Hydrology Report February 2009 Lee Catchment Flood Risk Assessment and Management Study Hydrology Report Checking and Approval Prepared by: Scott Baigent, Senior Hydrologist Linda Hemsley, Hydrologist Paul Dunne, Assistant Project Manager 15 Apr 2008 Checked by: M Clare Dewar 25 Feb 2009 Project Manager Approved by: Richard Crowder 25 Feb 2009 Project Director Contents amendment record Issue Revision Description Date Signed 0 1 Draft table of contents for comment June 07 MCD 1 0 Draft to OPW for comment Nov 07 MCD 2 0 Final Report Feb 08 MCD 3 0 Final Report following OPW comments April 08 MCD 3 1 Final Report with updated MRFS flows Feb 09 MCD Halcrow Group Ireland Ltd has prepared this report in accordance with the instructions of the Office of Public Works for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. Halcrow Group Ireland Limited 3A Eastgate Road, Eastgate, Little Island, Cork Tel +353 21 452 4418 Fax +353 21 452 4419 www.halcrow.com © Halcrow Group Ireland Limited 2009 i Lee Catchment Flood Risk Assessment and Management Study Hydrology Report Acknowledgements The Lee Catchment Flood Risk Assessment and Management Strategy is being undertaken by Halcrow Group Ireland Limited with support from MarCon Computation International Ltd, J B Barry & Partners Ltd and Brady Shipman Martin. This hydrology report has been prepared by Halcrow Group Ltd and J B Barry & Partners Ltd. The meteorological and hydrological analyses presented in Sections 5 and 6 of this report were undertaken by J B Barry & Partners Ltd. MarCon Computations International BRADY SHIPMAN MARTIN Ltd ii Lee Catchment Flood Risk Assessment and Management Study Hydrology Report Executive Summary The Office of Public Works and its partners, Cork City Council and Cork County Council, are undertaking a catchment-based flood risk assessment and management study of the Lee Catchment – the Lee Catchment Flood Risk Assessment and Management Study (CFRAMS). The main output from this study will be flood maps and a Catchment Flood Risk Management Plan, which will identify a programme of prioritised studies, actions and works to manage the flood risk in the Lee catchment in the long-term. The plan will also make recommendations in relation to appropriate development planning. The Lee CFRAMS is the primary pilot project for a new national approach to flood risk management. This report details the hydrological assessment that has been undertaken for this study with the objective of determining hydrological inputs for the Lee and its tributaries for specific design events and future scenarios. This is based on a review and analysis of historic flood information and use of meteorological and hydrometric records. The Flood Studies Report (FSR) and Flood Estimation Handbook (FEH) methodologies have been used to enable determination of design hydrological inputs considering potential future catchment changes likely to influence flood risk. Hydraulic model calibration and verification events have been identified and integration of the hydrology and hydraulic modelling undertaken. The analysis presented in this report is concerned with the estimation of extreme flows, which will form the basis for subsequent flood level and mapping stages of the Lee CFRAMS. An extensive review of historical flood related documents has highlighted that there are a number of urban and rural areas at risk of flooding within the Lee catchment from both tidal and fluvial flood mechanisms. Flow, rainfall and tidal gauge data from the catchment and historic flood documentation has allowed at least two calibration/verification events for five of the eight river models representing the main rivers and tributaries in the catchment to be generated. The Lee catchment was sub-divided into 56 sub-catchments in total to ensure representation of the hydrological processes in the catchment is at a scale and resolution appropriate to this study. Three types of hydrological inflows (hydrographs, steady flows and lateral flows) were identified to be used to feed into the hydraulic models; these included the use of lateral inflows in all urban areas to reduce uncertainty. The study will identify both the existing risk and potential future risk of flooding to communities. There are a number of drivers that can influence future flood risk in the Lee catchment, the main drivers have been identified as being climate change, afforestation and urbanisation. These drivers have been extensively investigated and two future flood risk management scenarios have been proposed, a Mid Range Future Scenario and a High End Future Scenario. The outputs from this hydrological assessment will inform the subsequent stages of this study, in particular the hydraulic modelling and flood mapping stages. Knowledge of the hydrological processes and historic flooding gained from this work will support the decision making process for the flood risk management options, including the potential of reviewing the operation of the hydroelectric dams before and during flood events. iii Lee Catchment Flood Risk Assessment and Management Study Hydrology Report Table of contents Checking and Approval ........................................................................................................... i Contents amendment record................................................................................................... i Acknowledgements................................................................................................................. ii Executive Summary................................................................................................................ iii Table of contents..................................................................................................................... v List of figures ......................................................................................................................... vii List of tables............................................................................................................................ ix Glossary ................................................................................................................................... x 1. Introduction................................................................................................................. 1 1.1. Background .............................................................................................................. 1 1.2. Objectives................................................................................................................. 1 1.3. Approach .................................................................................................................. 2 2. Data collection ............................................................................................................ 4 2.1. Introduction............................................................................................................... 4 2.2. Topographical data................................................................................................... 4 2.3. Hydrometric data ...................................................................................................... 5 2.4. Meteorological data .................................................................................................. 8 2.5. Tidal data.................................................................................................................. 8 2.6. Mapping data............................................................................................................ 9 3. Description of the Lee Catchment .......................................................................... 11 3.1. Upper Lee catchment............................................................................................. 12 3.2. Lower Lee catchment............................................................................................. 13 3.3. River Bride catchment............................................................................................ 15 3.4. Glashaboy River catchment ................................................................................... 15 3.5. Carrigtohill catchment............................................................................................. 17 3.6. Owennacurra River catchment............................................................................... 18 3.7. Owenboy River catchment ..................................................................................... 19 3.8. Tramore River catchment....................................................................................... 20 3.9. Cork Harbour catchment ........................................................................................ 20 v Lee Catchment Flood Risk Assessment and Management Study Hydrology Report 3.10. Operation of Carrigadrohid and Inishcarra hydroelectric dams ............................. 21 4. Review and analysis of historic floods .................................................................. 24 4.1. Introduction............................................................................................................. 24 4.2. Flood events........................................................................................................... 24 4.3. Summary of Flood Mechanisms............................................................................. 24 4.4. Selection of calibration