Newlands Cross Upgrade EIS - European Investment Bank

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South Dublin County Council N7 Newlands Cross Upgrade Environmental Impact Statement Arup Consulting Engineers • The aquatic environment: designated conservation areas associated with each catchment, aquatic and riparian habitats and fisheries value; • Water Quality: biological quality. 3.13.2 Drainage 3.13.2.1 Option 1 – Overpass The N7 mainline Drainage East of Newlands Cross shall tie in with the existing drainage and continue to its existing outfall at the Red Cow Roundabout. The N7 mainline Drainage West of Newlands Cross shall outfall to the existing outfalls, southeast of Newlands Cross for westbound and median drainage, and into the Fonthill Road drainage network for eastbound verge drainage. The drainage on the structure shall be composed of composite kerb drains, outfalling to the mainline drainage network. The Fonthill Road drainage shall tie in to existing drainage and therefore continue to outfall to the Camac River catchment, where longitudinal gradients allow. The ramps from Fonthill Road to the N7 mainline shall outfall to the existing drainage network. The Belgard Road drainage network shall continue to outfall to the watercourse adjacent to it. The ramps from Belgard Road to the N7 mainline shall also outfall at this location. The discharge peak flows at each outfall will be increased post construction due to increased impermeable road surfaces; therefore flow regulation may be required depending on the capacity of receiving watercourses. This may be provided by means of an attenuation pond, or by oversizing drainage pipes and using a flow regulating device (such as HydroBrake or equivalent) where the proposed drainage outfalls. In summary, the overpass option allows both the existing outfalls to be utilised, and the additional flow generated by the upgrade will be split between them. 3.13.2.2 Option 2 – Underpass The N7 mainline drainage shall have a low point at Newlands Cross some depth below the existing ground level. Where longitudinal gradients necessitate, all mainline drainage should converge at this point. From there it needs to outfall to a point of the same level or lower. To do this requires piping the drainage in a deep trench pipe towards the Red Cow roundabout. This method would be expensive and cause a lot of disruption during construction, but would be cheaper to operate and easier to maintain. The alternative is to install a pumping station at the southeast of Newlands Cross, and pump the drainage to the existing outfall. Fonthill Road drainage should outfall to existing drainage, and thus the Camac River catchment. Ramps to the west of Newlands Cross should be piped to the outfall at Belgard Road. Ramps to the east of Newlands Cross should outfall to the existing drainage and outfall at the Red Cow Roundabout. Belgard Road drainage should outfall to the adjacent outfall location. Should attenuation be required, there is area available in the lands made available for a pond at the location. Attenuation may also be provided through the use of oversized pipes, and flow control devices such as HydroBrake or equivalent. Drainage on the structure shall be composed of composite kerb drains, and will connect with the Belgard road drainage network, and thus outfall to the adjacent watercourse. It would be necessary to meticulously maintain drains with this option as blocked drains could result in a severe flooding problem in the event of heavy or sustained rainfall. December 2007 Page 63
South Dublin County Council N7 Newlands Cross Upgrade Environmental Impact Statement Arup Consulting Engineers In summary, the underpass option makes it difficult to utilise existing outfalls without pumping. There is little land available to install a pump at the Fonthill road outfall, therefore most of the mainline drainage needs to be discharged at the Belgard road watercourse. This will significantly increase the discharge flows at this location. 3.13.3 Water Quality 3.13.3.1 Option 1 – Overpass There is the potential to significantly impact on the water quality of Ballymount Park and the Camac River during construction. The extent of the risk depends on the proximity of construction activity to the watercourse and sensitivity of the watercourse. If mitigation measures are implemented the potential risk is minimised. There are operational risks from road runoff, winter maintenance and accidental spillage. Runoff from such sources would be split between the Camac River and Ballymount Park. Again, if mitigation measures are implemented the potential risk is minimised. 3.13.3.2 Option 2 – Underpass The construction and operational risks here are the same as for the other option. However, the requirement for a pump to remove water from the underpass means that, rather than a constant flow, pollutants will be pulsed/pumped in large quantities into the drainage catchments. This is coupled with the fact that all drainage from the underpass option will drain to the Ballymount Park catchment rather than being split between the two catchments as with the other options. 3.13.4 Aquatic Ecology 3.13.4.1 Option 1 – Overpass Operational runoff from the overpass will be on a constant basis which will enable ecosystems to adjust over time. However, if mitigation measures are implemented the potential risk is minimised. 3.13.4.2 Option 2 – Underpass Runoff from the underpass could overload the system. Also, a pulse of water at the wrong time i.e. during spawning, could be critical for the wildlife. Again, however, if mitigation measures are implemented the potential risk is minimised. 3.14 Economics Davis Langdon PKS were commissioned by Arup Consulting Engineers to prepare a cost estimate of both options for the proposed development. 3.14.1 Summary of Costs A summary of the construction costs for both the Overpass and Underpass Options are depicted in Table 3.18. Table 3.18: Overall Summary of Construction Costs OPTION OVERPASS UNDERPASS Total Construction Cost incl. VAT and PVC €53,064,067 €78,759,566 December 2007 Page 64
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PLATES
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APPENDIX A5 - TRAFFIC AND TRANSPORT
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0 kilometres 2 Scale N.T.S Ordnance
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APPENDIX A8 - NOISE AND VIBRATION
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APPENDIX A9 - AIR QUALITY
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APPENDIX A11 - ECOLOGY
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APPENDIX A12 - ARCHAEOLOGY, ARCHITE
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