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 14.4.2 Construction Impacts 14.4.2.1 Drainage Culverts and Bridges There are no culverts or drainage related bridges located within the boundaries of the Newlands Cross scheme. Carriageway Drainage It is proposed to utilise the existing drainage outfalls post construction where practical. The outfall peak flows will be increased post construction due to increased impermeable road surfaces. In accordance with the NRA DMRB Volume 4 9 , HD33/96, Section 6.3, the most severe event a drainage system must accommodate is a 1 in 5 year storm. Estimated maximum peak flows pre and post construction are indicated in Table 14.4. These estimated maximum peak flows indicate that flow regulation is required to ensure that the capacity of receiving watercourses will not be impacted upon. Oversized pipes and Hydrobrake flow regulation devices will be utilised in this capacity to maintain the existing flow rates. The drainage system will be designed to the 1 in 5 year storm event standard as required but the oversized pipes and Hydrobrake flow regulation system can accommodate up to a 1 in 100 year storm event. An overflow outfall will be incorporated into the Ballymount Park outfall so that in the occurrence of a storm event exceeding a 1 in 100 year Average Return Interval, the excess flow will enter the Ballymount Park surface water system which will act as a natural attenuation mechanism upstream of the Camac River. This level of flow regulation is cognisant of the Greater Dublin Strategic Drainage Study policy for stormwater management (GDSDS Volume 3 Chapter 4 Paragraph 4.2 – Dublin City Council Stormwater Management Policy). Table 14.4 Estimated Maximum Flow Rates Pre and Post Construction (Flow Rates L/Sec). Pre-Construction Storm Event Ballymount Outfall Fonthill Outfall 2 Year Storm 300 160 5 Year Storm 360 200 50 Year Storm 600 300 Post Construction 2 Year Storm 360 230 5 Year Storm 430 310 50 Year Storm 630 430 Outfalls It is proposed that the upgraded road drainage networks will discharge to the same receiving watercourses as it does currently. As a result of the proposed attenuation measures, the impact of the proposed scheme on the flow is not significant for any of the receiving watercourses. 14.4.2.2 Water Quality Construction activities pose a significant risk to watercourses. The main contaminants arising from construction activities include: • Silt: elevated silt loading in surface water discharge may result from construction activities. Elevated silt loading leads to long term damage to aquatic ecosystems by clogging the gills of fish and smothering spawning grounds. Chemical contaminants bind to the organic particles attached to silt which can lead to increased bioavailability of these contaminants. Silt also stunts aquatic plant growth, limiting dissolved oxygen supplies and December 2007 Page 227
South Dublin County Council N7 Newlands Cross Upgrade Environmental Impact Statement Arup Consulting Engineers reducing the aquatic ecosystems quality. Silt can also contribute to flooding when it deposits, reducing the carrying capacity of the system and potentially causing blockages. • Hydrocarbons: accidental spillage from construction plant and storage depots. • Faecal coliform contamination from inadequate containment and treatment of on-site toilet and washing facilities. The above risks are significant and are evaluated on the basis that the mitigation measures outlined in section 14.5 are carried out. The extent of the risk of these impacts is determined by the proximity of the construction activity to the watercourse, and the sensitivity of the watercourse. Installation of oil / petrol interceptors, and the spill containment facilities will not pose a risk to the local watercourse as there is a sufficient distance between the outfall and the larger surface water features in Ballymount Park. 14.4.2.3 Aquatic Ecology The impacts on aquatic ecology are specific to each watercourse and are discussed below: Ballymount Park and the Camac River At present, surface water from the N7 at Newlands Cross as far as the M50, and the northern section of the Belgard Road drain to Ballymount Park and subsequently the Camac River. The upgraded junction will also drain to this surface water system. There is a risk of siltation to Ballymount Park associated with the junction upgrade earthworks, installation of petrol interceptors and the emergency spill containment facilities which could potentially result in a minor adverse impact. The emergency spill containment facilities will be installed on both sides of the junction. The structures will be two 20 m³ tanks located at both the Fonthill Road outfall and the Ballymount Park outfall, these tanks will be buried and grassed over, thereby confining impacts to the construction phase. This will constitute a potential minor adverse impact. Dodder River The Dodder River will not be affected during the construction phase of the project. The N7 and Newlands Cross Junction are a significant distance from this river system to have no impact during construction. Construction work on the northern section of the Belgard Road will not affect the Dodder River as surface water drains to Ballymount Park in this area. Poddle River The Poddle River will not be affected during the construction phase of the project. The N7 and Newlands Cross Junction are a significant distance from this river system to have no impact during construction. Construction work on the northern section of the Belgard Road will not affect the Poddle River as surface water drains to Ballymount Park in this area. 14.4.3 Operational Impacts 14.4.3.1 Drainage Culverts and Bridges There are no culverts or drainage related bridges located within the boundaries of the Newlands Cross scheme. December 2007 Page 228
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Page 288: PLATES
Page 292: APPENDIX A5 - TRAFFIC AND TRANSPORT
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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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Newlands Cross Upgrade EIS - European Investment Bank

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