River Dodder Catchment Flood Risk Management Plan

RIVER DODDER CATCHMENT FLOOD RISK MANAGEMENT PLAN

HYDROLOGICAL ANALYSIS REPORT

OCTOBER 2008

River Dodder Catchment Flood Risk Assessment and Management Study

Hydrological Analysis Report

DOCUMENT CONTROL SHEET

Client Dublin City Council

Project Title River Dodder Catchment Flood Risk Assessment and Management Study

Document Title Hydrological Analysis Report

Document No. MDW0259Rp0016

No. of DCS TOC Text List of Tables List of Figures This Document Appendices Comprises 1 1 92 1 1 6

Rev. Status Author(s) Reviewed By Approved By Office of Origin Issue Date

F01 Approval C. O’Donnell B. Elsaesser B. Elsaesser West Pier 31.10.08

River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

IMPORTANT DISCLAIMER – HYDROLOGICAL ANALYSIS

Please read below the disclaimer, and limitations associated with this report to avoid incorrect interpretation of the information and data provided.

DISCLAIMER

Dublin City Council, South Dublin County Council, Dun Laoghaire and Rathdown County Council and The Office of Public Works make no representations, warranties or undertakings about any of the information provided in this report including, without limitation, on its accuracy, completeness, quality or fitness for any particular purpose. To the fullest extent permitted by applicable law, neither the State, Dublin City Council, South Dublin County Council, Dun Laoghaire and Rathdown County Council nor The Office of Public Works nor any of their members, officers, associates, consultants, employees, affiliates, servants, agents or other representatives shall be liable for loss or damage arising out of, or in connection with, the use of, or the inability to use, the information provided in this report including, but not limited to, indirect or consequential loss or damages, loss of data, income, profit, or opportunity, loss of, or damage to, property and claims of third parties, even if Dublin City Council, South Dublin County Council, Dun Laoghaire and Rathdown County Council and The Office of Public Works has been advised of the possibility of such loss or damages, or such loss or damages were reasonably foreseeable. Dublin City Council, South Dublin County Council, Dun Laoghaire and Rathdown County Council and The Office of Public Works reserves the right to change the content and / or presentation of any of the information provided in this report at their sole discretion, including these notes and disclaimer. This disclaimer, guidance notes and conditions of use shall be governed by, and construed in accordance with, the laws of the Republic of Ireland. If any provision of these disclaimer, guidance notes and conditions of use shall be unlawful, void or for any reason unenforceable, that provision shall be deemed severable and shall not affect the validity and enforceability of the remaining provisions.

UNCERTAINTY

Although great care and modern, widely-accepted methods have been used in the preparation of this report there is inevitably a range of inherent uncertainties and assumptions made during the estimation of design flows. These can arise due to:

• a lack of recorded flood data at the specific location, • short periods of recorded flood flows where they have been recorded, • approximations in the hydrological analysis in representing physical reality, • assumptions in the hydrological analysis.

BEST AVAILABLE INFORMATION

Hydrological and Hydrometric datasets are constantly changing and subsequently the statistical analysis of these datasets is only correct at the time of assessment. The Hydrological Analysis described in this report used the best available information at the time utilising best practice methodologies but it is acknowledged that new methodologies and/or recently recorded data could have a minor impact on the estimated design flows. However under the requirements of the Floods Directive (2007/60/EC), and its transposing legislation (SI 122 of 2010) into Irish law, this report will be reviewed every 6 years and changes to methodologies and/or datasets will be incorporated within a subsequent revised version.

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TABLE OF CONTENTS

1 INTRODUCTION ...... 1 1.1 BACKGROUND ...... 1 1.2 OBJECTIVES OF REPORT ...... 1 2 CATCHMENT DESCRIPTION ...... 3 2.1 RIVER SYSTEM...... 3 2.1.1 Dodder Main Channel ...... 3 2.1.2 Tallaght Stream: ...... 4 2.1.3 Owendoher:...... 4 2.1.4 Whitechurch:...... 4 2.1.5 Little Dargle: ...... 4 2.1.6 Dundrum Slang: ...... 4 2.1.7 Bohernabreena Reservoir System ...... 4 2.2 WATER LEVEL GAUGING STATIONS...... 5 2.3 LAND USE DATA ...... 9 2.3.1 Current Development Scenario...... 9 2.3.2 Future Development Scenarios...... 11 2.3.3 Afforestation ...... 12 2.3.4 Deforestation ...... 13 2.3.5 Most Likely Future Development Scenario ...... 13 3 HYDROLOGICAL DATA ...... 14 3.1 METEOROLOGICAL DATA...... 14 3.2 HYDROMETRIC DATA ...... 17 3.3 COASTAL DATA...... 18 3.3.1 Joint Probability Assessment of Extreme Coastal and Fluvial Events...... 18 3.4 HISTORIC FLOODING DATA ...... 20 3.4.1 River Dodder Main Channel...... 20 3.4.2 Tributaries...... 21 3.5 CLIMATE CHANGE SCENARIOS...... 23 3.5.1 Effect of Climate Change on Precipitation ...... 23 3.5.2 Effect of Climate Change on Coastal Water Levels...... 28 4 METHODOLOGY...... 30 4.1 RATING CURVE ANALYSIS ...... 31 4.2 RAINFALL RUNOFF MODEL CONSTRUCTION & CALIBRATION...... 31 4.2.1 NAM Model...... 31 4.2.2 Urban Model...... 34 4.2.3 MIKE Combined Model ...... 34 4.3 SENSITIVITY ANALYSIS ...... 35 4.4 EXTREME VALUE ANALYSIS (EVA)...... 35 4.5 REGIONALISATION OF RAINFALL-RUNOFF MODELS ...... 36

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4.6 COASTAL DATA...... 36 4.7 SUMMARY OF METHODOLOGY ...... 37 5 ANALYSIS OF HYDROMETRIC AND METEOROLOGICAL DATA...... 39 5.1 RATING CURVE ANALYSIS ...... 39 5.1.1 Rating Curve Check ...... 39 5.1.2 Rating Curve Application...... 41 5.2 RAINFALL-RUNOFF MODEL CALIBRATION ...... 42 5.2.1 Waldron’s Bridge ...... 43 5.2.2 Willbrook Road ...... 47 5.2.3 Frankfort ...... 50 5.3 SENSITIVITY ANALYSIS ...... 51 5.3.1 Umax (Maximum Water Content in Surface Storage)...... 52 5.3.2 Lmax (Maximum Water Content in Root Zone Storage)...... 53 5.3.3 CQOF (Overland Flow Runoff Coefficient)...... 54 5.3.4 TOF (Root Zone Threshold Value for Overland Flow) ...... 55

5.3.5 CK1,2 (Time Constant for Routing Overland Flow)...... 56 5.3.6 Summary of Sensitivity Analysis ...... 57 5.4 EXTREME VALUE ANALYSIS (EVA) OF SIMULATED DISCHARGE USING HISTORIC RAINFALL RECORD...... 57 5.4.1 Waldron’s Bridge ...... 57 5.4.2 Willbrook Road ...... 58 5.4.3 Frankfort ...... 60 5.4.4 Estimated Return Period of Historic Events ...... 61 5.5 JOINT PROBABILITY ANALYSIS – SURGE RESIDUALS AND RAINFALL ...... 62 5.6 ADDITIONAL ANALYSIS ...... 65 5.6.1 Extreme Value Analysis (EVA) of Historic Annual Maxima Series ...... 65 5.6.2 Analysis of Rainfall Data ...... 69 5.6.3 Flood Studies Report Method...... 75 5.7 SUMMARY ...... 77 5.7.1 Rating Curve Analysis ...... 77 5.7.2 Rainfall-Runoff Model Calibration...... 77 5.7.3 Sensitivity Analysis...... 77 5.7.4 EVA of Simulated Discharge Using Historic Rainfall Record...... 77 5.7.5 Joint Probability Analysis – Surge Residual and Rainfall...... 78 5.7.6 Additional Analysis ...... 78 6 ESTIMATION OF DESIGN FLOODS ...... 81 6.1 PRESENT DAY SCENARIO...... 81 6.1.1 Regionalisation of Rainfall Runoff Model ...... 81 6.1.2 Summary of Parameters ...... 82 6.1.3 Extreme Flows...... 82 6.2 MOST LIKELY FUTURE SCENARIOS...... 84

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6.2.1 Future Scenario 1 – Assuming FULL Implementation of SUDS ...... 84 6.2.2 Future Scenario 2 – Assuming NO Implementation of SUDS...... 87 6.2.3 Climate Change Rainfall Sensitivity Analysis...... 89 7 CONCLUSION ...... 91

LIST OF FIGURES

Figure 1-1 Flood Damage from Hurricane Charlie...... 1 Figure 2-1 Orwell Weir ...... 6 Figure 2-2 Willbrook Road Water Level Gauging Station ...... 6 Figure 2-3 Frankfort Water Level Gauging Station ...... 7 Figure 2-4 Bohernabreena Water Level Gauging Station...... 7 Figure 2-5 River Dodder Catchment Schematic ...... 9 Figure 2-6 River Dodder Catchment and Sub-Catchment Boundaries...... 10 Figure 3-1 Example of Complete Data Series from Glenasmole (Castlekelly) Station...... 16 Figure 3-2 Example of Incomplete Data Series from Ballyboden Station...... 16 Figure 3-3 Typical Flow Hydrographs ...... 17 Figure 3-4: Mean Sea Level (MSL) Pressure Fields in hPa...... 19 Figure 3-5: Hourly rainfall during October 2004 storm surge event for Dodder catchment ...... 19 Figure 3-6 Contoured Representation of Interpolated Precipitation Factors ...... 24 Figure 3-7 Location of data points in relation to the Dodder Catchment ...... 25 Figure 3-8 Present Day and Average Future Rainfall Datasets for Bohernabreena NAM Catchment...... 27 Figure 4-1 Methodology Flow Chart...... 30 Figure 4-2 Example NAM Model Discharge File...... 33 Figure 4-3 Example Combined Model Discharge File ...... 34 Figure 4-4 Tide gauge record from Dublin North Wall ...... 36 Figure 4-5 Digital recorded surface elevation at Dublin with predicted water level and surge residual (different scale) ...... 37 Figure 5-1 Waldron’s Bridge Rating Curves (Gauge rated to 112 m3/s) ...... 40 Figure 5-2 Willbrook Road Rating Curves (Gauge rated to 6 m3/s) ...... 40 Figure 5-3 Frankfort Rating Curves (Gauge rated by EPA to 2 m3/s) ...... 41 Figure 5-4 Simulated V Recorded Discharge for Waldron’s Bridge Gauge Catchment ...... 44 Figure 5-5 Example of Attenuated Flow in Observed Discharge Hydrograph...... 45 Figure 5-6 Example of Step Changes in Flow in Observed Discharge Record...... 46 Figure 5-7 Individual Flood Events Used in Calibration for Waldron’s Bridge Gauge Catchment.... 46 Figure 5-8 Simulated V Recorded Discharge for Willbrook Road Gauge Catchment ...... 47 Figure 5-9 Recorded Rainfall for Sample Event ...... 48 Figure 5-10 Recorded Discharge for Sample Event ...... 49 Figure 5-11 Simulated V Observed Discharge for Frankfort Gauge Catchment ...... 50

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Figure 5-12 Individual Flood Events Used in Calibration for Frankfort Gauge Catchment...... 51 Figure 5-13 Graph of Sensitivity Analysis Results for Umax Parameter ...... 52 Figure 5-14 Graph of Sensitivity Analysis Results for Lmax Parameter ...... 53 Figure 5-15 Graph of Sensitivity Analysis Results for CQOF Parameter ...... 54 Figure 5-16 Graph of Sensitivity Analysis Results for TOF Parameter...... 55 Figure 5-17 Graph of Sensitivity Analysis Results for CK1,2 Parameter...... 56 Figure 5-18 EVA of Simulated Discharge for Waldron’s Bridge Gauge Catchment ...... 58 Figure 5-19 EVA of Simulated Discharge for Willbrook Road Gauge Catchment ...... 59 Figure 5-20 EVA of Simulated Discharge for Frankfort Gauge Catchment ...... 60 Figure 5-21 Probability Plot of Extreme Surge Residuals Used in Joint Probability Analysis ...... 63 Figure 5-22 Probability Plot of Extreme Rainfall Used in Joint Probability Analysis...... 63 Figure 5-23 Joint Probability of rainfall and surge residuals ...... 65 Figure 5-24 EVA of Annual Maxima Flows from Waldron’s Bridge Gauge...... 67 Figure 5-25 EVA of Annual Maxima Flows from Willbrook Road Gauge...... 68 Figure 5-26 EVA of Annual Maxima Flows from Frankfort Gauge...... 69 Figure 5-27 Probability Plot for Rainfall Data Recorded at Casement Aerodrome...... 70 Figure 5-28 Probability Plot for Rainfall Data Recorded at Dundrum Gauging Station ...... 71 Figure 5-29 Probability Plot for Rainfall Data Recorded at Glenasmole Gauging Station...... 72 Figure 5-30 EPA Recorded Discharge Hydrograph for Hurricane Charlie Event ...... 73 Figure 5-31 Recorded Rainfall Data for Hurricane Charlie Event...... 74 Figure 6-1 Present Day, Mid Range Future and High End Future Scenario Rainfall Datasets for Bohernabreena NAM Catchment...... 89

LIST OF TABLES

Table 2-1 Bohernabreena Lower Reservoir Storage ...... 5 Table 2-2: Current Development Scenario ...... 9 Table 2-3: GDSDS Future Development Scenario ...... 11 Table 2-4: Revised Future Development Scenario...... 12 Table 3-1: Rain Gauge Data Provided by Local Authorities ...... 14 Table 3-2 Extreme Rainfall Return Period Chart for Dundrum Rainfall Gauging Station ...... 14 Table 3-3: Rain Gauge Data Provided by Met Éireann ...... 15 Table 3-4 Hydrometric Data from EPA ...... 17 Table 3-5 Highest Ranked Floods at Waldron’s Bridge Gauge for the 20th Century...... 20 Table 3-6 Documents Relating to Historic Flooding in River Dodder Catchment...... 22 Table 3-7 Predicted Changes in Precipitation from Climate Change Models...... 25 Table 3-8 Summary of Expected Changes in Evaporation for the year 2100 ...... 26

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Table 3-9 Return Period Rainfall Depths for December and June ...... 28 Table 4-1 Contribution of rainfall gauge data to flow gauge catchments...... 32 Table 4-2 NAM Parameters Supplied by Water Framework Directive Project ...... 33 Table 5-1 Details of 1D Hydrodynamic Models of Gauging Stations...... 39 Table 5-2 Comparison of Discharges for Waldron’s Bridge Gauge (Gauge rated to 112m3/s) ...... 41 Table 5-3 Comparison of Discharge for Willbrook Road Gauge (Gauge rated to 6m3/s)...... 42 Table 5-4 Comparison of Discharges for Frankfort Gauge (Gauge rated to 2m3/s) ...... 42 Table 5-5 Rainfall Gauging Stations Contributing to Willbrook Road Gauge Catchment...... 49 Table 5-6 Statistics for Sensitivity Analysis of Umax Parameter ...... 52 Table 5-7 Statistics for Sensitivity Analysis of Lmax Parameter...... 53 Table 5-8 Statistics for Sensitivity Analysis of CQOF Parameter ...... 54 Table 5-9 Statistics for Sensitivity Analysis of TOF Parameter ...... 55 Table 5-10 Statistics for Sensitivity Analysis of CK1,2 Parameter...... 56 Table 5-11 Design Flows for Proposed Probability Distributions...... 58 Table 5-12 Design Flows for Proposed Probability Distributions...... 59 Table 5-13 Design Flows for Proposed Probability Distributions...... 60 Table 5-14 Estimated Return Periods for Historic Events Recorded at Waldron’s Bridge Gauge .... 61 Table 5-15 Estimated Return Periods for Historic Events Recorded at Willbrook Road Gauge ...... 62 Table 5-16 Estimated Return Periods for Historic Events Recorded at Frankfort Gauge ...... 62 Table 5-17 Joint Probability of rainfall and surge residuals ...... 64 Table 5-18 Annual Maxima Flows for Waldron’s Bridge Gauging Station...... 66 Table 5-19 Annual Maxima for Willbrook Road Gauging Station ...... 67 Table 5-20 Annual Maxima for Frankfort Gauging Station (modified from OPW, 2007) ...... 68 Table 5-21 Daily Rainfall Intensities for Proposed Probability Distributions ...... 70 Table 5-22 Daily Rainfall Intensities for Proposed Probability Distributions ...... 71 Table 5-23 Daily Rainfall Intensities for Proposed Probability Distributions ...... 72 Table 5-24 Catchment Characteristics Used in FSR Flood Estimation Method...... 75 Table 5-25 FSR Estimated Design Flood Flows...... 75 Table 5-26 FSR, Simulated Design and Annual Maxima Flows for Waldron’s Bridge Gauge Catchment...... 76 Table 5-27 FSR, Simulated Design and Annual Maxima Flows for Willbrook Road Gauge Catchment...... 76 Table 5-28 FSR, Simulated Design and Annual Maxima Flows for Frankfort Gauge Catchment..... 76 Table 5-29 Design Discharges for Gauge Catchments ...... 78 Table 5-30 Design Rainfall Intensities ...... 79 Table 5-31 FSR and Simulated Design Flows for Gauge Catchments ...... 79 Table 6-1 Present Day Scenario Summary Table ...... 82 Table 6-2 Design Event Coastal Water Levels ...... 82 Table 6-3 Present Day Design Flows for Rainfall Runoff Boundary Catchments (m3/s) ...... 83 Table 6-4 Future Scenario 1 Summary Table...... 84 Table 6-5 Design Event Coastal Water Levels for Future Scenario 1 ...... 85

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Table 6-6 Future Scenario 1 Design Flows for Rainfall Runoff Boundary Catchments (m3/s) ...... 86 Table 6-7 Future Scenario 2 Summary Table...... 87 Table 6-8 Future Scenario 2 Design Flows for Rainfall Runoff Boundary Catchments (m3/s) ...... 88 Table 6-9 Predicted Changes in Precipitation from Climate Change Models...... 89

APPENDICES

APPENDIX A Rainfall Data 4

APPENDIX B Historical Flooding Data 3

APPENDIX C Simulated Discharge Data 2

APPENDIX D Joint Probability Data 2

APPENDIX E Rainfall-Runoff Models 25

APPENDIX F Glossary

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LIST OF ABBREVIATIONS

Abbreviation Explanation C4i Community Climate Change Consortium for Ireland CFRAMS Catchment Flood Risk Assessment and Management Study CFRMP Catchment Flood Risk Management Plan DCC Dublin City Council DEFRA Department of the Environment, Food and Rural Affairs (UK) DLRCC Dun Laoghaire Rathdown County Council EPA Environmental Protection Agency EV Extreme Value EVA Extreme Value Analyses FSR Flood Studies Report GDA Greater Dublin Area GDSDS Greater Dublin Strategic Drainage Study GIS Geographical Information Systems IPCC Intergovernmental Panel on Climate Change OD Ordnance Datum mOD meters Ordanance Datum OPW Office of Public Works POT Peak over Threshold RCAMPC Regional Climate, Analysis, Modelling and Prediction Centre RR Rainfall Runoff SDCC South Dublin County Council SUDS SUstainable Drainage Systems WFD Water Framework Directive

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EXECUTIVE SUMMARY

The River Dodder has overflowed its banks on numerous occasions, notably during the 1986 ‘Hurricane Charlie’ event and again in 2000 and 2002. Consequently the development of a Catchment Flood Risk Management Plan (CFRMP) for the River Dodder catchment has been identified as a priority project due to existing flood risk. The River Dodder Catchment Flood Risk Assessment and Management Study (CFRAMS), which will inform the River Dodder CFRMP, aims to assess the spatial extent and degree of flood hazard and risk within the Dodder Catchment, examine future pressures that could impact on flood risk, and develop a long-term strategy for managing flood risk that is economically, socially and environmentally sustainable.

The objectives of this report are to describe a comprehensive hydrological analysis of the River Dodder main channel and its five major tributaries (Dundrum Slang, the Little Dargle, the Owendoher, the Whitechurch and the Tallaght Stream). The key purpose is to produce design discharge files for use in the hydraulic models for the present day scenario and future scenarios (2100).

During the data collection phase of this study, a large quantity of hydrological data was collated. Recorded rainfall data was supplied by Met Éireann, Dublin City Council and Dun Laoghaire Rathdown County Council. Recorded water levels and discharge data was supplied by the EPA for their three gauges within the river catchment; Waldron’s Bridge, Willbrook Road and Frankfort. RPS were also in possession of the coastal water levels used by the Dept. of Communications, Marine and Natural Resources (DCMNR) in the Irish Coastal Protection Strategy. An extensive amount of data on climate change scenarios was also gathered and expected future changes in precipitation and evaporation for the year 2100 estimated.

Hydrological (rainfall-runoff) models were created for the catchments draining to the three water level gauging stations (Waldron’s Bridge, Willbrook Road and Frankfort) in the River Dodder catchment. These hydrological models were produced using the rainfall-runoff module of MIKE 11 and are combined models comprising both NAM and Urban parameters. These models were then calibrated against recorded discharge data from the gauging stations. Long records of historical rainfall data were run through the calibrated hydrological models and simulated discharge files produced. Extreme Value (EV) Analyses were carried out on these simulated discharge files to generate design discharge values for events of various probabilities (of annual exceedance).

To describe the hydrological response of the River Dodder catchment on a smaller scale, the three calibrated rainfall-runoff models were sub-divided into fourteen ‘sub-catchment’ rainfall-runoff models. Historic rainfall data was run through these smaller models and EV analyses carried out on the resulting simulated discharge files to produce design discharges of known return period for the Present Day Scenario.

The future design horizon chosen for this study is the year 2100. ‘Sub-catchment’ rainfall-runoff models were created for two future scenarios: with and without the implementation of SUstainable Drainage Systems (SUDS). These models are based on the Present Day Scenario models with parameters altered as appropriate to reflect future catchment conditions. The rainfall datasets created for the Present Day Scenario rainfall-runoff models were modified to account for the predicted effects of climate change. These modified rainfall files were run through the future scenario rainfall-runoff models to produce design discharges of known return period.

The Present Day and Future scenario design flows will be used in the hydraulic modelling phase of the study to simulate extreme flood events.

Additional hydrological analyses were carried out to enhance the robustness of this study including analysis of the rating curves for the water level gauges (Waldron’s Bridge, Willbrook Road, Frankfort), EV Analysis of historic flow records, EV Analysis of catchment rainfall and flood predictions using the Flood Studies Report (FSR) method.

MDW0259Rp0016 i Rev F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

1 INTRODUCTION

1.1 BACKGROUND

The River Dodder has overflowed its banks on numerous occasions, notably during the 1986 ‘Hurricane Charlie’ fluvial event and again in 2002 in conjunction with a tidal event. During these events, particularly the 1986 event, extensive damage was caused in the lower reaches of the River, which flows through south Dublin to the river’s confluence with the Liffey Estuary.

The overriding purpose of the River Dodder Catchment Flood Risk Assessment and Management Study (CFRAMS) is therefore to assess the spatial extent and degree of flood hazard and risk within the Dodder Catchment, examine future pressures that could impact on flood risk, and develop a long- term strategy for managing flood risk that is economically, socially and environmentally sustainable. The outcome of the CFRAMS will be a Catchment Flood Risk Management Plan (CFRMP).

Figure 1-1 Flood Damage from Hurricane Charlie

1.2 OBJECTIVES OF REPORT

The objectives of this report are to describe a comprehensive hydrological analysis of the River Dodder main channel and its five major tributaries (Dundrum Slang, the Little Dargle, the Owendoher, the Whitechurch and the Tallaght Stream) as part of the River Dodder Catchment Flood Risk Management Plan. The analysis includes a detailed examination of the river catchment and the collation of all available hydrometric and meteorological data. With this information, rainfall-runoff models were established and calibrated before being used to generate design storm events for both present day and future catchment conditions (the year 2100). A joint probability analysis was also undertaken to assign return periods to combined storm surge (in the Dodder estuary) and extreme

MDW0259Rp0016 1 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report rainfall events. The key objective of the hydrological analysis is to produce design discharge files for use in the hydraulic models for the current catchment condition and future scenarios (2100).

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2 CATCHMENT DESCRIPTION

The River Dodder Catchment stretches from Ringsend in Dublin City, west as far as Tallaght and southwest as far as Kippure in the Dublin Mountains. The upper portion of the catchment from the source to Old Bawn in Tallaght includes the two Bohernabreena Reservoirs and spillways. This section is mainly rural while the lower catchment is already heavily developed with residential and industrial land uses. The river catchment encompasses the River Dodder as well as its five main tributaries; the Dundrum Slang, the Little Dargle, the Owendoher, the Whitechurch and the Tallaght Stream and covers a total area of 12,080 ha (120.8km2).

The River Dodder is renowned for its quick catchment response and flashy characteristics. Factors contributing to this flashy nature include; • Large rainfall events in the mountainous part of the river catchment; • Large catchment area compared to river length; • Geology and canalisation of upper catchment gives large percentage runoff.

2.1 RIVER SYSTEM

2.1.1 Dodder Main Channel

The River Dodder rises at Kippure in the Dublin Mountains at an elevation of 753.8mOD and flows in a north-westerly direction towards Tallaght where it changes course to a north-easterly direction and continues down through the urban areas of Rathfarnham, Milltown, Donnybrook and Ballsbridge before entering the Liffey estuary at Ringsend. The total length of the river is approximately 27km.

The Bohernabreena reservoir system is located in the upper reaches of the River Dodder at approximately 180mOD. Approximately 27.95km2 of the River Dodder catchment drains to the reservoirs while the remaining 92.85km2 drains directly to the river downstream of the lower reservoir spillway. The stretch of river upstream of the Lower Reservoir spillway is approximately 9.5km in length and falls at an average gradient of 1 in 15. The upper catchment topography is predominantly rural and mountainous in nature.

The stretch of river downstream of the reservoirs is approximately 17.5km in length and falls at an average gradient of 1 in 115. The lower reaches of the river are highly modified and canalised with walled banks in some areas, however in the newer urban areas there are large areas of parkland and riverside walks.

Below the weir at Ballsbridge, approximately 2 km from the confluence with the River Liffey, the River Dodder is considered tidal.

The river has five principal tributaries flowing southwards from the Dublin Mountains into the river. These are:

1. The Tallaght stream 2. The Owendoher River 3. The Whitechurch River 4. The Little Dargle River 5. The Dundrum River

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2.1.2 Tallaght Stream:

The Tallaght Stream rises at Knockannavea in the Dublin Mountains, south of Tallaght, at an elevation of approximately 390mOD. It flows in a northerly direction towards Jobstown and then flows east through Tallaght where it joins the River Dodder Main Channel. The stream is approximately 8.2km in length, falls at an average gradient of 1 in 25 and drains a catchment of approximately 12.9km2.

2.1.3 Owendoher:

The Owendoher rises at Kilakee in the Dublin Mountains, at an elevation of approximately 570mOD. It flows in a northerly direction through Edmondstown and Ballyboden before joining the River Dodder Main Channel at Bushy Park in Rathfarnham. The stream is approximately 9.9km in length, falls at an average gradient of 1 in 19 and drains a catchment of approximately 13.3km2.

2.1.4 Whitechurch:

The Whitechurch is a tributary of the Owendoher and rises between Tibradden and Kilmashogue Mountains at an elevation of approximately 480mOD. It flows in a northerly direction through Marley Park and St. Enda’s Park and onto Willbrook where it meets the Owendoher. The stream is approximately 7.7km in length, falls at an average gradient of 1 in 18 and drains a catchment of approximately 8.9km2.

2.1.5 Little Dargle:

The Little Dargle rises at Two Rock Mountain at an elevation of approximately 520mOD. It flows in a northerly direction through Ballinteer and Churchtown before joining the River Dodder Main Channel in Rathfarnham. The stream is approximately 8.5km in length, falls at an average gradient of 1 in 17 and drains a catchment of approximately 8.3km2.

2.1.6 Dundrum Slang:

The Dundrum Slang rises at Three Rock Mountain at an elevation of approximately 430mOD. It flows in a northerly direction through Dundrum and Windy Arbour before joining the River Dodder Main Channel in Milltown. The stream is approximately 8km in length, falls at an average gradient of 1 in 20 and drains a catchment of approximately 9.5km2.

2.1.7 Bohernabreena Reservoir System

The Bohernabreena reservoir system was constructed between 1883 and 1886 to serve as a water supply to Dublin city. It consists of two separate reservoirs, known as the Upper and Lower reservoirs.

Upper Reservoir The Upper Reservoir has a surface area of 0.23km2, a maximum capacity of 1.56x106 m3 and a catchment area of approximately 6.95km2. The waters from this catchment are clear and suitable for drinking and the reservoir is therefore used for water supply purposes. The water level in this reservoir depends on natural inflow and drinking water demands and it is not used for water storage during storm events.

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Lower Reservoir The Lower Reservoir has a surface area of 0.12km2, a maximum capacity of 0.5x106m3 and a catchment area of approximately 21km2. This reservoir can be drawn down to provide additional water storage preceding an expected storm event.

A 1600mm diameter valve at the Lower Reservoir outlet controls the water level and can lower it by up to 4m from a top water level of 148.3m (outlet weir crest level) to 144.3mOD. The storage offered by lowering the water level has been calculated and is presented in Table 2-1 below:

Water level Surface Area Effective Storage (mOD) (m2) (m3)

148.3 - 108,497.40

147.3 98,953.60 103,725.50

146.3 85,853.40 92,403.50

145.3 73,321.40 79,587.40

144.3 49,269.20 61,295.30 Total 337,011.70 Table 2-1 Bohernabreena Lower Reservoir Storage

2.2 WATER LEVEL GAUGING STATIONS

The Environmental Protection Agency (EPA) operates four water level gauging stations within the River Dodder catchment, namely Waldron’s Bridge, Willbrook Road, Frankfort and the new station at Bohernabreena Reservoir.

Waldron’s Bridge Gauging Station: Waldron’s Bridge water level gauging station is located upstream of Waldron’s Bridge on the River Dodder Main Channel in Rathgar, Co. Dublin. Dublin City Council installed the water level recorder in 1952 and upgraded the recording equipment in 2000. The gauging station is situated on the left bank of the river approximately 25m upstream of Waldron’s Bridge and 190m upstream of Orwell Weir. The river is approximately 15m wide at this point. A catchment area of circa 100km2 drains to Waldron’s Bridge gauging station. This catchment includes the catchments draining to Bohernabreena and Willbrook Road gauging stations. Figure 2-1 shows Orwell Weir.

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Figure 2-1 Orwell Weir

Willbrook Road Gauging Station: The water level gauging station at Willbrook Road is located on the Owendoher stream approximately 150m downstream of its confluence with the Whitechurch stream and immediately upstream of the footbridge to Glenbrook Park off Willbrook Road. The stream is approximately 5m wide at the site of the gauging station and is canalised with stone walls constructed at both the left and right banks. South Dublin County Council installed the water level recorder in 1980 and erected a channel control at the station in 1996. The recording equipment was updated in 2002. A catchment area of circa 23km2 drains to this gauging station. Figure 2-2 shows Willbrook Road water level gauging station.

Figure 2-2 Willbrook Road Water Level Gauging Station

Frankfort Gauging Station: Frankfort water level gauging station is located on the Dundrum / Slang stream approximately 135m downstream of the road bridge to Frankfort Park and 20m upstream of the road bridge to Frankfort Court off the Dundrum Road. The stream is approximately 4.3m wide at the gauging station with a vertical concrete wall at the left bank and a naturally sloping, vegetated right bank. South Dublin County Council erected a water level recorder at this site in 1982 and erected a channel control in

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1986. The recording equipment was updated in 2002. A catchment area of circa 6.4km2 drains to this gauging station. Figure 2-3 shows Frankfort water level gauging station.

Figure 2-3 Frankfort Water Level Gauging Station

Bohernabreena Gauging Station: The water level gauging station at Bohernabreena is located downstream of the Lower Reservoir spillways. It has only recently been installed (June 2005) and has yet to be calibrated. For this reason data from the Bohernabreena gauge is not being utilised in this study. Figure 2-4 shows Bohernabreena water level gauging station

Figure 2-4 Bohernabreena Water Level Gauging Station

All flow data available digitally from Waldron’s Bridge, Willbrook Road and Frankfort water level gauging stations was provided by the EPA for use in the River Dodder CFRAMS.

Figure 2-5 is a schematic of the River Dodder catchment showing the tributaries, gauging station locations and contributing catchments.

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Figure 2-5 River Dodder Catchment Schematic

2.3 LAND USE DATA

For the purposes of land-use assessment, the River Dodder catchment has been broken down into 18 sub-catchments (A to R) ranging in size for 37km2 to 0.8km2. The sub-catchments boundaries have been assigned based on the tributary catchment areas and on the rural / urban divide. The boundary between the rural and urban catchments was taken at 160mOD elevation as it is assumed that major future development will not occur beyond this point due to water supply restrictions and this elevation is also seen as a limit in terms of landscaping and conservation. Sub-catchments A, B, C, D, E, G, J, M, O, Q and R are classed as “urban” (u), while sub-catchments F, H, I, K, L, N and P are classed as “rural” (r). A map showing the River Dodder catchment and the sub-catchment boundaries with sub- catchment names can be seen in Figure 2-6.

2.3.1 Current Development Scenario

As part of the Greater Dublin Strategic Drainage Study (GDSDS) (Dublin Drainage Consultancy, 2002), a comprehensive Population and Land Use Study was undertaken. One element of this land use study involved the assessment of the 2002 development situation in the Greater Dublin Area. Given the time lapse since the preparation of the GDSDS Population and Land Use Study and given the level of development in the Greater Dublin Area in the intervening years, the GDSDS current development land use figures were revisited for this project.

To update the GDSDS land use figures to reflect the current level of development in the River Dodder catchment, the latest 1:1000 OSI mapping was overlaid on the GDSDS mapping and recent developments identified. The current level of development in the sub-catchments is illustrated in Table 2-2.

Sub- Catchment Development Development Catchment Area (km2) 2007 (%) 2007 (km2) Name A (u) 9.53 58.70 5.59 B (u) 4.81 54.10 2.60 C (u) 8.62 67.30 5.80 D (u) 7.32 88.28 6.46 E (u) 5.93 67.00 3.97 F (r) 37.46 3.00 1.13 G (u) 4.67 52.70 2.46 H (r) 9.37 5.00 0.49 I (r) 3.03 4.00 0.12 J (u) 1.50 78.20 1.17 K (r) 4.36 4.00 0.17 L (r) 5.30 4.00 0.21 M (u) 3.68 30.10 1.11 N (r) 1.03 9.00 0.09 O (u) 7.65 72.15 5.518 P (r) 0.83 5.00 0.06 Q (u) 3.26 85.34 2.79 R (u) 2.64 76.40 2.015 Table 2-2: Current Development Scenario

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Figure 2-6 River Dodder Catchment and Sub-Catchment Boundaries

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2.3.2 Future Development Scenarios

The GDSDS Population and Land Use Study included future development scenarios for 2011 and 2031. When these development scenarios are applied to the River Dodder sub-catchments in addition to the current development situation, the following table can be produced:

% Development GDSDS GDSDS Sub- Sub-Catchment Projected Projected Catchment 2007 Area (km2) Development Development Name 2011 2031 A (u) 9.53 58.70 68.60 68.60 B (u) 4.81 54.10 56.00 56.00 C (u) 8.62 67.30 74.70 74.70 D (u) 7.32 88.28 89.69 89.87 E (u) 5.93 67.00 67.30 67.30 F (r) 37.46 3.00 3.00 3.00 G (u) 4.67 52.70 55.30 64.50 H (r) 9.37 5.00 5.00 5.00 I (r) 3.03 4.00 4.00 4.00 J (u) 1.50 78.20 80.72 80.72 K (r) 4.36 4.00 4.00 4.00 L (r) 5.30 4.00 4.00 4.00 M (u) 3.68 30.10 30.10 37.60 N (r) 1.03 9.00 9.00 9.00 O (u) 7.65 72.15 72.59 72.59 P (r) 0.83 5.00 5.00 5.00 Q (u) 3.26 85.34 87.38 88.14 R (u) 2.64 76.40 76.81 78.14 Table 2-3: GDSDS Future Development Scenario

It can be seen from Table 2-3 that the projected development scenario for 2011 has nearly been realised within all “urban” sub-catchments. It would therefore appear that the future development scenarios proposed in the GDSDS were underestimated for the River Dodder catchment. However, it should be noted that the GDSDS scenarios were developed based on the 2002 census. The underestimation of recent population projections is discussed in the Department of the Environment and Local Government Circular SP1/07 – Revised National and Regional Population Targets to 2020.

To construct a realistic picture of future development in the sub-catchments, it has been assumed that development will continue at the current rate until 2100. To ascertain this, current annual rates of development were calculated for each sub-catchment between 2005 and 2007 and development figures for 2100 were extrapolated from these. The 2005 development figures were prepared for a Flood Impact Assessment carried out by RPS for a private developer for a site along the River Dodder (Flood Impact Assessment – Dodder River, RPS 2006).

A limit of 85% development was placed on the projections for urban sub-catchments with current development levels not already in excess of this figure. This limit was based on the observation that urban development is limited to approximately 87% to 89% in the inner city areas and approximately 83% to 85% in the urban areas outside the immediate city centre. Where this limit was already exceeded we retained the current development figures. It is assumed that no future development will occur in the “rural” sub-catchments since all of these are above the 160mOD elevation line.

Table 2-4 presents the projected future development percentages for each sub-catchment for the 2100 scenarios as extrapolated from current annual rates of development.

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% Development

Sub-Catchment Sub-Catchment Annual % 2005 2007 2100 Name Area (km2) Increase A (u) 9.53 52.40 58.70 3.15 85.00 B (u) 4.81 50.20 54.10 1.95 85.00 C (u) 8.62 62.60 67.30 2.35 85.00 D (u) 7.32 83.80 88.28 2.24 88.28 E (u) 5.93 66.60 67.00 0.20 85.00 F (r) 37.46 3.00 3.00 0.00 3.00 G (u) 4.67 49.50 52.70 1.60 85.00 H (r) 9.37 5.00 5.00 0.00 5.00 I (r) 3.03 4.00 4.00 0.00 4.00 J (u) 1.50 77.26 78.20 0.45 85.00 K (r) 4.36 4.00 4.00 0.00 4.00 L (r) 5.30 4.00 4.00 0.00 4.00 M (u) 3.68 27.20 30.10 1.45 85.00 N (r) 1.03 9.00 9.00 0.00 9.00 O (u) 7.65 66.34 72.59 3.13 85.00 P (r) 0.83 5.00 5.00 0.00 5.00 Q (u) 3.26 80.80 85.34 2.27 85.34 R (u) 2.64 71.10 76.40 2.65 85.00 Table 2-4: Revised Future Development Scenario

It should be noted that there is a high degree of uncertainty in projecting future development scenarios for the River Dodder Catchment. A limit of 85% has been placed on development in all urban sub- catchments which have not yet reached this level of urbanisation. It is assumed that a minimum of 15% of the catchments will be retained as “green-belt” areas. This is a conservative approach to future development projections as it ignores any potential effects from the introduction of SUstainable Drainage Systems (SUDS) into developments in the sub-catchments. However, the role SUDS could play as a flood risk management measure will be further investigated in this report.

2.3.3 Afforestation

Large numbers of studies have been carried out around the world on the effects of afforestation on stream flows and there is widespread agreement amongst researchers that this practice can alter water balances in catchments. Although the precise effect afforestation has on catchments is the subject of debate. Huang et al (2002) in the paper entitled “Runoff responses to afforestation in a Watershed of the Loess Plateau, China”, showed that afforestation reduced surface runoff and the reduction increased with the age of the trees, reaching a maximum reduction after approximately 15 years. However, the pre-planting land drainage often associated with afforestation can have the opposite effect causing increases in high and low flows (Flood Estimation Handbook, 1999). However, in time, as the trees mature and the drainage system becomes vegetated, the expected decrease in runoff flows may be realised.

The effects of afforestation of the upland areas on catchment hydrology will not be examined any further in this study as it is felt that the long-term prospects of such activities will improve on the current rainfall-runoff scenario with regard to flooding concerns. However, large areas of the catchment would need to be forested to see any appreciable benefit.

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2.3.4 Deforestation

It is generally accepted that deforestation increases the annual average flow in a catchment as shown by Bosch and Hewlett (1982) in their summary of 94 catchment experiments. The immediate effects of deforestation on catchment hydrology are similar to those of urbanisation e.g. increased flow peaks and decreased runoff times. Some of these effects can be attributed to deforestation practices, i.e. soil disturbance by plant machinery causing compaction and soil loss/ erosion. The longer term effects of deforestation depend on what replaces the forest, e.g. agriculture, development or new forest (Flood Estimation Handbook, 1999).

2.3.5 Most Likely Future Development Scenario

The most likely future scenario will see development in the River Dodder “urban” sub-catchments continue at current rates until it is capped due to a lack of available lands. It is not expected that any future development will occur in the “rural” sub-catchments above 160mOD elevation.

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3 HYDROLOGICAL DATA

This chapter details the hydrological data collated for use in this study. The data gathered includes historic rainfall, water level, discharge and coastal water level records as well as information on climate change scenarios.

3.1 METEOROLOGICAL DATA

All available digital rainfall gauge data from within the River Dodder catchment was requested from the relevant Local Authorities (Dublin City Council (DCC), Dun Laoghaire Rathdown County Council (DLRCC) and South Dublin County Council (SDCC)). A summary of the rain gauge data received from the Local Authorities is presented in Table 3-1.

Rain Gauge Start (Year) End (Year) Source Data Type Roundwood 2003 2007 DCC Hourly Bohernabreena 2003 2007 DCC Hourly Ballymore Eustace 2003 2007 DCC Hourly Donnybrook 2004 2007 DCC Minute Ballyboden 2004 2007 DCC Minute Sandyford 2003 2007 DLRCC Daily Table 3-1: Rain Gauge Data Provided by Local Authorities

In addition, Met Éireann have provided an extensive amount of data from their rainfall gauges throughout the River Dodder catchment and surrounding area. The OPW coordinated the provision of this data and Met Éireann kindly supplied it free of charge for use in the River Dodder CFRAMS. Table 3-3 overleaf presents a summary of the rain gauge data received from Met Éireann. The use of radar images of weather patterns was investigated, but Met Éireann advised that they are useful for qualative rather than quantative purposes and therefore are of limited value to this study.

Met Éireann also supplied Extreme Rainfall Return Period Charts for the rainfall gauges at Casement Aerodrome, Glenasmole, Dundrum and Phoenix Park. These will assist in the derivation of rainfall return period growth factors which will be used for the very small catchments with concentration times of less than one hour. Table 3-2 presents the Extreme Rainfall Return Period Chart for Dundrum rainfall gauging station.

Maximum rainfall (mm) of indicated duration expected in the indicated return period. Return Period (years) Duration 1/2 1 2 5 10 20 50 100 1 min 1.8 2.0 2.4 2.9 3.3 2 min 3.1 3.5 4.1 5.0 5.7 5 min 5.5 6.3 7.4 9.2 10.5 10 min 7.9 9.1 10.8 13.5 15.6 15 min 5.1 6.3 7.1 9.5 11.5 13.7 17.3 20 30 min 7.0 8.6 9.6 12.8 15.3 18.2 23 26 60 min 9.4 11.5 12.8 16.9 20.1 24 29 34 2 hour 12.8 15.6 17.1 22.1 26 30 37 42 4 hour 17.8 21.3 23.1 29 34 39 46 53 6 hour 21.7 25.8 28 35 40 46 55 62 12 hour 28.6 34 37 46 52 59 69 79 24 hour 36 42 46 56 64 72 84 95 48 hour 45 52 56 69 78 88 102 114 Table 3-2 Extreme Rainfall Return Period Chart for Dundrum Rainfall Gauging Station

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Rain Gauge Start (Year) End (Year) Source Data Type Dublin Airport 1941 1994 Met Éireann Daily Dublin Airport II 1994 2007 Met Éireann Daily Dublin Airport Check 1994 1997 Met Éireann Daily Dublin (Phoenix Park) 1941 2007 Met Éireann Daily Dublin (Glasnevin) 1941 2006 Met Éireann Daily Glenasmole D.C.W.W. 1941 2006 Met Éireann Daily Dun Laoghaire (People's Park) 1941 1989 Met Éireann Daily Dublin (Ringsend) 1941 2006 Met Éireann Daily Ballyedmonduff 1967 1984 Met Éireann Daily Ballyedmonduff House 1985 2007 Met Éireann Daily Stillorgan (Vartry House) 1941 1999 Met Éireann Daily Tallaght (St. Maelruain's) 1945 1950 Met Éireann Daily Peamont (San.) 1941 1979 Met Éireann Daily Glenasmole (Castlekelly) 1959 2007 Met Éireann Daily Glenasmole (Supt.'s Lodge) 1959 2006 Met Éireann Daily Dublin (Merrion Square) 1948 2007 Met Éireann Daily Leixlip (Gen. Stn.) 1949 2006 Met Éireann Daily Dublin (Ballsbridge) 1959 1972 Met Éireann Daily Killiney (Tedburn) 1961 1987 Met Éireann Daily Milltown (Golf Club) 1963 2000 Met Éireann Daily Ballyboden 1966 2001 Met Éireann Daily Tallaght 1966 1969 Met Éireann Daily Tibradden (Larch Hill) 1967 1990 Met Éireann Daily Clondalkin 1967 1985 Met Éireann Daily Rathcoole Saggart 1969 2001 Met Éireann Daily Rathfarnham (St. Columba Coll.) 1972 1995 Met Éireann Daily Dublin (Simmonscourt) 1972 2006 Met Éireann Daily Dublin (Pembroke Road) 1975 1978 Met Éireann Daily Brittas (Glenaraneen) 1975 2006 Met Éireann Daily Dublin (Dundrum) 1975 1995 Met Éireann Daily Blackrock 1982 1997 Met Éireann Daily Kippure (T.V. Trans. Stn.) 1962 1968 Met Éireann Daily Blackrock 1982 1985 Met Éireann Daily Dundrum (Dromartin) 1997 2001 Met Éireann Daily Dun Laoghaire 1997 2006 Met Éireann Daily Knocklyon (St. Colmcille's) 2004 2005 Met Éireann Daily Bray G.S. 1949 1993 Met Éireann Daily Enniskerry (Kilmalin) 1975 2006 Met Éireann Daily M. Ballinatona 1993 2006 Met Éireann Daily M. Sally Gap 1943 2006 Met Éireann Daily Casement Aerodrome 1954 2007 Met Éireann Daily Casement Aerodrome 01/01/1964 22/05/2007 Met Éireann Hourly

Table 3-3: Rain Gauge Data Provided by Met Éireann

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Figure A.1 in Appendix A shows the location of all the rainfall gauging stations used in this study.

The rainfall data received was of varying quality, with short, long, complete and incomplete data series. Figure 3-1 is an example of a long complete rainfall data series and is an example of a short incomplete data series with large gaps.

Figure 3-1 Example of Complete Data Series from Glenasmole (Castlekelly) Station

Ballyboden Hourly Rainfall [mm] 0 s f d . y l r u

13 o H _ n e d o

12 b y l l a B \ l l a f

11 n i a r \ a t a d \ 10 f f o n u R - l l a

9 f n i a R \ K M

8 \ s e l i r F h m / a

7 r m g o r m P \ : 6 C

0 2004 2005 2006 2007

Date

Figure 3-2 Example of Incomplete Data Series from Ballyboden Station

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The rainfall data was used in the Rainfall-Runoff computer models to simulate catchment response and stream discharge.

3.2 HYDROMETRIC DATA

Data from Waldron’s Bridge, Willbrook Road and Frankfort water level gauges was provided by the EPA for use in the River Dodder CFRAMS. Table 3-4 below lists the hydrometric data received from the EPA for this purpose.

Water Level Gauge Data Period Data Type Jan. 1986 - Dec. 1986 60 Min. Discharge and Water Level Jan. 1993 - Dec. 1994 60 Min. Discharge and Water Level Waldron's Bridge Jan. 1997 - Dec. 1998 60 Min. Discharge and Water Level Oct. 2000 - July 2007 15 Min. Discharge and Water Level Dec. 1980 - Dec. 1990 60 Min. Discharge and Water Level Jan. 1992 - Dec. 1992 60 Min. Discharge and Water Level Willbrook Road Jan. 1994 - Dec. 1994 60 Min. Discharge and Water Level Nov. 1996 - Jan. 2002 60 Min. Discharge and Water Level Apr. 2002 - Jul. 2007 15 Min. Discharge and Water Level Jan. 1986 - Jun. 1986 60 Min. Discharge and Water Level Jan. 1993 - Dec. 1994 60 Min. Discharge and Water Level Frankfort Jan. 1997 - Jul. 1998 60 Min. Discharge and Water Level Apr. 2002 - Dec. 2006 15 Min. Discharge and Water Level

Table 3-4 Hydrometric Data from EPA

Figure 3-3 presents typical flow hydrographs for each of the three gauging stations.

Typical Flow Hydrographs from Water Level Gauging Stations

W aldron's Bridge

25 Frankfort

Willbrook Road

20 / s 3 ] 15 F l o w 10 [ m

J J J O a F M M u u A N D n e a n l0 u c o e 0 b r a 0 5 g t0 v c 5 0 0 y0 5 0 5 0 0 5 5 5 Date 5 5 5

Figure 3-3 Typical Flow Hydrographs

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3.3 COASTAL DATA

Coastal water levels are recorded at a number of locations in Dublin. Long term observations are available from DCC and Dublin Port which recorded coastal water levels at Poolbeg Lighthouse. This gauge was later moved to North Wall Quay and recently an additional gauge has been installed at Kish Bank Lighthouse. RPS have analysed these values and have a peak over threshold data set for the period January 1980 to December 2004. In addition RPS have different return period coastal water levels used by the Dept. of Agriculture, Fisheries and Food (DAFF) in the Irish Coastal Protection Strategy Study. This data set will be used as downstream boundary conditions for the hydraulic model. Furthermore, the Dublin Coastal Flooding Protection Project, a recent study carried out by Royal Haskoning Ireland for DCC, will be used to provide extreme water levels for the tidal section of the Dodder.

3.3.1 Joint Probability Assessment of Extreme Coastal and Fluvial Events

The River Dodder is influenced by both tidal and fluvial events below the weir at Ballsbridge. While it is relatively easy to derive the return periods for each component separately, under certain circumstances both extreme coastal water levels (storm surge) and extreme discharges in the river can occur. This can result in back water effects potentially giving higher water levels compared to the levels expected from fluvial or coastal events on their own.

The likelihood of the combination of events is referred to as probability of joint occurrence or joint probability and needs to be addressed in this study. Essentially typical storm surges are associated with cyclones tracking in from the Atlantic over Ireland occasionally combined with zonal fronts associated with rainfall and strong winds as well as sudden changes in wind direction. One very good example is the October 2004 storm surge event, which gave large water levels along the south coast and Wexford area and combined with large rainfall gave rise to significant flooding in southern parts of Ireland. The meteorological condition of this event is given in Figures 3-4 and 3-5. Even in Dublin the total rainfall on the 27th & 28th of October 2004 reached a value of over 100mm, with most rain falling when wind speeds peaked.

Detailed analysis of the joint probability of extreme fluvial flows and extreme coastal water levels have been undertaken in the UK as part of a Research & Development project lead by the Department for Environment, Food and Rural Affairs (DEFRA). DEFRA have provided guidelines for various coastal regions of the UK. However these are not transferable to the Dodder catchment as meteorological conditions causing surge on the Western shores of the Irish Sea are different from those on the Eastern (UK) shores. Thus an analysis has been carried out based on recorded surge values at Dublin against observed rainfall for the Dodder catchment. Based on the combined occurrence of surge and precipitation, the χ dependency measure was derived, which provides the likelihood of joint occurrence. This analysis includes an estimate of the confidence intervals using Jack-Knife resampling.

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Figure 3-4: Mean Sea Level (MSL) Pressure Fields in hPa

Figure 3-5: Hourly rainfall during October 2004 storm surge event for Dodder catchment

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3.4 HISTORIC FLOODING DATA

This section outlines the various historic flooding data that was available. Maps outlining the historical flood extent of the River Dodder as well as isolated flooding points throughout the catchment are provided in Appendix B.

3.4.1 River Dodder Main Channel

The River Dodder has a history of regular flooding in the 20th Century, as evidenced in Table 3-5. This table presents a list of highest ranked floods at Waldron’s Bridge Gauge for the past 100 years (Cawley et al., 2005).

Peak Flow Date 3 (m /s) 25th August 1986 232 25th August 1905 198 5th November 2000 156 3rd September 1931 153 17th November 1965 139 19th December 1958 116 2nd December 2003 112 3rd February 1994 110 5th November 1982 106 9th April 1998 87 2nd November 1968 85 December-83 82 11th June 1993 81 26th August 1912 80 25th September 1957 74

Table 3-5 Highest Ranked Floods at Waldron’s Bridge Gauge for the 20th Century.

The peak flow rate from the 25th/26th August 1986 (Hurricane Charlie) was estimated at 232 m3/s by Hennigan (et al) in 1988. This estimate was based on a rating curve derived from flow measurements with a peak measured flow rate of 33 m3/s. However the EPA’s most recent estimate for the Hurricane Charlie event is 269 m3/s.

Information on flooding in the years before 1900 is difficult to garner, although Jack Keyes (1987) presents a table of historical flood events from 1880, 1883, 1891 and 1898. In addition, The Old Dublin Society Paper, (Dixon, 1953) references flooding on the River Dodder in 1739, 1787, 1794, 1802 and 1851. However, no gauged flow quantities are provided in either of these papers. It is mentioned in the EPA paper on flooding in the Dodder Catchment (Mac Cárthaigh, 2005) that the construction of Bohernabreena Reservoir in 1883 has helped to alleviate downstream flooding.

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3.4.2 Tributaries

There is very little published data available on flooding along the main tributaries of the River Dodder. The exception is a report on the Little Dargle River prepared in 1958 by Dublin Corporation. This document references flooding on the Little Dargle in September 1931, December 1956, September 1957 and February 1958. It also discusses proposed flood alleviation works which were put in place circa. 1958. Other information on flooding along the tributaries was gathered from the OPW website (www.floodmaps.ie).

Many documents relating to historic flooding in the River Dodder catchment were reviewed during the Hydrological Analysis phase of this study. A list of these documents is provided in Table 3-6.

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Reports on Historic Flooding

River Dodder Flooding Report - Dec 1986 (DCC)

Hurricane Charlie - An Overview Activities of An Foras Forbatha EPA Report on Flooding in the Dodder Catchment 26 August 1986 (Hurricane Charlie) and 2 December 2003 (EPA July 2005) Presentation of Analysis Carried Out by Drainage Design Division of Dublin County Council on The Dodder River With Particular Reference to Flooding Which Occurred on 25th/26th August 1986 to IEI

Dodder River - Flood Study (IEI Paper 1988)

Tidal Flooding of 1st February 2002 (IEI Paper)

River Dodder 1986 Floodplain

Photographs of Flooding During Hurricane Charlie

Photographs following Hurricane Charlie Flooding

Photographs of Flooding During 1987 Flood Event

River Dodder Flooding 1958 - Interim Report

Flooding at AIB Ballsbridge (Hurricane Charlie)

A Selection of Extreme Flood Events - The Irish Experience (IEI Paper)

Bridge Collapse - Causes, Consequences and Remedial Measures - 1987

The Engineer Journal Vol. 40 No. 11 - Hurricane Charlie - River Dodder Flooding

Hurricane Charlie - An Overview of Flooding in Dublin City Rivers

Hurricane Charlie Photos

Report on Little Dargle River

The use of Historical Data in Flood Frequency Estimation Hurricane Charlie - An Overview - Flooding in Dublin City Rivers - 25th/26th August 1986 Weather in Old Dublin (F.E. Dixon) - Old Dublin Society Dodder Investigation - Flood of the 19th December 1958 - Waterworks Dept. - 20th March 1959 Feasibility of Reservoir Control at Bohernabreena on the Dodder River - Second Report - Sept. 1978 - Dublin Corporation Main Drainage Department. R. Dodder - Proposed Improvement Works From Ballsbridge to Donnybrook - Dublin Corporation Sewer and Main Drainage Section - March 1966 - Report R. Dodder - Proposed Improvement Works From Ballsbridge to Donnybrook - Dublin Corporation Sewer and Main Drainage Section - 25th July 1966 - Drawings

Table 3-6 Documents Relating to Historic Flooding in River Dodder Catchment

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3.5 CLIMATE CHANGE SCENARIOS

3.5.1 Effect of Climate Change on Precipitation

Met Éireann provided output data taken from three climate change models and this was used to estimate the increase in precipitation across the Dodder Catchment for the required epochs and ultimately allow an assessment of how climate change may effect river flows. The data used was taken from the ECHAM 5 (EC5), Hadley Centre High Sensitivity (HAH) and Hadley Centre Low Sensitivity (HAL) models;

• ECHAM 5 is the fifth-generation general circulation model developed at the Max Plank Institute for Meteorology (MPIM) and is the most recent version of the ECHAM (European Centre Hamburg Model) series of models. The ECHAM models evolved from the spectral weather prediction model of the European Centre for Medium Range Weather Forecasts (ECMWF) (Roeckner et al., 2003). This model was downscaled to regional level using the RCA model. The model inputs are based on the A1B emissions scenario which predicts that global population will increase until 2050 and then decrease after that, coupled with rapid economic growth eventually leading to the introduction of new “cleaner” more efficient technologies. It also predicts that there will be a balanced reliance on all energy sources.

• The Hadley Centre is the climate change research department of the UK Met Office. The High and Low Sensitivity models are based on their Regional Climate Models (RCM) which take their input at the boundaries from the Hadley Centres Coupled Atmosphere-Ocean General Circulation Models (AOGCM’s), HadCM2 and HadCM3. Both the High and Low sensitivity models are based on the A2 emissions scenario with various parameters differing between the two models to enable assessment of the climates sensitivity. The A2 emissions scenario predicts a heterogeneous world with an underlying theme of self reliance and preservation of local identities with a continually increasing global population. Economic growth and technological change is assumed to be slow compared to other scenarios.

3.5.1.1 Data

The data was provided as a spatially varying factor of change in average monthly rainfall for 3 time periods 1961 - 2000, 2021 - 2060 and 2061 – 2100. These can be related to 1 past epoch and 2 future epochs 1980, 2040 and 2080 respectively. As the required epoch for the current project is 2100 an interpolation of the data was required to extract the relevant factors of change. This was done for each set of data from the 3 models to produce 3 different sets of factors.

Interpolation and Extrapolation Methodology

The data was provided as individual ASCII files for each month within each epoch for each model i.e. a total of 108 files. Each file contains 25,896 points in a 166 by 156 grid. The MATLab software package was used to undertake the interpolation and extrapolation;

• The data was firstly loaded into the package in monthly groups for all epochs from a particular model i.e. the January data for 1980, 2040 and 2080 was loaded as a group then February and so on. This effectively produced a 4 dimensional matrix within MATLab for each model i.e. each point has an I and J coordinate with an epoch and month attribute.

• A linear interpolation script was then applied to the 4th dimension in the matrix (the monthly mean for each epoch) and the relevant values for 2100 were extrapolated from this.

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A contoured representation of the data can be produced to allow a visual inspection of the climate change factors (Figure 3-6)

Figure 3-6 Contoured Representation of Interpolated Precipitation Factors

To identify the points applicable to the Dodder catchment an original point file was loaded into ArcGIS. This allowed a database to be exported containing only the relevant points with a record of their unique identifier and I J coordinates. Having a record of the I J coordinates allowed the values for specific points to be exported from the matrix. Three points were considered relevant to the Dodder catchment and their notional location shown in Figure 3-7.

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Figure 3-7 Location of data points in relation to the Dodder Catchment

The influence of each point on the final climate change factor was calculated by creating Thiessen Polygons based on the data point locations and determining the percentage of the catchment within each polygon. The resultant percentage influence was then applied to the required monthly factor of each point. The average of the three climate change model predictions is taken, for the purposes of this study, as the Mid Range Future Scenario predicted change in precipitation due to climate change for 2100 across the whole catchment. The Monthly Maximum climate change model predictions are taken as the High End Future Scenario figures. Table 3-7 presents the climate change model predictions along with the average and maximum predicted values for each month.

Hadley High Hadley Low Monthly Monthly CC_Model ECHAM5 Sensitivity Sensitivity Average Maximum

January 130% 118% 124% 124% 130% February 103% 132% 116% 117% 132% March 112% 112% 120% 115% 120% April 107% 108% 130% 115% 130% May 75% 113% 104% 97% 113% June 86% 79% 81% 82% 86% July 82% 86% 97% 88% 97% August 90% 78% 59% 76% 90% September 111% 102% 131% 114% 131% October 109% 97% 98% 102% 109% November 115% 145% 118% 126% 145% December 113% 126% 133% 124% 133% Total 103% 108% 109% 107% 118% Table 3-7 Predicted Changes in Precipitation from Climate Change Models

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3.5.1.2 Evaporation

There is limited information available on evaporation in most climate change information and data sets. Based on the assumption that average temperatures are expected to rise by around 3° by the year 2100, the evaporation is expected to increase for both summer and winter months.

The change in evaporation was estimated using an empirical method. The equation for calculating evaporation established by Thornwaite was used, which is based on average temperature for each month. This average temperature was increased by 3°C accordingly and the new evaporation rates were estimated. Again the percentage change was applied to the observed evapotranspiration time series from the present day scenario and used in the simulation.

Table 3-8 presents the expected change in evaporation for each month.

% Change in % Change in Month Month Evaporation Evaporation

January 54.0 July 21.4 February 46.2 August 21.4 March 46.2 September 23.0 April 40.4 October 32.2 May 29.3 November 40.4 June 24.8 December 57.0

Table 3-8 Summary of Expected Changes in Evaporation for the year 2100

3.5.1.3 Application of Data

Given that a rainfall-runoff model is being used, changes were applied to the rainfall datasets and new runoff events derived. This was carried out by applying the average predicted changes in rainfall-runoff pattern taken from Table 3-7 to the existing data set to produce Average Future rainfall data. It is expected that the summer rainfall will change from mostly frontal rainfall to convective rainfall and that longer drier periods will occur. On the other hand it is assumed that the number of extreme rainfall events (>20mm/day) will increase and in particular summer rainfall will become significantly heavier (concentrated). Based on these characteristics the reference rainfall data set was modified, obtaining more concentrated rainfall events all year and, where applicable, reducing the number of days with precipitation in the summer.

Figure 3-8 presents an example of Present Day and Average Future rainfall data sets for a typical year.

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Figure 3-8 Present Day and Average Future Rainfall Datasets for Bohernabreena NAM Catchment.

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3.5.1.4 Seasonal Variation in Rainfall

To demonstrate the effect of the climate change seasonal variations on rainfall, separate rainfall data sets were extracted for December and June from a typical historical rainfall record (1964 to 2006) and from artificially generated Average Future rainfall data for the year 2100.

Table 3-9 presents return period rainfall depths from an Extreme Value Analysis carried out on these historic and future rainfall datasets for the months of December and June.

Return Period [years] Bohernabreena NAM 2 5 10 25 50 100 200 1000 Catchment Rainfall Rainfall [mm/hr] Present Day Rainfall - 7.62 9.99 11.78 14.33 16.46 18.79 21.35 28.34 December Future Rainfall - 9.47 12.33 14.52 17.70 20.383 23.37 26.68 35.94 December Present Day Rainfall - 6.11 8.99 11.2 14.2 16.54 18.95 21.42 27.38 June

Future Rainfall - June 6.73 10.43 13.28 17.18 20.24 23.39 26.64 34.51

Table 3-9 Return Period Rainfall Depths for December and June

3.5.2 Effect of Climate Change on Coastal Water Levels

The recent publication and update on climate change provided by the Intergovernmental Panel on Climate Change (IPCC2007) has effectively confirmed some of its estimates in terms of increasing temperature in the next 100 years. Overall it can be assumed that mean global temperatures will rise between 1.5° and 3°C in the next century using different relatively moderate emission scenarios and global atmospheric models. This rise in temperature will have a significant impact on coastal water levels in several ways. In relation to changes in Ireland there are a number of factors which could or are likely to impact in the next 100 years.

Firstly, it is expected that the overall increase in temperature will increase the total volume of seawater and consequently cause a rise in sea level by between 0.1 and 0.4 metres. In addition, the melting of some of the glacial sheets covering parts of the northern hemisphere will add to this. The largest contribution will be from the Greenland Ice Sheet, which is largely situated above sea level at the moment and therefore will provide a net contribution to the volume of sea water. Estimates for the volumes expected from these sheets vary greatly, since the exact thickness of the ice sheet is unknown and the rate of thawing is difficult to estimate. This thawing is related to the rise in temperature and a long-term temperature increase by the above values is likely to cause most of the ice on Greenland to melt. If this happens global water levels are likely to rise in the order of 5 metres in the next 4 to 8 centuries. The recent IPCC publication (IPCC 2007) indicates that current water level with increase by 0.5m over the next 100 years, however a value of 0.8m (DEFRA 2006) is considered a maximum likely increase.

This overall increase in water level will have an impact on the way the tides and storm surges will propagate around Ireland and it is expected that this will increase the tidal range and the surge residual in some cases, though at this stage this has not been confirmed.

As a result of the changing global climate it is expected that Ireland will in future experience a milder but wetter climate with the frequency of storms shifting slightly further north and thus decreasing. However due to the increased temperature difference between the arctic waters and Europe, it is assumed that the intensity of storms will increase and thus more extreme surges will be experienced.

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The exact impact of this on Ireland is difficult to assess, since the track of each cyclone plays an important role in the actual surges observed along the shoreline. If the track of the cyclones does not change significantly, an increase in surge residual (the value the water rise above the normal tide due to the storm) in the order of 0.05 to 0.3 metres could be expected. However these surges would be less frequent, thus it is assumed at this stage that the surge residual will have no significant impact. Met Éireann are currently simulating storm surges based on past and future climate scenarios, with results shortly.

The above is complicated in Ireland by the change of land level in relation to surrounding area which is due to changes in climate after the last ice age. Essentially the mass of the glacial sheets which covered large parts of the northern hemisphere during the last ice age caused the tectonic plates to sink. Since the ice has mostly melted now the land level has risen in the more northern areas whereas southern parts of Ireland are sinking similar to a float being ballasted down on one end and now being relieved. This whole process is termed isostatic recovery and a detailed report on this and the relevance to Ireland is expected to be published later this year. From the information available to date it appears that Dublin is close to the pivot point of this isocratic recovery and therefore not affected by this change in land level.

For the purposes of this study a water level increase of 0.5m will be adopted for the Mid Range Future Scenario and 0.8m for the High End Future Scenario.

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4 METHODOLOGY

This chapter provides an overview of the methodology employed in the development of a hydrological model for the River Dodder Catchment. The methodology comprises simulating historic recorded rainfall falling on the river catchment under current catchment conditions and analysing the resulting discharge. Design discharges of known return period are then produced for current and future catchment conditions. These design discharges and derived design water levels will be used in the construction of the hydraulic models of the Dodder Main Channel and the five tributaries. Details of the analyses undertaken in the development of the hydrological model are provided in Chapter 5.

Figure 4-1 presents the methodology as a flow chart with the relevant report sections noted under each heading.

Figure 4-1 Methodology Flow Chart

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4.1 RATING CURVE ANALYSIS

Rating curve data for Waldron’s Bridge, Willbrook Road and Frankfort water level gauges was received from the EPA for use in the River Dodder CFRAMS. This data was supplied as a series of water levels (mOD) and corresponding flows (m3/s) for each of the three gauging stations. Maximum flow ratings were provided for each gauge indicating the maximum flow and corresponding level which have been verified by the EPA at that gauge. Any flows above this maximum value have been extrapolated by the EPA from the measured data.

The extrapolated sections of these rating curves were reanalysed for the purposes of this study and new Project Rating Curves produced for each of the water level gauges. The EPA Rating Curves and the Project Rating Curves were then used to assign discharge values to the twelve highest recorded water levels at each of the gauging stations and the results compared. The rating curve analysis is detailed in Section 5.1 of this report.

4.2 RAINFALL RUNOFF MODEL CONSTRUCTION & CALIBRATION

Rainfall-Runoff modelling was carried out using the rainfall-runoff (RR) module of the Mike 11 river modelling system software. This module allows different rainfall-runoff models to be applied to a single catchment to enable the reproduction of complex catchment responses. Given the mixed urban and rural natures of the gauge catchments, combined models were established for each catchment using both the NAM and Urban components of the RR module in Mike 11.

4.2.1 NAM Model

MIKE NAM hydrological models were established to simulate the rainfall-runoff response of the catchments draining to Waldron’s Bridge, Willbrook Road and Frankfort water level gauges.

To do this, recently recorded rainfall data (recorded within the past eight years) from a number of rain gauges within the gauge catchments was entered into the model and weighted according to their contribution relative to the catchment area using voronoi polygons. The rainfall was then temporally distributed according to the rainfall pattern from the hourly rainfall data recorded at Casement Aerodrome. The resulting rainfall files span the available data record length of the rainfall gauging stations used in the establishment of each RR model. Evapotranspiration data from 1980 to 2007 was also entered into the model to provide an accurate picture of the hydrological conditions in the catchment. Table 4-1 presents details of the contribution of rainfall gauge data to each of the gauge catchments.

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Water Level Gauge Stations

Waldron's Bridge Willbrook Road Frankfort Rainfall Gauge Stations (%) (%) (%)

Kippure (T.V. Trans. Stn.) 9.57 -

Glenasmole (Castlekelly) 17.30 6.91 -

Glenasmole (Supt.'s Lodge) 5.23 0.95 -

Glenasmole D.C.W.W. 6.98 - -

Brittas (Glenaraneen) 2.29 - -

Rathcoole - Saggart 0.77 - -

Tallaght (St. Maelruain's) 13.41 - -

Knocklyon (St. Colmcille's) 10.22 2.69 -

Ballyboden 7.82 20.86 -

Milltown (Golf Club) 3.45 - -

Dublin (Dundrum) 2.65 - -

Rathfarnham (St. Columba's) 6.01 13.15 2.34

Sandyford 0.14 - 48.57

Tibradden (Larch Hill) 13.05 54.40 -

Ballyedmonduff 0.79 - 2.81

Ballyedmonduff House 0.32 1.04 -

Dublin (Dundrum) - - 28.30

Dundrum (Dromartin) - - 17.98

Total 100% 100% 100%

Table 4-1 Contribution of rainfall gauge data to flow gauge catchments.

The NAM model software has an autocalibration function which was utilised for each of the gauge catchment rainfall-runoff models. Recorded discharge data from the appropriate gauge was entered into the model as part of the autocalibration process. The models were then run in autocalibration mode where the software allocated appropriate values to the NAM parameters and used the rainfall and evapotranspiration data to produce a discharge file as similar as possible to the actual gauged data.

The autocalibration exercise resulted in a roughly calibrated model. However the model did not simulate the summer peaks and large winter peaks accurately. In order to improve the calibrated models, NAM parameters prepared as part of study undertaken for the Water Framework Directive (WFD) Project were used in the model. This study provides NAM parameters based on catchment characteristics using various GIS layers. Table 4-2 presents the NAM parameters provided by the WFD for the gauge catchments.

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NAM Parameters

Water Level Gauge Catchment CQOF UMAX CKIF CKBFLOW

Willbrook Road Gauge Catchment 0.90 15 - 20 ~ 200 2892.80

Frankfort Gauge Catchment 0.90 15 - 25 200 - 300 2969.80

Waldron's Bridge Gauge Catchment 0.90 15 - 25 ~ 200 2596.10

Table 4-2 NAM Parameters Supplied by Water Framework Directive Project

The new NAM parameters provided by the WFD gave a slightly improved calibration plot.

Figure 4-2 shows a comparison between the NAM model discharge using the WFD parameters (in red) and the recorded discharge at Waldron’s Bridge (in blue).

Recorded Discharge [m^3/s] Simulated Discharge [m^3/s]

40 ) s / 35 3 m ( e

g 30 r a h c s i 25 D

0 March April May June July August September October November December 2002 2002 2002 2002 2002 2002 2002 2002 2002 2002 Date

Figure 4-2 Example NAM Model Discharge File

Figure 4-2 it can be seen that the summer and some of the large winter peaks in the NAM model are not predicting accurately. The steep short peaks during the summer months suggest a quick catchment response to rainfall events. A response such as this is indicative of runoff from an urbanised catchment with a large amount of impervious surface area and cannot be reproduced using a NAM hydrological model. Similar results were found for both the Willbrook Road and Frankfort gauge catchments. For this reason an Urban model was created for each of the gauge catchments and joined with the NAM models in Combined hydrological models.

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4.2.2 Urban Model

An Urban Runoff Model ‘B’ was established for each of the gauge catchments to reflect the rainfall- runoff characteristics of the contributing urban area under current catchment conditions. Urban Model B facilitates the input of five different catchment surface descriptions. Each surface can be described in terms of wetting, storage, infiltration capacity and hydraulic roughness. The gauge catchments were broken down into five different surface types and appropriate percentage areas applied to each.

The basis of the surface runoff calculations of Urban Runoff Model B is the kinematic wave computation. The concept of kinematic wave is used to solve unsteady, one-dimensional, gradually varied, open channel flow problems. In these models the runoff is calculated as flow in an open channel with only gravitational and frictional losses accounted for. The volume of the runoff is dictated by the size of the contributing catchment and hydrological losses while the shape of the runoff hydrograph is controlled by the length, slope and surface roughness of the catchments.

4.2.3 MIKE Combined Model

A Combined rainfall-runoff model was established by joining the NAM and Urban models together. This model combines the runoff from the NAM and Urban models into a single runoff series. The NAM and Urban models are specified separately and have separate rainfall time-series inputs. By slightly adjusting some of the parameters in each model, reasonable calibration plots were produced for each gauge catchment. An example discharge file from the Waldron’s Bridge gauge catchment is shown in Figure 4-3.

Figure 4-3 Example Combined Model Discharge File

The rainfall-runoff models were calibrated using recently recorded rainfall data (recorded within the past eight years) to ensure that the calibrated models accurately describe the hydrology of the catchment under current conditions. However, following calibration the entire available historic rainfall record was run through the calibrated models and simulated discharge data files produced.

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An analysis of the results of the rainfall-runoff models for the three water level gauge catchments is discussed in Section 5 of this report.

4.3 SENSITIVITY ANALYSIS

Throughout the calibration / verification process the rainfall-runoff model parameters were adjusted to produce the best fit possible between the recorded and simulated flow data. To ascertain the individual influence of the principal parameters on the simulated flow data a sensitivity analysis was undertaken on the calibrated Waldron’s Bridge rainfall-runoff model. The parameters analysed are Umax, Lmax, CQOF, TOF and CK1,2 and are discussed in detail in Section 5.3 of this report.

4.4 EXTREME VALUE ANALYSIS (EVA)

Extreme value analyses were carried out on the simulated discharge data files to determine design discharges of known return period for each of the three gauge catchments (Waldron’s Bridge, Willbrook Road and Frankfort). The analyses were carried out on the simulated discharge files by peak over threshold (POT) analysis also known as partial duration series. A threshold level was applied over which the events were selected for inclusion into the data series. An inter-event independence criterion of 24 hours was applied to the analyses meaning that only peak discharge events separated by at least 24hrs were selected for inclusion in the data series.

Candidate probability distributions were then fitted to the data. Seven distributions are available in the toolbox used by RPS CE for POT analysis and are as follows:

Weibull, Generalised Pareto, Gamma/Pearson Type 3, Log-Pearson Type 3, Log-normal, Exponential Truncated Gumbel For the estimation of the parameters relating to the probability distributions three methods were applied;

The method of moments The method of L-moments The maximum likelihood method.

The goodness of fit of the resulting distributions was then tested using five statistical methods;

Chi-squared, Kolmogorov-Smirnov test, standardised least squares criterion probability plot correction co-efficient Log-likelihood measure.

The uncertainty of these distributions was evaluated by application of the Jackknife resampling technique, which provided confidence limits for each event.

Details of the EVA carried out on the simulated discharge files from the gauge catchment calibrated rainfall-runoff models are contained in Section 5.4 of this report.

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4.5 REGIONALISATION OF RAINFALL-RUNOFF MODELS

The key objective of the hydrological analysis is to produce design discharge files for use in the hydraulic models of the River Dodder Main Channel and the five tributaries. Therefore, the rainfall- runoff models created for the three water level gauge catchments were sub-divided into 14 smaller models to describe the hydrological response on a smaller scale.

The 14 ‘sub-catchment’ rainfall-runoff models adopted the NAM and Urban model parameters from the three calibrated gauge catchment models (Waldron’s Bridge, Willbrook Road and Frankfort). Historic rainfall data from the rainfall gauging stations within each of the 14 ‘sub-catchment’ boundaries was entered into the models and weighted and distributed in the same manner as the larger catchments. These catchments produced discharge data based on present day catchment conditions and historic rainfall records.

An EVA was carried out on the simulated discharge files and design discharges of known return period were produced.

‘Sub-catchment’ rainfall-runoff models were also created for future (2100) catchment conditions using rainfall incorporating climate change estimates for two separate scenarios: with and without the full implementation of Sustainable Urban Drainage Systems. Design discharges were produced for each of these scenarios for a series of known return periods.

4.6 COASTAL DATA

At Dublin, all historic water levels from 1980-2000 were analysed by RPS staff and all water levels above 4m LAT (1.49mOD Malin Head) were derived. To supplement this data set, digital data from 2000-2007 was obtained, additionally extreme level analysis data generated for Dublin City Council was incorporated. However, there are a number of issues associated with the 1980-2000 data set. The data was recorded on paper via a tracing device until the end of 1999 and only then was the gauge converted to digital recording. Commonly, one entire week is recorded on one sheet which covers a drum turning once in 24 hours. Thus there are usually 13 flood and ebb tides on one sheet. An example of such a sheet is shown in Figure 4-4.

Figure 4-4 Tide gauge record from Dublin North Wall

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On a number of occasions, in particular under storm surge conditions, the recording is less clear and it is very difficult to distinguish the separate curves. This makes the analysis of different high water levels difficult and it was noted from the analysis undertaken in this study, that occasionally high water levels were associated with the wrong date. Furthermore, the location and datum of the gauge was altered during this period and the recording also changed from imperial to metric units.

In addition, the trace recorded by the gauge shows a significantly shorter period of oscillation (less than 3 hours). This is in part attributed to a poor damping of the gauge chamber and also to seiching effects observed in Dublin Bay. Figure 4-5 shows the recorded water level and the predicted tidal elevation, together with the surge residual (red line on different scale) to illustrate this point. The surge residual clearly shows the higher harmonics due to seiching.

Dublin Tidal Elevation [m] RPS residual [m] RPS predicted [m] 4.5

4.0 1.0

3.5

3.0 0.5

2.5

2.0 0.0

1.5

1.0 -0.5 00:00 12:00 00:00 12:00 00:00 12:00 00:00 2004-10-04 10-05 10-06 10-07

Figure 4-5 Digital recorded surface elevation at Dublin with predicted water level and surge residual (different scale)

Based on the 2000-2007 digital data set, RPS derived tidal predictions using harmonic analysis. The water levels for each of the recorded high waters were determined based on these tidal predictions and then the surge residuals calculated. The data was filtered, leaving only the extreme events, based on water levels over 4m LAT.

In order to perform the extreme value analysis and joint probability analysis, the rainfall data from the entire catchment had to be matched to the already tabulated tidal data. The rainfall was measured at 09.00 hours everyday, producing daily rainfall readings. Once matched, the data was then sorted to produce the most extreme events for both rainfall and surge residuals, to be used in the extreme value analysis. With regard to the joint probability analysis, all surge residuals were used, whether positive or negative. The joint probability analysis is discussed further in Section 5.5 of this report.

4.7 SUMMARY OF METHODOLOGY

The extrapolated sections of the EPA Rating Curves were reanalysed for the purposes of this study and new Project Rating Curves produced for each of the water level gauges. The EPA Rating Curves and the Project Rating Curves were then used to assign discharge values to the twelve highest recorded water levels at each of the gauging stations and the results compared.

Rainfall-runoff models were produced for the catchments draining to each of the water level gauges in the River Dodder catchment (Waldron’s Bridge, Willbrook Road and Frankfort). These models simulated historic recorded rainfall falling on the gauge catchments under current catchment conditions and produced expected discharges at the gauge locations. The simulated discharges were then calibrated against the recorded historical discharge records for each of the gauges.

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Extreme Value Analyses were carried out on the simulated discharge files to produce discharges of known return period for each of the three catchments.

The three rainfall-runoff models produced for the gauge catchment were sub-divided into 14 smaller models. These ‘sub-catchment’ models retained the NAM and Urban model parameters from the larger scale calibrated models and new historic recorded rainfall files were created for each of them. Extreme Value Analyses were carried out on the simulated discharge files produced using the ‘sub- catchment’ models to generate design discharges of known return period. Design discharges were also produced for future (2100) scenarios using projected 2100 catchment conditions and rainfall incorporating climate change estimates.

All historic coastal water levels at Dublin from 1980-2000 were analysed by RPS staff and all water levels above 4m LAT (1.49mOD Malin Head) were derived. To supplement this data set, digital data from 2000-2007 was obtained, additionally extreme level analysis data generated for Dublin City Council was incorporated. Extreme Value Analyses and Joint Probability Analyses were carried out on this data set in conjunction with rainfall data from the entire river catchment. The resulting design water levels will be used as boundary conditions in the hydraulic models.

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5 ANALYSIS OF HYDROMETRIC AND METEOROLOGICAL DATA

This chapter details the analyses carried out on the hydrometric and meteorological data sets. It includes the calibration of the rainfall-runoff models, the extreme value analyses carried out on the simulated discharge files and joint probability analysis of rainfall and coastal surge. It also covers additional analyses on water level rating curves, historic data sets, rainfall and flood study report predictions.

5.1 RATING CURVE ANALYSIS

Rating curve data for Waldron’s Bridge, Willbrook Road and Frankfort water level gauges was received from the EPA for use in the River Dodder CFRAMS.

Waldron’s Bridge (Station 09010) The rating curve data for Waldron’s Bridge gauge comprises flow and water levels for 104 dates between 1979 and 2004. The gauge datum is 27.4mOD (Malin Head) and it has been rated by the EPA to 112m3/s. The EPA advised that a keystone in the weir has fallen out twice between 1998 and 2000 and has a significant impact on the rating curve for low flows. The mean annual flood (Qbar) for the River Dodder at Waldron’s Bridge is approximately 54m3/s and the rated value is 207% of this.

Willbrook Road (Station 09009) The rating curve data for Willbrook Road gauge comprises flow and water levels for 119 dates between 1980 and 2005. The datum for this gauge changed a number of times during these 25 years and the EPA provided the datum levels and the dates at which they changed. The current datum is 45.88mOD (Malin Head) and the gauge has been rated by the EPA to 6m3/s. The mean annual flood 3 (Qbar) for the Owendoher Stream at Willbrook Road is approximately 14m /s and the rated value is 43% of this.

Frankfort (Station 09011) The rating curve data for Frankfort gauge comprises flow and water levels for 91 dates between 1982 and 2005. The gauge datum is 37.793mOD (Malin Head) and has been rated by the EPA to 2 m3/s. 3 The mean annual flood (Qbar) for the Dundrum/Slang Stream at Frankfort is approximately 3m /s and the rated value is 66% of this.

5.1.1 Rating Curve Check

The extrapolated sections of the rating curves provided by the EPA were reanalysed for the purposes of this study. To do this 1D hydrodynamic models were constructed for short reaches of the Dodder Main Channel, the Owendoher and the Dundrum / Slang in the vicinity of the water level gauges. Details of these hydrodynamic models are provided in Table 5-1 below. These models stretch upstream and downstream of each gauge and include any hydraulic features which may impact on the gauge readings.

No. of Cross- No. of Structures Water Level Gauge Length (m) Sections Weirs Culverts

Frankfort 90 8 2 1

Willbrook Road 504 25 4 5

Waldron’s Bridge 1031.4 19 2 1

Table 5-1 Details of 1D Hydrodynamic Models of Gauging Stations.

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To calibrate the 1D models they were run with varying flow rates and flow–level (Q-H) data was exported for comparison with the EPA rating curve. The models were calibrated to the EPA rating curves up to their respective rated flow value. Beyond the rated flow value the curves produced using the hydrodynamic models diverge from the EPA rating curves. Figure 5-1 to Figure 5-3 present the rating curves produced using the 1D models and EPA rating curve points for each of the water level gauges.

Waldron's Bridge Water Level Gauge - Rating Curve Comparison 31.0

30.5

] 30.0 d a e H n

i 29.5 l a M D

O 29.0 m [ l e v

e 28.5 L r e t a 28.0 EPA Measured Data W Extrapolated EPA Data 27.5 Modelled Data Bank Level

27.0 0 50 100 150 200 250 300 Discharge [m3/s]

Figure 5-1 Waldron’s Bridge Rating Curves (Gauge rated to 112 m3/s)

Willbrook Road Water Level Gauge - Rating Curve Comparison

48.0

47.8 EPA Measured Data Extrapolated EPA Data 47.6 Modelled Data 47.4

47.2

47.0

46.8

46.6

W 46.4 a t e

r 46.2 L e v

e 46.0 l [

m 0 5 10 15 20 25 30 O 3 D Discharge [m /s] BankM Level = 51.09mOD a l i

n H e a

Figured 5-2 Willbrook Road Rating Curves (Gauge rated to 6 m3/s) ]

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Frankfort Water Level Gauge - Rating Curve Comparison 40.0

39.8

39.6 ] d

a 39.4 e H n i l

a 39.2 M D

O 39.0 m [ l e

v 38.8 e L r e t

a 38.6

W EPA Measured Data 38.4 Extrapolated EPA Data Modelled Data 38.2 Bank Level

38.0 0246810121416 Discharge [m3/s]

Figure 5-3 Frankfort Rating Curves (Gauge rated by EPA to 2 m3/s)

5.1.2 Rating Curve Application

The 12 highest water levels recorded at Waldron’s Bridge, Willbrook Road and Frankfort water level gauges were extracted from the historic record provided by the EPA. Using the rating curves (Project Rating Curves) created from the 1D hydrodynamic models, flow values were assigned to each of the water levels. These flow values were then compared against those provided by the EPA. This data is presented in Table 5-2 to Table 5-4 below.

Waldron's Bridge Gauge Discharge – EPA Discharge – Observed Water Date & Time Rating Curve Project Rating Depth (m) 3 3 (m /s) Curve (m /s) 26/08/1986 02:00 3.020 268.79 251.08 05/11/2000 23:00 2.173 156.32 141.32 02/12/2003 08:45 1.780 112.02 101.10 03/02/1994 23:16 1.660 100.29 90.02 09/04/1998 07:03 1.470 81.44 73.33 11/06/1993 18:00 1.450 76.90 71.64 08/12/2000 03:15 1.400 75.80 67.30 29/10/2004 06:00 1.270 63.76 56.60 30/09/1998 18:43 1.250 62.10 55.12 18/11/1997 01:52 1.210 59.73 54.00 14/11/2002 17:45 1.201 58.98 53.31 30/06/1986 20:21 1.170 56.05 51.00 Table 5-2 Comparison of Discharges for Waldron’s Bridge Gauge (Gauge rated to 112m3/s)

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Willbrook Road Discharge – EPA Discharge – Observed Water Date & Time Rating Curve Project Rating Depth (m) 3 3 (m /s) Curve (m /s) 25/08/1986 23:15 1.619 31.79 26.38 09/04/1998 06:30 1.189 27.46 16.15 18/12/1997 07:45 1.145 25.04 15.21 05/11/2000 20:00 1.098 22.53 14.27 30/09/1998 16:30 1.078 21.56 13.93 29/10/2004 04:00 0.990 17.58 12.35 14/11/2002 05:30 0.970 16.72 11.97 15/10/2001 20:15 0.969 16.24 11.96 03/02/1994 22:45 0.968 15.30 11.96 17/11/1997 22:00 0.919 14.34 11.03 28/10/2004 02:45 0.909 12.50 10.86 03/04/1998 01:00 0.864 12.44 10.05 Table 5-3 Comparison of Discharge for Willbrook Road Gauge (Gauge rated to 6m3/s)

Frankfort Gauge Discharge – EPA Discharge – Observed Water Date & Time Rating Curve Project Rating Depth (m) 3 3 (m /s) Curve (m /s) 26/05/1993 10:15 1.308 9.40 8.59 11/06/1993 16:45 1.254 8.39 7.78 14/11/2002 17:30 1.153 6.67 6.36 18/12/1997 07:00 1.043 5.03 4.97 27/11/2002 11:00 0.969 4.07 4.07 03/04/1998 00:30 0.943 3.76 3.77 22/11/2002 02:30 0.898 3.25 3.28 15/05/1994 13:00 0.893 3.20 3.23 09/04/1998 05:00 0.878 3.03 3.07 11/06/1997 06:15 0.876 3.02 3.05 18/11/1997 00:30 0.871 2.96 3.00 03/02/1994 21:45 0.869 2.93 2.98 Table 5-4 Comparison of Discharges for Frankfort Gauge (Gauge rated to 2m3/s)

The new rating curves produced for this study were calibrated against the EPA rating curves up to the maximum rated flow value. For this reason the discharge values below the rated value should be very similar. Above the rated value the discharges produced using the new rating curves are lower than those produced using the EPA rating curves. At a given water level above the rated value, the new rating curves indicate that less flow passes through the channel. It is recommended that discharge values below the rated values of the gauges will be taken from the EPA rating curve, while those above the gauge rated values will be taken from the Project Rating curve.

5.2 RAINFALL-RUNOFF MODEL CALIBRATION

Rainfall-runoff models for the three gauge catchments were established as combined models using the Rainfall-Runoff (RR) module of the Mike 11 river modelling system software. These RR models were then calibrated against recorded discharge files. The calibration process involved adjusting the NAM and Urban model parameters to appropriately reflect the catchment characteristics and attempt to match the modelled discharge file to the recorded data. Recently recorded rainfall data (recorded within the past eight years) was used in the calibration process to create the simulated discharge files.

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The objectives of the model calibration were to have good match between:

1. Simulated and observed catchment runoff (i.e. a good water balance); 2. Simulated and observed hydrograph shapes; 3. Simulated and observed peak flows with respect to timing, rate and volume; 4. Simulated and observed low flows.

To assess the quality of model calibration the RR module in Mike 11 provides two parameters which compare the modelled discharge data with observed or recorded discharge data. The parameters are as follows:

A. Overall Water Balance Error: The difference between the average simulated and observed runoff. This is expressed as a percentage.

B. Overall Shape of Hydrograph (R2): A measure of the overall shape of the hydrograph based on the Nash-Sutcliffe coefficient. A perfect match corresponds to R2 = 1

Equation 5-1 Nash Sutcliffe Coefficient

Where Qsim,i = simulated discharge at time i Qobs,i = corresponding observed discharge Qobs = average observed discharge

5.2.1 Waldron’s Bridge

The rainfall-runoff model for Waldron’s Bridge gauge catchment was calibrated against recorded flow data from 19/10/2000 to 01/07/2007. Figure 5-4 presents the simulated and observed discharge files for Waldron’s Bridge gauge catchment following the calibration process.

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Recorded Discharge [m^3/s] Simulated Discharge [m^3/s]

160

150

140

130

120

110

) 100 s / 3

m 90 ( e g r 80 a h c s i 70 D

0 2001 2002 2003 2004 2005 2006

Date

Figure 5-4 Simulated V Recorded Discharge for Waldron’s Bridge Gauge Catchment

5.2.1.1 Difficulties in Calibrating Data

• Bohernabreena Lower Reservoir:

The Lower reservoir at Bohernabreena is generally used for flood control and compensation releases to the River Dodder. In addition to the natural attenuation the reservoir offers, the water level can be lowered to accommodate additional waters in anticipation of a flood event. The attenuation effect of the reservoir can be seen in the discharge records at Waldron’s Bridge gauging station as evidenced by Figure 5-5 below. The graph shows the observed discharge for a storm event (in red) and the modelled discharge for the same event (in blue). The peak of the blue hydrograph is higher than the red although the volume of both hydrographs is approximately equal, indicating that flow is attenuated in the natural drainage system.

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Simulated Discharge [m^3/s] Observed Discharge [m^3/s]

0 18:00 00:00 06:00 12:00 18:00 00:00 06:00 2002-11-20 11-21 11-22

Figure 5-5 Example of Attenuated Flow in Observed Discharge Hydrograph

The impact of the Lower reservoir on the discharge records will influence the ability to accurately calibrate the rainfall-runoff models to recorded discharge. This attenuation effect cannot be reproduced in the Rainfall-Runoff models and therefore has a negative effect on the correlation.

• Step Changes in Flow:

The recorded discharge pattern shows sudden step changes in flow during low flow conditions. As shown in

Figure 5-6, the flow suddenly changes within single time steps, giving an unusual appearance to the discharge file. The changes in flow are at a small scale, between 0.05m3/s and 0.4m3/s in this example, and do not impact on large flood volumes. However, the inability to match the recorded discharge profile reduces the correlation in the calibration process.

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Figure 5-6 Example of Step Changes in Flow in Observed Discharge Record

Due to the difficulties in obtaining a satisfactory calibration correlation, individual flood events with a range of peak discharges from the observed discharge record were compared with the simulated discharge data. These flood events are presented graphically in Figure 5-7.

Figure 5-7 Individual Flood Events Used in Calibration for Waldron’s Bridge Gauge Catchment

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The overall calibration parameter for the individual flood events used in the calibration is as follows:

R2: 0.795

A coefficient of this value indicates that there is a satisfactory correlation between the observed and simulated discharge data.

The differences between the observed and simulated discharge data are attributed to the spatial and temporal distribution of rainfall data within the River Dodder Catchment. The rainfall gauges collecting data in the Waldron’s Bridge catchment only record daily rainfall totals. The hourly temporal rainfall pattern was therefore taken from the rainfall recorded at the synoptic station at Casement Aerodrome. For this reason the intensity of rainfall events localised to the Waldron’s Bridge Catchment may not be accurately represented by the temporal distribution at Casement Aerodrome. However, the overall distribution of events between observed and simulated discharges are very similar and it is therefore accepted that the model is accurately representing the relationship between rainfall and runoff in the catchment.

The rainfall runoff model for Waldron’s Bridge gauge catchment will now be used to generate design rainfall events.

5.2.2 Willbrook Road

The rainfall-runoff model for Willbrook Road gauge catchment was calibrated against recorded flow data from 01/05/2002 to 31/12/2006. Figure 5-8 presents the simulated and observed discharge files for Willbrook Road gauge catchment following the calibration process.

Recorded [m^3/s] Simulated [m^3/s]

0 2002 2003 2004 2005 2006

Figure 5-8 Simulated V Recorded Discharge for Willbrook Road Gauge Catchment

From examination of the simulated and recorded discharge data it can be seen that the simulated discharge peaks do not coincide with the peaks in the observed record. However the simulated low

MDW0259Rp0016 47 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report flow data compares well with the observed data. It appears that the difficulty in calibrating peak discharges is due to the inaccuracy of the rainfall data input.

Figure 5-9 and Figure 5-10 present the recorded rainfall and recorded discharge for a sample event from the Willbrook Road gauge catchment calibration data. From these figures it can be seen that the rainfall does not accurately match the discharge. For example in early October 2005 there is a significant rainfall event with recorded rainfall depths up to 8.5mm which coincides with a maximum recorded discharge of 1.3m3/s. Then around the 20th October there is a recorded rainfall event with a maximum depth of 3.6mm which coincides with a recorded discharge of 1.8m3/s. These two events have similar antecedent conditions.

4 R a i n f

a 3 l l D e p t

h 2 ( m m

) 1

0 00:00 00:00 00:00 00:00 2005-10-15 10-25 11-04 11-14 Date Figure 5-9 Recorded Rainfall for Sample Event

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Recorded Discharge [m^3/s] 5.0

4.5

4.0

3.5

3.0

2.5 D i s

c 2.0 h a r g e 1.5

1.0

0.5

0.0 00:00 00:00 00:00 00:00 2005-10-15 10-25 11-04 11-14 Date

Figure 5-10 Recorded Discharge for Sample Event

As reviewed in Section 3 of this report, the rainfall contributing to Willbrook Road gauge catchment is recorded at the rainfall gauging stations presented here in Table 5-5.

Rainfall Gauge Stations Gauge Record Glenasmole (Castlekelly) 1959 - 2007 Glenasmole (Supt.'s Lodge) 1959 - 2006 Knocklyon (St. Colmcille's) 2004 - 2005 Ballyboden 1966 - 2007 Rathfarnham (St. Columba's) 1972 - 1995 Tibradden (Larch Hill) 1967 - 1990 Ballyedmonduff House 1985 - 2007

Table 5-5 Rainfall Gauging Stations Contributing to Willbrook Road Gauge Catchment

From Table 5-5 it can be seen that only Glenasmole (Castlekelly), Glenasmole (Supt’s Lodge), Ballyedmonduff House and Knocklyon (St. Colmcille’s) rainfall gauges were recording during the chosen calibration period. Of these four gauges, three are located above 180mOD which give a bias towards mountainous rainfall patterns and therefore do not accurately represent the rainfall characteristics of the whole catchment. The observed discharge records indicate that peak discharges are occurring at the time of recorded rainfall events, but they do not directly match each other in terms of time or intensity. It is therefore concluded that the rainfall data used in the calibration process is inaccurate and cannot reproduce the observed discharge record for this period.

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The rainfall-runoff model for Willbrook Road gauge catchment is producing simulated discharge data which is similar to the observed data in terms of frequency of discharge peaks. However the scale of the peaks differs to a significant degree. The NAM parameters used in the RR model were derived for the Water Framework Directive project and are judged to be accurate and the Urban parameters are taken directly from the catchment characteristics. Difficulties with the calibration exercise are attributed to inaccurate recorded rainfall data. However, further results derived using this RR model will be treated with caution.

5.2.3 Frankfort

The rainfall-runoff model for Frankfort was calibrated against recorded flow data from 01/04/2002 to 31/12/2006. Figure 5-11 presents the simulated and observed discharge files for Frankfort gauge catchment following the calibration process.

Recorded Discharge [m^3/s] Simulated Discharge [m^3/s] 0

7.0 s f d . D E N I B M O

6.0 C T R O F K N A R F \

5.0 n o i t a r b i l a c R R \ k r e

4.0 o g w \ r t r a o f k h n c a r s F i _ l l D a 3.0 f n i a R \ K M \ s e l i F

2.0 m a r g o r P \ : C 1.0

0.0 2002 2003 2004 2005 2006

Figure 5-11 Simulated V Observed Discharge for Frankfort Gauge Catchment

The catchment draining to Frankfort gauge is relatively small, as are flows in the Dundrum / Slang stream. Because of this it has proved difficult to achieve a satisfactory calibration as any small difference in flow between simulated and observed show up as large errors. For this reason individual flood events with a range of peak discharges from the observed discharge record were compared with the simulated discharge data. These flood events are presented graphically in Figure 5-12.

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Figure 5-12 Individual Flood Events Used in Calibration for Frankfort Gauge Catchment

The overall calibration parameter for the individual flood events used in the calibration is as follows:

R2: 0.767

A coefficient of this value indicates that there is a satisfactory correlation between the observed and simulated discharge data. The differences in observed and simulated discharge data are again attributed to the spatial and temporal distribution of rainfall data within the River Dodder Catchment as discussed in Section 5.2.1 of this report. The rainfall-runoff model for Frankfort gauge catchment will now be used to generate design rainfall events.

5.3 SENSITIVITY ANALYSIS

A sensitivity analysis was undertaken on the influence of the principal parameters used in the Waldron’s Bridge rainfall-runoff model on the output discharge. The parameters analysed are discussed further as follows:

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5.3.1 Umax (Maximum Water Content in Surface Storage)

The Umax parameter denotes the upper limit of the amount of water in the surface storage and is measured in millimetres. The Umax value in the Waldron’s Bridge calibrated rainfall-runoff model is 15mm. This value was adjusted by ± 20% and the model re-run. Figure 5-13 presents the discharge results for the three Umax scenarios (calibrated, -20% and +20%) for a sample year. Table 5-6 presents the associated statistical values for the peak discharges for the data record from 19/10/2000 to 01/07/2007 over a threshold value of 30m3/s.

Figure 5-13 Graph of Sensitivity Analysis Results for Umax Parameter

% Difference No. of Mean Max. % Difference from Parameter 3 from Calibrated 3 Events [m /s] [m /s] Calibrated Value Value Umax Calibrated 22 52.4 100.00% 164.9 100.00% Umax -20% 24 51.6 98.45% 165.2 100.15% Umax +20% 21 52.3 99.89% 164.5 99.78% Table 5-6 Statistics for Sensitivity Analysis of Umax Parameter

The results of this analysis indicate that a reduction in the Umax parameter causes an increase in the larger discharge peaks, i.e. when the precipitation fills the available surface storage volume it is converted into overland flow causing a significant effect on peak events while having a lesser effect on smaller events when the catchment is less saturated.

In contrast, when the Umax value is increased the peak discharge values are reduced indicating that the additional surface storage volume is accommodating more of the precipitation. Again there is a lesser effect on smaller discharge events.

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5.3.2 Lmax (Maximum Water Content in Root Zone Storage)

The Lmax parameter denotes the upper limit of the amount of water in root zone storage and is measured in millimetres. The Lmax value in the Waldron’s Bridge calibrated rainfall-runoff model is 120mm. This value was adjusted by ± 20% and the model re-run.

Figure 5-14 presents the discharge results for the three Lmax scenarios (calibrated, -20% and +20%) for a sample year. Table 5-7 presents the associated statistical values for the peak discharges for the data record from 19/10/2000 to 01/07/2007 over a threshold value of 30m3/s.

Figure 5-14 Graph of Sensitivity Analysis Results for Lmax Parameter

No. of Mean % Difference from Max. % Difference from Parameter 3 3 Events [m /s] Calibrated Value [m /s] Calibrated Value Lmax Calibrated 22 52.4 100.00% 164.9 100.00% Lmax -20% 26 51.7 98.59% 167.6 101.61% Lmax +20% 19 53.2 101.49% 162.7 98.68% Table 5-7 Statistics for Sensitivity Analysis of Lmax Parameter

The results of this analysis indicate that the reduction of the Lmax parameter produces an increase in the peak discharges, i.e. when the root zone storage volume is reduced there is an increase in overland flow which increases the peak discharges as seen in Figure 5-14.

An increase in the value of the Lmax parameter causes a reduction in the peak discharges indicating that the additional root zone storage reduces the amount of precipitation that contributes to overland flow. The adjustment of the Lmax parameter causes little or no change in the smaller discharge events.

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5.3.3 CQOF (Overland Flow Runoff Coefficient)

The CQOF parameter is the overland flow runoff coefficient that influences the amount of precipitation that contributes to overland flow and has a value between 0 and 1. The CQOF value in the Waldron’s Bridge calibrated rainfall-runoff model is 0.6. This value was adjusted by ± 20% and the model re-run. Figure 5-15 presents the discharge results for the three CQOF scenarios (calibrated, -20% and +20%) for a sample year. Table 5-8 presents the associated statistical values for the peak discharges for the data record from 19/10/2000 to 01/07/2007 over a threshold value of 30m3/s.

Figure 5-15 Graph of Sensitivity Analysis Results for CQOF Parameter

No. of Mean % Difference from Max. % Difference from Parameter 3 3 Events [m /s] Calibrated Value [m /s] Calibrated Value CQOF Calibrated 22 52.4 100.00% 164.9 100.00% CQOF -20% 13 53.3 101.77% 137.8 83.58% CQOF +20% 30 54.4 103.87% 190.5 115.50% Table 5-8 Statistics for Sensitivity Analysis of CQOF Parameter

The results of the analysis indicate that an increase in the CQOF parameter value produces a large increase in the peak discharge values and a slight decrease in the low flow discharges. Conversely a reduction in the CQOF parameter value results in a decrease in the peak discharge values coupled with a slight increase in the low flow discharges. The changes in low flow are due to the inter- relationship of other model parameters with the CQOF value.

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5.3.4 TOF (Root Zone Threshold Value for Overland Flow)

The TOF parameter is the threshold value for overland flow that influences when overland flow occurs and has a value between 0 and 1. The TOF value in the Waldron’s Bridge calibrated rainfall-runoff model is 0.55. This value was adjusted by ± 20% and the model re-run.

Figure 5-16 presents the discharge results for the three TOF scenarios (calibrated, -20% and +20%) for a sample year. Table 5-9 presents the associated statistical values for the peak discharges for the data record from 19/10/2000 to 01/07/2007 over a threshold value of 30m3/s.

Figure 5-16 Graph of Sensitivity Analysis Results for TOF Parameter

No. of Mean % Difference from Max. % Difference from Parameter 3 3 Events [m /s] Calibrated Value [m /s] Calibrated Value

TOF Calibrated 22 52.4 100.00% 164.9 100.00% TOF -20% 25 51.3 97.81% 167.8 101.75% TOF +20% 21 51.4 98.02% 160.3 97.21%

Table 5-9 Statistics for Sensitivity Analysis of TOF Parameter

This analysis indicates that a reduction in the TOF parameter value produces an increase in peak discharge values while an increase in the TOF value has the effect of reducing the peak discharge values. However, the effect of adjusting the TOF value becomes less influential for larger events.

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5.3.5 CK1,2 (Time Constant for Routing Overland Flow)

The CK1,2 parameter is the time constant for routing overland flow and influences the shape of the overland flow hydrograph. The flow is routed through two linear reservoirs in series with variable time constants. The CK1, value in the Waldron’s Bridge calibrated rainfall-runoff model is 3.5 and the CK2 value is 5. These values were adjusted by ± 20% and the model re-run. Figure 5-17 presents the discharge results for the three CK1,2 scenarios (calibrated, -20% and +20%) for a sample year. Table 5-10 presents the associated statistical values for the peak discharges for the data record from 19/10/2000 to 01/07/2007 over a threshold value of 30m3/s.

Figure 5-17 Graph of Sensitivity Analysis Results for CK1,2 Parameter

No. of Mean % Difference from Max. % Difference from Parameter 3 3 Events [m /s] Calibrated Value [m /s] Calibrated Value

CK1,2 Calibrated 22 52.4 100.00% 164.9 100.00%

CK1,2 -20% 30 51.9 98.95% 165.9 100.66%

CK1,2 +20% 16 56.4 107.61% 162.4 98.53%

Table 5-10 Statistics for Sensitivity Analysis of CK1,2 Parameter

This analysis indicates that a reduction in the CK1,2 parameter produces an increase in peak discharges, i.e. the hydrograph volume must be accommodated within a reduced time frame causing an increase in peak discharges. Conversely an increase in the CK1,2 parameter causes a reduction in peak discharge as the time axis of the hydrograph has been increased.

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5.3.6 Summary of Sensitivity Analysis

The sensitivity analysis undertaken on the calibrated Waldron’s Bridge rainfall-runoff model has shown that the number of events exceeding the threshold value of 30m3/s changes as the parameters are altered indicating a change in the number of peak events. The greatest change in mean and maximum flow values are seen when the CQOF parameter is altered.

5.4 EXTREME VALUE ANALYSIS (EVA) OF SIMULATED DISCHARGE USING HISTORIC RAINFALL RECORD

Historic rainfall files were created during the establishment of the Mike NAM rainfall-runoff (RR) models as discussed in Section 4.2 of this report. The entire available historic rainfall records were applied to the calibrated rainfall-runoff models to simulate present-day catchment response to historic rainfall events. Extreme value analyses were then carried out on the resulting discharge files to generate design discharges of known return period for each of the three gauge catchments.

The extreme value analyses were carried out on the simulated discharge files by peak over threshold (POT) analysis as detailed in Section 4.4 of this report.

5.4.1 Waldron’s Bridge

Historic rainfall data from 1963 to 2006 was run through the calibrated Waldron’s Bridge rainfall-runoff model. An EVA was carried out on the resulting discharge file with a threshold level of 60m3/s assigned to this data set for the POT analysis. The results are presented in graphical format in Figure 5-18. The best fitting probability distributions for this data set are the Log Pearson Type 3 (LP3/MOM), Truncated Gumbel (TGUM/ML) and Exponential (EXP1/MOM). Estimated design discharges for each of the three proposed probability distributions are presented in Table 5-11. Average quantiles and standard errors for these distributions are provided in tabular format in Appendix C.

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Figure 5-18 EVA of Simulated Discharge for Waldron’s Bridge Gauge Catchment

Probability Distribution Return Period LP3/LMOM TGUM/ML EXP1/MOM [years] 2 75.491 74.511 74.26 5 107.282 108.191 108.177 10 129.626 130.205 130.634 Design Flow 25 161.513 157.899 159.008 3 (m /s) 50 188.542 178.406 180.057 100 218.775 198.747 200.951 200 252.787 219.007 221.769 1000 349.681 265.926 269.991 Table 5-11 Design Flows for Proposed Probability Distributions

The EXP1/MOM probability distribution is favoured as the best fitting distribution to this data set.

5.4.2 Willbrook Road

Historic rainfall data from 1966 to 2006 was run through the calibrated Willbrook Road rainfall-runoff model. An EVA was carried out on the resulting discharge file with a threshold level of 7.5m3/s was assigned to this data set for the POT analysis. The results are presented in graphical format in

Figure 5-19.

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The best fitting probability distributions for this data set are the Weibull (WEI2/LMOM), Generalised Pareto (GP2/LMOM) and Log Pearson Type 3 (LP3/LMOM). Estimated design flows for each of the three proposed probability distributions are presented in Table 5-12. Average quantiles and standard errors for the best fitting probability distributions are provided in tabular format in Appendix C.

Figure 5-19 EVA of Simulated Discharge for Willbrook Road Gauge Catchment

Probability Distribution Return Period WEI2/LMOM GP2/LMOM LP3/LMOM [years] 2 8.089 8.13 8.115 5 10.581 10.405 10.394 10 12.46 12.228 12.331 Design Flow 25 14.993 14.967 15.349 3 (m /s) 50 16.962 17.37 18.08 100 18.979 20.121 21.286 200 21.042 23.28 25.061 1000 25.996 32.569 36.631 Table 5-12 Design Flows for Proposed Probability Distributions

The GP2/LMOM probability distribution is favoured as the best fitting distribution to this data set.

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5.4.3 Frankfort

Historic rainfall data from 1972 to 2006 was run through the calibrated Frankfort rainfall-runoff model. An EVA was carried out on the resulting discharge file and a threshold level of 3m3/s was assigned to this data set for the POT analysis. The results are presented in graphical format in

Figure 5-20.

The best fitting probability distributions for this data set are the Weibull (WEI2/ML), Generalised Pareto (GP2/LMOM) and Log Pearson Type 3 (LP3/MOM/LOG). Estimated design flows for each of the three proposed probability distributions are presented in Table 5-13. Average quantiles and standard errors for the best fitting probability distributions are provided in tabular format in Appendix C.

Figure 5-20 EVA of Simulated Discharge for Frankfort Gauge Catchment

Probability Distribution Return Period LP3/MOM/LOG WEI2/ML GP2/LMOM [years] 2 3.878 3.797 3.749 5 5.565 5.615 5.453 10 6.816 6.92 6.847 Design Flow 25 8.653 8.641 8.983 3 (m /s) 50 10.248 9.96 10.891 100 12.066 11.298 13.11 200 14.148 12.657 15.698 1000 20.256 15.885 23.514 Table 5-13 Design Flows for Proposed Probability Distributions

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The LP3/MOM/LOG probability distribution is favoured as the best fitting distribution to this data set.

5.4.4 Estimated Return Period of Historic Events

Using the EVA diagrams prepared above, return periods were assigned to the twelve largest events recorded at each of the water level gauges (taken from Section 5.1 of this report). These water levels were converted to discharges using the Project Rating Curves and the associated return periods caluculated using the best fitting distributions. The estimated return periods are presented in Table 5-14 to Table 5-16.

Waldron's Bridge Gauge Estimated Return Observed Water Discharge - Project Period from Date & Time 3 Depth (m) Rating Curve (m /s) EXP1/MOM Distribution (Years) 26/08/1986 02:00 3.020 251.08 719.24 05/11/2000 23:00 2.173 141.32 18.36 02/12/2003 08:45 1.780 101.10 4.79 03/02/1994 23:16 1.660 90.02 3.31 09/04/1998 07:03 1.470 73.33 1.89 11/06/1993 18:00 1.450 71.64 1.79 08/12/2000 03:15 1.400 67.30 1.55 29/10/2004 06:00 1.270 56.60 1.08 30/09/1998 18:43 1.250 55.12 1.03 18/11/1997 01:52 1.210 54.00 0.99 14/11/2002 17:45 1.201 53.31 0.97 30/06/1986 20:21 1.170 51.00 N/A Table 5-14 Estimated Return Periods for Historic Events Recorded at Waldron’s Bridge Gauge

Willbrook Road Gauge Estimated Return Observed Water Discharge - Project Period from Date & Time 3 Depth (m) Rating Curve (m /s) LP3/LMOM Distribution (Years) 25/08/1986 23:15 1.619 26.38 316.46 09/04/1998 06:30 1.189 16.15 30.64 18/12/1997 07:45 1.145 15.21 23.09 05/11/2000 20:00 1.098 14.27 17.11 30/09/1998 16:30 1.078 13.93 15.28 29/10/2004 04:00 0.990 12.35 8.69 14/11/2002 05:30 0.970 11.97 7.51 15/10/2001 20:15 0.969 11.96 7.49 03/02/1994 22:45 0.968 11.96 7.49 17/11/1997 22:00 0.919 11.03 5.14 28/10/2004 02:45 0.909 10.86 4.78 03/04/1998 01:00 0.864 10.05 3.34

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Table 5-15 Estimated Return Periods for Historic Events Recorded at Willbrook Road Gauge

Frankfort Gauge Estimated Return Discharge - Observed Water Period from Date & Time Project Rating Depth (m) 3 LP3/MOM/LOG Curve (m /s) Distribution (Years) 26/05/1993 10:15 1.308 8.59 23.88 11/06/1993 16:45 1.254 7.78 15.35 14/11/2002 17:30 1.153 6.36 6.91 18/12/1997 07:00 1.043 4.97 3.05 27/11/2002 11:00 0.969 4.07 1.73 03/04/1998 00:30 0.943 3.77 1.42 22/11/2002 02:30 0.898 3.28 1.00 15/05/1994 13:00 0.893 3.23 N/A 09/04/1998 05:00 0.878 3.07 N/A 11/06/1997 06:15 0.876 3.05 N/A 18/11/1997 00:30 0.871 3.00 N/A 03/02/1994 21:45 0.869 2.98 N/A Table 5-16 Estimated Return Periods for Historic Events Recorded at Frankfort Gauge

5.5 JOINT PROBABILITY ANALYSIS – SURGE RESIDUALS AND RAINFALL

A statistical analysis of a 27 year data set of rainfall and surge residual values was undertaken to establish the return periods for extreme rainfall and surge residual conditions for use in the joint probability analysis. The analysis was undertaken using the Extreme Value Analysis (EVA) toolbox provided within the MIKE software suite.

The results of the statistical analysis of the surge residuals are presented in graphical format in Figure 5-21.

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Figure 5-21 Probability Plot of Extreme Surge Residuals Used in Joint Probability Analysis

The results of the statistical analysis of the rainfall data are presented in graphical format in Figure 5-22.

Figure 5-22 Probability Plot of Extreme Rainfall Used in Joint Probability Analysis

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Estimated quantiles and standard errors for the best fitting probability distributions for the surge residual and rainfall data are provided in tabular format in Appendix D.

Based on the results of the analysis of the surge residual and rainfall data, a joint probability analysis was carried out to establish the range of rainfall and surge residual combinations which will provide a 1 in 100 year return period event. The joint probability analysis was carried out by establishing the correlation between rainfall and surge residuals for the joint exceedence return periods within the dataset. This correlation was applied to the extreme rainfall and surge residuals established from the extreme value analysis for the 1 in 100 year return period.

The results of the joint probability analysis are given for return period events ranging from 1 in 1 to 1 in 1000 years. The data is presented in tabular form in Table 5-17, followed by a figure illustrating the data in Figure 5-23.

Joint Return Period (years) Surge Level (m) 2 5 10 25 50 100 200 1000 Rainfall (mm) -0.5 47 65 78 96 110 125 140 176 -0.4 47 65 78 96 110 125 140 176 -0.3 47 65 78 96 110 125 140 176 -0.2 47 64 78 96 110 124 139 175 -0.1 46 64 77 95 109 123 138 174 0 45 63 76 93 107 121 136 171 0.1 42 59 72 89 103 117 131 166 0.2 38 55 67 84 97 111 125 160 0.3 35 50 62 78 91 104 118 152 0.4 31 47 58 74 86 98 112 145 0.5 23 40 52 69 81 94 107 138 0.6 13 31 44 61 74 88 101 133 0.7 n/a 19 32 50 64 78 92 126 0.8 n/a 4 18 37 51 66 81 116 0.9 n/a n/a 0 21 36 51 67 104 1 n/a n/a n/a 0 18 34 50 88 1.1 n/a n/a n/a n/a n/a 13 31 71 1.2 n/a n/a n/a n/a n/a n/a 7 51 1.3 n/a n/a n/a n/a n/a n/a n/a 28 Table 5-17 Joint Probability of rainfall and surge residuals

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Figure 5-23 Joint Probability of rainfall and surge residuals

5.6 ADDITIONAL ANALYSIS

5.6.1 Extreme Value Analysis (EVA) of Historic Annual Maxima Series

Extreme Value Analyses of the historic annual maxima series for each of the three gauging stations (Waldron’s Bridge, Willbrook Road and Frankfort) was carried out to assign probability and return period values to the historic events.

The extreme value analyses have been carried out using the EVA tool from the MIKE software suite. The candidate probabilities available for Annual Maxima analysis are as follows:

• Gumbel (EV Type 1) • Generalised Extreme Value • Weibull • Frechét (EV Type 2) • Generalised Pareto • Gamma/Pearson Type 3 • Log-Pearson Type 3 • Log-normal • Square Root Exponential

For the estimation of the parameters relating to the probability distributions three methods were applied; the method of moments, the method of L-moments and maximum likelihood method. The goodness of fit of the resulting distributions were then tested using five statistical methods; Chi- squared, Kolmogorov-Smirnov test, standardised least squares criterion, probability plot correction coefficient and Log-likelihood measure. The uncertainty of these distributions was evaluated by application of the Jackknife resampling technique, which provided confidence limits for each event.

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5.6.1.1 Waldron’s Bridge Gauge

Annual Maximum Values from the flow measurement gauge at Waldron’s Bridge between 1949 and 2007 have been collated from the EPA paper (Micheál Mac Cárthaigh, August 2005) and the IEI Dodder River Flood Study Report (P. Hennigan, J. McDaid, J. Keyes, Nov. 1988). These annual maximum values are presented in Table 5-18.

Annual Maximum Discharge at Waldron's Bridge Hydrometric Peak Flow Hydrometric Peak Flow Hydrometric Peak Flow Year** (m3/s) Year** (m3/s) Year** (m3/s) 1949* 58.05 1968* 84.95 1987 33.0 1950* 36.81 1969* --- 1988 --- 1951* 50.69 1970* --- 1989 18.6 1952* 35.68 1971* --- 1990 48.0 1953* 32.00 1972* --- 1991 14.2 1954* 50.77 1973* 37.94 1992 80.9 1955* 28.88 1974* 48.42 1993 110.0 1956* 64.85 1975* 33.98 1994 34.7 1957* 74.00 1976* 40.21 1995 46.1 1958* 116.10 1977* 46.44 1996 18.3 1959* 44.85 1978* 43.70 1997 86.5 1960* 67.96 1979* 41.78 1998 45.8 1961* 28.32 1980* 72.09 1999 32.9 1962* 23.79 1981* 57.62 2000 156.0 1963* 28.60 1982* 105.63 2001 41.1 1964* 21.80 1983* 82.27 2002 58.4 1965* 138.75 1984* 53.30 2003 112.0 1966* 44.74 1985 269.0 2004 63.8 1967* 50.40 1986 63.5 ------* There is uncertainty about the rating curve used to derive peak flows for these years and it is unclear whether these are hydrometric or calendar years. **A Hydrometric Year runs from 1st October in a given year to 30th September in the following year. Table 5-18 Annual Maxima Flows for Waldron’s Bridge Gauging Station

The results of this EVA are presented graphically in Figure 5-24. The EV Type 1 and 2 distributions are shown on the graph.

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Figure 5-24 EVA of Annual Maxima Flows from Waldron’s Bridge Gauge

5.6.1.2 Willbrook Road Gauge

Annual Maximum Values from Willbrook Road gauging station are provided in the EPA paper: ‘Flooding in the Dodder Catchment 22 June 2007’ (Mac Cárthaigh, 2007) and have been reproduced here in Table 5-19.

Annual Maximum Discharge at Willbrook Road Hydrometric Peak Flow Hydrometric Peak Flow Year (m3/s) Year (m3/s) 1980 7.79 1993 --- 1981 8.39 1994 --- 1982 10.9 1995 --- 1983 7.94 1996 9.1 1984 11.2 1997 27.5 1985 32.1 1998 12 1986 9.13 1999 9.73 1987 11.4 2000 22.9 1988 5.55 2001 16.3 1989 --- 2002 23 1990 --- 2003 12.6 1991 --- 2004 24.3 1992 --- 2005 4.97 Table 5-19 Annual Maxima for Willbrook Road Gauging Station

An EVA has been carried out on these annual maximum values as per those from Waldron’s Bridge gauge and the results are presented graphically in Figure 5-25.

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Figure 5-25 EVA of Annual Maxima Flows from Willbrook Road Gauge

5.6.1.3 Frankfort Gauge

Annual Maximum Values from Frankfort gauging station were provided by the OPW for use in this study and have been reproduced here in Table 5-20.

Annual Maximum Discharge at Frankfort Hydrometric Peak Flow Hydrometric Peak Flow Year (m3/s) Year (m3/s) 1985 7.73 1995 1.94 1986 2.57 1996 2.73 1987 5.85 1997 2.10 1988 1.94 1998 5.17 1989 1.69 1999 1.98 1990 3.64 2000 2.00 1991 1.96 2001 1.29 1992 6.86 2002 5.17 1993 2.92 2003 1.98 1994 2.83 2004 2.00 Table 5-20 Annual Maxima for Frankfort Gauging Station (modified from OPW, 2007)

An EVA has been carried out on these annual maximum values as per those from Waldron’s Bridge gauge and the results are presented graphically in Figure 5-26.

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Figure 5-26 EVA of Annual Maxima Flows from Frankfort Gauge

5.6.2 Analysis of Rainfall Data

A statistical analysis of the rainfall data sets from three rainfall gauging stations located within the River Dodder catchment was undertaken to establish the return periods for extreme rainfall conditions. The gauging stations analysed were Casement, Dundrum, and Glenasmole, with data set lengths of 44, 21, and 48 years respectively. The analysis was undertaken using the Extreme Value Analysis (EVA) toolbox provided within the MIKE suite.

The EVA facility in MIKE provides a large number of candidate distributions including Exponential, Generalised Pareto, Gumbel, Generalised Extreme Value, Weibull, Frechét, Gamma, Pearson Type 3, Log-Pearson Type 3, Log-Normal, and Square-Root Exponential distributions. The use of different parameter estimation methods was also investigated including the method of moments, maximum likelihood method, and method of L-moments. The method used for uncertainty analysis was the Jackknife re-sampling technique.

The extreme rainfall values were derived from the statistical analysis of the rainfall data from each of the three rainfall gauging station locations. Figure 5-27 to Figure 5-29 present the results of this statistical analysis in graphical format. Estimated rainfall intensities from each of the proposed probability distributions are presented in Table 2-1 to Table 5-22. Average quantiles and standard errors for the best fitting probability distributions are provided in tabular format in Appendix A. It should be noted that a 100 year return period flood event is not considered to result from a 100 year return period rainfall event.

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Figure 5-27 Probability Plot for Rainfall Data Recorded at Casement Aerodrome

Probability Distribution (Casement Rainfall) Return Period WEI2/ML GP2/ML GAM/ML [years] 2 35.595 36.106 35.438 5 47.049 46.162 47.642 10 57.082 55.084 57.938 Rainfall (mm/day) 25 71.645 68.933 72.236 50 83.464 81.221 83.376 100 95.871 95.325 94.705 200 108.801 111.514 106.174 Table 5-21 Daily Rainfall Intensities for Proposed Probability Distributions

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Figure 5-28 Probability Plot for Rainfall Data Recorded at Dundrum Gauging Station

Probability Distribution (Dundrum Rainfall) Return Period WEI2/MOM GP2/MOM LP3/MOM/LOG GP2/LMOM [years] 2 39.238 38.561 38.005 38.292 5 49.893 48.197 48.187 48.335 10 58.665 56.68 57.769 57.532 Rainfall (mm/day) 25 71.015 69.745 73.523 72.276 50 80.845 81.248 88.303 85.779 100 91.044 94.362 106.103 101.71 200 101.576 109.315 127.539 120.507 Table 5-22 Daily Rainfall Intensities for Proposed Probability Distributions

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Figure 5-29 Probability Plot for Rainfall Data Recorded at Glenasmole Gauging Station

Probability Distribution (Glenasmole Rainfall)

Return Period [years] LP3/MOM/LOG LN2/LMOM

2 54.166 53.817 5 70.197 74.592 10 85.839 97.914 Rainfall (mm/day) 25 112.379 141.81 50 138.015 187.374 100 169.674 246.089 200 208.756 320.604 Table 5-23 Daily Rainfall Intensities for Proposed Probability Distributions

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5.6.2.1 Hurricane Charlie Event

The Hurricane Charlie flood event of the 25th/26th August 1986 on the River Dodder is currently the largest event recorded at Waldron’s Bridge gauging station. The EPA records show that the water level recorded during this event equates to a discharge of 269m3/s. The water level gauge was not operating during the peak of the storm and the peak water level was recorded from a high water mark inside the gauge house.

Figure 5-30 presents the recorded discharge hydrograph from the Hurricane Charlie event from Waldron’s Bridge gauging station as recorded by the EPA.

Discharge Recorded at Waldron's Bridge Gauge for Hurricane Charlie Event

300

250 EPA Recorded Discharge Data

/ 200 s 3 )

150

100 D i s c h

a 50 r g e ( m 0

2 2 2 2 2 2 2 2 2 5 5 6 6 7 7 8 8 9 /0 /0 /0 /0 /0 /0 /0 /0 /0 8 8 8 8 8 8 8 8 8 /1 /1 /1 /1 /1 Date /1 /1 /1 /1 9 9 9 9 9 9 9 9 9 8 8 8 8 8 8 8 8 8 6 6 6 6 6 6 6 6 6 0 1 0 1 0 1 0 1 0 0 2 0 2 0 2 0 2 0 :0 :0 :0 :0 :0 :0 :0 :0 :0 0 0 0 0 0 0 0 0 0

Figure 5-30 EPA Recorded Discharge Hydrograph for Hurricane Charlie Event

The volume of the hydrograph in Figure 5-5 has been calculated as approximately 16.6x106 m3 for the 31 hour period between 08:00 on 25/08/1986 to 15:00 on 26/08/1986. A base-flow of 1.7m3/s has been deducted from the volume calculation. This base-flow is taken as a typical low flow value from the recorded discharge record.

Figure 5-31 presents the recorded rainfall data from the Hurricane Charlie event for the Waldron’s Bridge gauge catchment as prepared during the establishment of the Mike NAM rainfall runoff model.

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Rainfall Recorded in Waldron's Bridge Catchment for Hurricane Charlie Event

16.00 Recorded Rainfall Data

) 14.00 r h /

m 12.00 m ( 10.00 y t i s

n 8.00 e t n I 6.00 l l a f

n 4.00 i a

R 2.00 0.00

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 :0 7 9 1 3 5 9 1 3 1 3 5 7 9 1 5 0 0 1 1 1 17 1 2 2 0 0 0 0 0 1 13 1 25/08/1986 26/08/1986

Figure 5-31 Recorded Rainfall Data for Hurricane Charlie Event

A total of 157.9mm of rain fell on the Waldron’s Bridge gauge catchment in the 31 hour period. This rainfall equates to a volume of 15.75x106m3 falling on the whole catchment.

When the volume of the hydrograph is deducted from the volume of the rainfall there is a deficit of 0.85x106 m3. This calculation indicates that the rainfall depths recorded during Hurricane Charlie are insufficient to produce the flood event as recorded by the EPA at Waldron’s Bridge. The volume of the hydrograph is 105% of the volume of the rainfall. It is noted in the Meteorological Service Report on this event that two rainfall gauges were damaged during the storm and some records lost which would account for some of the missing rain. In addition there is anecdotal evidence that some of the rain gauges were overtopped during this event. However it would seem unlikely that this could be the complete explanation.

The Project Rating Curve assigns a discharge value of 251.1m3/s to the water level recorded during the Hurricane Charlie event. If the peak discharge of the hydrograph produced by the EPA is reduced to 251.5m3/s, then the volume of the hydrograph becomes 15.09x106m3. This revised hydrograph volume equates to 96% of the volume of the rainfall falling on the catchment.

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5.6.3 Flood Studies Report Method

For comparison purposes, the Flood Studies Report (FSR) Catchment Characteristics Method of flood prediction was applied to the three gauge catchments to estimate design flood flows for the present day scenario. Given the mixed urban and rural nature of the catchment, the methods within the FSR Supplementary Report Nos. 5 and 16 were used in conjunction with those in the original 1975 FSR documents. Some of the catchment characteristics used in the calculations are presented in Table 5-24 below:

Gauge Catchments Catchment Waldron's Willbrook Units Frankfort Characteristics Bridge Road Area km2 99.8 22.7 7.2

Soil - 0.42 0.43 0.31

SAAR mm 1100 1050 900

Length km 24.1 9.1 6.9

S1085 m/km 14.8 52.0 48.4

Urban % 20.8 9.6 50.2

Table 5-24 Catchment Characteristics Used in FSR Flood Estimation Method

Design flood estimations for each of the gauge catchments are presented in Table 5-25 below:

Gauge Catchments

Waldron's Willbrook Return Period Frankfort Bridge Road (Years) Q (m3/s) Q (m3/s) Q (m3/s) 2 50.30 11.45 3.99 5 64.60 15.10 4.94 10 72.54 17.61 5.30 25 81.01 19.63 5.82 50 87.36 21.39 6.10 100 96.13 23.75 6.55 200 103.26 25.73 6.88

Table 5-25 FSR Estimated Design Flood Flows

A standard factorial error of 1.47 has been applied to the estimated flows which equates to a confidence of approximately 68%.

5.6.3.1 Comparison with Simulated Discharge Results and Recorded Annual Maxima

Table 5-26 to Table 5-28 present design flows produced from the FSR Catchment Characteristics Method of flood prediction, those from the EVA of the Simulated Discharge Files (Sim) (see Section 5.4 of this report) and those from the EVA of Historic Annual Maxima Series (see Section 5.6.1 of this

MDW0259Rp0016 75 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report report). The design flows from the EVA of the simulated discharge files are taken from the probability distributions thought to be the best fit to the data.

Waldron's Bridge Gauge Catchment Return Period EV1 of Annual EV2 of Annual Sim (EXP1,MOM) FSR Maxima Maxima

(Years) Q (m3/s) Q (m3/s) Q (m3/s) Q (m3/s)

2 74.26 50.30 56.00 53.00

5 108.18 64.60 90.00 78.00

10 130.63 72.54 114.00 102.00

25 159.01 81.01 146.00 143.00

50 180.06 87.36 165.00 170.00

100 200.95 96.13 185.00 210.00

200 221.77 103.26 210.00 257.00

Table 5-26 FSR, Simulated Design and Annual Maxima Flows for Waldron’s Bridge Gauge Catchment.

Willbrook Road Return Period EV1 of Annual EV2 of Annual Sim (GP2/LMOM) FSR Maxima Maxima (Years) Q (m3/s) Q (m3/s) Q (m3/s) Q (m3/s) 2 8.13 11.45 14.50 14.00

5 10.41 15.10 19.90 19.25

10 12.23 17.61 24.60 24.00

25 14.97 19.63 30.50 30.50

50 17.37 21.39 34.70 35.50

100 20.12 23.75 N/A N/A

200 23.28 25.73 N/A N/A

Table 5-27 FSR, Simulated Design and Annual Maxima Flows for Willbrook Road Gauge Catchment.

Frankfort Return Period Sim EV1 of Annual EV2 of Annual FSR (LP3/MOM/LOG)) Maxima Maxima

(Years) Q (m3/s) Q (m3/s) Q (m3/s) Q (m3/s)

2 3.88 3.99 3.41 3.13 5 5.57 4.94 4.52 4.35 10 6.82 5.30 5.60 5.45 25 8.65 5.82 6.92 7.08 50 10.25 6.10 7.96 8.41 100 12.07 6.55 N/A N/A 200 14.15 6.88 N/A N/A

Table 5-28 FSR, Simulated Design and Annual Maxima Flows for Frankfort Gauge Catchment.

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It is evident from Table 5-26 and Table 5-28 that the design flows produced using the EVA analysis of the simulated discharge files (Sim) are consistently larger than those produced using the FSR catchment characteristic method. The Sim flows compare well with the EV1 and EV2 flows produced from the EVA of the historic annual maxima The results in Table 5-27 indicate that the Sim flows compare well with the FSR flows, but are lower than those produced from the EVA of the historic annual maxima. This is to be expected given the difficulties in calibrating the rainfall runoff model to the recorded flow data. The simulated design flows are favoured for use in this study over the FSR flows as they are the product of recorded rainfall data and calibrated catchment response and they compare well with the results of the EVA on the recorded annual maxima.

5.7 SUMMARY

5.7.1 Rating Curve Analysis

It is recommended that discharge values below the rated values of the water level gauges will be taken from the EPA rating curve, while those above the gauge rated values will be taken from the project rating curve.

5.7.2 Rainfall-Runoff Model Calibration

The rainfall-runoff models created for the three gauge catchments (Waldron’s Bridge, Willbrook Road and Frankfort) were calibrated to recorded discharge data from each gauge. Waldron’s Bridge: Individual flood events calibrated well with discharge records and a calibration factor (R2) of 0.795 was achieved which indicates a good correlation. Willbrook Road: There was difficultly calibrating this model due to the inaccuracy of the rainfall data input. However, the model is producing simulated discharge data which is similar to the observed data in terms of scale and frequency of discharge peaks. The NAM parameters were derived from the Water Framework Directive Project and are judged to be accurate and the Urban parameters are taken directly from the catchment characteristics. Therefore this model is deemed to satisfactorily calibrated. Frankfort: Individual flood events calibrated well with discharge records and a calibration factor (R2) of 0.767 was achieved which indicates a good correlation.

5.7.3 Sensitivity Analysis

A sensitivity analysis was carried out on the principal parameters of the calibrated Waldron’s Bridge rainfall-runoff model. The results of this analysis indicate that the greatest change in mean and maximum discharge can be achieved by altering the Overland Flow Runoff Coefficient (CQOF) parameter.

5.7.4 EVA of Simulated Discharge Using Historic Rainfall Record

Historic recorded rainfall data was run through the calibrated rainfall-runoff models and simulated discharge files were generated. An EVA was carried out on each simulated discharge file and design discharge events of known return period for produced for each gauge catchment. Table 5-29 presents the design discharges for each of the three gauge catchments. The probability distribution with the best fit was chosen for each.

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Gauge Catchment

Return Period Waldron's Bridge Willbrook Road Frankfort [years] (EXP1/MOM) (GP2/LMOM) (LP3/MOM/LOG)

2 74.26 8.13 3.878 5 108.177 10.41 5.565 10 130.634 12.23 6.816

Design Flow 25 159.008 14.97 8.653 3 [m /s] 50 180.057 17.37 10.248 100 200.951 20.12 12.066 200 221.769 23.28 14.148 1000 269.991 32.569 20.256

Table 5-29 Design Discharges for Gauge Catchments

5.7.5 Joint Probability Analysis – Surge Residual and Rainfall

A joint probability analysis was carried to establish the correlation between surge residual and rainfall for joint exceedence return periods.

5.7.6 Additional Analysis

Additional analysis was undertaken to enhance the robustness of the study. However the results of these analyses will not be used in the generation of design discharges for use in the hydraulic models of the River Dodder Main Channel and the five tributaries.

5.7.6.1 EVA of Historic Annual Maxima

An EVA was carried out on the annual maxima record for each of the three water level gauging stations (Waldron’s Bridge, Willbrook Road and Frankfort). The results of this provide probabilities and return periods for historic discharge events.

5.7.6.2 Analysis of Rainfall Data

A statistical analysis of the rainfall data sets from three rainfall gauging stations within the River Dodder catchment was carried out to establish return periods for extreme rainfall in the catchment. Table 5-30 presents the design rainfall intensities for each of the three rainfall gauging stations. The probability distribution with the best fit was chosen for each.

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Rainfall Gauging Stations Return Period Casement Dundrum Glenasmole [years] (WEI2/ML) (GP2/LMOM) (LP3/MOM/LOG)

2 35.595 38.292 54.166 5 47.049 48.335 70.197 10 57.082 57.532 85.839 Rainfall (mm) 25 71.645 72.276 112.379 50 83.464 85.779 138.015 100 95.871 101.71 169.674 200 108.801 120.507 208.756 Table 5-30 Design Rainfall Intensities

An analysis was carried out on the Hurricane Charlie Flood event of 25th/26th August 1986, comparing the recorded discharge hydrograph from Waldron’s Bridge gauge with the recorded rainfall in the catchment draining to the gauge. The result of this analysis indicates that the recorded rainfall was insufficient to cause the recorded peak discharge.

When the Project Rating Curve was used to determine the peak discharge for the Hurricane Charlie Event, it correlated much better with the recorded rainfall in the catchment.

5.7.6.3 Flood Studies Report Method

For comparison purposes, the Flood Studies Report (FSR) Catchment Characteristics Method of flood prediction was applied to the three gauge catchments (Waldron’s Bridge, Willbrook Road and Frankfort) to estimate design flood flows for the present day scenario.

Design flows produced using the FSR Catchment Characteristics Method of flood prediction are presented in Table 5-31 alongside the design flows produced from the EVA analysis of the simulated discharge files (Sim) (see Section 5.4 of this report).

Gauge Catchment Design Flows

Waldron's Bridge Willbrook Road Frankfort Return Sim Sim Sim Period FSR FSR FSR (EXP1/MOM) (GP2/LMOM) (LP3/MOM/LOG) (Years) Q (m3/s) Q (m3/s) Q (m3/s) Q (m3/s) Q (m3/s) Q (m3/s) 2 74.26 50.30 8.13 11.45 3.878 3.99 5 108.177 64.60 10.405 15.10 5.565 4.94 10 130.634 72.54 12.228 17.61 6.816 5.30 25 159.008 81.01 14.967 19.63 8.653 5.82 50 180.057 87.36 17.37 21.39 10.248 6.10 100 200.951 96.13 20.121 23.75 12.066 6.55 200 221.769 103.26 23.28 25.73 14.148 6.88

Table 5-31 FSR and Simulated Design Flows for Gauge Catchments

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The simulated design flows are favoured for use in this study as they are the product of recorded rainfall data and calibrated catchment response and they compare well with the results of the EVA on the recorded annual maxima.

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6 ESTIMATION OF DESIGN FLOODS

This chapter details the estimation of design floods for use in the hydraulic models. It includes the downscaling or regionalisation of the rainfall-runoff models, information on the future scenarios chosen for this study as well as design discharges of known return period for Present Day and Future Scenarios.

6.1 PRESENT DAY SCENARIO

6.1.1 Regionalisation of Rainfall Runoff Model

In order to establish a comprehensive and detailed hydraulic model of the River Dodder and its major tributaries it is necessary to provide rainfall runoff boundaries at regular intervals. These boundaries feed discharge hydrograph data into the model to simulate flood events. The MIKE suite of software allows the insertion of six types of boundaries in the MIKE 11 hydrodynamic models as follows:

• Open; • Point Source; • Distributed Source; • Global; • Structures; • Closed.

The two boundary types which will be utilised in this study are the point source and the distributed source. The point source boundary provides an input hydrograph at a single location while the distributed boundary divides the input hydrograph data equally between computational points along a specified river reach.

The proposed rainfall runoff boundary locations in the River Dodder catchment are presented in Figure E.1 in Appendix E.

Rainfall- Runoff (RR) models were created for the catchment draining to each RR boundary location. The parameters in these models are based on the calibrated RR models for the three gauge catchments, with alterations to the catchment length and time of concentrations as required. In total 15 RR models were created.

Historic rainfall data from the rainfall gauging stations within the RR boundary catchments was entered into each model and weighted according to their contribution relative to the catchment area using voronoi polygons. The rainfall was then distributed according to the rainfall pattern from the hourly rainfall data recorded at Casement Aerodrome.

The RR models for each RR boundary catchment produced a discharge file based on present day catchment conditions and historic rainfall records.

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6.1.2 Summary of Parameters

Table 6-1 presents the parameters used in the Present Day Scenario.

Present Day Scenario Further Parameter Description Information Land Use Scenario Current Development (2007) Section 2.3 Rainfall Data Historic Rainfall Records Section 3.1 Coastal Data Current Coastal Water Levels Section 3.3

Table 6-1 Present Day Scenario Summary Table

The coastal water levels used as downstream boundary conditions for the River Dodder Main Channel hydraulic model are presented in Table 6-2 below. These figures are taken from the Irish Coastal Protection Strategy Study (ICPSS) commissioned by the Dept. of Agriculture, Fisheries and Food (DAFF) for the Dublin Port Area.

Design Event ICPSS Water Return Period Level (mOD) (Years)

2 2.46 5 2.58 10 2.67 20 2.76 50 2.88 100 2.97 200 3.07 1000 3.28

Table 6-2 Design Event Coastal Water Levels

6.1.3 Extreme Flows

An Extreme Value Analysis (EVA) was carried out on discharge files produced from the RR models for each of the RR boundary catchments using the same methods as described in Section 5.4 of this report.

The best fitting probability distribution was chosen from each EVA and design flows for 2, 5, 10, 25, 50, 100, 200 and 1000 year return periods allocated. The design flows for each RR boundary catchment are presented in Table 6-3 and graphs of the chosen distributions are shown in Appendix E.

MDW0259Rp0016 82 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

Return Period (years) Catchment RR Boundary Catchment Probability Distribution 2 Area (km ) 2 5 10 25 50 100 200 1000 Log Pearson Type 3 Tallaght NAM_Present Day 6 6.90 9.08 10.75 13.23 15.41 17.89 20.74 29.14 (LP3/LMOM) Log Pearson Type 3 Bohernabreena NAM_Present Day 28 37.08 47.11 56.55 71.98 86.49 104.04 125.32 193.81 (LP3/LMOM) Dodder Main Channel NAM_Present Log Pearson Type 3 14.6 19.02 22.34 26.00 31.42 36.12 41.46 47.54 65.20 Day (LP3/LMOM) Log Pearson Type 3 Owendoher NAM_Present Day 11.1 14.10 18.32 21.39 25.82 29.58 33.80 38.53 52.00 (LP3/LMOM) Log Pearson Type 3 Whitechurch NAM_Present Day 8.1 10.43 12.74 15.34 19.33 22.90 27.07 31.94 46.75 (LP3/LMOM) Little Dargle NAM_Present Day Generalised Pareto (GP2/ML) 4 7.12 8.80 10.67 13.42 15.79 18.47 21.50 30.19 Log Pearson Type 3 Dundrum NAM_Present Day 1.42 3.00 4.08 5.12 6.86 8.55 10.65 13.24 21.91 (LP3/LMOM) Log Pearson Type 3 Tallaght URBAN_Present Day 6.9 4.70 6.41 8.18 11.00 13.64 16.83 20.70 33.23 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN_1_Present Day 3.3 1.95 2.85 3.65 4.87 5.99 7.32 8.91 13.93 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN_2_Present Day 10.2 6.97 9.41 12.18 16.92 21.60 27.54 35.10 61.63 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN 3+4_Present Day 11.92 5.77 8.04 10.01 13.20 16.22 19.89 24.37 39.03 (LP3/LMOM) Owendoher_Whitechurch Generalised Pareto (GP2/ML) 2.9 2.95 4.33 5.43 7.07 8.51 10.15 12.03 17.53 URBAN_Present Day Log Pearson Type 3 Little Dargle URBAN_Present Day 4.3 4.54 5.89 6.89 8.37 9.65 11.09 12.73 17.47 (LP3/LMOM) Dundrum URBAN 1+2_Present Day Generalised Pareto (GP2/ML) 8.08 6.593 9.601 11.996 15.575 18.697 22.254 26.32 38.18

Table 6-3 Present Day Design Flows for Rainfall Runoff Boundary Catchments (m3/s)

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6.2 MOST LIKELY FUTURE SCENARIOS

The future design horizon chosen for this study is the year 2100. By this time is it predicted that development within the River Dodder Catchment will have increased significantly (as discussed in Section 2.3) and that climate change impacts on rainfall and coastal water levels will be actualised (as discussed in Section 3.5).

The use of SUstainable Drainage Systems (SUDS) is recommended in the Greater Dublin Strategic Drainage Study Environmental Management Policy. It is therefore envisaged that SUDS will continue to be implemented in the River Dodder Catchment in the future. SUDS minimise the impact of urban development on flooding by attenuating surface runoff at source and releasing it slowly and will therefore have a significant effect on future surface runoff. For this reason the Future Scenario with Full Implementation of SUDS (Scenario 1) is examined in this study. However, given future unknowns with regards to policy implementation and SUDS maintenance, the Future Scenario with No Implementation of SUDS (Scenario 2) is also explored.

Rainfall-runoff models were created for each of the 15 RR boundary catchments for both of the future scenarios. These rainfall-runoff models are based on the Present Day Scenario models with parameters altered as appropriate to reflect future catchment conditions. The Rainfall-Runoff boundary locations are the same as those for the Present Day Scenario and are presented in Figure E.1 in Appendix E.

The rainfall datasets created for the Present Day Scenario RR models were modified to account for the predicted effects of climate change. The future rainfall files were then run in the RR models and a discharge file was produced for each catchment model for both future scenario conditions.

6.2.1 Future Scenario 1 – Assuming FULL Implementation of SUDS

6.2.1.1 Summary of Parameters

Table 6-4 presents the parameters used in Future Scenario 1.

Future Scenario 1 Further Parameter Description Information Land Use Scenario Future Development (2100) Section 2.3 Historic Rainfall Records + Rainfall Data Climate Change (Mid Range Sections 3.1 & 3.5 Future Scenario) ICPSS + Climate Change (Mid Coastal Data Range Future Scenario) Section 3.5

SUDS Full implementation of SUDS -

Table 6-4 Future Scenario 1 Summary Table

The coastal water levels used as downstream boundary conditions for the River Dodder Main Channel hydraulic model for Future Scenario 1 are presented in Table 6-5 below. These figures are a combination of water levels taken from the Irish Coastal Protection Strategy Study (ICPSS) and climate change increases (+0.5m).

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Design Event ICPSS Water Return Period Level + Climate (Years) Change (mOD)

2 2.96 5 3.08 10 3.17 20 3.26 50 3.38 100 3.47 200 3.57 1000 3.78

Table 6-5 Design Event Coastal Water Levels for Future Scenario 1

6.2.1.2 Extreme Flows

An Extreme Value Analysis (EVA) was carried out on discharge files produced from the RR models for each of the RR boundary catchments for Future Scenario 1 using the same methods as described in Section 5.4 of this report.

MDW0259Rp0016 85 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

Catchment Return Period (years) RR Boundary Catchment Probability Distribution 2 Area (km ) 2 5 10 25 50 100 200 1000 Log Pearson Type 3 Tallaght NAM_Future SUDS 6 8.35 10.62 12.30 14.73 16.79 19.11 21.71 29.09 (LP3/LMOM) Log Pearson Type 3 Bohernabreena NAM_Future SUDS 28 46.21 58.95 69.25 84.87 98.69 114.63 133.06 187.95 (LP3/LMOM) Log Pearson Type 3 Dodder Main Channel NAM_Future SUDS 14.6 23.75 27.96 32.16 37.95 42.70 47.86 53.51 68.88 (LP3/LMOM) Log Pearson Type 3 Owendoher NAM_Future SUDS 11.1 18.74 25.06 29.79 36.77 42.83 49.72 57.60 80.66 (LP3/LMOM) Log Pearson Type 3 Whitechurch NAM_Future SUDS 8.1 13.67 16.43 19.43 23.86 27.71 32.09 37.10 51.74 (LP3/LMOM) Little Dargle NAM_Future SUDS Generalised Pareto (GP2/ML) 4 9.04 11.10 13.26 16.23 18.63 21.20 23.94 31.09 Log Pearson Type 3 Dundrum NAM_Future SUDS 1.42 3.63 4.78 5.82 7.50 9.06 10.93 13.17 20.21 (LP3/LMOM) Log Pearson Type 3 Tallaght URBAN_Future SUDS 6.9 6.10 8.33 10.86 15.20 19.51 24.99 31.98 56.67 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN_1_Future SUDS 3.3 3.53 4.68 5.89 7.83 9.64 11.82 14.47 23.05 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN_2_Future SUDS 10.2 9.12 12.22 15.53 20.93 26.06 32.34 40.08 65.70 (LP3/LMOM) Log Pearson Type 3 Dodder Urban_3+4_Future SUDS 11.92 7.53 10.22 12.87 17.45 22.01 27.79 35.15 60.95 (LP3/LMOM) Owendoher_Whitechurch URBAN_Future Generalised Pareto (GP2/ML) 2.9 3.77 5.77 7.40 9.88 12.09 14.65 17.63 26.57 SUDS Log Pearson Type 3 Little Dargle URBAN_Future SUDS 4.3 6.00 7.80 8.94 10.46 11.66 12.95 14.33 17.96 (LP3/LMOM) Dundrum Urban_1+2_Future SUDS Generalised Pareto (GP2/ML) 8.08 8.78 12.46 15.78 21.35 26.78 33.58 42.13 71.47

Table 6-6 Future Scenario 1 Design Flows for Rainfall Runoff Boundary Catchments (m3/s)

MDW0259Rp0016 86 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

6.2.2 Future Scenario 2 – Assuming NO Implementation of SUDS

6.2.2.1 Summary of Parameters

Table 6-7 presents the parameters used in Future Scenario 2.

Future Scenario 2 Further Parameter Description Information

Land Use Scenario Future Development (2100) Section 2.3

Historical Rainfall Records + Rainfall Data Climate Change (Mid Range Sections 3.1 & 3.5 Future Scenario) ICPSS + Climate Change (Mid Coastal Data Section 3.5 & 6.2.1 Range Future Scenario)

SUDS No implementation of SUDS -

Table 6-7 Future Scenario 2 Summary Table

6.2.2.2 Extreme Flows

An Extreme Value Analysis (EVA) was carried out on discharge files produced from the RR models for each of the RR boundary catchments for Future Scenario 2 using the same methods as described in Section 5.4 of this report.

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Catchment Return Period (years) RR Boundary Catchment Probability Distribution 2 1 Area (km ) 2 5 10 25 50 100 200 1000 Log Pearson Type 3 Tallaght NAM_Future No SUDS 4.35 6.03 7.65 8.86 10.61 12.11 13.79 15.67 21.06 (LP3/LMOM) Log Pearson Type 3 Bohernabreena NAM_Future No SUDS 28.00 46.21 58.95 69.25 84.87 98.69 114.63 133.06 187.95 (LP3/LMOM) Log Pearson Type 3 Dodder Main Channel NAM_Future No SUDS 10.66 18.73 21.66 24.65 28.81 32.24 35.98 40.09 51.30 (LP3/LMOM) Log Pearson Type 3 Owendoher NAM_Future No SUDS 9.37 16.65 21.68 25.49 31.10 35.96 41.48 47.78 66.09 (LP3/LMOM) Log Pearson Type 3 Whitechurch NAM_Future No SUDS 5.30 10.39 12.56 15.06 18.89 22.34 26.37 31.10 45.53 (LP3/LMOM) Generalised Pareto Little Dargle NAM_Future No SUDS 3.03 6.19 8.19 9.77 12.14 14.20 16.54 19.22 27.02 (GP2/ML) Log Pearson Type 3 Dundrum NAM_Future No SUDS 0.83 2.11 2.76 3.34 4.27 5.12 6.13 7.32 11.00 (LP3/LMOM) Log Pearson Type 3 Tallaght URBAN_Future No SUDS 8.55 9.29 12.43 15.80 21.26 26.45 32.80 40.62 66.45 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN_1_Future No SUDS 5.76 6.78 8.87 11.02 14.40 17.51 21.23 25.68 39.76 (LP3/LMOM) Log Pearson Type 3 Dodder URBAN_2_Future No SUDS 11.57 12.00 15.79 19.76 26.10 32.02 39.16 47.82 75.74 (LP3/LMOM) Log Pearson Type 3 Dodder Urban_3+4_Future No SUDS 11.92 8.85 11.75 14.61 19.49 24.28 30.31 37.89 64.01 (LP3/LMOM) Owendoher_Whitechurch URBAN_Future No Generalised Pareto 7.54 9.35 13.95 17.66 23.28 28.24 33.95 40.54 60.10 SUDS (GP2/ML) Log Pearson Type 3 Little Dargle URBAN_Future No SUDS 5.29 7.85 10.89 12.75 15.19 17.11 19.15 21.33 27.05 (LP3/LMOM) Generalised Pareto Dundrum Urban_1+2_Future No SUDS 8.64 11.2 16 20.9 28 35.6 45 55.5 92 (GP2/ML) Table 6-8 Future Scenario 2 Design Flows for Rainfall Runoff Boundary Catchments (m3/s)

1 The catchment areas have changed to reflect the increase in urban and decrease in rural allocations in Future Scenario 2.

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6.2.3 Climate Change Rainfall Sensitivity Analysis

As part of a sensitivity analysis a High End Future Scenario rainfall data-set was produced for the Bohernabreena NAM rainfall-runoff model using the maximum predicted change in precipitation for each month shown in Table 6-9 (taken from Table 3-7 in Section 3.5 of this report).

Monthly Monthly CC_Model CC_Model Maximum Maximum January 130% July 97% February 132% August 90% March 120% September 131% April 130% October 109% May 113% November 145% June 86% December 133%

Table 6-9 Predicted Changes in Precipitation from Climate Change Models

Figure 6-1 presents the rainfall data sets for the Bohernabreena NAM catchment for the Present Day, Mid Range Future and High End Future Scenarios for a typical year.

Figure 6-1 Present Day, Mid Range Future and High End Future Scenario Rainfall Datasets for Bohernabreena NAM Catchment.

MDW0259Rp0016 89 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

The High End Future Scenario rainfall data-set was then run in the Bohernabreena NAM rainfall-runoff model and a simulated discharge file produced. An extreme value analysis was carried out on this discharge file and design discharges of known return period calculated.

The 1 in 100 year return period design discharge produced using the High End Future Scenario Rainfall data sets will be compared against those produced using the Average Future Rainfall data sets for the Bohernabreena NAM rainfall-runoff model and a percentage increase calculated. This percentage increase in discharge will be applied to all flows contributing to the Dodder Main Channel model. The Dodder Main Channel model will then be run for the 1 in 100 year return period event and the resulting flood outline examined.

MDW0259Rp0016 90 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

7 CONCLUSION

To summarise, this report has outlined the process and results of the Hydrological Analysis undertaken for the River Dodder CFRAMS. The report has detailed the data collected, the method and type of analysis undertaken and the results of this analysis.

A number of conclusions can be drawn for the analyses as follows:

There were some difficulties in calibrating the rainfall-runoff models to recorded discharge data and it is concluded that this is due to insufficiencies in regard to the spatial and temporal distribution of rain gauges in the River Dodder catchment. However, it was possible to adequately calibrate the model with the available data.

From examination of the latest climate change research relating to Ireland, it is concluded for the 2100 Future Scenario that the mean annual precipitation will increase. It is expected that there will be more concentrated rainfall events all year and a reduction in the number of days with precipitation during the summer.

From analysis of the rating curves for the three water level gauges in the River Dodder Catchment (Waldron’s Bridge, Willbrook Road and Frankfort) provided by the EPA it is concluded that the extrapolated portion of the curves do not match the modelled Project Curves prepared for this study. For example the EPA rating curve for Waldron’s Bridge rates the Hurricane Charlie event (25th/26th August 1986) at approximately 269m3/s while the modelled Project Curve rates it at approximately 251m3/s.

The Flood Studies Report Catchment Characteristics Method of flood prediction was applied to the three gauge catchments to estimate design flood flows for the Present Day Scenario. These flows were then compared against those produced from the simulated model results. It is concluded from this analysis that the simulated design flows are favoured for use in this study as they are the product of recorded rainfall data and calibrated catchment response.

The results of the Hydrological Analysis provide design discharges of known return period for Present Day and Future Scenarios. These design discharge will be used as inputs to the hydraulic models of the Dodder Main Channel and its five major tributaries (the Dundrum Slang, the Little Dargle, the Owendoher, the Whitechurch and the Tallaght Stream).

MDW0259Rp0016 91 Rev. F01 River Dodder Catchment Flood Risk Management Plan Hydrological Analysis Report

References

Bell, A.K., Elsaesser, B., Glasgow, G. (2007). “Climate change scenarios and impact on catchment and rainfall runoff response”.IHP Conference Tullamore 2006.

Cawley, A.M. , Fitzpatrick, J., Cunnane, C. and Sheridan, T. (2005). “A Selection of Extreme Flood Events- The Irish Experience”. National Hydrology Seminar, 2005.

Dixon, F.E. (1959). “Weather in Old Dublin”-The old Dublin Society Paper. Dublin Historical Records.

Flood Estimation Handbook – Procedures for Flood Frequency Estimation. Institute of Hydrology, 1999

Hennigan, P., McDaid, J., Keyes, J., (Dublin City Council and Dublin Corporation) (1988).”Dodder River- Flood Study”. A paper presented to the Civil Division, IEI, November,1988.

Huang, M., Zhang, L., Gallichand, J. (2003). “Runoff responses to afforestation in a watershed of the Loess Plateau, China”.

Intergovernmental Panel on Climate Change (Bernstein, L. et al.) (2007). “Climate Change 2007- Synthesis Report – An Assessment of the Intergovernmental Panel on Climate Change” (IPCC Valencia, Spain, 12-17 November 2007).

Keyes, J. (1987). “Floods in Dublin City Rivers 25th /26th August 1986”. Institute of Engineers Ireland, 1987.

Mc Grath, R., Nishimura, E., Nolan, P., Semmler, T., Sweeney, C. and Wang, S. (2005) “Climate Change: Regional Climate Model Predictions for Ireland”. EPA Report.

Mac Cárthaigh, M. ( 2005): ‘Flooding in the Dodder Catchmen 26th of August 1986 (Hurricane Charlie) & 2nd of December 2003t’. EPA paper

Mac Cárthaigh, M. (2007): ‘Flooding in the Dodder Catchment 22 June 2007’. EPA paper

www.floodmaps.ie www.c4i.i

MDW0259Rp0016 92 Rev. F01 APPENDIX A

Rainfall Data

A1 Extreme Value Analysis Results of Rainfall Gauging Stations

Probability Distribution (Casement Rainfall)

Return Period [years] WEI2/ML GP2/ML GAM/ML

2 35.595 36.106 35.438 5 47.049 46.162 47.642 10 57.082 55.084 57.938 Estimated Quantile 25 71.645 68.933 72.236 50 83.464 81.221 83.376 100 95.871 95.325 94.705 200 108.801 111.514 106.174 2 35.382 35.945 35.23 5 47.09 46.029 47.772 10 57.466 55.17 58.399 Average Quantile 25 72.597 69.614 73.176 50 84.901 82.61 84.698 100 97.827 97.665 96.42 200 111.299 115.063 108.292 2 1.211 1.17 1.282 5 3.103 2.894 3.266 10 5.214 4.552 5.582 Standard Deviation 25 8.833 7.522 9.206 50 12.119 10.62 12.201 100 15.823 14.709 15.339 200 19.909 20.061 18.582 CHISQ 4.889 3.037 8.593 Goodness-of-fit SLSC 0.051 0.052 0.061 Statistics PPCC1 0.968 0.971 0.964 PPCC2 0.975 0.981 0.964

Table A.1: Rainfall derived for various return periods for Casement Rainfall Gauging Station

A2 Probability Distribution (Dundrum Rainfall)

Return Period WEI2/MOM GP2/MOM LP3/MOM/LOG GP2/LMOM [years] 2 39.238 38.561 38.005 38.292 5 49.893 48.197 48.187 48.335 10 58.665 56.68 57.769 57.532 Estimated Quantile 25 71.015 69.745 73.523 72.276 50 80.845 81.248 88.303 85.779 100 91.044 94.362 106.103 101.71 200 101.576 109.315 127.539 120.507 2 39.185 38.794 37.645 38.179 5 49.403 47.795 47.507 47.979 10 57.795 55.406 57.18 57.051 Average Quantile 25 69.615 66.778 74.021 71.991 50 79.041 76.585 91.038 86.237 100 88.841 87.654 113.347 103.854 200 98.986 100.23 143.263 125.872 2 2.43 2.234 2.489 2.198 5 4.59 4.125 4.677 4.329 10 6.871 6.355 8.192 7.304 Standard Deviation 25 10.667 10.826 17.399 14.198 50 14.083 15.712 30.301 22.76 100 17.939 22.269 52.481 35.635 200 22.214 30.947 91.533 54.82 CHISQ 11.185 6.37 3.778 4.889 Goodness-of-fit SLSC 0.048 0.045 0.038 0.037 Statistics PPCC1 0.972 0.977 0.985 0.982 PPCC2 0.978 0.986 0.991 0.99

Table A.2: Rainfall derived for various return periods for Dundrum Rainfall Gauging Station

A3 Probability Distribution (Glenasmole Rainfall)

Return Period [years] LP3/MOM/LOG LN2/LMOM

2 54.166 53.817 5 70.197 74.592 10 85.839 97.914 Estimated Quantile 25 112.379 141.81 50 138.015 187.374 100 169.674 246.089 200 208.756 320.604 2 54.207 54.038 5 69.39 75.343 10 84.697 99.538 Average Quantile 25 111.92 145.639 50 139.956 194.067 100 177.373 257.129 200 228.652 338.005 2 2.664 2.308 5 5.265 7.108 10 8.83 13.9 Standard Deviation 25 19.69 29.052 50 36.651 46.969 100 68.312 72.391 200 129.699 107.513 CHISQ 24.75 20.375 Goodness-of-fit SLSC 0.046 0.053 Statistics PPCC1 0.973 0.985 PPCC2 0.981 0.971

Table A.3: Rainfall derived for various return periods for Glenasmole Rainfall Gauging Station

A4 APPENDIX B

Historic Flooding Data

B1 

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Project Figure Issue Details River Dodder Flood Risk Management Plan Dodder B2.1 Drawn: A. Anderssen Project No. MDW0259 Catchment Title Checked: C. O'Donnell File Ref. Area Historic Flooding Locations - Overview Approved: G. Gillespie MDW0259Mi0021F01 Scale: 1:60,000@A3 Drawing No. Rev.

Date: March'07 Mi0021 F01

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Date: March'07 Mi0022 F01

Notes 1. This drawing is the property of RPS Consulting Engineers. It is a confidential document and must not be copied, used, RPS Consulting Engineers Ph: 01-2884499 or its contents divulged without prior written consent. West Pier Business Campus, Fax: 01-2835676 2. All levels are referred to Ordnance Datum, Malin Head. Dun Laoghaire, E: [email protected] 3. Ordnance Survey Ireland Licence No. EN 0005007 Co Dublin W: www.rpsgroup.com/ireland Copyright Government of Ireland. ght ght ed, or its or ed, doubt ask. doubt t is Head. F01 MDW0259 Rev. Ph: Ph: 01-2884499 Fax: 01-2835676 E: [email protected] W: www.rpsgroup.com/ireland MDW0259Mi0028F01 Mi0028 Project No. Project File Ref. Drawing No. Legend Legend Legend Legend Legend Legend Legend Legend Legend Legend Historic Flooding Points Watercourses Historic Fluvial Flooding Area Flood Flow Direction Dodder Catchment Boundary A. Anderssen G. Gillespie C. O'Donnell NTS@ A3 River Dodder Flood Locations - 2 of 2 - 2 Locations Historic Flooding Historic confidentiala us copied, must be not and document divulged consent. prior contents written without 2. All levels Datum, referred to are MalinOrdnance 3. NOTonly,figured TOif dimensions use SCALE, in Copyri Survey 4. Ordnance 0005002 EN No. Licence Ireland 1. This drawing is the property of property is the - RPS MCOS Ltd. 1. I drawing This Government of Ireland. Risk Management Plan 30 March 2007 March 30 See Figure B2.1 Figure See study the area of extent forfull RPS Consulting Engineers Engineers Consulting RPS Campus, West Pier Business Laoghaire, Dun Dublin Co % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % % Date: Project Title B2.3 Figure Details Issue Drawn: Checked: Approved: Scale: Notes Dodder Catchment Area                                                                                                                                                         

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D D D D D D APPENDIX C

Simulated Discharge Data Extreme Value Analysis of Simulated Discharge Data

Probability Distribution (Waldron's Bridge Simulated)

Return Period [years] LP3/LMOM TGUM/ML EXP1/MOM

2 75.491 74.511 74.26 5 107.282 108.191 108.177 10 129.626 130.205 130.634 Estimated 25 161.513 157.899 159.008 Quantile 50 188.542 178.406 180.057 100 218.775 198.747 200.951 200 252.787 219.007 221.769 1000 349.681 265.926 269.991 2 75.399 73.718 74.258 5 107.146 108.707 108.172 10 129.309 131.808 130.626 Average 25 160.63 160.784 158.996 Quantile 50 186.863 182.194 180.043 100 215.846 203.411 200.935 200 247.997 224.533 221.75 1000 337.016 273.432 269.967 2 3.279 3.039 1.946 5 6.497 6.559 6.575 10 9.419 9.249 9.639 Standard 25 15.555 13.609 13.512 Deviation 50 22.363 17.265 16.384 100 31.388 21.087 19.236 200 43.016 25.007 22.077 1000 82.819 34.312 28.657 CHISQ 2.25 3 3 KS 0.063 0.065 0.063 Goodness- SLSC 0.021 0.023 0.022 of-fit Statistics PPCC1 0.995 0.997 0.997 PPCC2 0.996 0.997 0.997 LLM -216.027 -211.13 -211.137

Table B.1: Design Flows for Simulated Discharge from Waldron’s Bridge Gauge Catchment

C1 Probability Distribution (Willbrook Road Simulated)

Return Period [years] LP3/MOM/LOG WEI2/ML GP2/LMOM

2 8.089 8.13 8.115 5 10.581 10.405 10.394 10 12.46 12.228 12.331 Estimated 25 14.993 14.967 15.349 Quantile 50 16.962 17.37 18.08 100 18.979 20.121 21.286 200 21.042 23.28 25.061 1000 25.996 32.569 36.631 2 8.085 8.125 8.108 5 10.601 10.424 10.388 10 12.475 12.279 12.299 Average 25 14.974 15.042 15.234 Quantile 50 16.898 17.415 17.847 100 18.854 20.053 20.868 200 20.843 22.963 24.361 1000 25.57 30.725 34.701 2 0.15 0.134 0.156 5 0.556 0.531 0.547 10 0.957 0.863 0.956 Standard 25 1.609 1.555 1.756 Deviation 50 2.18 2.394 2.608 100 2.809 3.604 3.73 200 3.492 5.277 5.185 1000 5.255 11.6 10.327 CHISQ 6 6 6 KS 0.116 0.103 0.11 Goodness-of- SLSC 0.04 0.038 0.033 fit Statistics PPCC1 0.989 0.991 0.992 PPCC2 0.989 0.991 0.992 LLM -69.255 -68.871 -69.815

Table B.2: Design Flows for Simulated Discharge from Willbrook Road Gauge Catchment

C2 Probability Distribution (Frankfort Simulated)

Return Period [years] LP3/MOM/LOG WEI2/ML GP2/LMOM

2 3.878 3.797 3.749 5 5.565 5.615 5.453 10 6.816 6.92 6.847 Estimated 25 8.653 8.641 8.983 Quantile 50 10.248 9.96 10.891 100 12.066 11.298 13.11 200 14.148 12.657 15.698 1000 20.256 15.885 23.514 2 3.9 3.769 3.745 5 5.602 5.628 5.465 10 6.808 6.978 6.878 Average 25 8.522 8.768 9.029 Quantile 50 9.965 10.142 10.92 100 11.566 11.538 13.076 200 13.349 12.956 15.521 1000 18.32 16.328 22.423 2 0.215 0.176 0.172 5 0.461 0.445 0.456 10 0.611 0.674 0.643 Standard 25 0.838 1.026 0.984 Deviation 50 1.061 1.327 1.424 100 1.358 1.656 2.117 200 1.755 2.01 3.146 1000 3.237 2.916 7.377 CHISQ 10.571 7.905 6.762 KS 0.095 0.069 0.078 Goodness-of- SLSC 0.046 0.045 0.049 fit Statistics PPCC1 0.961 0.973 0.949 PPCC2 0.977 0.977 0.973 LLM -64.403 -58.094 -58.397

Table B.3: Design Flows for Simulated Discharge from Frankfort Gauge Catchmen

C3 APPENDIX D

Joint Probability Data

Extreme Value Analysis of Surge Residuals for Joint Probability Analysis

Probability Distribution (Surge Residuals)

Return Period GAM/LMOM EXP1/LMOM [years] 2 0.697 0.697 5 0.804 0.818 10 0.875 0.898 Estimated 25 0.962 1 Quantile 50 1.027 1.075 100 1.091 1.149 200 1.154 1.224 1000 1.301 1.396 2 0.697 0.697 5 0.805 0.818 10 0.875 0.898 Average 25 0.962 0.999 Quantile 50 1.027 1.074 100 1.091 1.149 200 1.154 1.223 1000 1.301 1.395 2 0.015 0.015 5 0.029 0.028 10 0.039 0.036 Standard 25 0.052 0.047 Deviation 50 0.063 0.055 100 0.073 0.062 200 0.084 0.07 1000 0.108 0.088 CHISQ 10.324 6.27 Goodness-of- SLSC 0.022 0.014 fit Statistics PPCC1 0.997 0.998 PPCC2 0.997 0.998

Table C.1: Surge Residuals for Various Return Periods

D1 Extreme Value Analysis of Rainfall for Joint Probability Analysis

Probability Distribution (Rainfall)

Return Period GP2/MOM LN2/MOM LP3/LMOM [years] 2 46.394 45.853 45.748 5 62.762 69.508 65.696 10 78.288 96.231 86.884 Estimated 25 104.085 147.084 126.314 quantile 50 128.555 200.565 168.058 100 158.321 270.358 223.949 200 194.53 360.11 298.795 1000 311.759 670.588 585.711 2 46.979 46.161 45.852 5 62.165 70.356 66.391 10 75.831 97.919 88.867 Average 25 97.534 150.869 132.461 quantile 50 117.368 207.085 180.983 100 140.872 281.069 249.507 200 168.876 377.031 347.206 1000 257.523 714.09 777.641 2 3.407 4.069 3.587 5 6.78 10.611 9.353 10 10.907 19.325 18.107 Standard 25 19.45 38.28 40.347 deviation 50 29.128 60.491 71.499 100 42.586 91.971 124.766 200 61.056 135.574 216.355 1000 133.487 304.756 785.418 CHISQ 11.105 14.526 15.053 Goodness- SLSC 0.054 0.027 0.033 of-fit statistics PPCC1 0.967 0.971 0.973 PPCC2 0.984 0.993 0.989

Table C.2: Rainfall for Various Return Periods

D2 APPENDIX E

Rainfall-Runoff Models

E1 Extreme Value Analysis Plots for Rainfall-Runoff Boundary Catchment Simulated Flows – Present Day Scenario

Figure E1.1 EVA Plot for Tallaght NAM RR Simulated Flows – Present Day

Figure E1.2 EVA Plot for Bohernabreena NAM RR Simulated Flows – Present Day

E2 Figure E1.3 EVA Plot for Dodder Main Channel NAM RR Simulated Flows – Present Day

Figure E1.4 EVA Plot for Owendoher NAM RR Simulated Flows – Present Day

E3 Figure E1.5 EVA Plot for Whitechurch NAM RR Simulated Flows – Present Day

Figure E1.6 EVA Plot for Little Dargle NAM RR Simulated Flows – Present Day

E4 Figure E1.7 EVA Plot for Dundrum NAM RR Simulated Flows – Present Day

Figure E1.8 EVA Plot for Tallaght URBAN RR Simulated Flows – Present Day

E5 Figure E1.9 EVA Plot for Dodder URBAN 1 RR Simulated Flows – Present Day

Figure E1.10 EVA Plot for Dodder URBAN 2 RR Simulated Flows – Present Day

E6 Figure E1.11 EVA Plot for Dodder URBAN 3+4 RR Simulated Flows – Present Day

Figure E1.12 EVA Plot for Owendoher Whitechurch URBAN RR Simulated Flows – Present Day

E7 Figure E1.13 EVA Plot for Little Dargle URBAN RR Simulated Flows – Present Day

Figure E1.14 EVA Plot for Dundrum URBAN 1+2 RR Simulated Flows – Present Day

E8 Extreme Value Analysis Plots for Rainfall-Runoff Boundary Catchment Simulated Flows – Future Scenario 1

Figure E1.15 EVA Plot for Tallaght NAM RR Simulated Flows – Future Scenario 1

Figure E1.16 EVA Plot for Bohernabreena NAM RR Simulated Flows – Future Scenario 1 Error!

E9 Figure E1.17 EVA Plot for Dodder MC NAM RR Simulated Flows – Future Scenario 1

Figure E1.18 EVA Plot for Owendoher NAM RR Simulated Flows – Future Scenario 1

E10 Figure E1.19 EVA Plot for Whitechurch NAM RR Simulated Flows – Future Scenario 1

Figure E1.20 EVA Plot for Little Dargle NAM RR Simulated Flows – Future Scenario 1

E11 Figure E1.21 EVA Plot for Dundrum NAM RR Simulated Flows – Future Scenario 1

Figure E1.22 EVA Plot for Tallaght Urban RR Simulated Flows – Future Scenario 1

E12 Figure E1.23 EVA Plot for Dodder Urban 1 RR Simulated Flows – Future Scenario 1

Figure E1.24 EVA Plot for Dodder Urban 2 RR Simulated Flows – Future Scenario 1

E13 Figure E1.25 EVA Plot for Dodder Urban 3+4 RR Simulated Flows – Future Scenario 1

Figure E1.26 EVA Plot for Owendoher Whitechurch Urban RR Simulated Flows – Future Scenario 1

E14 Figure E1.27 EVA Plot for Little Dargle Urban RR Simulated Flows – Future Scenario 1

Figure E1.28 EVA Plot for Dundrum Urban 1+2 RR Simulated Flows – Future Scenario 1

E15 Extreme Value Analysis Plots for Rainfall-Runoff Boundary Catchment Simulated Flows – Future Scenario 2

Figure E1.29 EVA Plot for Tallaght NAM RR Simulated Flows – Future Scenario 2

Figure E1.30 EVA Plot for Bohernabreena NAM RR Simulated Flows – Future Scenario 2

E16 Figure E1.31 EVA Plot for Dodder MC NAM RR Simulated Flows – Future Scenario 2

Figure E1.32 EVA Plot for Owendoher NAM RR Simulated Flows – Future Scenario 2

E17 Figure E1.33 EVA Plot for Whitechurch NAM RR Simulated Flows – Future Scenario 2

Figure E1.34 EVA Plot for Little Dargle NAM RR Simulated Flows – Future Scenario 2

E18 Figure E1.35 EVA Plot for Dundrum NAM RR Simulated Flows – Future Scenario 2

Figure E1.36 EVA Plot for Tallaght Urban RR Simulated Flows – Future Scenario 2

E19 Figure E1.37 EVA Plot for Dodder Urban 1 RR Simulated Flows – Future Scenario 2

Figure E1.38 EVA Plot for Dodder Urban 2 RR Simulated Flows – Future Scenario 2

E20 Figure E1.39 EVA Plot for Dodder Urban 3+4 RR Simulated Flows – Future Scenario 2

Figure E1.40 EVA Plot for Owendoher Whitechurch Urban RR Simulated Flows – Future Scenario 2

E21 Figure E1.41 EVA Plot for Little Dargle Urban RR Simulated Flows – Future Scenario 2

Figure E1.42 EVA Plot for Dundrum Urban 1 RR Simulated Flows – Future Scenar

E22 APPENDIX F

Glossary

Glossary of Terms

The sum of evaporation and transpiration. It is the water lost to the Evapotranspiration atmosphere from the ground surface.

A method for estimating the precision of sample statistics by using Jack-Knife Re-sampling subsets of available data.

Modelling software produced by DHI used for the Hydrological and MIKE Hydraulic modelling components of the River Dodder CFRMP.

The abbreviation of the Danish “Nedbør-Afstrømnings-Model”, NAM meaning precipitation-runoff-model.

SAAR Standard Average Annual Rainfall

Soil The index of how the soil may accept infiltration

An offshore rise in water level associated with a low pressure Storm Surge weather system. It is caused primarily by high winds pushing up the ocean's surface.

A sequence of management practices and control structures SUstainable Drainage designed to drain surface water in a more sustainable fashion than Systems (SUDS) some conventional techniques.

The slope of the main stream between 10% and 85% of its length S 1085 measured from the catchment outlet (m/km)

A partition of space into cells. Each cell is an area in which the Voronoi Polygons contained points are closer to the enclosed site than to any others.