COMHAIRLE CONTAE ÁTHA CLIATH THEAS
SOUTH DUBLIN COUNTY COUNCIL

south dublin county council crest

MEETING OF SOUTH DUBLIN COUNTY COUNCIL

Monday, December 14, 2009

QUESTION NO. 9

QUESTION: Councillor D. Keating

To ask the Manager to report on the recent major flooding event within the South Dublin County Council area and the affects on the local community. Would the Manager report on what measures can be taken to allay concerns, including possible impacts of such flooding, including possible dangers to our bridges, and what measures will the Manage take to stress test our bridges, including Lucan Bridge as an example, where serious flooding occurred in one of our rivers, the River Liffey?

REPLY:

There was no flooding caused by the Liffey in South Dublin during recent rainfall events.

Flooding Context

(a) The recent flood event of 29/11/09 and how it impacted on South Dublin was different from normal flooding events. This needs to be understood in the context of the management of the River Liffey.

Heavy rain started to fall on the Liffey Catchment at approximately 8pm on 28/11/09 and continued until 2pm on 29/11/09. The vast majority of this rain fell on the upper and middle Liffey catchment. From approximately midday on 29/11/09 the ESB, through the media started issuing flood alert warnings about the Liffey Catchment, especially through County Kildare. The ESB also contacted officials in South Dublin, Kildare, Fingal and Dublin City at this time to get an understanding of what was happening on the ground. This allowed the ESB to make informed decisions about water releases from the three dams on the catchment. Based on feedback on river levels and monitoring of reservoir levels the ESB ceased releasing water from Poulaphouca and Golden Falls to ensure they did not contribute to already the already rapidly rising Liffey. The middle Liffey through Kildare rose steadily causing flooding but based on discussions with Kildare County Council and the ESB, South Dublin flood management team was able to assess the river at Lucan and make informed decisions. Flooding did not happen simultaneously along the Liffey and Kildare County Council were reporting to South Dublin on locations where water levels had peaked throughout the evening of 29/11/09. The surge was visible moving through Kildare and thus the ESB and South Dublin, Fingal and Dublin City Council could assess and plan for potential river rises.

Due to the amount of water in the Liffey the discharge from the dam in Leixlip was increased steadily throughout the evening. The discharge went steadily from 80 tonnes / per second to 97 to 103 to 106 to 113 to a maximum of 116 tonnes per second. Officials could see what impact this was having through Lucan and inform Dublin City accordingly. Even though rain ceased at approximately 2pm on 29/11/09, peak flow did not arrive in Lucan until 10.30pm that night. At that stage increase discharge could be balanced with a reduction in tributary inflows thus Lucan only experienced road flooding along Strawberry Beds in Fingal. This was relayed to Dublin City and they were in a position to stand down their flood management team at 11pm approximately.

It should be noted that NO flooding occurred as a result of the Liffey in South Dublin

Examples of rainfall rates

Casement Rainfall rate
29/11/09 30.9 mm
28/11/09 4.7 mm
19/11/09 19.2 mm
18/11/09 14.8 mm
4/5 Nov 2000 84 mm
Hurricane Charlie 89.2 mm
   

The return period rainfall for a 12, 24 and 48 hour period are

Duration 2yrs 5yrs 10yrs 20yrs 30yrs 50yrs 75yrs
12hrs 28 35.3 40.3 45.6 48.9 53.3 57.1
24hrs 36.9 45.4 51.2 57.2 61 65.9 70.1
48hrs 45.3 54.7 61.0 676.5 71.5 76.5 81.1

The flood event of 29/11/09 is an example of where rainfall falling some distance from a location can at a later time cause flooding downstream and this is how the Liffey flooding on 29/11/09 occurred through Lucan. This was be managed by way of communication between the ESB and the stakeholder Local Authorities.

(b) The other risk of flooding in South Dublin occurs where heavy rainfall falls directly on a catchment and the flooding happens very quickly through the river and stream or through the drainage systems. This is more difficult to manage.

With regard to future planning – both the Liffey and Dodder studies will produce flood mapping which in turn will become part of the County Development Plan. The GDSDS and the Greater Dublin Regional Drainage Code of practice has recommendations with regard to development buffer distances from rivers and streams.

Severe weather alerts are received by the Council in advance of severe weather, due to the localized nature of some downpours  - a warning does not mean flooding will occur and also a lack of warning does not mean flooding will not occur. The weather alert allows crews to be informed that there is a possibility that an emergency response may be required.

It is impossible to eliminate flooding as there are many variables that cause it.  Pipes / Culverts sized appropriately a decade ago may not be capable of dealing with higher flows being experienced today as a result of more and longer high rainfall events, rainfall events occurring over a longer period with heavy saturation of existing areas also increases the possibility.   Bridges and culverts can be subject to blockages with debris from time to time. Experience has shown that even garden furniture or building material on the banks of a stream can be collected and cause blockages during rises in stream levels.  The drainage crews do carry out a daily screen cleaning rota and the frequency of this is increased during bad weather. SDCC has a programme in place that best manages the risk of flooding and can provide advice with regard to flood prevention and emergency services where flood has occurred.

Bridge Inspections in SDCC

Bridges in South Dublin are divided into 2 categories, those on National Primary/ National Secondary routes and those on Non National Routes.

There are 62 bridges on NP/NS routes and the NRA is responsible for their maintenance. However SDCC get a yearly allocation to carry out a routine inspection on the bridges and to carry out routine works on them.

‘Principal inspections’ are carried out by NRA itself and any major works needed are carried out by the NRA.

The Non National bridges are maintained by SDCC.

In 2009, SDCC commissioned Roughan O’ Donovan structural engineers

to survey all the Non National road bridges and set up a data base.

89 bridges in total were surveyed and their condition was rated from 0 to 5 (0 being new and 5 being critical.) and in the process rated each of the following components of each bridge.

·        Bridge surface

·        Footway/Median

·        Parapet/Safety Barrier

·        Wing walls/Spandrel walls/Retaining walls.

·        Abutments

·        Deck

·        Riverbed

·        Structure in general

SDCC has no Level 5 rated bridge, one Level 4 rated bridge (which is closed), and four bridges rated at Level 3.

Roads Dept. have carried out a visual inspection on these four Level 3 rated bridges in the last week and have also inspected Lucan, Firhouse, Oldbawn and Rathfarnham Bridges.

Early in 2010, SDCC intends to employ a structural engineer to carry out a ‘Principal inspection’ on all the bridges rated Level 3 and upwards to assess what if any remedial action is needed. As part of the assessment, all bridges carrying a high volume of traffic over large flows of water will be inspected including Lucan, Firhouse, Oldbawn and Rathfarnham Bridges.