Proposed Alterations and Extension to 17 St. John's Road, Sandymount, Dublin 4

Transcription

1 Proposed Alterations and Extension to 17 St. John's Road, Sandymount, Dublin 4 Engineering Report April, Issue 1 Casey O Rourke Associates Structural & Civil Consulting Engineers 77 Merrion Square Dublin 2 Tel: Fax: E: info@cora.ie

2 Table of Contents 1 Introduction General Proposed Scheme Reference Publications used in the production of this Report 1 2 Foul Waste Discharge 1 3 Surface Water Discharge 2 4 Flood Risk Assessment Scope Sources of Potential Flooding Conclusion 4 5 Appendix A Error! Bookmark not defined. Existing Public Record Drawings Error! Bookmark not defined. 6 Appendix B 5 Flooding Local Area Report 5 7 Appendix C 7 Soakaway Calculations 7 8 Appendix D 9 Non-Return Valve 9 9 Appendix E 12 Flood Barrier Details 12

3 1 Introduction 1.1 General This report addresses the foul and surface water drainage for the proposed development works to the site at 17 St. John's Road, Sandymount, Dublin 4 The site currently houses a 2 storey residential end of terrace property. The house is currently drained by means of combined drains, conveying both foul and surface water drainage to the existing private drain located to the front of the property on St. Johns Road. The location of the private drain is indicated on CORA drawing 1422-C Proposed It is proposed to construct a single storey extension to the rear of the property. A review of the Office of Public Works records in relation to flooding reveals that there is no history of flooding on the site. There is however records for flood events occurring at the intersection of Park Avenue and St. Johns Road. A flood risk assessment has been carried out and details are contained in Section 4. The extended property will provide 3No. double bedrooms and 1No. single bedrooms bringing the maximum bedspaces to 7. For the purposes of calculations, a PE of 7 will be adopted. 1.2 Proposed Scheme In order to comply with modern drainage standards, it is proposed to drain the foul and surface water generated from the proposed extension using a completely separate network. The rainwater falling on the roof areas will flow by gravity via the surface water network. The foul sewage generate will flow by gravity to the existing connection to the private sewer at the front of the site. Surface water from the extension and rear pitch of the main roof will be collected by a gravity system and re-used within the development by implementing rainwater harvesting. 1.3 Reference Publications used in the production of this Report Code of Practice Wastewater Treatment and Disposal Systems Serving Single Houses (p.e. 10) Greater Dublin Strategic Drainage Study Volumes 1 to 6 Greater Dublin Regional Code of Practice for Drainage Works Version 6.0 Technical Guidance Documents Part B Recommendations for Site Development Works for Housing Areas 2 Foul Waste Discharge In order to calculate the volume of foul sewage generate on site, the following calculations have been adopted: No. of Bedspaces (House): 7 Population Equivalent: 7PE Wastewater Loading: 225Litres/person/day Casey O Rourke Associates Issue No. 01 April 2014 Page 1 of 14

4 Flow Rate: 1,575Litres/day The above discharge volume represents a discharge flow (1 DWF) of approximately 0.035Litres/second and a design flow (6 DWF) of 0.22Litres/second which is well within the capacity of the proposed 100mm diameter connection which installed at a fall of approximately 1:80 which would have an approximate capacity of 6.3Litres/second. Details of the proposed network are detailed on drawing number 1361 C01. 3 Surface Water Discharge It is proposed to dispose of all surface water generated on the roof areas, including the roof to the single story extension along with the existing pitched roof from the original building by means of a soakaway located in the rear garden. Rainwater falling on hard landscaping areas will be discharged directly to ground by means of a permeable paving systems such as gravel, permeable paving etc. A preliminary design of the soakaway has been carried out based on an overall roof area of 140m 2. Calculations in accordance with BRE 365 indicated a soakaway of 4m x 2m x 0.61m deep will adequately cater for the surface water generated on the site. The storm water sewers have been designed using the Modified Rational Method and a storm return period of 2 years in accordance with the recommendations of Recommendations for Site Development Works for Housing Areas. Calculations in accordance with BRE 365 are contained within Appendix B of this report. 4 Flood Risk Assessment The purpose of the flood risk assessment is to both identify and quantify the potential of flooding of property from all potential flood risks including coastal, pluvial, pluvial flooding and from public sewers and groundwater. The risk will be quantified on a numerical scale from 1 (Very Low) to 5 (Very High) 4.1 Scope The compilation of the following flood risk assessment is based on a desk study compiling information obtained from the Local Authority along with Dublin City Council Flood Resilience City Projects Office. Information was also obtained from Fingal East Meath Flood Risk Assessment and Management Study (FEM-FRAMS), Ordnance Survey maps, On-site topographical survey and the Office of Public Works (OPW) flood maps and reports. 4.2 Sources of Potential Flooding The following potential sources of flooding have been reviewed and evaluated for this particular site: Casey O Rourke Associates Issue No. 01 April 2014 Page 2 of 14

5 4.2.1 Coastal Reviewing the records contained on the OPW s floodmaps.ie website, there are no records of flooding to the site. A copy of the record results are contained within appendix A. The ground floor level of the existing development is 1.60m AOD to the front. The level is lower than desired. Given the proximity of the site to the seafront, coastal flooding cannot be ruled out. In order to offer protection to the site during periods of high tides, it is proposed to install flood barriers to the front and rear entrance doors. It is also proposed to replace the existing suspended timber floors with a lined concrete floor to prevent the entry of flood water through floor vents etc. Based on the above, we believe that the risk of coastal flooding to the site can be rated as 4 (High) Fluvial Fluvial or river flooding occurs where the capacity of the conveying watercourse (river or stream) is exceeded as a result of heavy rainfall or blockage and water overspills the banks of the water course. Given that there are no watercourses in the vicinity of the site, the risk posed from Fluvial sources can be classed as 1 (Very Low) Pluvial Pluvial or over ground flooding occurs as a result of water flowing over the surface of the ground which can occur where the infiltration of the receiving ground is exceeded. The minimum proposed floor level of the property is 1.60m. Whilst the finished floor level of the property is 460mm above the road level to St. Johns Road, In order to minimise the risk of pluvial flooding, the introduction of flood barriers (as outlined in above) is to be carried out. Given the above, the risk of pluvial flooding can be classed as 4 (High) Flooding from Public Sewers The site is served by a 225mm diameter foul sewer located to the east of the property on St. Johns Road. It is proposed to install a non-return valve on the outfall sewer from the site to prevent back flow into the property. An "Aco Quatrix" valve is proposed to the outfall located in the front garden of the property which will prevent back flow in the event of a sewer surcharge. Given the above, the risk of flooding from the public sewer can be classed as 3 (Average). Casey O Rourke Associates Issue No. 01 April 2014 Page 3 of 14

6 4.2.5 Flooding from Ground Water Sources Flooding as a result of groundwater occurs when the level of the underlying water table rises, as a result of sustained rainfall, to a level exceeding the ground level. Given the likely permeable nature of the soil on the site coupled with the topographical levels would make flooding as a result of ground water highly unlikely. Given the above, the risk of flooding from ground water sources can be classed as 2 (Low). 4.3 Conclusion Based on the information outlined above, we are of the opinion that the current property, in it's current configuration has a high risk of flooding in th future. Whilst there is no history of flooding to the site is on record, measures are to be adopted to minimise the risk of flooding in the future. The measures include: Installation of flood barriers to all entrance doors. Installation of concrete ground floor slab. Installation of non-return valve to outfall sewer. Taking the above remedial measures into account, the site has an overall risk level of 3 (Average). Casey O Rourke Associates Issue No. 01 April 2014 Page 4 of 14

7 5 Appendix A Flooding Local Area Report Casey O Rourke Associates Issue No. 01 April 2014 Page 5 of 14

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9 6 Appendix B Soakaway Calculations Casey O Rourke Associates Issue No. 01 April 2014 Page 7 of 14

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11 7 Appendix C Non-Return Valve Casey O Rourke Associates Issue No. 01 April 2014 Page 9 of 14

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14 8 Appendix D Flood Barrier Details Casey O Rourke Associates Issue No. 01 April 2014 Page 12 of 14

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