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  • 1. Santry River Flooding at Raheny Assessment and Solutions Report DOCUMENT CONTROL SHEETClient: Dublin City CouncilProject Title: Santry River Flooding at RahenyDocument Title: Assessment and Solutions ReportDocument No: MDW0484Rp0002 DCS TOC Text List of Tables List of Figures No. of AppendicesThis DocumentComprises: 1 1 25 - - 3Rev. Status Author(s) Reviewed By Approved By Office of Origin Issue DateA01 Approval J Hobbs Jean Hobbs Jerry Grant West Pier 18 Jun 2012 R KaneA02 Approval J Hobbs Jean Hobbs Jerry Grant West Pier 20 Jun 2012 R KaneA03 Approval J Hobbs Jean Hobbs Jerry Grant West Pier 25 Jun 2012 R KaneA04 Approval J Hobbs Jean Hobbs Jerry Grant West Pier 28 Jun 2012 R Kane rpsgroup.com/ireland
  • 2. Santry River Flooding at Raheny Assessment & Outline Solutions Report TABLE OF CONTENTS1 INTRODUCTION ........................................................................................................................ 1 1.1 INTRODUCTION ................................................................................................................ 1 1.2 SANTRY RIVER CATCHMENT ............................................................................................. 1 1.3 PREVIOUS AND ONGOING STUDIES ................................................................................... 4 TH2 FLOOD EVENT OCTOBER 24 2011...................................................................................... 6 2.1 INTRODUCTION ................................................................................................................ 6 2.2 RAINFALL EVENT ............................................................................................................. 6 2.3 FLOODING AT RAHENY ..................................................................................................... 83 TOPOGRAPHICAL SURVEY .................................................................................................. 11 3.1 TOPOGRAPHICAL SURVEY DATA ..................................................................................... 114 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 .................................................................................................. 185 CONCLUSIONS AND RECOMMENDATIONS ....................................................................... 19 5.1 SUMMARY CONCLUSIONS ............................................................................................... 19 5.2 OUTLINE RECOMMENDED OPTIONS ................................................................................. 20 APPENDICESAPPENDIX A EPA Preliminary Assessment - Selected CatchmentsAPPENDIX B Topographical Survey Drawings – February March 2012APPENDIX C Flood MapsMDW0484Rp0002 i Rev A04
  • 3. Santry River Flooding at Raheny Assessment & Outline Solutions Report1 INTRODUCTION1.1 INTRODUCTIONFollowing widespread, heavy rain on 24th October 2011 and subsequent flooding events almostcountry-wide, but particularly in the eastern and northern parts of Ireland, RPS was commissioned byDublin City Council (DCC) to assess and report on the flooding incident which occurred on the SantryRiver at Raheny Village on the night of October 24th 2011. This assessment meets the followingobjectives:- • 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 CATCHMENTThe Santry River catchment lies to the north of Dublin City, with the river flowing south-eastwards fromthe airport to Dublin Bay, discharging opposite Bull Island. The catchment, which has an area ofapproximately 1,360ha, is 11km long and typically about 1km wide.Approximately 60-70% of the catchment is urbanised – mainly residential, but with some largeindustrial areas, particularly either side of the M50 motorway. There are also recreational areas, manyadjacent to the river. Upstream of the M1 the catchment is predominantly rural, but with a lot of recentdevelopment between the M50 and the M1. Whilst all of the rural areas are within the Fingal CountyCouncil 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 drainagenetworks discharging into the river.The river commences near Dublin Airport and flows in a south-easterly direction, crossing the M50 atjunction 4. It then flows through open ground and recent development, before entering the main urbanarea upstream of Swords Road. The Santry crosses under the M1 and continues eastwards, pastvarious 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 Roadand Main Street. The Santry River continues in a south-easterly direction, and near the church at TheVillage, 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 ofculvert, which passes under James Larkin Road. The culvert, which is 130m long and has twinsections 2.3m wide by 1.3m high, discharges into Dublin Bay, just north of the western end ofCauseway Road – the link to Bull Island. Only this downstream part of the river is affected by tidalconditions.MDW0484Rp0002 1 Rev A04
  • 4. Santry River Flooding at Raheny Assessment & Outline Solutions ReportThere have been several occurrences of flooding in the flood plain in the Harmonstown Road area andalso in parkland upstream of the DART crossing at Raheny. This is associated with the floodingdownstream 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 Rahenyvillage has occurred on several occasions in the past – typically once every 5 years. Most recently theRaheny Village area was flooded on 24th October 2011. Extreme rainfall events in August 2008 andJuly 2009 flooded premises on Howth Road and supported a request to the OPW for funding for flooddefences.The Santry River, its main tributaries and its catchment boundary, are presented in Figure 1-1overleaf.MDW0484Rp0002 2 Rev A04
  • 5. Santry River Flooding at Raheny Assessment & Outline Solutions ReportFigure 1-1 Santry River Catchment Raheny: Location of flooding on th 24 October 2011MDW0484Rp0002 3 Rev A04
  • 6. Santry River Flooding at Raheny Assessment & Outline Solutions Report1.3 PREVIOUS AND ONGOING STUDIES1.3.1 GDSDSPrevious flooding assessment work has been conducted on the Santry River Catchment through theGreater Dublin Strategic Drainage Study (GDSDS) 2001 – 2005. During this study a type 1 (allnodes) river and surface water model was built for the Santry River in InfoWorks, using 2002 assetand topographical survey data.The InfoWorks CS model was used to identify pressure points on the existing system and todemonstrate if increasing the overall capacity of the system to accommodate existing and future flowsis feasible in areas where growth is expected to cause a problem. The results of this modelassessment produced Drainage Development Options (DDO) for the Santry River Catchment in theGDSDS Final Report 2005.DDO recommendations in the GDSDS to overcome local flooding from the piped network includedupsizing 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 fromdevelopment and the effects of climate changed.Two flood plain deficiencies were identified during the GDSDS, based on predicted flooding for 100year 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 itwould be necessary to consider the needs for and the details of the DDOs further, prior to developingany improvements. This was deemed particularly important at the time, due to some uncertainties inthe system layout and in the contributing areas. It was recommended in the GDSDS Final Report thatfurther site investigations be carried out, to obtain more detailed information and to enable the modelto be updated.1.3.2 ECFRAMThe Santry River Catchment is also within the remit of the Office of Public Works (OPW) recentlycommissioned Eastern Catchment Flood Risk Assessment and Management Study (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 DublinArea. The National Catchment Flood Risk Assessment and Management (CFRAM) Programme wasdeveloped to meet the requirements of the EU Floods Directive (2007/60/EC), as well as to deliver oncore components of the 2004 National Flood Policy.MDW0484Rp0002 4 Rev A04
  • 7. Santry River Flooding at Raheny Assessment & Outline Solutions Report 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 thanconcentrating on one select area. Under normal circumstances, the current ongoing Eastern CFRAMwould inform on the benefits or otherwise of providing flood protection for the Raheny area. However, asevere flood event occurred in Oct 2011 and any solutions proposed would have to cater for thishappening again, as a minimum. These outline solutions have been put forward in our report and mustalso be taken into consideration during the ongoing ECFRAMS. MDW0484Rp0002 5 Rev A04
  • 8. Santry River Flooding at Raheny Assessment & Outline Solutions Report2 FLOOD EVENT OCTOBER 24TH 20112.1 INTRODUCTION thFollowing the widespread heavy rain on 24 October 2011 and subsequent flooding events almostcountry-wide, but particularly in the eastern and northern parts of Ireland, the Dublin RegionalInspectorate of the Environmental Protection Agency (EPA) produced a report entitled “A PreliminaryAssessment of the Flooding Which Occurred on 24 October 2011 in Selected Catchments on the EastCoast” - November 2011. Unfortunately the EPA preliminary assessment did not refer to the SantryRiver Catchment.However, the EPA report does give a clear indication of flood flows experienced in other rivers on thesame day in Dublin City - such as the Tolka and Dodder River at Botanic Gardens and Waldron’sBridge respectively. There are similarities between the Santry and Tolka River Catchments, in thatthey would have some comparable catchment characteristics (in the urban context). Refer to thAppendix A for further detail. Peak flow on the Tolka River recorded on 24 October 2011 did notreach peak levels recorded during the extreme flooding events of 2001 and 2003. It also appears that thriver catchments in south Dublin and county Wicklow experienced more extreme rainfall on 24October 2011.2.2 RAINFALL EVENT thDuring Monday 24 October 2011, a spell of very heavy rainfall, brought about by a slow movingfrontal depression centred over Ireland, affected eastern parts of the country, in particular the GreaterDublin Area. Met Eireann produced a report on this heavy rainfall incident, concluding that it wasparticularly severe in the Dublin area due to the Wicklow Mountains causing a process calledorographic uplift, in addition to coastal convergence of isobars off the east coast – forcing moist air torise, condense and then fall as rain.It is noted that Met Eireann’s weather station at Casement Aerodrome, Baldonnell, Co. Dublin th(approximately 30km south of the Santry River Catchment) set a new record of 82.2mm of rain on 24October 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 fallingbetween 3pm and 7pm. According to Met Eireann, this 4-hour interval of heavy rainfall correspondsapproximately 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 thrainfall total of 69.1mm for 24 October 2011, with 42.9 mm falling in a 4-hour interval, correspondingapproximately to a 1 in 40 year rainfall event. In addition, rainfall readings from weather station within ththe Dublin City Council administrative area at Grange, recorded, between 5pm and 9pm on 24October 2011, rainfall totalling at 45.8mm. Refer to Table 2.1 overleaf for further detail. It is worthnoting however that 24mm of rain was recorded in one hour (from 7 - 8pm) at Grange, slightly morethan the wettest hour on record (since records began 1954) at Casement Aerodrome, but with a timelag of 3 hours.Initial analysis of the available measurements resulted in the EPA concluding that although significantamounts of rainfall affected many areas in Eastern and Northern parts of Ireland, the greater DublinArea received by far the most rainfall. Met Eireann weather stations at both Dublin Airport and PhoenixPark recorded lesser intensities of rainfall than Casement Aerodrome, indicating that south Dublin, dueto its proximity to the Wicklow Mountains, may have been more severely affected by the rainfall thannorth Dublin.However, as noted above, Grange weather station’s wettest hour recorded (24mm) exceeded the thCasement Aerodrome wettest hour (23mm) recorded on 24 October 2011. This demonstrates thespatial variation in intensity of rainfall that can occur in storm events within catchments.MDW0484Rp0002 6 Rev A04
  • 9. Santry River Flooding at Raheny Assessment & Outline Solutions ReportTable 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 October 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. Thecritical duration storm for the Santry Catchment at Raheny has been identified by hydraulicmodelling (GDSDS) as 4 hours. The critical duration storm is defined as the rainfall duration whichinfluences the maximum peak runoff rate in a river. Therefore, it can be concluded that winter rainfallconditions on the 24th October 2011, on an already saturated catchment, were characteristic of stormconditions likely to cause maximum flood flow conditions at Raheny.MDW0484Rp0002 7 Rev A04
  • 10. Santry River Flooding at Raheny Assessment & Outline Solutions Report2.3 FLOODING AT RAHENY thDuring Monday 24 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 asa result of this intense rain, which fell on an already saturated catchment. The catchment was already thsaturated due to preceeding wet weather conditions over the week and weekend leading up to the 24October 2011 – resulting in a higher than normal amount of rainfall runoff flowing straight to the riverrather than being naturally attenuated by the landscape. thImmediately after the flood event of 24 October 2011, RPS, as the OPW’s Consulting Engineer forthe Eastern Catchment Flood Risk Assessment and Management Study (CFRAMS) project, begancollecting information on the flood event in areas along the east coast, including the Santry River at theaforementioned flood affected locations. Figure 2-1 below shows information collated regarding the thextent of the Santry River flood event on 24 October 2011 at Raheny. th Figure 2-1 Flood Event at Raheny Village 24 October 2011 – Santry RiverRPS staff also carried out site visits to Raheny in the weeks following the flooding event, in order tocarry out an assessment of events. This included liaising with Dublin City Council personnel and anumber of flood-affected residents. As a result of these accounts we can report on the approximate thmechanisms of the flooding event on October 24 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.MDW0484Rp0002 8 Rev A04
  • 11. Santry River Flooding at Raheny Assessment & Outline Solutions Report • 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 - RahenyMDW0484Rp0002 9 Rev A04
  • 12. Santry River Flooding at Raheny Assessment & Outline Solutions Report Figure 2-3 Entrance to Main Street Culvert - RahenyAs a result of this flood event, the ground floors of 6 no. properties on Howth Road, 1 no. apartmentblock on Watermill Road and 10 no. properties on Main Street Raheny were inundated to a level ofapproximately 300mm, including a pre-school/crèche facility.MDW0484Rp0002 10 Rev A04
  • 13. Santry River Flooding at Raheny Assessment & Outline Solutions Report3 TOPOGRAPHICAL SURVEY3.1 TOPOGRAPHICAL SURVEY DATAA local topographical survey was commissioned in February 2012 to assist RPS with the assessmentof the flooding incident at Raheny, in particular to enable full comparison between the detailed designproposals for the flood management works and the present-day structures at the same location. Thesurveys, 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 Surveydrawings, produced by Land Surveys Ltd, are included in Appendix B. Figure 3-1 Scope of Topographical Survey undertaken in February 2011MDW0484Rp0002 11 Rev A04
  • 14. Santry River Flooding at Raheny Assessment & Outline Solutions Report4 HYDRAULIC ASSESSMENT4.1 INFOWORKS CS MODELAs discussed earlier, the GDSDS produced a calibrated InfoWorks CS model of the Santry Rivercatchment. This model was used to calculate the run-off to the river from various rainfall events. The2011 fully developed catchment model was chosen and was used to generate input flows into a newtruncated MIKE 11 model of the Raheny Area. Flow hydrographs and water levels were extracted fromthe InfoWorks model and used as boundary conditions in the MIKE 11 model.4.1.1 RainfallRecorded rainfall data from the rain gauge situated at the Grange Tank was used to simulate theOctober 24th 2011 storm. Design storm events were generated within InfoWorks using the UK rainfallgenerator tool. This method is based on local parameters selected from the Ireland maps in the FloodStudies 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 werealso generated using this same tool to account for future scenarios with climate change. Criticalduration of the downstream end of the system was determined in the GDSDS study to be 240 minutesso this was chosen as the storm duration for these simulations. thHeavy rainfall in the days leading up to the October 24 event would have contributed to higher than ndnormal run-off to the Santry River. Because of this, rainfall data taken from midday on the 22 ofOctober at the Grange Tank rain gauge was used in order to accurately represent the catchment thresponse to the October 24 event. A truncated flow hydrograph (below) was then extracted from theInfoWorks model and used as the main boundary input at the start of the MIKE 11 model. The peakflow at the upstream boundary of the model was 10.11m3/sec.MDW0484Rp0002 12 Rev A04
  • 15. Santry River Flooding at Raheny Assessment & Outline Solutions Report th Fig 4.1: Model Input Hydrograph upstream of Harmonstown Road for October 24 Event.In addition to the upstream boundary flow hydrograph, 7 other inflow hydrographs from thesurrounding surface water catchments between Harmonstown Road and Main Street were included inthe model. These were extracted from InfoWorks CS and inserted into the MIKE 11 model at the threlevant locations. The peak flow at the Main Street culvert during the October 24 event was11.24m3/sec. th Fig 4.2: Inflow Hydrographs for subcatchments for October 24 Event.MDW0484Rp0002 13 Rev A04
  • 16. Santry River Flooding at Raheny Assessment & Outline Solutions Report4.2 1-DIMENSIONAL MIKE 11 MODELA 1-D hydrodynamic model was created in MIKE 11 for the Santry River in the vicinity of Rahenyvillage. 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 wascarried out as part of the works. It was augmented by cross section data from the InfoWorks CSmodel. The total length of the modelled river is approximately 1.5km. The model was generated from20 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.MDW0484Rp0002 14 Rev A04
  • 17. Santry River Flooding at Raheny Assessment & Outline Solutions Report Figure 4.3 1-D Model extents and Boundary Locations4.3 2-DIMENSIONAL MIKE 21 MODELA 2-D hydrodynamic model was established in MIKE 21 for the Santry River. The 2-D model domainused in the MIKE 21 model is based on the LiDAR data made available for this study by DCC. Thisdata provides details of the river floodplains and facilitates modelling of out of bank flow. The modeldomain is approximately 1.7km x 1.4km in size with cell dimensions of 2m x 2m.MDW0484Rp0002 15 Rev A04
  • 18. Santry River Flooding at Raheny Assessment & Outline Solutions Report Fig 4.4: 2-D Model Domain4.4 MIKE FLOOD COUPLED HYDRAULIC MODELA MIKE FLOOD model was established for the Santry River. This MIKE FLOOD Model coupled theMIKE 11 (1-D) and MIKE 21 (2-D) models producing a hydrodynamic model that is fully dynamicallylinked, 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 modelalong these lateral link boundaries. Links were established for the left bank and the right bankseparately, to take account of the different elevations of each bank. Sections that do not allow overspillinto the flood plain, such as walled bridges and culverts, were not coupled to the 2D model. The laterallinks are modelled as weir structures and the depth of water at the crest at which spill occurs is set at0.1m for all models.4.5 MODEL CALIBRATIONFor 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 datafor the MIKE Flood model was extracted from the Infoworks CS model. The 2-D floodplain output fromthis model was compared against flooding recorded in the Raheny area by DCC staff after the October24th 2011 event (Figure 4.6 and Figure 4.7 below).MDW0484Rp0002 16 Rev A04
  • 19. Santry River Flooding at Raheny Assessment & Outline Solutions Report Fig 4.5: Fallen tree in Santry River (40m upstream of Main Street) thThe initial model runs predicted less flooding than was observed on 24 October 2011. However, asignificant tree blockage (Figure 4.5) was discovered during a site visit by RPS staff in between HowthRoad and Main Street. This blockage was subsequently modelled and after an iterative process, goodcorrelation between the observed and modelled floodplains was achieved. th Fig 4.6: Observed October 24 2011 flood extents th Fig 4.7: Modelled October 24 2011 FloodplainMDW0484Rp0002 17 Rev A04
  • 20. Santry River Flooding at Raheny Assessment & Outline Solutions Report4.6 MODELLED SCENARIOS thAfter a reasonable level of calibration was achieved for the October 24 2011 event, a number ofscenarios were modelled in MIKE FLOOD. These included: • th October 24 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 stormsThe GDSDS report suggested that the flooding at Raheny could be mitigated by providing in-linestorage upstream of Harmonstown Road. RPS assessed this option and also examined the possibilityof providing flood plain storage at the playing fields between the DART Bridge and Harmonstown Roadin addition to minor flood defence works along the channel between Howth Road and Main Street.MDW0484Rp0002 18 Rev A04
  • 21. Santry River Flooding at Raheny Assessment & Outline Solutions Report5 CONCLUSIONS AND RECOMMENDATIONS5.1 SUMMARY CONCLUSIONS5.1.1 October 24th 2011 eventAfter carrying out a hydraulic assessment of the Santry River at Raheny RPS can conclude the thfollowing regarding the October 24 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 3 1 2,300m . 3 3. The peak flow in the river immediately upstream of the Main Street culvert was 11.24m /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) eventAfter carrying out a hydraulic assessment of the Santry River at Raheny RPS can conclude thefollowing 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 3 1800m . 3 3. The peak flow in the river immediately upstream of the Main Street culvert was 12.08m /s 4. Top water level in the river immediately upstream of Main Street was 14.81mOD.5.1.3 1% AEP (including 25% additional rainfall) eventAfter carrying out a hydraulic assessment of the Santry River at Raheny RPS can conclude thefollowing 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 3 2,800m . 3 3. The peak flow in the river immediately upstream of the Main Street culvert was 12.46m /s1 The floodplain volume includes the artificially higher water levels resulting from the fallen tree in channel whichhas since been removed by DCC.MDW0484Rp0002 19 Rev A04
  • 22. Santry River Flooding at Raheny Assessment & Outline Solutions Report 4. Top water level in the river immediately upstream of Main Street was 14.91mOD.5.2 OUTLINE RECOMMENDED OPTIONSRPS recommends providing attenuation on the Santry River upstream of the DART Bridge. This isconsidered to be the most viable option as the Santry River already has in this area an undevelopedfloodplain. To do so, a throttle would have to be installed to limit the flows through the channel and tohold back flows in the river floodplain. Two possible throttle combinations were modelled andpreliminary costings have been presented. The first being a 1200mm diameter pipe and retaining wallat the Harmonstown Road culvert, the second being a 1200mm diameter pipe and embankment at thepedestrian 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 GardensThis solutions involves a 1200mm diameter throttle pipe being installed in the river at the existingfootpath crossing of the Santry River adjacent to Lein Gardens. Throttling flows at this location willallow the utilization of the basin-like natural topography of the playing fields for flood storage. It alsomeans that a large volume can be stored at relatively shallow depths. This will also require theconstruction of an embankment which would be constructed along the line of the existing footpath tocreate a dam to store the water. An embankment of 2.5m height above existing top of bank level would 3provide additional storage of approximately 23,000m during the 1% AEP + 25% Rainfall event andwould not require the excavation of large quantities of spoil. The peak water level immediatelyupstream 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 EventMDW0484Rp0002 20 Rev A04
  • 23. Santry River Flooding at Raheny Assessment & Outline Solutions Report5.2.2 Option B: Throttle at Harmonstown RoadRPS propose that a 1200mm diameter throttle pipe is installed in the river upstream of HarmonstownRoad culvert. Throttling flows at this location will allow the utilization of the entire river channel forstorage. In the current situation, the water depth in the river channel for the Q100 +25% flows is19.69mOD whereas it is 22.86mOD for this proposed throttled solution. A possible additional storage 2of 62m 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 3option will provide additional storage of 26,600m . Flood waters will be stored within the steep naturalfloodplain channel and as a result flood depths will be greater. The peak water level immediatelyupstream 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 EventFlow Hydrographs at Raheny for the 1% AEP +25% rainfall event for the existing scenario and for bothattenuation options as are presented below in Figure 5.3. Peak flows at Main Street Raheny have 3 3 3reduced from 12.9 m /s for the existing scenario to 10.9 m /s for Option A and 9.2 m /s for Option B.The difference in peak flows at Raheny between Option A and Option B arise due to the amount ofstorage volume available at each throttle respectively. For Option A, storage is limited by the height ofthe embankment which would have to be built to create a dam to store the water in the playing fieldopposite Lein Gardens. For the purposes of this report, this has been limited to 2.5m with a view to thebuildability of such a structure. For Option B, the throttling flow to a higher level (4m) is practical due tothe 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.MDW0484Rp0002 21 Rev A04
  • 24. Santry River Flooding at Raheny Assessment & Outline Solutions Report 14 12 10 Flow (m3/s) 8 6 4 2 0 12:00 13:00 14:00 15:00 16:00 Time Existing Option A (weir & embankment) Option B (weir) Figure 5.3 Flow Hydrographs at Raheny for 1% AEP + 25% Rainfall EventRPS 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.MDW0484Rp0002 22 Rev A04
  • 25. Santry River Flooding at Raheny Assessment & Outline Solutions Report • 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 Raheny5.3 COST ESTIMATESTable 5.1a and 5.1b below show a summary of cost estimates for the outline solutions proposed atHarmonstown Road and Raheny. In addition to the estimated construction costs below, someprovision should be made within the DCC budget to provide for inspection and routine maintenance ofthe river environment at the proposed throttle and between Main St and Howth RoadMDW0484Rp0002 23 Rev A04
  • 26. Santry River Flooding at Raheny Assessment & Outline Solutions Report5.3.1 Option A - Throttle Adjacent to Lein Gardens and Associated Flood DefencesTable 5.1a Description (Option A) Length Cost Estimate (€) Rebuild boundary walls and localised strengthening of - 20,000 existing walls (Southern Bank) 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 - 15,000 headwalls) Local realignment and clearance of Santry River - between Howth Road and 25m downstream of Main 7,500 Street culvert Construction Costs € 185,990 Detail Design Costs € 27,898 Option A Total Costs € 213,888MDW0484Rp0002 24 Rev A04
  • 27. Santry River Flooding at Raheny Assessment & Outline Solutions Report5.3.2 Option B – Throttle at Harmonstown Road and Associated Flood DefencesTable 5.1b Description (Option B) Length Cost Estimate (€) Rebuild boundary walls and localised strengthening of - 20,000 existing walls (Southern Bank) 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 - 20,000 (including headwall) Local realignment and clearance of Santry River between - 7,500 Howth Road and 25m downstream of Main Street culvert Local Landscaping upstream of Harmonstown Road - 5,000 Construction Costs € 156,240 Design Costs € 23,436 Option B Total Costs € 179,676MDW0484Rp0002 25 Rev A04
  • 28. APPENDIX AEPA Preliminary Assessment - Selected Catchments
  • 29. APPENDIX BTopographical Survey Drawings – February March 2012
  • 30. APPENDIX CFlood Maps