River Dodder Catchment Flood Risk Assessment & Management Study
River Dodder Catchment
The River Dodder is one of Dublin’s best known and most important rivers. The river rises in the Dublin Mountains and in its upper reaches it forms a reservoir system which is an integral part of the water supply to Dublin. It flows down through the suburban areas of Tallaght and Rathfarnham and through the city areas of Donnybrook and Ballsbridge before discharging into the Liffey Estuary at Ringsend. The lower section of the river is tidal up to Ballsbridge. There are a number of tributaries draining into the River Dodder with the significant ones being the Tallaght Stream, the Owendoher, the Whitechurch, the Little Dargle and the Dundrum Slang. The Dodder’s surrounding parklands are an extremely important amenity to Dublin and the river is widely used by fishermen and a variety of sporting and recreational interests over its 26 Km long course.
Minister Brian Hayes of the OPW launched the Dodder CFRAMS Consultation process. It was from this process that 42 submissions were received and are currently being processed.
Launch in South Dublin County Council’s Tallaght Library at 9.30am on Tuesday 6th March 2012
|Pictured here are (left to right) Philomena Poole, County Manager, Minister Brian Hayes TD, Mayor Cllr Caitríona Jones & Dermot Finch Senior Engineer, Environmental Services.|
Launch in Dun Laoghaire - Rathdown County Council’s County Hall 7th March 2012 at 11.30am
|Pictured here are (left to right) Ms Kathleen Holohan, Deputy County Manager, An Cathaoirleach, Councillor John Bailey, Minister Brian Hayes TD and Mr Frank Austin, Director of Services,|
Launch in Dublin City Council’s Civic Offices Thursday 8th March 2012 at 11.30am.
|Pictured here are (left to Right) Michael Phillips City Engineer and President of Engineers Ireland 2012/13, Minister Brian Hayes TD, Deputy Lord Mayor Cllr. Maria Parodi, and Gerard O’Connell Project Engineer.|
Comments were invited from the public until 8th June 2012 on the flood maps, draft Plan, the SEA and the NIS. All these documents are available on this webpage, and the hard copies were on display until 6th June 2012 at the following locations:
- Planning Registry Section of Dublin City Council, Civic Offices, Wood Quay, Dublin 8.
- Civic Offices, South Dublin County Council, Tallaght, Dublin 24.
- County Hall, Dun Laoghaire/Rathdown County Council, Marine Road, Dun Laoghaire, Co. Dublin.
- Ringsend Library, Irishtown, Ringsend, Dublin 4.
- Ballsbridge (Pembroke) Library, Anglesea Road, Ballsbridge, Dublin 4.
- Clondalkin, Council Offices, South Dublin County Council.
- Dundrum Area Office, Dundrum, Dun Laoghaire Rathdown Council.
- Office of Public Works Offices, 17-19 Lower Hatch Street, Dublin 2
What is CFRAMS?
A CFRAMS is a Catchment Flood Risk Assessment and Management Study and its purpose is to manage flood risk to the area being studied. The objectives of a CFRAMS are:
- Identify and map existing and potential future flood hazard risk within the catchment.
- Identify viable structural and non-structural measures and options for managing the flood risk.
- Build a strategic information base necessary for making informed decisions in relation to managing flood risk.
- Develop an environmentally, socially and economically appropriate long term strategy (Catchment Flood Risk Management Plan, CFRMP) to manage the flood risk and help ensure safety and sustainability of communities in the catchment.
- Carry out a Strategic Environmental Assessment (SEA) and Appropriate Assessments (Natura Impact Assessment) to ensure that environmental issues and opportunities for enhancement are considered.
- To comply with the requirements of the EU Directive on the assessment and management of flood risks (2007/60/EC – the “Floods Directive”). Transposed into Irish Law as SI 122 of 2010.
CFRAMS are to be carried out for the whole of Ireland. This will consist of dividing Ireland into smaller manageable areas in which to study. The Dodder CFRAMS is one of four pilot studies in Ireland and is the first comprehensive study undertaken with a view to producing a single flood risk management strategy for the whole Dodder catchment.
Dodder CFRAMS: Outcomes
The assessment of potential flood risk management scenarios determined that there were a number of viable solutions to alleviate those areas at greatest risk of flooding. The potential solutions are now available for public consultation and feedback and described in detail below.
- No properties are flooded within the Tallaght stream catchment during the computer modelled design flood event (1% AEP (Annual Exceedance Probability) or 1:100 year event). However during the 24th of October 2011 a number of areas flooded in Tallaght, these areas were flooded before the flood waters made their way into the Tallaght stream. SDCC are proposing some local works.
- Continued maintenance required.
- No properties are flooded within the Owendoher stream catchment during the computer modelled design flood event.
- Woodside flooded on the 24th October 2011. SDCC are applying for funding to combat this under the minor works scheme.
- Continued maintenance required.
- A small number of properties are at risk from flooding in the lower reach of the Little Dargle. Flood embankments are currently proposed as a potential flood alleviation measure.
- There are properties at risk from flooding in both the upper and lower reaches of the Whitechurch.
- Individual property protection is currently proposed as a potential flood alleviation measure.
- Flood walls, dredging and removal of weirs is proposed along the Whitechurch Stream from St. Enda’s Park to Willow Brook Lawn.
- The Dundrum Slang stream is currently protected up to a major flooding event which has a 2% probability (1 in 50 year) of occurring in any given year.
- Results from the national framework assessment show that there is no viable flood risk management option available for the entire Dundrum Slang.
- As flows in the Slang exceeded those of hurricane Charlie a review of this tributary will be carried out under the Eastern CFRAMs Study.
- There are properties at risk from flooding in the upper, middle and lower reaches of the River Dodder.
- Flood walls and embankments from Donnybrook Bridge to New Bridge at Lansdowne are currently proposed as a potential solution.
- Flood embankments and walls are proposed starting at the Dundrum Slang confluence and finishing at the Clonskeagh Road.
- Two large retention ponds are proposed at quarries at Firhouse / Tallaght to combat global warming.
- As part of the Flood Risk Management Plan a River Dodder Maintenance Plan was prepared for the river and its main streams.
History of Flooding
The River Dodder has a history of flooding and is known as a "flashy" river with a quick response to rainstorms. This is due to its source being in the Dublin mountains which provides it with a steep gradient. In the last century it has overflowed its banks on numerous occasions causing damage to adjacent properties. One of the most severe floods in recent times occurred on 25th August 1986 (Hurricane Charlie) with well over 300 properties affected by the flooding, leading to much human misery and anxiety. During this storm the reservoir dams in Bohernabreena were within millimetres of being overtopped. To secure the dams new spillways have been constructed at the Bohernabreena reservoirs to cater for the "Probable Maximum Flood". This work was completed in 2006. Another most notable flooding event occurred in February 1st 2002 when there was a significant high tide, over 600 properties were flooded on the lower Dodder downstream of Landsdowne Road Bridge. The recent flooding event on the 24th October 2011, a similar number of properties flooded throughout the catchment.
In order to increase the level of protection on the River Dodder, the following work was carried out to the most vulnerable areas:
- In 1986 flood walls were constructed at Donnybrook.
- In 2003 work was started on the Dodder Estuary at Fitzwilliam Quay and Stella Gardens.
- In 2006 work was completed on the Bohernabreena Reservoir Spillways.
- Ongoing Lower Dodder Flood Alleviation Work 2007 – 2015.
The river has been the subject of a number of investigations starting in 1844 and the most significant were those carried out in 1986 and 1987. There has never been a fully inclusive analysis of the river’s behaviour under a full range of potential floods and certainly there has been no investigation of the catchment’s response to severe rainfall in its existing, or likely future, development conditions. It was proposed in 2005 by the relevant Local Authorities that a flooding study of the entire River Dodder Catchment be carried out from South Dublin and Dun Laoghaire Rathdown County Councils through to where it meets the River Liffey Estuary. A proposal for carrying out the study was forwarded to the Office of Public Works (OPW). Following review and consultation with the Local Authorities the OPW approved funding for the completion of the study.
In January 2007 following completion of the appropriate procurement process, Dublin City Council appointed RPS Consulting Engineers as Service Provider (Consultants) to carry out the study. A detailed brief, prepared by the Local Authorities, OPW and other relevant stakeholders was supplied to RPS Consulting Engineers which outlines in detail all the items of work to be completed by the Consultants to carry out the study. The study is project managed by Dublin City Council, and funded by the OPW. The direction of the study is guided by a Steering Group which comprises of representatives of the OPW, Dublin City Council, Dun Laoghaire Rathdown County Council, South Dublin County Council and other Stakeholders as deemed necessary.
The key outputs from the Study will be flood hazard and risk maps, and a "Catchment Flood Risk Management Plan" (CFRMP). The main aim of the Study is to undertake a comprehensive Flood Risk Assessment of the River Dodder catchment and develop Flood Risk Management measures for the catchment. The study included the development of a robust computer model representing the hydrological and hydraulic characteristics of the River Dodder catchment. This model was used for the mapping of the flood risk. Using model results, the technical and economic merit of various options for flood management, taking account of current and future land developments, is presented within the draft report. Consideration of the latest findings in relation to the effect of Climate Change on relevant issues such as rainfall, tide levels and river flows was also undertaken. As part of the Flood Risk Management Plan a River Dodder Maintenance Plan was prepared for the river and its main tributaries.
Strategic Environmental Assessment (SEA)
In accordance with the recently introduced EU regulations a draft Strategic Environmental Assessment (SEA) was undertaken as part of the study running in parallel with the development of the Flood Risk Management Plan. The objectives of the SEA are to consider the environmental constraints, and opportunities, within the catchment, and to look at the environmental consequences of choosing one option relative to the impact of choosing a reasonable alternative option, or options, at a strategic level in order to minimise any environmental impact(s). An integral part of the SEA process is consultation with the public and all relevant stakeholders. The process included public information events.
A detailed topographical survey of the river and its main tributaries was carried out, to provide current data for the building of a computer model of the river, by Dublin City Council’s Surveying and Mapping Section. This survey entailed surveying of cross sections of the channels every 100-150 metres and detailed dimensional surveys of all relevant hydraulic structures including bridges, weirs, walls and river outfalls. The survey data was provided to RPS Consulting Engineers following their appointment and they will use this data to build the computer model of the river. An aerial (LiDAR) survey of the river and tributaries was commissioned by the OPW to collect elevation levels of the ground surface and to construct a digital terrain model (DTM) of the catchment. This DTM will be used by the Consultants in conjunction with the computer model to predict the flood routing and produce the flood plain mapping of the river and tributaries.
Flood Hazard and Flood Risk Mapping
The Study has produced draft flood hazard maps showing the extent and depth of computer modelled floods for a number of extreme events (including the 100-year event for the fluvial portion of the river and the 200-year event for the tidal portion of the river). Protected areas of the river catchment are also analysed. Draft Flood risk maps were produced from these flood hazard maps indicating areas of potential economic risk from flooding, e.g. Properties, roadways, buildings, farmlands, businesses, parks, etc.
Flood Risk Management Measures
RPS Consulting Engineers identified and developed measures to manage the existing and potential future risks within the Dodder Catchment. The measures considered included structural and non-structural measures and should be of localised application and benefit (for example, structural flood defences or floodplain preservation through land zoning for a specific area) or of broader application and benefit (such as the use of distributed upland storage or a catchment-wide flood forecasting and warning system). Through assessment of the full range of potentially viable measures against a range of criteria including technical, economic and environmental, the Consultants developed preferred options for the flood risk management strategy for the Dodder Catchment which comprised a mix of localised and broader measures. Alternative measures of similar merit for a particular location or broad application may be put forward within the strategy, with final selection and development to be undertaken through future design and assessment work.
Flood Risk Management Plan
The draft Flood Risk Management Plan sets out the flood risk assessment analysis and the adopted flood risk management strategy for use by the OPW, Local Authorities and other relevant bodies in relation to decision making on flood risk management issues and other issues that may impact upon, or be impacted by, flood risk management activities.
Public Information days
The Dodder Study project team arranged Public Information days where they met with stakeholders and members of the public to present key findings and recommendations and sought their views at the following locations, dates and times:
- South Dublin County Council’s Tallaght Library on Wednesday 30th June 2010 from 3pm to 8pm
- Dublin City Council’s main foyer in Civic Offices, Wood Quay on Thursday 1st July 2010 from 3pm to 8pm
- Wilfield / Guilford Conference Suite, Mount Herbert Hotel, Herbert Road, Sandymount, Dublin 4 on Wednesday 28th July 2010 from 3pm to 8pm.
Where are we now
The points below show the remainder of the CFRAMS process:
- The Final Dodder CFRAM Plan was adopted by Dublin City Council, South Dublin County Council and noted by Dun Laoghaire-Rathdown County Council in November 2014.
- Work is continuing on the Lower Dodder from Ballsbridge up to the Smurfit Weir.
- Planning Permission to develop local areas (flood cells) in the Final Plan is to be progressed.
- Development of Public Awareness with local representatives and residents associations is ongoing.
Final Dodder CFRAM Plan
- Final Plan
- Dodder Maps Part 1
- Dodder Maps Part 2
- Dodder Maps Part 3
- Dodder Maps Part 4
- Dodder Maps Part 5
- Dodder Maps Part 6
- Dodder Maps Part 7
- Dodder Maps Part 8
- Dodder Maps Part 9
- Dodder Maps Part 10
- Dodder Maps Part 11
- Dodder Maps Part 12
- Dodder Maps Part 13
- Dodder Maps Part 14
- Dodder Maps Part 15
- Dodder Maps Part 16
- Dodder Maps Part 17
- Dodder CFRAMS SEA Statement