The Dublin Coastal Protection Project

Transcription

1 Coastal Engineering 245 The Dublin Coastal Protection Project I. Cooke 1, A. D. Maguire 2, O. McManus 3 & B. Bliek 4 1 Posford Haskoning Ltd. 2 Dublin City Council 3 Posford Haskoning Ltd. 4 Svasek Hydraulics Abstract In February 2002 the City of Dublin experienced severe flooding as a result of what was believed to be a combination of unusually high tides and meteorological surges. The disruption to the city prompted the Dublin City Council to put in place a programme of works of which the Dublin Coastal Flooding Protection Project is a seminal part. Subsequent analysis of tidal records and meteorological conditions has shown that the return period for the event of 1 st February 2002 was not unusual. This paper will describe the development of a coastal flood forecasting system for the City of Dublin taking account of the state of coastal defences along the coast line as well as the Rivers Tolka, Dodder and Liffey. It will describe the role of numerical modelling to derive sea state conditions along the coastline affected. When coupled with a finite element hydrodynamic model, a model of the Rivers Liffey, Dodder and Tolka and overtopping, enable forecasts of future high tides and potential flooding to be made and actions taken. The emphasis is on the practical application of the results of the numerical modelling within a coastal flood forecasting system. The Paper will also describe the role of the project within the European Strategies & Actions For Emergency Risk management initiative (SAFER) and the development of best practice within a wider European context. The SAFER project has been constituted to promote a risk management approach to flooding focused on three central themes of: (a) provision of flood hazard and risk information; (b) flood emergency response planning and management delivered at the local level through (c) flood partnerships. Keywords: SAFER, coastal flooding, joint probability, Dublin, flood forecasting, flood protection, asset survey, flood partnerships, Interreg IIIB, themes.

2 246 Coastal Engineering 1 Introduction Flooding of any type, especially in urban conurbations imposes a significant burden on the local population in terms of the economic and social stresses experienced. This was the case when a severe low pressure system (934mb) located off the North West coast of Ireland, combined with spring high tides on 1 st February 2002, to produce water levels which led to widespread flooding and disruption across a large area of the City of Dublin. Dublin City Council in association with Fingal County Council, and supported by the Department of Communications Marine and Natural Resources, the Office of Public Works and the European SAFER initiative, have commissioned the Dublin Coastal Flooding Protection Project (DCFPP) as a means to investigate the protection against future flooding. Posford Haskoning has been commissioned as the service provider for the study, the principal features of which have been to: Identify the flooding mechanisms identify the extent of the assets at risk from coastal flooding; identify flood compartments; prepare flood hazard and flood risk maps; Identify programmes of risk reduction & mitigation works formulate an interim flood forecasting system; and identify a phased programme of capital expenditure aimed at providing a 1:200 year standard of protection to The study also plays a key role within the European SAFER Initiative (Strategies & Actions For Emergency Risk management) and the development of best practice within a wider European context. The SAFER project has been constituted to promote a risk management approach to flooding focused on three central themes of: provision of flood hazard and risk information; flood emergency response planning and management delivered at the local level through flood partnerships. These themes broadly mirror a new national strategy for flood hazard management currently proposed by the Irish Government, which includes: i) a move towards a capital expenditure profile based on the identification of population and assets at risk; ii) the use of technology to promote early identification of flooding, residual risk; and iii) a programme of information dissemination to make populations at risk more aware of the measures available for self help. An undertaking of this sort necessitates a substantial input in terms of data collection and collation. This paper will not focus on these elements, but will,

3 Coastal Engineering 247 instead, concentrate on the major components that have contributed to the overall success of the project to date. Figure 1: Location map. 2 The Role of SAFER The SAFER initiative is an INTERREG IIIB approved project. The INTERREG initiative seeks to ensure that national boundaries do not become barriers to the balanced development and integration of Europe, by encouraging co-operation between areas to their mutual advantage.

4 248 Coastal Engineering Individual countries within the EU have and are developing their own responses to flood risk management, often to meet very specific local needs. The SAFER project, however, is an innovative proposal that seeks to develop a best practice approach to flood risk management based on three themes, which can be used or adapted (as necessary) across the wider EU Community. The recent expansion of the Community therefore makes trans-national co-operation a necessity if the results of such work and funding are to achieve the maximum benefit. The three themes referred to are: Theme 1 Flood Hazard Information: This deals with the creation of hazard maps, determination of flooding frequency,. Figure 2 shows an example of a flood hazard map. Figure 2: Flood hazard map for the Clontarf frontage. Theme 2 Flood Emergency Response: The building of new defences or the use of temporary demountable defences to counter flooding is only one manifestation of the flood emergency response theme. Of equal importance is

5 Coastal Engineering 249 the development of seamless systems to forecast potential flooding, which in turn feeds into the appropriate call out and response by the authorities. The means whereby the members of the public are informed is also a development of this theme an issue that is discussed later in this paper. Theme 3 Flood Partnerships: Partnerships are currently in place with national and regional agencies such as the Office of Public Works and the Department of Communications Marine and Natural Resources. The Dublin Coastal Flooding Protection Project provides for the first time, the flood hazard information on Dublin s coastal flooding risk. Many of the working relationships between regional and national stakeholders have been fostered through the various workshops held during the course of the study. It has also laid the groundwork for partnerships to be established with local communities, and the further development of these latter partnerships is ongoing. The Dublin Coastal Flooding Protection Project (DCFPP) forms the major constituent of Dublin City s work on the SAFER project and also within the overall Dublin Flooding initiative (see Figure 3). In addition to Dublin City Council, there are four other partners contributing to the SAFER project: Gewasserdirektion Neckar in Germany, who are also the lead partners; Forestry Commission of Scotland Federal Office for Water and Geology of Switzerland; and; Ecole Polytechnique Federale, also of Switzerland. Progress is monitored through a series of Partnership Boards and local steering committees. 3 The Dublin Coastal Flooding Protection Project 3.1 Survey of coastal assets The importance of the survey of coastal assets becomes apparent once the needs for capital expenditure are considered and when the assets at risk located within the flood compartments are taken into account. The survey was undertaken early on in the study, however, in order for the data collected to be of use, the assets were first divided into management units, and thereafter sub-divided into cells. A management unit comprises a coherent section of frontage, which is determined by its exposure. For example the area known as Clontarf extends from the northern boundary of the port, to the wooden bridge that provides access to the southern half of Bull Island. The bridge therefore constitutes a logical break between the Clontarf frontage and that immediately behind Bull Island. A cell comprises a defence type e.g. concrete wall, masonry wall, dunes, rock armour. To collect the data for the asset survey, it was necessary for an experienced engineer to walk the full length of the study frontage, taking photographs of the defence types, sketching cross sections, noting particular details such as

6 250 Coastal Engineering cracking, deterioration or corrosion (including Accelerated Low Water Corrosion), access points, beach condition etc. Initially, the collection of asset condition data was limited to the extent of the study, although this was later extended under a separate commission, to include the whole of the Fingal County Council coastline. Figure 3: Dublin Flooding Initiative schemes. The details were then entered into a database such as is seen in Figure 4. The database is a live document and is now installed with both Dublin City and

7 Coastal Engineering 251 Fingal County Councils. In the case of Dublin City Council, the database has been used to identify priority emergency works, and also to identify future capital expenditure based on both exposure to flooding and asset condition. Figure 4: Asset database. 3.2 Joint probability analysis The Interim Flood Forecasting System (which is described in the next section) to forecast coastal flooding is built upon a detailed joint probability analysis of tides and meteorological conditions, as well the results of a series of numerical models. (The hydraulic tidal modelling and joint probability analyses were carried out by Svasek Hydraulics, formerly part of the Royal Haskoning Group.) The joint probability analysis is discussed below; however, the numerical models are excluded at this stage as they are essentially mechanical in nature. Their place within the forecast system itself is illustrated in Figure 5. The joint probability analysis was a crucial element of the study, in that it was the first step in understanding the seriousness of the events leading to the flooding of 1 st February Digital and analogue tidal records were converted into a time series of tidal heights. Filtering removed the effects of atmospheric influences such as surge. The remaining tidal components where then decomposed into more than 102 tidal harmonics, from which representative tides could be derived. Similarly a time series of surge was produced and a joint probability analysis undertaken.

8 252 Coastal Engineering Annual Prediction of Astronomical Tide (once per year) External Forecasts Forecasts of Winds/Waves and Surges (once every 12 hours for the next 36 hours) Tide Level Forecasts at Port of Dublin Central Data Storage (Dublin) Wind/Wave Forecasts at Offshore Points Tide Level Forecasts at Port of Dublin and other warning points (Internal Triton) Wind/Wave Forecasts at Offshore Points (Internal Triton) (3 hourly) Nearshore Global Water Levels Offshore Wave Conditions Tide Level Interpolation to Specific Sites Site Specific Nearshore Water Levels Wave Transformation Matrices Site Specific Nearshore Wave Conditions Site Specific Water Levels Wave Overtopping Matrices Nearshore Wave Data at Specific Sites Forecasts from External Sources Comparison with Site Specific Flood Warning Triggers Site Specific Overtopping Forecasts (mean, peak and volume) Central Data Store Base Data Displayed in Triton System Processed Data Displayed in Triton System Proposed Warnings for Specific Site Processes in Triton System Key Figure 5: Data flow through the TRITON flood forecast system. The probability analysis showed that the return period of the February 2002 event was of the order of 1:70 a not infrequent event. Moreover within the period that tidal records have been collected at Dublin Port (80 years) there have

9 Coastal Engineering 253 been tides of a similar magnitude to the February 2002 event of which 4 have occurred in the last 14 years see table below. Year Tide Level Year Tide Level NB. The values in the table above are ranked by tide level. Those highlighted have occurred with the last 14 years To arrive at the return period event, the astronomical tide and surge components are combined, assuming that both parameters are statistically independent; an assumption which is valid given that the mechanisms that give rise to each are independent (Pugh and Vassie 1980). To avoid bias, the full range of astronomic tide and surge conditions are used. In this way the joint occurrences of a storm and neap tide are properly taken into account. To combine the probabilities, a matrix was created in MS Excel with fine intervals on both tide and surge. The probability of occurrence of each cell in the matrix is determined by multiplying the probabilities of occurrence of both parameters for that specific cell. The matrix is then sorted on combined water level i.e. astronomic tide and surge, and then the occurrence frequencies of all events giving rise to the same combined water level are added together. The combined probability curve is shown in Figure 6. Whilst the analysis of tides and surges was proceeding, a separate review of sea Mean Sea Level (MSL) was undertaken, to determine the effect of sea level rise on the long term trends and also on the design of potential capital works schemes (Sweeney 2003; IPCC 2001; UKCIP ). Trends in sea level rise have been incorporated in the joint probability analysis. The review of sea level rise concluded: Based upon the recommendations of the Intergovernmental Panel on Climate Change (2001) the Medium-High scenario should be used as a baseline for design. Annual average sea level rise of 4.15mm/year should be used to the end of This includes an allowance of 0.3mm/yr for land subsidence.

10 254 Coastal Engineering Joint probability of surge and tidal high water 1 exceedance probability yr 0.14 yr 1.4 yr 14 yr 140 yr 1400 yr yr High water (m) P(H>=Hw) MSL-0.1m MSL+0.1m Figure 6: Joint probability of astronomic tide and surge. Analysis of the records of the February 2002 event also revealed the occurrence of a tidal seiche, approximately 1 hour before high tide with a period of between 45mins 90mins. The seiche is not present in all tides. Further investigation into the phenomenon suggests that the shape of Dublin Bay with certain atmospheric conditions plays a role in creating the seiche. Under normal daily conditions, the seiche height is approximately 0.1m which, within the accuracy of the accuracy of the mathematical filtering, may be considered as noise. Seiches only become significant when the height exceeds this level. This happens mainly during the winter months. Although the February 2002 event was an extreme event in its own right, the sieche component was not significant. However the seiche that occurred two days later was the highest recorded during the period Interim flood forecasting system The components of an early warning system For any flood forecasting and warning system to deliver benefit to the communities that it is intended to protect, it must be viewed as a coherent entity, if the overall objective is to reduce the risk to life and property of the general public. The benefits of issuing flood warnings to the public are only realised if the dissemination is effective.

11 Coastal Engineering 255 The established thinking on flood forecasting and warning systems looks at six components: detection; forecasting. warning. response evaluation; and improvement The above components are linked and must not be considered in isolation. The first three components are looked at below. The last three elements, i.e. response, evaluation and improvement, can only occur if adequate records of flood events are maintained. At the coastal margin detection takes the form of wind/wave/surge forecasts. Along river systems, detection will include access to flow and/or level data in real time with forecast precipitation supplied by the UK Met Office or Met Eireann. Forecasting compares what is happening (or what is forecast to happen) with the standards of existing defences. Forecasting looks at where and when the defences might be exceeded and for how long. Having forecast the exceedence, the next action must address the question of What will the impact be?. To do this requires access to an asset survey database and flood risk maps. A flood warning terminology that is adopted (and understood) by all interested parties is an essential pre-requisite for any warning system. Without this it would be impossible to convey the seriousness of a message from which the necessary actions must flow. When developing the terminology the needs of the general public, the broadcast media and the professional and emergency services must be taken into consideration. The first three elements of detection, forecasting and warning are present in the TRITON interim forecasting system. The remaining elements will be dealt with in Section TRITON user interface The forecasting of extreme tide levels and potential coastal flooding events draws on the individual elements of the study. These elements, namely: The joint probability analysis of tides and surges to determine the return period of the February 2002 event also enables the prediction of extreme tide levels, against which the standard of protection of the current defences can be assessed. The defence asset survey database described above. Numerical modelling of tides, waves provide significant detailed information about the distribution of the wave and tidal conditions across the study frontage under normal and extreme tides. Numerical modelling of overtopping provides information against which the standards of defence can be compared, and from which the trigger level criteria are derived.

12 256 Coastal Engineering Flood compartment mapping, enables the assets at risk to be identified in a consistent manner, as well as being able to assess the effectiveness of the options proposed to mitigate future flooding. Offshore wave forecasts. These are supplied by the UK Met Office for a point off Dublin Bay. Figure 7: General over view page. Figure 8: Detailed information sheet held beneath general overview page.

13 Coastal Engineering 257 Each element, whilst important as a decision making tool, does not provide sufficient information to enable a coherent forecast to be made of potentially destructive tides and wave conditions. The interim forecast system employs the TRITON user interface to combine the results of the individual components in a manner that allows the operations staff of Dublin City Council and Fingal County Council to develop appropriate protocols and strategies for dealing with future flooding events. Figure 5 shows how the above are combined to produce a flood forecast. Figures 7 and 8 respectively show a general overview page in which the current status of the warning points is illustrated, and then a more detailed view of a specific warning point Flood warning and trigger criteria The DCFPP Interim Flood Forecast System employs a four tier warning approach with each tier getting progressively more serious: All Clear Flood Watch Flood Warning (which is further sub-divided into two categories) Severe Flood Warning This categorisation is similar to that employed by the Environment Agency in the UK, but as the system expands, the categorisations may change. Each warning category is activated according to a preset series of trigger criteria. For example, along the Clontarf frontage the following thresholds have been determined: Flood Watch - based on Mean overtopping rate where 0.5l/s/m < Mean OT < 0.1l/s/m Flood Warning A - Mean Overtopping rate >0.1l/s/m Flood Warning B - Mean Overtopping rate > 1.5l/s/m Severe Flood Warning - Overtopping volume > 10,000m 3 During the course of the study it has emerged that the setting of the trigger criteria are themselves dependent on: the state of the tide, the peak overtopping rate, the mean overtopping rate and the overtopping volume. Notwithstanding this it is clear that a standard value for each warning point is not possible, since for any site subject to wave action, any of the above could trigger the warning categories. The TRITON system allows the threshold values to be set for each warning point individually, and to be adjusted as onsite experience allows Fluvial warnings Water levels along the Rivers Tolka, Liffey and Dodder were also required, for the warning system to operate across all sections of the frontage subject to flooding. The Tolka was the subject of a separate study and therefore is not discussed here. The response of the Rivers Liffey and Dodder to combinations

14 258 Coastal Engineering of tide and fluvial discharge were studied using a numerical model called ZWENDL. The wide range of surge, tide and fluvial discharge conditions are summarised and presented in a macro program shown in Figure 8. Forecasts of potential flooding are treated separately to the TRITON system. 4 Future actions 4.1 Dissemination of flood warnings to the appropriate authorities The interim flood forecast system described in this paper has been promoted and developed by Dublin City Council and its partners in response to the flooding which occurred in February However, there exists a potential conflict of interest in that the issue of a flood warning is primarily for the benefit of the public and should be made by an independent competent body with the necessary expertise to interpret the forecast warnings alongside the meteorological drivers. Dublin City Council in this regard has a vested interest in responding to the warnings and as such do not have the expertise necessary to interpret the information that would contribute to the issue of the warning. With this in mind, and recognising that the interim flood forecasting system is potentially the template for a national flood forecasting system, Dublin City Council have approached a third party with a view to them assuming responsibility for the running, maintenance and issue of a viable flood warning system. Discussions on the mechanics of this are at an early stage and at the time of writing this paper no firm decisions have been taken with respect to the protocols or appropriate operating model. Both the UK and Australia operate a system of flood forecast warnings from which a working model can be developed. 4.2 Dissemination of warnings to the public In the previous section the first three elements of the early warning system detection, forecasting and warning are dealt with. The remaining elements of response, evaluation have yet to be completed, as they are principally areas that fall within the remit of Dublin City Council. The response strategy will vary according to the target group to which it is directed. For the warnings to have meaning, the members of the public should be familiar with the symbols used, and should be encouraged to prepare their own flood emergency plans using the literature available. The natural response of the public on hearing a warning is to want to verify its authenticity. In England and Wales the Environment Agency (EA) has established Floodline ; which is a contracted out service that can both endorse the flood warning messages as well as provide back ground information. The Scottish Environmental Protection Agency has also joined the Floodline service. If the broadcast media are to cooperate in disseminating warnings, a protocol must be established under which they will receive the warnings within a specific

15 Coastal Engineering 259 timescale. The protocol must also include an agreement from the media that they will not exercise editorial control over the wording of the messages. Finally, the emergency services as well as other organisations responding to a warning must be willing to amend their operating procedures to accommodate any flood warnings received. A forecast system is only as good as the information it receives. It is therefore imperative that a procedure for receiving and acting upon feedback is implemented. This requires a round-table discussion with all parties, including representatives from local communities. As the system is not yet fully operational, we are not able to report on this element. 4.3 Flood partnerships Through this project Flood Partnerships have been developed with national and regional agencies, and have laid the foundation for partnerships with local communities, which are seen as key to the on-going collection and dissemination of information. Contact thus far with members of the public in affected areas of Dublin has been positive. 4.4 Emergency services response The emergency services play an important role in aiding communities affected by flooding. However, with the introduction of a flood forecasting system which will deliver warnings of potential flooding, there is a need for a shift in the way in which resources are utilised during any response. This also applies to the Council s operational staff. Lines of communication will need to be established and tested as necessary, along with feedback to improve the response efficiency. These aspects are outside of the scope of this paper. 5 Conclusions The continued use of sea defences as the primary means of protection against coastal flooding is unsustainable without a detailed understanding of mechanisms causing the flooding, and the attendant risk. Similarly, the isolated development of systems to analyse and predict flooding is unsustainable, where expertise is available from within the European Union. The approach outlined in this paper draws on expertise gained from across Europe. This has enabled the Dublin Coastal Flooding Protection Project to benefit from the close collaboration of European partners in the development of a flood forecasting system, and in the development of a risk based approach to flooding. The interim flood forecasting system will initially be used to forecast flooding for the City of Dublin, however, the system has the capacity to expand and become a national standard. Notwithstanding this, there are challenges ahead in establishing an independent authority that will be responsible for maintaining, monitoring and improving the system, whilst at the same time having responsibility for issuing warnings.

16 260 Coastal Engineering Members of the public in affected areas must be informed about the risks and how they are to be mitigated, as well as what steps they can themselves take. A programme of public exhibitions and regular leaflets, as well as the formation of flood partnerships will assist in disseminating the information. Flood partnerships at a community level are seen as key to the success of any on-going collection and dissemination of information. References [1] Sweeney, J (2003). Climate Change Scenarios and Impacts for Ireland [2] IPCC (2001). Climate Change 2001: The Scientific Basis. IPCC Climate Change the IPCC Scientific Assessment. [3] UKCIP02 (2002). Climate Change Scenarios for the UK. Scientific Report, April. [4] Pugh, D.T and Vassie, J.M (1980). Applications of the Joint Probability Method for Extreme Sea Level Computations, in Proceedings Institute Civil Engineers, December 1980; 69: page