Flood alleviationbrief170214

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Flood alleviationbrief170214

  1. 1. Regional Projects & Emergency Services Division, Environment & Engineering Department, Room 409, 68/70, Marrowbone Lane, Dublin 8. Rannog Regiunach Tionscadalí agus Seirbhisi Eigeandala, Roinn Comhshaoil agus Innealtóireachta Seomra 409, 68/70 Sraid Naomh Mhuire, Baile Átha Cliath 8. T: +353 (0) 1 222 4302 F: +353 (0) 1 4546435 E. gerry.oconnell@dublincity.ie Ref:-OPW261113ol1 Re: Clontarf Promenade Development and Flood Alleviation Measures - Alfie Byrne Road to Wooden Bridge. Preliminary Flood Alleviation Project Brief Clontarf Promenade between Alfie Byrne Road and the wooden Bridge Bull Wall Road, is an expanse of linear park, cycle track, walkways, trees, public seating, viewing points, art features, etc over a length of 3.5km and varying width of generally 30-40m. A flood defence embankment was proposed to be constructed in 2011 but due to public disquiet it was rejected. Subsequently, the local residents and businesses met with Dublin City Council to try and find a way forward for development of the Promenade including some form of flood defences. Following the recommendation of a Conciliator between Clontarf Residents Association (CRA), Clontarf Business Association (CBA) and various departments of Dublin City Council (DCC) a Joint Working Group (JWG) was set up in February 2013 to try and agree a strategy for the development of this portion of the Clontarf Promenade including flood alleviation measures. This group has had nine meetings in total to date. The first four concentrating on the development of the community and city asset which is the linear park, it’s recreational, environmental and linkage values for the people of Clontarf, Dublin City and visitors. The next four meetings discussed the need for flood alleviation measures, the requirements of any flood defence and the building blocks of the height of any coastal flood defence; static flood level, wave height, wave overtopping, climate change allowance and freeboard (safety) allowance. It was agreed by the JWG that some form of flood defence was required, but any proposal should minimise the visual impact to the Promenade, both with respect to its magnificent views, existing features and with regard to the health and safety of people using this amenity, particularly at night. A vision statement was produced by the JWG of the current assets of the Promenade Park and ideas for future enhancement. The JWG thought that many of the existing features of the Promenade were sufficiently good to be retained and any development should not be such as to make it unrecognisable from its current character. It was also recognised by the JWG that a preliminary proposal for the flood alleviation measures would have to be agreed first as these will probably have a significant effect on development of the Promenade. At meetings five to eight which included advice from various consultants and the Office of Public Works, the JWG considered various flood alleviation options for the Promenade, raising the existing sea wall, putting a floodwall in the green area of the promenade, the rejected 2011 proposal and putting a floodwall adjacent to the footpath beside the Clontarf roadway. 1
  2. 2. It was noted that the existing sea wall does not extend the full length of the Promenade and is absent for circa 300m at the western end where it meets the Alfie Byrne Road. Following this analysis, the JWG agreed that, based on the available data the flood alleviation proposal which is most likely to minimise the visual impact to the Promenade Park and leave as much of it as possible for current usage or development would be a dual flood defence comprising the existing sea wall and a secondary defence as close as possible to Clontarf Road and that this proposal would be put forward for computer modelling analysis. It was also agreed that in order to ensure that this strategy operates along the full length of the Promenade, an extension of the existing sea wall of similar height would be necessary at the Alfie Byrne Road end. This dual defence strategy involves the following: leaving the existing sea wall generally as it is but extending it to minimum health and safety levels (1.1m, 3’ 8”) at the Alfie Byrne Road end where it is missing and providing a second wall with flood gates as near as possible to Clontarf roadway for as much of the Promenade as it would be required. The existing sea wall defends against the current 1 in 200 year tide level of 3.15-3.2m Malin Head and also breaks up any overtopping waves. The Promenade area will be sacrificed to flood waters in extreme events and its length and grass surface will be used to advantage to contain and absorb flood waters. The secondary wall will only have to cater for secondary waves, climate change and freeboard thus reducing it’s height to a minimum. The car parks and all of the flood gates will be closed following warnings of very high tides; which are normally issued two days in advance. This brief is therefore to carry out sufficient hydraulic design and structural design to produce preliminary drawings for discussion by the JWG and the public in general in order to finalise a preliminary planning proposal for this revised Promenade development scheme and to carry out preliminary costings for the scheme to ensure that funding can be provided for it. Other options to protect certain higher risk areas of the promenade from flooding first, require a full flood scheme to be developed, agreed and a funding stream for it secured. The vision statement for the Promenade by the JWG is attached, plus a landscape analysis presentation, by DCC Parks and Landscape Division which divides the Promenade into eight cells and highlights the main current assets of each cell. The nodal points at each end and where major roadways, (Vernon Avenue, Oulton Road, Hollybrook Road/St. Lawrence Road, Castle Avenue/Haddon Road & Conquer Hill Road), meet the Promenade are of particular importance. Brief for consultant. 1. Read all previous reports on Clontarf Promenade Development from McCarthy Hyder (liquidated), Royal Haskoning (now Royal Haskoning DHV) and others supplied. 2. Review 100 year and 200 year static water levels. 3. Review local generated wave heights, speeds and periods for current 100 year and 200 year events. Use information from RPS Belfast and Royal Haskoning DHV in this review. 4. Review OPW requirements for static seawater height, wave height, wave over topping, climate change and freeboard allowances. 5. Use joint probability to calculate required defence levels at the existing sea wall and adjacent to pathway beside Clontarf Road for 100 year and 200 year events. These calculations are to be based on a dual defence system, using the existing sea wall, extended to the corner adjacent to Alfie Byrne Road at a height of 1.1m above ground 2
  3. 3. level and a mainly new wall on the sea side of Clontarf Road Footpath beside the Promenade. This analysis is to include the breaking up of the primary waves at the sea wall, filling up of the promenade area with sea water, generation of secondary waves in the Promenade area to the second wall which will have a number of closable floodgates to allow pedestrian access. Escape of floodwaters seaward after flood event and the use of nonreturn valves. Calculations of the height of the second wall should take into account the distance between it and the sea wall and the beneficially role the Promenade will play in alleviating flooding. The primary objective is to reduce the height of the secondary flood wall to its minimum while maintaining the statutory flood protection levels required by the Office of Public Works. 6. Carry out preliminary structural design of the new walls required. Comment of the strength of the existing sea wall. Carry out a review of the aesthetic design of the defences, including review of possible artistic approaches, material finishes, plus maintenance and cost implications. 7. Produce preliminary drawings in plan with at least 20 cross-sections from the sea side of the existing buildings on the opposite side of Clontarf Road with top levels and ground levels of each proposed defence. Typical wall thicknesses and foundation cross-sections are also required for proposed walls. These drawings will provide accurate flood defence levels required to inform the subsequent design process. 8. Consultancy budget. Preliminary costings estimate of flood alleviation elements of the scheme. 9. Allow for presentation of findings to Joint Working Group and one public consultation day with subsequent alterations to documents. 10. Advise on revised EIS/EIA to An Bord Pleanala. Advise on best planning procurement route. 11. Put preliminary programme together for hydraulic analysis, preliminary design, promenade development preliminary design, planning procedures, detailed design, contractor procurement, construction, maintain period, handover. Look at programme with EIS requirements and programme if EIS is not required. 12. Comment on areas of promenade suitable for significant development post flood defence construction and access to these, taking vision statement into account. Consider in particular the node points at both end of the Promenade and the junctions of Clontarf Road with other significant roadways. Gerard O’Connell, Engineer-in-Charge, Regional Projects & Emergency Services Division. 3

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