Pluvial Flooding Works
Pluvial (Monster rain) rainfall Projects
Following the thunder storms and flooding of August 2008 and July 2009, DCC have initiated a number of projects which are aimed at reducing the flood risk of the areas which were flooded.
Pluvial flooding can be defined as flooding that results from rainfall-generated overland flow, before the runoff enters any watercourse or sewer. It is usually associated with high intensity ‘extreme or monster’ rainfall events (typically >30mm/h) resulting in overland flow and ponding in depressions in the topography. In urban situations underground sewerage/drainage systems and surface watercourses may be completely overwhelmed.
We are planning to have a number of Swales constructed, and have constructed a number so far. A swale is a depressed land form, a gradual depression, similar to a ditch, however it is much wider than deep. The use of a swale is to carry or hold flood waters. Swales can allow infiltration of water and nutrients down slope of it into the ground. A swale should have no sharp edges and should also have gentle slopes to enable easy grass cutting, and use as a public amenity.
These are our projects, to date:
Glendhu Park and Park Road
Following an extreme flood event on August 9th 2008 in Glendhu Park, eight houses experienced flooding and many more came within inches of being flooded as the entire estate experienced high levels of road flooding. The flooded area was at the lowest point of the estate, some 8.8metres below the level of the Navan Road. The works were designed to reduce the flood levels in the estate should there be a repeat of this rainfall event.
Two Swales were constructed to provide a volume of surface water storage to ensure that no flooding will recur in a similar 100 year pluvial event (i.e. an event which is statistically estimated to occur on average once every 100 years, but like all statistics could happen at any time). These are to be located at:
- Glendhu Park - 1200m3 of open flood storage
- Park Road - 500m3 of open flood storage
Following a severe storm event the swales are designed to self-drain back into the existing surface water sewer system, thus ensuring that there will be no significant water present in the swales other than during and following a major flood event. In designing new works, national design standards were applied which in this instance in the Greater Dublin Area is to prevent property flooding up to a 1 in 100 year storm event (1% AEP), 60 minute event.
In each case the works involved the excavation of existing soil to the required level, removal off-site of excavated materials, associated pipe works, and final reinstatement / landscaping.
Workshop / Public meetings were held on the following dates:
- 11th November 2008 - The November Central Area Committee meeting discussed the flood of August 8th 2008
- 4th December 2008 – Meeting with representatives from Glendhu at DCC’s Cabra area office
- 5th May 2009 - Meeting with representatives from Glendhu at DCC’s Cabra area office
- 24th June 2009 - Letters to local residents were delivered – Re: “Proposed Flood Alleviation Swales”
- Notices of the proposed development were published on 18th January 2010 and the plans and particulars were displayed at the Civic Offices, at the Public Library on Navan Road and at the Cabra Area Office
- 13th October 2009 – Central Area Committee agreed to the initiation of the Part 8 procedure for these flood relief measures
A final report was completed by December 2009, Dublin City Council Planning department recommended to the City Council in 26th March 2010 that the project be given approval, and it subsequently received Part 8 Planning Permission by the City Council on 17th May 2010.
The construction stage of the project started on May 31st 2010, and was substantially completed by September 2010.
In all around 20 houses were flooded in Ballygall Crescent and Griffith Road in August 2009. Some local improvements on the drainage system were carried out. They were:
- Ballygall Crescent, a 20m length of Surface water pipe was diverted which has improved drainage deficiencies
- Extra gullies were also installed in Ballygall Crescent
- New and upgraded pipework was constructed between Griffith Avenue Extension and the Finglas Road to connect to the Finglas River
There is also a proposal to construct a swale for the storage of c. 2,300 m3 of flood waters in a public park along Glasanaon Road. These works will involve the excavation of existing soil to the required level, removal off-site of excavated materials, associated pipe works, and final reinstatement / landscaping. Construction and maintenance of these swales should result in no property flooding in the 100 year - 60-minute event (i.e. an event which is statistically estimated to occur on average once every 100 years, but like all statistics could happen at any time). Options are currently being analysed and a report is being compiled, which will form part of a Part 8 Planning Application in the coming months.
UPDATE (May 2013)
Part 8 Planning Permission for a proposed Flood Storage Swale at Ballygall Parade / Glasanaon Road, Finglas, Dublin 11 is now at the Public Consultation phase of the Part 8 Planning process. The relevant Part 8 documents (and newspaper advert) are available to download below:
A submission or observation in relation to the proposed development, dealing with the proper planning and sustainable development of the area in which the development would be situated may be made, in writing, to the Executive Manager, Planning Department, Dublin City Council, Civic Offices, Wood Quay, Dublin 8, before 16.30 hrs on Monday 8th July, 2013.
Cabra West – Dingle Road, Carnlough Rd & Drumcliffe Drive
Following the extreme flood event of 9th August 2008 Dublin City Council has analysed this catchment and is proposing solutions to reduce the possibility of future flooding. Some work has already been carried out in the area and it is proposed to install two swales (green area hollows) to further reduce the possibility of flooding. The proposed locations are:
- Killala Road which is proposed to have a flood water storage of 2200m3
- Drumcliffe Road, which is proposed to have a flood storage of 950m3
You can download below 3D drawings showing the proposed swales in the green area of Killala Road & Drumcliffe Road:
The swales are designed to prevent residential flooding up to the 100-year pluvial event (i.e. an event which is statistically estimated to occur on average once every 100 years, but like all statistics could happen at any time). When the local drainage pipes fill or surcharge, which happens approximately once every 10 years, the surplus water will go into these proposed swales by way of the new underground pipes. After the storm abates the water finds its way back into the drainage system by way of a new outlet pipe back into the surface water sewer network. The swales will normally be relatively dry as per any green area.
The swales would fill up in a 100-year, or greater, event. In lesser events they would partially fill and rainwater would start to enter the swale only in a greater than one in 10 year storm event. The new swales would be covered with topsoil and grassed as per existing green area.
Dublin City Council intends applying for planning permission for these two swales through the Part 8 procedure, which gives the general public and other organisations an opportunity to comment. You views are very welcome. We are hoping that the construction of these swales will start this summer (2011).
Cabra East – Leix / Cuala Road
12 houses and 2 kitchen extensions were flooded during the event of August 2008. A full analysis of the sewer network in this area is currently taking place. There are c. 2570 houses and c. 13,472m of sewers on the network. A 225mm overflow was constructed in Imaal Road from the combined sewer into the Bradogue stream culvert, which will reduce flood levels there.
To the left is a photo of flooding on Offaly Road.
Santry River - Proposed Flood Alleviation
Following the extreme flood events of 9th August 2008 & 24th October 2011, Dublin City Council has put a lot of effort into analysing the flooded areas and proposing solutions to reduce the possibility of future flooding in Raheny Village. Some work has already been carried out in this area and in addition, a Catchment Study of the Santry River was commissioned. This was carried out by RPS Consulting Engineers on behalf of Dublin City Council.
The Flood Defence Unit circulated letters to residents in Dublin 5 on 1st October 2012 to make them aware of a proposed engineering solution to alleviate flooding in Raheny Village.
The Letter and accompanying drawing can be downloaded below: