Bandon Flood Relief Scheme Draft Hydraulic Report 7D Bandon EIA - Hydraulics Report.pdf.pdf · This...

Bandon Flood Relief Scheme Draft Hydraulic Report October 2011

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JBA Office

JBA Consulting 24 Grove Island Corbally Limerick Ireland

JBA Project Manager

Elizabeth Russell

Revision History

Revision Ref / Date Issued Amendments Issued to

Initial Issue V1 - 09 May 2011 - OPW Steering Group

Initial Issue V2 - 3rd October 2011

Following comments OPW Steering Group

Contract

This report describes work commissioned by the Office of Public Works, under the Bandon Flood Relief Scheme, by a letter dated 3rd December 2010. OPW’s representative for the contract was Michael Collins. Wolfram Schlüter and Kevin Frodsham of JBA Consulting carried out this work.

Prepared by ..................................................Wolfram Schlüter BSc MSc PhD CEng C.WEM MCIWEM

Senior Engineer

.......................................................................Kevin Frodsham BSc BSc DipMath MSc PhD FGS ARSM

Senior Analyst

Reviewed by .................................................Chris Smith BSc PhD CEnv MCIWEM C.WEM MCMI

Principal Analyst

Purpose

This document has been prepared as a hydraulic report for the Office of Public Works. JBA Consulting accepts no responsibility or liability for any use that is made of this document other than by the Client for the purposes for which it was originally commissioned and prepared.

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Acknowledgments

The OPW provided considerable data, including river flows and levels and historic accounts of flooding. The EPA also provided information on river gauges.

Copyright

© JBA Consulting Engineers and Scientists Limited 2012

Carbon Footprint

262g

A printed copy of the main text in this document will result in a carbon footprint of 206g if 100% post-consumer recycled paper is used and 262g if primary-source paper is used. These figures assume the report is printed in black and white on A4 paper and in duplex.

JBA is working to achieve carbon neutrality and the carbon emissions from our activities are offset.

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Executive Summary

WYG Ireland and JBA Consulting were commissioned by the Office of Public Works (OPW) to assess the flood risk within Bandon Town and develop a flood relief scheme and other measures to manage this risk. This hydraulic modelling report is one of a series being produced under the first stage of the project.

Bandon Town, in County Cork, has a long history of serious flooding. Flooding is primarily due to high flows in the Bandon River and Bridewell River exceeding the channel capacity. Surface water flooding associated with heavy rainfall and exceedance of the drainage system is also a problem, which is being addressed under a separate study.

The focus of the study is Bandon Town, and rivers included in the hydraulic model are the Bandon River, the Bridewell River, the Millstream, the Inishannon and the Brinny River.

The final hydraulic model provides a good representation of the flood characteristics at Bandon Town, as it has been verified against recent flood events, namely the November 2009 event and the January 2011 event.

Flood flow hydrographs based on the flows in the final hydrology report have been run through the hydraulic models. The design flows provide a great deal of confidence for the River Bandon, as these are based on 50 years of annual maximum data record from the Bandon Gauge and apply the latest Flood Study Update (FSU) methodology. The January 2011 event resulted in relatively minor flooding at Bandon and was estimated as approximately 1 in 10 year return period event. Modelling results show that a 50 year return period event would cause flooding at Bandon and the November 2009 event is estimated to be in the range of a 1 in 200 year return period.

Modelling results show that Bandon Town is subject to flood risk, mainly due the Bandon River exceeding its channel capacity at a number of locations along the left and right river bank (looking downstream). There are a number of gaps in the existing flood defences, which allow flood waters to exit the river channel and overspill into the low lying areas of Bandon Town. The main gaps are at the pedestrian bridge on the Bandon River, the access gaps in the existing wall downstream of the Riverside Shopping Centre and along the walkway on the right bank downstream of Bandon Bridge. The existing earth embankment in the vicinity of LIDL was also found to overtop at a number of locations and there is concern with regard to its structural stability during flood conditions.

The influence of the Bandon Weir on flood levels at Bandon Town is relatively small. Similarly the impact of the walkway downstream of Bandon Bridge is relatively small on the channel capacity and flood levels.

Bandon Bridge itself has a significant influence on water levels during flood events and water levels are approximately 0.5m higher upstream in comparison to downstream of the bridge. Modelling results confirm that Bandon Town is located in the floodplain of the Bandon River with only minor flooding originating from the Bridewell River. It is thought that surface water ponding resulted in flooding of New Road along the Bridewell during the November 2009 event, There is also no evidence to suggest that the angle of the confluence from the Bridewell River with the Bandon River is having a significant impact on flood levels along the Bridewell or the Bandon River. Findings also show that Bandon Town is not influenced by the tide and this is due to the level difference between the river bed and the tide, which is more than five meters. Results also confirm that the existing culvert on the Milllstream is significantly undersized resulting in flooding of the local area. Sensitivity testing has been undertaken on various model parameters and findings show that the uncertainty in flow has the biggest influence. Detailed freeboard analysis has also been carried out and the recommended freeboard level provides for the model uncertainty.

Once the model was established for the existing case, a number of potential flood alleviation measures were considered, namely raised defence walls, dredging of the river bed, removing Bandon Weir, providing storage upstream and providing a compound or flood relief channel, as well as a combination of the dredged and defended options. Following extensive analysis of various combinations of options and stakeholder consultation, the preferred option is likely

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to be a combination of flood containment and dredging and this report will be finalised once the preferred option has been identified.

A summary of the findings for each of the modelled option is as follows:

Increasing the standard of protection using raised defences offers the potential to significantly reduce flood risk in Bandon Town, however it also results in a significant increase in water levels and associated flood risk upstream, which is contrary to best practise guidelines.

Extensive dredging would be required if this option was to be adopted as the only flood alleviation measure. This consists of removing more than 2m of bed material for a distance of 4km and resulting in approximately 200,000m3 of material to be removed.

Removal of the weir upstream of Bandon Bridge was found to have little impact on water levels in the town.

The option of providing flood storage upstream in the catchment was assessed and results show that vast quantities of water (8-13million m3) would be require to be retained with extensive environmental, economic and social implication. As a result this option was not assessed any further.

Provision of a compound channel was assessed and this could only be provided in restricted areas, outside of Bandon Town and modelling results showed negligible improvement in terms of flood risk.

The freeboard allowance has been assessed using the combined option of flood containment and dredging. Results show that a freeboard of 0.34- to 0.59m for hard defences is required to account for model uncertainty, such as model roughness and hydrological flow estimation.

Flood extents and depths will be used to assess the number of properties at risk, the depth of flooding and the potential economic damages to those properties. This economic assessment will be crucial in the option appraisal process. This report will be updated once the preferred option has been identified.

Contents


Executive Summary.......................................................................................................... iv

1. Introduction.......................................................................................................... 1

1.1 Context of the Study .............................................................................................. 1 1.2 Scope of this Report .............................................................................................. 1 1.3 Description of Study Area...................................................................................... 2 1.4 Potential Sources of Flood Risk ............................................................................ 5 1.5 Flood History.......................................................................................................... 5 1.6 Flood Defences...................................................................................................... 5 1.7 Available Data........................................................................................................ 6 1.8 Presentation of Return Periods ............................................................................. 7

2. Hydraulic Boundaries.......................................................................................... 8

2.1 Overview................................................................................................................ 8 2.2 Fluvial Hydrology ................................................................................................... 8 2.3 Lateral Catchments................................................................................................ 10 2.4 Design Flow Estimates .......................................................................................... 10 2.5 Tidal Boundaries.................................................................................................... 11

3. Hydraulic Modelling Overview ........................................................................... 13

3.1 Overview................................................................................................................ 13 3.2 Model Extents ........................................................................................................ 13 3.3 Flood Event Probability.......................................................................................... 17 3.4 Options Overview .................................................................................................. 17

4. Medium Priority Watercourse Model ................................................................. 18

4.1 Introduction ............................................................................................................ 18 4.2 1D Model Development and Schematisation ........................................................ 18 4.3 Boundary Conditions ............................................................................................. 18 4.4 Model Limitations................................................................................................... 18 4.5 ISIS Modelling Results........................................................................................... 18

5. High Priority Watercourse Model....................................................................... 18

5.1 Model Development............................................................................................... 18 5.2 1D (ISIS) Model Component ................................................................................. 18 5.3 2D (TUFLOW) Model Component ......................................................................... 18 5.4 HPW Model Boundary Conditions ......................................................................... 18 5.5 Existing Risk Model Runs...................................................................................... 18 5.6 Model Verification .................................................................................................. 18 5.7 Existing Risk Design Events.................................................................................. 18 5.8 Climate Change Impacts ....................................................................................... 18 5.9 Sensitivity Testing.................................................................................................. 18 5.10 Model Limitations................................................................................................... 18

6. Options Assessment ........................................................................................... 18

6.1 Flood Containment using Raised Defences .......................................................... 18 6.2 Dredging ................................................................................................................ 18 6.3 Weir Removal ........................................................................................................ 18 6.4 Removal of Walkway ............................................................................................. 18 6.5 Confluence of Bridewell River to Bandon River .................................................... 18 6.6 Storage Option....................................................................................................... 18 6.7 Compound Channel............................................................................................... 18 6.8 Freeboard Analysis................................................................................................ 18 6.9 Summary of Flood Alleviation Options .................................................................. 18

Contents


7. Performance of Preferred Option....................................................................... 18

8. Summary and Conclusion .................................................................................. 18

Appendices

A. Modelling Hydrographs

B. Sensitivity Analysis


List of Figures

Figure 1-1 Bandon River catchment ..........................................................................3

Figure 1-2 Model Extent showing HPW and MPW model and HEPs ......................4

Figure 1-3 Stone wall protecting Bandon Town (right bank). .................................5

Figure 1-4 Earth embankment (Riverview)................................................................5

Figure 1-5 Gap in defence for pedestrian access (left bank). .................................6

Figure 1-6 Gap in defence for pedestrian access at bridge (right bank). ..............6

Figure 1-7 Gap in flood defences at Bandon Bridge (left bank). ............................6

Figure 1-8 Gap in flood defences at access ramp, Bandon Bridge (left bank). ....6

Figure 1-9 Stone wall protecting Bandon Town. ......................................................6

Figure 1-10 Gap in flood defences at walkway (left bank).......................................6

Figure 2-1 Study area ..................................................................................................9

Figure 2-2 Inflow hydrographs at HEP01 - Bandon River........................................11

Figure 2-3 Design tide levels - Kinsale Harbour (OPW, 2011).................................12

Figure 3-1 Model schematic........................................................................................15

Figure 4-1 Fluvial 100 year event combined with tidal 2 year and tidal 200 year events .........................................................................................................18

Figure 5-1 Active 2D model domain and LiDAR topography ..................................18

Figure 5-2 2D roughness map for Bandon................................................................18

Figure 5-3 Modelled and observed outlines for the Nov 2009 event ......................18

Figure 5-4 Modelled and observed flood depths for the Nov 2009 event ..............18

Figure 5-5 Modelled peak flood levels during the Nov 2009 event.........................18

Figure 5-6 Modelled sequence of flooding during the Nov 2009 event. ................18

Figure 5-7 Modelled flood depths for the Jan 2011 event .......................................18

Figure 5-8 Flooding Threshold upstream and downstream of Bandon Bridge ....18

Figure 5-9 Flooding Threshold at key locations.......................................................18

Figure 5-10 Flood outlines from selected design events ........................................18

Figure 5-11 Peak flood depths during the 1% AEP event........................................18

Figure 5-12 Peak flood velocities in Bandon during the 1% AEP event ................18

Figure 5-13 Climate change impact showing the 1% AEP flood outlines..............18

Figure 6-1 Impact of increasing the standard of protection on the 1% AEP flood outlines. ....................................................................................................18

Figure 6-2 Impact of increasing the standard of protection on 1% AEP flood levels. ..................................................................................................................18

Figure 6-3 Comparison of dredge option with existing flood level (1%AEP) ........18

Figure 6-4 Impact of the 1m dredging option on the 1% AEP flood outlines. .......18

Figure 6-5 Impact of the 1m dredging option on the 1% AEP flood levels. ...........18

Figure 6-6 Combination of dredging & flood containment (1%AEP)......................18


Figure 6-7 Impact of weir removal on the 1% AEP flood outlines. .........................18

Figure 6-8 Impact of weir removal on the 1% AEP flood levels. .............................18

Figure 6-9 Impact of walkway removal on the 1%AEP flood levels........................18

Figure 6-10 Compound Channel, RS22000B................................................................18

Figure 6-11 Compound Channel, RS21500B................................................................18

Figure 6-12 Compound Channel, Results in Longsection .........................................18

List of Tables

Table 1-1 Return Periods and Annual Exceedance Probability ..............................7

Table 2-1 Scheme Design Flow Estimates (m3/s) (JBA, 2011) ................................10

Table 2-2 Present Day Tidal Still Water Levels for Kinsale Harbour. .....................12

Table 3-1 Modelled watercourse extents...................................................................13

Table 5-1 2D roughness categories in the Bandon model ......................................18

Table 6-1 Model runs of dredging options ................................................................18

Table 6-2 Model runs of combined options ..............................................................18

Table 6-3 Storage volume required for 100 year design flow .................................18

Table 6-4 presents the input parameters with a brief description. Please refer to Appendix B for further details on the sensitivity testing that was used to derive the appropriate uncertainty levels. Table 6-4: Description of Input Parameters ......................................................................18

Table 6-5 Details of Freeboard calculation - US of Bandon Weir ...........................18

Table 6-6 Details of Freeboard calculation - US of Bandon Bridge........................18

Table 6-7 Details of Freeboard calculation - DS of Bandon Bridge........................18

Table 6-8 Details of Freeboard calculation - US of Bandon Weir ...........................18

Table 6-9 Details of Freeboard calculation - US of Bandon Bridge........................18

Table 6-10 Details of Freeboard calculation - DS of Bandon Bridge......................18

Table 6-11 Summary of modelling results and defence heights for the selected flood alleviation options....................................................................18


Abbreviations

1D One Dimensional (modelling)

2D Two Dimensional (modelling)

AEP Annual Exceedance Probability

AM Annual Maximum

AMAX Annual Maximum

APSR Area of Potential Significant Risk

DoEHLG Department of the Environment, Heritage and Local Government

EPA Environmental Protection Agency

FEH Flood Estimation Handbook

FSR Flood Studies Report

FSU Flood Studies Update

GS Gauging Station

HEFS High End Future Scenario

HEP Hydrological Estimation Point

HPW High Priority Watercourse

ISIS Hydrology and hydraulic modelling software

JBA JBA Consulting – Engineers & Scientists

mOD Metres above Ordnance Datum

MPW Medium Priority Watercourse

MRFS Mid-Range Future Scenario

OD Ordnance Datum

OPW Office of Public Works

POT Peaks Over a Threshold

QBAR Mean Annual Maximum Flood

QMED Median Annual Flood (with return period 2 years)

QMEDrural Median annual maximum flood in the as-rural state

Tp Time to Peak

TUFLOW Two-dimensional Unsteady FLOW (a hydraulic model)

WYG White Young Green

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1. Introduction

1.1 Context of the Study Bandon Town, in County Cork, has a long history of serious flooding. Flooding is primarily due to high flows in the Bandon River and Bridewell River exceeding the channel capacity. Surface water flooding associated with heavy rainfall and exceedance of the drainage system is also a problem, which is being addressed under a separate study. It is reported that river levels can be exacerbated by high tides in the Bandon River estuary and this has been assessed as part of this study. The highest recorded flooding occurred in November 2009. Serious flooding has also occurred in the town in 1975, 1978, 1982, 1986, 1988, 2004, 2005 and 2006. A smaller flood event occurred following commencement of this study, in January 2011. As a result, Bandon Town has been identified by the OPW as an Area of Potentially Significant Risk (APSR).

Following recommendations contained in a report1 produced following the 2009 flood event, WYG Ireland and JBA Consulting were commissioned by the Office of Public Works (OPW) to assess the flood risk within Bandon Town and develop a flood relief scheme and other measures to manage this risk. An assessment of the potential for significant increase in this risk due to climate change, ongoing development and other future pressures was also required. The whole project will comprise five stages:

Stage I - Feasibility study and preparation of a flood risk management plan

Stage II - Public exhibition

Stage III - Detailed design, confirmation and tender

Stage IV - Construction

Stage V - Handover of works

This hydraulic report is one of a series being produced under Stage I of the project and should be read in conjunction with the hydrology report, which provides details on the design flow estimation as well as information on a number of important aspects for the hydraulic analysis. The hydrology and hydraulic assessment are closely linked through various mechanisms. Hydrological flow estimation points for example, are located at fixed locations along each watercourse to anchor the hydraulic modelling results with the hydrological flow estimation. Hydraulic modelling has also influenced the hydrology through sensitivity testing that was undertaken as part of the rating curve analysis for the Bandon and Curranure Gauge.

1.2 Scope of this Report

1.2.1 Project Brief

Key tasks identified in the project brief for the hydraulic analysis are:

Develop dynamic hydraulic models for HPWs and MPW and their floodplains (please see Section 1.3 for description of model type and Figure 1-2 showing the model extents)

Analyse historic events and estimate design and potential future flood events.

Adopt 1-dimensional modelling for the channel reaches of the HPWs and channel and floodplain modelling for the MPW.

Adopt 2-dimensional modelling for the floodplain of the HPWs.

Undertake model calibration according to the HEPs to ensure hydrological continuity.

Calibrate and verify the fluvial hydraulic models against a number of suitable past flood events.

1 WYG Ireland, Bandon Flood Relief Scheme Scoping Report, 2009


Undertake sensitivity tests for each and all forms of modelling as appropriate to determine the robustness and sensitivity.

Undertake hydraulic modelling for the existing condition and for the MRFS for the annual exceedence probability (AEP) of 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1%.

Undertake hydraulic modelling for the existing condition and for the HEFS for the annual exceedence probability (AEP) of 10%, 1%, and 0.1%.

This report details the work undertaken to complete these tasks and presents the results of the analysis.

1.2.2 Content and Key Tasks

This report provides details on the hydraulic model development covering the HPWs and MPW of the Bandon River. Details are provided on model calibration of the November 2009 event and the January 2011 event. Investigation into the potential impact of various flood mitigation measures are outlined for a range of AEP and associated flood depth mapping is provided. Sensitivity testing has also been undertaken to test the model robustness and sensitivity of the various model parameters.

Survey information on the Bridewell River is outstanding for the Oliver Plunket Culvert and newly constructed flood protection walls and the existing model will be extended to include the Bridwell River and Millstream, once this information is available.

1.2.3 Report Structure

This section provides an outline of the study and sets its context with regards to the project brief and key deliverables as well as providing background information on the study area and data availability. Section 2 provides details on the hydraulic modelling boundaries and presents the design flow estimates that were derived from the hydrological assessment (JBA, 2011)2. An overview of the hydraulic modelling approach is presented in Section 3. Section 4 provides details on the 1D (ISIS) model, which covers the MPWs. Section 5 presents details of the 2D (TUFLOW) model, which covers the APSR of Bandon Town, corresponding to the HPWS. Initial findings of flood alleviation options are presented in Section 6 and Section 7 provides a conclusion of the hydraulic modelling assessment to date.

1.3 Description of Study Area A detailed description of the study areas is provided in the report 'Bandon Flood Relief Scheme - Hydrology Report' and the following presents a brief summary for ease of reference:

The subject of this study is Bandon Town and surrounding area, which has been defined as an Area of Potential Significant Flood Risk (APSR). The main watercourses within the town boundary of Bandon are defined as High Priority Watercourses (HPWs) and these are the Bandon River, the Bridewell River and the Mill Stream. Medium Priority Watercourses (MPWs) are defined outside the town boundary and along a longer reach of the Bandon River, which extends past Inishannon to the tidal dominated estuary. The Brinny River and the Inishannon River, both tributaries of the Bandon, are also defined as MPWs. Figure 1-1 presents the Bandon River Catchment and Figure 1-2 shows the extents of the HPW and MPW model as well as the hydrological flow estimation points.

It should be noted that the model accuracy increases significantly for the HPW model due to much closer river section spacing and also the model methodology, which consists of a 1 dimensional river channel model that is linked to a 2 dimensional floodplain model. The MPW model is based on a 1 dimensional model for the river and floodplain.

2 JBA, 2011: Bandon Flood Relief Scheme Draft Hydrology Report, April 2011


Figure 1-1 Bandon River catchment

Bandon River Catchment


Figure 1-2 Model Extent showing HPW and MPW model and HEPs

Area for Further Assessment


1.4 Potential Sources of Flood Risk The main potential sources of flooding to Bandon Town and surrounding area investigated in this study is associated with overtopping of the natural banks and/or defences of the HPWs and MPWs during either high fluvial flow or high sea level events.

Surface water flooding in direct response to high rainfall is an additional potential source of flooding to Bandon but has not been investigated in this study.

1.5 Flood History A detailed analysis of the flood history for Bandon Town and surrounding area is provided in Section 2 of the report 'Bandon Flood Relief Scheme - Draft Hydrology Report'. It is noted that the November 2009 is the most extreme event that has been recorded at the Bandon Gauge and for which approximate flood extent mapping is available. This event is estimated to be of approximately 1 in 200 years return period with an estimated peak flow of approximately 410m3/s.

The January 2011 is the most recent significant event, which is estimated to be of approximately 1 in 10 years return period with a peak flow of approximately 180m3/s.

Both events were adopted for model calibration.

1.6 Flood Defences As a consequence of the historic flooding noted in Section 1.5, there are several sections of man-made flood defences alongside the Bandon River. These flood defences include an earth embankment alongside the commercial area of Riverview on the right bank of the Bandon River (Figure 1-4 Earth embankment (Riverview). and an extensive system of stone walls defending the centre of the town itself (Figure 1-3). There are however a number of gaps along the stone wall and gaps for pedestrian access at the Bandon Bridge and the Footbridge (Figure 1-5 to 1-9).

The flood defences are represented in detail in the HPW model and are based on the out-of-bank survey that was specifically undertaken to capture any variations in flood defence levels. Gaps in the flood defences are modelled as link from the 1 dimensional ISIS model to the 2 dimensional TUFLOW model by specifying its elevation and width and thereby allowing a realistic representation.

Figure 1-3 Stone wall protecting Bandon Town (right bank).

Figure 1-4 Earth embankment (Riverview).


Figure 1-5 Gap in defence for pedestrian access (left bank).

Figure 1-6 Gap in defence for pedestrian access at bridge (right bank).

Figure 1-7 Gap in flood defences at Bandon Bridge (left bank).

Figure 1-8 Gap in flood defences at access ramp, Bandon Bridge (left bank).

Figure 1-9 Stone wall protecting Bandon Town.

Figure 1-10 Gap in flood defences at walkway (left bank).

1.7 Available Data

1.7.1 Hydrometric Data

There are a number of gauging stations located in the Bandon River catchment and a data review has been undertaken and findings are presented in Section 3 of the report 'Bandon


Flood Relief Scheme - Draft Hydrology Report'. A rating review has also been undertaken for gauging station 20001 and 20002.

1.7.2 Tidal Data

The report "Irish Coastal Protection Strategic Study" (RPS, 2011) was examined and design tide levels for Kinsale Harbour were adopted as downstream boundary for the hydraulic model development.

1.7.3 Topographic Data

New topographic channel survey and bank survey data along HPWs and MPWs were collected for this study by Maltby Land Surveys Ltd in October 2010. Additional survey is being undertaken on recently constructed flood defence walls along the Bridewell River, as well as a CCTV survey of the 'Oliver Plunket Culvert'. Filtered and unfiltered LiDAR data at a 2m resolution was also available and was used to define the floodplain topography.

It is noted that local dredging works has been undertaken in the vicinity of Bandon Bridge, since the topographical survey has been undertaken. However, the extent of these works is not anticipated to affect modelling results significantly.

1.7.4 Previous Studies

No previous models for the study area were available for incorporation into this study.

1.8 Presentation of Return Periods The probability of a flood event is classified by its Annual Exceedance Probability (AEP) or return period (in years). A 0.5% AEP flood will occur on average once every 200 years and has a 1 in 200 chance of occurring in any given year.

Expressing flood frequency as AEP can be helpful when presenting results to members of the public who may associate the concept of return period with a regular occurrence rather than an average recurrence interval.

Table 1-1 Return Periods and Annual Exceedance Probability

Return Period (years)

Annual Exceedance Probability (%)

2 50

5 20

10 10

20 5

50 2

100 1

200 0.5

1000 0.1


2. Hydraulic Boundaries

2.1 Overview The modelled design events required for the Bandon Flood study were:

Fluvial existing condition 50%, 20%, 10%, 5%, 2%, 1%, 0.5% and 0.1% AEP and also testing for MRFS (20% flow increase as an allowance for possible climate change).

Fluvial existing condition for the annual exceedence probability (AEP) of 10%, 1%, and 0.1% to test for the HEFS (30% flow increase as an allowance for possible climate change).

Details of the fluvial hydrology of the Bandon River catchment has been analysed using the Flood Studies Update (FSU) and Flood Study Report (FSR) traditional methods. This analysis is presented in the report 'Bandon Flood Relief Scheme - Hydrology Report' and design flows are reproduced in Table 2-1 for ease of reference.

Tidal flood risk was analysed by generating suitable extreme sea level tidal graph boundaries using findings from the ICPSS. The method of creating tidal graphs for Kinsale Harbour is discussed in Section 2.5.

2.2 Fluvial Hydrology A hydrological assessment was required to derive peak flow estimates and inflow hydrographs for the relevant design flow events along for each of the HEPs in order that the fluvial flood risk could be modelled.

In addition a QMED (50% AEP) estimate was also required to act as a moderate fluvial inflow when modelling tidal (extreme sea level) events. The approach and methodology used to derive the fluvial hydrological estimates is summarised in the following sections.

2.2.1 Flow Estimation Points and Catchment Descriptors

The initial number of flow estimation points was increase from 13 to 17 during our hydrological analysis and this is detailed in the report 'Bandon Flood Relief Scheme - Hydrology Report'. These points are located at the upstream limits of the hydraulic model, at the junction of tributaries and at a number of other key points along the Bandon River. The location and a brief description of the HEPs is provided in Table 2-1 and illustrated in Figure 3-1 and the study area is presented in Figure 2-1.

For each of the points, a catchment has been delineated based on the Ordnance Survey's National Height Model. The catchments have been reviewed and cross checked against those given as catchment descriptors under the Flood Studies Update programme3.

3 Compass Geomatics (2009) Work Package 5.3 - Preparation of Digital Catchment Descriptors, Flood Studies Update


Figure 2-1 Study area

Legend Study Area


2.3 Lateral Catchments Lateral catchments provide lateral inflows to the hydraulic model from intervening areas between the flow estimation points. These inflows are from non-point sources such as overland flow, urban runoff or watercourses that are too small to be included as a sub-catchment. Inflows from lateral catchments tend to be adjusted during the modelling phase of mapping studies to ensure that they correspond with the hydrological estimates at the flow estimation points. Where there are inflows of particular note, the lateral inflow to the model have been weighted to reflect this and applied at cross-sections between the corresponding HEPs.

2.4 Design Flow Estimates The Statistical Method is the preferred method to estimate peak flows and for medium-sized (sub-) catchments such as those at Bandon. Details of the design flow analysis are provided in the hydrology report and Table 2-1 has been extracted for ease of reference.

Table 2-1 Scheme Design Flow Estimates (m3/s) (JBA, 2011)4

Return Period

Growth Factor (GEV)

HE

P01

HE

P02

HE

P03

HE

P04

HE

P05

HE

P06

HE

P07

HE

P08

HE

P09

HE

P10

HE

P11

HE

P14

HE

P15

HE

P16

HE

P17

2 1.00 116 117 4.9 122 4.3 0.7 3.0 129 14.4 135 1.5 2.8 1.6 15.2 1.0

5 1.33 155 155 6.5 162 5.7 1.0 4.0 172 19.1 180 2.0 3.7 2.1 20.2 1.4

10 1.60 186 187 7.8 194 6.9 1.2 4.9 206 23.0 216 2.4 4.4 2.5 24.3 1.6

50 2.37 276 277 11.5 288 10.2 1.8 7.2 306 34.1 320 3.6 6.6 3.8 35.9 2.4

100* 2.79 325 326 13.6 339 12.0 2.1 8.5 360 40.2 377 4.2 7.7 4.4 42.3 2.9

200* 3.28 382 383 16.0 399 14.1 2.4 10.0 423 47.2 443 4.9 9.1 5.2 49.7 3.4

1000* 4.73 551 553 23.0 575 20.3 3.5 14.4 610 68.1 639 7.1 13.1 7.5 71.7 4.8

* Return Period exceeds record lengths and data should be treated with caution

2.4.1 Hydraulic Model Hydrographs

For unsteady model simulations, a set of flood hydrographs are required within ISIS. For the Bandon Flood study, the November 2009 has been used to generate design hydrographs for the HEPS along the Bandon River, whereas the FSR RR method was adopted for the tributaries. The time to peak on the Bandon River was monitored as 48h during the November 2009 event.

Inflow hydrographs from each tributary were adjusted as follows:

The FSR RR hydrographs were scaled to fit the upstream inflow estimates (see Table 2-1).

A time lag of 12 to 37 hours was added to allow overlap of the peak flow from each tributary with the peak flow from the Bandon River, as conservative estimate.

Lateral Inflows were added (as required) to match the design flow values for each HEP.

Example inflow hydrographs into the Bandon River are shown in Figure 2-2 and modelling results at specific HEPs are provided in Appendix A.

4 JBA, 2011: Bandon Flood Relief Scheme Hydrology Report, September 2011


Daily rainfall data was available from two stations throughout the Bandon River Catchment. In order to improve the critical storm duration higher resolution rainfall data would be required and no adjustment to the critical duration was carried out.

Figure 2-2 Inflow hydrographs at HEP01 - Bandon River

0

100

200

300

400

500

600

0 20 40 60 80 100

Flow (m3/s)

Time (hours)

Q1000

Q200

Q100

Q50

Q20

Q10

Q5

Q2

The hydraulic and hydrological analysis is subject to a number of uncertainties, such as the channel and floodplain roughness, representation of structures, estimation of the annual flood flow, estimation of the rating curve and estimation of growth curve.

Final design flow for culvert and bridge sizing are provided in the hydrology report (JBA, 2011)5.

2.5 Tidal Boundaries In order to simulate tidal inundation processes as accurately as possible, it is necessary to apply time varying hydraulic boundaries in the form of design tidal graphs. Observed time series data (e.g. Mean High Water Springs {MHWS}) or readily calculated time series data (e.g. Highest Astronomical Tides {HAT}) are often suitable for modelling low return period tidal events. However, since extreme sea level events are normally a consequence of major storm surges, design tidal curves for extreme tidal events are typically created by combining a storm surge curve to an observed low return period tidal series. The production of a design tidal graph requires a scaled storm surge to be combined with an underlying astronomical tide series in such a way as to produce a tide series in which the peak sea level matches the estimated, design extreme sea level for a specified coastal location.

2.5.1 Extreme Sea Levels

The most recent set of tidal still water Extreme Sea Level (ESL) estimates for the Bandon study area are available in draft format from the OPW6, as shown in Table 2-2. Figure 2-3 presents an example of the downstream boundary when running fluvial scenarios.

5 JBA, 2011: Bandon Flood Relief Scheme Hydrology Report, September 2011

6 OPW, 2011: Irish Coastal Protection Strategic Study.


Table 2-2 Present Day Tidal Still Water Levels for Kinsale Harbour.

Figure 2-3 Design tide levels - Kinsale Harbour (OPW, 2011)

‐1.5

‐1.0

‐0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 50 100 150 200

MSL

Duration (hours)

Design Tide ‐ Kinsale Harbour

0.5% AEP

Appendix B provides details of the sensitivity testing on tidal flooding. Results show that Bandon Town is not influenced by tidal pattern and also that the fluvial flooding is the dominating factor in terms of flood risk to the lower reaches of the Bandon River. As a result the 50% AEP tide water levels have been adopted as nominal downstream boundary.

AEP 50% 20% 10% 5% 2% 1% 0.5% 0.1%

to Malin 1.95 2.04 2.11 2.17 2.26 2.32 2.39 2.54


3. Hydraulic Modelling Overview

3.1 Overview The study area to be included in the hydraulic model includes the Bandon River channel and its tributaries as shown in Figure 3-1. Medium Priority Watercourse (MPW) extents upstream and downstream of Bandon have been modelled using the one-dimensional modelling software ISIS to simulate channel and floodplain flows. High Priority Watercourse (HPW) extents have been modelled using ISIS for the river channels dynamically linked to two-dimensional modelling software TUFLOW for modelling floodplain flows.

Linked 1D-2D hydraulic models are capable of increasing the accuracy of model predictions relative to 1D only models since linked 1D-2D models are capable of more accurately defining local flow patterns across floodplains. The model for the HPWs is an ISIS-TUFLOW model using ISIS v3.4 linked to TUFLOW 2009-07-AE-iSP.

3.2 Model Extents The hydraulic modelling extent is shown in Figure 3-1 and detailed in Table 3-1. The 1D model of the river channel extends from approximately 4km upstream of Bandon to approximately 5km downstream of Inishannon. The confluence with the Irish Sea at Kinsale Harbour was represented in ISIS using digital terrain model data for completeness. The 2D model domain covers the HPW extent through Bandon Town.

Table 3-1 Modelled watercourse extents

Watercourse Priority Upstream Limit Downstream Limits Comments

Bandon MPW 145380, 54746 147783, 54971 Upstream of the Bandon HPW

Bandon HPW 147783, 54971 150503, 55927 Through Bandon town.

Bandon MPW 150503, 55927 156832, 53026 Downstream of the Bandon HPW

Bridewell HPW 147568, 53015 149296, 55074 DS limit is confluence with the Bandon

Bridewell Trib (unnamed)

HPW 147206, 53686 147770, 53897 DS limit is confluence with the Bridewell

Mill Stream HPW 150710, 55088 150232, 55407 DS limit is confluence with the Bandon

Brinny MPW 152325, 58847 153188, 57332 DS limit is confluence with the Bandon

Inishannon MPW 155612, 57497 154787, 56894 DS limit is confluence with the Bandon


Figure 3-1 Model schematic


3.3 Flood Event Probability The representation of flood event probability is classified by Annual Exceedance Probability AEP (%) or return period (in years). As explained in Section 1.8, a 0.5% AEP flood will occur on average once every 200 years and has a 1 in 200 chance of occurring in any one year.

Flood risk management and planning guidelines recommend that assessment considers the 1% AEP for fluvial flood risk and the 0.5% AEP for tidal flood risk.

The lower reach of the Bandon River is influenced hydraulically by both tidal and fluvial waters. Joint probability refers to the chance of two or more conditions occurring at the same time. However, sensitivity runs showed that Bandon Town is not influenced by tidal levels (see Section 4.5.1) and no joint probability analysis is undertaken.

3.4 Options Overview In addition to establishing the existing risk, a range of scenarios of potential flood alleviation options have been assessed and these are outlined below with results presented in Section 6. The focus of this assessment has been on the HPW model for Bandon Town.

3.4.1 Existing Risk

Initially, the existing risk model was calibrated to recordings from the November 2009 event and January 2011 event. This was undertaken as an iterative process following the rating review for gauging station 20001 and 20002 and testing modelled data with recordings in combination with sensitivity testing. This base model was assessed for the full range of design flow events. The Existing Risk model at Bandon Town is detailed in Section 5.5.

3.4.2 Defended Option

The Fully Defended option was developed for Bandon Town using the HPW model by adjusting the 2D model and artificially restricting inflow into the 2D domain. This option was assessed for the 100 year design return period and details are provided in Section 6.1.

3.4.3 Dredging Option

The Dredging option was tested in the HPW model by adjusting the cross section data in the 1D domain. This option was assessed for the 100 year design return period details are provided in Section 6.2.

3.4.4 Weir Removal Option

The Weir Removal option was developed in the HPW model by removing the relevant ISIS nodes from the 1D domain. This option was assessed for the 100 year design return period and details are provided in Section 0.

3.4.5 Storage Option

The Storage Option considers storage of flood waters in the catchment that would result in a reduction in peak flow and flow volume upstream of Bandon Town and details are presented in Section 6.6.

3.4.6 Compound Channel Option

The Compound Channel option considers providing flood alleviation by diverting excess flood waters into a compound or flood relief channel. Details of this option are presented in Section 6.7.


4. Medium Priority Watercourse Model

4.1 Introduction A one-dimensional model was constructed in ISIS to model channel flow and estimate river levels in the MPW model for specific flood events. The MPW model is quite coarse with cross sections up to 1km apart but structures are included in the model. Roughness values have been set separately for the river channel and floodplain. The output from the model includes water level and flow at each cross section. The water levels have been used to create flood outlines but these are relatively coarse due to the sparse cross section data. The MPW model overlaps with the HPW model through the town of Bandon. Within this section of overlap the HPW model results should be used in favour of those from the MPW model.

4.2 1D Model Development and Schematisation A 1D hydraulic model of the MPWs was developed in ISIS (v3.4) with the following model extents;

Bandon River - Sixty-five cross-sections were surveyed across a chainage of 16,633 metres upstream from the intertidal reach of the Bandon River, commencing approximately 10 km upstream from Kinsale Harbour.

Brinny River - Seven cross sections were surveyed extending for a chainage of 2,065 metres upstream from the confluence with the Bandon River.

Inishannon Stream- Nine cross sections were surveyed extending for a chainage of 1,156 metres upstream from the confluence with the Bandon River.

River channel sections were taken from surveyed cross sections collected by Maltby Land Surveys Ltd in October 2010 as part of the study. The nomenclature used to label river sections within the model is a combination of watercourse name and unique identifier that increases with chainage. Hence, 26500B is the upstream-most node on the Bandon River. Similarly, 3000BR is the upstream-most node on the Brinny, 1600M is the upstream- most node on the Millstream and 2000I is the upstream- most node on the Inishannon. In addition to these labels, several suffixes have been used in the form of u….d etc to represent the upstream and downstream faces of bridges and other structures present.

Some key features of the 1D (ISIS) element of the Bandon model include:

Hydraulic resistance within the 1D channel system was defined using Manning's 'n' roughness coefficients based upon experienced judgement and reference to Chow (1959)7. Manning's 'n' values within most of the channel were set to 0.040 and the relatively coarse cross section spacing did not lend itself to setting individual channel roughness values for each cross section.

The Bandon Bridge and the Footbridge, within the HPW section of the Bandon River have been included within the ISIS model since both are considered to have the potential to influence the local flood risk. The Bandon Bridge was modelled as an ARCH bridge unit in ISIS with an associated overtopping SPILL. The footbridge upstream of the Bandon Bridge was also modelled as an ARCH bridge in combination with a BLOCKAGE unit to represent the flow restriction of its piers.

For model stability reasons, it was necessary to add a number of interpolate sections between the surveyed river sections.

4.3 Boundary Conditions HEPs are located at fixed locations along each watercourse to anchor peak flows from hydraulic modelling results with the hydrological flow estimation. Lateral inflows were added at location to present additional runoff or flow from minor streams that were not explicitly modelled. The model boundaries for the MPW model consist of:

7 Chow VT, Open Channel Hydraulics (1959)


A Flow-Time boundary unit that provides inflow into the Bandon River at HEP01. This represents direct inflow from the upstream catchment. Additional lateral inflow was considered necessary in order match flows at HEP02.

A Flow-Time boundary unit that provides inflow into the Bandon River at HEP03. This represents direct inflow from the Bridewell River, which is not explicitly modelled in the MPW model but is required to match flows at HEP04.

A Flow-Time boundary unit that provides inflow into the Bandon River downstream from HEP14. This represents direct inflow from the Millstream, which is not explicitly modelled in the MPW model but is required to match flows at HEP08. Additional lateral inflows were considered necessary in order match flows at HEP08, which also includes flows estimated at HEP17.

A Flow-Time boundary unit that provides inflow into the Brinny River at HEP09. This represents direct inflow from the upstream catchment. Additional lateral inflow was considered necessary in order match flows at HEP16.

A Flow-Time boundary unit that provides inflow into the Inishannon River at HEP11. This represents direct inflow from the upstream catchment. Additional lateral inflow was considered necessary in order match flows at HEP15.

A Head-Time boundary unit was attached to the downstream boundary of the Bandon River to represent the tide level at Kinsale harbour.

4.4 Model Limitations All hydraulic model results are prone to uncertainties due to factors such as uncertainties in the key model inputs (e.g. flows, sea levels, topography), modelling parameters (e.g. roughness), the modelling software used, model stability and the nature of the assumptions used in the modelling. Some particular limitations of the Bandon MPW model that future users may wish to be aware of is its relatively coarse cross-section spacing which ranges up to 1km. Cross sections also had to be extended using LiDAR data and the downstream section to Kinsale Harbour has been provided as a routing model based on DTM data.

The outputs from the model, such as water levels and flood outlines, should therefore only be used for high level studies where a broad overview is required. More detailed cross section data would be required if the model was required to deliver more detailed results.

4.5 ISIS Modelling Results Appendix A provides design flow hydrographs at specific HEPs throughout the MPW model for the existing risk and the MRFS. Flood mapping for the full range of return periods for the existing risk model will be presented in the flood mapping report.

The ISIS model is effectively an undefended model since it uses the full width of the floodplain for conveyance regardless of the presence of embankments or walled protection. This approach was taken due to the relatively coarse cross section spacing in the majority of the MPW model, which could not accurately represent local flood protection embankments. The undefended flood maps produced from the MPW model are therefore likely to have quite conservative flood extents.


4.5.1 Tidal Influence

The tidal influence was tested using the 100 year fluvial design event with both the 2 year and 200 year tide levels. Figure 4-1 shows the maximum resultant flood levels in the lower reach of the Bandon River from these two scenarios. These show that the Bandon River is influenced by tidal levels up to chainage 16000, which is located upstream of the confluence with the Inishannon River, approximately 6km downstream of Bandon Town. Bed levels in Bandon are typically around 11.0m AOD i.e. several metres higher than the predicted extreme tidal levels. It therefore seems unlikely that extreme tides will be able to influence flooding within Bandon Town. Sea level rise due to climate change could alter this situation in future years but the rise in sea level would need to be large to extend several kilometres further upstream.

Figure 4-1 Fluvial 100 year event combined with tidal 2 year and tidal 200 year events

Long Section: 26500B - 965BRuD - Maximum Stage; 10 - 60 h.

Node Label

1700

0Bu

1600

0B

1593

0B

1500

0Bu

1400

0B

1300

0B

1266

6Bi

1233

3Bi

1200

0B

1175

0Bi

1150

0Bi

1125

0Bi

1100

0B

1075

0Bi

1050

0Bi

1025

0Bi

1000

0B

Ele

vatio

n (m

AD

)

8

7.5

7

6.5

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

0.5

0

-0.5

-1

-1.5

-2

-2.5

-3

-3.5

-4

-4.5

Fluvial 100 with Tidal 2

Fluvial 100 with Tidal 200


5. High Priority Watercourse Model

5.1 Model Development The High Priority Watercourse (HPW) in this study is centred on the town of Bandon and includes the Bandon River and two of its tributaries, the Bridewell and the Millstream. The flood risk from this watercourse system has been analysed by creating an existing risk (defended) model of the HPWs through the following stages:

Model creation (by trimming and adding a 2D domain to the MPW model).

Model verification (by calibrating the HPW model against data obtained from the November 2009 and January 2011 events).

Sensitivity testing (to test the robustness of the HPW model to key model parameters).

Simulation of design runs.

5.2 1D (ISIS) Model Component The 1D (ISIS) component of the HPW model is based on the MPW model and was generated from the MPW model by making the following changes:

The MPW model was shortened to prevent any instability from outside of the HPW model area from affecting the HPW model performance. The 1D component of the HPW model therefore extends from approximately 3.5 kilometres upstream of Bandon Weir (Chainage 26500) to the Ballylangley area (Chainage 20900), approximately 1.3 kilometres downstream of the gauging station in Bandon.

A normal flow boundary was placed at the downstream end of the HPW model and calibrated against the rating at this location that was being predicted by the MPW model (further details in Section 5.4). The inflow into the HPW model has also been effectively reduced to the design inflow into the Bandon River the Bridewell River, its tributary and the Millstream.

Extended cross-sections through the HPW area were removed from the model by making use of deactivation panel markers in ISIS. This step was necessary in order that the floodplains in this area could be satisfactorily modelled as a 2D (TUFLOW) domain.

Bridge overtopping spill units and Bandon Weir were constrained to be limited to the width of the channel in the new linked model.

Additional spill units were added to local access bridges on the Bridewell River, the footbridge on the Band River and Bandon Bridge to enable the flow through low points adjacent to each bridge to be modelled directly into the 2D (TUFLOW) domain.

The ISIS inflow units were separated into separate IED files.

5.3 2D (TUFLOW) Model Component A 2D (TUFLOW) model domain was linked to the ISIS component of the HPW model in order to improve floodplain representation in the vicinity of Bandon. This section summarises the geometry and configuration of the 2D (TUFLOW) domain.

An active 2D domain of approximately 1.97 square kilometres was created encompassing the full extent of the HPWs reach of the Bandon River (Figure 5-1), from approximately 1.2 kilometres upstream of Bandon Weir (Chainage 24000) to the Ballylangley area (Chainage 20900), approximately 1.3 kilometres downstream of the gauging station in Bandon. Both up and downstream extents were chosen to be sufficiently distant from the town centre for floodplain flow routes through the town centre to be adversely affected by the boundary conditions. Laterally the domain boundaries were configured along high ground so that floodwater did not pond up against the domain boundary.


Figure 5-1 Active 2D model domain and LiDAR topography

Ordnance Survey Licence No. EN0021011 © Ordnance Survey of Ireland / Government of Ireland

The existing risk HPW model has been created with cell sizes of four and ten metres. Sensitivity testing showed that the flood outlines created using the two different cell sizes are almost identical (see Appendix B). The results presented in this chapter have been largely derived from the four metre model but given the much shorter run time of the ten metre model, the options models and sensitivity tests have been carried out using the ten metre model.

Filtered LIDAR, which was flown specifically for this project, was the principal topographic source by which the TUFLOW Digital Terrain Model (DTM) was defined.

The ISIS and TUFLOW components of the model were linked along the length of both banks of the Bandon River, the Bridewell River and the Millstream. The elevations of these bank crest lines within the model were interpolated between successive spot heights, which were obtained primarily from the Maltby survey, along the bank crests. Since the bank crest survey (with points often less than one metre apart) was generally of a higher definition than the model grids (four or ten metres), careful manipulation of the bank crest data was required to ensure that the model representation of the survey was appropriate. In addition, some of the small (sub cell-size) gaps in the defences, such as the wall gaps at the footbridge and near Bandon Bridge, were included in the model by using additional ISIS spill units rather than modifying the bank crest lines. However, it should be noted that not all gaps could be included in this way. Since the detailed Maltby bank crest survey extends along much but not all of the modelled reach, bank crest levels beyond the limits of this survey had to be interpolated between the surveyed bank levels from successive river sections with some additional points based on LiDAR.

A small number of additional alterations to the core TUFLOW DTM were made. These include:

The floor level of each building was set according the findings from a threshold survey specifically carried out for this project..

LIDAR Elevation (m AOD)

<13.00

13.00 - 15.00

15.00 - 17.00

17.00 - 22.00

22.00 - 28.00

>28.00

1D (ISIS) Nodes

Bandon Weir

Bandon Gauging Station

Active 1D Domain Extent

Active 2D Domain Extent


The deck of Bandon Bridge was interpolated using TUFLOW z-shapes to cover the filtered LiDAR gap across this structure so enabling lateral flow along the bridge deck to be simulated.

The lateral embankment upstream of LIDL was placed in the model as a TUFLOW z-line based upon the surveyed (Maltby) crest levels.

Some local smoothing of ground levels was carried out to improve model stability using TUFLOW z-pt patches.

A 2D roughness map was defined on the basis of OSI data. This enabled the preferential flow routes along roads and around buildings to be captured at a high definition within Bandon village by setting contrasting high (1.00) Manning's 'n' values for buildings and low (0.025) values for roads configured against a natural background roughness of 0.05. These roughness values have been set using experience and following Syme (2008)8 l and also the calibration events (see Section 5.6). Figure 5-2 shows the 2D roughness map for the Bandon model and Table 5-1 contains the allocated roughness values (as Manning's 'n' equivalent values).

Figure 5-2 2D roughness map for Bandon


8 Syme, W.J. (2008): Flooding in Urban Areas - 2D Modelling Approaches for Buidlings and Fences. Engineers Australia, 9th National Conference on Hydraulics in Water Engineering, Australia

Land Use/ Roughness Categories

Buildings

Roads, Tracks & Paths

General Natural Surface


Table 5-1 2D roughness categories in the Bandon model

Land Use Type

Roughness Value (Manning's ‘n’ Equivalent)

General Natural Surfaces 0.050

Buildings 1.000

Roads, tracks, paths & pavements 0.025

5.4 HPW Model Boundary Conditions The HPW model currently contains the following hydraulic boundaries:

An upstream flow-time (ISIS QTBDY) boundary has been applied to the upstream-most node of the ISIS component of the model. This represents direct inflow from the upstream catchment and has been configured to the estimated magnitude of the design flow at Bandon gauging station. This was considered a reasonable approach since any attenuation of the flood wave through Bandon Town is modelled to be negligible. The shape of the design inflow hydrographs was taken from the November 2009 event as conservative measure.

A normal flow (ISIS NCDBDY) boundary has been applied at the downstream end of the ISIS model. This boundary has been defined with a manual slope of 0.0015 following calibration against a rating curve that was obtained from the MPW model at this location. A normal flow downstream boundary was favoured over a flow-head (rating curve) boundary for the HPW model since some of the options (e.g. dredging) had the capacity to significantly affect the existing rating at this location.

The connection between 1D (ISIS) and 2D (TUFLOW) has been enabled by using:

HX boundaries along both banks of the Bandon River and the two tributaries to allow the overtopping volume to be calculated by TUFLOW based on the relative water level between ISIS and TUFLOW domains.

Local SX boundaries at the wall gaps at the footbridge and Bandon Bridge and local access bridges along the Bridewell River to allow the ISIS calculated overtopping flow through these narrow gaps to be passed directly to the TUFLOW domain. This is a simple mass balance boundary allowing flow to be passed between the 1D and 2D domain.

5.5 Existing Risk Model Runs

5.5.1 Run Settings

The linked ISIS-TUFLOW model was run with the following settings;

ISIS v3.4 and TUFLOW release 2010-10-AB-iSP-w32.

A fixed ISIS (1D) timestep of 3 second and TUFLOW (2D) model timestep of 3 seconds was found to be appropriate for all design runs using the ten metre model grid. The time step generally had to be halved in order to run the four metre models.

All existing risk models were found to start satisfactorily with a single set of ISIS initial conditions.

Default run time parameters were found to be appropriate for all model runs.

All existing risk model runs based on a ten metre model grid took less than one hour in contrast to those based on a four metre grid that took between 4.5 and 8.5 hours.

5.5.2 Convergence and Stability Issues

Except for low flow conditions at the start of low return period events, there is only one short period of ISIS non-convergence during the passage of the flood wave. This is associated with the initiation of flow through the wall gap at the footbridge in the 10 metre grid models. There are a small number of TUFLOW negative depth warnings during some of the design


event simulations and TUFLOW mass balance errors (MBEs) were always low (<1%) following initial wetting. Evidence of model instability is therefore largely restricted to some localised oscillatory spill between ISIS and TUFLOW during the 0.5% and 0.1% AEP events. Since these small-scale localised instabilities only affect the most extreme events and can not be seen to have a significant adverse impact on the final model results, the results of the Bandon ISIS-TUFLOW model are considered to be sufficiently stable for the purpose of this study.

5.6 Model Verification The existing risk, HPW model has been verified against both the November 2009 and January 2011 events. This section lists the outcome of this model verification/ calibration process.

5.6.1 November 2009 Event

The November 2009 event, which is believed to have had a return period of approximately 200 years along the Bandon River, caused widespread damage in Bandon, principally from overtopping of the Bandon River. The stage series at Bandon gauging station, the approximate extent of flooding and the approximate flood depths at five locations were recorded during the event and were made available for this study for calibration purposes.

The November 2009 event was modelled by running a flow hydrograph with the same shape as that recorded during the November 2009 flood wave through the model. The magnitude of this inflow hydrograph was scaled to that which was necessary to match the observed peak flood level at Bandon gauging station during the November 2009 event. Thus a combined model inflow of 405m3/s was found to produce a peak flood level of 15.31 mAOD at the location of Bandon gauging station. Figure 5-3 and Figure 5-4 show the outcome of running the November 2009 event hydrograph through the existing risk HPW model. From Figure 5-3 it can be seen that there is a good agreement in flood outlines between the model output and the recorded flood extent. Figure 5-4 also shows that the modelled flood depths are in good general agreement with the data recorded during the event. The HPW model is therefore capable of providing a reasonable simulation of the November 2009 event. Any differences between observed and modelled outlines shown in these diagrams could easily be explained by factors such as; the presence of local floodplain characteristics (non-flood defence walls, solid buildings etc) that can not be realistically incorporated into the hydraulic model; poor local LIDAR quality (especially with regard to recording the ground levels of buildings) and/or errors in the data collection process.

Figure 5-3 Modelled and observed outlines for the Nov 2009 event

Modelled Nov 2009 Outline

River

Observed Nov 2009 Outline



Figure 5-4 Modelled and observed flood depths for the Nov 2009 event


Figure 5-5 shows the modelled peak flood profile along the Bandon River during the November 2009 event. This illustrates that Bandon Bridge exerts a significant control on upstream flood levels at least as far as Bandon Weir during an event of this magnitude. However, flood levels are not predicted to reach a level sufficient to directly overtop the parapet of Bandon Bridge. Instead the bridge is by-passed on both banks with the left bank by-pass being established comparatively early on during the event. The footbridge is modelled to be overtopped during this event and floodwater is modelled to flow through the wall gap(s) at the footbridge at up to 11m3/s.

Figure 5-5 Modelled peak flood levels during the Nov 2009 event

Flood Depth (metres)

>1.80

1.40-1.80

1.00-1.40

0.60-1.00

0.20-0.60

0.00-0.20

Bandon Gauge

Bandon Bridge

Footbridge

Bandon Weir


The predicted sequence of flooding for the November 2009 event is presented in Figure 5-6. The event peaks on the 19th November at 21:45 and the three snapshots illustrate that at 12:45, flooding is largely restricted to natural floodplain areas both up and downstream of Bandon with some localised encroachment into low-lying areas (e.g. the children's playground) adjacent to the river. The next snapshot at 16:15 illustrates what happens when the flood level on the Bandon River is sufficient to begin flooding Bandon Town centre. Floodwater is modelled to approach the town centre via a variety of routes; overtopping of the LIDL embankment, by-passing both Bandon Weir and Bandon Bridge on the left bank, flowing through the wall gap(s) at the footbridge, and by backward-directed flow of floodwater that has overtopped the undefended right bank downstream of Bandon Bridge. At a later stage of this event direct overtopping of the existing flood wall is also modelled to occur. The last snapshot shows the peak of the event at 21:45 predicting that at this stage flow would be generally parallel to the Bandon River across the full width of the floodplain with flood levels in the town centre being little different from those in the main channel.


Figure 5-6 Modelled sequence of flooding during the Nov 2009 event.

19thNov09 16:15

19thNov09 12:45

19thNov09 21:45


5.6.2 January 2011 Event

The January 2011 event, which is believed to have had a return period between 1 in 5 to 1 in 10 years9 along the Bandon River, caused minor flooding within Bandon Town centre but any damage is believed to have originated from either surface water flooding rather than by overtopping of the Bandon River itself.

The January 2011 event was simulated by scaling the November 2009 event hydrograph to produce the recorded peak flood level at the Bandon gauging station. Thus a combined model inflow of 187m3/s was found to produce a peak flood level of 14.04 mAOD at the approximate location of Bandon gauging station.

Figure 5-7 illustrates the flood depth map produced by the HPW model for the January 2011 event. The model predicts that overtopping during the January 2011 event should have been largely restricted to natural floodplain areas both upstream and downstream of Bandon with negligible flooding of the town. This appears consistent with the flood report of the January 2011 event and also serves to demonstrate that the HPW model does not significantly under predict the threshold of flooding associated with the Bandon River in Bandon.

9 Table 2.5 Hydrology Report, JBA Consulting, September 2011


Figure 5-7 Modelled flood depths for the Jan 2011 event


5.7 Existing Risk Design Events The Existing Risk model has been assessed using a number of return period events and findings are discussed in this section.

Figure 5-8 and 5-9 provide water level estimation at key locations along the Bandon River in comparison to the flood threshold level for a range of return periods. It can be seen that some areas typically flood during the10 year return period event or less (Roundabout on Ring Road and Walkway downstream of Bandon Bridge). It should be noted that only few properties are affected at these locations.

The flooding threshold for Bandon Town is better represented by the threshold at the Lidl Store, which corresponds to a return period between 1 in 20 to 1 in 50 years.

The flooding threshold in the vicinity of Lidl is 16.9m, which is the first area that gets flooded upstream of Bandon Town. Flooding downstream of Bandon Town occurs about 1hour earlier along the walkway with a threshold of 14.1 to 14.6m dependant on the location. Overland flood route then develop from either ends of Bandon Town and the majority of properties at flood risk are inundated approximately 3 hours later. At that time the Bandon River has breached its banks at a number of other locations, namely the access gaps of the Bandon Footbridge, the access gap upstream of Bandon Bridge on the left bank and access gaps along the Riverside Shopping centre.

Flood extents and depths will be used to assess the number of properties at risk, the depth of flooding and the potential economic damages to those properties. This economic assessment will be crucial in the option appraisal process

Figure 5-8 Flooding Threshold upstream and downstream of Bandon Bridge

13.0

13.5

14.0

14.5

15.0

15.5

16.0

16.5

17.0

1 10 100

Elevation

Return Period (1 in _ years)

Flooding Threshold

Water Level Threshold

Access gap US of BandonBridge LOB

Walkway DS of Bandon

Bridge ROB


Figure 5-9 Flooding Threshold at key locations

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

19.0

1 10 100

Elevation

Return Period (1 in _ years)

Flooding Threshold

Water Level Threshold

Lidl ROB

Roundabout on

Ring Road

Footbridge LOB

Figure 5-10 shows the modelled flood outlines from four key, existing risk, design events. For events below 5% AEP, flooding is limited to natural floodplain areas, a small number of low-lying areas within the town itself plus the N71 downstream of the town centre. However, Figure 5-10 shows that the flood threshold to many parts of Bandon Town centre is modelled to have a return period of 1 in 50 years (2%AEP) as can be seen by the significant increase in the modelled flood outlines in comparison to the 1 in 20 year event (5% AEP).

Flooding during a 1% AEP event is modelled to affect much of Bandon Town centre with flood water entering via a wide variety of routes as noted previously for the November 2009 event. Peak flood depths during this event are modelled to exceed 1.00m across many parts of the town centre (Figure 5-11) and roads are modelled to act as significant conduits for flow with velocities exceeding 1.0m/s along several roads within the town centre (Figure 5-12). This would present an extreme hazard within the town.

.


Figure 5-10 Flood outlines from selected design events


Figure 5-11 Peak flood depths during the 1% AEP event


Figure 5-12 Peak flood velocities in Bandon during the 1% AEP event


5.8 Climate Change Impacts According to best practise climate change predictions are assessed using the High End Future Scenario (HEFS) and the Medium Risk Future Scenario (MRFS). Existing design flows are increased by 30% and 20% for the HEFS and MRFS, respectively. Figure 5-13 shows the potential impact of climate change on the 1% AEP flood outline that consequently seems to be quite insensitive to climate change. However, the same is not true for flood depths, which typically increase by in the order of 0.60m during the MRFS scenario and 0.90m during the HEFS scenario. Therefore it would appear that whereas few additional properties would likely be at risk during future events under existing conditions, there would still be a significant increase in flood risk arising from increased flood depths (and velocities). The final flood alleviation scheme will be tested for the MRFS and HEFS.

Each flood mitigation response will have a different ability to manage climate change, and this will be tested for the preferred option. Climate change is a serious threat to Bandon and should be addressed within in any future scheme.


Figure 5-13 Climate change impact showing the 1% AEP flood outlines


5.9 Sensitivity Testing This section provides details on Sensitivity Testing on the HPW model, which has been undertaken on a number of model parameters in order to test the strength of the model predictions. The sensitivity tests have been carried out by running the 1% AEP design hydrograph through the 10m grid, existing risk model.

The sensitivity of the fluvial models to the following parameters was analysed:

2D Cell size

Channel (1D) roughness

Floodplain (2D) roughness

Downstream boundary

Bridge Afflux

Weir parameters

Flow

Detailed modelling results of the model sensitivities analysis is supplied in Appendix B. In summary, the modelled flood risk within Bandon Town centre has been found to be:

Relatively insensitive to the 2D cell size and the weir modelling approach.

Quite sensitive to both the channel and floodplain roughness and flow.

Locally sensitive to the slope of the normal flow downstream boundary and the bridge afflux approach; the more conservative approach for bridge afflux is currently being used in the model.

Flow estimation is quite uncertain and modelling results show greatest level of uncertainty.

It should be noted that since the 1% AEP flood outline largely fills the available floodplain at Bandon, model sensitivity has been assessed on the basis of changes to 1% AEP floodplain depths and channel flood levels in addition to changes in the flood outlines. Results from the sensitivity testing have been adopted in order to inform the Freeboard Analysis, which is discussed in Section 6.8 below.

5.10 Model Limitations All hydraulic model results are prone to uncertainties due to factors such as uncertainties in the key model inputs (e.g. flows, sea levels, topography), modelling parameters (e.g. roughness), the modelling software used, model stability and the nature of the assumptions used in the modelling. The key strengths of the HPW model are considered to be that the model utilises detailed bank crest survey data through Bandon Town centre and that the model appears capable of simulating the November 2009 event and the January 2011 event to a reasonable level of accuracy, providing a comprehensive and well calibrated model for the existing condition. Any remaining uncertainties are taken account of by means of Freeboard allowance, as discussed in Section 6.8.


6. Options Assessment

An appraisal of various potential Flood Relief Schemes at Bandon has been carried out and findings show that dredging of the Bandon River in combination with flood containment using raised defences offers the most viable Flood Relief Option. The options appraisal is based on social, economic, environmental and cultural heritage factors, all of which are discussed in detail in a separate report on Options Appraisal.

A summary of the hydraulic analysis of the options assessment is provided as follows:

Flood containment using raised defences offers the potential to significantly reduce flood risk in Bandon Town, however it also results in a significant increase in water levels and associated flood risk upstream, which is contrary to best practise guidelines10.

The option of dredging was found to provide a significant reduction in flood levels. However, extensive dredging would be required if this option was to be adopted as standalone measure and this would consist of removing more than 2m of bed material for a distance of 4km and resulting in more than 200,000m3 of material to be removed.

Removal of the weir upstream of Bandon Bridge was found to reduce water levels upstream of the weir with little impact on water levels in the town.

Provision of a compound channel was assessed and this could only be provided in restricted areas, outside of Bandon Town and modelling results show negligible improvement in terms of flood risk at Bandon Town.

The option of providing flood storage upstream in the catchment was assessed and results show that vast quantities of water (8-13million m3) would be require to be retained with extensive environmental, economic and social implications.

This section illustrates the impact of each scenario on the 1% AEP design event relative to the modelled existing risk.

6.1 Flood Containment using Raised Defences This has been investigated by raising the existing defence crests above maximum flood levels and by adding new defence lines both up and downstream of Bandon to protect the majority of local properties from flooding. The proposed standard of protection is set at the 1% AEP and this scenario enables us to predict both the level to which the defences would need to be set to protect Bandon to protect against this design event and also provides us with an indication of the magnitude of any up and downstream impacts that might result from installing this scheme.

Figure 6-1 shows the location of the modelled defences that were glass-walled and the consequence of defending Bandon Town centre on the 1% AEP flood outline. As expected all defended properties are protected and there is no major increase in the flood extent elsewhere. Figure 6-2 shows the modelled flood levels along the Bandon River for both scenarios. This demonstrates that there would be an upstream impact of increasing the standard of protection to Bandon but that any downstream impact would be negligible during a 1% AEP event.

10 The Planning System and Flood Risk Management – Guidelines for Planning Authorities, November 2009.


Figure 6-1 Impact of increasing the standard of protection on the 1% AEP flood outlines.


Figure 6-2 Impact of increasing the standard of protection on 1% AEP flood levels.

6.2 Dredging Dredging has been investigated by creating and running three different dredging depth and extents and an overview of these model runs are presented in Table 6-1.

This option considers lowering the river bed of the Bandon River immediately downstream of the Bandon Weir by 1m, 2m and 3m and providing a constant slope of 1 in 1000m for a length of approximately 3km, 4km and 5.2km, respectively. The volume of bed material that would be required to be removed is estimated at approximately 110,000m3, 195,000m3 and 432,000m3 and the majority of which is expected to be bedrock rather than gravel.

Existing Risk With Defences to 1% AEP Modelled Defence Lines

Bandon Gauge

Bandon Bridge

Footbridge

Bandon Weir

Existing Risk Scenario

With Defences to 1% AEP


Table 6-1 Model runs of dredging options

Run Dredge Volume (m3) Channel Length (km)

Dredging 1m 110,000 3.0

Dredging 2m 195,000 4.0

Dredging 3m 432,000 5.2

Figure 6-3 presents a comparison of the dredging options with the existing condition for the 1%AEP. While some of these scenarios are somewhat extreme, it does provide an indication of the extent to which dredging alone can reduce flood risk in the Bandon area.

Figure 6-3 Comparison of dredge option with existing flood level (1%AEP)

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23100

22900

22770

22530

22470

22200

21800

21300

Elevation (mOD)

Chainage (m)

Dredging Options

Bed Level Ground Level (RB)Existing Dredge 1mDredge 2m Dredge 3m

The new bed profile of the 1m dredge option is illustrated in Figure 6-5, which may be compared against the existing bed profile that is depicted in Figure 6-2 and cross sections are provided in Appendix C.

Figure 6-4 shows the modelled consequence of the 1m dredge option on the 1% AEP flood outline. The flood extent is modelled to be reduced in response to the substantial increase in channel capacity downstream of Bandon Weir. However, flooding is still modelled to affect parts of the town centre. Figure 6-5 shows the modelled flood levels along the Bandon River for both existing risk and dredged scenarios. This demonstrates that dredging could potentially bring about a significant reduction (~1.5 metres) in peak flood levels in Bandon River alongside the town centre with only limited up or downstream impacts during a 1% AEP event.


Figure 6-4 Impact of the 1m dredging option on the 1% AEP flood outlines.


Figure 6-5 Impact of the 1m dredging option on the 1% AEP flood levels.

6.2.1 Dredging and Flood Containment

A combination of flood containment and dredging was assessed for some of the less invasive dredging options with up to 1m removal of bed material and an overview of the options is presented in Table 6-2.

Existing Risk With Dredged Channel

Bandon Gauge

Bandon Bridge

Footbridge

Bandon Weir


With Dredged Channel


Table 6-2 Model runs of combined options

Run Dredge Volume (m3) Channel Length (km)

Combination 0.2m 15,300 0.5

Combination 0.7m 28,000 0.6

Combination 0.7m ext 46,000 1.4

Combination 1m 110,000 3.0

Figure 6-6 presents the results of these runs in comparison to the existing condition and the option of providing flood defences only. The ground level provides an indication of the flooding threshold in the existing condition.

Results show that the option of removing 0.2m of bed material for a length of 500m (Combination 0.2m) would protect Bandon Town from flooding due to the raised defences and effectively result in similar flood levels as in the existing condition.

Increasing the level of dredging to 0.7m, which would require 28,000m3 of bed material to be removed, would reduce flood levels along Bandon Town by approximately 0.5m and there would be no significant improvement when extending this level of dredging to a channel length of 1.4km (Combination 0.7m ext).

A further reduction in flood levels would be achieved when extending the dredging to 1m and for a channel length of 3km.

Figure 6-6 Combination of dredging & flood containment (1%AEP)

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23100

22900

22770

22530

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22200

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Elevation (mOD)

Chainage (m)

Combination of Dredging & Flood Containment

Bed Level Ground Level (RB)

Existing Combination 0.2m

Combination 0.7m Combination 0.7m ext

Defences Only Combination 1m

6.3 Weir Removal The removal of Bandon Weir has been investigated by creating and running a model scenario with the weir removed from the ISIS component of the model. Removing the weir also required some re-profiling of the main channel for approximately 300 metres upstream of the


weir to create a realistic new bed gradient. The new bed profile is illustrated in Figure 6-8, which may be compared against the existing bed profile that is depicted in Figure 6-2.

Figure 6-7 shows the modelled consequence of removing Bandon Weir on the 1% AEP flood outline. The flood extent is modelled to be slightly reduced upstream of the Weir in response to the weir removal. However, downstream of the weir and through Bandon Town centre, there is very little difference between this scenario and the existing risk model. Figure 6-8 shows the modelled flood levels along the Bandon River for both existing risk and weir removal scenarios. This demonstrates that while weir removal would be likely to reduce flood levels upstream of the present weir location, this scenario appears unlikely to offer much potential for reducing flood levels within Bandon Town centre and further downstream.

Figure 6-7 Impact of weir removal on the 1% AEP flood outlines.


Figure 6-8 Impact of weir removal on the 1% AEP flood levels.


With Bandon Weir Removed

Bandon Gauge

Bandon Bridge

Footbridge

Bandon Weir


6.4 Removal of Walkway The option of removing the walkway downstream of Bandon Bridge has been assessed within the ISIS model component by widening the main channel for the appropriate section. Modelling results show a minor reduction in water levels for along this section in the order of 4 cm. Figure 6-9 presents a water surface profile of comparing this option with the existing condition for the 1% AEP.

Figure 6-9 Impact of walkway removal on the 1%AEP flood levels

6.5 Confluence of Bridewell River to Bandon River Improving the hydraulic capacity of the confluence from the Bridewell River to the Bandon River has been suggested to be a contributing factor to the flooding at Bandon Town.

This option has been assessed as part of the sensitivity testing by combining the 1% AEP flow from the Bridewell River with the 50% AEP flow from the Bandon River (see Appendix B for detailed modelling results).

Findings show that the Bridewell River on its own does not result in flooding during the 1% AEP event and it is the Bandon River that controls water level along the downstream reach of the Bridewell River. Also, the additional inflow from the Bridewell River to the Bandon River is less than 5% of the total river flow and does not significantly influence flood levels. Although, the precautionary approach was taken to coincide the timing of the flood peak for the Bridewell and the Bandon River, in reality this is very unlikely to occur due to the difference in critical duration.

As a result in can be concluded that improving the confluence of the Bridewell River and to the Bandon River would not influence flood levels significantly.

6.6 Storage Option This option considers storage of flood waters in the catchment that would result in a reduction in peak flow and flow volume upstream of Bandon Town. The amount of flood waters that would be required to be stored in order to alleviate flooding at Bandon Town mainly depends on the flow duration and peak flow in the Bandon River.

Long Section: 23400B - 19250B - Maximum Stage; 10 - 120 h.

Node Label

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0B

2320

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The flood width hydrograph from the November 2009 event was adopted for this assessment and the storage estimate is based on the 100 year design flow. Typically storage options would be provided in combination with other flood alleviation measures and it useful to consider a range of storage options, depending on the level of protection that is provided elsewhere. Table 6-3 provides storage volume estimate, depending on the level of protection.

Storage provided above: Storage required for 100 year event

Return Period Peak Flow Volume Duration Area (2m deep)

(years) (m3/s) (m3) (h) (km2)

10 181 12,999,997 18 2.55

20 213 11,469,734 15 2.39

50 274 7,727,560 10 1.97

Table 6-3 Storage volume required for 100 year design flow

No property damage was reported during the January 2011 event and this is estimated to correspond to the 5 to 10 year return period. Assuming that Bandon Town is currently protected for a 10 year design flood; the storage volume that would be required to alleviate Bandon Town during the 100 year design flow is therefore estimated to be approximately 13 million cubic metres. This volume of water would require an area of approximately 2.6km2 with an average depth of approximately 2m.

Table 6-3 also presents alternative scenarios of storage above the 20 year return period and 50 year return period (i.e. if the level of protection would be raised to this return period by other means) and the corresponding storage volumes would be 11.5 and 7.7 million cubic metres, respectively.

6.7 Compound Channel A Compound Channel has been assessed by increasing the channel width and effectively creating a two stage channel. This two stage channel could only be provided along a limited river section downstream of Bandon Town for 500m from River Section 22000B to 21500B and would consist of removing approximately 25,000m3 of material from the right bank. Figure 6-11 and 6-8 presents a comparison of the existing and modified river section.

Figure 6-10 Compound Channel, RS22000B

0 50 100 150 200 250 300 3508

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Station (m)

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vatio

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Legend

Ground - Comp Geom 22

Bank Sta - Comp Geom 22

Merge Range

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Bank Sta

.06 .04 .06

Legend Ground Model Removed Section


Figure 6-11 Compound Channel, RS21500B

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Ground - Comp Geom 22

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Merge Range

Ground

Bank Sta

.06 .04 .06

Figure 6-12 presents a comparison of with the existing condition in long section. Modelling results show that the compound channel would reduce water levels up to 0.4m locally but not affect Bandon Town.

Figure 6-12 Compound Channel, Results in Longsection

Long Section: 26500B - 18000Bas - Maximum Stage; 10 - 120 h.

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Legend Ground Model Removed Section


6.8 Freeboard Analysis Freeboard is a factor of safety usually expressed in height above a flood level for purposes of floodplain management. Freeboard is typically applied to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood, such as uncertainty of the effect of bridges, hydrological uncertainty, uncertainty in model roughness etc.

The OPW typically apply a Freeboard of 0.3m for hard defences and 0.5m for soft defences. For this study, a scheme specific freeboard allowance is provided and this has been undertaken for three distinct sections along the Bandon River, namely upstream of Bandon Weir, from Bandon Weir to Bandon Bridge and downstream of Bandon Bridge. These sections have been selected due to the difference in the model uncertainty associated with each section.

The freeboard allowance is estimated as follows.

Where:

FB is the Freeboard Allowance in meters;

A1 to A4 is the uncertainty in water level prediction for each input type (please refer to Table 6-4 for details).

Table 6-4 presents the input parameters with a brief description. Please refer to Appendix B for further details on the sensitivity testing that was used to derive the appropriate uncertainty levels.

Table 6-4: Description of Input Parameters

Parameter Type Decription

A1 Hard Defence settlement Zero for hard defences and 0.3m for soft defences to allow for settlement

A2 Roughness Model roughness increased by 20% to take account of uncertainty

A3 Afflux Bandon Br Uncertainty in modelling approach, which is set to orifice calculation when bridge is surcharge as conservative approach

A4 Hydrology bounds

Uncertainty in hydrology has been assessed by sensitivity analysis of the rating curve by varying the model roughness which was found to result in 15% increase in flows.

Two different potential scheme options have been adopted as basis for the freeboard analysis and results are presented below.

6.8.1 Flood Containment using Raised Defences

Freeboard analysis has been undertaken for the option of flood containment using raised defence and the corresponding results are presented in this section. It should be noted that an additional allowance of 0.3m would have to be made for sections where soft defences, such as earth embankments would be proposed. Table 6-5 to 6-6 present details of the freeboard analysis for the option of flood containment. It should be noted that the input values were taken as average water level increase for each section and maximum values were found to be vary in the order of 0,1m.

Findings show that the recommended freeboard allowance for this option would be 0.3m for the section downstream of Bandon Bridge but significantly upstream of Bandon Weir (0.59m) and Bandon Bridge (0.98m). The uncertainty in the water level estimation is mainly influenced by uncertainty in the hydrological flow estimation and the roughness estimation of the channel and it is the effect of Bandon Bridge that results in the increased level of uncertainty.

24

23

22

21 AAAAFB


Table 6-5 Details of Freeboard calculation - US of Bandon Weir

Parameter Type Unit Value

A1 Hard Defence settlement (m) 0.00

A2 Roughness (m) 0.35

A3 Afflux Bandon Br (m) 0.06

A4 Hydrology bounds (m) 0.48

FB Calculated Freeboard allowance (m) 0.59

Table 6-6 Details of Freeboard calculation - US of Bandon Bridge







Table 6-7 Details of Freeboard calculation - DS of Bandon Bridge







6.8.2 Combined Option of Dredging and Flood Containment

The freeboard analysis has also been undertaken for the combined option of 1m dredging and flood containment (see Table 6-2). This provides a comparison to results presented in Section 6.8.1 indicating the different level of uncertainty. Table 6-8 to 6-9 present details of the freeboard analysis for this option. Similarly as for Section 6.8.1, input values were taken as average water level increase for each section and maximum values were found to be vary in the order of 0,1m.

Table 6-8 Details of Freeboard calculation - US of Bandon Weir








Table 6-9 Details of Freeboard calculation - US of Bandon Bridge







Table 6-10 Details of Freeboard calculation - DS of Bandon Bridge







Results show that the freeboard for each section would be higher than the 0.3m typically applied by the OPW, albeit only marginally for the section upstream of Bandon Weir. It should be noted that Bandon Bridge does not surcharge for this scheme option and as a result, the uncertainty associated with the modelling approach is reduced to zero.

6.9 Summary of Flood Alleviation Options Potential flood alleviation options have been considered for the Bandon River. Flood containment using raised defences offers the potential to alleviate flooding at Bandon Town, However it would also result in increased water levels of up to 0.5m along a significant length of the Bandon River, which is not acceptable under best practise guidelines11. As a result flood containment as stand-alone flood alleviation option would not be acceptable.

Dredging of the Bandon River bed has been tested using a number of scenarios and results show that extensive dredging would be required in order to reduce water levels sufficiently enough to provide flood alleviation to Bandon Town. In order to deliver the 2m reduction in flood level that would avoid significant remediation of existing walls or extension of flood walls in Bandon the scale of works both in length of river dredged and the volume of material excavated demonstrates that dredging on its own is not practicable and would required extensive river rehabilitation.

The removal of the weir upstream of Bandon appears to have little impact on water levels in the town. It does, however, reduce water levels upstream that could contribute to the flooding problem. Weir removal could therefore form part of a comprehensive alleviation scheme if other benefits (e.g. environmental and geomorphological) are apparent.

Upstream flood storage has been assessed and the volumes of storage required have been shown to be in the order of 8-13 million cubic metres. This is a consequence of the high design flows, long duration events and the low threshold of flooding. This volume of storage is very unlikely to be achievable in the Bandon catchment.

A compound channel downstream of Bandon was considered as an option that would require removal of approximately 25,000m3 of material. Modelling results show that water levels would reduce locally by up to 0.4m. However, no reduction in flood levels at Bandon Town was achieved.

11 The Planning System and Flood Risk Management – Guidelines for Planning Authorities, November 2009.


Other options consisted of removing the Walkway downstream of Bandon Bridge and improving the confluence of the Bridewell River with the Bandon River and neither of these options were found to significantly influence flood levels.

The extreme hazard in Bandon would suggest that structural options will be required to mitigate the risk but non structural responses to flooding should also be considered in conjunction with the structural options listed. This would involve the existing flood warning system and flood forecasting, as well as emergency response planning, development control and public preparedness.

The combination of measures consisting of flood containment and dredging is the likely outcome and details of existing and proposed flood levels in comparison to potential flood defence heights is presented in Table 6-11 overleaf. It should be noted that the proposed defence heights incorporate the freeboard allowance for the relevant section as estimated in Table 6-8 to 6-9. Please refer to Table 6-1 and Table 6-2 for an overview of the different dredge options and combination of flood containment and dredging, respectively.


Table 6-11 Summary of modelling results and defence heights for the selected flood alleviation options

Location Chainage Section Ground

Level (LB)Ground

Level (RB)Existing

Flood LevelDefenses

OnlyDredge

1mDredge

2mDredge

3mCombination

0.2mCombination

0.7mCombination

0.7m extCombination

1mLidl Carpark 13231 23100B 16.0 16.7 17.3 17.7 17.3 17.3 17.3 17.4 17.4 17.4 17.4

US of Weir 13052 22900B 15.3 16.2 17.1 17.6 17.1 17.1 17.1 17.2 17.2 17.2 17.2

DS of Weir 12927 22770B 15.9 15.8 16.6 17.2 15.3 14.6 13.7 16.5 16.1 16.0 15.5

US of Bandon Bridge 12655 22530B 14.5 15.7 16.2 16.7 15.0 14.3 13.4 16.2 15.7 15.6 15.1

DS of Bandon Bridge 12595 22470B 14.8 14.5 15.4 15.6 14.8 14.2 13.2 15.6 15.4 15.3 14.9

Gauge Station 12347 22200B 14.2 14.0 15.1 15.0 14.6 13.9 13.1 15.0 15.0 15.0 14.6

Roundabout on Ring Road 11895 21800B 12.4 12.5 14.2 14.4 14.0 13.3 12.5 14.3 14.3 14.2 14.1

WWTP 11315 21300B 12.4 13.2 13.5 13.5 13.4 12.5 11.8 13.5 13.5 13.5 13.5

Location Chainage SectionGround

Level (LB)Ground

Level (RB)Existing

Flood DepthDefenses

OnlyDredge

1mDredge

2mDredge

3mCombination

0.2mCombination

0.7mCombination

0.7m extCombination

1m

Lidl Carpark 13231 23100B 16.0 16.7 0.6 1.3 0.9 0.9 0.9 1.0 1.0 1.0 1.00

US of Weir 13052 22900B 15.3 16.2 0.9 1.7 1.3 1.3 1.3 1.3 1.3 1.3 1.32

DS of Weir 12927 22770B 15.9 15.8 0.8 1.9 0.0 0.0 0.0 1.3 0.8 0.7 0.20



Gauge Station 12347 22200B 14.2 14.0 1.1 1.6 1.1 0.5 0.0 1.5 1.6 1.5 1.12


WWTP 11315 21300B 12.4 13.2 0.3 0.8 0.7 0.0 0.0 0.8 0.8 0.8 0.84

Location Chainage Section Ground Level (LB)

Ground Level (RB)

Existing Flood Depth

Defenses Only

Dredge 1m

Dredge 2m

Dredge 3m

Combination 0.2m

Combination 0.7m

Combination 0.7m ext

Combination 1m

Lidl Carpark 13231 23100B 16.0 16.0 1.3 2.0 1.6 1.6 1.6 1.7 1.7 1.7 1.72

US of Weir 13052 22900B 15.3 15.3 1.8 2.6 2.1 2.1 2.1 2.2 2.2 2.2 2.18

DS of Weir 12927 22770B 15.9 15.9 0.8 1.9 0.0 0.0 0.0 1.3 0.8 0.7 0.19



Gauge Station 12347 22200B 14.2 14.2 0.9 1.3 0.8 0.2 0.0 1.3 1.3 1.2 0.86


WWTP 11315 21300B 12.4 12.4 1.1 1.6 1.5 0.6 0.0 1.6 1.6 1.6 1.62

Flood Level (1% AEP)

Heights of Defences (1% AEP) - Right Bank

Heights of Defences (1% AEP) - Left Bank


7. Performance of Preferred Option

The preferred option has been selected based on the hydraulic modelling results presented in Section 6 and also the Social, Economic, Environmental and Cultural Heritage aspects, all of which have been assessed and reported in a separate report.

The scheme option will be designed to cater for the 1%AEP and this will be tested for climate change condition as part of the MRFS and HEFS as well as the November 2009. Results of the freeboard analysis have been presented in Section 6.8 and the scheme option will include the specific freeboard allowance for the relevant section of the river.

This chapter will be updated and completed once the preferred option is established.


8. Summary and Conclusion

This report provides findings from the hydraulic modelling that has been undertaken as part of the Bandon Flood Relief Scheme.

Two separate models have been developed using a 1D (ISIS) model for the MPWs and a 1D-2D (ISIS-TUFLOW) model for the HPW of the Bandon River. Model calibration for the HPWs was undertaken using the November 2009 event as well as the January 2011 event and a good fit has been achieved to both flood extent and local water depths. Design events have been run through the hydraulic models based on the flows in the hydrology report. Flood extents and depths will be used to assess the number of properties at risk, the depth of flooding and the potential economic damages to those properties. This economic assessment will be crucial in the option appraisal process.

Modelling results show that Bandon Town is subject to flood risk, mainly due the Bandon River exceeding its channel capacity at a number of locations along the left and right river bank (looking downstream). There are a number of gaps in the existing flood defences, which allow flood waters to exit the river channel and overspill into the low lying areas of Bandon Town. The main gaps are at the pedestrian bridge on the Bandon River, the access gaps in the existing wall downstream of the Riverside Shopping Centre and along the walkway on the right bank downstream of Bandon Bridge. The existing earth embankment in the vicinity of LIDL was also found to overtop at a number of locations and there is concern with regard to its structural stability during flood conditions.

Once the model was established for the existing case, a number of potential flood alleviation measures were considered, namely a defended option, dredging of the river bed, removing Bandon Weir, providing storage upstream and providing a compound or flood relief channel, as well as a combination of the dredged and defended options. Following extensive analysis of various combinations of options and stakeholder consultation, the preferred option is likely to be a combination of flood containment and dredging and this report will be finalised once the preferred option has been identified.

A summary of the findings for each of the modelled option is as follows:

Increasing the standard of protection using raised defences offers the potential to significantly reduce flood risk in Bandon Town, however it also results in a significant increase in water levels and associated flood risk upstream, which is contrary to best practise guidelines.

Extensive dredging would be required if this option was to be adopted as the only flood alleviation measure. This consists of removing more than 2m of bed material for a distance of 4km and resulting in approximately 200,000m3 of material to be removed.

Removal of the weir upstream of Bandon Bridge was found to have little impact on water levels in the town.

The option of providing flood storage upstream in the catchment was assessed and results show that vast quantities of water (8-13million m3) would be require to be retained with extensive environmental, economic and social implication. As a result this option was not assessed any further.

Provision of a compound channel was assessed and this could only be provided in restricted areas, outside of Bandon Town and modelling results showed negligible improvement in terms of flood risk.

The freeboard allowance has been assessed using the combined option of flood containment and dredging. Results show that a freeboard of 0.34m to 0.59m for hard defences is required dependant on the section and to account for uncertainty in the hydrological flow estimation and model roughness.

This chapter will be completed once the preferred option is established.

Bandon Flood Relief Scheme Draft Hydraulic Report 7D Bandon EIA - Hydraulics Report.pdf.pdf · This...

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