Structural measures form the preferred options to be pursued for the Little Dargle Analysis Unit and the four APSRs/flood cells in the Dodder catchment where the flood risk is greatest. Details of the preferred options to be pursued are given in the option description sheets included as Appendix E.
Flood defences are proposed in the form of flood walls and/or embankments, with the type of defence determined by space availability, defence height and visual impact. One preferred option (at St Enda’s & Tara Hill) includes improvement of channel conveyance along with flood walls. Other structural measures assessed generally resulted in a lower MCA score than an option based solely on walls and embankments, nevertheless, these will be investigated in more detail as components of a scheme at the next stage of development in order to optimise the solution.
Improvement in channel conveyance, usually to remove minor or localised constrictions that could limit flow, may be considered if not already proposed, as part of the preferred options’ works, at detailed design stage.
For any structural works, operation and maintenance procedures should be prepared and budget provision made. The cost estimates include for this and it will be important to continue the effective functioning of any structure and prolong its design life. Flood walls need little attention other than periodic inspection and repair as necessary. Embankments are susceptible to settlement and crest degradation where they are accessible to people, animals or vehicles, and they need more frequent inspection and rectification of any defects. Where defences incorporate gates or other mechanical components, regular inspection and maintenance will be provided. Dredged channels can silt up again reducing the channel capacity. Regular inspection and proactive maintenance will be required to sustain the channel conveyance.
8.4.1 Preferred Option details
At Analysis Unit level a preferred option was found for the Little Dargle which consists of 32m of earth embankment approximately 1m high at the downstream extent of the stream. No other preferred options were found at AU level and the level of assessment was therefore focused on the smaller APSR/flood cell level.
Several preferred options were found at APSR/flood cell level. On the Dodder River the area downstream of Donnybrook, excluding the existing protected areas from the current and scheduled flood defences, was considered. A preferred option was found which consists of 1,683m of flood wall and 351m of earth embankment on both sides of the river bank averaging 1.2m high. Further upstream the area encompassing Shanagarry apartments and the Smurfit site was considered at APSR level and a preferred option found. This option consists of 250m of flood wall and 120m of earth embankment on both sides of the river bank. One other area along the Dodder River was identified at APSR level for which a preferred option was found, Orwell Gardens. This option consists of a short 24m section of flood wall and 29m of embankment.
All of the Dodder tributaries were assessed and various areas identified at APSR level. Of these, one APSR on the Whitechurch Stream (called Tara Hill & St Endas) produced a preferred option. This option consists of 285m of flood wall, averaging 0.7m high, 702m of dredging along the stream and the removal of two weirs.
8.4.2 Existing Defences
The Study has identified a number of existing defence assets on the Lower Dodder. Proactive maintenance of these defences should be undertaken where relevant.
8.5 INDIVIDUAL RISK RECEPTORS
Flood risk management of the individual risk receptors is subject to discussion with their owners to agree an appropriate course of action and responsibility for it. From Section 6, it is unlikely that flood protection for individual assets would be justified, except if they are within the coverage of the preferred options for the respective APSR or Analysis Unit.
Several of the risk receptors are in the Lower Dodder, Donnybrook APSR, where the preferred option is for permanent flood defence works and, if implemented, this will solve the problem for the receptors or infrastructures at risk. The proposed timescale for
implementation of preferred option is by 2015. The owners of the assets have the option to take action to fit their own programme and resources.
The Dundrum Shopping Centre and theatre, ESB sub-station, Dodder Road Lower and N81 Tallaght Bypass do not fall within the protection of APSR defences and no positive BCR option could be identified for these individual risk receptors.
Proactive planning for diversion arrangements for flooded roads and alternative bus services for flooded railways will alleviate the situation for transport infrastructure. For utilities infrastructure such as water and waste water treatment plants, flood alleviation can be achieved through provision of flood defences, maintenance of existing defences, or emergency planning for closure of the plants during floods and alternative supply arrangements, or even closure and re-location of the plant. The owners of the receptors, usually the local authorities, will be consulted to agree the action to take.
Table 8-2 anticipates the possible outcome of discussions of the risk receptors with their owners, and adoption of the FRMP components in Table 8-1.
The maintenance element of the system and its defences is a key support measure to both existing and proposed hard defences and to the safeguarding of channel conveyance. The purpose of this maintenance plan is to provide the participating Local Authorities with a tool to enable them to direct risk based detailed inspections and prioritised maintenance programmes to manage effectively the reaches of the Dodder within their respective operational controls.
Table 8-2 Possible Solutions for Individual Risk Receptors
Risk receptor Owner AU/APSR Possible solution
AIB Centre
Ballsbridge (off Merrion Road and Serpentine Road)
AIB Group Dodder/DS of
Donnybrook
APSR defences
AVIVA Rugby & Soccer Stadium
IRFU & FAI (LRSDC) Dodder/DS of Donnybrook
APSR defences Royal Dublin Society
(RDS)
RDS Dodder/DS of
Donnybrook
APSR defences Shelbourne Park Dog
Track Irish Greyhound Board / Shelbourne Greyhound Stadium Limited Dodder/DS of Donnybrook APSR defences Marian College School Marist Brothers managed by the Board of Management Dodder/DS of Donnybrook APSR defences
American, Israeli and
Czech Republic Embassy's on Northumberland Road. Respective countries governments Dodder/DS of Donnybrook APSR defences Church of Ireland, Anglesea Road.
Church of Ireland Dodder/DS of Donnybrook
APSR defences Dublin Bus, Beaver
Row, Donnybrook.
Dublin Bus Dodder/DS of
Donnybrook
Risk receptor Owner AU/APSR Possible solution Dart Line at Lansdowne Road Bridge and Serpentine Avenue crossing.
Irish Rail Dodder/DS of
Donnybrook
APSR defences
Merrion Cricket Pitch Occasionally used as parking for events in RDS.
Merrion Cricket Club Dodder/DS of Donnybrook
APSR defences
Leinster and Old
Wesley Rugby
football ground, Donnybrook.
Old Wesley RFC Dodder/DS of
Donnybrook APSR defences Major Roadways disrupted. Shelbourne Road, Merrion Road, Northumberland Road, Donnybrook Road (N11), Stillorgan Road (N11), Clonskeagh Road.
Local Authority Dodder/DS of
Donnybrook
APSR defences
Dundrum Shopping Centre & Theatre
Crossidge Developments
Dundrum Slang Localised flood defences. ESB sub-station,
Dodder Road Lower,
ESB Dodder Localised flood defences
or relocation of sub- station. Short term
arrangements for
temporary road
diversion.
N81 Tallaght Bypass Local Authority Tallaght Short term arrangements for temporary road diversion.