South Galway FRS Feasibility

South Galway (Gort Lowlands) Flood Relief Scheme Feasibility Report February 2021 South Galway (Gort Lowlands) Flood Relief Scheme Feasibility Study Contents 1.0 Introduction and Background ..................................................................................................... 1 1.1 History of Flooding .................................................................................................................. 1 1.2 South Galway Hydrology & Hydrogeology .............................................................................. 2 1.3 Previous Studies ...................................................................................................................... 2 1.3.1 South Galway Flood Study Report .................................................................................. 3 1.3.2 Trinity College Dublin (TCD) Research and Modelling .................................................... 3 1.3.3 Geological Survey Ireland (GSI) Groundwater Flood Project .......................................... 3 1.4 South Galway (Gort Lowlands) Flood Relief Scheme .............................................................. 4 1.5 Feasibility Study ...................................................................................................................... 5 1.6 Report Structure ..................................................................................................................... 5 2.0 Flood Risk Assessment ................................................................................................................ 6 2.1 Scheme Hydrology & Hydrogeology ....................................................................................... 6 2.1.1 TCD Hydraulic Model of Gort Lowlands .......................................................................... 6 2.1.2 GSI Groundwater Flood Project ...................................................................................... 6 2.1.3 Ryan Hanley Input ........................................................................................................... 6 2.2 Flood Mapping, Flood Levels and Duration ............................................................................ 9 2.2.1 Design Flood Levels and Sensitivity Analysis................................................................. 15 2.2.2 Flood Duration .............................................................................................................. 15 2.3 Assessment of Flood Risk ...................................................................................................... 16 2.3.1 Zoning of Flood Plains ................................................................................................... 16 2.3.2 Properties Flood Risk .................................................................................................... 18 2.3.3 Cultural Heritage Sites Flood Risk ................................................................................. 22 2.3.4 Slatted Shed Flood Risk ................................................................................................. 24 2.3.5 Agricultural Land Flood Risk .......................................................................................... 25 2.3.6 Access and Transportation Flood Risk .......................................................................... 25 2.4 Flood Risk Assessment Summary .......................................................................................... 35 3.0 Emerging Engineering Scheme ................................................................................................. 36 3.1 Introduction .......................................................................................................................... 36 3.2 Catchment Overview ............................................................................................................ 37 3.2.1 Establishing Extents of the Emerging Engineering Scheme .......................................... 37 3.2.2 Scheme Design Criteria ................................................................................................. 38 3.3 Flood Risk Management Measures ....................................................................................... 41 3.3.1 Overview ....................................................................................................................... 41 3.3.2 Description of Measures ............................................................................................... 47 3.4 Flood Hydrographs ................................................................................................................ 60 South Galway (Gort Lowlands) Flood Relief Scheme Feasibility Study 4.0 Environmental Assessment ....................................................................................................... 68 4.1 Introduction .......................................................................................................................... 68 4.2 Significant Environmental Risks ............................................................................................ 69 4.2.1 Land Use, Social and Material Assets ............................................................................ 69 4.2.2 Cultural Heritage ........................................................................................................... 74 4.2.3 Biodiversity.................................................................................................................... 75 4.2.4 Water Resources ........................................................................................................... 78 4.2.5 Landscape and Visual .................................................................................................... 78 4.3 Conclusion ............................................................................................................................. 79 5.0 Preliminary Cost Benefit Analysis ............................................................................................. 80 5.1 Cost Estimate ........................................................................................................................ 80 5.2 Scheme Benefits .................................................................................................................... 80 5.3 Cost Benefit Analysis Summary ............................................................................................ 81 6.0 Conclusions ............................................................................................................................... 82 7.0 Recommendations .................................................................................................................... 86 South Galway (Gort Lowlands) Flood Relief Scheme Feasibility Study Index of Tables and Figures Tables Table 2.1: Design Flood Levels (Current Scenario) and Historic Maximum floods in the study area floodplains ........................................................................................................................ 13 Table 2.2: Target maximum flood levels in the study area floodplains and approximate difference compared to the calculated 1% AEP flood levels. ............................................................ 14 Table 2.3: Grouped Floodplain Zones ................................................................................................ 16 Table 2.4: Residential Properties Flooding at the various Flood Annual Exceedance Probabilities (Current Scenario) ............................................................................................................ 19 Table 2.5: Additional Residential Properties at High Flood Risk for the various Flood Annual Exceedance Probabilities (Current Scenario) ................................................................... 20 Table 2.6: Non-Residential Properties Flooding for the various Flood Annual Exceedance Probabilities (Current Scenario) ....................................................................................... 21 Table 2.7: Flood Risk at Thoor Ballylee Castle ................................................................................... 22 Table 2.8: Number of slatted building complexes relative to AEP .................................................... 25 Table 2.9: Roads at Flood Risk within the Study Area ....................................................................... 29 Table 2.10: Rural Communities at Risk of being Cut-off due to groundwater flooding ...................... 30 Table 2.11: Main Road Flood Duration for the design AEP ................................................................. 32 Table 2.12: Flood Risk at the Railway line at Castletown .................................................................... 34 Table 3.1: Flood Relief Schemes in the Study Area ........................................................................... 37 Table 3.2: Scheme Design Criteria ..................................................................................................... 40 Table 3.3: Scheme Overview ............................................................................................................. 46 Table 3.4: Description of proposed works ......................................................................................... 58 Table 6.1: Overview of Emerging Engineering Scheme ..................................................................... 83 Table 6.2: Overview of Works that will not form part of Emerging

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