Office of Public Works Study to Identify Practical Measures to Address
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Office Of Public Works Study to identify practical measures to address flooding on the Dunkellin River including the Aggard Stream TOBIN CONSULTING ENGINEERS in association with ROYAL HASKONING Engineering Services, Tobin Consulting Engineers, Office of Public Works Fairgreen House, Head Office, Fairgreen Road, Galway Trim, Co. Meath, Royal Haskoning, 20 Lower Baggot Street, Dublin 2 June 2010 Study to Address Flooding on the Dunkellin River & Aggard Stream REPORT PROJECT: Study to identify Practical Measures to address Flooding on the Dunkellin River and Aggard Stream CLIENT: Engineering Services, Office Of Public Works, Head Office, Trim, Co. Meath. COMPANY: TOBIN Consulting Engineers in association with Royal Haskoning c/o TOBIN Consulting Engineers Fairgreen House, Fairgreen Road, Galway. www.tobin.ie i Study to Address Flooding on the Dunkellin River & Aggard Stream DOCUMENT AMENDMENT RECORD Client: Engineering Services – Office of Public Works Project: Study of Practical Measures to address Flooding on the Dunkellin River and Aggard Stream Title: Report on Measures to address Flooding on the Dunkellin River and Aggard Stream PROJECT NUMBER: 5968 DOCUMENT REF: 5968- TR01 Dunkellin Flood Alleviation Study B Final Issue MMcD 27-6-10 MMcD 27-6-10 MFG 28-6-10 A Issue to Client MMcD 10-6-10 MFG 10/6/10 MFG 10/6/10 Revision Description & Originated Date Checked Date Authorised Date Rationale TOBIN Consulting Engineers i Study to Address Flooding on the Dunkellin River & Aggard Stream TABLE OF CONTENTS 1 EXECUTIVE SUMMARY ...................................................................................1 2 INTRODUCTION ...............................................................................................3 2.1 BACKGROUND AND SCOPE OF THIS STUDY .............................................................. 3 2.2 EXTENT, LOCATION AND DESCRIPTION OF THE CATCHMENTS.............................. 5 3 EXISTING CATCHMENT CHARACTERISTICS................................................7 3.1 TOPOGRAPHY & HYDRAULIC CONTROLS ALONG THE DUNKELLIN RIVER ............ 7 3.1.1 Zone 1 – Craughwell River .................................................................................................... 9 3.1.2 Zone 2 – Rahasane Turlough .............................................................................................. 12 3.1.3 Zone 3 – Rinn Bridge to Kilcolgan ...................................................................................... 14 3.2 TOPOGRAPHY & HYDRAULIC CONTROLS ALONG THE AGGARD STREAM........... 17 3.3 EXISTING CONDITION OF THE DUNKELLIN RIVER AND AGGARD STREAM .......... 21 3.4 HISTORICAL FLOODING WITHIN THE CATCHMENTS ............................................... 23 3.4.1 Flood Event in January 2005 ............................................................................................... 23 3.4.2 Flood Event in November 2009 ........................................................................................... 24 3.5 HISTORICAL FLOWS REGIMES, PREVIOUS STUDIES & PROPOSED ARTERIAL DRAINAGE DESIGNS ............................................................................................................. 31 3.5.1 Historical Flow Regimes and Previous Studies.................................................................... 31 3.5.2 Formerly Proposed Arterial Drainage Schemes................................................................... 32 4 FLOOD FREQUENCY ANALYSIS..................................................................33 4.1 HYDROMETRIC STATIONS AND DATA USED IN THIS STUDY.................................. 33 4.2 METHODOLOGY............................................................................................................ 35 4.3 STATISTICAL METHODS .............................................................................................. 38 4.3.1 Extreme Value Distribution Type 1 ...................................................................................... 38 4.3.2 Extreme Value Distribution Type 1 (Gumbel)- Method of Moments ..................................... 38 4.3.3 Index Flood Method............................................................................................................. 39 4.3.4 Results of the Statistical Analysis ........................................................................................ 39 4.3.5 Discussion of Statistical Results at each Hydrometric Station.............................................. 47 4.4 ESTIMATED RETURN PERIOD FOR THE NOVEMBER 2009 EVENT......................... 50 4.5 CLIMATE CHANGE & FUTURE FLOW SCENARIOS.................................................... 51 5 DEVELOPMENT OF THE HYDRAULIC MODEL............................................54 5.1 MODEL BUILD................................................................................................................ 54 ii Study to Address Flooding on the Dunkellin River & Aggard Stream 5.2 KEY FEATURES OF THE MODEL................................................................................. 57 5.3 MODEL ASSUMPTIONS & ROUGHNESS CO-EFFICIENT........................................... 58 5.3.1 Model Assumptions ............................................................................................................. 58 5.3.2 Model Roughness Coefficient.............................................................................................. 58 5.3.3 Sensitivity Tests................................................................................................................... 59 5.4 MODEL CALIBRATION .................................................................................................. 60 5.5 STRATEGIC SCHEME MODELLING ............................................................................. 61 6 TESTING OF POSSIBLE FLOOD ALLEVIATION MEASURES .....................64 6.1 STAGE 1 - OVERVIEW OF POSSIBLE FLOOD ALLEVIATION MEASURES ............... 64 6.2 STAGE 2 - TESTING OF POSSIBLE FLOOD ALLEVIATION MEASURES ................... 66 6.2.1 Impact of Flood Alleviation Measures considered................................................................ 66 6.3 SUMMARY OF MODELLING SCENARIOS.................................................................... 81 6.4 MODELLING WORK ALONG THE AGGARD STREAM AND LACKAN CHANNEL....... 83 6.5 STAGE 3- DEVELOPMENT OF THE RECOMMENDED FLOOD ALLEVIATION SCHEME.................................................................................................................................. 84 6.6 ALTERNATIVE FLOOD MEASURES CONSIDERED .................................................... 91 7 ESTIMATED CAPITAL COSTS AND ASSESSMENT OF SCHEME BENEFITS 92 7.1 ESTIMATED CAPITAL COST OF THE PREFERRED SCHEME ................................... 92 7.2 ASSESSMENT OF SCHEME BENEFITS....................................................................... 96 7.2.1 Flood Damage to Residential Properties ............................................................................. 96 7.2.2 Road Disruption................................................................................................................... 98 7.2.3 Other Cost Benefits ............................................................................................................. 99 7.3 COST BENEFIT RATIO.................................................................................................. 99 8 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS .........................100 8.1 SUMMARY & CONCLUSIONS ..................................................................................... 100 8.2 RECOMMENDATIONS................................................................................................. 102 Appendices Appendix No. 1 Drawings Appendix No. 2 Graphical Representation of Hydrographs Appendix No. 3 Hydrometric Data Appendix No. 4 Main Culverts along the Aggard Stream and Monksfield River iii Study to Address Flooding on the Dunkellin River & Aggard Stream 1 EXECUTIVE SUMMARY Following the invitation to tender from the Office of Public Works, dated 8th February 2010 and the submission of a Tender proposal on 19th February 2010, TOBIN Consulting Engineers in association with Royal Haskoning Ireland, were commissioned on 10th March 2010 to undertake a Study to : “Identify Measures to address flooding on the Dunkellin River from Craughwell to Kilcolgan to include the Aggard Stream and its tributaries up to Cregaclare”. Following on from this study, Tobin Consulting Engineers in association with Royal Haskoning have proposed a costed package of flood remediation works along the Dunkellin River and the Aggard Stream to Cregaclare, which will accommodate the 100 Year Flood, with an allowance for climate change. These works are based on a developed hydraulic model of the Dunkellin River and the Aggard Stream, which includes the various bridge structures, culverts, and other hydraulic controls, and which has been used to test the proposed measures, in two Strategic Schemes, against the expected benefits of including these measures in the overall package of works. The Report describes the catchment topography, and the channel profile, from the headwaters, through Craughwell, into the Rahasane/Rinn Turlough system, and downstream to the sea at Kilcolgan. It recognises the karstic nature of the limestone bedrock, and draws upon previous work in describing its impact on Rahasane Turlough in particular. The flood events of January 2005 and November 2009 are described, to define the extent of flood damage, traffic and