Eastern CFRAM Study HA10 Hydraulics Report ‐ DRAFT FINAL

Eastern CFRAM Study HA10 Hydraulics Report Avoca Model

DOCUMENT CONTROL SHEET

Client OPW

Project Title Eastern CFRAM Study

Document Title IBE0600Rp0028_HA10 Hydraulics Report

Model Name Avoca

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

D01 Draft Various J. Canavan S Patterson G Glasgow Belfast Feb 2014

D02 Draft Various J. Canavan S Patterson G Glasgow Belfast 20/06/2014

D03 Draft Various J. Canavan S Patterson G Glasgow Belfast 13/10/2014

F01 Draft Various J Canavan S Patterson G Glasgow Belfast 23/12/2014 Final F02 Draft Various J Canavan S Patterson G Glasgow Belfast 13/08/2015 Final F03 Draft Various J Canavan S Patterson G Glasgow Belfast 02/08/2016 Final

IBE0600Rp0028 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Table of Reference Reports

Report Issue Date Report Reference Relevant Section Eastern CFRAM Study Flood December IBE0600Rp0001_Flood Risk Review_F02 3.11.5 Risk Review 2011 Eastern CFRAM Study July 2012 IBE0600Rp0005_HA10 Inception Report_F02 4.3.2 Inception Report UoM10 Eastern CFRAM Study November IBE0600Rp0003_HA10_Hydrology 4.8 Hydrology Report UoM10 2013 Report_D01 Eastern CFRAM Study HA10 May 2013 IBE0600Rp0019_HA10 Wicklow Survey 1.6 Wicklow Survey Contract Contract Report_F01

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4 HYDRAULIC MODEL DETAILS

4.3 AVOCA MODEL

4.3.1 General Hydraulic Model Information

(1) Introduction:

The Eastern CFRAM Flood Risk Review (IBE0600 Rp0001_Flood Risk Review_F02) highlighted Avoca as an Area for Further Assessment for fluvial flooding based on a review of historic flooding and the extents of flood risk determined during the PFRA.

The Avoca model represents the lower reaches of the Avoca River catchment, beginning just upstream of the Avoca AFA, approximately 5km upstream of its confluence with the Aughrim River and ending at Arklow, where the river flows into the sea. The model also includes a number of small tributaries, which directly affect the Avoca AFA. The Avoca catchment is relatively flat in comparison to other catchments within HA10, with an S1085 value of 6 m/km. Land use within the catchment is a mix of pasture (32%), peatlands (37%) and forested areas (31%) with the only significant urbanisation at the villages of Rathdrum and Avoca. The catchments of the three smaller tributaries that affect the AFA range in size from 3 to 12 km2. These catchments are predominantly steep (S1085 of 32 to 54 m/km) and made up of pasture (82 – 100%).

Note Arklow AFA is not being addressed under this Eastern CFRAM Study as it is being progressed via a separate OPW study.

The Avoca model itself is ungauged along its reaches however there is an FSU pivotal site gauging station approximately 9km upstream of the modelled reaches at Rathdrum (10002 – EPA) and also a gauging station on Arklow town bridge (10042 – OPW) and on the Aughrim River upstream of the confluence point with the Aughrim River on Model 12 at Knocknamohill (10028 – EPA). The Rathdrum gauging station was given a B classification under FSU and has been subject to rating review. There is some uncertainty with the current EPA rating although the FSU, EPA and CFRAM Study rating review rating all show that the observed Qmed value is significantly higher than that which is predicted using the FSU physical catchment descriptors. In light of this catchment descriptor based estimates for all of the HEPs in the Avoca catchment have been adjusted upwards based on the relationship between the physical catchment descriptor based estimate of Qmed and the observed value (FSU plus additional years based on EPA rating) giving an adjustment factor of 1.31. This is consistent with the average adjustment factor for HA10 pivotal sites. Refer to UoM 10 Hydrology Report (IBE0600Rp0003_HA10_Hydrology Report) for full details on hydrology estimation for this model.

The river network through Avoca AFA is HPW including the Avoca River (upstream reach of the Aughrim River) and tributaries (refer to Error! Reference source not found.). They were therefore modelled as 1D-2D using the MIKE suite of software to adequately represent floodplain flow conditions. The remainder of the Avoca River is designated as a MPW and 1D modelling was carried out for this reach (9km of the

IBE0600Rp0028 4.3-1 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL watercourse towards the downstream limit). The mapping limit of the model is at the M11 (XS ID:1028M00315D chainage 10290m) as the Arklow model is downstream and is a separate OPW study..

Channel markers have been located at the right and left banks of all cross sections. Flow within these markers is calculated by the 1D model component; however when the water level rises sufficiently to meet the bank markers flow can enter the 2D domain which represents the floodplain. Refer to Section Error! Reference source not found. for further details on model schematisation.

(2) Model Reference: HA10_AVOC11

(3) AFAs included in the model: Avoca

(4) Primary Watercourses / Water Bodies (including local names):

Reach ID Name

1028M AVOCA RIVER

1040M AVOCA TRIB 1

1040A AVOCA TRIB 2

1039M AVOCA TRIB 3

1039A AVOCA TRIB 4

1039B AVOCA TRIB 5

1039C AVOCA TRIB 6

1038M AVOCA TRIB 7

(5) Software Type (and version):

(a) 1D Domain: (b) 2D Domain: (c) Other model elements: MIKE 11, 2011 MIKE 21 - Rectangular Mesh, MIKE FLOOD, 2011 2011

4.3.2 Hydraulic Model Schematisation

(1) Map of Model Extents:

Figure 4.3.1 and Figure 4.3.2 illustrate the extent of the modelled catchment, river centre line, HEP locations and AFA extents as applicable. The catchment contains 7 Upstream Limit HEPs, 8 Tributary HEPs, 1 gauging station Arklow town bridge (10042 – OPW) and 1 downstream limit point.

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Figure 4.3.1: Map of Model Extents

Figure 4.3.2: Detail of AFA Extents

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(2) x-y Coordinates of River (Upstream extent):

River Name x y 1028M AVOCA RIVER 319955 181268 1040M AVOCA TRIB 1 320693 180966 1040A AVOCA TRIB 2 321190 180265 1039M AVOCA TRIB 3 321999 178537 1039A AVOCA TRIB 4 321936 179419 1039B AVOCA TRIB 5 321462 178215 1039C AVOCA TRIB 6 321652 178523 1038M AVOCA TRIB 7 319452 179283

(3) Total Modelled Watercourse Length: 20.1 kilometres (approx.)

(4) 1D Domain only Watercourse Length: 9.3 km (5) 1D-2D Domain 10.8 km

(approx.) Watercourse Length: (approx.)

(6) 2D Domain Mesh Type / Resolution / Area: Rectangular / 5 metres / 14.4 km2

(7) 2D Domain Model Extent: Figure 4.3.3 illustrates the modelled extents and the general topography of the catchment. The grid illustrates the 2D extent, the 1D model is illustrated as a light-blue line. Buildings are excluded from the mesh and therefore represented as white spaces. Refer to Chapter 3 for details on representation of buildings in the model.

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Figure 4.3.3: 2D Domain Model Extent

Figure 4.3.4 shows an overview drawing of the model schematisation. Figure 4.3.5 to Figure 4.3.8 show detailed views. The overview diagram covers the model extents, showing the surveyed cross-section locations, AFA boundary and river centre line. It also shows the area covered by the 2D model domain. The detailed views are provided where there is the most significant risk of flooding. These diagrams include the surveyed cross-section locations, AFA boundary and river centreline. They also show the location of the critical structures as discussed in section 4.3.3 (1), along with the location and extent of the links between the 1D and 2D models. For clarity in viewing cross-section locations, the diagrams show the full extent of the surveyed cross-sections. Note that the 1D model considers only the cross-section between the 1D-2D links.

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Figure 4.3.4: Overview Drawing of Model Schematisation

Figure 4.3.5: Detailed Area of Model Schematisation showing Critical Structure (1 of 4)

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Figure 4.3.6: Detailed Area of Model Schematisation showing Critical Structure (2 of 4)

Figure 4.3.7: Detailed Area of Model Schematisation showing Critical Structure (3 of 4)

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Figure 4.3.8: Detailed Area of Model Schematisation showing Critical Structure (4 of 4)

(8) Survey Information

(a) Survey Folder Structure:

First Level Folder Second Level Folder Third Level Folder

Murphys_E10_M11_WP2_121127_ GIS and Floodplain Photos Flood Defence Register 1028M Floodplain Photos and Avoca Shapefiles Murphy - Surveyor Name Structure Register E10 - Eastern CFRAM Study Area, Surveyed Cross Section Lines Hydrometric Area 10

M011 - Model Number 11 V0_20121012_Photos_Videos

WP2 - Work Package 2 _Ascii 121127 - Date issued (27

November 2012) Drawings and PDFs 1028M - River Reference Photos (Naming convention is in the format of Cross-Section ID and orientation - upstream, downstream, left bank or right bank)

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(b) Survey Folder References:

Reach ID Name File Ref.

1028M AVOCA Murphys_E10_M11_WP2_121127_1028M

1040M AVOCA TRIB 1 Murphy_E10_M11_WP2_1040M_20120910

1040A AVOCA TRIB 2 Murphy_E10_M11_WP2_1040A_20120911

1039M AVOCA TRIB 3 Murphy_E10_M11_WP2_1039M_20120910

1039A AVOCA TRIB 4 Murphy_E10_M11_WP2_1039A_20120910

1039B AVOCA TRIB 5 Murphy_E10_M11_WP2_1039B_20120910

1039C AVOCA TRIB 6 Murphy_E10_M11_WP2_1039C_20120910

1038M AVOCA TRIB 7 Murphy_E10_WP2_M11_V0_20120910_1038m

(9) Survey Issues: In some locations the surveyors were unable to gain access to record all information (these areas are discussed in section 4.3.6 (1)).The surveyors made efforts to acquire survey information and no further action was required having established areas were inaccessible due to overgrown vegetation.

4.3.3 Hydraulic Model Construction

(1) 1D Structures (in-channel along See Appendix A. 1 modelled watercourses): Number of Bridges and Culverts: 33

Number of Weirs: 3

The survey information recorded includes a photograph of each structure, which has been used to determine the Manning's n value. Further details are included in Chapter 3.5.1. A discussion on the way structures have been modelled is included in Chapter 3.3.4.

The location of critical structures included in the model is presented in Figure 4.3.4 to Figure 4.3.8. Details of these structures are also presented in Appendix A.1.

The structure 1040M00066D (Figure 4.3.9) on Avoca Tributary 1 at chainage 266m floods during 1% and 0.1% AEP events. This is shown to have an impact on a housing development in the upstream reach of the channel.

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Figure 4.3.9: 1040M00066D - Looking Downstream

On Avoca Tributary 2, the bridge 1040A00015D (Figure 4.3.10) at chainage 2126m restricts flow during design runs of 1% and 0.1% AEP, resulting in some out of bank flooding upstream, impacting a property on the right bank during the 0.1% AEP.

Figure 4.3.10:1040A00015D - Looking Downstream

On Avoca Tributary 3, the structure1043M0094I (chainage 1348m), shown in Figure 4.3.11 does not have sufficient capacity to convey flow during simulated fluvial events of 10%,1% and 0.1% AEP causing flooding to the adjacent area and impacting a number of properties.

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Figure 4.3.11:Cross Section 1039M00094I

On Avoca Tributary 3 at chainage 1629m (Figure 4.3.12). Flow is restricted and flooding occurs upstream during 1% and 0.1% AEP events.

Figure 4.3.12:Cross Section 1039M00068D

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Bridge 1038M00022D is located on the Avoca Tributary 7 at chainage 1058m (Figure 4.3.13). Flow is restricted and flooding occurs upstream during all simulated %AEP events. No properties are affected by this flooding.

Figure 4.3.13:1038M00022D - Looking Downstream

The structure 1038M00008D shown in Figure 4.3.14 is located near the downstream extent of Avoca Tributary 7, the bridge restricts the flow significantly as the river floods during all simulated AEP events.

Figure 4.3.14:1038M00008D - Looking Downstream

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On the Avoca River, the bridge 1028M01274D (Figure 4.3.15) at chainage 716m restricts flow during design runs of all AEP events, resulting in extensive out-of-bank flooding.

Figure 4.3.15:1028M01274D - Looking Downstream On the Avoca River, at cross-section 1028M01185D (Figure 4.3.16) at chainage 1594m; flow is restricted flow during all simulated % AEP fluvial events. This results in some flooding upstream.

Figure 4.3.16: 1028M01185D - Looking Downstream

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On Avoca River (ID 1028M01033D,Figure 4.3.17) at chainage 3115m restricts flow during all % AEP fluvial events, resulting in extensive flooding upstream.

Figure 4.3.17: 1028M01033D - Looking Downstream

On the Avoca River, flooding occurs due to the bridge 1028M00842D as shown in Figure 4.3.18 (located at chainage 5022m). This structure restricts flow causing substantial flooding during all design AEP events. This flooding breaches both banks, impacting Woodenbridge Golf Club on the left bank.

Figure 4.3.18: 1028M00842D - Looking Downstream

On Avoca River Bridge 1028M00433D (Figure 4.3.19) at chainage 9115m. Out-of-bank flooding occurs during simulated fluvial events of 10 %, 1% AEP and 0.1% AEP. This flooding is quite extensive on both banks.

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Figure 4.3.19: 1028M00433D - Looking Downstream

Near the downstream extent of the Avoca River, 1028M00315D (Figure 4.3.20) at chainage 10307m restricts flow during simulated fluvial events of 10%, 1% and 0.1% AEP, resulting in extensive flooding upstream.

Figure 4.3.20: 1028M00315D - Looking Downstream

(2) 1D Structures in the 2D domain None (beyond the modelled watercourses):

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(3) 2D Model structures: None

(4) Defences:

Type Watercourse Bank Model Start Chainage Model End (approx.) Chainage (approx.)

Wall 1028M (Avoca Left 1551 1590

INEFFECTIVE River)

Railway embankment 1028M (Avoca Right 1084 & 1404 &

INEFFECTIVE River) 1590 2652

The defence wall (left bank) was considered ineffective, this is discussed further in section 4.3.5.

The railway embankment (right bank) was considered ineffective in order to conservatively show the maximum flood extents in its absence, this is discussed further in section 4.3.5.

(5) Model Boundaries - Inflows:

Full details of the flow estimates are provided in the Hydrology Report (IBE0600Rp0028_HA10 Hydrology Report_D01 - Section 4.8 and Appendix D). The boundary conditions implemented in the model are shown in below:

Table 4.3.1: Model Boundary Conditions

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Figure 4.3.21: 1% AEP Modelled Upstream Hydrograph- Aughrim River - HEP 10_98_4_RPS

Figure 4.3.21 is an example of the 1% AEP inflow hydrograph for the upstream HEP on the Avoca River (HEP 10_98_4_RPS). The model node IDs are located at the upstream extent of each watercourse. Open inflows were applied at the nodes. Appendix A.3 contains further details of comparison of estimated flows with simulated flows in the model.

The model flow at checkpoints was examined during initial development runs .The fluvial inflow 10_98_4_RPS and point source inflow 10_1339_3_RPS on the Avoca River were delayed to ensure peaks of the hydrographs coincided to ensure the peak flows were maximised.

On Avoca tributary 2 and Avoca tributary 7 inflows 10_902_3_RPS and 10_133_2 were delayed and distributed source inflows 10_250_4_RPS & 10_250_6_RPS and 10_133_2 & 10_227_4 were adjusted to ensure the flows at each checkpoint were anchored.

(6) Model Boundaries – Q-h relationship boundaries at the downstream model extent of the Avoca Downstream Conditions: River (chainage 13412m). This relationship is based on critical flow conditions at this location, and is plotted below. These boundaries are set to a ‘dummy’ water level value of -5.583mOD Malin (approximately equal to the bed level) at the start of the simulation. However this value is ignored once the simulation commences and the level of this boundary varies in time based on dynamic calculations within the model.

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Figure 4.3.22: Q-h Relationship at Avoca River (Chainage 13412m)

The Q-h boundary was used as it allows the flows to be compared accurately. If a tidal boundary was used then the downstream HEP points would be subject to tidal influences, this means the discharge in the river could not be reliably measured as there is inflow from the coast acting in the opposite direction to the fluvial flow. This boundary was also used because the mapping output for Avoca is approximately 1km upstream of the point of tidal influence.

(7) Model Roughness: (see Chapter 3.6.1 'Roughness Coefficients')

(a) In-Bank (1D Domain) Minimum 'n' value: 0.020 Maximum 'n' value: 0.050

(b) MPW Out-of-Bank (1D) Minimum 'n' value: 0.040 Maximum 'n' value: 0.050

(c) MPW/HPW Out-of-Bank Minimum 'n' value: 0.030 Maximum 'n' value: 0.067

(2D) (Inverse of Manning's 'M') (Inverse of Manning's 'M')

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Figure 4.3.23:Map of 2D Roughness (Manning's n)

Figure 4.3.23 illustrates the roughness values applied within the 2D domain of the model. Roughness in the 2D domain was applied based on land type areas defined in the Corine Land Cover Map with representative roughness values associated with each of the land cover classes in the dataset.

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(d) Examples of In-Bank Roughness Coefficients

Figure 4.3.24: Avoca Trib 7 - 1038M00078I Figure 4.3.25: Avoca Trib 3 - 1039M00122I_UP

Manning's n = 0.050 Manning's n = 0.013

No vegetation in channel, the river bed has cobbles Concrete lined channel, straight and free of debris stones and boulders.

Figure 4.3.26: Avoca River - 1028M00802_UP Figure 4.3.27: Avoca Trib 1 - 1040M00019I_UP

Manning's n = 0.045 Manning's n = 0.060

Clean winding some pools and shoals. The river bed with cobbles, stones and boulders.

4.3.4 Sensitivity Analysis

To be completed for final version of report.

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4.3.5 Hydraulic Model Calibration and Verification

(1) Key Historical Floods (from IBE0600Rp0005_HA10 Inception Report_F02 unless otherwise specified):

(a) Nov 2000 Flooding occurred in Avoca, Bray, Arklow, Aughrim and Wicklow during November 2000 caused by heavy rain and gale force winds. In Avoca on November 6th, flooding and fears over the integrity of the bridge resulted in the town being cut off. Rising flood waters resulted in a car being washed into a river.

The Avoca model itself is ungauged along its reaches data at Rathdrum (10002 – EPA) and Arklow town bridge (10042 – OPW) is not available therefore only data recorded at Knocknamohill (10028 – EPA) could be used to validate this flood event. This gauging station is located on the Aughrim River upstream of the confluence point with the Avoca River on Model 12. The flood event on November 5th 2000 was estimated at approximately 20-10% AEP.

Design rainfall frequency was estimated using the FSU Depth Duration Frequency model (FSU WP 1.2 ‘Estimation of Point Rainfall Frequencies’). The closest rainfall gauge with data available for this event is Avoca (Copper Mines) daily station (approximately 1km upstream of the AFA). Recordings from this station indicate that 77mm of rain fell on the 5th November 2000, equating to a rainfall event of approximately 7% AEP. As this gauge is not located within the AFA extents and only has a data recording resolution of 24 hours, this rainfall frequency should be treated with caution.

The modelled flood extent shows flooding in Avoca downstream of the Avoca bridge during a 10% AEP event and higher which helps corroborate this historical flood event.

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Figure 4.3.28: All AEP Extents Downstream of Avoca Bridge

(b) Aug 1986 On the 25/26 of August 1986 Hurricane Charlie occurred. On the 26 of August rainfall was in excess of 100mm in the 24 hour period in many areas. In Avoca, flooding of regional roads occurred. Lions Bridge, approximately 4km upstream of Avoca, was washed away due to flood flows in the river. Damage was also caused to Avoca Bridge following undermining of a pier on the upstream side.

Using this rainfall duration and depth information a design rainfall frequency was estimated using the FSU Depth Duration Frequency model (FSU WP 1.2 ‘Estimation of Point Rainfall Frequencies’). This gave a rainfall return period of 2.3%AEP (45 years). The location at which the rainfall was recorded is not stated within the report and the closest hourly rainfall gauges are at Casement and Oak Park. The data at Oak Park does not extend back to the event and from a review of the hourly data at Casement it can be shown that on this date a maximum of 89.9mm was recorded over 24 hours. Due to the unknown location of the data and the unknown antecedent catchment conditions it is not possible to estimate with any accuracy the peak flow and hence the flood return period of the event.

The Avoca model itself is ungauged along its reaches however there is a gauging station at Arklow town bridge. (10042 – OPW). The peak flow recorded at this location is a 10 -6.67% AEP event. This partially validates the model results as the model shows flooding at the Avoca Bridge during 1% and 0.1% AEP events. Lions Bridge is approximately 2.6km upstream of the modelled watercourse.

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(c) Nov 1965 A flood event occurred in Ashford/Rathnew, Avoca, Bray, Arklow, Aughrim and Wicklow on the 16th of November 1965. Flooding occurred of a large section of the railway line at Avoca causing it to be uprooted and ending up in the river. Damage occurred to a bridge in the Vale of Avoca due to flooding on the main railway line between Dublin and Rosslare Harbour.

The Avoca model itself is ungauged along its reaches however there is an FSU pivotal site gauging station approximately 9km upstream of the modelled reaches at Rathdrum (10002 – EPA)

The modelled flood extent shows flooding on the right bank of the Avoca river at all 1% and 0.1% AEP events, this has an impact on the gardens and properties in Woodview Park.

Summary of Calibration

It should be noted that this AFA has no quantitative hydrometric data from these events that can be used to undertake detailed model calibration. Gauging station data, from outside the modelled extent and AFA extent, was used were possible to verify model results.

As historical flood data for specific events is not available for the Avoca AFA, model calibration was not possible. Limited qualitative support for the model results has been achieved based on the data available, despite the lack of calibration and verification data, the model is considered to be performing satisfactorily for design event simulation. Please refer to Section 4.3.5(2) for an update on the flooding mechanisms.

Model flows were checked against the estimated flows at HEP check points where possible to ensure they were within an acceptable range. For example at HEP 10_1612_2_RPS, the estimated flow during the 0.1% AEP event was 552m3/s and the modelled flow was 573m3/s giving a percentage difference of 3.6. A good correlation is found at many of the checkpoints. Full flow tables can be found in Appendix A.3.

The mass error in the model was calculated to ensure it was within acceptable limits. The mass error for this model was calculated at 0.03% during the 1% AEP design run. This is within an acceptable range.

(2) Post Public Consultation Updates:

Following informal public consultation and formal S.I. public consultation periods in 2015, it was noted that flooding should be more frequent in the area of the confluence of Tributary 2 and Tributary 3. The following updates to the model were carried out.

 Additional links between the M11 and M21 file along the Avoca River.

 Manning's n values of 1039M00094I, 1039M00068I & 1039M00065I structures located on the Tributary 3 were increased from 0.013 to 0.025 & 0.02 respectively. The Manning's n value of structure 1039A00012I located on the Tributary 4 was also increased from 0.013 & 0.02. Additionally the Manning's n value of Bridge 1028M01185D located on the Avoca River was

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increased from 0.025 to 0.03.

 Markers were repositioned on some cross sections of Tributary 1, Tributary 2, Tributary 3, Tributary 4, Tributary 5, Tributary 7 and Avoca River.

These changes resulted in increased flooding in the area of the confluence of Tributary 2 and Tributary 3 as shown in Figure 4.3.29. The model results are further supported by the Key Historical Flooding discussed in Section 4.3.5 (1). The model was updated and check flows recalculated with a revised set of flood hazard and risk mapping issued as Final to reflect this change.

Trib 2

Trib 3

Figure 4.3.29 Modelled Flood Extent of the Confluence of Tributary 2 and Tributary 3

(3) Standard of Protection of Existing Formal Defences:

Defence Type Watercourse Bank Modelled Standard of Reference Protection (AEP)

1. Wall (ineffective) Avoca River Left Provides no protection ID:A10_AVO_ at 10%AEP or greater 001

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2. Railway embankment Avoca River Right Crest level is above

(ineffective) the 0.1% AEP event however this built structure was excluded from the model to conservatively show maximum natural flood extent.

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Figure 4.3.30: Flood Defences within Avoca AFA

The ineffective structures which were initially considered as defences in Avoca are shown in Figure 4.3.29. The first is a wall on the left bank of the Avoca River upstream of the bridge 1028M01185D at chainage 1591m. The retaining wall is approximately 38m long and the crest of the wall is approximately 0.66m wide. This flood defence is considered to be ineffective as it is overtopped at flood events of 10% AEP or greater, therefore no areas are currently benefitting from the defence, no changes were made and the wall was retained within the model files.

The second flood structure is classified as an informal, ineffective, therefore it was removed from all defended model runs. The railway embankment is on the right bank of the Avoca River and the section that was removed extends from chainage 1084 to 1404 and from cross section 1028M01185D at approximate chainage 1590.526 to 1028M01080 at approximate chainage 2652m. The embankment was removed by amending the cross sections within the 1D model and the 5m rectangular mesh was changed to represent the scenario before the defence was constructed. This approach was discussed with the LA and OPW and agreed as a a conservative approach.

(4) Gauging Stations:

The nearest gauging station to the model extent is (b) 10002 Rathdrum

The Rathdrum gauging station is located over 9km upstream of the Avoca AFA and is located on the Avonmore River which is an unmodelled watercourse. As a result it should be noted that this area has no

IBE0600Rp0028 4.3-26 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL hydrometric data with which to undertake model calibration.

(5) Other Information:

NA

4.3.6 Hydraulic Model Assumptions, Limitations and Handover Notes

(1) Hydraulic Model Assumptions:

(a) Assumptions had to be made regarding the bed dimensions of certain cross sections along the Avoca River. The assumptions were based on OS mapping and long sections provided by surveyor. Many areas could not be surveyed due to dense vegetation. The data was utilised in order to provide the most accurate results possible. This was the best use of the information that was available.

 sections 1028M01120 to 1028M01128 could not be surveyed because of rapid flows.

 sections 1028M01299 to 1028M01274D there was no survey access due to a steep rise along the right bank.

 the confluence point of Avoca Tributary 2 (1040A) and Avoca Tributary 1 (1040M) could not be surveyed as a result of access issues.

 dense vegetation prevented cross sections 1040M00022 to 1040M00052 on Avoca Tributary 1 from being surveyed and also impacted the lower end of Avoca Tributary 3 (1039M) as cross sections downstream of 1039M00032 could not be surveyed.

 only one cross section could be observed on Avoca Tributary 6 (1039C) because the channel runs into heavy scrub. The assumptions were based on OS mapping and long sections provided by surveyor. Many areas could not be surveyed due to dense vegetation. The data was utilised in order to provide the most accurate results possible. This was the best use of the information that was available.

(b) Various assumptions had to be made regarding structures along the watercourses as survey information could not be obtained. Along the Avoca River at Arklow Townbridge (1028M001222D) strong currents prevented the centre of the channel from being surveyed. On Avoca Tributary 4 (1039A) no manholes could be located along the 200m culvert, as a result the route was estimated using OS mapping and the drawing provided by the surveyors.

(c) The input hydrograph and point source hydrograph at chainage 5813m for the Avoca River were timed to ensure the peak flows were maximised. On Avoca tributary 2 and Avoca tributary 7 inflows 10_902_3_RPS and 10_133_2 were delayed and distributed source inflows 10_250_4_RPS, 10_250_6_RPS and 10_133_2 & 10_227_4 were adjusted to ensure the flows at each checkpoint were anchored.

(d) The downstream section of the Avoca River was modelled 1D as it is a designated Medium Priority Watercourse. To represent the floodplain cross sections were extended using the available LiDAR and MIKE HYDRO software to supplement the topographical survey data. The LiDAR was also applied in the

IBE0600Rp0028 4.3-27 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL model to provide ground elevations between each cross section for mapping flood hazard instead of interpolating between the extended cross sections. As this reach of the model is 1D raw output shows flooding in low lying areas across each cross section, (see below) this creates flood ponds that are not connected to the watercourse and would not flood in reality, consequently these areas of flooding were removed in the model output post processing.

Figure 4.3.31:1%AEP event raw output illustrating flood ponds

(e) The in-channel roughness coefficients were selected based on normal bounds using photos received from the channel and structure survey and have been reviewed during the calibration process - it is considered that the final selected values are representative.

(2) Hydraulic Model Limitations and Parameters:

(a) The calibration of the model could be improved if more historical information was available.

(b) A grid resolution of 5 metres has been selected. It is considered that the 5m resolution is best suited for modelling purposes, e.g. reducing run times while maintaining sufficient detail of the modelled area and floodplain.

(c) It should be noted that observed flooding along the MPW may be represented less accurately than within the AFA. The MPW is modelled using cross section data only; it was found during the Draft modelling stage that the cross sections did contain enough data on the left and right banks. As water levels increased, the floodplain could not be accurately represented as water was not able to spill as required. During the draft final modelling stage, the necessary cross sections on the Avoca River starting from chainage 4016m were extended with the use of the NDHM to provide enough information of the floodplain and allow water to spill as necessary. Background mapping from the NDHM was applied to the MPW which allowed for more accurate floodplain representation between the 1D cross sections

(d) There is a minor instability during all AEP events at cross section 1028M00315D at chainage 10307m. Error! Reference source not found. shows the instability during a 1% AEP event. This instability is due

IBE0600Rp0028 4.3-28 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL to the bed level at this structure. Due to the close proximity of the structures it was not possible to eradicate these instabilities without making disproportionate alterations to the channel geometry. The impact of this instability was reviewed and it was concluded that it does not cause erroneous out-of-bank flooding.

Minor instability at chainage 10307m (ID: 1028M00315D)

Figure 4.3.32: 1% AEP Model Discharge Levels

(e) The link exponential smoothing factor was reduced from the default value of 1.0 to 0.8. This introduces a smoothing of the water levels transferred from the 1D model to the 2D model, steep gradients are smoothed through the links, dampening out oscillations and reducing any instability. The smaller the value the greater the smoothing. This value of 0.8 is considered appropriate as a very small smoothing factor will cause a time lag in the transfer of the values from one model component to the other.

(f) The depth tolerance shown below was changed in some instances from the default value of 0.1 to 0.2. Lateral links can cause instability when the difference in water levels between MIKE 11 and MIKE 21 is similar, this can be avoided by changing the depth tolerance factor.

MIKE 11

Timestep (seconds) 1

Wave Approximation High Order Fully Dynamic

Delta 0.85

MIKE 21

Timestep (seconds) 1

Drying / Flooding (metres) 0.02 / 0.03

Eddy Viscosity (and type) Constant eddy viscosity fluxed based formulation

IBE0600Rp0028 4.3-29 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

of 0.5

MIKE FLOOD

Link Exponential Smoothing Factor 0.8 Tributary 4, Tributary 5 and Tributary 7

(where non-default value used)

Lateral Length Depth Tolerance (m) 0.2 Tributary 5 and Tributary 7

(where non-default value used)

(3) Design Event Runs & Hydraulic Model Handover Notes:

(a) The Network and Cross Section files linked to the Mike 11 setup are identical for all event runs.

(b) The model runs best when the Mike 11 initial condition is set to Steady State.

(c) The capacity of Avoca Tributary 1 was generally found to be adequate with 10%AEP. During a 1% and 0.1% event, flooding occurs due to a lack of capacity in structure 1040M00066D located at chainage 266m along this stretch of river. This is shown to have an impact on a housing development adjacent to the R754 in the upstream reach of the channel as shown in Figure 4.3.33

Figure 4.3.33: Flood Extent During All AEP Events

(d) Very little flooding occurs along the upper reaches of Avoca Tributary 2. A small amount of flooding occurs during the 1% and 0.1% event further downstream due to the bridge 1040A00015D at chainage 684m restricting the flow.

(e) Flooding occurs on Avoca Tributary 3 during all flood events. The flooding impacts all of the residential properties (during a 0.1% AEP event) located on the right bank in Brook Meadows, just upstream of

IBE0600Rp0028 4.3-30 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL culvert 1039M00094I (chainage 1357m), due to a combination of inadequate channel capacity during low AEP events and a lack of capacity in the culvert. The ESB station, Brooklawn and Cherrybrook are all affected as shown in Figure 4.3.34 The channel capacity is also insufficient downstream of this culvert, with properties affected during all events. The culverts 1039M00068I and 1039M00065I also have insufficient capacity for 10%, 1% and 0.1% flows.

(f) The Avoca River shows notable out-of-bank flooding when conveying higher return period flows. This may have an impact on a number of properties within the AFA at higher return periods.The downstream section of the Avoca River was modelled 1D as it is a designated Medium Priority Watercourse. Some sections of the river have insufficient capacity and structures (see Figure 4.3.4 to Figure 4.3.8 for all critical structures) along the reach are restrictive during higher return periods causing water levels to build up. Woodenbridge golf course floods during 1% and 0.1% return periods.

(g) Avoca Tributary 4 floods during all AEP events. Structures (1039A00046I and 1039A00012I) on this watercourse cause a restriction to the flow during this event. The properties affected during each AEP event are shown in Figure 4.3.34.

Figure 4.3.34: Flood Extent During All AEP Events

(h) The capacity of Avoca Tributaries 5 and 6 was generally found to be adequate during all flood events.

(i) Flooding occurs throughout the lower reaches of Avoca Tributary 7 (see Figure 4.3.35), east of the R752, at all return periods. Out-of-bank flooding is quite extensive, however the topography shows that this land appears to act as a natural floodplain and there is no impact on any properties during higher flood events.

IBE0600Rp0028 4.3-31 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Figure 4.3.35: Flood Extent During All AEP Events

(j) The mapping extent for this model is 10307m. This was selected as the Arklow model downstream is being progressed via a separate OPW study.

(k) There are some instabilities within the model as discussed within the Hydraulic Model Limitations and Parameters Section. The main instability occurs along the Avoca River (chainage 10307m) at a structure (ID: 1028M00315D) along the MPW where there is an increase in bed level. There are no other instabilities on the other reaches. These reamining instabilities cause a slight fluctuation in water level but do not have an overall impact on the flood extent.

(4) Hydraulic Model Deliverables:

Please see Appendix A.4 for a list of all model files provided with this report.

(5) Quality Assurance:

Model Constructed by: Jen Canavan

Model Reviewed by: Stephen Patterson

Model Approved by: Malcolm Brian

IBE0600Rp0028 4.3-32 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Appendix A. 1

Structure Details – Bridges and Culverts

OPENING SPRING HEIGHT MANNINGS RIVER BRANCH CHAINAGE ID LENGTH (m) HEIGHT (m) WIDTH (m) SHAPE FROM INVERT (m) N

Irregular x1 1.3 1.8 N/A AVOCA TRIB1 266.256 1040M00066D 6.48 0.025 AVOCA TRIB1 728.208 1040M00019I 13.641 Circular x1 1.5 1.5 N/A 0.013 AVOCA TRIB2 683.667 1040A00015D 6.91 Arch x1 2.11 3.08 0.96 0.025 AVOCA TRIB2 737.225 1040A00011D 3.95 Arch x1 1.07 1.94 0.37 0.025 AVOCA TRIB2 784.546 1040A00006D 9.07 Arch x1 2.3 1.72 1.46 0.025 AVOCA TRIB5 153.014 1039B00045I 7.88 Circular x1 0.5 0.5 N/A 0.013 AVOCA TRIB5 373.414 1039B00029D 8.81 Irregular x1 0.82 0.58 N/A 0.025 AVOCA TRIB5 568.537 1039B00004I 11.05 Circular x1 0.45 0.45 N/A 0.013 AVOCA TRIB3 555.644 1039M00175D 9.298 Irregular x1 0.84 1.015 N/A 0.02 AVOCA TRIB3 768.782 1039M0015I 20.248 Circular x1 0.75 0.75 N/A 0.013 AVOCA TRIB3 905.865 1039M00140D 2.671 Irregular x1 1.66 1.718 N/A 0.02 AVOCA TRIB3 1052.26 1039M00125I 16.49 Circular x2 1.0 1.0 N/A 0.013 AVOCA TRIB3 1080.521 1039M00122I 26.213 Circular x2 1.0 1.0 N/A 0.013 AVOCA TRIB3 1356.932 1039M00094I 17.796 Circular x2 0.9 0.9 N/A 0.013 AVOCA TRIB3 1634.546 1039M00068I 11.334 Circular x3 0.6 0.6 N/A 0.013 AVOCA TRIB3 1651.878 1039M00065I 33.358 Circular x3 0.6 0.6 N/A 0.013 AVOCA TRIB4 994.312 1039A00020D 1.5 Irregular x1 0.91 3.02 N/A 0.02 AVOCA TRIB4 1057.376 1039A00016D 3.4 Irregular x1 1.86 2.10 N/A 0.02 AVOCA TRIB4 1149.75 1039A00012I 112.64 Circular x1 0.45 0.45 N/A 0.013

IBE0600Rp0028 4.3-33 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

RIVER AVOCA 716.2 1028M0127D 9.12 Irregular x1 0.91 36.92 N/A 0.025 AVOCA TRIB7 491.837 1038M00078I 13.25 Circular x1 0.45 0.45 N/A 0.013 AVOCA TRIB7 1057.958 1038M00022D 15.37 Irregular x1 1.37 1.049 N/A 0.025 AVOCA TRIB7 1207.955 1038M00008D 4.673 Irregular x1 0.86 1.32 N/A 0.025 Arch x3 6.08, 6.03, 14.36, 13.81, 3.6, 2.86, 3.78 RIVER AVOCA 1594.141 1028M01185D 7.23 6.01 13.52 0.025 RIVER AVOCA 3115.372 1028M01033D 5.18 Irregular x1 5.95 30.09 N/A 0.025 RIVER AVOCA 3923.318 1028M00952D 5.9 Irregular x1 6.79 36.73 N/A 0.025 RIVER AVOCA 5021.729 1028M00842D 2.9 Irregular x1 6.67 31.72 N/A 0.025 RIVER AVOCA 5444.469 1028M00800D 2.03 Irregular x2 4.51, 4.73 21.62, 25.90 N/A 0.025 RIVER AVOCA 5698.813 1028M00775D 3.17 Irregular x1 5.9 32.25 N/A 0.025 RIVER AVOCA 9114.538 1028M00433D 7.6 Irregular x1 4.95 70.38 N/A 0.025 Arch x20 3.53, 3.48, 4.10, 4.89, 4.75, 2.61, 2.54, 2.65, 3.44, 3.51, 4.78, 6.97, 6.86, 3.51, 2.57, 2.52, 3.87, 3.83, 6.73, 6.86, 7.01, 2.76, 2.58, 2.65, 3.78, 3.92, 6.99, 6.99, 6.93, 2.55, 2.55, 2.52, 3.86, 3.75, 6.92, 6.89, 6.96, 2.47, 2.27, 2.35, 3.75, 3.51, 5.07, 4.74, 4.90, 2.13, 1.95, 1.95, 3.55, 3.57, 4.72, 4.80 2.04, 2.18 3.30, 3.00, 2.79, 3.00, RIVER AVOCA 12228.602 1028M00122D 11.6 3.23, 2.99 0.025 RIVER AVOCA 10307.013 1028M00315D 32.8 Irregular x1 5 83.47 N/A 0.025 *AVOCA TRIB4 758.737 1039A00046I 212.24 Circular x1 0.75 0.75 N/A 0.013 *Denotes structures incorporated as closed cross-sections only. (and therefore not included in the Network file).

** Structure ID Key: D – Bridge Upstream Face E – Bridge Downstream Face I – Culvert Upstream Face J – Culvert Downstream Face

IBE0600Rp0028 4.3-34 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Structure Details - Weirs RIVER CHAINAGE ID MANNINGS N TYPE BRANCH 0.05 Broad Crested Weir AVOCA TRIB2 641.931 1040A00017W 0.04 Broad Crested Weir AVOCA TRIB2 724.476 1040A00012W 0.05 Broad Crested Weir AVOCA TRIB2 745 1040A00010W

* Note that all the other weirs in the Network file are overtopping weirs which form part of a composite structure with the culvert/bridge at the corresponding chainage.

IBE0600Rp0028 4.3-35 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Appendix A. 2

Solid Black line indicates the Right Bank

Dashed Black Line indicates the Left Bank

Dashed Red Line indicates the Peak Water Level

Minor instability at chainage 10307m (ID: 1028M00315D)

Avoca River 0.1% AEP Fluvial Flow

The Avoca River is the largest reach associated with the Avoca model. No model instabilities were identified along this reach.All instabilities are highlighted and discussed within the report. There are no instabilities on the other watercourses.

IBE0600Rp0028 4.3-36 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Appendix A. 3

IBE0600 EAST CFRAM STUDY RPS Peak Water Flows Model Flow Diff River Name & Chainage AEP Check Flow (m3/s) (m3/s) (%) AVOCA TRIB1 822.7 10% 6.45 6.27 -2.81 10_76_2_RPS 1% 10.91 11.40 +4.47 0.1% 17.86 19.11 +7.02 AVOCA TRIB2 784.5 10% 3.29 3.63 +10.40 10_250_6_RPS 1% 5.83 6.47 +11.03 0.1% 10.06 11.19 +11.27 AVOCA TRIB3 2202.61 10% 3.00 3.12 +3.87 10_209_3_RPS 1% 5.61 5.67 +1.03 0.1% 10.18 10.61 +4.22 AVOCA TRIB4 1079.21 10% 0.85 0.77 -8.94 10_182_1 1% 1.59 1.35 -14.84 0.1% 2.89 2.69 -6.78 AVOCA TRIB3 680.197 10% 0.97 1.09 12.06 10_279_1_RPS 1% 1.82 2.06 12.91 0.1% 3.30 3.91 18.33 AVOCA TRIB7 1072.05 10% 2.53 2.11 -16.72 10_227_4 1% 4.73 4.80 +1.52 0.1% 8.57 8.77 +2.38 RIVER AVOCA 6533.79 10% 238.79 240.85 +0.86 10_1268_2 1% 363.24 369.79 +1.80 0.1% 541.99 557.29 +2.82 RIVER AVOCA 11549 10% 243.23 246.59 +1.38 10_1612_2_RPS 1% 369.99 379.03 +2.44 0.1% 552.06 572.85 +3.76

The table above provides details of the flow in the model at every HEP intermediate check point and modelled tributary. These flows have been compared with the hydrology flow estimation and a percentage difference provided. In general there is good correlation between estimated and modelled flows during all AEPs as the greatest percentage difference is 18%.

The estimated and modelled flows at the downstream end of the Tributary 1 at check point 10_76_2_RPS correlate well for all model design runs. Due to flooding at this confluence point, some flooding from the Avoca River may have been included in the calculation resulting in a slight increase in modelled flows ranging from +4% during a 1% AEP event and +7% during a 0.1% AEP event.

10_1268_2 and the downstream checkpoint on the Avoca River also shows a good correlation as the percentage difference is below 10% during all AEP events.

IBE0600Rp0028 4.3-37 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

At HEP 10_279_1_RPS on Avoca Trib 3 the modelled flows are 12 - 18% higher than the hydrological estimates. Upstream of this point the model inflows were simplified to resolve differences between the hydrological estimates and the surveyed river. The modelled flows at 12 - 18% higher are reflective of this but are considered to be moderately well anchored at this point where the catchment is only 1.5km2 and there is fair uncertainty in the hydrological estimates. Further downstream at HEP 10_209_3_RPS the flows are shown to be well anchored with the differences reduced to between 1 - 4%.

The table also shows a negative percentage difference between the modelled and estimated flows at the downstream end during all AEP events on Tributary 4 and the 10% AEP event on Tributary 7. In this instance it is predominantly due to the effect of structure 1038M00008D restricting flow slightly and reducing the measured peak. On Tributary 7 during the 1% and 0.1% event, there is additional flow that comes out of bank upstream of 10_227_4 that is being taken into account at the 10_227_4 HEP, resulting in smaller difference in comparison to the 10% AEP event.

IBE0600Rp0028 4.3-38 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Appendix A. 4

MIKE FLOOD MIKE 21 MIKE 21 RESULTS HA10_AVOC11_MF_DES_22_Q10 HA10_AVOC11_M21_ DES_22_Q10 HA10_AVOC11_M21_ DES_22_Q10 HA10_AVOC11 _MF_ DES_22_Q100 HA10_AVOC11_M21_ DES_22_Q100 HA10_AVOC11_M21_ DES_22_Q100 HA10_AVOC11_MF_ DES_22_Q1000 HA10_AVOC11_M21_ DES_22_Q1000 HA10_AVOC11_M21_ DES_22_Q1000 HA10_AVOC11_DFS2_bldgs_5 (BATHYMETRY) HA10_AVOC11_Resistance (RESISTANCE)

MIKE 11 ‐ SIM FILE & RESULTS FILE MIKE 11 ‐ NETWORK FILE MIKE 11 ‐ CROSS‐SECTION FILE MIKE 11 ‐ BOUNDARY FILE HA10_AVOC11_M11_ DES_22_Q10 HA10_AVOC11_NWK_ DES_23 HA10_AVOC11_XNS_ DES_23 HA10_AVOC11_BND_ DES_5_Q10 HA10_AVOC11_M11_ DES_22_Q100 HA10_AVOC11_BND_ DES_5_Q100 HA10_AVOC11_M11_ DES_22_Q1000 HA10_AVOC11_BND_ DES_5_Q1000 MIKE 11 ‐ DFS0 FILE MIKE 11 ‐ HD FILE & RESULTS FILE HA10_AVOC11_DFS0_Q10 HA10_AVOC11_HD_DES_22_Q10 HA10_AVOC11_HD _DES_22_Q10 HA10_AVOC11_DFS0_Q100 HA10_AVOC11_HD_DES_22_Q100 HA10_AVOC11_HD_ DES_22_Q100 HA10_AVOC11_HD_DES_22_Q1000 HA10_AVOC11_HD _DES_22_Q1000 HA10_AVOC11_DFS0_Q1000

IBE0600Rp0028 4.3-39 Rev F03 Eastern CFRAM Study HA10 Hydraulics Report - DRAFT FINAL

Appendix A. 5

GIS DELIVERABLES

GIS Deliverables - Hazard Flood Extent Files (Shapefiles) Flood Depth Files (Raster) Water Level and Flows (Shapefiles) Fluvial Fluvial Fluvial E04NFCDF0

E04EXFCD001F0 E04DPFCD001F0 E04EXFCD010F0 E04DPFCD010F0 E04EXFCD100F0 E04DPFCD100F0

Flood Zone Files (Shapefiles) Flood Velocity Files (Raster) Flood Defence Files (Shapefiles) Fluvial Fluvial N/A E04ZNA_FCDF0 E04VLFCD001F0 E04ZNB_FCDF0 E04VLFCD010F0 E04VLFCD100F0

GIS Deliverables - Risk Specific Risk - Inhabitants (Raster) General Risk - Economic (Shapefiles) General Risk-Environmental (Shapefiles) Fluvial E04RIFCD001F0 E04RIFCD010F0 E04RIFCD100F0

IBE0600Rp0028 4.3-40 Rev F03