South Western CFRAM Study
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South Western CFRAM Study Hydrology Report, Unit of Management 19 June 2016 The Office of Public Works South Western CFRAM Study 296235 IWE CCW R015 C C:\Users\pig44561\Desktop\296235-IWE-CCW-R015-C-Hydrology Report UoM19.docx 24 June 2016 Hydrology Report, South Western CFRAM Study Unit of Management 19 Hydrology Report,Unit of Management 19 June 2016 The Office of Public Works Trim Co. Meath Mott MacDonald, 5 Eastgate Avenue, Eastgate, Little Island, Cork, Ireland T +353 (0)21 4809 800 F +353 (0)21 4809 801 W www.mottmac.com South Western CFRAM Study Hydrology Report,Unit of Management 19 Issue and revision record Revision Date Originator Checker Approver Description Standard A September 2013 M Piggott R Gamble R Gamble Draft B February 2014 M Piggott R Gamble R Gamble Draft Final C June 2016 M Piggott C Hetmank C Hetmank Final This document is issued for the party which commissioned it and We accept no responsibility for the consequences of this for specific purposes connected with the above-captioned project document being relied upon by any other party, or being used only. It should not be relied upon by any other party or used for for any other purpose, or containing any error or omission any other purpose. which is due to an error or omission in data supplied to us by other parties. This document contains confidential information and proprietary intellectual property. It should not be shown to other parties without consent from us and from the party which commissioned it.. 296235 /IWE/CCW/R015/C 24 June 2016 C:\Users\pig44561\Desktop\296235-IWE-CCW-R015-C-Hydrology Report UoM19.docx S outh Western CFRAM Study Hydrology Report,Unit of Management 19 Contents Chapter Title Page Executive Summary i 1 Introduction 1 1.1 Context of the CFRAM Study __________________________________________________________ 1 1.2 SW CFRAM Study Process ___________________________________________________________ 1 1.3 Report Structure ____________________________________________________________________ 2 1.4 Flood Probabilities __________________________________________________________________ 3 2 Description of the Study Area 5 2.1 Extent ____________________________________________________________________________ 5 2.2 Rivers ____________________________________________________________________________ 7 2.3 Coastal Features ___________________________________________________________________ 7 2.4 Topography _______________________________________________________________________ 8 2.5 Rainfall ___________________________________________________________________________ 8 2.6 Geology _________________________________________________________________________ 12 2.7 Land Use ________________________________________________________________________ 12 3 Data Collection and Review 13 3.1 Available Data ____________________________________________________________________ 13 3.2 River Gauge Data __________________________________________________________________ 13 3.3 Rainfall Data ______________________________________________________________________ 16 3.4 Coastal Data ______________________________________________________________________ 19 4 Historical Flood Review 21 4.1 Historical Flood Events ______________________________________________________________ 21 4.2 Historical Flood Mechanisms _________________________________________________________ 24 4.3 Historical Flood Frequency Estimates __________________________________________________ 25 5 Design Flows 28 5.1 Overview ________________________________________________________________________ 28 5.2 Definition of Sub-Catchments _________________________________________________________ 28 5.3 Flood Frequency Analysis ___________________________________________________________ 31 5.4 Hydrograph Generation _____________________________________________________________ 36 5.5 Coastal Conditions _________________________________________________________________ 39 6 Calibration, Sensitivity and Uncertainty 42 6.1 Flow Determination for Model Calibration ________________________________________________ 42 6.2 Uncertainty and Sensitivity Testing_____________________________________________________ 48 7 Summary of Design Hydrology 50 296235 /IWE/CCW/R015/C 24 June 2016 C:\Users\pig44561\Desktop\296235-IWE-CCW-R015-C-Hydrology Report UoM19.docx South Western CFRAM Study Hydrology Report,Unit of Management 19 8 Considerations for Hydrological and Hydraulic Model Integration 53 8.1 Inflows __________________________________________________________________________ 53 8.2 Downstream Conditions _____________________________________________________________ 54 9 Hydrogeomorphology 55 9.1 Approach ________________________________________________________________________ 55 9.2 Assessment ______________________________________________________________________ 55 9.3 Impact on Flood Risk _______________________________________________________________ 58 10 Joint Probability 59 10.1 Overview ________________________________________________________________________ 59 10.2 Fluvial-Fluvial Dependence __________________________________________________________ 59 10.3 Fluvial-Coastal Dependence _________________________________________________________ 61 11 Future Scenarios 62 11.1 Potential Climate Changes ___________________________________________________________ 62 11.2 Potential Catchment Changes ________________________________________________________ 62 11.3 Design Future Scenario Conditions ____________________________________________________ 64 12 Conclusions, Key Findings and Recommendations 66 12.1 Conclusions and Key Findings ________________________________________________________ 66 12.2 Recommendations _________________________________________________________________ 68 Glossary 69 296235 /IWE/CCW/R015/C 24 June 2016 C:\Users\pig44561\Desktop\296235-IWE-CCW-R015-C-Hydrology Report UoM19.docx South Western CFRAM Study Hydrology Report,Unit of Management 19 Executive Summary The Office of Public Works (OPW) is undertaking six catchment-based flood risk assessment and management (CFRAM) studies to identify and map areas across Ireland which are at existing and potential future risk of flooding. Mott MacDonald Ireland Ltd. has been appointed by the OPW to assess flood risk and develop flood risk management options in the South Western River Basin District. This hydrology report is one of a series of reports being produced as part of the South Western Catchment Flood Risk Assessment and Management Study (SW CFRAM Study). This report details the assessment of the hydrological conditions at the following locations within Unit of Management 19: Ballingeary Castlemartyr Killeagh The downstream reaches of the River Womanagh This report does not review or update flows for the wider Lee catchment which have been assessed separately under the River Lee Pilot CFRAM Study. A review and analysis of historical flood events, hydrometric data and hydrogeomorphological processes has highlighted flooding issues to urban areas and nationally important infrastructure from the River Womanagh, Upper Lee, Bunsheelin River and a number of smaller tributaries. The Flood Studies Update methodologies have been used to determine the current design peak flows for eight specified flood probabilities. Rainfall-runoff modelling has been used to derive the critical hydrograph at Ballingeary that results in flooding to the town. The FSU UPO-ERR Gamma curve has been applied to derive the characteristic flood hydrographs across the Womanagh catchments. Corresponding coastal conditions have been developed for the design fluvial events. Calibration events were identified in Castlemartyr, Killeagh, and Ballingeary where there was sufficient historical flood data. Potential future catchment changes relevant to the Upper Lee and Womanagh catchment have been assessed including changes in urban development, land use and hydrology related to global climate change. Two future scenarios have been developed from this analysis, a Mid Range Future Scenario and High End Future Scenario, which have been used to develop potential future flows and extreme sea levels. The resultant design flood hydrographs and coastal conditions will be used as input to the hydraulic models. The knowledge of the hydrological processes and the historical flooding issues across Unit of Management 19 established in this report will support the development of sustainable and appropriate flood risk management options in those areas at greatest flood risk. 296235/IWE/CCW/R015/C 24 June 2016 i C:\Users\pig44561\Desktop\296235-IWE-CCW-R015-C-Hydrology Report UoM19.docx South Western CFRAM Study Hydrology Report,Unit of Management 19 1 Introduction 1.1 Context of the CFRAM Study Flooding is a natural process that occurs throughout Ireland as a result of extreme rainfall, river flows, storm surges, waves, and high groundwater. Flooding can become an issue where the flood waters interact with people, property, farmland and protected habitats. Flood risk in Ireland has historically been addressed through the use of structural or engineered solutions (arterial drainage schemes and / or flood relief schemes). In line with internationally changing perspectives, the Government adopted a new policy in 2004 that shifted the emphasis in addressing flood risk towards: A catchment-based context for managing risk; More pro-active flood hazard and risk assessment and management, with a view to avoiding or minimising future increases in risk, such as that which might arise from development