Dublin City Council Report on the Extreme Flooding of 24th October 2011.Posted February 14th, 2012
Progress Report to the South East Area Committee on the Extreme Pluvial Flooding Event that Occurred on 24th October 2011.
The flooding that occurred on the 24th October 2011 was the result of an extreme rainfall event that exceeded the carrying capacity of existing river and drainage systems within Dublin City.
The City Council received over 1300 reports of flooding on roads and properties throughout the City. Investigations by the Drainage Department to determine the full extent of the effects of the flooding and to identify suitable remedial measures are ongoing. The length of time required to complete this work will be considerable due to the limited resources, both financial and staffing, that are currently available to the Department.
The following reports which address the nature of the event and the manner in which resources were deployed to handle the emergency have already been presented to the City Council and the Environment & Engineering Strategic Policy Committee.
• Report No. 338/2011 considered by the Council at the Monthly Meeting held on 7th November 2011 on Dublin City Flood Risk and interim report on extreme pluvial flooding event affecting East Coast and Dublin City on 24th November 2011.
• Progress Report on Extreme Event Pluvial Flooding 24th October 2011 considered by the Environment & Engineering Strategic Policy Committee at a meeting in December 2011.
• Progress Report on Extreme Event Pluvial Flooding 24th October 2011 (Incorporating the report from Jacobs titled Interim Review and Recommendations following the Dublin flood event of 24th October 2011) considered by the Environment & Engineering Strategic Policy Committee at a meeting in January 2012.
The purpose of this report is to brief the Area Committee on the specific drainage issues that have arisen in the South East Area as a result of the rainfall event of 24th October.
Flooding in the South East Area
The City Council has received reports of flooding at 130 road locations in the South East Area. The list of these locations is provided in Appendix 1 of the report.
Principal Sources of Flooding
The principal sources of flooding within the South East Area were:
• The Dodder River
• The (piped) Swan River and its overflow to the Dodder estuary.
• The Poddle River
• Incapacity in the drainage network
Flooding Related to the River Dodder
Of all the major rivers in Dublin the river Dodder falls the greatest height over the shortest distance. It rises on the slopes of Kippure mountain and descends through a distance of 26km to discharge into the River Liffey near Ringsend. The swift transfer of rainfall into the river channel combined with the high degree of urbanisation within the catchment downstream of Bohernabreena makes the river system highly susceptible to flooding during
periods of extreme rainfall events. Current estimates indicate that the peak volume of water which flowed through the River Dodder during the most recent flooding event was approximately 250m3/sec downstream of Ballsbridge.
The majority of flooding in the Dodder catchment resulted from the river overtopping its banks and flooding areas that had been historically flood plains. While isolated flooding reports were received in the Dartry Road and Milltown areas, the greatest number of flooding reports was received from the Stillorgan Road in Donnybrook to Newbridge (Lansdowne Road Bridge). The river is tidal from upstream of Ballsbridge and for the remainder of its course to where it discharges into the Liffey at Ringsend.
The current estimate of premises flooded within the Dodder catchment stands at 192 dwellings in addition to 136 other buildings/non-residential ground floor units.
The principal locations where property flooding occurred as a result of the Dodder River were as follows:
Marian College (swimming pool and residential building)
Canon Place (12 Properties)
Herbert Road (1 Property)
Railway Cottages (15 Properties)
AVIVA stadium car park and grounds
Sweepstakes site (130 Properties)
Ballsbridge Park (The car parks of three apartment blocks)
Herbert Cottages (26 Properties)
Ballsbridge Avenue (18 Properties)
Dodder View Cottages (43 Properties)
Beatty’s Avenue (8 Properties)
Granite Place (1 Property)
Main RDS building (15% of the floor area)
Anglesea Road, Dunluce, Somerset (54 Properties, cricket club pavilion)
Licensed Vintners Association (Basement and wall collapse)
Junction of Anglesea Road and Simmonscourt Road -The Church of Ireland near the junction was also flooded
Leinster Rugby Ground
Bective Rangers Ground
Old Wesley Ground
Bective Tennis Courts
Eglington Road (3 back gardens)
Clonskeagh (Public House)
Strand Terrace (5 Properties)
The Dropping Well Public House
Dartry Cottages (4 Properties)
There are three tidal flood gates located at Londonbridge Road, Lansdowne Village and Newbridge Avenue and two demountable flood barriers located on the Aviva Stadium side of the river. These were designed to protect local areas against tidal flooding events. On the night of 24th October the predicted tide level was below that which necessitated closure of these units.
Following reports on the night of rising river levels a decision was taken to activate these defences. The gates were closed at approximately 10pm and the demountable flood barriers were erected approximately half an hour later.
There was a delay in closing the tidal flood gates as difficulties were encountered in retrieving the locking pins which were stored in a kiosk adjacent to the Aviva Stadium. The Drainage Division has reviewed the protocol for closure of these gates and will in future be closing the gates on foot of both tidal and pluvial adverse weather forecasts subject to staff availability. This warning will correspond roughly to a yellow Tidewatch Report.
Flooding Related to the River Swan
The Swan River comprises a network of small pipelines which drain much of the old Rathmines and Pembroke drainage catchment area. It originates as a very small overflow of the River Poddle in the vicinity of Kimmage Manor and drains the areas of Terenure, Rathgar, Rathmines and parts of Ballsbridge before discharging into the River Dodder near the Aviva Stadium. The river has a total channel length of over 17km and is culverted for almost its entire length.
Significant numbers of flooding reports were received from the Rathgar, Rathmines and Ranelagh areas. These areas are notable not only for the age of the drainage but for the age of their building stock, many dating from the 19th century. In particular extensive flooding, including basement flooding took place at the following locations:
Chelmsford Road (1 Property)
O’ Connell Gardens (1 Property)
Palmerstown Villas (1 Property)
Grosvenor Place (2 Properties)
Havelock Square (3 Properties)
Kennilworth Square (3 Properties)
Effra Road (1 Property)
Derrynane Gardens (1 Property)
Leinster Road (1 Property)
Leinster Square (7 Properties)
Most of the flooding associated with the Swan River related to properties containing basements or rooms below street level.
Flooding Related to the River Poddle
The flow of water from the Tymon Stream splits upstream of Kimmage Manor into two sections, the River Poddle and the Lakelands overflow. On October 24th screens located at the Lakelands overflow blocked with debris carried down during the flood causing the bulk of the upstream flood waters to flow into City section of the Poddle. The debris carried in this flow in turn blocked the screen at Gandon Hall resulting in extensive flooding in the locality. The majority of flooding occurred within the South Central Area, However flooding did occur within the South East Area at Mount Harold Terrace where 7 premises were flooded.
The design of river screens is currently being examined and minor alterations have been made to the Gandon Hall screen. The operation of all screens and the installation of screens that allow for overtopping is currently being examined.
Flooding Related to the Drainage Network
At a number of locations in the South East Area flooding resulted where the capacity of the drainage network was unable to cope with the discharges to the piped system. The most serious flooding occurred at the following locations.
Ailsbury Park (2 Properties)
Ailsbury Road (1 Property)
Eglinton Road (1 Property)
Elgin Road (1 Property)
Gullistan Cottages (1 Property)
Highfield Road (6 Properties)
Irishtown Road (1 Property)
Landsdowne Road/Shelbourne Road Junction (3 hotels)
Macken Street (2 Properties)
Magennis Place (1 Property)
Pearse Grove (1 Property)
Pearse House (1 Property)
Pearse Square (1 Property)
Pearse Street (2 Properties)
Pembroke Cottages (1 Property)
Rathmines Road Lr. (1 Property)
St. Alban’s Park (3 Properties)
St. John’s Road East (8 Properties)
Suffolk Street (4 Properties)
Sydney Parade Ave. (1 Property)
Wilton Place (1 Property)
Flood Alleviation Measures in the South East Area
The River Dodder Catchment Flood Risk Management (CFRAM) Study is at draft Final Report Stage as reported to SE Area recently and is programmed to go to 3 months public consultation next month in the three Local Authority Areas following submissions and comments it will come before the 3 Councils for adoption this autumn. Significant flood defences along the lower banks of the Dodder have already been put in place through the efforts of the City Council Flood Defence Unit and the Office of Public Works (OPW) and these saved dozens of dwellings from flooding on 24th October 2011.
The proposed flood protection works include raising and replacing existing flood walls and embankments in certain locations on the west/ north bank of the river to cater for the one in a hundred year river event or the worst 200 year combined tidal and river event in the tidal region. All of these works are currently envisaged to be provided within 50m of the river banks. Certain flood alleviation measures may be classified as emergency works in the near future with a view to their quick implementation.
The OPW has responsibility for co-ordinating flood works by central Government Agencies and has agreed to discuss with Department of Environment, Community and Local Government (DECLG) on how to address schemes which may not fall under OPW remit but which might be eligible for funding under DECLG Water Services Investment Programme (WSIP). This particularly refers to flooding associated with underground “rivers” many of which are fully piped. In particular flooding associated with River Swan has been referred to OPW to determine eligibility for capital funding from the appropriate Government
As part of the Eastern CFRAMS, which is due to report in December 2015, the OPW has agreed to fast-track portions of a study of the river Poddle in order to identify potential improvement schemes provided that it does not interfere with the statutory deadlines of the overall study. The flood alleviation schemes required on the Poddle will require the co-operation of the City Council, South County Dublin Council, OPW and Eastern CFRAMS consultants. A meeting was held on Friday 20th January 2012 with a view to progressing the necessary works.
The Drainage Network
Flood Resilient City Project (FRCP)
The FRCP currently underway will identify areas that are vulnerable to the threat of pluvial flooding and develop the most appropriate strategies to protect those areas against flooding. A final report will be available by mid 2012. A number of pilot areas have been selected for detailed attention. And in the South East Area Pearse Square is one of the areas for which a specific report will be prepared.
Rathmines and Pembroke Drainage Scheme
The aim of the Rathmines & Pembroke Sewerage Scheme is to develop a rehabilitation strategy for the combined foul and surface water sewer serving the Rathmines and Pembroke catchment. To this end a Drainage Area Plan was developed, its objective being to identify and prioritise cost-effective solutions that address the hydraulic, environmental and structural deficiencies of the system. The current aim of the project is to review the Drainage Area Plan and out of this to develop a number of Construction Schemes to implement the solutions identified and to improve the performance of the system. A Project Engineer has been recently been appointed to progress this scheme.
Gully Management Programme
Dublin City Council radically revised its gully cleaning programme in 2004. A new database was introduced which records the date and location of every single gully cleaning operation. All of the city’s 54,000 gullies are cleaned on average once per annum. Gullies on the main thoroughfares are cleaned more frequently, in some cases once every six to eight weeks. In addition, during adverse weather conditions and in particular on receipt of severe weather forecasts, normal gully cleaning work is suspended and the crews travel to areas which have historically flooded in the past to deal with any visible flooding and also carry out precautionary gully cleaning.
Flood Resilient Communities
The City Council will endeavour to progress alleviation schemes to minimise the adverse effects of flooding within Dublin City. However it is not feasible to afford protection to every location for extreme pluvial events. The City Council’s sandbag policy (Appendix 2) points out that the prime responsibility for the protection of premises at risk of flooding rests with the owners of those properties.
In Scotland a model has been developed by the Scottish Flood Forum that has enabled over 200 Community Flood Groups to plan and prepare for flood events.
Recently the Flood Resilient City Office (FRCO) in conjunction with the Central Area Office invited the Scottish Flood Forum to meet with residents in the East Wall area with a view to establishing a Community Resilience Group to deal with floods. This type of model provides individuals and communities with an effective and efficient means of protecting properties from floods by:
• Establishing local area flood watch systems
• Establishing awareness of factors contributing to flood risks.
• Developing local community flood action
• Minimizing the danger of flooding within the local area.
• Assisting at times of flooding and supporting people who have experienced flooding to ensure effective support is available to assist recovery.
• Meeting regularly to ensure that flood preventive methods are being maintained and monitored.
• Monitoring and reporting to the City Council those areas which are at risk of flooding through lack of maintenance or repair.
• Raising the awareness of personal and collective actions to limit the occurrences of flooding.
• Engaging with the City Council and other organisations to reduce the risk of flooding.
• Developing a local community flood awareness training programme
• Promoting flood protection equipment and materials to prevent further flooding to property
• Arranging free surveys of properties and giving specific advice on the most appropriate means of flood protection.
• Providing advice on the type of protection and suitability of products that will minimize the risk of flooding to properties.
• Making recommendations on minor repairs to properties that may prevent entry of water.
• Facilitating substantial reductions in the cost of flood defence products such as flood gates through bulk buying.
The City Council has explored the possibility of establishing Flood Resilience Groups in the recently flooded areas as the most appropriate means of facilitating the protection of homes and businesses from flood events. The Flood Resilience Cities Office (FRCO) has created a trans national learning alliance between Dublin City Council and the Scottish Floods Forum (SFF). A workshop between City Council staff and the Director of the SFF was held on Thursday 12th Jan 2012. The purpose of establishing the Alliance is towards developing a Dublin Flood Forum that will encourage and support the establishment of Community Resilience Groups across the Dublin region in areas that are faced with the threat of flooding. This will be advanced through the Area Committees.
Of particular benefit in establishing flood resilience communities in the South East Area is in combating coastal flooding and the flooding of basements. In the case of the latter many of the premises that were flooded in the South East Area were basement properties.
The 2005 Greater Dublin Strategic Drainage Study highlighted a number of issues with regard to basements and the associated flooding risks. Throughout the city many properties have basements with connections to old sewers and culverts. When these sewers become surcharged the flows often enter basements as these basements are built below the surcharge levels. These basements can be protected by backflows from surcharge by the installation of small pumping stations or anti flooding devices.
The City Council has a Basement Policy which advises property owners on all aspects of drainage concerning basements. The policy strongly advises against any basements in new developments as these areas will always be liable to flooding. The basements in older dwellings in areas like Rathgar, Rathmines and Ranelagh are a legacy that will always remain susceptible to flooding. Rooms that may have been once used as storage areas when the buildings were originally constructed have, in many cases, been redeveloped as apartments or other living accommodation.
The Council has produced an advisory leaflet on Basement Flooding (Appendix 3) advising owners of basement properties to install Non-Return Valves or, where possible, a separate drainage system which is then pumped to the public sewer. In both instances the objective is to break the direct connection to the main sewer as in flood conditions the level in the sewer will rise, in many cases to a level above any drain inlets in the particular basement, and the sewer will back-up and flood the basement.
An important measure to reduce flooding within these areas will be the actions taken by property owners themselves, whether in the installation of non-return valves or pumped connections to the main sewer, in many cases it is actions they will undertake themselves that will protect their property from flooding.
Dublin City Council’s Sandbag Policy
The City Council does not provide or distribute sandbags to individual premises at risk of flooding. The prime responsibility for the protection of such premises rests with the owners of those properties.
The primary role of the City Council during pluvial events is to manage the drainage network in order to minimise the extent of flooding to the general public. The supply and distribution of sandbags would present a considerable impediment to this task. In addition the inevitable increase in telephone requests for sandbags to emergency call centres would seriously interfere with the ability of those centres to cope with major flooding events.
The use of sandbags has become established in the public’s mind as an effective flood protection measure. This is reflected in demands for the City Council to make sandbags available to householders and businesses at risk of flooding.
The Council maintains strategic stocks of sandbags at a small number of locations. These amount to around 9,000 at various locations including Clontarf, Sandymount, Glendhu Park, and the drainage depots at Marrowbone Lane and Bannow Road. The stocks at these sites are maintained for strategic purposes and play a useful role in areas when dealing with flood events which have sufficient advance warning.
General advice to property owners on dealing with floods is provided by the OPW in booklet format and on the website www.flooding.ie. The OPW advice recommends property owners at risk of flooding to have a supply of sandbags close at hand. The advice notes also acknowledge that sandbags can be difficult to deploy during flood events and can also pose health risks if contaminated with sewage.
A major report on of the serious pluvial flooding that occurred in the UK in 2007, known as The Pitt Review, concluded the following with regard to the role of sandbags as a means of protecting individual properties during flood events:
• While it is clear that sandbags have a useful role in certain types of floods when used strategically, their benefits are less clear when they are used by householders to protect individual properties. This weakness is further heightened by their relative inefficiency when compared with alternative dedicated flood defence products that have been developed in recent years, such as floodgates and airbrick covers.
• Extensive evidence of public over-reliance on sandbags which often proved of little value in protecting against flooding.
• Many householders and business owners put time and energy into obtaining and installing sandbags which would have been better spent on other activity such as moving possessions to safety and deploying door boards.
• Sandbags can be effective when it is marginal, as to whether water enters a house or not, but in relation to large volumes of water they are largely ineffective, contrary to public perception.
To supply sandbags to all properties at risk of flood during sudden rainfall events would require a level of resources that is much greater than is currently available to the City Council. Even if such resources were provided the deployment of sandbags in sufficient time to prevent significant flooding of properties, particularly during monster rainfall events would be logistically impossible. During a flood event, invariably the transport network is very busy, which means that DCC crews have great difficulty in reaching certain areas to deploy pumps, close flood gates, or deliver sandbags. If there is little notification that an event will occur, delivering sandbags would not be possible.
The provision of sandbag stores at specific locations around the City that could be accessed locally by residents on foot of flood warnings would require considerable investment by the City Council to manage and maintain. The unpredictable nature of flood warnings which can average 4-5 a year would result in sandbags being deployed more often than required, leading to the unnecessary expense of maintaining the required stock of bags at each location. The transportation and placement of sandbags from local containers would still require a considerable effort by local residents and they would be unlikely to be in position in time to prevent flooding to most properties subject to sudden rainfall events. Furthermore, if sandbags were deployed at certain locations, there is no guarantee that the people who need them will get them. During a flood, panic generally sets in, and those who are not in risk of flood could easily exhaust the supply of bags at the expense of those in need.
Owners of properties that are at risk of flooding are encouraged to keep where possible, their own stock of empty sandbags together with sufficient stocks of sand to fill bags at times of potential flooding. Preferably owners should invest in the provision of suitable proprietary flood gates and covers to protect openings such as doors, windows and vents. Dublin City Council does not have financial provision in the 2012 budget for purchase of any additional sandbags.