between the 2D domain, which calculates the overland flow, and the 1D drainage network in which storm water is conveyed underground. The 1D/2D links were located at each manhole relevant to the pipe network where road gullies would normally drain the overland flow to. Each manhole was connected to the 2D domain via a pit channel, broadly representing the system ‘road gulley + pipe’ discharging into the manhole chamber. The pit channels were defined using ground level and manhole chamber invert levels. The rate at which flow is passed from the 2D domain to the 1D domain, or conversely, expelled out of the 1D domain and on to the 2D domain when surcharging occurs within the drainage network, is governed by:

• The number of road gullies draining the overland flow to a particular manhole;

• The calculated depth at the manhole location as the difference in water level at the manhole location calculated in the 1D and 2D model domains – if the water level calculated is higher in the 2D domain water will flow into the drainage system; if it is higher in the 1D domain, water will flow out of the drainage system onto the ground surface; and

• A depth-discharge relationship at each 1D/2D link set up by the model user.

7 For further details, refer to TUFLOW User Manual BUILD 2010-10-AA, pp.4-56 8 It should be noted that the GDSDS Infoworks CS models were re-run with the same rainfall profiles used in the Type 2 models to generate these 1D boundary conditions.



A depth-discharge relationship was derived from British Highway Agency design standard tables for road gullies9. The determination of appropriate depth-discharge characteristics for road gullies is very complex because it depends upon numerous factors including longitudinal road gradient, road crossfall gradient and grating type. Figure 2.4 shows the depth-discharge curves adopted in this study for two types of road gullies. ‘Type F’ corresponds to the gullies in roads with a flat longitudinal gradient (typical slope of 1/300) whilst ‘Type S’ corresponds to gullies in roads having a steep longitudinal gradient (typical slope of 1/50). It should be noted that both curves have been capped at a maximum discharge of 0.015 m3/s (15 l/s). This is in accordance with the guidance from ‘Spacing of Road Gullies’ 9 which suggests that maximum flow rate that can be accepted by a gully pot without surcharge is about 15 l/s, if the outlet pipe (i.e. the pipe connecting the gully pot to the manhole chamber) has a diameter of 150mm.

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1

Depth of flow above gulley grate (m)

Cap

ture

d F

low

(m

3/s)

Gulley "Type F"

Gulley "Type S"

Figure 2.4: Depth-discharge Curves Adopted for Road Gullies in the Type 2 Models

2.3 Verification of the Type 2 Hydraulic Models (St ep 2)

Verification of the Type 2 models was carried out based on the information available. The following sections outline the verification process and how the information was used to check model performance.

Due to the paucity of records of flood depth from pluvial flooding within the Pilot Areas, no formal calibration of the Type 2 models was possible. Instead, verification of the models was carried out through:

• Model performance checks; and

• Comparison of areas of flooding predicted by the models with historic flooding records.

9 Highways Agency design standard HA 102/00 ‘Spacing of Road Gullies’



2.3.1 Model Performance

TUFLOW hydraulic modelling software provides run performance guidance, along with levels of acceptable error ranges and convergence thresholds that should be achieved during each model run. The concept of an acceptable error range has been adopted by the developers of the software, as numerical errors occur due to the quality of the data used, limitations of the software and underlying equation solving schemes. Run performance has been monitored throughout the Type 2 model build process and then during each simulation carried out, to ensure the optimum model convergence at any computed time step. In particular, the Cumulative Mass Balance Error reports associated with both 1D and 2D domains have been considered. For all the simulations undertaken, the latter parameters were found acceptable, staying within the +/-1% tolerance range recommended by the software developers. In addition, model outputs have been thoroughly reviewed to:

• Track any sign of instability or inconsistency between simulated events of increasing magnitude.

• Ensure energy loss coefficients throughout the drainage system and velocity/ depth distribution across the modelled area were sensible in both 1D and 2D domains.

2.3.2 Verification against Historical Flood Events

The Type 2 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 and in particular within the Pilot Areas. Flood incidents recorded following the August 2008 and July 2009 events were input by Dublin City Council into a database, and this has been further complemented as part of this project via mailing and review of questionnaires to households located in areas which experienced flooding. Type 2 model outputs (flood extent and depth) were compared to the recorded August 2008 and July 2009 flood incident locations to verify the soundness of the model predictions. Results of these comparisons are further detailed and displayed in Section 1.5 of the site specific appendices associated with each Pilot Area (Appendices A, B, C, D and E). It should be noted that a further significant event occurred on 24 October 2011. However, as flood incident records were still being reviewed at the time of carrying out the Type 2 modelling, it was not possible to use this event for model verification. However, for information purposes, the recorded incidents from this event at each of the Pilot Areas are displayed for the 2% (Annual Exceedance Probability (AEP), 3 hour duration modelled event on Figures 3.4B and 3.4C in each of the Pilot Area reports (Appendices A – E).



2.4 Modelled Scenarios (Steps 2, 3, 6)

Following the model build and verification exercise (Step 2), the first stages of Step 3 and also Step 6 of the detailed pluvial flood risk assessment are initiated to develop the various model scenarios required to assess each of the Pilot Areas.

Figure 2.5: Model Build and Modelled Scenarios Ste ps Type 2 hydraulic models were run to simulate pluvial flooding associated with a series of storm events with the following annual probabilities (with equivalent return periods shown in brackets): 20% (1 in 5 years), 10% (1 in 10 years), 5% (1 in 20 years), 2% (1 in 50 years), 1.3% (1 in 75 years), 1% (1 in 100 years) and 0.5% (1 in 200 years). In terms of a modelled scenario, the possible impact of climate change is an important consideration. It should be noted that Climate Change applied to a 1% annual probability event is considered to equate approximately to a 0.5% annual probability event (1 in 200 years). The 0.5% annual probability event can therefore be compared with the 1% event to assess sensitivity to, and the impact of, Climate Change. All storm events simulated were based on three-hour duration as this is considered representative of typical storms likely to trigger flooding incidents within Dublin City. Rainfall profiles associated with the above events were derived using the methodology described in Section 3.2.4 of the City-wide Pluvial Flood risk Assessment Report (Volume Two). Two different types of scenarios were adopted to estimate the extent of pluvial flooding across the Pilot Areas: 1. Baseline Scenarios (Step 2-3), including:

• Do Minimum Scenario: assuming ‘Existing’ conditions within the modelled area and, in particular, maintenance and normal operation of the drainage system i.e. no blockages to gullies or pipes within the combined and storm drainage networks.

• Do Nothing Scenario: entailing no maintenance of the drainage system and therefore assuming 90% blockage to all inlets to the combined and storm drainage networks (i.e. 1D/2D links in the model).

2. Corrective Measure Scenarios (Step 6): assuming various flood mitigation

options tested with the Type 2 hydraulic models. For each Pilot Area, measures under consideration were initially grouped under Options A, B and C. A ‘preferred option’ was finally selected based on a comparison of the performance of each option. The various measures considered and the selection process undertaken for the preferred options are detailed in Section 4.



Table 2.4 shows the matrix of storms/scenarios run with the Type 2 hydraulic models. Table 2.4: Matrix of Rainfall Events Run with the T ype 2 Hydraulic Model for each Pilot Area.

Annual Probability Scenario

20% 10% 5% 2% 1.3% 1% 0.5%

Baseline Scenarios Do Minimum ��

Do Nothing ��

Corrective Measures Option A ��

Option B ��

Option C ��

Preferred Option ��

2.5 Type 2 Model Outputs (Steps 2, 3, 6)

The Type 2 hydraulic models can output 2D results, such as flood level, depth, velocity and hazard rating at regular intervals throughout a simulation. In addition, maximum values associated with these outputs can be produced as 2D grids of 2.5m resolution. These 2D grids can be easily processed into flood maps if required. It should be noted that flood hazard rating is automatically calculated by the Type 2 models as a function of depth and velocity (with a debris factor included) following the DEFRA methodology10. This approach is similar to the one adopted for the Type 1 City-wide model and a comprehensive definition of flood hazard rating is given in Section 4.1 of the Volume Two City-wide Pluvial Flood Risk Assessment Report. The Type 2 models can also produce 1D results in the form of flow, velocity and water level time series for each pipe included in the storm drainage network. Maximum values associated with these outputs are also available.

Examples of Type 2 model outputs are presented and discussed in the site specific appendices associated with each Pilot Area (Appendices A, B, C, D and E). These outputs represent Steps 3 and 6 of the detailed pluvial flood risk assessment methodology illustrated in Figure 1.1. When mapping/displaying model outputs, low value categories of flood depth (less than 100mm), velocity (below 0.1m/s) and hazard rating (below a value of 0.75, also referred to as low hazard) are filtered out and not shown on the maps. This is for clarity purposes, to avoid reporting all surfaces being considered as flooded and to assist with the identification of high risk areas. However, model data relating to these low value categories is contained within the model outputs and can be displayed should this be required.

10 DEFRA (2008) Supplementary Note on Flood Hazard Ratings and Thresholds for Development Planning and Control Purposes



2.6 Key Model Assumptions and Limitations (Step 2)

2.6.1 Model Grid Resolution

It should be noted that small topographic features, for example, kerbs and traffic calming measures are not explicitly represented within the Type 2 hydraulic models. The 5m cell size of the model grid and the resolution of the LiDAR data (2m) mean that elevation differences between roads and kerbs are unlikely to be well represented in the model. Whilst refined representation of these features in the model may influence shallow surface flows, this has not been attempted here due to the scale and complexity of modelling undertaken.

2.6.2 Representation of the Drainage Systems

A certain number of assumptions associated with the representation of the storm drainage network and its interaction with the overland runoff are inherent to the Type 2 modelling approach adopted in this study. Although some of them have already been discussed in previous sections, it is worthwhile summarising them here:

• The Type 2 hydraulic models do not allow for a detailed representation of the surface water entering the drainage network via road gullies and pipes discharging into the main sewers. Instead it is assumed (i.e. in modelling terms) that rainfall runoff enters into the system at the manholes located near the road gullies.

• The representation of the drainage network in the 1D domain relies entirely on the pipe/manhole data included in the GDSDS Infoworks CS models of the GDSDS catchments. It is assumed that the latter datasets (e.g. pipe dimensions, invert levels, gradients) are correct.

• Although the manhole data extracted from the GDSDS Infoworks CS models is a comprehensive dataset, further assumptions had to be made about the manhole chamber shapes and dimensions. Thus, all manholes were considered to be rectangular shape with a minimum length (in the direction of the flow) of 1.05m and a width, automatically calculated by TUFLOW, depending on the number and size of the incoming/outgoing pipes.

• Whilst it is considered that the 1D representation of the drainage network using ESTRY is comprehensive and offers a realistic representation of the operation of the drainage system under flood conditions, the 1D component of the Type 2 models should not be regarded as equivalent to the GDSDS Infoworks CS models within the Pilot Areas. The latter have been verified in more detail than the ESTRY representation. In addition, the Infoworks CS models are likely to provide more realistic results under normal flow conditions. This is because, when compared with ESTRY, Infoworks CS software offers some additional features better adapted to a detailed representation of sewer networks.

2.6.3 Model Calibration

As referred to in Section 2.3, no formal calibration of the Type 2 hydraulic models against observed flood levels was undertaken. Instead, verification of the models was carried out through model performance checks and comparison of areas of flooding predicted by the models with historic flooding records. This verification demonstrates that the models provide sensible results deemed appropriate to assess the flood risk and corrective measures within the Pilot Areas. However, whilst the Type 2 hydraulic models should be regarded as robust tools to help manage pluvial flood risk across the



Key learning outcomes from Section 2 with regard to the Detailed Pluvial Modelling for the Pilot Areas are as follows:

• The Type 2 Hydraulic Modelling approach is described including the data used to construct the model, and the development of the series of five models (one for each Pilot Area) based on a 5m grid cell size. The model includes both overground (2D) and underground (1D) elements which are dynamically linked.

• The key aspects of the 2D overground model domain are: o 2D grid schematisation; o hydraulic friction of different land surfaces; o natural rainfall infiltration into the ground; and o boundary conditions including the rainfall input to the model, inflow to

the model from outside the model boundary, and downstream boundary conditions.

• The key aspects of the 1D underground model domain are: o 1D schematisation; o Sewer network boundary conditions; and o representation of the links between the 1D and 2D model domains to

allow, for example, the 1D model (underground) to become surcharged, leading to flow coming out of manholes and onto the 2D (overground) model.

• The performance of the models was verified against two flood events – August 2008 and July 2009. The model was then used for different design events covering seven AEPs; from the 20% AEP event to the 0.5% AEP event. The storm duration in all cases is 3 hours.

• Data from the October 2011 flood event in Dublin was not available for verification of the Type 2 models. However, for information purposes, the flood incident records for this event have been included in the Pilot Area flood depth maps (Figures 3.4B and 3.4C of Appendices A-E).

• All models were set up to adopt two types of scenarios; Baseline Scenario (the ‘Do Nothing’ and the ‘Do Minimum’ Scenarios), and Corrective Measures Scenario (to test the effectiveness of different measures).

• Type 2 hydraulic model outputs produce both 2D and 1D results which can be easily processed into flood maps:

o 2D: flood level, depth, velocity and hazard rating at regular intervals throughout a simulation;

o 1D: in the form of flow, velocity and water level time series for each pipe included in the storm drainage network.

Such model outputs are displayed for each Pilot Area in Appendices A–E.

• Key model assumptions and limitations associated with the detailed pluvial (Type 2) modelling are outlined for consideration when reviewing the model outputs.

Pilot Areas, consideration should be given to the calibration of the models prior to using any for informing detailed design of flood relief measures.



SECTION 3 BASELINE SCENARIO DETAILED PLUVIAL FLOOD HAZARD AND RISK MAPPING

3.1 Pluvial Flood Depth and Hazard Rating Mapping ( Step 3)

On completion of the model builds (Step 2) and the Baseline Scenario model runs, the Baseline Scenarios of pluvial flood depth and hazard rating for each Pilot Area are mapped to illustrate existing conditions. These mapping outputs are then used to develop the pluvial flood risk maps for each Pilot Area (Section 3.3, Step 4).

Figure 3.1: The Baseline Scenario Mapping Outputs U sed to Inform the Existing Risk to Receptors within each Pilot Area (Step 4).

The model files associated with the Do Minimum (existing conditions) and Do Nothing Baseline Scenario model runs listed in Table 2.4 were reviewed for each Pilot Area. Observations specific to each Pilot Area as well as mapped examples of the detailed model outputs for the Baseline Scenarios are included in Section 2.1.1 of Appendices A – E. The mapped examples for each Pilot Area show the maximum flood depth, velocity and flood hazard rating (combined depth and velocity) predicted across each modelled area for the 2% AEP (1 in 50yr) event.

As mentioned in Section 2.5, when mapping/displaying model outputs, low value categories of flood depth (less than 100mm), velocity (below 0.1m/s) and hazard rating (below a value of 0.75, also referred to as low hazard) are filtered out and not shown on the maps. This is for clarity purposes, to avoid reporting all surfaces being considered as flooded and to assist with the identification of high risk areas. However, model data relating to these low value categories is contained within the model outputs and can be displayed should this be required. The model outputs for the Baseline (Existing) Scenario were reviewed to identify the likely food mechanisms at key locations. This also allowed for the identification of flooding ‘hotspots’ or ‘problem areas’ where significant depths or high hazard ratings were predicted. The following section discusses this review process and outlines the various model elements that were considered as part of the interpretation of the model outputs. Section 5.2 of this report, provides a summary of the key flood mechanisms and characteristics of pluvial flooding and specific ‘hot spot’ or ‘problem areas’ highlighted by the Type 2 (detailed) modelling.



3.2 Flood Mechanisms (Step 3)

To help identify the appropriate corrective measures for each Pilot Area (Steps 5 and 6), the Baseline (Existing) Scenario model outputs were reviewed to ascertain the likely flood mechanisms at key locations across the modelled areas. Details of these flood mechanisms for Pilot Area are outlined in Section 2.1.2 of Appendices A – E. TUFLOW SMS viewer software11 was used to display and review model outputs (depth, velocity and hazard rating) over a series of time-steps to assess the flood mechanisms across the duration of a modelled event. Figure 2.3 of each of the Pilot Area Appendices (A – E) conveys the development of pluvial flooding through the 2% AEP 3 hour duration event under Existing (Do – Minimum) conditions by displaying model outputs at three time-steps; one at 1 hour, one at 1.75 hours and one at 3 hours. The mapping outputs for each time-step displayed in Figure 2.3 of Appendices A-E show both the overland flow parameters (three-maps for each time-step) as well as the drainage system status (one map for each time-step). The elements shown in each of these mapping outputs are described in more detail below. Overland flow elements The pluvial flooding (above ground) elements shown for each time-step are:

• Flood depth, with velocity vectors showing overland flowpaths overlaid on the same plan;

• Flow velocity for the overland flow element; and

• Flood hazard rating – based on the combination of flood depth and flow velocity (with a debris factor included).

Drainage system elements The drainage elements shown for each time-step indicated the state of the drainage network with regard to whether the drainage system and associated manholes are surcharged or not:

• Pipes are represented in blue where the system runs at less than capacity. Conversely, a purple colour has been allocated to the pipes running at full capacity. For clarity, the arrows showing the normal direction of the flow within the pipes have been removed (however, these are shown in Figure 1.3 of each Pilot Area specific Appendix).

• Manholes are represented as red dots where the storm drainage network surcharges and causes or contributes to flooding on the ground surface. Where the system is still running at or less than capacity, this is indicated by green dots at the manholes. Where ‘green’ manholes are associated with purple pipes, this indicates the drainage system is running a full capacity but is still acting as a ‘sink’ and able to drain flow from the ground surface.

11 http://www.tuflow.com/SMS%20TUFLOW%20Interface.aspx



An example of the drainage system elements which are illustrated for each Pilot Area is provided in Figure 3.2 below.

Figure 3.2: Examples of the Baseline Drainage Syst em Elements Used to Assess the Flood Mechanisms at each Type 2 Pilot Area.

3.3 ‘Existing’ Pluvial Flood Risk Mapping (Step 4)

The pluvial flood depth mapping produced from the Baseline ‘Existing’ Scenario model runs (Step 3) was used in conjunction with the receptor groups identified in Volume Two: City-wide Pluvial Flood Risk Assessment to assess and map the potential pluvial flood risk for each of the Pilot Areas. The Existing pluvial flood risk maps were then reviewed with the Existing hazard rating and depth model outputs to inform the appraisal of corrective measures and options (Steps 5 and 6).

Figure 3.3: Existing Risk Maps used to Inform Appra isal of Measures

The Type 2 modelling risk assessment was based upon the susceptibility of receptors to pluvial flooding during a particular design event. The methodology applied is the



same as that used for the Type 1 modelling risk assessment which is described in Section 4.2 of Volume Two. A key difference is in the detail of the approach and data used - the resolution for the Type 2 modelling risk assessment was based on a finer 5m grid, as opposed to a 25m grid for the Type 1 modelling risk assessment. The outputs from the risk assessment were achieved using Geographical Information System (GIS) tools by first assigning pluvial flood depth values derived from the Type 2 mapping to individual receptors.

As with the City-wide Pluvial Flood Risk Maps presented in Volume Two, the following receptor groups were appraised and used in conjunction with the Type 2 Depth mapping as the basis of the risk assessment:

• Risk to Human Health and Critical Infrastructure • Risk to the Economy • Risk to the Environment and Cultural Heritage

The risk classification of High, Medium, Low and Marginal was assigned to each receptor in the ‘Human Health and Critical Infrastructure’ and the ‘Environment and Cultural Heritage’ receptor groups using the approach shown in Figure 4.2 and the classes displayed in Table 4.5 of Volume Two. Due to higher level of detailed provided as part of the Type 2 risk assessment, the ‘Risk to Economy Maps’ display the actual total present value (PV) damages (refer to Section 4.4.2 for further information). This allows for a more accurate interpretation of the current risk to economy. The flood risk maps based on the ‘Existing’ situation are shown in Figures 2.4 of the Pilot Area Appendices (A-E). Basement Properties In the absence of a comprehensive database of basement locations, basements (below-ground) properties were not specifically identified within the Type 2 models. However, as a trial, the Dublin North Central Pilot Area model (Appendix B) was used to consider how the representation of basements within the Type 2 GIS-based pluvial flood risk assessment could further inform/influence receptor risk ratings at this scale. Basement location data recorded as part of the GDSDS for the Dublin North Central Pilot Area was reviewed and then supplemented following a walk-over survey of the Pilot Area undertaken by Dublin City Council12. Similar to the approach used for the City-wide flood risk maps, the new combined basement dataset for the Dublin North Central Pilot Area was used to introduce an ‘uplift’ to the risk bands applied to the Human Health & Critical Infrastructure Risk Maps at locations were property receptors were located. The presence of a basement location triggered an increase in an existing risk rating to the next higher rating, either ‘medium’ or ‘high’ risk. Figures 3.4A and 3.4B below provide examples of how this was applied to the Dublin North Central Pilot Area.

12 A key point observed during this walkover was that some of the basements within this Pilot Area may have been converted to cellars and therefore were not obvious during the walk-over survey.



Figure 3.4A – Human Health & Critical Infrastructur e Risk Map Showing Risk Ratings for ‘Non-Basement’ Receptors.

Figure 3.4B – Human Health & Critical Infrastructur e Risk Map Showing Uplifted Risk Rating for Updated Basement Location Records

An uplift was applied to the risk rating of the Human Health & Critical Infrastructure receptors for which basement records exist.



Key learning outcomes from Section 3, with regard to Baseline Scenario Detailed Pluvial Flood Hazard and Risk , are as follows:

• Model outputs for the Baseline Scenarios inform the identification of the characteristics of each Pilot Areas, such as the existing flood mechanisms and flooding ‘hotspots’ or ‘problem areas’ where significant flood depths are predicted.

• TUFLOW SMS Viewer software is used to display and review model outputs (depth, velocity and hazard rating) over a series of time-steps to assess the flood mechanisms across the duration of a modelled event. Appendices A-E include maps which illustrate the results of these time series for each Pilot Area.

• Using the risk assessment methodology defined for the City-wide pluvial flood risk assessment (Volume Two), pluvial flood risk maps for the Baseline Scenario are developed based in the Type 2 model outputs (flood depth).

• Although below-ground properties were not been specifically identified within the Type 2 models, the Dublin North Central Pilot Area (Appendix B) was used to trial the use of GIS to inform the risk assessment of such properties at this scale.



SECTION 4 MODELLING APPRAISAL OF CORRECTIVE MEASURE S

The following sections describe the basis for the identification, screening and selection of the ‘flood resilience and adaptation measures’ (corrective measures) from those introduced in Volume Three: Pluvial Flood Risk Management; Code of Practice 2 (Step 5). This process informed the technical and economic appraisal of various options identified for each of the Pilot Areas using the detailed pluvial model (Step 6), i.e. the ‘With Scheme’ model runs.

Figure 4.1: Option Appraisal – ‘With Scheme’ Runs

4.1 Basis for Identifying Mitigation Measures (Step 5)

Figure 4.2 provides a high level summary of the different flooding sources, mechanisms and impacts which is a useful reference point for considering different mitigation measures to reduce flood risk. It shows the typical Source-Pathway-Receptor approach to flood risk management. Source – The origin of the flood hazard e.g. heavy rainfall. Pathway – The route that the flood hazard takes to reach a receptor. Receptor – The entity that is harmed e.g. a person, property, or infrastructure. For example, in the event of heavy rainfall (the source), floodwater from a river flows over the top of the river bank and across the floodplain (the pathway) and inundates housing (the receptor). This combination of factors leads to material damage (the consequence). Consequence – An impact such as economic, social or environmental damage.



Sourcee.g. rainfall, river flow, surface water, sewer capacity

exceeded, high tide, waves

Pathwaye.g. overtopping or breach of defences, sewer

overflow, roads, floodplain inundation

Receptore.g. residential and commercial property, transport

infrastructure, utilities infrastructure, green space etc..

Consequencee.g. damage to property and infrastructure, loss of

business, stress, loss of life.

Typical Source-Pathway-Receptor Approach

Sourcee.g. rainfall, river flow, surface water, sewer capacity

exceeded, high tide, waves

Pathwaye.g. overtopping or breach of defences, sewer

overflow, roads, floodplain inundation

Receptore.g. residential and commercial property, transport

infrastructure, utilities infrastructure, green space etc..

Consequencee.g. damage to property and infrastructure, loss of

business, stress, loss of life.

Typical Source-Pathway-Receptor Approach

Figure 4.2: Source-Pathway-Receptor Approach To those that are affected by flooding, it is the consequence or impact that is most clearly identified. However, to develop appropriate mitigation measures – measures to reduce the impact of flooding in future – it is important to understand the source-pathway-receptor links to enable mitigation measures to be targeted appropriately.

4.2 Mitigation Measures under Consideration (Step 5 )

A wide range of measures have been identified as part of the Dublin FloodResilienCity Project and are included within Volume Three, Appendix V3-B, Code of Practice 2 – Flood Resilience and Adaptation Measures. This Code of Practice groups the various measures according to the following categories:

• Generic and Early Win Measures – those which can be applied universally across the whole of the Pilot Area and aim to raise the overall level of resilience to pluvial flood risk. They apply to areas of high risk and lesser risk and therefore cover areas where the level of risk does not justify investment in Site Specific measures. Those measures which are low cost or easy to implement and yet achieve a significant benefit may be categorised as Early Wins.

• Community Flood Resilience Measures require engagement and participation at community or householder level. Specific actions may be required to implement building resistance and resilience measures and ownership of that responsibility will be an important element of effective resilience raising.

• Site Specific Measures are normally applicable only in areas of identified high risk. They are likely to involve capital investment and there may be an ongoing maintenance commitment.

Table 4.1a presents an initial 29 measures considered for Dublin City. Each measure is referenced with at flood risk management (FRM) code (e.g. ‘FRM 4’) which is used



throughout this Volume and its Appendices when referring to a measure. In this table, the measures are grouped according to the broad category headings adopted in the Code of Practice 2: Flood Resilience and Adaptation Measures (Volume Three). Similar to the matrix presented in Figure B1.1 of Code of Practice 2, Table 4.1a below identifies the primary responsibility for the implementation of these measures. Table 4.1a – Corrective Measures under Consideratio n ( * indicates potential innovative solution; X* indicates shared responsibility when applied in community-wide schemes)



4.3 Initial Screening of Measures for Trialling in Pilot Areas (Step 5)

Following the identification of individual measures to manage pluvial flooding (Table 4.1a), an initial screening assessment using a multi-criteria matrix approach was undertaken. The purpose of this initial screening was to determine which measures would be taken forward for a more detailed assessment within the Type 2 models for each Pilot Area. No modelling or economic appraisal was undertaken at the initial screening stage as the primary aim was to discount unsuitable or inappropriate measures at an early stage. The measures have been screened using the following criteria:

• Technical overview – ‘is it likely to reduce flood risk during an extreme rainfall event?’

• Environmental overview – ‘is it likely to be environmentally acceptable?’

• Social overview – ‘is it likely to be socially acceptable?’

• Sustainability overview – ‘is it a sustainable measure’

• Economic / indicative cost – ‘is the cost reasonable/not prohibitively high?’ A number of the measures listed in Table 4.1a are non-structural measures which cannot be modelled within TUFLOW and are therefore the first measures to be ‘screened out’. These measures are highlighted in Table 4.1b as ‘non-structural measures relevant to spatial planning / building control’ and ‘good practice’. The former should be considered for all new developments in the study area to alleviate future increases in flood risk as a result on new development. By their nature, being implemented over many years giving incremental improvements in flood risk reduction; these non-structural measures aim to reduce the pluvial flood risk through the city in the longer term. The ‘good practice’ measures noted in Table 4.1b are important to reduce the flood risk to properties in the city and increase resilience. Both sets of measures are recommended but are not relevant for further evaluation using the detailed (‘Type 2’) models. It is a recommendation of this report that should a Flood Risk Management Scheme be taken forward for more detailed appraisal within the Pilot Areas, that these non-structural measures are considered an integral part of any scheme to reduce and manage pluvial flood risk in the area.



Table 4.1b – Corrective Measures under Consideratio n Following Initial Screening Assessment (measures ‘screened out’ are h ighlighted in grey) (* indicates potential innovative solution)



On reviewing the potential options for each Pilot Area, the emphasis was on identifying ‘early wins’ and measures that are clearly appropriate based on the findings of the modelling. The options identified do not necessarily represent the complete solution to the pluvial flood problems identified; they are recommended as part of a city-wide solution, and in many cases will be most effective when combined with other measures. For all areas, early consideration and emphasis must be given to measures that reduce the risk of injury and risk to life . For garden flats/basements and below-ground facilities consideration should be given to the possibility of installing water level warning alarms and means to ensure quick response (FRM 2) in the event of a flood in addition to Internal Resilience Measures. Where External Resistance Measures (FRM 15) are recommended for a Pilot Area, Internal Resilience Measures (FRM 16) should also be considered. Table 4.2 overleaf summarises the outcome of the high level screening exercise which identified 17 measures as appropriate for further consideration in the Type 2 Pilot Areas. The individual measures which passed the initial screening process were then brought together to form pluvial flood risk management options (groups of mitigation measures across the Pilot Areas).

Volume Four: Detailed Pluvial Flood Risk Assessment of Pilot Areas


Table 4.2 – High Level Screening of Initial Flood R isk Management Measures

Volume Four: Detailed Pluvial Flood Risk Assessment of Pilot Areas


Table 4.2 (continued) – High Level Screening of Ini tial Flood Risk Management Measures

Notes: Technical Feasibility Initial Assessment

3 - Feasible with significant benefits U: Unacceptable

2 - Feasible with some benefits -2: Severe negative outcome

1 - Feasible but marginal benefit -1: Moderate negative outcome

0 - Not feasible or not relevant 0: Neutral

1: Moderate positive outcome

2: High positive outcome



Key learning outcomes from Section 4, with regard to Modelling Appraisal of Corrective Measures , are as follows:

• Initial screening, using a multi-criteria matrix approach, of the flood resilience and adaptation (corrective) measures identified in Volume Three: Pluvial Flood Risk Management is undertaken to identify which measures are taken forward for a more detailed assessment within the Type 2 models (and ultimately, development into flood risk management options).

• Non-structural measures which cannot be modelled are firstly ‘screened out’. This includes measures relevant to spatial planning / building control’ and ‘good practice’.

• Following the initial screening exercise, 17 measures are identified as appropriate for further consideration using the models of the Type 2 Pilot Areas.



SECTION 5 OPTION TECHNICAL AND ECONOMIC APPRAISAL

In order to identify the most appropriate set of mitigation measures, three flood risk management options (Options A, B and C) were developed for each Pilot Area based on appropriate combinations of the selected measures listed in Table 4.2. The viability and effectiveness of these options were then assessed using the detailed model (technical appraisal). Identification of a ‘preferred option’ was based on this assessment as well as an economic appraisal. The preferred option could be any one of the A, B or C options, or a combination of these options based on the outcome of the modelling evaluation of each. Figure 5.1 below shows the link between the use of the ‘With-Scheme’ model runs from Step 6 and the selection of the preferred option (Step 7).

Figure 5.1: With Scheme Model Outputs Used to Infor m Preferred Option Selection As part of the evaluation of potential options, the following were considered:

• Initial multi-criteria scoring analysis of measures;

• Site inspection information collated by Jacobs and Dublin City Council staff on different occasions as part of the review to identify approximate locations of properties and areas which are susceptible to pluvial flooding (e.g. garden flats/basements, access to underground parking and other below ground infrastructure);

• Detailed modelling showing likely flow routes; and

• Consultation with stakeholders.

5.1 Flood Risk Management Options (Step 6)

The selection of the combinations of measures was based on the review of the Baseline (Existing) Scenario model outputs and the existing flood mechanisms and ‘hot-spots’ or ‘problem areas’ identified for each Pilot Area (refer to Section 3). Further to the initial (high-level) screening undertaken (Step 5), the following criteria were used for the more detailed evaluation of Options A, B and C for each Pilot Area and to identify the ‘preferred option’:

• Technical (refer to Section 5.2) – ‘to what extent is the option likely to reduce flood risk?’

• Economic (refer to Section 5.3) – ‘is the cost of the scheme less than the resulting economic benefit?’



To facilitate this more detailed evaluation, conceptual designs of Options A, B and C were developed using GIS. A series of ‘With Scheme’ model runs (as outlined in Section 2.4) were then carried out for Options A, B and C relevant to each Pilot Area. Figure 5.2 below provides a summary of this evaluation process.

Figure 5.2: Technical and Economic Appraisal Proces s for the Selection of the ‘Preferred’ Option. Details of the options considered for each Pilot Area are outlined in Section 3.2 of Appendices A – E.

5.2 Technical Assessment (Step 6)

The verified TUFLOW hydraulic model has, where appropriate, been used to assess to what extent each of the three potential flood risk management options (A, B and C) are likely to reduce flood risk during a 2% AEP event. Attention was paid to key ‘hotspot’ or problem areas and how the measures would reduce flood levels and subsequently flood risk. This allowed the effectiveness of the individual mitigation measures within each option to be assessed on the basis of the reduction in water levels and extents (based on the pluvial flood depth mapping). Following this assessment, if an optimised FRM option was not identified, i.e. if Option A, B or C was not sufficient as a ‘preferred’ option, a fourth option was developed based on a combination of the most effective mitigation measures from each of the trialled options (A, B and C). In this instance, the TUFLOW model was re-run with the new ‘preferred’ / ’optimised’ FRM option. On completion of technical appraisal an economic assessment was then carried out.



5.3 Economic Assessment (Step 6)

Introduction The second stage in the evaluation of the options involved a detailed economic assessment. This involved calculating the overall damages (cost of flooding) based on the Baseline ‘Do-Minimum’ (‘Existing’) and the Baseline ‘Do – Nothing’ scenarios to provide an indication of the damage costs associated with flooding under the existing conditions. The cost of implementing each FRM option13 is then estimated so that the cost of damages remaining after an option has been put in place can be determined. The benefit / cost ratio is then used as an indicator of the economic viability of an option. For a FRM option to be economically viable, its total benefit in terms of damage avoided (reduction in damages) must be greater than the cost of the measure, i.e. the benefit / cost ratio must be greater than 1. The calculations relevant to each Pilot Area are outlined in Appendices A – E. Methodology Damage Assessment The calculation of economic benefits is undertaken using standardised guidelines and figures, provided in the Flood and Coastal Defence Project Appraisal Guidance (FCDPAG)14 published by DEFRA in the UK, and also the Middlesex University’s Flood Research Centre’s ‘Multi-Coloured Manual’15 (MCM). The approach outlined in these guidance documents refers to an economic as opposed to financial assessment. For example, residential property damages take account of depreciation of the value of contents (e.g. replacing items like-for-like, as opposed to purchasing new items). Indirect damages to non-residential properties (i.e. loss of business) are generally not included as this is assumed to be displaced elsewhere within the economy. The exception is for highly specialised businesses, in which case loss of business could represent a true loss to the national economy. This can be considered on a case-by-case basis. In accordance with the project specification, a 50 year appraisal period and fixed discount rate of 4% were assumed. The economic ‘benefits’ for each of the three flood risk management options (or the ‘Do something’ options) are expressed as damage avoided when compared to the Do-nothing damages. It should be noted that vehicle damage is excluded from the Multi-coloured Handbook damage calculations, and therefore was also not considered as part of the economic assessment methodology used for this project. A GIS-based Flood Depth Estimation System (FDES; Jacobs, 2007) tool was used to calculate the ‘Existing’ damages and also the ‘With Scheme’ damages by using the flood levels from the TUFLOW hydraulic models developed for the technical appraisal.

13 Excluding VAT 14 Department for Environment, Food and Rural Affairs (2004) Flood and Coastal Defence Project

Appraisal Guidance, FCDPAG3 Economic Appraisal, Supplementary Note to Operating Authorities. Revisions to Economic Appraisal on: Reflecting socio-economic equity in appraisal, and appraisal of human related intangible impacts of flooding.

15 Flood Hazard Research Centre, The Benefits of Flood and Coastal Defence: Techniques and Data for 2003, Middlesex University (known as Multi Coloured Manual (MCM).



It is important to note during the application of FDES that apartment blocks may influence the total costs at a location by adding multiple property entries at one property location. To minimise this for the purpose of FDES, a screening exercise was undertaken to correct this for a number of apartment complexes. It was not possible to screen out all apartments based on the information available. For each option scenario (A, B, and C), four return periods were modelled: a 1 in 10 year (10% AEP), a 1 in 50 year (2% AEP), a 1 in 100 year (1% AEP) and 1 in 200 year (0.5% AEP). The damages occurring in each event were combined, along with the probabilities of occurrence, to derive an Average Annual Damage (AAD) figure. This was then used to calculate the total present value (PV) damages, over a 50 year appraisal period assuming a 4% discount rate.

The FDES tool allows for the assessment of the overall economic damages associated with flooding which is comprised of the following components:

• Direct property damage: damage to fabric and contents of building;

• Indirect damage: indirect costs incurred as a result of flooding (e.g. temporary accommodation);

• Intangible damage: human health impacts (physical and psychological); and

• Emergency services: emergency works and recovery operations The following sections provide a breakdown of each of the components listed above and how these were accounted for as part of this assessment.

Direct Property Damage Approximately 80% of the total damages are derived from direct property damages. The following items make up the variables which contributed to the calculation of the Direct Property Damages for the Pilot Areas. (Figure 5.3 shows the relationship between the direct property damage and depth of flooding).

• Property type: Residential or commercial (residential properties were not subdivided). The depth-damage data presented in the MCM for code 199 ‘average residential’ were applied to all residential properties. Average depth-damage data for non-residential properties were calculated by taking the average across all non-residential property types (MCM codes 200-900).

• Depth: Damage accrued for flood depths greater than 0.15m16. No ‘negative’ flood depths were included – i.e. only properties which show flood waters crossing the threshold were assumed to incur any damages. This is standard for a surface water assessment as this type of flooding tends to be ‘flashy’ in nature and potentially does not cause extensive damage to foundations.

• Duration: Cost of long duration (>12 hours) is greater than short duration (<12 hours). All events were assumed to be short duration (<12 hours).

• Market Value: Residential property damages were limited (or ‘capped’) to an estimated market value17. This means that the total present value flood damages over the appraisal period cannot exceed the market value of the property. The application of this market value limit however was not required for

16 All property thresholds are assumed to be 0.15m above ground level, i.e. 0.15m has been subtracted from all flood depths provided (based on a number of sensitivity variations, including a 0.1m and 0.25m thresholds). 17 The average residential property value estimated for use in this study was €347,936



residential properties within the Pilot Areas as no single property incurred greater damages than the estimated average market price. Non-residential property damages were not capped due to insufficient information about the type of properties to adequately estimate a market value.

Figure 5.3 – Graph Showing Relationship Between Dir ect Property Damage (Average Residential) and Depth of Flooding. Indirect Property Damage The following items make up the variables which contribute to the calculation of the Indirect Property Damages (Indirect Damages increase with the depth of flooding): Residential properties

• Includes the cost of temporary accommodation and use of blowers/dehumidifiers (damage increases with flood depth).

Non-residential properties

• Indirect damages (i.e. loss of business) have not been included. Generally these are small compared with direct losses; however this should be considered on a case-by-case basis, for example where the economic sector is highly specialised (see discussion on Economic Assessment above).

Intangible Damage and Emergency Services These are dependent on the following:

• Intangible damages are only accrued for residential properties (these damages are set equal to the ‘direct’ residential property damages);

• Damage increases with frequency of flooding; and

• Risk to life (mortality) is assessed separately18.

18 Risk to Life was not assessed as part of the FDES assessment tool. However the risk maps accompanying this report provide a good indication of the risk to Human Health and Critical Infrastructure



Based on the data available at the time of this study, small local business property types were not differentiated from the wider non-residential properties. Therefore intangible damages were not calculated for small businesses. Emergency Service costs are calculated as 5.7% of the capped direct property damage. Figure 5.4 shows the relationship between the intangible damages (AAD) and event frequency / Annual Exceedance Probability (AEP) of flooding.

Figure 5.4 - Graph Showing Relationship Between Intangible Damag es and the Annual Exceedance Probability of Flooding. FRM Option Implementation Costs To calculate the benefit cost ratio of each FRM option (A, B, C and Preferred) the estimated costs associated with the implementation of the FRM need to be compared to the benefits (i.e. damages avoided with the FRM option in place). The estimation of costs associated with each FRM was generated based on the following approach:

• Using the conceptual designs developed for each FRM option (A, B and C), a FRM Unit Cost spreadsheet was generated to calculate the costs associated with each of the options elements (individual measures).

• The construction costs of the individual measures were calculated by multiplying the unit costs introduced Volume Three: Pluvial Flood Risk Management; Code of Practice 2 (per m, property, m² etc) with the dimensions identified for the measure (length, area, number of units etc).

• The construction cost of each FRM option was then calculated by combining the construction costs of the individual measures in the FRM option.

• A number of uplifts associated with construction maintenance and implementation were then applied to the construction cost calculated for each option:

o Design & Survey (24%);

o Planning & Environmental (9%);



o Land & Legal (2%); and

o Project Management & Site Supervision (7%).

• The resulting cost for each FRM option was then uplifted further using the following elements to estimate the total cost of implementing each option.

o Optimism Bias (30%); and

o PV Future Maintenance. Cost details relevant to each Pilot Area are outlined in Appendices A – E. Benefit / Cost Ratio On collation of the information referred to above, the following steps are followed to calculate the overall costs/damages associated with both the Baseline Scenarios: Do Nothing and Do Minimum (Existing), and the With Scheme (Corrective) Scenario: Option A, B and C (as outlined in Section 2.4) for each Pilot Area.

• Prepare property data: classification as residential or commercial, and property valuation;

• Determine maximum flood depths at each property for each flood event (10% AEP, 2% AEP, 1% AEP and 0.5% AEP) using the Baseline and With Scheme depth mapping outputs;

• Determine direct and indirect property damages for each flooded property;

• Determine intangible and emergency service costs for each flooded property;

• Sum all damage components to derive overall economic cost of flooding. The benefit / cost ratio can then be calculated to provide an indicator of the economic viability of an option. As noted above, for a flood risk management option to be economically viable, its total benefit in terms of damage avoided must be greater than the cost of the measure i.e. the benefit / cost ratio must be greater than 1. Section 3.3 in each Pilot Area Specific Appendix summarises the economic benefits estimated for each of the flood risk management options, along with the estimated benefit / cost ratio. The results presented in the Appendices assume all properties have a threshold level of 0.15m above footpath level. This has been taken as broadly representative of thresholds and may not always be the case. Where thresholds are lower (as observed at some locations) damages may be higher. Estimated costs for the options assessed for the Pilot Areas (A, B, C and Preferred) are based on unit cost rates available at the time of this study which were applied on other recent similar projects.



Key learning outcomes from Section 5, with regard to Option Technical and Economic Appraisal , are as follows:

• Baseline Scenario model outputs and flood mechanisms are reviewed to identify combinations of the selected measures to form three possible flood risk management options for each Pilot Area.

• GIS is used to develop conceptual designs for each Option. Using these designs, the cost of constructing/implementing each option is then be estimated.

• The viability and effectiveness of the options is then assessed using the Type 2 models (technical assessment) and economic benefit calculation tools.

• As part of this options appraisal process, the model outputs for the 10%, 2%, 1% and the 0.5% AEP ‘With Scheme’ model runs were all considered.

• Following this assessment and on identification of the ‘preferred option’ for each Pilot Area, pluvial flood maps are then produced to illustrate the effects (and benefits) of the implementation of the identified flood risk management options.

• The conceptual design developed in GIS for the ‘preferred option’ is included in the Pilot Area Appendices (A-E).

5.4 With Scheme Pluvial Flood Maps (Steps 7, 8)

On completion of the technical and economic appraisal, the preferred option for each of the Pilot Areas was identified. The preferred option for each Pilot Area is then used to develop the ‘With-Scheme’ pluvial flood depth, hazard rating and risk maps to illustrate the effects of the implementation of the identified flood risk management options.

Figure 5.5 – Preferred Option Used to Produce ‘With Scheme’ Risk Maps As with the Baseline ‘Existing’ Scenario Pluvial Flood Risk Maps (refer to Section 3.3), receptor groups were used in conjunction with the Type 2 ‘With Scheme’ depth mapping as the basis for the ‘With Scheme’ risk assessment. Appendices A – E detail the results of this appraisal process specific to each Pilot Area and provide a series of maps to illustrate the effectiveness of the preferred options in reducing flood risk to identified ‘hot spot’ areas.



SECTION 6 SUMMARY OF EXISTING PROBLEM AREAS AND THE PREFERRED FRM OPTIONS

The following sections provide a summary of the key flood mechanisms and characteristics of pluvial flooding and specific ‘hot spot’ or ‘problem areas’ highlighted by the Type 2 (detailed) modelling for each of the Pilot Areas. The combination of corrective measures identified to mitigate the identified flood risk associated with these flood mechanisms (in addition to other wider non-structural measures and ‘good practice’ measures – see Section 4.2) is also summarised. Further details for each Pilot Area are included in Appendices A, B, C, D and E of this Volume.

6.1 Dublin South East Pilot Area – Georges Quay & S outh Inner City

The outputs of the Type 2 modelling as detailed in Appendix A have highlighted specific problem areas within the Dublin South East Pilot Area, and provide an indication of local flood hazard and risk areas, which may be summarised as follows:

• Basement property flooding: Although basements were not specifically represented in the model, pluvial flooding across the Pilot Area drains to the topographic low spots (e.g. Pearse Square, Macken Street) with these properties being at a significantly higher risk.

• In addition, within the Dublin South East Pilot Area, a large number of areas/properties (without basements) are located in topographic ‘low spots’ compared to the surrounding ground level making these areas particularly susceptible to pluvial accumulation.

• The Dart line which runs through the north of the Dublin South East Pilot Area acts as an obstruction to overland flow preventing accumulated flood water to drain south (Clarence Street, Macken Street).

• The existing road network plays an important role at conveying surface flow, specifically at low lying junctions which form the confluence of flow paths which feed surrounding low lying areas/properties (e.g. James Street and James Place).

• There is a north/south divide in the response from the drainage system. To the south of a line approximately parallel to the Dart Line and running from Trinity College to Grand Canal Street Lower, the system mostly runs at less than capacity. To the north of the aforementioned line, the drainage system operates at full capacity and manhole surcharging occurs

• High tide plays a key role in influencing water levels in the River Liffey which impounds the outfalls on Georges Quay and City Quay resulting in backing up in the system and affects all the pipes located to the north of the divide line as the land above them is relatively flat (compared to the higher ground levels to the south).

In general, pluvial flooding in the Dublin South East Pilot Area, north of the Dart line is influenced by the capacity of the drainage system in addition to surface runoff from upstream areas which reaches the low ponding areas and is obstructed by the Dart line.



The preferred option identified (as illustrated in the Schematic Plan in Figure A3.4A of Appendix A) comprises the following set of mitigation measures:

• External Resistance Measures;

• SuDS storage (including underground storage);

• Surface Conveyance (Streets as Streams Roads as Rivers);

• Below Ground Conveyance;

• Basement Specific Measures;

• Access Protection; and

• Rain Gardens;

The preferred option includes storage areas (SuDs storage and underground storage) in the vicinity of Stephen’s Green East, Iveagh Garden, York Street, Stephen’s Place, Trinity College, Fenian Street, Lower Grand Canal Street, Pearse Square, Ely Place Upper and Pearse Grove. Also, the existing green space adjacent to Stable Lane was included as a storage area. External Resistance Measures include flood walls, raised kerbing and access protection. Basement specific measures are proposed for properties across the Dublin South East Pilot Area (including Pearse Square, Macken Street, Stephen’s Place, Hatch Place, Cuffe Lane, Protestant Row, Leason Lane, Molesworth Street, Stephens Green Lower, Merrion Square West and Trinity College).

6.2 Dublin North Central Pilot Area – Marino and Fa irview

The outputs of the Type 2 modelling, as detailed in Appendix B highlight that pluvial flooding in Dublin North Central is influenced by the capacity of the drainage system in addition to surface runoff from upstream areas which reaches the low topographic spots. The road network plays a key role in conveying surface flow which ponds in low–lying areas. Three key flood mechanisms identified as contributing to flooding in the Dublin North Central Pilot Area can be summarised as follows:

• Pluvial accumulation in low lying areas;

• Pluvial runoff conveyed by road network; and

• Drainage system surcharging. The preferred option identified (as illustrated in the Schematic Plan in Figure B3.4A of Appendix B) comprises the following set of mitigation measures:

• Access Protection;

• External Resistance Measures; and

• Surface Conveyance (Streets as Streams Roads as Rivers).

External Resistance Measures include flood walls and raised kerbing. Access Protection (raised ramping) is proposed at Malahide Road, Addison Road, Gadogan Road and Fairview Strand, which are also proposed as surface conveyance routes.



6.3 Dublin South Central Pilot Area – Carrickfoyle Terrace

The outputs of the Type 2 modelling, as detailed in Appendix C, have highlighted specific problem areas within the Dublin South Central Pilot Area, and provide an indication of local flood hazard and risk areas, which may be summarised as follows:

• Overland flow across the area flowing in a generally north to south direction leading to flooding at low topographic spots in Lady’s Lane, Shannon Terrace and Kearn’s Place.

• Flood depth increases rapidly at these areas as overland flow cannot flow further south as it is impounded by a wall running along the right bank of the Cammock River.

• Manhole surcharging occurs corresponding to the deeply flooded areas of Carrickfoyle Terrace and Turvey Avenue. This leads to additional surcharged manholes further upstream.

In general, pluvial flooding in the Dublin South Central Pilot Area is influenced by the capacity of the drainage system in addition to surface runoff from upstream areas which reaches the low topographic spots within Lady’s Lane, Kearns Place and Shannon Terrace. The preferred option identified (as illustrated in the Schematic Plan in Figure C3.4A of Appendix C) comprises the following set of mitigation measures:



• External Resistance Measures; and

• Access Protection. This option includes three outfall pipes at Lady’s Lane, Shannon Terrace and Kerns Place to allow ponding pluvial flow to be discharged into the Cammock River. In addition, individual property resistance measures (raised kerbing and access protection) are proposed along the Kilmaiham Road (until Mount Brown), Lady’s Lane and Kearns Place.

6.4 Dublin North West Pilot Area – Kippure Park

The outputs of the Type 2 modelling, as detailed in Appendix D, have highlighted specific problem areas within the Dublin North West Pilot Area, and provide an indication of local flood hazard and risk areas, which may be summarised as follows:

• Overland flow (in a general north - south direction) leads to flooding at low ponding locations in Kippure Park and continuing to the Tolka valley.

• Flood depth increases rapidly at Kippure Park as overland flow cannot flow further south as it is impounded by an embankment separating the residential properties from the Tolka Valley Road.



• Kippure Park is also flooded due to incoming surface water runoff from the north routed along Cardiffsbridge Road, and floodwater overtopping the embankment at Kippure Park.

• Manhole surcharging occurs corresponding to the deeply flooded areas of Kippure Park and Tolka Valley Road, as well as along Cardiffsbridge Road and St Helena’s Road. Backing up leads to additional surcharged manholes further upstream.

In general, pluvial flooding in Dublin North West Pilot Area is influenced by the capacity of the drainage system in addition to surface runoff from upstream areas which reaches the low ponding areas within Kippure Park. The preferred option identified (as illustrated in the Schematic Plan in Figure D3.4A of Appendix D) comprises the following set of mitigation measures:



• Below Ground Conveyance; and

• Access Protection.

This option includes flood walls and raised kerbing within the Kippure Park area and Tolka Valley Park area, in addition to the redirection of flow by Surface Conveyance and Below Ground Conveyance measures at Kippure Park.

6.5 Dublin Central Pilot Area – East Wall

The outputs of the Type 2 modelling, as detailed in Appendix E, have highlighted specific problem areas within the Dublin Central Pilot Area, and provide an indication of local flood hazard and risk areas, which may be summarised as follows:

• Seaview Avenue East, West Road, Barnabas Gardens, Hawthorn Terrace, Fairfield Avenue, Church Road, St Mary’s Road North, East Road and Irvine Terrace all show significant flood hazard levels.

• Due to the local topography of the East Wall Pilot Area pluvial accumulation plays a key roll in contributing to flooding across the Pilot Area.

• The existing road network plays a major role in conveying additional rainfall volume from north to south with surface water accumulating in the southern part of the catchment (with the railway embankment acting as an obstruction allowing flood depths to build up).

• The drainage system throughout East Wall is at full capacity two hours into the event modelled 2% AEP (1 in 50 annual chance) despite the operation of pumps. As a result, manhole surcharging occurs throughout the catchment.



The preferred option identified (as illustrated in the Schematic Plan in Figure E3.4A of Appendix E) comprises the following set of mitigation measures. The main elements as indicated in the Schematic Plan in Figure E3.4A of Appendix are:


• SuDS storage;

• Aggregated Micro Storage;



• Access Protection;

• Pumping Station; and

• Gulley Enhancement. This Option includes flood walls and raised kerbing for properties at Seaview Avenue, Fairfield Road, Church Road, East Road, Merchant’s Road and Killan Road. Also, storage areas are proposed in green spaces at the Russell Avenue East Roundabout, Caledon Road and Merchant Square. A new pumping station is proposed south of the East Road Bridge with a link to Church Road and Irvine Terrace to reduce the accumulative levels in the south of the catchment.

6.6 Conclusions and Recommendations

Following the application of the FRM process it is important to note that the ‘preferred option’ does not provide a complete solution to the flooding issue in each of the Pilot Areas (i.e. residual risk of flooding is still a factor). One of the key benefits of the appraisal of these measures is that it has allowed for the effectiveness of the measures applied to each of the Pilot Areas to be assessed.

It is recommended that wider generic and good practice measures including Awareness Raising and Education, Flood Warning and Enhancement of Flood Emergency Management, Planning and Development Control, Development Policy and Urban Land-use Management, Other Generic SuDS, Rainwater Harvesting, Vegetation management, and Enhanced Maintenance are also considered as an integral part of any scheme to reduce and manage pluvial flood risk. These measures provide cost effective ways of In addition where External Resistance Measures are specified Internal Resilience Measures should also be considered – these should also be considered for basement properties. Basement Specific Measures are also recommended. It is recognised that Dublin City Council is likely to proceed with the trial application of a number of corrective mitigation measures in the Pilot Areas. This will require further detailed consideration of measures selected for trialling at specific locations. Feedback from these trials should then be taken into account in the development of any Flood Risk Management Scheme.



Key learning outcomes from Section 6, with regard to Summary of Existing Problem Areas and the Preferred FRM Options , are as follows:

• A summary of the following information is provided:

o The key flood mechanisms and characteristics of pluvial flooding, and specific ‘hot spot’ or ‘problem areas’ highlighted by the Type 2 (detailed) modelling;

o The ‘preferred option’ identified; and

o Details of the combination of corrective measures and where these are recommended within the Pilot Area.

• Further details for each Pilot Area are included in Appendices A, B, C, D and E of this Volume.

• In addition to the identified options, wider generic and good practice measures are recommended. Also, consideration of internal resilience measures (which are primarily the responsibility of the community) is highlighted as important in areas where external resistance measures are recommended. This can include basement-specific measures.

VOLUME 4 Detailed Pluvial Flood Risk Assessment of Pilot Areas

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