Fingal East Meath Flood Risk Assessment and Management Study PROJECT: Fingal East Meath Flood Risk Assessment and Management Study DOCUMENT: HYDROLOGY REPORT DATE: January 2010 Fingal East Meath Flood Risk Assessment and Management Study Hydrology Report Checking and Approval Prepared by: Keshav Bhattarai December 2009 Senior Hydrologist Checked by: Scott Baigent December 2009 Associate / Senior Hydrologist Checked by: Jenny Pickles December 2009 Principal Water Management Consultant Approved by: Anne-Marie Conibear January 2010 Project Manager Contents amendment record Issue Revision Description Date Signed 1 0 1st draft for review Oct ‘09 2 1 Final Jan ‘10 3 2 Final Report Apr ‘10 (additional text in response to OPW comments) Halcrow Barry has prepared this report in accordance with the instructions of Fingal County Council, Meath County and the OPW for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. Halcrow Barry Tramway House, 32 Dartry Road, Dublin 6, Ireland Tel +353 1 4975716, Fax +353 1 4975716 www.halcrow.com www.jbbarry.ie © Halcrow Barry, Fingal County Council, Meath County Council & Office of Public Works, 2010 i Fingal East Meath Flood Risk Assessment and Management Study Hydrology Report ii Fingal East Meath Flood Risk Assessment and Management Study Hydrology Report Executive Summary Fingal County Council (FCC), in conjunction with Meath County Council (MCC) and the Office of Public Works (OPW), are undertaking a flood risk assessment and management study in Fingal and East Meath – the Fingal East Meath Flood Risk Assessment and Management Study (FEM FRAMS). Halcrow Barry (HB) was commissioned to carry out the work on behalf of FCC/MCC/OPW. The main report from this study – a Flood Risk Management Plan – will identify a programme of prioritised studies, actions and works to manage flood risk in the Fingal and East Meath (FEM) study area. This Hydrology Report, together with the Preliminary Hydrology Report published in February 2009, details the hydrological assessment that has been undertaken for this study with the objective of determining hydrological inputs for the 23 rivers and streams in the study area that are to be modelled, for specific design events and future scenarios. The hydrology is based on a review and analysis of historic flood information and use of meteorological and hydrometric records. The Flood Studies Report (FSR), Flood Estimation Handbook (FEH) and the Irish Flood Studies Update (FSU) methodologies have been used to enable the determination of design hydrological inputs which also consider potential future catchment changes likely to influence flood risk. 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 FEM FRAMS. To distribute these flows along the river reach, the HPWs (High Priority Watercourses) and MPWs (Medium Priority Watercourses) sections of the 23 rivers and streams that are to be modelled, have been further sub-divided into a total of 270 sub-catchments. Catchment characteristics of these sub-catchments have been extracted using GIS automation tools aided by manual checking. Design inflows at these sub-catchments are calculated using the catchment characteristics, FSU-based rainfall inputs and applying the FSSR 16 and IOH 124 unit hydrograph methods. The total routed inflows from all the upstream sub-catchments at the gauging stations will be reconciled with the statistical method estimated design floods at the gauging stations using iterative simulations in the river hydraulic models. Hydraulic model calibration and verification events have been identified by reviewing the information on historic floods in the study area including photographs of flood events or their aftermaths. It should also be noted that most of the hydrometric stations in the study area were inoperational between 1995 and 2001 and thus the recent flooding events do not have corresponding hydrometric information. This meant that calibration of only three river hydraulic models out of the total 23 river and stream models was possible. To assist in the future model calibration and flood forecasting in the rivers, Halcrow Barry has developed a priority list of hydrometric gauging stations that should be installed or re-activated in the catchment. The FEM FRAM study will identify both the existing risk and potential future risk of flooding in the study area. There are a number of drivers that can influence future flood risk in the study area, the main drivers have been identified as being climate change and increasing 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 and, 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. iii Fingal East Meath Flood Risk Assessment and Management Study Hydrology Report iv Fingal East Meath Flood Risk Assessment and Management Study Hydrology Report Table of Contents Executive Summary................................................................................................................ iii Table of Contents .....................................................................................................................v 1. Introduction................................................................................................................. 1 1.1 Background...................................................................................................................... 1 1.2 General description of the study area.............................................................................. 2 1.3 Objectives of FEM FRAMS .............................................................................................. 4 1.4 Objective and approach of hydrological analysis............................................................. 5 2. Data Collection............................................................................................................ 7 2.1 Introduction ...................................................................................................................... 7 2.2 Rainfall data ..................................................................................................................... 7 2.3 Hydrometric data.............................................................................................................. 7 2.4 Historic flood data .......................................................................................................... 10 2.5 Tidal data ....................................................................................................................... 10 2.6 Mapping data ................................................................................................................. 11 2.7 Topographic survey data ............................................................................................... 11 2.8 LiDAR data..................................................................................................................... 11 2.9 Land use and soil data................................................................................................... 12 3. Review and analysis of historic floods .................................................................. 13 3.1 Collection of historic flood data ...................................................................................... 13 3.2 Flooding mechanism in the study area .......................................................................... 13 3.3 Summary of flooding mechanism................................................................................... 15 4. Meteorological Analysis .......................................................................................... 16 4.1 Introduction .................................................................................................................... 16 4.2 Analysis of rainfall data .................................................................................................. 16 4.2.1. Data used.......................................................................................................... 16 4.2.2. Annual maximum hourly rainfall series .............................................................. 16 4.2.3. Annual maximum two day rainfall series ........................................................... 17 4.2.4. Summary of meteorological analysis ................................................................. 20 v Fingal East Meath Flood Risk Assessment and Management Study Hydrology Report 5. Hydrological Analysis .............................................................................................. 21 5.1 Introduction .................................................................................................................... 21 5.2 Rating curve review ....................................................................................................... 21 5.3 Index flood...................................................................................................................... 24 5.3.1. Median annual maximum flood