Galway Bay South East Hydrology Report
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Western CFRAM Unit of management 29 – Galway Bay South East Hydrology Report Final report December 2014 Office of Public Works Trim Co. Meath JBA Consulting 24 Grove Island Corbally Limerick Ireland JBA Project Manager Sam Willis BSc MSc CSci CEnv MCIWEM CWEM Revision History Revision Ref / Date Issued Amendments Issued to Draft v1.0 OPW Draft v 2.0 Updated to reflect OPW 24 July 2014 comments received on all draft UoM reports Final v3.0 Updated to reflect OPW 12 December 2014 comments received from OPW Final v4.0 Updated to reflect OPW 25 March 2015 comments received on final UoM reports Contract This report describes work commissioned by the Office of Public Works, by a letter dated (28/07/11). The Office of Public Works’ representative for the contract was Rosemarie Lawlor. Duncan Faulkner, Colin Riggs, Paige Garside, Lucy Barker and Kevin Haseldine of JBA Consulting carried out this work. Prepared by .................................................. Duncan Faulkner MSc DIC MA FCIWEM C.WEM CSci Head of Hydrology Reviewed by ................................................. Jonathan Cooper BEng MSc DipCD CEng MICE MCIWEM C.WEM MloD Director Purpose This document has been prepared as a final report for the Office of Public Works. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared. 2011s5232 Western CFRAM UoM29 Final Hydrology Report v4.0.docx i JBA Consulting has no liability regarding the use of this report except to the Office of Public Works. Copyright Copyright is with the Office of Public Works. All rights reserved. No part of this report may be copied or reproduced by any means without the prior written permission of the Office of Public works. Legal Disclaimer This report is subject to the limitations and warranties contained in the contract between the commissioning party (the Office of Public Works) and JBA. 2011s5232 Western CFRAM UoM29 Final Hydrology Report v4.0.docx ii Executive Summary This report describes the hydrological analysis carried out as part of the Catchment-Based Flood Risk Assessment and Management Study (CFRAM) for the Western River Basin. It covers unit of management (UoM) 29, Galway Bay South East. The brief calls for a comprehensive and detailed hydrological analysis that places particular emphasis on flood flow estimation for the main flood risk areas (termed AFAs, Areas for Further Assessment) and the watercourses that flow through these areas (termed HPWs, High Priority Watercourses). In UoM 29, the AFAs are Oranmore, Athenry, Gort, Loughrea and Kinvarra. Kinvarra is subject to coastal flood risk, while the other four AFAs are vulnerable to fluvial risk, and Gort is also at risk of groundwater flooding. The principal objective of the hydrological study is to derive best estimates of design fluvial flood parameters including peak flows, hydrographs and flood volumes, for all hydrological estimation points. The study also includes derivation of design coastal flood parameters for AFAs subject to significant coastal flood risk. The word “design” here refers to a quantity that is expected to be exceeded with a specified probability or frequency, as opposed to a measured river flow or sea level for any particular date and time. Design flood parameters are estimated by statistical analysis or modelling. The report includes a review of the hydrological data available in the study area. There are several gauging stations that measure river level, some of which have rating equations that enables conversion of level into flow. None of the AFAs has a flow gauging station on site, although there are level gauges at Oranmore and Lough Rea. Loughrea, Athenry and Gort have flow data available either further downstream or upstream on the same watercourse, although there are limitations associated with each gauge. A variety of methods are available for estimation of design floods. The approach taken for most parts of the Western CFRAM study area is to base the analysis closely on the recorded flow data, in accordance with the methods developed during the Flood Studies Update research. The implementation of the FSU research project has not yet been completed and so it has been necessary to develop software to apply some of the methods. Peak flows have been estimated from statistical analysis of annual maximum flows. At locations without flow data, design flows have been estimated indirectly from physical properties of the catchment, combined with transfer of data from representative gauged catchments both locally and further afield throughout Ireland. For the most extreme design floods (annual probabilities below 1%), the statistical analysis has been supplemented with an extended flood growth curve from the Flood Studies Report rainfall-runoff method. The design flows have been derived by direct analysis of flood data so they will naturally be consistent with that data. However design flows have been checked to identify any results that fall outside expected ranges; these included confirmation that growth factors are within expected ranges, that AEPs for observed events implied in the flood frequency curves are appropriate and that there was spatial consistency between design flows. Several approaches have been trialled for the estimation of design flood hydrographs, and the results assessed using techniques such as analysis of percentage runoff and flood volumes. The recommended approach for most watercourses is to derive the shape of design hydrographs using the rainfall-runoff method from the Flood Studies Report. A different method of flood estimation has been recommended for the AFA at Loughrea, using the Flood Studies Report rainfall-runoff method in conjunction with flood routing to account for the attenuation that occurs as the flood hydrograph passes through Lough Rea. 2011s5232 Western CFRAM UoM29 Final Hydrology Report v4.0.docx iii Methods used to estimate design flood hydrographs at each AFA AFA Watercourse QMED Growth curve Distribution Hydrograph method method shape Catchment Carrowmoneash, Descriptors Ballynageeha, FSR rainfall- Oranmore (altered from Pooled General Logistic Rocklands, Data Transfer – runoff Moneyduff Pivotal 29004) Catchment Descriptors Generalised FSR rainfall- Athenry Graigabbey River (altered from Pooled Extreme Value Data Transfer – runoff Pivotal 29004) Loughrea St Clearan’s River n/a: Design flood hydrographs downstream of Lough Rea outlet to be and Lough Rea estimated using the FSR rainfall-runoff method, routed through the lough. outlet channel Tonaroasty Data Transfer Pooled General FSR rainfall- (tributary that does – Pivotal Logistic runoff not discharge from 29007 lough) To be confirmed General FSR rainfall- River Gort, Pooled, to be Gort in hydraulics Logistic, to be Ballyhugh confirmed runoff, to be report confirmed confirmed The design flood hydrographs will form inflows to the hydraulic models that are being used to predict flood levels, depths and extents. It has been necessary to reconcile flows within the model with hydrological estimates of flow to ensure consistency through the river systems, and consider the main assumptions and sources of uncertainty in the design flows, and how these are translated into the model. As well as design flows for the present-day situation, the study has produced a set of flows for two future scenarios, which have considered climate change impacts on both river flows and sea levels and the impact of increased urbanisation. It is considered that land use change, in the form of changes to forestry practice, will have little impact on flood risk in the UoM, so this has not been accounted for. To provide a downstream boundary condition for hydraulic models of rivers that enter the sea, design tidal graphs have been created by combining information on extreme sea levels with design surge shapes and design astronomical tide curves. Detailed records of the calculations are provided in the appendices, along with a table of the design peak flows. The report is accompanied by digital deliverables which provide the design flows for all locations, along with further information on the methods used at each location. The Hydrology Report for UoM 29 should be read in conjunction with the Hydraulic Modelling Report for UoM 29, and the specific modelling reports for each AFA, which detail the application of the hydrology to the specific river reaches. 2011s5232 Western CFRAM UoM29 Final Hydrology Report v4.0.docx iv Contents Legal Disclaimer ................................................................................................................... ii Executive Summary ............................................................................................................. iii 1 Introduction .............................................................................................................. 1 1.1 Background ................................................................................................................ 1 1.2 Objectives of hydrological study ................................................................................ 1 1.3 Report structure ......................................................................................................... 1 1.4 Unit of management 29 - Galway Bay South East .................................................... 1 2 Hydrology of the study area ..................................................................................