This report has been prepared with the objective of identifying practical measures to address flooding on the Clare River and on its main tributary, the Abbert River, within a study area from Lough Corrib to Corofin in County Galway. The study was commissioned as result of severe damage and disruption caused by flooding in the area in November 2009.
The brief from the Office of Public Works (OPW) required the study to include the following;
- A hydrographic survey from Lough Corrib to Claregalway to establish whether the presence of siltation significantly reduces the conveyance capacity of the channel.
- An examination of existing data on the Clare and Abbert Rivers to compare the Arterial Drainage Scheme design with the existing channel at critical locations.
- An examination of the available Hydrometric Stations data in the area with particular reference to the recent flood event.
- The development of a numeric hydraulic model of the study area, using available data augmented with a series of cross sections taken at selected locations.
- Calibration of the hydraulic model using data gathered from the recent flood.
- The development of an outline flood damage (improvement works benefit) analysis.
- Recommendation of a number of engineering measures to form the basis of a proposed alleviation option to eliminate or reduce flooding in the study area from a similar sized event to the November 2009 flood.
In order to fulfil the brief, the study comprised the following elements;
- A review of the Clare River catchment including description of the various Arterial Drainage Scheme works, physical characteristics of the Clare and Abbert Rivers, the control of water levels in Lough Corrib, catchment geology and hydrogeology, ecology, meteorology, urbanisation and land use zoning and cultural heritage.
- An analysis of historical floods in the study area.
- An analysis of the various parameters pertaining to the November 2009 flood event including analysis of rainfall events and return periods.
- The collection and assessment of extensive datasets from the Environmental Protection Agency, the Office of Public Works and Met Eireann relating to Clare River flows and water levels, Abbert River flows and water levels, Lough Corrib levels, rainfall intensities and operational data relating to the Salmon Weir sluice barrage in Galway city on the Corrib River.
- Surveying of cross-sections of the Clare and Abbert River channels and flood plains.
- Surveying of all of the bridges and other hydraulic features along the study area river reaches.
- The comparison of existing bed levels and cross sectional areas to design levels and cross sectional area in both the Clare and the Abbert Rivers.
- The collection of anecdotal evidence relating to peak flood levels and other pertinent information.
- The estimation of flood flows relating to November 2009, the 100 year flood and the 100 year flood incorporating a climate change allowance.
- An hydraulic analysis of the Clare and Abbert River channels including the development of a HECRAS 1D model.
- Calibration of the hydraulic model using the anecdotal evidence collected for the November 2009 flood event.
- The use of the hydraulic model to analyse potential flood risk management measures to eliminate or reduce the impact of a flood of similar magnitude to that of November 2009.
- The preparation of an outline flood damage analysis for the November 2009 flood event in order to calculate the economic damages consequent to this flood.
- Extrapolation of the November 2009 flood damage analysis for events of other magnitudes in order to calculate the economic risk posed by various flood events and to calculate the benefit accruing from putting various flood risk management measures in place.
- The evaluation of various alleviation options made up of a number of flood risk management measure, by carrying out initial viability screening on technical, economic, environmental and social acceptability grounds.
- For flood risk management measures deemed to be potentially viable, a more detailed viability assessment was carried out which included cost benefit analysis, an assessment of upstream or downstream flood risk, an assessment of environmental impact, climate change impact and future maintenance requirements.
- The drawing up of a programme of flood risk management measures satisfying the detailed viability assessment.
The main outcome of the elements of works listed above is a recommended programme of measures to deal with identified flood risk locations complete with cost estimates, estimates of economic benefits accruing as a result of the implementation of the measures and the calculation of benefit to cost ratios.
Another outcome of the study is that it is not possible to provide viable flood risk management measures for the areas at risk of flooding adjacent to the Abbert River and for one particular area in Miontach North adjacent to the Clare River.
The full description of the study is divided into eight chapters contained in Volume 1. Volume 2 contains Appendices pertinent to the study.
Chapter 1 is an introduction to the report and sets out the aims of the study.
Chapter 2 discusses the general catchment characteristics of the Clare River catchment as well as the study area. It also discusses the Lough Corrib catchment and the operation of the sluice gates at the Salmon Weir in Galway city in relation to levels on Lough Corrib. The final section of the chapter describes significant flood events in the study area including a description of the flood of November 2009.
Chapter 3 describes the various datasets collected in order to carry out the study including a description of the survey of the rivers as well as the collection of anecdotal evidence from residents and public representatives from the study area.
Chapters 4 and 5 analyse the hydrology and the hydraulics of the Clare and Abbert River. The hydrological element focuses on estimating pertinent flood flows at the various hydrometric gauges.
In Chapter 5, relating to the hydraulic analysis, the development of the hydraulic model is described as well as an analysis of various model runs both for calibration purposes and to test various flood risk management measures.
An outline damage analysis is provided in Chapter 6 for the November 2009 flood events, for the Design Flood event and for the Design Flood Event with a Climate Change allowance factored in. It also describes the results of the preliminary Flood Risk Assessment.
Chapter 7 provides detail on the various flood risk management measures proposed and an initial and more detailed viability assessment process, based on which the measures were assessed.