Limerick Smarter Travel Route 2 Planning Application. Flood Risk Assessment Report

Transcription

1 Planning Application March 2014

2 Document Control Document Number RP002- Flood Risk Assessment Revision Date Prepared Checked Approved R0 (1 st issue) 5 March 2014 A Prendergast S. Ryan C. Lewis R1 (2 nd issue) 31 March 2014 A Prendergast S. Ryan C. Lewis Page 1 March 2014

3 Table of Contents 1.0 Introduction Background Nature of proposed development Summary of the Project The Planning System and Flood Risk Management Guidelines Flood Risk Identification Establishing Flood Risk Sources of Information OSI Historical Flood Maps OPW Flood Hazard Identification Mapping Estimates of flood zone and flood risk Indicative Flood Zone Maps Walkover Survey Overview of the Hydrology of the River Shannon Overview of the Hydrology of the River Groody Initial Flood Risk Assessment Sources of Flooding Flood Zone Vulnerability Potential Impact of Development on Flooding Elsewhere Flood Risk Management Conclusion Appendix A Site Photographs Appendix B CFRAM Maps Appendix C Jacobs Map Page 2 March 2014

4 1.0 Introduction 1.1 Background PUNCH Consulting Engineers was appointed by Limerick Smarter Travel to complete the planning application for the proposed Limerick Smarter Travel Route 2. Limerick Smarter Travel s Route 2 is located on the bank of the River Shannon stretching from Guinness Bridge to the University of Limerick. A Site Specific Flood Risk Assessment (FRA) study for the proposed route has been prepared in conjunction with the planning application. The FRA has been carried out in full compliance with the requirements of The Planning System & Flood Risk Management Guidelines published by the Department of the Environment in November Nature of proposed development The project involves the upgrade of an existing pathway between the University of Limerick and the Guinness Bridge over a distance of approximately 1.6 km. The path is located on the route of a historical towpath along the bank of the river Shannon. The primary aim of the project is to restore and improve the existing pathway and encourage its use a pedestrian and cyclist commuter link. In order to achieve these aims, the following measures are proposed; Upgrade the surface and alignment of the existing path; Improve lighting and security measures; Widen/ bypass a number of narrow footbridges along the route; 1.3 Summary of the Project This project will primarily involve the following elements; Upgrade of the existing path from Guinness Bridge to beyond Reboge Bridge to provide a 3m wide path (bitmac surface) for combined cycle and pedestrian use. Upgrade of the existing path from Bridge 6 to the University of Limerick Boat House to provide a 3m wide path (lightweight decking with slip resistance surfacing) for combined cycle and pedestrian use. Construction of two new footbridges (lightweight decking with slip resistance surfacing) along the route Widening of the two existing footbridges along the route. For further details, please refer to Planning Drawings Page 3 March 2014

5 2.0 The Planning System and Flood Risk Management Guidelines In September 2008 The Planning System and Flood Risk Management Guidelines were published by the Department of the Environment, Heritage and Local Government in Draft format. In November 2009, the adopted version of the document was published. The Flood Risk Management Guidelines give guidance on flood risk and development. The guidelines recommend a precautionary approach when considering flood risk management in the Planning system. The core principle of the guidelines is to adopt a risk based sequential approach to managing flood risk and to avoid development in areas that are at risk. The sequential approach is based on the identification of flood zones for river and coastal flooding. The guidelines include definitions of Flood Zones A, B, and C as described below. It should be noted that these do not take into account the presence of flood defences, as there remain risks of overtopping and breach of the defences. Zone A (high probability of flooding) is for lands where the probability of flooding is greatest (greater than 1% or 1 in 100 for river flooding and 0.5% or 1 in 200 for coastal flooding). Zone B (moderate probability of flooding) refers to lands where the probability of flooding is moderate (between 0.1% or 1 in 1000 and 1% or 1 in 100 for river flooding and between 0.1% or 1 in 1000 and 0.5% or 1 in 200 for coastal flooding). Zone C (low probability of flooding) refers to lands where the probability of flooding is low (less than 0.1% or 1 in 1000 for both river and coastal flooding). When a flood zone has been identified, the guidelines set out the different types of development appropriate to each zone. Exceptions to the restrictions of development due to potential flood risks are provided for through the use of the Justification Test, where the Planning need and the sustainable management of flood risk to an acceptable level must be demonstrated. This recognises that there will be a need for future development in existing towns and urban centres than lie within flood risk zones, and that the avoidance of all future development in these areas would be unsustainable. A three-staged approach to undertaking a FRA is recommended in the guidelines: Flood Risk Identification (Stage 1) Identification of any issues relating to the site that will require further investigation through a Flood Risk Assessment. Initial Flood Risk Assessment (Stage 2) Involves establishment of the sources of flooding, the extent of the flood risk, potential impacts of the development and possible mitigation measures. Detailed Flood Risk Assessment (Stage 3) Assess flood risk issues in sufficient detail to provide quantitative appraisal of potential flood risk of the development, impacts of the flooding elsewhere and the effectiveness of any proposed mitigation measures. This report addresses the requirements for both Stages 1 and Page 4 March 2014

6 3.0 Flood Risk Identification The proposed development consists of the upgrade and widening of an existing walk and cycle way from Guinness Bridge to the University of Limerick (U.L.) Boat House. The existing levels of the towpath will be maintained. The development includes the provision of two new bridges along the route and widening of two existing bridges. The proposed route is located along an existing towpath on the bank of the River Shannon. The towpath has been in use for over 200 years and has been subjected to significant improvements in the past. These improvements have led to the increased usage of the pathway as a link between the University, the Corbally area, and the City centre. Both recreational users and commuters use the path on a daily basis. Figure 1 illustrates the location of the proposed route relative to the river. Proposed Location of Limerick Smarter Travel - Route 2 Location of Proposed Development U.L. Boat House Corbally Guinness Bridge Plassey Bridge (Black Bridge) Canal Groody River Reboge Towpath Proposed Pathway Limerick City Centre Historical Towpath Figure 1 Location of Proposed Development relative to the River Shannon Page 5 March 2014

7 3.1 Establishing Flood Risk Sources of Information Limerick Smarter Travel Route 2 Various sources were examined in order to establish the flood risk to the pathway. Historical maps were used to establish historic flood plains and areas liable to flooding in the vicinity of the path. Figures 2 and 3 are images taken from historical mapping available on OSI.ie and illustrate areas liable to flooding OSI Historical Flood Maps Figure 2 Extract from the 25 historical map showing flooding in the vicinity of the development Figure 3 Extract from the 6 historical map showing flooding in the vicinity of the development Page 6 March 2014

8 3.1.2 OPW Flood Hazard Identification Mapping The OPW Flood Hazard Mapping Website identifies previous flood events but is not a complete record. The map below indicates that the proposed path is subject to flooding from the River Shannon. Pathway highlighted in red Figure 4 OPW Flood maps indicating multiple flood events in the area Page 7 March 2014

9 The most recent flood events along this stretch of the River Shannon occurred in 2009, 1999, and 1990, and as recently as January A report prepared by Limerick County Council in 2010 which documents the flooding of the Lower Shannon area of County Limerick in November 2009 stated that the area affected by flooding stretched from Montpelier village downstream to the confluence with the Groody River at Plassey. Plassey Bridge (Black Bridge) and the route of the towpath were closed during this period due to flooding. 3.2 Estimates of flood zone and flood risk Indicative Flood Zone Maps CFRAM Mapping The CFRAM (Catchment Flood Risk Assessment and Management) programme is a national programme which to-date has produced a series of Preliminary Flood Risk Assessments (PFRA), which cover the entire country, see or for the Shannon area. It must be noted that these maps are also indicative however, currently river channels across the country are being surveyed and it is hoped to produce flood hazard mapping in the future using data gathered in these surveys. The OPW note that these maps are indicative and they have been developed using simple and cost-effective methods that are suitable for the PFRA. They should not be used for local decision-making or any other purposes without verification. Route of existing pathway Figure 5 CFRAM maps of the Limerick City/ Reboge/ Castletroy area (image taken from showing that the site is at risk in a 1 in 100 year fluvial event. Refer to Appendix B for the full map. Areas of pluvial flooding (brown) can also be noted in the vicinity of the site of the proposed development.) Page 8 March 2014

10 3.3 Walkover Survey The proposed site has been visited by PUNCH Consulting Engineers on a number of occasions during the preliminary design process. On 9 th January 2014, PUNCH Consulting Engineers visited the site to establish any potential sources of flooding, likely routes of floodwaters and the site s key features. The site was revisited on 6 th February 2014 for further inspection. The following were established on site: o o o o o o The bank of the river Shannon on which the towpath is located is overgrown with mature trees and hedgerow. There were no visible restrictions to flow in the main channel of the River Shannon near the proposed development. An open drain exists at the landside of the towpath. The drain provides flood storage capacity in times of high river levels. The drain is relatively dry for most of the year. The drain contained water at a similar level to the Shannon on both visits. The topography of the route is such that there is a fall in the path from the Northern End (Guinness Bridge) towards U.L. boathouse. There were no visible restrictions to flow in the main channel of the Groody River in the vicinity of the pathway. An open wetland area exists at the landside of a section of the towpath. This area of land acts as a floodplain in times of high water levels in the Shannon and Groody rivers. The area was visibly flooded on both visits. Refer to Appendix A for site photographs Page 9 March 2014

11 3.4 Overview of the Hydrology of the River Shannon The River Shannon is both the longest and largest river in Ireland. The River Shannon rises in the Shannon Pot in County Cavan and generally flows in a southwesterly direction towards the Shannon Estuary where it enters the Atlantic Ocean. The catchment upstream of Limerick is greater than 10,000 km 2. There are three major lakes, Lough Allen, Lough Ree, and Lough Derg on the Shannon. There is a very limited fall between Lough Allen and Lough Derg (circa 13m over 200km). The presence of the lakes and the small fall between the outlet of Lough Derg and inlet of Lough Allen has the effect of attenuating flows in the catchment. The lag time between runoff from rainfall in the middle and upper sections of the Shannon catchment and increased flows in Limerick may be in the order of days. Downstream of Lough Derg a hydroelectric scheme was built in the 1920 s. A weir at Parteen splits the flow of the River Shannon into two separate flows. One flow continues in the natural channel via Castelconnell and the second flow is diverted through a man-made headrace to Ardnacrusha electricity generating station. The flow passes through the power station and rejoins the natural channel just upstream of Limerick. Floodwaters pass down the natural channel by opening sluice gates in Parteen Weir. Following the construction of the scheme the natural channel receives a base compensation flow of up to 400m 3 /s with the next 400 m 3 /s used for electricity generation at Ardnacrusha Hydroelectric Power station. However, we note that the volumes released are dependent on the levels at Parteen Weir. For example, in January/ February 2014 we understand that additional flows of up to 200m 3 /s were released down the natural channel. The natural and man-made channels are illustrated in Figure 6. Flow in the natural channel of the Shannon is influenced by the operation of the power station at Ardnacrusha, and receives reduced flows since the construction of the Scheme in the 1920 s. This reduction in flow may have exacerbated the accumulation of alluvial material and subsequent encroachment of riparian woodland along this stretch of the river. The Mulkear confluence with the River Shannon is located approximately 2km upstream of the proposed development area and carries a considerable silt load during flood events. This sediment is deposited at and downstream of the Mulkear/ Shannon confluence due to the reduction in water velocity when it enters the much larger and regulated Shannon channel. Sections of the old river, such as at Castleconnell, have been subject to extensive remodelling and vegetation control to cater for salmon angling requirements under the reduced post-shannon Scheme flows. Other parts of the river have become derelict in channel maintenance terms. The conveyance capacity of the channel is now much reduced and is no longer able to cope with flood water releases from Parteen Weir and runoff into the residual catchment (e.g. from the Mulkear sub-catchment). This has caused a number of flooding issues in the Clonlara area in recent years with farmland and occupied houses affected Page 10 March 2014

12 The proposed Limerick Smarter Travel Route 2 will follow a section of the old Limerick Navigation Scheme, as illustrated below. Ardnacrusha ESB Station Man-made Channel Parteen Weir Natural Channel Tow Path Figure 6 River Shannon Channels Page 11 March 2014

13 3.5 Overview of the Hydrology of the River Groody The river Groody drains an area to the southeast of Limerick. Land use in the catchment is predominantly agricultural (dairy farming) with some urbanisation in the Singland and Castletroy areas. Large sections of the catchment are relatively flat and much of the soil in the catchment can be considered to have a relatively poor winter rainfall acceptance potential. The OPW has carried out a drainage scheme in the catchment and the channels in the catchment are now maintained by the OPW. Unfortunately, there is no flow monitoring in the Groody Catchment. A water level recorder (OPW Hydrometric Stn 25012) does exist on the Groody on the Dublin Road however; there is no rating curve for this monitoring station. The highest water level observed on the Dublin Road gauge was on the 27 th of November 2009 at 6.36 m however, we are of the opinion, that this water level was driven by the high water levels on the river Shannon at the time rather than extreme flows in the river Groody. The Met Éireann monthly weather reports light showers on 27 th of November 2009 however, the Shannon River was still in flood from the extreme rainfall the week before. Due to the lack of observed flow data in the Groody catchment Punch Consulting Engineers used the Flood Studies Report 6 variable equation to estimate the flow in the catchment. Initial estimates of Q bar (average maximum flow in any given year) suggest that the Groody Catchment will generate a Q bar of 2.5 l/s/ha which is larger but comparable to a Q bar estimate of 1.72 l/s/ha on the river Mulkear at Abington observation station (OPW Hydrometric Stn 25003). Due to the ungauged nature of the Q bar estimate, the Flood Studies Report recommend multiplying Q bar by 1.47 to increase confidence levels and using a growth factor of 1.96 to convert Q bar to Q 100 (1:100 year event). The OPW also recommend multiplying Q 100 by 1.6 due to the fact the catchment is a drained catchment and also to allow for a 20% increase in flows due to climate change. Using these factors, initial estimates suggest that the flow during a 1:100 year event at the outfall of the Groody Catchment is circa 80 m 3 /s. We also note that the entire area is considered to be in Flood Zone A (at risk from a 1:100 year event). Under a Flood Risk Assessment, it can be argued the development type is suited to Flood Zone A once consideration is given as to manage the residual flood risk (i.e. closure of the towpath to the public in times of high flow). A Flood Risk Assessment must also demonstrate that the development will not result in a loss of flood storage. Therefore, any loss in flood storage due to new embankments, structures, etc. must be compensated for within the site of the proposed development Page 12 March 2014

14 4.0 Initial Flood Risk Assessment 4.1 Sources of Flooding When carrying out a flood risk assessment one should consider all the potential flood risks and sources of floodwater at the site. In general the main flood sources are: o Fluvial Fluvial flooding is the result of a river exceeding its capacity and excess water spilling out onto the adjacent floodplain. This is the primary source of flood risk to the proposed development. o Pluvial (Overland flow e.g. due to sewer surcharging, blocked gullies, etc. usually associated with high intensity rainfall) This is not considered a risk to the proposed development. o Coastal flooding The section of river Shannon adjacent to the site is not tidal; therefore, this is not considered a risk. o Groundwater flooding This is not considered a risk to the site. 4.2 Flood Zone Having considered the historical evidence of flooding in Sections and and with reference to Figure 5 above it is clear that the entire pathway is in Flood Zone A. Flood Zone A is defined in the Planning System and Flood Risk Management Guidelines as the Flood Zone where the probability of flooding from rivers is highest (greater than 1% or 1 in 100). It is estimated that the 1 in 100 year flood level along this stretch of the River Shannon is 6.50m O.D. (Ref: Planning Application 04/3646 University of Limerick Pedestrian Bridge ESB Hydraulic Model) Additionally, a flood level of 7.33m O.D. was recorded at the Living Bridge on the University campus in 2009 (Ref: U.L. Records). A topographical survey of the site of the proposed development shows the ground levels of the site range from 5.8m to 7.3m, thus confirming that the path is at risk of flooding and is in Flood Zone A (1 in 100 year flood event). 4.3 Vulnerability Table 3.1 of the Planning System and Flood Risk Management Guidelines for Planning Authorities gives a detailed classification of vulnerability of different types of development. Amenity open space, outdoor sports, and recreation are classified as water compatible development. This proposal essentially involves the upgrade and widening of an existing riverside walk and is therefore considered as a flood compatible development (refer to Tables 3.1 and 3.2 of the Guidelines below) Page 13 March 2014

15 Vulnerability Class Highly Vulnerable development (including essential infrastructure) Land Uses and types of development which include Garda, ambulance, fire stations, and command centres required to be operational during flooding Hospitals Emergency access and egress points Schools Dwelling houses, student halls of residence and hostels Residential institutions such as residential care homes, children s homes and social services homes Caravans and mobile home parks Dwelling houses designed, constructed or adapted for the elderly or, other people with impaired mobility Essential infrastructure, such as primary transport and utilities distribution, including electricity generating power stations and sub-stations, water and sewage treatment, and potential significant sources of pollution in the event of flooding. Less vulnerable development Buildings used for: retail, leisure, warehousing, commercial, industrial, and non-residential institutions Land and buildings used for holiday or short-let caravans and camping subject to specific warning and evacuation plans Land and buildings used for agriculture and forestry Waste treatment (except landfill and hazardous waste) Mineral working and processing Local transport infrastructure Water-compatible development Flood control infrastructure Docks, marinas and wharves Navigation facilities Ship building, repairing and dismantling, dockside fish processing and refrigeration and compatible activities requiring a waterside location Water-based recreation and tourism (excluding sleeping accommodation) Lifeguard and coastguard stations Amenity open space, outdoor sports and recreation and essential facilities such as changing rooms Essential ancillary sleeping or residential accommodation for staff required by uses in this category (subject to a specific warning and evacuation plan) Table 1 Classification of vulnerability of different types of development (Extract from The Planning System & Flood Risk Management ) Page 14 March 2014

16 Flood Zone A Flood Zone B Flood Zone C Highly vulnerable development (including essential infrastructure) Less vulnerable development Water-compatible development Justification Test Justification test Appropriate Justification Test Appropriate Appropriate Appropriate Appropriate Appropriate Table 2 Matrix of vulnerability versus flood zone to illustrate appropriate development and that required to meet the Justification Test (Extract from The Planning System & Flood Risk Management ) 4.4 Potential Impact of Development on Flooding Elsewhere The proposed lightweight decking will be constructed above the existing pathway. The surface of the deck will be approximately 300mm above the level of the existing pathway. However, the supporting structure is open and will allow floodwater to flow between the river and the towpath drain. Therefore, the proposed structure will not affect flood storage volume (i.e. there will be no reduction in storage capacity along the proposed route). 4.5 Flood Risk Management Flood risk management under the EU Floods Directive aims to minimise the risks arising from flooding to people, property, and the environment. Minimising risk can be achieved through structural measures that block or restrict the pathways of floodwaters, such as river defences or non-structural measures that are often aimed at reducing the vulnerability of people and communities such as flood warning, effective flood emergency response, or resilience measures for communities or individual properties. In this case, the proposed works involve upgrade of an existing path. The finished level of the path will be similar to the existing levels. The path and bridges will be subject to flooding due to their location in Flood Zone A. However, the construction materials proposed have been chosen to limit any damage. While it is accepted that the path may suffer from flooding, current procedures whereby the path is closed during extreme events will be continued Page 15 March 2014

17 5.0 Conclusion All existing information has been reviewed regarding the flood risk in the area. Historical evidence of flooding at the site was found. OSI historical mapping indicates that the site has flooded previously while the OPW flood hazard mapping has several flood events in the area on record. Indicative flood maps of the area were consulted to establish the flood zone. The CFRAM flood maps and the historical data available indicate that the proposed development lies within Flood Zone A. A Stage 3 detailed flood risk assessment was deemed unnecessary in this case, as the proposal to upgrade the existing river pathway is considered compatible with its location in Flood Zone A, as set out in Table 3.1 and 3.2 of the Flood Risk Management Guidelines Page 16 March 2014

18 Appendix A Site Photographs Photograph 1 Steps adjacent to towpath 5 th June 2013 Photograph 2 General View of the River Shannon adjacent to the Towpath 5 th June Page 17 March 2014

19 Photograph 3 Towpath UL to Bridge 4 9 th January 2014 Photograph 4 River Shannon adjacent to the towpath between Reboge Bridge and Bridge 6 9 th January Page 18 March 2014

20 Photograph 5 General view of the River Shannon adjacent to the towpath 9 th January 2014 Photograph 6 General view of the River Shannon and Lands adjacent to the Groody Bridge 9 th January Page 19 March 2014

21 Limerick Smarter Travel Route 2 Photograph 7 Photograph General View of Towpath adjacent to U.L. Boat House - 6th February 2014 General View of Groody Bridge U.L. side 6th February 2014 Page 20 March 2014

22 Appendix B CFRAM Maps Page 21 March 2014

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24 Appendix C Jacobs Map Page 22 March 2014

25 Limerick City (b) Km The PFRA Flood Extents shown are indicative. They have been developed using simple and cost-effective methods that are suitable for the PFRA. They should not be used for local decision-making or any other purpose without verification. Legend Flood Extents (PFRA, 2010) FINAL CAR 37 A3 Shannon CFRAM Study Limerick City (b) Jul 01, 2011