Eastern CFRAM Study HA09 Hydraulics Report – DRAFT FINAL

Eastern CFRAM Study HA09 Hydraulics Report Clane Model

DOCUMENT CONTROL SHEET

Client OPW

Project Title Eastern CFRAM Study

Document Title IBE0600Rp0027_HA09 Hydraulics Report

Model Name Clane

Rev. Status Modeller Reviewed By Approved By Office of Origin Issue Date

D01 Draft M Wilson S. Patterson G, Glasgow Limerick/Belfast 03/06/2014

F01 Draft Final M.Wilson S.Patterson G. Glasgow Belfast 13/03/2015

F02 Draft Final M.Wilson S.Patterson G. Glasgow Belfast 13/08/2015

F03 Draft Final M.Wilson S.Patterson G. Glasgow Belfast 05/08/2016

IBE0600Rp0027 Rev F03 Eastern CFRAM Study HA09 Hydraulics Report – DRAFT FINAL

Table of Reference Reports

Report Issue Date Report Reference Relevant Section Eastern CFRAM Study Flood Risk December IBE0600Rp0001_Flood Risk Review_F02 3.5.5 Review 2011 Eastern CFRAM Study Inception August 2012 IBE0600Rp0008_HA09 Inception 4.3.2 Report UoM09 Report_F02 Eastern CFRAM Study Hydrology September IBE0600Rp0016_HA09_Hydrology 4.11 Report UoM09 2013 Report_F01 Eastern CFRAM Study HA09 Liffey November 2147_REP_130109_SC2 Survey Report 1.7 Survey Contract Report 2012 v3-1

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4 HYDRAULIC MODEL DETAILS

4.4 CLANE MODEL

4.4.1 General Hydraulic Model Information

(1) Introduction:

The Eastern CFRAM Flood Risk Review (IBE0600 Rp0001_Flood Risk Review_F02) highlighted Clane in the Liffey catchment as an AFA for fluvial flooding based on a review of historic flooding and the extents of flood risk determined during the PFRA.

The Clane model (Model 6A) represents the portion of the middle Liffey affecting the Clane AFA and two tributaries called the Gollymochy River and the Cott watercourse. The contributing catchment of the middle Liffey at the downstream boundary of the model is 337 km2 (excluding the upper catchment, the effect of which is as considered in Chapter 6 of the UoM 09 hydrology report).

Newbridge and Naas are upstream of Clane which is upstream of Celbridge. The River Liffey is connected upstream from Clane (Model 6A) to Newbridge (Model 8). The downstream boundary flows from Model 8 are used to provide the upstream boundary flows of Model 6A. The watercourses from Naas (Model 6B) flow into the River Liffey within Model 6A. The flows from model 6B are represented as point flows into model 6A. The River Liffey is connected downstream from Clane (Model 6A) to Celbridge (Model 3B). The upstream boundary water levels from Model 3B are used to provide the downstream boundary of Model 6A. Figure 4.4.1 shows the overarching relationship between the Clane model and the surrounding models.

There are no gauging stations located directly on the modelled reaches of the Clane model with the nearest gauging station located approximately 10km downstream at Celbridge (09006 – ESB). There are some issues with the quality of data at this gauging station however ESB provided rating and spot gauge information which indicates that there is confidence in the rating up to and above Qmed. The inflow hydrographs to the middle Liffey models have been derived based on observed dam release flow data at Golden Falls (upper Liffey catchment) combined with hydrologically derived inflows to the middle Liffey. The combined flow on the Liffey resulting from both mechanisms has been modelled from Golden Falls to the reservoir at Leixlip through the Newbridge, Clane and Celbridge models. To ensure that the correct frequency conditions are being achieved on the middle Liffey the modelled flows have been validated against the available data at the Celbridge gauging station within the Celbridge model. This is reported on in Section 4.3.1.

No catchment rainfall run-off models have been developed for the Clane model as the available gauge data on the main channel of the Liffey includes the intermittent releases from the dams upstream and as such calibration of the models could not be achieved. Tributaries which enter the model laterally are both small and ungauged and as such calibration of catchment models could not be achieved.

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The majority of the Clane model has been identified as a HPW and therefore modelled as 1D-2D using the MIKE suite of software. The last 4km of the River Liffey, within the Clane model, have been designated as MPW. This type of watercourse requires a minimum of 1D modelling however given that the proportion of MPW is small within the Clane model and that the next reach of the River Liffey in the Celbridge model has been modelled as 1D-2D the decision was taken to model this remaining 4km of the River Liffey as 1D-2D also.

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.

(2) Model Reference: HA09_CLAN6A

(3) AFAs included in the model: CLANE

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

Reach ID Name

09COTT COTT

09GOLL GOLLYMOCHY RIVER

09LIFF RIVER LIFFEY

(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)

4.4.2 Hydraulic Model Schematisation

(1) Map of Model Extents:

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and Error! Reference source not found. illustrate the extent of the modelled catchment, river centre line, HEP locations and AFA extents as applicable. This reach of the Liffey catchment contains 2 Upstream Limit HEPs, 1 Downstream Limit HEP and 5 Tributary HEPs.

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Figure 4.4.1: Clane model Overview in the context of HA09

Figure 4.4.2: Map of Model Extents in vicinity of Clane AFA

Figure 4.4.3 provides an overview drawing of the model schematisation. Figures 4.4.4 and 4.4.5 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 areas are provided where there is the most significant risk of flooding. These diagrams include the surveyed cross-section locations, AFA boundary and river centre. They also show the location of the critical structures as discussed in Section 4.4.3, along with the location and extent of the links between the 1D and 2D models. For clarity in viewing cross-section locations, the model schematisation 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.

On the River Liffey, flood mapping covers the entire modelled reach between the upstream and downstream model extents.

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Figure 4.4.3: Overview of model schematisation

Figure 4.4.4: Clane Model schematisation AFA overview

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Figure 4.4.5: Clane Model schematisation AFA overview

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

River Name x y 09COTT COTT 285242.7 227803.17 09GOLL GOLLYMOCHY RIVER 286659.04 229687.93 09LIFF RIVER LIFFEY 282949.64 218829.55

(3) Total Modelled Watercourse Length: 25.34 (km)

(4) 1D Domain only Watercourse Length: - (5) 1D-2D Domain 25.34 (km) Watercourse Length:

(6) 2D Domain Mesh Type / Resolution / Area: Rectangular / 5 metres / 108 km2

(7) 2D Domain Model Extent:

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represents the modelled extents and the general topography of the catchment within the 2D model domain. The river centre-line is shown in pale blue with red areas representing blocked cells i.e. river centre lines, buildings or the area beyond the 2D model domain. There was no further post processing of the data contained within the mesh required. Changes in the vertical scale of this map are outlined by the index; all levels have been set to OD Malin (metres).

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Figure 4.4.6: Diagram of 2D Model Extent

(8) Survey Information

(a) Survey Folder Structure:

First Level Folder Second Level Folder Third Level Folder

Murphy_E09_M06A_WP4_120830 GIS and Floodplain Floodplain Photos and

Clane Photos Shapefiles

Murphy: Surveyor Name Structure_Register E09: Eastern CFRAM Study Area, Hydrometric Area 9 Ascii

M06A: Model Number 06A Drawings and PDFs

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09YEOM: River Reference Photos (Naming WP4: Work Package 4 convention is in the

Version: Most up to date format of Cross-Section ID and orientation - 120830: Date Issued (30th AUG 2012) upstream, downstream,

left bank or right bank)

(b) Survey Folder References:

Reach ID Name File Ref.

09COTT COTT Murphy_E09_M06A_WP4_120801_09COTT

09GOLL GOLLYMOCHY RIVER Murphy_E09_M06A_WP4_120801_09GOLL

09LIFF RIVER LIFFEY Murphy_E09_M06_WP4_120801_09LIFF_G

Murphy_E09_M06A_WP4_120801_09LIFF_H

Murphy_E09_M06_WP4_120830_09LIFF_I

Murphy_E09_M08_WP4_120830_09LIFF_J

(9) Survey Issues:

At the culvert crossing on Prosperous Road at the Hazelhill Nursing Home the levels stated in the ACAD drawing are contradictory at XS 09COTT00153E. The pipe invert level is given as 72.3mOD and the bed level at approx the same level is given as 71.6mOD. The surveyor addressed these comments and updated the survey drawings. This was checked with the assumed levels used in the hydraulic model.

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4.4.3 Hydraulic Model Construction

All structures within the 1D model that have potential to overtop, such as bridges and culverts, were given an overtopping weir representative of the associated parapet or deck. This allows for flood water to overtop a surcharged structure and avoids creating an artificially high backwater profile. Overtopping weirs were applied to all bridges and culverts in the Longwood model.

(1) 1D Structures (in-channel along See Appendix A.2 modelled watercourses): Number of Bridges and Culverts: 41

Number of Weirs: 4

The survey information recorded includes a photograph of each structure, which was 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.

A series of culverts are located along the Cott Stream some of which, due to location and size effect the water levels during flood events. The Figure 4.4.7 shows the location of critical structures identified along this reach of the Cott Stream in the Butterstream Commons area.

Figure 4.4.7: Location of critical structures in Butterstream Commons area

The culvert shown in Figure 4.4.8 is 0.75m in and surcharges during flood events which causes water to back up along the river channel where out of bank flooding occurs. There is significant flooding during the 0.1% AEP event and a relatively small amount during the 1% AEP event.

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Figure 4.4.8: Critical structure 09COTT00248I identified on the Cott Stream at chainage 1408m.

Figure 4.4.9 shows a 1m diameter culvert. During the 0.1% and 1% AEP events the water is restricted through this culvert which causes the water to back up in the channel. Out of bank flooding occurs when this happens on the left bank which floods the road. This remains a localised incident during the 1% AEP event but during the 0.1% AEP event the water flows overland to join other flooding downstream.

Figure 4.4.9: Critical structure 09COTT00202I identified on the Cott Stream at chainage 1860m.

The culverts shown in Figure 4.4.10, 09COTT00178I and 09COTT00175I are located 25m apart. Both are an irregular shape with the upstream culvert approximately 1m wide and 0.7m deep and the downstream culvert approximately 1.5m wide and 0.5m deep. Both culverts restrict the flow causing out of bank flooding immediately upstream during the 0.1% and 1% AEP events. The flood water then flows along the adjacent road but is unable to join with the Cott Stream again.

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Figure 4.4.10: Critical structures identified on the Cott Stream at chainages 2112m and 2137m.

The bridges shown in Figure 4.4.11, 09COTT00155D and 09COTT00152D are located approximately 25m apart. Both bridges restrict the flow through them and cause a backwater effect upstream. This results in out of bank flooding on the right hand bank.

Figure 4.4.11: Critical structures identified on the Cott Stream at chainage 2328m and 2354m

The two culverts shown in Figure 4.4.12, ref 09COTT00138I, at chainage 2428m form part of a flood alleviation scheme along with the overspill weir also shown. While there is no flooding caused by these culverts they are critical structures in alleviating the flood risk.

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Figure 4.4.12: Critical structure identified on the Cott Stream at chainage 2482m.

A temporary culvert, 09COTT00081I, 0.9m in diameter has been placed at chainage 3090m as seen in Figure 4.4.13. The flow is restricted through this culvert and causes out of bank flooding both on the left and right hand banks for 0.1%, 1% and 10% AEP flood events.

Figure 4.4.13: Critical structure identified on the Cott Stream at chainage 3090m.

The culvert shown in Figure 4.4.14, 09GOLL00289I, is a 1.05m diameter culvert. The culvert is causes a restriction to the flow and the water to back up upstream of it. During the 0.1% AEP event out of bank flooding occurs on both the left and right banks.

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Figure 4.4.14: Critical structure identified on the Gollymochy Stream at chainage 1376m.

The culvert shown in Figure 4.4.15, 09GOLL00120I, is a twin culvert with two 1.2 diameter culverts. For events of 10% AEP or greater the flow is restricted through this structure which causes the water to back up along the channel upstream and results in out of bank flooding on both banks. At least on property is affected by the 1% and 0.1% AEP events at this location as a result of this structure.

Figure 4.4.15: Critical structure identified on the Gollymochy Stream at chainage 3107m.

The culvert shown in Figure 4.4.16, 09GOLL00024D, is a stone arch approximately 1.3m wide and 2.7m high. For events of 10% AEP or greater the flow is restricted through this structure resulting in the water backing up along the channel upstream. Due to the culvert being close to the confluence with the River Liffey the water levels from the River Liffey play an important role also is controlling the water levels upstream of this culvert. As a result of the culvert restricting flow and high water levels from the River Liffey out of bank flooding occurs on the right banks immediately upstream of the culvert. One property is affected by this flooding mechanism.

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Figure 4.4.16: Critical structure identified on the Gollymochy Stream at chainage 4045m.

A significant weir is located on the River Liffey at chainage 53495m, reference 09LIFF03634W. The weir spans the river diagonally and results in a bed level drop of approximately 3m. The effect of the weir continues even during a 0.1% AEP flood event, at no point is it drowned out. The result is high water levels upstream of the weir which results in out of bank flooding on both banks for all of the three events modelled (10%, 1% & 0.1% AEP). The resulting flooding on the left bank flows overland affecting some properties and fields before returning to the River Liffey.

Figure 4.4.17: Critical structure identified on the River Liffey at chainage 53495m.

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

(3) 2D Model structures: None

(4) Defences:

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

Culvert bypass Cott Both 2280m 2985m

(5) Model Boundaries - Inflows:

Full details of the flow estimates are provided in the Hydrology Report (IBE0600Rp0016_HA09_Hydrology Report_F01-Section 4.11 and Appendix D). The boundary conditions implemented in the model are shown in Table 4.4.1 and Error! Reference source not found.8.

Table 4.4.1: Model Boundary Conditions

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Figure 4.4.18: Inflow Hydrograph for River Liffey (09_1239_6_RPS) for the 0.1% AEP Event

The main inflow to the Clane model is shown in Error! Reference source not found. and represents the flow in the River Liffey during a 0.1%AEP event. The hydrograph shows a series of peaks which represents two flood scenarios. The first event which peaks at approx 90m/s3 shows the 0.1%AEP event where the contributing catchment starts downstream of the two ESB dams (Pollaphuca and Golden Falls). The following two peaks represent the subsequent dam release which would normally follow a large flood event. The last peak which plateaus around 160m/s3 represents a controlled release of flow from the dams as might arise after a 0.1%AEP event has past.

(6) Model Boundaries – The water levels generated from the adjacent Celbridge model at its Downstream Conditions: upstream boundary were taken as the downstream boundary condition on the River Liffey. The influence of the water levels in the Celbridge model in creating a backwater effect along the River Liffey was accounted for in this way. Figure 4.4.19 shows the downstream boundary water levels based on the Celbridge upstream boundary for the 0.1%, 1% and 10% AEP flood events.

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Figure 4.4.19: Downstream Boundary Condition (Water Level / Time Plot extracted from Lower Liffey Model (at 09LIFF00862))

(7) Model Roughness:

(a) In-Bank (1D Domain) Minimum 'n' value: 0.035 Maximum 'n' value: 0.05

(b) MPW Out-of-Bank (1D) Minimum 'n' value: 0.035 Maximum 'n' value: 0.040

(c) MPW/HPW Out-of-Bank Minimum 'n' value: 0.035 Maximum 'n' value: 0.060

(2D)

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Figure 4.4.20: Map of 2D Roughness (Manning’s n) in Model Domain

Error! Reference source not found. 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.

(d) Examples of In-Bank Roughness Coefficients (based on Table 3.3 in Chapter 3.5.1)

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Figure 4.4.21: Example of roughness Figure 4.4.22: Example of roughness coefficients on the Cott Stream at cross coefficients on the Cott Stream at cross sections 09COTT00187 sections 09COTT00127

Manning’s n value: 0.050 Manning’s n value: 0.035

Straight but with weeds and stones Clean straight, some stones

Figure 4.4.23: Example of roughness coefficients on the River Liffey at cross sections 09LIFF03634w

Manning’s n value: 0.035

Clean straight, slightly sluggish

Figure 4.4.24

Figure 4.4.25

4.4.4 Sensitivity Analysis

To be completed.

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4.4.5 Hydraulic Model Calibration and Verification

(1) Key Historical Floods

(a) Nov 2009. The review indicated that flooding occurred in parts of Kildare and Dublin in November 2009 following heavy rainfall.

In Clane, the flooding was caused by the River Liffey and Butterstream (Cott) overflowing, coupled with the inability of the local drains to convey the floods. The Hazelhall Nursing Home on Prosperous Road required evacuation. Sandbags were needed at Butterstream housing estate; however Loughbollard and Cois Aoibhinn estates were flooded with sewage in the floodwaters. Millicent and Prosperous roads were also flooded and the rural area of Loughanure was entirely blocked off.

A flood alleviation scheme has been carried out in the area of Hazelhall Nursing Home and Butterstream housing estate. A bypass culvert was laid downstream of the nursing home increasing the overall capacity of the watercourse. The model shows that no out of bank flooding is occurring at this location. Given the changes in this location the historical data cannot be used to calibrate the model.

(b) Nov 2002. Widespread flooding occurred in mid November 2002 as a result of heavy and prolonged rainfall. The rainfall event was approximately a 2% AEP however no records are available to predict the equivalent flood event. Flooding was severe in some parts as the catchments were already somewhat saturated following high levels of rainfall in October and early November. The Clane area was also affected by excessive rainfall where the sewage pumping station was flooded. The Clane- Celbridge road and Clane-Maynooth road were also impassable as was Clane village. Sandbags were needed to protect properties in the vicinity.

The model confirms that the sewage pumping station is at risk in a 0.1%AEP event however the roads to Celbridge and Maynooth are not shown as flooding. There is not sufficient detail to ascertain the source or location of flooding or the flooding mechanism that caused flooding to these roads in November 2002.

th (c) Aug 1986. Hurricane Charlie occurred on 25/26 August 1986 and was deemed exceptional with large rainfall totals accompanied by strong to gale force winds.

There is no further detailed data for Clane relating to this event.

(d) Dec 1954. Gale force winds, reaching speeds of up to 65mph, and torrential rain caused widespread flooding throughout Dublin and parts of Kildare in early December 1954. The flooding was reported to be the worst for many years with an estimated AEP of 1.1%.

In Clane Village, a tributary of the River Liffey overflowed making roads in the area

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impassable. No additional details were provided.

It is assumed that the tributary referred to is the Cott Stream also known as the Butterstream as this watercourse has historically presented the greatest flood risk. However as mentioned before a flood alleviation scheme has been carried out on the Cott stream which has been incorporated into the model. This historical record is therefore of limited value in calibrating the model to the Cott Stream.

An ESB flood report of the 1954 flood provided recorded levels along certain parts of the River Liffey. For the Clane area levels were recorded at Alexandra Bridge as shown in Figure 4.4.24. The levels shown are recorded around 224.7ft OD Poolbeg upstream of the weir and 224.63ft OD Poolbeg upstream of the bridge. Converted to mOD Malin these levels are 65.79mOD and 65.77mOD respectively. These levels were compared with the modelled 1% AEP event, which is the closest event to the predicted 1954 event at 1.1% AEP. The model levels are 66.08mOD upstream of the weir and 65.93mOD upstream of the bridge. This gives a difference of 0.29m and 0.16m higher than the recorded levels from the 1954 event which shows good agreement between the model and the 1954 flood event.

Figure 4.4.26: Recorded flood levels during the 1954 flood event at Alexandra Bridge

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Summary of Calibration

Some historical information is available for the Clane area which showed the Cott stream to be a source of flood risk to the surrounding area historically. A flood alleviation scheme was carried out in 2010 to address these flood risk issues making the majority of the historical records of limited use in verifying the model. Recorded water levels on the River Liffey during the 1954 flood were compared with the modelled levels and found to be in agreement.

A comparison between estimated and modelled flows at HEP points was carried out and the details of which are presented in Appendix A.3.

A mass balance plot was carried out to assess the difference between the discharge into the model and the volume of water stored with the discharge out of the model. Results showed a difference of 0.6%. This is within the acceptable limits as stated in the Environment Agency's Fluvial Design Guide.

Whilst anecdotal information and available data has been used to the best extent possible, overall there is little or poor data to calibrate the model to and observation of more events would be necessary to reduce the uncertainty in model results.

(2) Public Consultation Comments and Response:

Following informal public consultation and formal S.I. public consultation periods in 2015, it was noted that the model required updating. The following updates to the model were carried out.

 A review of head loss factors on all culverts and weirs along the Gollymochy Stream;

 A review of the Manning’s n values for seven culverts along the Gollymochy Stream (289I, 233D, 209I, 111D, 97D, 70D & 24D);

 A review of the Manning’s n values for the cross-sections along the Gollymochy Stream.

These changes resulted in increased flood extents along the Gollymochy Stream – see Figure 4.4.27 model was updated and check flows recalculated with a revised set of flood hazard and risk mapping issued as Final to reflect these changes.

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Figure 4.4.27: Modelled Fluvial Flood Extents along the Gollymochy Stream

(3) Standard of Protection of Existing Formal Defences:

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

1 Culvert Cott (Butterstream) / 1% AEP

This defence is a below ground, culvert relief system.

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Figure 4.4.28: Culvert relief system on the Cott (Butterstream) at ch 4282m

Figure 4.4.29: Location of culvert relief system on the Cott (Butterstream)

(4) Gauging Stations:

There are no gauging stations located within the Clane model. A gauging station used to be located on the River Liffey near Straffan at the downstream extent of the model, however it has been

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(5) Other Information:

Local Authority consultation on 22nd May 2014 identified the control mechanisms in place within the relief scheme – preliminary maps reviewed at the workshop were subsequently updated to address these comments.

4.4.6 Hydraulic Model Assumptions, Limitations and Handover Notes

(1) Hydraulic Model Assumptions:

(a) A review of the roughness coefficients was carried out for all the watercourses in the model and extended into the out of bank areas along the 1D section of the River Liffey. Roughness values were based on survey data and from experience gathered from other calibrated models which required roughness adjustment.

(b) The culvert located in the Cott Stream at chainage 2338m starts as an arch but changes to a 1.5m diameter concrete pipe as shown in Figure 4.4.27. This culvert was represented in the model by the 1.5m diameter pipe, see Figure 4.4.28, as this was established as the critical geometry controlling the head loss through the structure.

Figure 4.4.30: Topographical survey of the upstream and downstream face of culvert 09COTT00155D

Figure 4.4.31: Model representation of culvert 09COTT00155D

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(c) Where secondary channels are present on the River Liffey and where an island is effectively created the cross sections extend over both channels to account for this, as shown in Figure 4.4.29.

Figure 4.4.32: Example of secondary channel in the River Liffey modelled in 1D element

However one secondary channel has been excluded from the model between chainage 52720m - 53088m as show in Figure 4.4.30. This stretch of the River Liffey has been designated as a MPW, as such the level of detail is lower than that of the HPW. Where features of the river impact on flood flows, levels and mechanism it is important to include them in the model, however the secondary channel, which splits at a near right angle to the flow of the river, does not principally convey flow or store water and is therefore not included it in the 1D part of the model. The presence of the secondary channel is accounted for within the 2D domain and flooding in and around this feature is accounted for in this way.

Figure 4.4.33: Example of secondary channel in the River Liffey modelled in 2D domain

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(2) Hydraulic Model Limitations and Parameters:

(a) A 2 second time-step for both the MIKE 11 and MIKE 21 models has been selected in order to achieve a successful model simulation for all return periods.

(b) An exponential smoothing factor of 0.2 and a lateral link roughness value of 0.07 was required to establish a stable model as the simulation passed from a the 1D model to the 2D model.

MIKE 11

Timestep (seconds) 2

Wave Approximation Fully Dynamic

Delta 0.85

MIKE 21

Timestep (seconds) 2

Drying / Flooding depths (metres) 0.02 / 0.03

Eddy Viscosity (and type) 0.25 (Flux Based)

MIKE FLOOD

Link Exponential Smoothing Factor 0.2 (Gollymochy Stream)

(where non-default value used) 0.2 (Cott Stream)

0.2 (River Liffey 46,252m - 46,955m)

Lateral Length Depth Tolerance (m) 0.2 (Gollymochy Stream)

(where non-default value used) 0.2 (Cott Stream)

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

The River Liffey inflow hydrograph represents both a flood scenario under normal conditions and also a dam release scenario from the Pollaphuca and Golden Falls reservoirs. There are therefore two flood events which take place within each model run.

The first flood effects the River Liffey and the tributaries, the Cott and Gollymochy Streams. The second flood (dam release) occurs on the River Liffey but also affects the downstream reaches of the two tributaries as flood water backs up both watercourses.

The Cott stream presents the greatest risk to the Clane AFA and shows the most out of bank flooding which affects flood receptors. Flow is controlled and flooding occurs due to a series of undersized culverts along the lane travelling through the Butterstream Commons as shown in Figure 4.4.31.

The culvert located on the Cott Stream at chainage 1408m (ref 09COTT00248I, see Section 4.4.3(1)) produces a significant head water upstream of it however minimal flooding occurs as a result during the 1% AEP event. Culvert 09COTT00202I has also been designated as a critical structure due to the restricted flow through it. There is minimal out of bank flooding at this location however.

At culverts 09COTT00178I & 09COTT00175I (see Section 4.4.3(1)) out of bank flood occurs on both

IBE0600Rp0027 4.4-28 Rev F03 Eastern CFRAM Study HA09 Hydraulics Report – DRAFT FINAL banks which then flows overland along the adjacent lane on the left hand bank and the gardens on the right. Near the junction with Prosperous road the overland flow is directed through the Butterstream Business Park and surrounding houses and school before crossing the Prosperous Road and finding its way back to the Cott Stream.

The flood alleviation scheme, which includes culvert 09COTT00138I at chainage 2482m (see Section 4.4.3(1)) can convey a 1% AEP event. A small amount of out of bank flooding occurs during the 0.1%AEP event but is contained to the field adjacent.

Figure 4.4.34: Location of critical structures and flood extents in Butterstream Commons area

The temporary culvert on the Cott Stream at chainage 3090m, downstream of Clane GAA club (see Section 4.4.3(1)) also causes some out of bank flooding which affects at least one property during the 1% AEP event.

The lower reach of the Cott Stream as it flows adjacent to Main Street seems to have adequate capacity during the 1% AEP event however during the dam release scenario the flow along the River Liffey as it reaches Clane backs up into the Cott Stream and out of bank flood occurs to the area around Main Street. During the 0.1% AEP event out of bank flooding occurs which affects Main Street, this is supplemented by overland flow from further upstream and also by the water backing from the River Liffey. The resulting flood extent can be seen in Figure 4.4.32.

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Figure 4.4.35: Location of flood extents in lower reach of the Cott Stream

Flooding occurs from the Gollymochy Stream however as it is a more rural stream there are less receptors that are affected. Culverts at chainage 1376m, 3107m and 4045m (see Section 4.4.3(1)) all restrict the flow and cause out of bank flooding. A shown in Figure 4.4.33 the most significant of these three culverts is at Higgins Lane, 09GOLL00120I, at chainage 3107m where at least one property is at risk during the 1% AEP event. Please refer to Section 4.4.5(2) for updates to the final model and flood extents.

Figure 4.4.36: Location of critical structure and flood extents at Higgin's Lane

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(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: Mark Wilson

Model Reviewed by: Stephen Patterson

Model Approved by: Andrew Jackson

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APPENDIX A.1 CRITICAL STRUCTURES

Spring Length Height Width Height from Manning’s Branch Chainage ID (m) Opening Shape (m) (m) invert (m) n 1 Cott 1153.95 09COTT00273I_culvert 21.31 Circular 0.75 - - 0.013 2 Cott 1409.36 09COTT00248I 172.17 Circular 0.75 - - 0.013 3 Cott 1731.73 09COTT00217I_culvert 84.75 Circular 1 - - 0.013 4 Cott 1866.083 09COTT00202I_culvert 9.2 Circular 1 - - 0.013 5 Cott 2025.9 09COTT00186I_culvert 17.29 Cross Section DB 0.794 1.18 - 0.013 6 Cott 2112.378 09COTT00178I_culvert 3.97 Cross Section DB 0.762 0.88 - 0.013 7 Cott 2137.44 09COTT00175I_culvert 7.63 Cross Section DB 0.694 1.56 - 0.013 8 Cott 2180.099 09COTT00171I_culvert 10.99 Cross Section DB 0.678 2.05 - 0.013 9 Cott 2217.816 09COTT00167I_culvert 12.2 Cross Section DB 0.727 1.7 - 0.013 10 Cott 2338.377 09COTT00155D_bridge 14.09 Circular 1.5 - - 0.013 11 Cott 2354 09COTT00152D_bridge 2.3 Cross Section DB 0.593 1.72 - 0.013 12 Cott 2501 09COTT00138I 434.68 Circular 0.85 - - 0.013 13 Cott 2403.822 09COTT00151I_culvert 60.19 Circular 0.85 - - 0.013 14 Cott 3090.206 09COTT00081I_culvert 5.17 Circular 0.9 - - 0.013 15 Cott 3181.124 09COTT00072I_culvert 53.7 Circular 1.5 - - 0.013 16 Cott 3418.809 09COTT00052D_bridge 7.75 Cross Section DB 1.458 3.69 - 0.013 17 Cott 3467.921 09COTT00048D_bridge 4.08 Cross Section DB 0.761 2.72 - 0.013 18 Cott 3542.417 09COTT00041I_culvert 13.75 Cross Section DB 0.98 1.82 - 0.013 19 Cott 3598.218 09COTT00035D_bridge 8.21 Cross Section DB 1.015 1.5 - 0.013 20 Cott 3660.555 09COTT00031I_culvert 58.32 Cross Section DB 1.346 3.03 - 0.013 21 Cott 3712.792 09COTT00023D_bridge 4.17 Cross Section DB 1.151 2.1 - 0.013 22 Cott 3743.188 09COTT00020D_bridge 4.21 Cross Section DB 1.25 2.1 - 0.013 23 Cott 3764.331 09COTT00018D_bridge 10.67 Cross Section DB 1.107 2.04 - 0.013

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Spring Length Height Width Height from Manning’s Branch Chainage ID (m) Opening Shape (m) (m) invert (m) n 24 Cott 3813.53 09COTT00014I_culvert 37.19 Cross Section DB 1.4 3.2 - 0.013 25 Cott 3924.072 09COTT00002D_bridge 1.31 Cross Section DB 1.976 5.39 - 0.013 26 Gollymochy (River) 1378.314 09GOLL00289I_culvert 5.41 Circular 1.05 - 0.017 27 Gollymochy (River) 1973.764 09GOLL00233D_bridge 11.25 Cross Section DB 1.88 1.56 1.105 0.017 28 Gollymochy (River) 2149.329 09GOLL00209I_culvert 5.45 Cross Section DB 1.2 2.25 - 0.014 29 Gollymochy (River) 3109.204 09GOLL00120I_culvert 4.85 Circular 1.2 - - 0.013 30 Gollymochy (River) 3188.594 09GOLL00111D_bridge 6.48 Cross Section DB 1.283 3.77 - 0.017 31 Gollymochy (River) 3328.741 09GOLL00097D_bridge 8.92 Cross Section DB 1.929 2.09 1.129 0.017 32 Gollymochy (River) 3608.07 09GOLL00070D_bridge 9.16 Cross Section DB 1.759 1.84 1.419 0.017 33 Gollymochy (River) 4057.317 09GOLL00024D_bridge 3.9 Cross Section DB 2.707 1.29 1.99 0.017 34 Liffey (River) 34294.97 09LIFF05551D_bridge 11.9 Cross Section DB 5.36 33.75 3.663 0.013 35 Liffey (River) 36934.28 09LIFF05282D_bridge 3.99 Cross Section DB 4.341 28.81 2.791 0.013 36 Liffey (River) 40335.43 09LIFF04942D_bridge 10.14 Cross Section DB 7.498 34.96 - 0.013 37 Liffey (River) 41027.95 09LIFF04872D_bridge 7.15 Cross Section DB 4.745 37.77 2.403 0.013 38 Liffey (River) 43844.42 09LIFF04592D_bridge 7.38 Cross Section DB 4.929 30.2 2.327 0.013 39 Liffey (River) 46897.32 09LIFF04293D_bridge 7.36 Cross Section DB 4.07 31.3 2.2 0.013 40 Liffey (River) 51749.94 09LIFF03815D_bridge 4.91 Cross Section DB 4.182 22.56 - 0.013 41 Liffey (River) 53578.39 09LIFF03629E_bridge 11.75 Cross Section DB 6.047 37.48 2.675 0.013

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APPENDIX A.2

LONG SECTIONS

River Liffey 0.1% AEP event

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River Liffey 0.1% AEP event

Confluence

Confluence Critical Structure chainage 53495m

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Cott Stream 0.1% AEP event

Critical Structure chainage 2482m

Critical Structure chainage 3090m Critical Structure chainage 1408m

Critical Structure chainage 1866m

Critical Structures chainage 2112m & 2137m Critical Structures chainage 2328m & 2354m Critical Structure chainage 3090m

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Gollymochy Stream 0.1% AEP event

Critical Structure chainage 4045m

Critical Structure chainage 1376m

Critical Structure chainage 3107m

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APPENDIX A.3 FLOWS

PEAK WATER FLOWS

AFA Name CLANE Model Code HA09_CLANE

Peak Water Flows

River Name & Chainage AEP Check Flow (m³/s) Model Flow (m³/s) Diff (%) COTT 1065.72 10% 0.64 0.66 3.28 09_1649_1_RPS 1% 1.18 1.22 3.05 0.1% 2.10 2.17 3.29 COTT 3924.07 10% 1.65 1.95 18.36 09_1649_9_RPS 1% 3.06 2.69 12.03 0.1% 5.45 5.08 6.72 GOLLYMOCHY (RIVER) 1282.84 10% 1.57 1.62 2.93 09_429_1 1% 2.91 3.09 6.09 0.1% 5.19 6.45 24.26 GOLLYMOCHY (RIVER) 4234.5 10% 3.72 3.24 12.85 09_200_2_RPS 1% 6.88 5.93 13.74 0.1% 12.27 7.09 42.22 LIFFEY (RIVER) 34746.27 10% 88.43 92.5 4.6 09_1519_16_RPS-ds 1% 120.58 125.04 3.69 0.1% 151.58 158.06 4.28 LIFFEY (RIVER) 53017.9 10% 93.17 82.94 10.98 09_1601_3_RPS 1% 125.72 105.77 15.87 0.1% 158.33 139.26 12.04

The table above provides details of flow in the model at every HEP inflow, check point and modelled tributary. These flows have been compared with the hydrology flow estimation and a percentage difference provided.

The modelled flows in the Cott Stream correlate well with the check flow at the upstream and downstream extents. It can be seen in the diagram below that overland flow occurs for the 1% and 0.1% AEP events, this has the affect of attenuating some flow and therefore results in a lower modelled flow compared with the check flow at the downstream extent. The effect of this is not as pronounced during the 0.1% AEP event as some of the flood water returns to the Cott Stream before the downstream extent. During the 1% AEP event however a higher proportion of the overland flow ponds and does not return to the watercourse. The result is a higher percentage difference between

IBE0600Rp0027 4.4-38 Rev F03 Eastern CFRAM Study HA09 Hydraulics Report – DRAFT FINAL the modelled and check flows for the 1% AEP event. Overall, with the largest percentage difference being 18.36%, we can be confident that the model is well anchored to the HEPs on the Cott Stream.

The 10% and 1% AEP modelled flows on the Gollymochy stream correlate well with the check flows with a difference of between 2.93 and 13.74% showing that the modelled is well anchored to the hydrological estimates. There is a greater discrepancy in the values for the 0.1% AEP with differences of 24.26 and 42.22%. For the check point 09_429_1, the model values are all higher due to a backwater effect from a flow restricting culvert downstream of this section. With greater flows, water backs up further and this explains the increasing percentage difference with progression through the increasing return periods for this check point. At the downstream check point, 09_200_2_RPS, model flows are lower than check flows, again becoming increasingly different with each increase in return period. This can be explained by the out of bank flooding which occurs upstream of this point, with most of this water not returning into the channel but rather ponding on land or flowing into the River Liffey.

The modelled flows in the River Liffey correlate well with the check flows at the upstream and downstream extents. While there are many trib HEPs along this reach of the River Liffey there are no other check flows on the River Liffey itself. The highest percentage difference between modelled and check flows, at 15.87%, is during the 1% AEP event at the downstream reach. Overall the flow percentage differences are relatively low and show that the model is well anchored to the HEPs on the River Liffey.

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APPENDIX A.4 FILES

MIKE FLOOD MIKE 21 MIKE 21 RESULTS HA09_CLAN6A _MF_DES_3_Q10 HA09_CLAN6A_M21_DES_3_Q10 HA09_CLAN6A_M21_DES_Q10_3 HA09_CLAN6A _MF_ DES_3 _Q100 HA09_CLAN6A_M21_DES_3_Q100 HA09_CLAN6A_M21_DES_Q100_3 HA09_CLAN6A _MF_ DES _3_Q1000 HA09_CLAN6A_M21_DES_3_Q1000 HA09_CLAN6A_M21_DES_Q1000_3 HA09_CLAN6A_DFS2_BATHY_3 HA09_CLAN6A_DFS2_FPR

MIKE 11 - SIM FILE & RESULTS FILE MIKE 11 - NETWORK FILE MIKE 11 - CROSS-SECTION FILE MIKE 11 - BOUNDARY FILE HA09_CLAN6A_M11_DES_3_Q10 HA09_CLAN6A_NWK_DES_9 HA09_ CLAN6A__XNS_DES_7 HA09_CLAN6A_BND_DES_Q10 HA09_CLAN6A_M11_DES_3_Q100 HA09_CLAN6A_BND_DES_Q100 HA09_CLAN6A_M11_DES_3_Q1000 HA09_CLAN6A_BND_DES_Q1000 MIKE 11 - DFS0 FILE MIKE 11 - HD FILE & RESULTS FILE HA09_CLAN6A_DFS0_0.1%AEP HA09_CLAN6A_HD_DES_Q10 HA09_CLAN6A_DFS0_1%AEP HA09_CLAN6A_HD_DES_Q100 HA09_CLAN6A_DFS0_10%AEP HA09_CLAN6A_HD_DES_Q1000 HA09_CLAN6A_DFS0_0.1%AEP_ NewbridgeInput_1 HA09_CLAN6A_DFS0_CELBRIDGE_WL

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GIS DELIVERABLES - HAZARD FLOOD EXTENT FILES (SHAPEFILES) FLOOD DEPTH FILES (RASTER) WATER LEVEL AND FLOWS (SHAPEFILES) FLUVIAL FLUVIAL FLUVIAL E13EXFCD100F0 E13DPFCD100F0 E13NCCDF0 E13EXFCD010F0 E13DPFCD010F0 E13EXFCD001F0 E13DPFCD001F0

FLOOD ZONE FILES (SHAPEFILES) FLOOD VELOCITY FILES (RASTER) FLOOD DEFENCE FILES (SHAPEFILES) E13ZNFCD100 E13VLFCD100F0 E13ZNFCD010 E13VLFCD010F0 E13ZNFCD001 E13VLFCD001F0

GIS DELIVERABLES - RISK SPECIFIC RISK - INHABITANTS (RASTER) GENERAL RISK - ECONOMIC (SHAPEFILES) GENERAL RISK-ENVIRONMENTAL (SHAPEFILES) FLUVIAL ONE UoM MAP E13RIFCD100F0 E13RIFCD010F0 E13RIFCD001F0

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