mdw0484 rp0002a04

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DOCUMENT CONTROL SHEET

Client: Dublin City Council

Project Title: Santry River Flooding at Raheny

Document Title: Assessment and Solutions Report

Document No: MDW0484Rp0002

DCS TOC Text List of Tables List of Figures No. of Appendices This Document Comprises:

1 1 25 - - 3

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

A01 Approval J Hobbs R Kane

Jean Hobbs Jerry Grant West Pier 18 Jun 2012







Santry River Flooding at Raheny

Assessment and Solutions Report

Santry River Flooding at Raheny Assessment & Outline Solutions Report

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TABLE OF CONTENTS

1 INTRODUCTION........................................................................................................................ 1

1.1 INTRODUCTION ................................................................................................................ 1

1.2 SANTRY RIVER CATCHMENT............................................................................................. 1

1.3 PREVIOUS AND ONGOING STUDIES ................................................................................... 4

2 FLOOD EVENT OCTOBER 24 TH 2011...................................................................................... 6

2.1 INTRODUCTION ................................................................................................................ 6

2.2 RAINFALL EVENT ............................................................................................................. 6

2.3 FLOODING AT RAHENY ..................................................................................................... 8

3 TOPOGRAPHICAL SURVEY ............................... ................................................................... 11

3.1 TOPOGRAPHICAL SURVEY DATA ..................................................................................... 11

4 HYDRAULIC ASSESSMENT ............................... ................................................................... 12

4.1 INFOWORKS CS MODEL ................................................................................................. 12

4.2 1-DIMENSIONAL MIKE 11 MODEL................................................................................... 14

4.3 2-DIMENSIONAL MIKE 21 MODEL................................................................................... 15

4.4 MIKE FLOOD COUPLED HYDRAULIC MODEL ................................................................. 16

4.5 MODEL CALIBRATION ..................................................................................................... 16

4.6 MODELLED SCENARIOS.................................................................................................. 18

5 CONCLUSIONS AND RECOMMENDATIONS .................... ................................................... 19

5.1 SUMMARY CONCLUSIONS ............................................................................................... 19

5.2 OUTLINE RECOMMENDED OPTIONS ................................................................................. 20

APPENDICES

APPENDIX A EPA Preliminary Assessment - Selected Catchments

APPENDIX B Topographical Survey Drawings – Februar y March 2012

APPENDIX C Flood Maps


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1 INTRODUCTION

1.1 INTRODUCTION

Following widespread, heavy rain on 24th October 2011 and subsequent flooding events almost country-wide, but particularly in the eastern and northern parts of Ireland, RPS was commissioned by Dublin City Council (DCC) to assess and report on the flooding incident which occurred on the Santry River at Raheny Village on the night of October 24th 2011. This assessment meets the following objectives:-

• Description of flood event of the 24th October 2011 including floodplain extents, flow paths, properties flooded – refer Section 2 ;

• Review and hydraulic assessment of the existing river using detailed hydraulic modeling techniques, using updated topographical and LiDAR survey data – refer Section 3 ;

• Conclusions reached during detailed hydraulic assessment and summary of typical flood management options that would need to be considered in further detail in order to reduce flood risk at Raheny – refer Section 4 and 5 .

1.2 SANTRY RIVER CATCHMENT

The Santry River catchment lies to the north of Dublin City, with the river flowing south-eastwards from the airport to Dublin Bay, discharging opposite Bull Island. The catchment, which has an area of approximately 1,360ha, is 11km long and typically about 1km wide.

Approximately 60-70% of the catchment is urbanised – mainly residential, but with some large industrial areas, particularly either side of the M50 motorway. There are also recreational areas, many adjacent to the river. Upstream of the M1 the catchment is predominantly rural, but with a lot of recent development between the M50 and the M1. Whilst all of the rural areas are within the Fingal County Council administrative area, the majority of the urban sections are within Dublin City Council’s area.

Most of the river flows in steep sided open channel, with short culverts and bridges at road crossings. Due to the layout of the catchment there are many small and medium sized surface water drainage networks discharging into the river.

The river commences near Dublin Airport and flows in a south-easterly direction, crossing the M50 at junction 4. It then flows through open ground and recent development, before entering the main urban area upstream of Swords Road. The Santry crosses under the M1 and continues eastwards, past various sports grounds and parks. The river then flows parallel to Greencastles and Springdale Roads, through more parkland.

The river downstream of the DART crossing flows through the older, more urbanised area of Raheny, where the channel is wide and shallow and is a mixture of natural channel and concrete sides – constructed because properties are located close to the river. In this area it passes under Howth Road and Main Street. The Santry River continues in a south-easterly direction, and near the church at The Village, off Watermill Road, it enters a 210m long culvert, 3.5m wide by 1.8m high.

After a further brief section of open channel, with wooded banks, the river enters a final section of culvert, which passes under James Larkin Road. The culvert, which is 130m long and has twin sections 2.3m wide by 1.3m high, discharges into Dublin Bay, just north of the western end of Causeway Road – the link to Bull Island. Only this downstream part of the river is affected by tidal conditions.


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There have been several occurrences of flooding in the flood plain in the Harmonstown Road area and also in parkland upstream of the DART crossing at Raheny. This is associated with the flooding downstream of the DART bridge at Raheny where historically the double culvert under Main Street, has regularly become silted, restricting the flow. Flooding of nearby roads and shops in the Raheny village has occurred on several occasions in the past – typically once every 5 years. Most recently the Raheny Village area was flooded on 24th October 2011. Extreme rainfall events in August 2008 and July 2009 flooded premises on Howth Road and supported a request to the OPW for funding for flood defences.

The Santry River, its main tributaries and its catchment boundary, are presented in Figure 1-1 overleaf.


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Figure 1-1 Santry River Catchment

Raheny: Location of flooding on 24th October 2011


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1.3 PREVIOUS AND ONGOING STUDIES

1.3.1 GDSDS

Previous flooding assessment work has been conducted on the Santry River Catchment through the Greater Dublin Strategic Drainage Study (GDSDS) 200 1 – 2005. During this study a type 1 (all nodes) river and surface water model was built for the Santry River in InfoWorks, using 2002 asset and topographical survey data.

The InfoWorks CS model was used to identify pressure points on the existing system and to demonstrate if increasing the overall capacity of the system to accommodate existing and future flows is feasible in areas where growth is expected to cause a problem. The results of this model assessment produced Drainage Development Options (DDO) for the Santry River Catchment in the GDSDS Final Report 2005.

DDO recommendations in the GDSDS to overcome local flooding from the piped network included upsizing existing pipes and laying new pipes to transfer flows to different parts of the system. Improvements were designed to the 2031 Scenario, which provided for increased flows arising from development and the effects of climate changed.

Two flood plain deficiencies were identified during the GDSDS, based on predicted flooding for 100 year design storms. DDOs to overcome the river flooding problems are as follows:

• removal of a weir;

• clearing existing channels – in particular a silted culvert in Raheny;

• attenuation of river flows, with storage upstream of Harmonstown Road.

It should be noted that because of the nature of the strategy study it will was recommended that it would be necessary to consider the needs for and the details of the DDOs further, prior to developing any improvements. This was deemed particularly important at the time, due to some uncertainties in the system layout and in the contributing areas. It was recommended in the GDSDS Final Report that further site investigations be carried out, to obtain more detailed information and to enable the model to be updated.

1.3.2 ECFRAM

The Santry River Catchment is also within the remit of the Office of Public Works (OPW) recently commissioned Eastern Catchment Flood Risk Assessment and Management Stud y (CFRAMS), due for completion in 2016.

RPS have been appointed as Consultants for the Eastern CFRAM study. Around 1.6 million people, 40% of Ireland’s population, live in the Eastern district with the majority living in the Greater Dublin Area. The National Catchment Flood Risk Assessment and Management (CFRAM) Programme was developed to meet the requirements of the EU Floods Directive (2007/60/EC), as well as to deliver on core components of the 2004 National Flood Policy.


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The main aims of the Eastern CFRAM Study are to:

• assess flood risk, through the identification of flood hazard areas and the associated impacts of flooding;

• identify viable structural and non-structural measures and options for managing the flood risks for localised high-risk areas and within the catchment as a whole;

• prepare a strategic Flood Risk Management Plan (FRMP) and associated Strategic Environmental Assessment (SEA) that sets out the measures and policies that should be pursued to achieve the most cost effective and sustainable management of flood risk;

• ensure that full and thorough public and stakeholder consultation and engagement is achieved.

The Eastern CFRAMS will therefore assess entire Santry River Catchment in a holistic way rather than concentrating on one select area. Under normal circumstances, the current ongoing Eastern CFRAM would inform on the benefits or otherwise of providing flood protection for the Raheny area. However, a severe flood event occurred in Oct 2011 and any solutions proposed would have to cater for this happening again, as a minimum. These outline solutions have been put forward in our report and must also be taken into consideration during the ongoing ECFRAMS.


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2 FLOOD EVENT OCTOBER 24TH 2011

2.1 INTRODUCTION

Following the widespread heavy rain on 24th October 2011 and subsequent flooding events almost country-wide, but particularly in the eastern and northern parts of Ireland, the Dublin Regional Inspectorate of the Environmental Protection Agency (EPA) produced a report entitled “A Preliminary Assessment of the Flooding Which Occurred on 24 October 2011 in Selected Catchments on the East Coast” - November 2011. Unfortunately the EPA preliminary assessment did not refer to the Santry River Catchment.

However, the EPA report does give a clear indication of flood flows experienced in other rivers on the same day in Dublin City - such as the Tolka and Dodder River at Botanic Gardens and Waldron’s Bridge respectively. There are similarities between the Santry and Tolka River Catchments, in that they would have some comparable catchment characteristics (in the urban context). Refer to Appendix A for further detail. Peak flow on the Tolka River recorded on 24th October 2011 did not reach peak levels recorded during the extreme flooding events of 2001 and 2003. It also appears that river catchments in south Dublin and county Wicklow experienced more extreme rainfall on 24th October 2011.

2.2 RAINFALL EVENT

During Monday 24th October 2011, a spell of very heavy rainfall, brought about by a slow moving frontal depression centred over Ireland, affected eastern parts of the country, in particular the Greater Dublin Area. Met Eireann produced a report on this heavy rainfall incident, concluding that it was particularly severe in the Dublin area due to the Wicklow Mountains causing a process called orographic uplift, in addition to coastal convergence of isobars off the east coast – forcing moist air to rise, condense and then fall as rain.

It is noted that Met Eireann’s weather station at Casement Aerodrome, Baldonnell, Co. Dublin (approximately 30km south of the Santry River Catchment) set a new record of 82.2mm of rain on 24th October 2011 – the greatest daily total for the month of October since records began there in 1954. The majority of the rainfall is recorded to have fallen between 2pm and 8pm, with 65.7mm falling between 3pm and 7pm. According to Met Eireann, this 4-hour interval of heavy rainfall corresponds approximately to a 1 in 80 year event. . In addition, 23mm of rain was recorded in one hour (from 4 - 5pm) – giving the wettest hour on record at Casement Aerodrome.

In comparison, Dublin Airport weather station, adjacent to the Santry River Catchment recorded a daily rainfall total of 69.1mm for 24th October 2011, with 42.9 mm falling in a 4-hour interval, corresponding approximately to a 1 in 40 year rainfall event. In addition, rainfall readings from weather station within the Dublin City Council administrative area at Grange, recorded, between 5pm and 9pm on 24th October 2011, rainfall totalling at 45.8mm. Refer to Table 2.1 overleaf for further detail. It is worth noting however that 24mm of rain was recorded in one hour (from 7 - 8pm) at Grange, slightly more than the wettest hour on record (since records began 1954) at Casement Aerodrome, but with a time lag of 3 hours.

Initial analysis of the available measurements resulted in the EPA concluding that although significant amounts of rainfall affected many areas in Eastern and Northern parts of Ireland, the greater Dublin Area received by far the most rainfall. Met Eireann weather stations at both Dublin Airport and Phoenix Park recorded lesser intensities of rainfall than Casement Aerodrome, indicating that south Dublin, due to its proximity to the Wicklow Mountains, may have been more severely affected by the rainfall than north Dublin.

However, as noted above, Grange weather station’s wettest hour recorded (24mm) exceeded the Casement Aerodrome wettest hour (23mm) recorded on 24th October 2011. This demonstrates the spatial variation in intensity of rainfall that can occur in storm events within catchments.


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Table 2.1 outlines the hourly rainfall recorded at the Grange Tank Weather Station.

Gauge Location GRANGE Date/Time mm

23/10/2011 12:00 0 23/10/2011 13:00 2 23/10/2011 14:00 2.6 23/10/2011 15:00 0.6 23/10/2011 16:00 0.8 23/10/2011 17:00 0.6 23/10/2011 18:00 0.8 23/10/2011 19:00 1.8 23/10/2011 20:00 1.4 23/10/2011 21:00 0.6 23/10/2011 22:00 1.8 23/10/2011 23:00 1.4 24/10/2011 00:00 2 24/10/2011 01:00 0.4 24/10/2011 02:00 0.2 24/10/2011 03:00 0 24/10/2011 04:00 0 24/10/2011 05:00 0.2 24/10/2011 06:00 0 24/10/2011 07:00 0 24/10/2011 08:00 0 24/10/2011 09:00 0 24/10/2011 10:00 0 24/10/2011 11:00 0.4 24/10/2011 12:00 1.8 24/10/2011 13:00 1.8 24/10/2011 14:00 4.6 24/10/2011 15:00 8.2 24/10/2011 16:00 3.8 24/10/2011 17:00 4 24/10/2011 18:00 4.6 24/10/2011 19:00 8.4 24/10/2011 20:00 24 24/10/2011 21:00 8.8 24/10/2011 22:00 0.2 24/10/2011 23:00 0 25/10/2011 00:00 0

Rain over 4hr Period 5pm to 9pm 45.8 Rain over 6hr Period 2pm-8pm 53.6

Total Rain over 24hrs 76.4 Total Rain over 32hrs 85.6

Table 2.1 – Rain Gauge Readings for 23rd & 24th Oct ober 2011 (data provided by DCC)

It is worth noting too, that the heavier rainfall is recorded to have fallen over a 4 to 6 hour duration. The critical duration storm for the Santry Catchment at Raheny has been identified by hydraulic modelling (GDSDS) as 4 hours . The critical duration storm is defined as the rainfall duration which influences the maximum peak runoff rate in a river. Therefore, it can be concluded that winter rainfall conditions on the 24th October 2011, on an already saturated catchment, were characteristic of storm conditions likely to cause maximum flood flow conditions at Raheny.


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2.3 FLOODING AT RAHENY

During Monday 24th October 2011, a spell of very heavy rain fell on the entire Santry River Catchment. The Howth Road and Raheny Main Street areas adjacent to the Santry River experienced flooding as a result of this intense rain, which fell on an already saturated catchment. The catchment was already saturated due to preceeding wet weather conditions over the week and weekend leading up to the 24th October 2011 – resulting in a higher than normal amount of rainfall runoff flowing straight to the river rather than being naturally attenuated by the landscape.

Immediately after the flood event of 24th October 2011, RPS, as the OPW’s Consulting Engineer for the Eastern Catchment Flood Risk Assessment and Management Study (CFRAMS) project, began collecting information on the flood event in areas along the east coast, including the Santry River at the aforementioned flood affected locations. Figure 2-1 below shows information collated regarding the extent of the Santry River flood event on 24th October 2011 at Raheny.

Figure 2-1 Flood Event at Raheny Village 24 th October 2011 – Santry River

RPS staff also carried out site visits to Raheny in the weeks following the flooding event, in order to carry out an assessment of events. This included liaising with Dublin City Council personnel and a number of flood-affected residents. As a result of these accounts we can report on the approximate mechanisms of the flooding event on October 24th 2011 at Raheny as follows:-

• Commercial properties on Howth Road were flooded as a result of a boundary wall collapse which allowed floodwaters to enter the access laneway and flood premises from the rear. This resulted in flooding to basements and ground floors.

• The ground floor of the apartment complex (and the rear gardens of several properties) on Watermill Road flooded as a result of both collapsed walls and the absence of impermeable walls between properties.


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• Properties on the northern bank of the river at Main Street flooded due to overtopping of the river bank.

• Number 14 Main Street flooded as a result of a collapsed wall beside the house.

• Properties on the downstream side of Main Street flooded due floodwaters escaping from the river on eastern side of Main Street. Floodwaters then overtopped the southern bank of the river and escaped via the collapsed wall beside number 14. Floodwater also overtopped the northern river bank and escaped via access lane between premises No. 8 and No. 10. Floodwaters then crossed Main Street and flooded 7 No. premises. In addition the absence of impermeable boundary walls on the downstream side of the river may have contributed to flooding at these premises.

Figure 2-2 Collapsed Boundary Wall - Santry River d /s of Howth Rd Culvert - Raheny


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Figure 2-3 Entrance to Main Street Culvert - Raheny

As a result of this flood event, the ground floors of 6 no. properties on Howth Road, 1 no. apartment block on Watermill Road and 10 no. properties on Main Street Raheny were inundated to a level of approximately 300mm, including a pre-school/crèche facility.


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3 TOPOGRAPHICAL SURVEY

3.1 TOPOGRAPHICAL SURVEY DATA

A local topographical survey was commissioned in February 2012 to assist RPS with the assessment of the flooding incident at Raheny, in particular to enable full comparison between the detailed design proposals for the flood management works and the present-day structures at the same location. The surveys, undertaken by Land Surveys Ltd., included:

• Detailed cross-section surveys along the Santry River every between Main Street and Howth Road;

• Survey of control structures within the river, such as the Harmonstown Road, Howth Road and Main Street Bridges, including upstream and downstream cross sections.

The survey data provided to RPS in March 2012 included:

• AutoCAD section drawings of all cross-sections.

• AutoCAD elevation drawings of all control structures.

• AutoCAD plan drawings of the Stream/Rivers showing cross-section locations and spot heights.

• ASCII files data of all cross-sections for use in the hydraulic model.

The scope of the topographical survey is shown in Figure 3-1 below. Detailed Topographical Survey drawings, produced by Land Surveys Ltd, are included in Appendix B.

Figure 3-1 Scope of Topographical Survey undertaken in February 2011


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4 HYDRAULIC ASSESSMENT

4.1 INFOWORKS CS MODEL

As discussed earlier, the GDSDS produced a calibrated InfoWorks CS model of the Santry River catchment. This model was used to calculate the run-off to the river from various rainfall events. The 2011 fully developed catchment model was chosen and was used to generate input flows into a new truncated MIKE 11 model of the Raheny Area. Flow hydrographs and water levels were extracted from the InfoWorks model and used as boundary conditions in the MIKE 11 model.

4.1.1 Rainfall

Recorded rainfall data from the rain gauge situated at the Grange Tank was used to simulate the October 24th 2011 storm. Design storm events were generated within InfoWorks using the UK rainfall generator tool. This method is based on local parameters selected from the Ireland maps in the Flood Studies Report(1975).

• M5-60 (5 yr 1 hr rain): 16.5mm • Rainfall ratio: 0.3 • Catchment area: 1227Ha • NAPI: Summer: 15, Winter: 25 • Antecedent depth: 10 • Location: England & Wales (generally considered to be appropriate for

Dublin)

This tool was used to simulate the Q100 event. Events with an additional 10% and 25% rainfall were also generated using this same tool to account for future scenarios with climate change. Critical duration of the downstream end of the system was determined in the GDSDS study to be 240 minutes so this was chosen as the storm duration for these simulations.

Heavy rainfall in the days leading up to the October 24th event would have contributed to higher than normal run-off to the Santry River. Because of this, rainfall data taken from midday on the 22nd of October at the Grange Tank rain gauge was used in order to accurately represent the catchment response to the October 24th event. A truncated flow hydrograph (below) was then extracted from the InfoWorks model and used as the main boundary input at the start of the MIKE 11 model. The peak flow at the upstream boundary of the model was 10.11m3/sec.


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Fig 4.1: Model Input Hydrograph upstream of Harmons town Road for October 24 th Event.

In addition to the upstream boundary flow hydrograph, 7 other inflow hydrographs from the surrounding surface water catchments between Harmonstown Road and Main Street were included in the model. These were extracted from InfoWorks CS and inserted into the MIKE 11 model at the relevant locations. The peak flow at the Main Street culvert during the October 24th event was 11.24m3/sec.

Fig 4.2: Inflow Hydrographs for subcatchments for O ctober 24 th Event.


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4.2 1-DIMENSIONAL MIKE 11 MODEL

A 1-D hydrodynamic model was created in MIKE 11 for the Santry River in the vicinity of Raheny village. The MIKE 11 model comprises:

• A Simulation file which contains details of the simulation period, timestep and provides a link to other MIKE 11 files.

• A Network file containing the location of the river channel and any branches and details of hydraulic structures on the river (culverts, bridges, etc).

• A Cross-Section file containing all river channel cross-sectional information. This information was taken from the topographical survey data prepared for this study. The roughness values assigned to the river channel varied from a Manning’s n of 0.035 to 0.08 based on visual inspection.

• A Boundary file containing all boundary conditions applied to the model. For the model created in this study the boundary editor file contains the steady state flow inputs calculated in section 2.2.

• A Hydrodynamic file containing details of the hydrodynamic parameters adopted in the simulation.

The Santry River 1-D model was constructed using the data from the topographical survey which was carried out as part of the works. It was augmented by cross section data from the InfoWorks CS model. The total length of the modelled river is approximately 1.5km. The model was generated from 20 no. cross-sections and includes 4 culvert/weir structures.

Figure 4.3 presents the 1-D model extent as well as identifying the boundary locations.


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Figure 4.3 1-D Model extents and Boundary Location s

4.3 2-DIMENSIONAL MIKE 21 MODEL

A 2-D hydrodynamic model was established in MIKE 21 for the Santry River. The 2-D model domain used in the MIKE 21 model is based on the LiDAR data made available for this study by DCC. This data provides details of the river floodplains and facilitates modelling of out of bank flow. The model domain is approximately 1.7km x 1.4km in size with cell dimensions of 2m x 2m.


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Fig 4.4: 2-D Model Domain

4.4 MIKE FLOOD COUPLED HYDRAULIC MODEL

A MIKE FLOOD model was established for the Santry River. This MIKE FLOOD Model coupled the MIKE 11 (1-D) and MIKE 21 (2-D) models producing a hydrodynamic model that is fully dynamically linked, exchanging data in each time step of the simulation.

The links between the 1D/2D model were established as lateral links for the entire length of the model. Once the water level in the MIKE 11 model exceeds the bank height it spills into the MIKE 21 model along these lateral link boundaries. Links were established for the left bank and the right bank separately, to take account of the different elevations of each bank. Sections that do not allow overspill into the flood plain, such as walled bridges and culverts, were not coupled to the 2D model. The lateral links are modelled as weir structures and the depth of water at the crest at which spill occurs is set at 0.1m for all models.

4.5 MODEL CALIBRATION

For calibration purposes, the coupled hydraulic model was run for the October 24th 2011 event. Rainfall records of this event were collected from Grange Storage Tank which is maintained by DCC. The rain event was input into the existing GDSDS Infoworks CS Santry River model. Boundary data for the MIKE Flood model was extracted from the Infoworks CS model. The 2-D floodplain output from this model was compared against flooding recorded in the Raheny area by DCC staff after the October 24th 2011 event (Figure 4.6 and Figure 4.7 below).


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Fig 4.5: Fallen tree in Santry River (40m upstream of Main Street)

The initial model runs predicted less flooding than was observed on 24th October 2011. However, a significant tree blockage (Figure 4.5 ) was discovered during a site visit by RPS staff in between Howth Road and Main Street. This blockage was subsequently modelled and after an iterative process, good correlation between the observed and modelled floodplains was achieved.

Fig 4.6: Observed October 24 th 2011 flood extents

Fig 4.7: Modelled October 24 th 2011 Floodplain


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4.6 MODELLED SCENARIOS

After a reasonable level of calibration was achieved for the October 24th 2011 event, a number of scenarios were modelled in MIKE FLOOD. These included:

• October 24th event without the tree blockage

• 1% AEP (Annual Exceedance Probability) + 10% additional rainfall event

• 1% AEP + 25% additional rainfall event

• Solution scenarios for the above design storms

The GDSDS report suggested that the flooding at Raheny could be mitigated by providing in-line storage upstream of Harmonstown Road. RPS assessed this option and also examined the possibility of providing flood plain storage at the playing fields between the DART Bridge and Harmonstown Road in addition to minor flood defence works along the channel between Howth Road and Main Street.


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5 CONCLUSIONS AND RECOMMENDATIONS

5.1 SUMMARY CONCLUSIONS

5.1.1 October 24 th 2011 event

After carrying out a hydraulic assessment of the Santry River at Raheny RPS can conclude the following regarding the October 24th 2011 event:

1. Flooding at Raheny Village was caused by a lack of capacity in the River channel to convey flows downstream. This was exacerbated by a fallen tree in the river which reduced the conveyance further and artificially raised flood levels. A combination of bank overtopping and the collapse of the boundary walls led to a number of properties being flooded.

2. The volume of the floodplain between Main Street and Howth Road has been estimated at 2,300m3. 1

3. The peak flow in the river immediately upstream of the Main Street culvert was 11.24m3/s at 8.00pm

4. Top water level in the river immediately upstream of Main Street was 14.67mOD.

5.1.2 1% AEP (including 10% additional rainfall) ev ent

After carrying out a hydraulic assessment of the Santry River at Raheny RPS can conclude the following regarding the 1% AEP (including 10% additional rainfall) event:

1. Flooding at Raheny Village would be caused by a lack of capacity in the River channel to convey flows downstream. A combination of bank overtopping and the absence of impermeable boundary walls would lead to a number of properties being flooded.

2. The volume of the floodplain between Main Street and Howth Road has been estimated at 1800m3.

3. The peak flow in the river immediately upstream of the Main Street culvert was 12.08m3/s


5.1.3 1% AEP (including 25% additional rainfall) ev ent

After carrying out a hydraulic assessment of the Santry River at Raheny RPS can conclude the following regarding the 1% AEP (including 25% additional rainfall) event:

1. Flooding at Raheny Village would be caused by a lack of capacity in the river channel to convey flows downstream. A combination of bank overtopping and the absence of impermeable boundary walls would lead to a number of properties being flooded on Main Street and Howth Road.

2. The volume of the floodplain between Main Street and Howth Road has been estimated at 2,800m3.

3. The peak flow in the river immediately upstream of the Main Street culvert was 12.46m3/s

1 The floodplain volume includes the artificially higher water levels resulting from the fallen tree in channel which has since been removed by DCC.


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5.2 OUTLINE RECOMMENDED OPTIONS

RPS recommends providing attenuation on the Santry River upstream of the DART Bridge. This is considered to be the most viable option as the Santry River already has in this area an undeveloped floodplain. To do so, a throttle would have to be installed to limit the flows through the channel and to hold back flows in the river floodplain. Two possible throttle combinations were modelled and preliminary costings have been presented. The first being a 1200mm diameter pipe and retaining wall at the Harmonstown Road culvert, the second being a 1200mm diameter pipe and embankment at the pedestrian river crossing of the Santry River at the playing fields upstream of the DART Bridge. Results of these simulations are presented in Appendix C.

5.2.1 Option A: Throttle adjacent to Lein Gardens

This solutions involves a 1200mm diameter throttle pipe being installed in the river at the existing footpath crossing of the Santry River adjacent to Lein Gardens. Throttling flows at this location will allow the utilization of the basin-like natural topography of the playing fields for flood storage. It also means that a large volume can be stored at relatively shallow depths. This will also require the construction of an embankment which would be constructed along the line of the existing footpath to create a dam to store the water. An embankment of 2.5m height above existing top of bank level would provide additional storage of approximately 23,000m3 during the 1% AEP + 25% Rainfall event and would not require the excavation of large quantities of spoil. The peak water level immediately upstream of the Main Street culvert in Raheny for this solution is 14.507mOD.

Figure 5.1 Proposed Throttled Floodplain at playing fields for 1% AEP + 25% Rainfall Event


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5.2.2 Option B: Throttle at Harmonstown Road

RPS propose that a 1200mm diameter throttle pipe is installed in the river upstream of Harmonstown Road culvert. Throttling flows at this location will allow the utilization of the entire river channel for storage. In the current situation, the water depth in the river channel for the Q100 +25% flows is 19.69mOD whereas it is 22.86mOD for this proposed throttled solution. A possible additional storage of 62m2 per m length of river can thus be achieved within the river channel if the flows are throttled. This solution will require the construction of a weir wall (max height 4m) within the river channel. This option will provide additional storage of 26,600m3. Flood waters will be stored within the steep natural floodplain channel and as a result flood depths will be greater. The peak water level immediately upstream of the Main Street culvert in Raheny for this solution is 14.198mOD.

Figure 5.2 Throttled Floodplain at Harmonstown Road for 1% AEP + 25% Rainfall Event

Flow Hydrographs at Raheny for the 1% AEP +25% rainfall event for the existing scenario and for both attenuation options as are presented below in Figure 5.3. Peak flows at Main Street Raheny have reduced from 12.9 m3/s for the existing scenario to 10.9 m3/s for Option A and 9.2 m3/s for Option B.

The difference in peak flows at Raheny between Option A and Option B arise due to the amount of storage volume available at each throttle respectively. For Option A, storage is limited by the height of the embankment which would have to be built to create a dam to store the water in the playing field opposite Lein Gardens. For the purposes of this report, this has been limited to 2.5m with a view to the buildability of such a structure. For Option B, the throttling flow to a higher level (4m) is practical due to the steep natural floodplains which exist between the playing fields upstream of Harmonstown Road. As such Option B provides greater storage and thus lower downstream flood levels than Option A.


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0

2

4

6

8

10

12

14

12:00 13:00 14:00 15:00 16:00

Time

Flo

w (m

3/s)

Existing Option A (weir & embankment) Option B (weir)

Figure 5.3 Flow Hydrographs at Raheny for 1% AEP + 25% Rainfall Event

RPS also recommends the following measures be undertaken:

• The realignment of the Santry River immediately upstream of the Main Street culvert inlet. This is to remove both 45° bends that immediately before the culvert and thus reduce head losses at the culvert inlet. This will also ensure that during ordinary flow conditions, water will flow equally through both culvert barrels which will reduce the likelihood of one of the culvert barrels silting up. The Main Street culvert should also be cleaned to remove all the silt deposits that have collected within the culvert.

• The construction of a 70m long earth embankment and wall structure on the northern side of the Santry River from the Main Street culvert to the existing steep embankment approximately 50m downstream of the Howth Road culvert outlet. The wall/embankment shall be between 0.5m and 1.5m high and shall prevent water escaping out of the river channel and flowing onto Main Street in Raheny.

• An earth embankment should be constructed between Main Street culvert and Watermill Court apartments rear boundary fence (Approx 35m) on the southern river bank. All collapsed boundary/floodwalls on the southern river bank should also be reconstructed. These walls should be built to a level of at least 500mm above the top water level for the 1% AEP + 25% rainfall event. Existing walls must be capable of safely withstanding the top water levels for the above event. If this is not the case, they should be rebuilt.

• Clearance of the river channel and banks in between Howth Road and 25m downstream of the Main Street culvert in Raheny. There are significant amounts of debris and low-lying vegetation overgrowing into the channel in this section of the stream. Clearance of this vegetation could reduce the risk of blockages in the channel, reduce associated friction losses and increase the flood carrying capacity of the watercourse.


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• Replacing rear boundary walls of properties No. 17 to No. 29, Main Street with solid a flood wall between 800mm and 1m in height. This will prevent water flowing into these properties through their rear gardens.

Figure 5.4 Proposed Additional Flood Defences at Ra heny

5.3 COST ESTIMATES

Table 5.1a and 5.1b below show a summary of cost estimates for the outline solutions proposed at Harmonstown Road and Raheny. In addition to the estimated construction costs below, some provision should be made within the DCC budget to provide for inspection and routine maintenance of the river environment at the proposed throttle and between Main St and Howth Road


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5.3.1 Option A - Throttle Adjacent to Lein Gardens and Associated Flood Defences

Table 5.1a

Description (Option A) Length Cost Estimate (€)

Rebuild boundary walls and localised strengthening of existing walls (Southern Bank)

- 20,000

Earth embankment (Southern Bank) 35m 16,625

RC wall on northern bank (1-1.5m high) 15m 18,965

Earth embankment (Northern Bank) 55m 20,900

RC wall downstream Main Street 35m 44,250

2.0m embankment along footbridge 90m 42,750

1200mm diameter throttle at footbridge (including headwalls)

- 15,000

Local realignment and clearance of Santry River between Howth Road and 25m downstream of Main Street culvert

- 7,500

Construction Costs € 185,990

Detail Design Costs € 27,898

Option A Total Costs € 213,888


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5.3.2 Option B – Throttle at Harmonstown Road and A ssociated Flood Defences

Table 5.1b

Description (Option B) Length Cost Estimate (€)

Rebuild boundary walls and localised strengthening of existing walls (Southern Bank)

- 20,000

Earth embankment (Southern Bank) 35m 16,625

RC wall at culvert (Northern Bank) 15m 18,965

Earth embankment (Northern Bank) 55m 20,900

RC wall downstream of Main Street 35m 44,250

500mm embankment at Harmonstown Road 20m 3,000

1200mm diameter throttle at Harmonstown Road culvert (including headwall)

- 20,000

Local realignment and clearance of Santry River between Howth Road and 25m downstream of Main Street culvert

- 7,500

Local Landscaping upstream of Harmonstown Road - 5,000

Construction Costs € 156,240

Design Costs € 23,436

Option B Total Costs € 179,676

APPENDIX A

EPA Preliminary Assessment - Selected Catchments

APPENDIX B

Topographical Survey Drawings – February March 2012

APPENDIX C

Flood Maps

mdw0484 rp0002a04

Technology

1200mm diameter

outline solutions

outline solutions

rebuild boundary

epa preliminary

impermeable

raheny assessment

raheny assessment