Flood Analysis of the Clare River Catchment Considering Traditional Factors and Climate Change AUTHOR Pierce Faherty G00073632 A Thesis Submitted in Part Fulfilment for the Award of M.Sc. Environmental Systems, at the College of Engineering, Galway Mayo Institute of Technology, Ireland Submitted to the Galway Mayo Institute of Technology, September 2010 .... ITUTE Of TECHNOLOGY DECLARATION OF ORIGINALITY September 2010 The substance of this thesis is the original work of the author and due reference and acknowledgement has been made, when necessary, to the work of others. No part of this thesis has been accepted for any degree and is not concurrently submitted for any other award. I declare that this thesis is my original work except where otherwise stated. Pierce Faherty Sean Moloney Date: 1 7 - 01" 10__ Abstract The main objective of this thesis on flooding was to produce a detailed report on flooding with specific reference to the Clare River catchment. Past flooding in the Clare River catchment was assessed with specific reference to the November 2009 flood event. A Geographic Information System was used to produce a graphical representation of the spatial distribution of the November 2009 flood. Flood risk is prominent within the Clare River catchment especially in the region of Claregalway. The recent flooding events of November 2009 produced significant fluvial flooding from the Clare River. This resulted in considerable flood damage to property. There were also hidden costs such as the economic impact of the closing of the N17 until floodwater subsided. Land use and channel conditions are traditional factors that have long been recognised for their effect on flooding processes. These factors were examined in the context of the Clare River catchment to determine if they had any significant effect on flood flows. Climate change has become recognised as a factor that may produce more significant and frequent flood events in the future. Many experts feel that climate change will result in an increase in the intensity and duration of rainfall in western Ireland. This would have significant implications for the Clare River catchment, which is already vulnerable to flooding. Flood estimation techniques are a key aspect in understanding and preparing for flood events. This study uses methods based on the statistical analysis of recorded data and methods based on a design rainstorm and rainfall-runoff model to estimate flood flows. These provide a mathematical basis to evaluate the impacts of various factors on flooding and also to generate practical design floods, which can be used in the design of flood relief measures. The final element of the thesis includes the author’s recommendations on how flood risk management techniques can reduce existing flood risk in the Clare River catchment. Future implications to flood risk due to factors such as climate change and poor planning practices are also considered. A cknowledgements The author would like to thank the following people for all their time and support in the preparation of this thesis. Mr. Sean Moloney, Department of Civil Engineering, GM1T Galway, for his expertise and supervision throughout the duration of the project. Mr. Kevin Rodgers, Western Regional Fisheries Board, for providing invaluable information and access to GIS software to produce essential catchment maps. The Office of Public Works, especially, Mr. Tim Pasley, for providing hydrometric data and significant knowledge on the Clare River catchment. My family for their continued encouragement and support. Glossary AEP Annual Exceedance Probability AOGCM Atmospheric and Oceanic Global Climate Model APSR Areas with Potentially Significant Risk AR4 IPCC’s Fourth Assessment Report on Climate Change CFRAMS Catchment Flood Risk Assessment and Management Study CO2 Carbon Dioxide DoEHLG Department of Environment, Heritage and Local Government EIA Environmental Impact Assessment FRA Flood Risk Assessment FSR Flood Studies Report GCM Global Climate Model GHG Greenhouse Gas GIS Geographic Information System GSI Geographical Survey Ireland HEFS High-End Future Scenario IPCC Intergovernmental Panel on Climate Change LAP Local Area Plan MRFS Mid-Range Scenario M-5 M5 rainfall is the rainfall depth with a return period of 5-years mAod metres Above ordnance datum. This will be taken as mAod Malin for the purpose of this report unless otherwise stated NSS National Spatial Strategy OPW Office of Public Works PFRA Preliminary Flood Risk Assessment ppb parts per billion ppm parts per million PSFRM Planning System and Flood Risk Management Guidelines RFRA draft Regional Flood Risk Appraisal for the west region RPG draft Regional Planning Guidelines for the west region SAC Special Area of Conservation SEA Strategic Environmental Assessment SFRA Strategic Flood Risk Assessment s.m.d. Soil moisture deficit SRES Special Report on Emissions Scenario SUDS Sustainable Urban Drainage System TAR IPCC’s Third Assessment Report on climate utc coordinated universal time WRFB Western Regional Fisheries Board Table of Contents A bstract....................................................................................................................................i Acknowledgements................................................................................................................ii Glossary...................................................................................................................................iii Chapter 1 Introduction.....................................................................................................1 Chapter 2 Clare River Catchment Characteristics.....................................................6 2.1 Topography and Hydrology of Catchment.....................................................7 2.2 Geology and Hydrogeology of Catchment...................................................11 Chapter 3 Flood Events in the Clare River Catchment........................................... 16 3.1 Historical Flooding......................................................................................... 17 3.1.1 November 1968................................................................................. 17 3.1.2 February 1990....................................................................................18 3.1.3 Winter 1990-1991............................................................................. 20 3.1.4 December 1999................................................................................. 21 3.1.5 January 2005......................................................................................23 3.1.6 December 2006................................................................................. 24 3.1.7 November 2009................................................................................. 27 Chapter 4 Frequency Analysis.......................................................................................34 4.1 Hydrometric Stations and Data Used............................................................35 4.2 Methodology................................................................................................... 36 4.2.1 E V1 Method of Moments................................................................ 37 4.2.2 Frequency Factor...............................................................................39 4.2.3 EV1 Distribution using Gringorten (Probability Plotting)........... 40 4.2.4 Comparison of Probability Plotting with Lognormal Distribution Fitted by Frequency Factor Method..........................40 4.2.5 Standard Error and Confidence Limits............................................41 4.3 Results of Statistical Analysis....................................................................... 42 4.3.1 Discussion of Results........................................................................47 Chapter 5 Climate Change and Rainfall..................................................................... 49 5.1 Climate Change............................................................................................... 50 5.1.1 Predicting Climate Change.............................................................. 52 5.1.2 Climate Change and Flooding..........................................................54 5.2 Key Precipitation Indicators...........................................................................62 5.3 Precipitation Data from Clare River Catchment.......................................... 63 5.4 Analysis of Precipitation Data....................................................................... 65 5.4.1 Milltown............................................................................................ 65 5.4.2 Glenamaddy......................................................................................72 5.5 Summary...........................................................................................................78 Chapter 6 Land Use......................................................................................................... 80 6.1 Influence of Land Use Changes on Flood Risk............................................81 6.2 Planning System and Flood Risk Management Guidelines........................ 88 6.2.1 Flood Risk Assessment....................................................................92 6.2.2 Justification Test.............................................................................. 98 6.3 Area Planning Guidelines on Flood Risk...................................................