EU IVB FloodResilienCity Project Final Report - Dublin

Transcription

1 Final Report - Dublin Non-Technical Summary September 2012 Tom Leahy, FRC Project Director, Tony Maguire, FRC Project Manager.

2 Dublin City Council is one of eleven partner organisations, drawn from eight European cities, which form the Interreg IVB flood risk management good practice project known as FloodResilienCity (FRC). The overriding aim of the Dublin work package programme in this project is to assist in the development of a sustainable pluvial flood risk management strategy for Dublin. Jacobs Engineering Ireland Ltd was appointed to provide support with technical aspects of the Dublin FRC project. This Non Technical Summary provides the background to the Dublin FRC Project and a summary of the Dublin FRC Project findings which are documented within the five technical report Volumes referenced below: Final Report Non Technical Summary Technical Report: Volume One Volume Two Volume Three Volume Four Volume Five Rainfall and Forecasting City-Wide Pluvial Flood Risk Assessment Pluvial Flood Risk Management Detailed Pluvial Flood Risk Assessment of Pilot Areas Pluvial Flood Alerting and Warning System Integration A Glossary and Abbreviations register is enclosed with the above Volumes. 1. INTRODUCTION 1.1 Context within the Dublin Flood Initiative Dublin is at risk from a number of flooding hazards and since 2002, under the Dublin Flood Initiative, Dublin City Council (DCC) has initiated a number of projects to address these hazards, namely: Coastal flooding of areas adjacent to the coast or tidal estuaries. Fluvial flooding due to river bank overtopping and/or flood defence collapse. Drainage flooding due to failure or inadequacies or flow exceedance (surcharging) of the drainage system. Dam Break flooding associated with dam failure, either actual failure or high discharge released when in danger of over topping. Rigorous inspection procedures are designed to minimise this risk. And most recently Pluvial flooding resulting from surface water runoff and ponding in low spots following intense rainfall. The collective strategy for managing all of these flood risks and combinations thereof is known as the Dublin Flood Initiative (DFI). The DFI addresses those flood risks and develops a unified and fully integrated flood risk management strategy. It also aims to raise the level of awareness of all aspects of flood risk management among policy makers, professionals and the general public. The DFI also recognises specific aspects of flood risk associated with basement properties and underground car parks. The first four flood hazards have already been addressed by previous DCC strategic studies. The remaining hazard, pluvial flooding, is being addressed through this, the Dublin FRC project. 1.2 FRC Project Background In the early 2000 s DCC recognised that to successfully address the many types of flood risk across Dublin, transnational knowledge and shared experience at European level were required. Dublin has benefited from being a participant in a number of EU Interreg transnational projects. This involvement started with the EU Interreg Programme IIIB SAFER project, which addressed coastal flood risk. Dublin is currently participating in the EU Interreg IVB Final Report Dublin 1

3 Programme 2007/13 as one of eleven partner organisations forming part of the EU Interreg IVB flood risk management good practice project; FloodResilienCity (FRC). The programme aims to promote an innovative approach to flood risk management and prevention based on the concept of the 4A s: Awareness, Avoidance, Alleviation and Assistance. DCC are further developing this concept for sustainable flood risk management through their Beyond the 4As initiative which incorporates Awareness raising among politicians and policy makers, professionals, and the public of all stages of the flood risk management process: Assessment of existing and emerging flood 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. 1.3 FRC Dublin Project Background Pluvial flooding (also referred to as surface water flooding aka Monster Rain ) occurs when heavy rainfall results in water flowing across the ground surface, and the capacity of local drainage (both natural and man-made) is overwhelmed - surface ponding occurs sometimes to a significant depth and high velocities where slopes are steep can also be a hazard. The route the water takes and the depth of flooding will depend on local features and can be difficult to predict. Several significant pluvial flood events have occurred in recent years, notably in 2002, 2008, 2009, 2010 and most recently on 24th October This type of flooding appears to be occurring with increasing frequency and severity and it is possible that this may, in part, be associated with climate change, and it is an objective of the study to examine the rainfall component separately from other factors such as increased development and impermeable areas. The overriding aim of the Dublin work packages in the FRC Project programme is to assist in the development of a Pluvial Flood Risk Management strategy for Dublin the Dublin FRC Project. This will not only consider the increasing flood risk from pluvial flooding to the city but will play a key role in contributing to the wider Interreg IVB FRC programme through the development of innovative approaches and techniques in pluvial flood risk management. 1.4 Links with European Practice In addition to its role in the FRC project Dublin has forged close links with a number of European flood risk management research and good practice programmes, including initiatives by the European Water Association (EWA) in relation to better understanding pluvial flood risk management. This in turn has linked with the work of the European Commission Working Group F addressing implementation issues associated with the Floods Directive. A key outcome from the Working Group F programme was a full report concluding that pluvial flooding is recognised as a growing flooding threat across all of Europe, but particularly in Western and Northern Europe. Final Report Dublin 2

4 1.5 Scope of the Dublin FRC Project The extent of the Dublin FRC Project study area is shown in Figure 1 which also indicates the five administrative areas within Dublin City. The technical findings of this project are detailed in five report Volumes as listed above. Figure 2 (on the following page) provides a brief summary of the content of each Volume and illustrates the relationships between the Volumes using both one-way and two-way arrows, with arrow size indicating relationship scale. Figure 1: Dublin FRC Project Study Area DCC set the following broad requirements for the project: Improve pluvial flood risk understanding through assessment of the characteristics of previous pluvial flood events. Review and scoping of elements required for an information management system for pluvial flood forecasting and warning. Review existing rainfall monitoring equipment and assess the scope for introducing new technologies in hardware and information handling and processing. City-wide overview modelling of pluvial flooding and preparation of hazard and risk mapping (Type 1 maps). Detailed modelling of pluvial hazards and risk mapping for several pilot areas and use of these models for the assessment of mitigation options (Type 2 maps). Development of two Codes of Practice, the first, on Spatial Planning and Building Regulations and, the other on Flood Resilience and Adaptation Measures to support pluvial flood risk management. Assessment of relevant corrective actions and mitigation measures for the areas modelled in detail such that pilot trialling of these measures could be undertaken by DCC and performance monitored. Final Report Dublin 3

5 Non-Technical Summary Figure 2: Dublin FRC Final Report Volumes - Summary of the content of each Volume and the relationships between each (arrow size indicates relationship scale) Final Report Dublin 4

6 2. RAINFALL AND FORECASTING Volume One 2.1 Understanding Rainfall and Flooding In order to develop and outline requirements for a pluvial flood forecasting and warning system it was necessary to first understand the nature of rainfall which may result in pluvial flooding across Dublin. This involved analysis of high intensity rainfall events associated with pluvial flooding in Dublin in recent years and the weather patterns which produced these extreme rainfalls. The approach taken to analyse these events considered both the frequency / rarity of an event and the likely impacts, (focused on density of property and infrastructure). The predicted damage impact of a storm is of primary importance and in this respect it is helpful to be able to categorise events such that, when a particular extreme rainfall event is forecast, the likely impact across Dublin can be assessed and used to inform the flood emergency response. A storm categorisation system was therefore developed using rainfall characteristics and predicted impacts and outcomes from city-wide pluvial flood risk mapping (Volume Two) across Dublin. 2.2 Characteristics of Rainfall Over Dublin Since 1954 there have been at least 12 extreme rainfall events sufficient to cause flooding in Dublin, with the highest hourly intensity of around 75mm/hr being recorded in June Five of these events have occurred between 2002 and Analysis of these recent pluvial events, including November 2002, August 2008, July 2009, July 2010 and most recently on 24 October 2011, indicates two types of weather pattern which have given rise to significant and potentially major pluvial flooding across Dublin: Short Duration Summer Convective Events - typified by the August 2008 and July 2009 events. Events of this nature are more likely to occur during the summer with typical 24hr total rainfall of around 50-70mm and peak intensity of 10-35mm/hr usually under 4hrs duration. Flood risk is primarily pluvial but can also involve flooding from sewer systems. These are characterised by rapid onset and hence are more difficult to forecast than the long duration frontal events. Whilst the likelihood of intense short duration convective rainfall may be forecast for a general area, the exact timing, duration and location cannot be forecast accurately. Longer Duration Frontal Events typified by the November 2002 and October 2011 events. Such events can occur at any time of the year with typical total 24hr rainfall of 70-90mm and peak intensity 10-30mm/hr over 6-12hrs duration or longer. These are often characterised by combined pluvial, fluvial and sewer flooding. Such events may be more amenable to forecasting. The recent events reviewed can all be regarded as extreme with estimated probabilities of the order of 1% Annual Exceedance Probability (AEP) (1 in 100 annual chance of occurrence) although the durations of the events are different. 2.3 Pluvial Storm Categorisation Rapid identification of likely impact areas for any forecast event can be informed by use of a series of risk maps prepared from the city-wide Type 1 modelling of pluvial flooding (Volume Two). As noted, the categorisation of rainfall events is based on the differing impacts of the various events rather than the quantum of rain falling in a particular time period as it is acknowledged that consideration of rainfall rarity alone, though relevant, may not always be a good indicator of the likely impact of an event. The proposed categorisation system is summarised in Table 1. Final Report Dublin 5

7 Table 1: Proposed Pluvial Flood Forecasting Categorisation System This system and grouping of maps can help benefit flood emergency response by developing the event categorisation maps to convey the potential severity of an event and the required response relevant to a forecast through the use of Pluvial Flood Categories. The impacts of a range of rainfall events were considered and then grouped into four families of storms (Storm Family A, B, C and D) according to the similarity of their impact. Each storm family can be assigned to a single set of event categorisation maps which indicate the predicted impact of flooding. If the predicted rainfall amount and storm duration are known at a sufficient level of detail (from a rainfall forecast), a look-up table can then be used to identify the event categorisation maps relevant to the forecast at that time. The format of the look-up table is presented in Table 2 and Figure 3 illustrates an example extract of a pluvial categorisation map Table 2: Impacts Look-up Table of Storm Family Figure3: Pluvial Categorisation Map (example extract) The relevant set of maps can be accessed at any stage during the flood forecasting process. As the forecasting accuracy improves and lead time reduces so too can the definition of impact areas and severity through progressive access to the map sets. This is considered to be an innovative approach to pluvial flood warning as there is no known practice to draw on. 2.4 Pluvial Flood Forecasting and Warning The specification for a Dublin flood forecasting system involved a review of the effectiveness of recent forecasts, key elements required, and integration with existing flood forecasting and warning systems for other types of flooding. This enabled a functional specification of system requirements to be prepared. A three tier system of alerting and warning is proposed from be aware to be alert and then take action as set out below (see also Table 1). Final Report Dublin 6

8 BE AWARE Early warning forecast of heavy rainfall (24-48hr lead time). Main purpose is to ensure readiness. BE ALERT Heavy rainfall warning (12 to 24 hour lead-time). Warning information aimed at initiating preparedness actions. TAKE ACTION Event flash warning (2 to 3 hours lead-time). This stage seeks to confirm or modify the earlier forecast, and to add necessary detail on location and intensity. NB: These levels of forecasting are not currently available but a number of initiatives are being pursued with the forecasting agencies and IBM (see later) to bridge the forecasting gap between synoptic/regional scale storm forecasting and dedicated city scale rainfall forecasting tailored to the flood hazard maps. Analysis of recent events has confirmed the suitability of using Table 2 as a system for preliminary trigger levels for each tier of the warning system (Table 3). These flood warnings would be issued based on the impacts and risks for each Storm Family as described above. Table 3: Preliminary trigger levels (mm) The nature of pluvial flood risk is that it can be highly variable over relatively short distances. The map extract from Figure 3 illustrates this, showing Category 4 square (Extreme Pluvial Flood Warning) adjacent to, for example, a Category 1 square (Pluvial Flood Watch) (each coloured square is 100m x 100m). Provision of flood warnings at this resolution is not appropriate, and therefore, the spatial extent/distribution of flood warnings (and flood emergency resources) should be focused on wider geographic Risk Sectors. These will aggregate the risk squares and thus identify the dominant risk level within the wider Risk Sectors depending on various operational factors. The Risk Sectors will be taken, in the first instance, to mirror the model boundaries used for the city-wide models (see section 3.3) while operational experience is built up. The development of an effective pluvial flood warning system will require significant infrastructure, systems, and staff development. In support of this, a functional specification for a pluvial flood warning system is proposed with features identified that are either: Mandatory, Preferred, Advantageous or Recommended. The proposed system will require direct access to Met Éireann forecasts and radar data, and automatic receipt and processing of data from DCC and third party raingauges in near real time. A GIS based user interface will link with other forecasting and warning systems; allow advanced data sharing with permitted users; include access to the categorisation map sets; and be accessible from multiple locations. In addition to tailored warning messages and highlighting of warning locations on the GIS, it is anticipated that the system can be linked with road traffic warning signage. Implementation aspects are discussed in Volume Five: Pluvial Flood Alerting and Warning System Integration. Likely Future Climatic and Other Trends Climate change impacts are considered by examining previous relevant studies as well as rainfall records for Dublin back to around Whilst there are some trends that are observable in various rainfall patterns (both positive and negative) there is insufficient evidence from the data to reliably extrapolate any trends into the future and it is proposed that further monitoring based on a network of high quality reference rainfall stations and sub-daily analysis is undertaken on an ongoing basis. However, recent studies in the UK indicate the following key patterns are anticipated: Heavy rainfall events Increase in rainfall depth, particularly in winter; Final Report Dublin 7

9 potential increase in combined sewer overflows; Increased runoff Increased runoff on slopes; changes to sediment and nutrient load. These general patterns may also be anticipated in Ireland. Development planning also has a significant influence on future pluvial flood risk in terms of increased permeable area and sewer infrastructure resulting in more rapid runoff. On the basis of current Development Plans, the annual growth in housing from 2012 to 2022 is estimated to be approximately 2% to 3.5%. How this growth is implemented will influence the extent to which pluvial flood risk is affected. 3. PLUVIAL FLOOD RISK ASSESSMENT AND MAPPING Volume Two: City-wide Pluvial Flood Risk Assessment; and Volume Four: Detailed Pluvial Flood Risk Assessment of Pilot Areas Pluvial flood modelling, risk assessment and mapping were undertaken at two spatial scales City-wide and Pilot Areas (1 3 km 2 ). The City-wide (Type 1) Modelling identified Dublin city s overall vulnerability to pluvial flooding using a two dimensional (2D) hydraulic model to produce flood depth, hazard and risk maps appropriate to the scale of the DCC administrative area. The approaches include both GIS-based techniques for identifying flow paths and ponding areas ( Dry mapping technique), and a high level hydraulic modelling approach ( Wet mapping technique). This methodology for high-level, city-wide assessment of flood risk has been successfully applied across the UK, including areas of London, Kent, Lancashire, Sussex, Devon and Berkshire. Based on outcomes of the city-wide assessment five Pilot Areas were identified to be taken forward for detailed (Type 2) modelling. 3.1 City-wide Preliminary Mapping of Flowpaths and Ponding The preliminary dry mapping allowed definition of surface water catchments and also informed early site inspection of likely high hazard areas, which, together with a review of records of past pluvial inundation, enabled a better understanding to be gained of pluvial flood risk across Dublin. 3.2 City-wide Preliminary Risk Assessment Key pluvial flood risk issues identified during site inspections include: A large proportion of residential and commercial areas have low access thresholds, with many at and below ground level. Many basement properties (estimated to be 18,000) which can be acutely vulnerable in flooding and represent a heightened risk to life and limb. The flat topography over extensive areas means that shallow pluvial flooding could occur relatively quickly and affect many properties. Multiple and inter-related flood hazards exist at many locations - understanding of these flood mechanisms and interrelationships is important in developing effective mitigation measures appropriate to the specific location. Railway lines are constructed on embankments in many areas. These generally act as barriers to surface flow movement but in many places important flowpath connections through underpasses and bridges were identified. Sections of rail or road infrastructure in cuttings or at dips at bridge crossings can also be particularly vulnerable to pluvial flooding and this potential risk was observed at various locations. Some potentially deep ponding areas were observed - also some steep areas where high surface water velocities could be a hazard. Final Report Dublin 8

10 Overall, this preliminary assessment of a high level of risk at many of the sites that were inspected confirmed the need for the further more detailed risk assessment. An extract from the city-wide pluvial flood depth mapping which resulted from the modelling of a 1% AEP 3hr duration event is shown in Figure City-Wide (Type 1) Pluvial Modelling and Mapping For the city-wide hydraulic modelling, a 2D pluvial model was constructed based on a 25m cell size and using TUFLOW surface flow modelling software. The 2D elements of this type of model represent the above-ground flow of water. A total of six separate models (Figure 4) were constructed based on surface flow catchment boundaries. Figure 5: Extract from City-Wide Depth (m) Mapping (1% AEP 3hr event) 3.4 City-wide Risk Analyses and Mapping Figure 4: Model Boundaries for City-wide Modelling This modelling simulated the application of rainfall to a high-level overground hydraulic model which took below-ground drainage capacity into account using an approach specifically developed for this study based on sewer capacity equivalent rainfall which used model results from earlier drainage studies and testing of equivalence figures in a trial catchment. A number of model runs were completed for various storm events, and model verification was undertaken using the August 2008 and July 2009 flood events. Insufficient fully verified data was available at the time to use the October 2011 event for verification nevertheless a good correlation between incident locations and modelled areas indicating higher risk was apparent. Using the city-wide depth mapping outputs sets of risk maps were prepared to indicate the potential impact of pluvial flood events in terms of: Risk to Human Health and Critical Infrastructure Risk to the Environment and Cultural Heritage Risk to the Economy. Risk to human health was expressed in terms of the number of properties that might be affected together with critical infrastructure such as schools, hospitals, care homes, rail stations, police and fire stations, key roads and utilities facilities including electricity substations. Basement properties - considered to be particularly vulnerable were also included. Risk to the economy was based on damage costs, both tangible and intangible and used a Flood Depth Estimation System (FDES), a GIS based tool which determines an economic valuation of flood damages for each individual property within a study area. Risk to the environment and cultural heritage was based on a number of receptors including Final Report Dublin 9

11 for example Natura 2000 sites and, Sites of Monuments. The number of receptors and the extent of damages within each model grid cell was assessed for each of the three types of risk described above, and a risk rating assigned. Adjacent model cells of similar risk were then merged to form Areas of Risk expressed as High, Medium, Low and Marginal risk. Where available, basement records were used to trigger elevation of the rating to the next highest level. An example from the city-wide depth risk mapping for a 1% AEP 3hr duration event is shown in Figure 6. underlying drainage system as indicated in Figure 7. Rain falling over the study area Resulting in: Overland Flow (Exceedance) Piped Flow (Conveyance) Figure 7: Schematic of Detailed Modelling Process Figure 6: City-Wide Risk Mapping Economic Damage 3.5 Detailed (Type 2) Pluvial Flood Modelling of Pilot Areas Following appraisal of the City wide model results and taking account of the level of risk and other aspects such as land ownership and the practicality of trialling various mitigation measures, five Pilot Areas within DCC s administrative boundaries were identified for further detailed investigation of potential pluvial flood risk. This more detailed level of assessment quantifies the estimated pluvial flood depth, hazard and risk in the Pilot Areas, and enables location-specific measures to mitigate pluvial flood risk to be identified and assessed. Advancing from the City-wide models, the detailed models for the Pliot Areas have used a finer 5m grid 2D TUFLOW integrated model of surface flows above-ground fully linked with a 1- dimensional (1D) ESTRY model of the Surface flow inputs at the model boundaries were derived from the city-wide model. Drainage system inputs were derived from the Greater Dublin Strategic Drainage Study modelling. Model verification was carried out against historic flooding records for the August 2008 and July 2009 events and generally good agreement was achieved. To represent baseline conditions, the same range of storm events was run as for the citywide (Type 1) model. Outputs from the detailed modelling included flood depth, velocity and hazard associated with surface flows; and flow, velocity and water levels in the below-ground drainage system. Locations where there is exchange of flow between the surface and below-ground systems are indicated on the maps. Using the same methodology applied to the City-wide (Type 1) risk assessment (as described in Section 3.4), sets of risk maps were prepared to indicate the potential impact of pluvial flood events at the Pilot Areas. The detailed (Type 2) models were then used to test the effectiveness of different measures as identified in the Code of Practice for Flood Resilience and Adaptation Measures (Volume Three, Appendix V3-B). 3.6 Appraisal of Flood Resilience and Adaptation Measures in Pilot Areas Final Report Dublin 10

12 Following initial screening, selected measures appropriate to each Pilot Area were assessed against the baseline condition using the Type 2 models to determine the level of benefit and thereby assist in determining a preferred set of measures for each area. The initial screening process used a multicriteria matrix approach. After discounting any measure which would clearly not be practical to implement, the remaining measures were then scored against Technical, Environmental, Social, Sustainability and Economic criteria bearing in mind the particular characteristics of each Pilot Area. Individual measures which passed the initial screening process were then brought together to form flood risk management options (groups of mitigation measures across the Pliot Areas). The detailed models for each Pilot Area were then used to assess the benefits and effectiveness of each option. Outputs from this appraisal for each Pilot Area include schematic plans for the preferred options and with scheme pluvial flood depth, hazard and risk maps which can be compared against those for the baseline conditions (without scheme) to illustrate predicted benefits associated with the preferred option. 4. PLUVIAL FLOOD RISK MANAGEMENT Volume Three: Pluvial Flood Risk Management; Volume Five: Pluvial Flood Alerting and Warning System Integration 4.1 Introduction Development of an overall pluvial flood risk strategy for Dublin is largely based on the hazard assessment and risk analyses; the mapping (presented in Volumes Two and Four); the development of the pluvial flood warning system (Volume Five) and the guidance provided in the two Codes of Practice (CoP) (as provided by Volume Three): - CoP1: Spatial Planning and Building Regulations outlines how the identification of pluvial flood hazard areas, in addition to pluvial flood risk assessment and management can be integrated into the application of sustainable planning and building control activities in Dublin City for both new and modified/extended developments. - CoP2: Flood Resilience and Adaptation Measures outlines measures suitable for managing surface and overland flows and creating resilience in existing buildings. The measures can also be considered for new development in conjunction with CoP1. CoP2 outlines how responsibility for these measures is shared between Government agencies, developers and the community. Implementation of a pluvial flood warning system is also fundamental to raising the overall resilience of Dublin to pluvial flooding. This is considered further together with developments in information management which will support pluvial flood risk management. 4.2 CoP1: Spatial Planning and Building Regulations The policies developed under CoP1 in relation to pluvial flooding and the spatial planning process are designed to be complementary to national, regional and local planning and building control policy and guidance. Effective planning is viewed as a potentially cost-effective and sustainable means of mitigating the risk from pluvial flooding and has the potential to be an early win measure. In particular, CoP1 is designed to be consistent with and complement: - National Planning System and Flood Risk Management Guidelines (DEHLG/OPW, 2009) which are based on avoidance of development in areas at risk of flooding and a sequential approach when assessing locations for new development based on avoidance, reduction, and mitigation of risk. - Greater Dublin Area Regional Planning Guidelines for the period in which several policies and recommendations are relevant to pluvial flooding and promote the use of green infrastructure. - Greater Dublin Strategic Drainage Study - Regional Drainage Policies which include Final Report Dublin 11

13 policies on SuDS, climate change and risk of basement flooding; and the Greater Dublin Regional Code of Practice for Drainage Works based on these policies. - Dublin City Council Development Plan which builds on the national and regional planning guidelines and promotes the need for flood resilient urban and building design and ability to adapt to climate change. - Building Control Regulations which are in place to assist the enforcement and compliance with national Building Regulations, aspects of which are of potential relevance to pluvial flood risk management. The policy elements covered under CoP1 include policy recommendations to facilitate the application of the principles of the sequential approach and justification test as set out in DEHLG/OPW Guidelines (2009) to assess pluvial flood hazard and risk and identify any necessary mitigation. In relation to the application of the sequential approach in the planning system, the designation of Pluvial Flood Zones is proposed these supplement the use of fluvial/coastal zones recommended in the DEHLG/OPW Guidelines (2009), and ensure an appropriate assessment of pluvial flood hazard / risk is triggered in areas where fluvial/coastal flood risk may be low or absent. In relation to Building Control, it is proposed that powers to require additional features of design / materials / construction are strengthened with regard to new build, extension or refurbishment of existing buildings or change of use. This links closely with guidance provided in CoP2 on improving flood resistance and resilience of buildings. Particular attention is focused on measures in relation to basement properties including alarm systems and evacuation routes. 4.3 CoP2: Flood Resilience and Adaptation Measures In CoP2, guidance on the evaluation of measures is firstly provided and basic principles are set out. In evaluating appropriate measures, opportunities for benefits additional to flood risk mitigation are sought including amenity enhancement, biodiversity enhancement, carbon reduction/sequestration, waste re-use, and recreational enhancement. Guidance on measures is broadly grouped under three main categories: - Generic and Early Win Measures those which can be applied universally across the whole of the Dublin administrative area and aim to raise the overall level of resilience to pluvial flood risk, or reduce or mitigate the 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 are categorised as Early Wins. - Community Flood Resilience Measures which require engagement and participation at community or householder level. Specific actions may be required to implement building resistance and resilience measures and it is considered that ownership of that responsibility will be an important element of effective resilience raising. Consultation with Community Flood Groups has commenced during the project including workshops which specifically focused on resistance and resilience measures. CoP2 also includes guidance specifically for community groups and householders on Resistance and Resilience Measures that may be appropriate for a particular type of property. - Site Specific Measures which are normally applicable only in areas of identified high risk. These are likely to involve capital investment and there may be an ongoing maintenance commitment. The guidance and referenced case study examples are based on a review of relevant literature and developing good practice in Ireland, the UK, more widely in Europe and globally. In the guidance a summary table is provided at the end of each section under the three main headings which summarises the scope of application of each measure as well as the relative advantages and risks, indicative costs, likely level of maintenance required and Final Report Dublin 12

14 which stakeholder body is likely to have responsibility for implementation. Innovative approaches are identified including, for example, the use of variable storage source control devices (Figure 8), and specific measures where there may also be scope for realising a certain amount of carbon sequestration. A number of these approaches can be trialled by DCC in the selected detailed model areas. Figure 8: Flexible storage Concept for Community Flood resilience Source Control Particular attention is being given to measures to deal with pluvial flood risk to basement properties where, for example, in addition to other measures, some form of self actuated riser step may be appropriate to seal gaps where the access threshold is at pavement level and permanent raising of the threshold level is not practical (Figure 9). Given that flooding can arise both from surface runoff and via the drainage system, and hence occur rapidly and to a considerable depth, it is also important to consider some form of automated household alarm system and that a safe route for rapid evacuation exists. Figure 9: Example of Low Access Threshold at a Basement Wherever possible, opportunities are encouraged to identify GreenWaterSpace where storage (perhaps with carbon sequestration potential) may be provided in existing urban green space areas linked by bioswales along green corridors, where possible, or designated pluvial surface flow-routes. Such designated flow-routes are based on the concept of Streets-as-Streams / Roads-as-Rivers whereby roads and other features of the urban fabric are designed to control overland flow and direct it safely through the urban environment using such features as speed-bumps, raised kerbs or more formalised road profiling. Issues to be considered include traffic management and signage linked to the pluvial flood warning system (as documented in Volume Five); reprofiling opportunities as part of planned road surface maintenance programmes; and safety issues associated with raised kerbs or pedestrian and traffic use during flow-routing periods. 4.4 Flood Alerting and Warning System Integration As part of the functional specification for a flood forecasting and warning system summarised earlier (from Volume One), specific requirements for integration and augmentation of the existing raingauge network across Dublin were assessed. This included a review of the basic rainfall data capture systems and infrastructure, including raingauges, rainfall sensors and the use of rainfall radar systems, and how this data might be used in a pluvial flood forecasting and warning system. The proposed approach will realise the potential of as much existing data as possible, while acknowledging the possible limitations of that data and the use to which it can be put, together with recognition of the challenges posed by the urban environment. The proposed expanded raingauge network is based on: Final Report Dublin 13

15 Existing DCC raingauges already on or to be transferred on to the telemetry system. Proposed locations for new DCC raingauges linked to the telemetry system. Met Éireann and National Roads Authority raingauges to be accessed by the proposed system. In order to provide sub-hourly data, Tipping Bucket Raingauges are proposed as these are able provide a reliable source of near real time data. Weighing Principle Gauges are also being considered. As part of their Smart City programme, IBM has commenced trialling their Deep Thunder system which is an innovative technology to bridge the current gap between the synoptic and regional scale and the city scale. As an integral part of the overall Information Management System for Pluvial Flood Warning, GPRS (General Packet Radio Service) communication is viewed as offering considerable advantages in terms of cost and efficiency of field data transfer. The potential is also available for active flood warning signage and operation of flood management equipment in the field linked to the forecasting and warning system as well as live feed access to CCTV and still imagery at specific flood hotspots. Following a review of existing information management and data processing technologies available, a system to facilitate pluvial flood warning are proposed. Such a system will require a range of user levels including administrators, who can make updates, and end users with a read only view. Certain configurations can be included to share certain datasets with the public also. This could be high level warning information provided on the DCC website or shared more widely via an RSS (Rich Site Summary) feed which the public could use to keep up to date on flood information. 5. DISCUSSION AND CONCLUSIONS 5.1 Key Conclusions Dublin is perhaps unique in that the flood risk from many different sources including pluvial flooding is high and it is crucially important that the systems and measures to address each of these types of flooding are closely integrated. In addition, and with specific regard to pluvial flooding, Dublin has major challenges to address. These include: The very high proportion of basement properties which are potentially at risk. The relatively flat topography in many areas which when coupled with low property threshold levels (and basements and below ground infrastructure) increases the risk during less extreme rainfall events. The variable topography in other areas which includes some deep ponding areas and also some steep slopes where high surface water velocities could be a hazard. The modelling and mapping which has been undertaken for the Dublin FRC project will support several needs in addressing pluvial flood risk across Dublin: The city-wide pluvial flood depth and hazard mapping (Volume Two) will inform future spatial planning together with the guidance provided in CoP1 (Volume Three). Spatial planning is viewed as having considerable potential to be a cost-effective and sustainable means of mitigating the risk from pluvial flooding. The city-wide pluvial risk mapping will inform the identification of higher risk areas such that location specific mitigation measures can be prioritised in conjunction with generic and community resilience measures described in CoP2 (Volume Three). The more detailed modelling and mapping undertaken for Pilot Areas enables the assessment of preferred mitigation Final Report Dublin 14

16 options based on groups of measures in each of these areas. To inform flood emergency response the city-wide modelling also enables the preparation of a set of pluvial categorisation maps showing flood risk zones (Volume Three). The use of these maps complements the provision of a pluvial flood warning system for which requirements, including a functional specification, have been determined (Volumes One and Five). As part of an integrated flood warning system which makes best use of emerging technology in monitoring and information management systems, this will play a key role in raising the overall level of resilience to, and management of, pluvial flooding across Dublin. Community Workshops have already demonstrated the benefit of effective community engagement. Building on this will be important in raising awareness of pluvial flood risk and effective application of community resilience measures. The FloodResilienCity programme, and other European links such as through the European Water Association, have proven to be highly effective vehicles for sharing of knowledge and good practice in pluvial flood risk management and have fostered innovative approaches and thinking. This can continue to build through forthcoming planned conferences, seminars and workshops. development of good practice in pluvial flood risk management as well as wider and integrated flood management practice in Europe. Local Flood Resilience Groups In 2011 the FRC project established a Transnational Learning Alliance with the Scottish Flood Forum (SFF) to help advance a Dublin model for local community resilience groups. The SFF is an independent organisation funded by the Scottish Government and works with: Scottish Environment Protection Agency Local Authorities Local Communities The FRC and SFF, working with the S.A.F.E.R. flood partnership in East Wall promoted a flood information day and equipment trade fair which was held in November. This proved extremely informative and was well received by the local community. Based on this experience and working with other city council departments, the FRC has advanced a programme for the establishment of a number of local community resilience groups and a Dublin Flood Forum which could be operative in Way Forward The work undertaken for the Dublin FloodResilienCity Project forms the basis of a Pluvial Flood Risk Management Strategy for Dublin. Ongoing development and implementation of this strategy will allow Dublin to continue to grow and realise its full potential as a Flood Resilient City of the future. In doing so, and as part of a fully integrated strategy for all flood risks under the Dublin Flood Initiative, Dublin can act as an exemplar city in contributing to continual Final Report Dublin 15