CARDIFF COUNCIL AGENDA ITEM: 3 CYNGOR CAERDYDD

Transcription

1 CARDIFF COUNCIL AGENDA ITEM: 3 CYNGOR CAERDYDD ENVIRONMENTAL SCRUTINY COMMITTEE: 17 th May 2011 PRELIMINARY FLOOD RISK ASSESSMENT Purpose of report 1. To provide Members with information of the Preliminary Flood Risk Assessment currently being developed for Cardiff. Background 2. A European Directive introduced the Flood Risk Regulations which came into force on 10 December These imposed strict timetables on member states for the completion of three elements of work which will finally culminate in the publication of Flood Risk Management Plans for River Basin districts, of which there are 11 in England and Wales, and river catchments, which number 77 in England and Wales. There are 9 river catchments in Wales and those that are relevant to Cardiff are the Taff/ Ely and the Rhymney / Ebbw catchments. 3. The Regulations identify the Environment Agency and Local Authorities as lead authorities to work with partners such as water companies to manage flood risk and to reduce the consequences of flooding on human health, economic activity, cultural heritage and the environment. The Regulations set out a 6 year cycle of assessment and production of mapping and plans with the Environment Agency assigned responsibility for main river, the sea and reservoirs and lead Local Flood Authorities (all councils in Wales), responsible for all other sources of flooding including surface water runoff, ground water and ordinary watercourse flooding. 4. The Flood Risk Regulations impose dates for the completion of 3 separate elements of work for the management of risk associated with flooding to be completed as follows Preliminary Flood Risk Assessment by 22 December 2011 Flood Risk and Flood Hazard Map by 22 December

2 Flood Risk Management Plans by 22 December The Environment Agency Wales is tasked with collating the 22 Preliminary Flood Risk Assessment s covering the whole of Wales and they must submit a summary Preliminary Flood Risk Assessment by the above deadline. In order that they have sufficient time to meet their deadline they in turn have imposed their own deadline of 22 June 2011 for Lead Local Flood Authorities to submit reports. 6. The report on Cardiff's Preliminary Flood Risk Assessment is due for submission to the Environment Agency by the 22 June The Environment Agency has produced a guidance document which has been used by all local authorities to draft their Preliminary Flood Risk Assessment s. It should be noted that the scope of the Preliminary Flood Risk Assessment covers only local flood risk, that of tributaries, surface water run off and feeder streams and therefore excludes flooding risk from the main rivers, the sea and large raised reservoirs. Those watercourse risks are being assessed by the Environment Agency and a consolidated report covering both is to be produced by the Environment Agency by the Welsh Assembly Government prescribed deadlines. 8. The Council has engaged a consultant, Hyder Consulting Ltd, to prepare its Preliminary Flood Risk Assessment and the draft version of this document is attached as Appendix 1. The document consists of: A text document prepared to a standard format. A map of the county showing the area of significant flood risk. A schedule of known flood events that have occurred in the past. 9. The costs of preparing the Assessment have been met from a 22,000 grant assigned to Cardiff from the Welsh Assembly Government from which all authorities in Wales benefited. The potential for future additional funding for further aspects of work associated with the Flood Risk Regulations is not know at this time. Liaison with the Welsh Assembly Government officials will continue to understand and develop the next stage of the process. 2

3 10. Environment Agency Wales supplied all Lead Local Flood Authorities with maps of their areas showing Indicative Flood Risk areas based on a national methodology which has been stipulated by the Assembly Government. To ensure a consistent and proportionate approach Environment Agency Wales have identified 1 km squares above the flood risk threshold using the Flood Map for Surface Water. The 1 km squares identify areas where more than 200 people, or more than one critical service or more than 20 nonresidential properties are predicted to be flooded to a depth greater than 0.3m under the 0.5% annual probability surface water event. With this as a starting point the consultant has drawn on all available information from many sources (as set out in Appendix 2) some internal to the Council and others external to assess whether the indicative flood risk map is an accurate representation of the actual flood risk. 11. In undertaking a review of the maps supplied by Environment Agency Wales there has been an analysis of all known historical flooding events both small and large and also potential future flooding based on a 1 in 100 year event. The Preliminary Flood Risk Assessment in Appendix 1 is the conclusion of this analysis. 12. The analysis identified three new 1km squares that should have been included but these have not increased the area covered by the original indicative flood risk area proposed by Environment Agency Wales. Two of the new squares cover the Whitchurch Brook and members will be aware that European funding has already been allocated to a project aimed to reduce the flood risk posed by the Brook. The project is currently awaiting match funding from the Assembly before it can proceed. 13. It should also be noted that in analysing the future flood risk the consultants have reviewed information available from Strategic Flood Consequence Assessments which have been undertaken to support proposals contained within the Local Development Plan. Further flood assessment work in relation to the Local Development Plan is ongoing but is unaffected by the conclusions of the Preliminary Flood Risk Assessment. 14. The Preliminary Flood Risk Assessment report is a high-level screening exercise which must consider flooding which has or could have a significant harmful consequence for human health, economic activity, the environment and cultural heritage and the map illustrating the area of flood risk will have to be refined with further study. The Plan presents a summary of potential consequences noting; the number of people and 3

4 critical services at risk, impacts on roads or rail infrastructure, that a number of Sites of Special Scientific Interest, Scheduled Monuments and Listed Buildings. 15. The next stage stipulated by the Regulations will be a more detailed and comprehensive analysis to prepare flood hazard maps and flood risk maps which will be used to develop the Councils Flood Risk Management Plan that must be submitted to the Environment Agency by the 22 June 2015, progress and delivery on this stage will be the subject of a future report to the Executive. Issues 16. Councillor Delme Bowen, Executive Member for Traffic & Transportation has been invited to attend the meeting. At the meeting officers representing City Services will be in attendance to deliver a presentation and answer Member questions. Way Forward 17. Members may wish to consider whether there are any issues or comments on this item which they would like to pass on to the Executive. Members may also wish to consider if and when this issue should be reconsidered by the Committee. Legal Implications 18. The Scrutiny Committee is empowered to enquire, consider, review and recommend but not to make policy decisions. As the recommendations in this report are to consider and review matters there are no direct legal implications. However, legal implications may arise if and when the matters under review are implemented with or without any modifications. Any report with recommendations for decision that goes to Executive/Council will set out any legal implications arising from those recommendations. All decisions taken by or on behalf the Council must (a) be within the legal powers of the Council; (b) comply with any procedural requirement imposed by law; (c) be within the powers of the body or person exercising powers of behalf of the Council; (d) be undertaken in accordance with the procedural requirements imposed by the Council e.g. Scrutiny Procedure Rules; (e) be fully and properly informed; (f) be properly motivated; (g) be taken having regard to the Council's fiduciary duty to its taxpayers; and (h) be reasonable and proper in all the circumstances. 4

5 Financial Implications 19. The Scrutiny Committee is empowered to enquire, consider, review and recommend but not to make policy decisions. As the recommendations in this report are to consider and review matters there are no direct financial implications at this stage in relation to any of the work programme. However, financial implications may arise if and when the matters under review are implemented with or without any modifications. Any report with recommendations for decision that goes to Executive/Council will set out any financial implications arising from those recommendations. RECOMMENDATIONS The Committee is recommended to: 1. Take account of the information received at the meeting, and; 2. Report any comments to the Executive for their consideration. Mike Davies Head of Service Scrutiny, Performance & Improvement 11 th May

6 Cardiff Council PRELIMINARY FLOOD RISK ASSESSMENT DRAFT REPORT

7 Hyder Consulting (UK) Limited HCL House Fortran Road St Mellons Business Park St Mellons Cardiff CF3 0EY United Kingdom Tel: +44 (0) Fax: +44 (0) Cardiff Council PRELIMINARY FLOOD RISK ASSESSMENT DRAFT REPORT Author S Hooton Checker R Green Approver N Evans Report No 5000-UA NE-R-01 Date 5 May 2011 This report has been prepared for Cardiff Council in accordance with the terms and conditions of appointment for the Preliminary Flood Risk Assessment dated February Hyder Consulting (UK) Limited ( ) cannot accept any responsibility for any use of or reliance on the contents of this report by any third party. Hyder Consulting (UK) Limited Page 2

8 Executive Summary This report has been prepared by Hyder Consulting (UK) Limited on behalf of Cardiff Council to help manage local flood risk and deliver the requirements of the Flood Risk Regulations 2009 ( the Regulations ). Cardiff Council is defined as a Lead Local Flood Authority (LLFA) under the Regulations. The Preliminary Flood Risk Assessment (PFRA) which comprises this document, the supporting spreadsheets and GIS layer, meets the first stage of the requirements of the Regulations. The PFRA process is aimed at providing a high level overview of flood risk from local flood sources, including surface water, groundwater and ordinary watercourses. As a LLFA, Cardiff Council must submit the PFRA to Environment Agency Wales (EAW) for review by the 22 June The methodology for producing the PFRA has been based on EAW guidance (EAW, 2010) and guidance provided by the Department of Environment, Food and Rural Affairs (DEFRA) and the Welsh Assembly Government (WAG) (DEFRA/WAG, 2010). EAW has used a national methodology, which has been set out by WAG, to identify indicative Flood Risk Areas in Wales. Of the eight indicative Flood Risk Areas that have been identified nationally, one is located within the Cardiff Council boundary. Within the finalised Flood Risk Area, the Regulations require Cardiff Council to produce Flood Risk and Hazard Maps and Flood Risk Management Plans by June 2013 and June 2015 respectively. In order to develop a clear overall understanding of the flood risk across Cardiff, flooding information and records of historical events have been collected from a range of key stakeholders, including the Council, Emergency Services, Dwr Cymru Welsh Water and Community Councils. Records relating to over 1000 flood events caused by flooding from local sources were collected and analysed. Each historical flood record has been analysed. However, no historical flood events are judged to have met the significant harmful consequences criteria which are defined in this report as a flood event which local resources were insufficient to manage and which made national headline news and could occur again. This PFRA has also reviewed the future flood risk to Cardiff, by analysing various local and national data sets. Based on this it is considered that there is no justification for the refinement of the EAW indicative Flood Risk Area for Cardiff. However, the review has highlighted that the boundary of the EAW indicative Flood Risk Area for Cardiff extends into the Vale of Glamorgan Council area and an indicative Flood Risk Area for part of Rhonda Cynon Taff extends into the Cardiff Council area. It is recommended that the indicative Flood Risk Areas are refined along administrative boundary lines. Consultations with the Vale of Glamorgan and Rhonda Cynon Taff Councils are ongoing in order to gain formal agreement to these changes and agree consistency across LLFA PFRAs.

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10 Contents 1 Introduction General Aims and Objectives Study Area LLFA Responsibilities Introduction LLFA Responsibilities Stakeholder Engagement Public Engagement Further Responsibilities Methodology and Data Review Overview Data Collection Process Data Collected Data Limitations Licensing and Restrictions Past Flood Risk Overview Description of Past Historical Floods Significant Harmful Consequences Future Flood Risk Locally Agreed Surface Water Information Ordinary Watercourses Groundwater Other Sources of Flooding Local Drainage Capacity Consequences of Future Floods Climate Change and Long Term Developments Review of Indicative Flood Risk Areas Overview EAW Identified Blue Squares Review of Blue Squares Consultations with adjacent LLFAs Identification of Flood Risk Areas Hyder Consulting (UK) Limited

11 8 Next Steps References Figures Figure 1-1 Cardiff Council Administrative Area... 2 Figure 3-1 Data Collection Pro forma... 6 Figure 4-1 Summary Map showing locations of Historical Flood Events (yellow dots)... 9 Figure 5-1 Locally agreed surface water information (FMfSW) Figure 6-1 1km squares where the flood risk has been assessed to be above the threshold Figure 6-2 Proposed additional 1km squares that are above the flood risk threshold Figure 6-3 Proposed 1km squares that are above the flood risk threshold Figure 7-1 EAW indicative Flood Risk Area Figure 7-2 EAW indicative Flood Risk Area with blue squares Figure 7-3 Proposed Flood Risk Area Tables Table 2-1 Key Stakeholders... 3 Table 3-1 Timeline for PFRA Completion... 5 Table 3-2 Summary of Historic Data Received... 6 Table 3-3 Additional Data from Stakeholders... 7 Table 4-1 Summary of Historical Flooding Records Table 5-1 Summary of Information on future floods that has been reviewed Table 5-2 Summary of the consequences of flooding on human health Table 5-3 Summary of the consequences of flooding on economic activity Table 5-4 Summary of the consequences of flooding on environmental criteria Table 6-1 Summary of flood risk in blue squares Table 6-2 Details for the three additional 1km squares Hyder Consulting (UK) Limited Page ii

12 Annexes Annex 1 Records of past floods and their significant consequences Annex 2 Records of future floods and their consequences Annex 3 Records of Flood Risk Areas and their rationale Annex 4 Review Checklist Annex 5 GIS layer of Flood Risk Area Hyder Consulting (UK) Limited

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14 1 Introduction 1.1 General Cardiff Council has commissioned Hyder Consulting (UK) Limited to undertake this Preliminary Flood Risk Assessment (PFRA). Under the Flood Risk Regulations 2009 ( the Regulations ), Lead Local Flood Authorities (LLFAs) are responsible for undertaking a Preliminary Flood Risk Assessment (PFRA) to provide an evidence base with which to identify areas of significant flood risk (Flood Risk Areas) from local sources such as surface runoff, groundwater and ordinary watercourses. Under the Regulations there is a six year flood risk management planning cycle based on a four stage process of: undertaking a PFRA; identifying Flood Risk Areas; preparing flood hazard and risk maps for Flood Risk Areas preparing flood risk management plans for Flood Risk Areas. This PFRA report and annexes form the first two stages of the process. The scope of the PFRA only covers local flood risk and therefore excludes flooding from main rivers, the sea and large raised reservoirs. This study therefore only considers flooding from surface runoff, groundwater and ordinary watercourses and any interaction these have with drainage systems and other sources of flooding including sewers, the sea and main rivers. A definition of these sources of flooding is as follows: Surface runoff rainwater (including snow and other precipitation) which is on the surface of the ground and has not entered a watercourse, drainage system or public sewer. Groundwater water which is below the surface of the ground and in direct contact with the ground or subsoil. Ordinary Watercourses stream, brook, ditch or non-public sewer which is not a main river. 1.2 Aims and Objectives The PFRA is a high level screening exercise which aims to identify areas where there is a significant flood risk. The PFRA involves: Collecting information from relevant organisations regarding past and potential future floods within the study area; Assembling the information into the PFRA report; Reviewing and amending, if necessary, the existing EAW Flood Risk Areas. The PFRA also aims to inform and support the local flood risk management strategy which is required to be produced under the Flood and Water Management Act Hyder Consulting (UK) Limited Page 1

15 1.3 Study Area The study area, shown in Figure 1-1, is the administrative boundary of Cardiff Council which is an area of approximately 158 km 2 and contains a population of around 336,200. The information collated for this PFRA is for areas located within the Cardiff study area. Cardiff falls within the Ely, Taff and Rhymney main river catchments. Two Catchment Flood Management Plans (CFMPs) cover the Cardiff area: the Taff and Ely CFMP (EAW, 2009) and the Eastern Valleys CFMP (EAW, 2009), which includes the River Rhymney. In Cardiff there are estimated to be 148,109 residential properties, 8,011 non-residential properties and 1,341 critical services (hospitals, nursing/care/retirement homes, electricity sub stations, schools, Police, Fire and Ambulance Stations, prisons and sewerage treatment works). This information has been obtained from the EAW National Receptor Database (NRD), a dataset which contains a number of GIS layers categorised into themes of information. Figure 1-1 Cardiff Council Administrative Area Hyder Consulting (UK) Limited Page 2

16 2 LLFA Responsibilities 2.1 Introduction The preparation of a PFRA is just one of several responsibilities of LLFAs under the new legislation. This section provides a brief overview of the responsibilities Cardiff Council are obliged to fulfil under their role as a LLFA. 2.2 LLFA Responsibilities During the review of the summer 2007 flooding, Sir Michael Pitt stated that the role of local authorities should be enhanced so that they take on responsibility for leading the coordination of flood risk management in their areas. As the designated LLFA, Cardiff Council is therefore responsible for leading local flood risk management across Cardiff. 2.3 Stakeholder Engagement Meetings have been held with representatives attending from Strategic Planning, Emergency Management and Drainage Management departments within Cardiff Council along with WAG, DCWW and IDB. The importance of working together is reflected in Regulation 35 of the Regulations and Section 13 of the Flood and Water Management Act. Table 2-1 lists the key stakeholders that were contacted during the preparation of the PFRA: Table 2-1 Key Stakeholders 1 EAW Stakeholder 2 British Waterways 3 Network Rail 4 Dwr Cymru Welsh Water 5 Caldicot and Wentlooge Levels Internal Drainage Board 6 South Wales Fire Service 7 South Wales Police 8 Tongwynlais Community Council 9 St Mellons Community Council 10 Radyr and Morganstown Community Council 11 Pentyrch Community Council 12 Lisvane Community Council 13 Welsh Assembly Government 2.4 Public Engagement Local public knowledge is also considered valuable and information has been documented by Cardiff Council and community councils. Public meetings attended by Cardiff Council have been held following local flood events and the Council has provided Hyder Consulting with Hyder Consulting (UK) Limited Page 3

17 relevant information (such as meeting minutes and plans) for use in the preparation of the PFRA. 2.5 Further Responsibilities Aside from forming partnerships and coordinating and leading on local flood management, there are a number of other key responsibilities that have arisen for LLFAs from the Flood & Water Management Act and the Flood Risk Regulations. These responsibilities include: Investigating flood incidents LLFAs have a duty to investigate and record details of significant flood events within their area. This duty includes identifying which authorities have flood risk management functions and what they have done or intend to do with respect to the incident, notifying risk management authorities where necessary and publishing the results of any investigations carried out. Asset Register LLFAs also have a duty to maintain a register of structures or features which are considered to have an effect on flood risk, including details on ownership and condition as a minimum. The register must be available for inspection and the Secretary of State will be able to make regulations about the content of the register and records. SuDS Approving Body LLFAs are designated the SuDS Approving Body (SAB) for any new drainage system, and therefore must approve, adopt and maintain any new sustainable drainage systems (SuDS) within their area. Local Strategy for Flood Risk Management LLFAs are required to develop, maintain, apply and monitor a local strategy for flood risk management in its area. The local strategy will build upon information such as national risk assessments and will use consistent risk based approaches across different local authority areas and catchments. This PFRA will be a key resource when developing the Local Flood Risk Management Strategy for Cardiff. Works powers LLFAs have powers to undertake works to manage flood risk from surface runoff and groundwater, consistent with the local flood risk management strategy for the area. Designation powers LLFAs, as well as district councils and the EA have powers to designate structures and features that affect flooding or coastal erosion in order to safeguard assets that are relied upon for flood or coastal erosion risk management. Hyder Consulting (UK) Limited Page 4

18 3 Methodology and Data Review 3.1 Overview The PFRA is based on existing and readily derivable data. Table 3-1 shows the key stages of the PFRA process. Table 3-1 Timeline for PFRA Completion Activity Indicative Completion Date 1 Set up governance & develop partnerships February Determine appropriate data systems February Collate information on past & future floods and their consequences March - April Determine locally agreed surface water information April Complete preliminary assessment report document May Record information on past & future floods with significant consequences in spreadsheet May Illustrate information on past and future floods May Review indicative Flood Risk Areas May Identify Flood Risk Areas May Record information including rationale May Data Collection Process The key stakeholders listed in Table 2-1 were consulted to obtain any relevant data that they hold. Figure 3-1 shows a sample of the pro forma spreadsheet that was issued to the Community Councils, Network Rail, Fire Service and the Police to assist in the collection of historic data. Hyder Consulting (UK) Limited Page 5

19 Figure 3-1 Data Collection Pro forma 3.3 Data Collected A brief summary of the historic flooding data received from the various organisations is shown in Table 3-2. Any gaps in this data have also been summarised in this table. The data sets were compared with each other and where several organisations had recorded the same event duplications were removed to ensure that double counting did not occur. Table 3-2 Summary of Historic Data Received Organisation Summary of Data Provided Type of flooding Gaps in data DCWW 814 records of internal and external sewer flooding in Cardiff. Sewer flooding as a result of hydraulic overload of the system. This includes 466 internal (DG5) flooding records and 348 external flooding records. Flooding details limited to just internal or external flooding. No information on flood duration. Cardiff Council 26 flooding records for properties affected in Rhiwbina in June Fluvial from an ordinary watercourse. Limited information on flood extent and total number of properties affected. Historical flooding record for Whitchurch Brook (between 1990 and 2009).10 known events. Fluvial from an ordinary watercourse. Flooding record is relatively recent. No detailed information on the duration, extent and number of properties affected. 145 historical flooding locations/events. Surface water and fluvial from an ordinary watercourse. No information on duration, cause and number of properties affected. Relatively recent and frequent events. Hyder Consulting (UK) Limited Page 6

20 WAG 26 flooding records for historic sewer flooding. 604 properties and 3 schools affected. Sewer. No information on duration or date of event. No details of cause. 13 flooding records for historic surface water flooding. 267 properties affected. Surface Water No information on duration or date of event. No details of cause. Fire Service Police Service Network Rail British Waterways IDB Tongwynlais Community Council St Mellons Community Council Radyr and Morganstown Community Council Pentyrch Community Council Lisvane Community Council 107 records. Records have been screened and, where needed, removed due to some records relating to domestic burst pipes etc. Significant events appear to have occurred on 1 September 2008 and 6 June No data received. No data received. No data received. No historical flooding records within the Council boundary. Groundwater, surface water, fluvial and sewer. 3 commercial properties flooded. Fluvial from an ordinary watercourse. 5 locations of regular flooding identified. Surface water. 4 locations of regular flooding identified. 2 properties flooded. Surface water and fluvial from an ordinary watercourse. 1 property flooded. Surface water. 3 locations of regular flooding identified. Surface water, sewerage and fluvial from an ordinary watercourse. Limited record only going back to August No information has been provided on the duration or details of number of people affected. Limited number of events and events recorded are relatively recent. Mainly minor road and nuisance flooding. Limited information on the dates of the events. Along with the historic data detailed in Table 3-2 additional information, as outlined in Table 3-3, was provided by Cardiff Council and EAW. Table 3-3 Additional Data from Stakeholders Information 1 Cardiff Strategic Flood Consequences Assessment (SFCA) Source Cardiff Council 2 National Receptor Dataset EAW 3 Indicative Flood Risk Areas EAW 4 Areas Susceptible to Surface Water Flooding (AStSWF) EAW 5 Flood Map for Surface Water (FMfSW) EAW 6 Historic Flood Map EAW Hyder Consulting (UK) Limited Page 7

21 7 Areas Susceptible to Groundwater Flooding EAW 8 Cluster Boundaries EAW 9 Flood Zone 3 Map EAW 10 Taff and Ely Catchment Flood Management Plan (CFMP) EAW 11 Eastern Valleys CFMP EAW 12 Severn Estuary Flood Risk Management Strategy EAW 3.4 Data Limitations The PFRA is limited to a certain extent by the quality of the data that has been received. In general the historical flooding information is only for relatively recent events and there is significant variation in the quality, accuracy and detail of the data from the different organisations. Some key gaps in the data sets have been highlighted in Table 3-2. A large proportion of the data was not available in a geo-referenced format, this data was subsequently geo-referenced relying on location descriptions and postcodes to try and accurately allocate the National Grid Reference. Cardiff Council do not currently have a formal database for documenting flood records. It is understood that records from some past events may have been lost or destroyed and held with individuals rather than in formal datasets. In the future it is proposed to develop a GIS system which will record all flooding and drainage geo-referenced data collected as part of the PFRA. This GIS system will enable the LLFA to access and interrogate existing information on past and future floods as well as create new flooding records. 3.5 Licensing and Restrictions The use of some of the data provided by EAW and DCWW is restricted to Cardiff Council in the preparation of the PFRA. Hyder Consulting (UK) Limited Page 8

22 4 Past Flood Risk 4.1 Overview The flood information received from the various stakeholders, detailed in Table 3-2, resulted in over 1000 historic flood records within the study area. The data was reviewed and a master spreadsheet of the appropriate information was compiled. This master data set was then loaded into GIS software to enable the historic flood event data to be viewed in its graphical location along with the National Receptor Database. Analysing the information in its GIS format enabled a logical assessment of the overall flood risk. This section summarises the findings of the review of the historical flooding records in order to extract relevant information on past events with significant harmful consequences. 4.2 Description of Past Historical Floods General A summary map showing the locations of known historical flooding is provided in Figure 4-1 and Table 4-1 provides an overview of the historical flooding records by receptor type. Figure 4-1 Summary Map showing locations of Historical Flood Events (yellow dots) Hyder Consulting (UK) Limited Page 9

23 Table 4-1 Summary of Historical Flooding Records Receptor Type Number of recorded events Residential Properties 341 Critical Services (schools) 9 Non- Residential Properties 156 Open areas (gardens, parks, roads etc) Surface Water Flooding Surface water flooding can occur when heavy rainfall overwhelms the drainage capacity of local areas. The historic records show that localised surface water flooding is widely distributed throughout Cardiff, which is consistent with the general topography of the area Ordinary Watercourses Flooding from ordinary watercourses occurs when a channel cannot cope with the water draining into it from the surrounding land. This can happen, for example, when heavy rain falls on an already waterlogged catchment. A key historic flooding event occurred in June 2009 when 29 residential properties, 5 non-residential properties and 1 critical service (school) were inundated across Cardiff. In addition to the reported property flooding there were 71 records of flooding of open areas. In this event the flooding was predominantly in the area of the Whitchurch Brook located in the northern area of the city. The flooding that occurred in this event was shown to be caused by the flow capacity of this ordinary watercourse being exceeded as a result of a rainfall event with an annual probability of between 3.33% and 5% Groundwater Flooding Groundwater flooding occurs when water levels in the ground rise above surface levels. It is most likely to occur in areas underlain by permeable rocks, called aquifers. These can be extensive, regional aquifers, such as chalk or sandstone, or may be more local sand or river gravels in valley bottoms underlain by less permeable rocks. There is no information on historic groundwater flooding which suggests that the risk of groundwater flooding is low in Cardiff Sewer Flooding Sewers can become overwhelmed by heavy rainfall which can cause flooding. The likelihood of flooding depends on the capacity of the local sewerage system. Land and property can be flooded with water contaminated with raw sewage as a result. Rivers can also become polluted by sewer overflows. A key sewer flooding event occurred in July 2001 with 230 residential properties, 129 non-residential properties and 3 critical services (schools) inundated across Cardiff. The records for this event were received from DCWW s DG5 register but, due to lack of information dating back to 2001, no other details, apart from summary information in the Cardiff Strategic Flood Consequences Assessment (SFCA) (Atkins, 2009), were available for this event. In addition to the reported property flooding there were 177 records of flooding of open areas. The flooding Hyder Consulting (UK) Limited Page 10

24 that occurred in this event was shown to be caused by hydraulic overload of the sewers as a result of a 1.67% annual probability rainfall event which exceeded the design capacity of the sewers (generally designed to a 3.33% annual probability standard). Once a property has been identified on DCWW s DG5 register, it typically means that the water company can put funding in place to take it off the DG5 register and, as such, a recurrence of a similar flood event is unlikely. 4.3 Significant Harmful Consequences National guidance issued by WAG sets thresholds for defining Flood Risk Areas where the risk of flooding is deemed to be significant. No guidance has been issued for defining locally significant harmful consequences as it is up to each LLFA to set its own definition. As agreed with Cardiff Council in this PFRA significant harmful consequences have been defined as the consequences of a flood event which local resources were insufficient to manage and which made national headline news and could occur again. For the localised surface water flooding and both the July 2001 and June 2009 flood events, Cardiff Council has confirmed that local resources were able to cope with the flooding experienced. Therefore, in line with the definition above, the PFRA has confirmed that there are no known past floods which are considered to have had significant harmful consequences. Therefore, no events with significant harmful consequences have been recorded in Annex 1. Hyder Consulting (UK) Limited Page 11

25 5 Future Flood Risk This section summarises all relevant information on future floods. Table 5-1 outlines the relevant information on future flooding that has been reviewed as part of the PFRA. Table 5-1 Summary of Information on future floods that has been reviewed No Information Source Use in Study 1 Flood Map for Surface Water (FMfSW) 2 Flood Zone 3 Map (excluding main river extents) 3 Areas Susceptible to Groundwater Flooding 4 Cardiff Strategic Flood Consequences Assessment (SFCA) 5 Extracts of the Whitchurch Brook Project Appraisal Report (PAR) EAW EAW EAW LLFA LLFA Assess future flood risk associated with surface water flooding Assess future flood risk associated with flooding from ordinary watercourses Assess future flood risk associated with groundwater flooding In the SFCA simplistic assessments have been undertaken to assess the flood risk from surface water and from ordinary watercourses. These assessments are mainly related to future developments. The Flood Map for Surface Water is considered to be more accurate for defining the future flood risk associated with surface water flooding and therefore the SFRA predictions on future flooding have only been used to provide background information. Only summary extracts of the findings of the PAR have been obtained. No detailed information to quantify the future flood risk associated with the Whitchurch Brook has been provided. 5.1 Locally Agreed Surface Water Information No strategic studies have been undertaken to specifically assess the flood risk from surface water in Cardiff. In the Cardiff SFCA (Atkins, 2009) reference has been made to known past surface water flooding locations, however no surface water flood maps have been produced. EAW has produced a national assessment of surface water flood risk in the form of two national mapping datasets. The first generation national mapping, Areas Susceptible to Surface Water Flooding (AStSWF) contains three susceptibility thresholds for a rainfall event that has a 0.5% annual probability of occurrence. The national methodology has since been updated to produce the Flood Map for Surface Water (FMfSW), a revised model with two flood events (3.33% and 0.5% annual probabilities) and two depth thresholds (greater than 0.1m and greater than 0.3m deep). The 0.5% event and 0.3m deep threshold has been used by the EA to produce indicative Flood Risk Areas and adopted by the LLFA to identify properties and critical services at future risk of surface water flooding. As no local information has been produced the FMfSW national dataset forms the locally agreed surface water information. The AStSWF dataset was not adopted as it was viewed to be less Hyder Consulting (UK) Limited Page 12

26 accurate, as the estimates of surface water flows did not consider infiltration and the effects of sewers. In addition the FMfSW dataset takes into account buildings which were not represented in the AStSWF dataset. The locally agreed surface water information is shown in Figure 5-1 and illustrates areas that are predicted to flood to a depth greater than 300mm associated with a 0.5% annual probability rainfall event. Figure 5-1 Locally agreed surface water information (FMfSW) 5.2 Ordinary Watercourses There are four key ordinary watercourses in Cardiff: the Whitchurch Brook, Fairwater Stream, Roath Brook and Nant Glandulas. Although designated as ordinary watercourses the EAW Flood Map covers these watercourses from the point at which their catchment areas exceed 3km². As these four watercourses are an important source of flooding in Cardiff the main river sections from the flood map have been removed in order to derive information about flooding from ordinary watercourses (see Section 5.6). A number of other ordinary watercourses are present in the area, many of which are partly culverted under the densely developed areas of the city. The Dock Feeder runs from an offtake from the River Taff at Blackweir to Bute Dock though sections of open canalised channels and culverts. It is understood that no flood studies have been undertaken for this watercourse. The Wentlooge Levels within the Cardiff Council boundary extends from the Rhymney River in the west to St Mellons Business Park in the East. The Wentlooge Levels comprise an area of reclaimed land. As part of the reclamation and subsequent management, an extensive system of ditches (reens) exists to drain the low lying land. A high level study was undertaken as part of the Cardiff SFCA (Atkins, 2009) to indicatively assess the risk of flooding from the reen system. This assessment only predicted significant flooding of low lying areas outside the Cardiff Council boundary. Therefore, in addition to the active management and regular maintenance of the reen system by the IDB, it is considered that there is no significant risk of flooding from the reens within the Cardiff Council boundary. Hyder Consulting (UK) Limited Page 13

27 5.3 Groundwater It is understood through consultations with key stakeholders that there is no local information on future groundwater flooding for Cardiff. The risk of groundwater flooding is considered to be low, and this is confirmed by the Taff and Ely CFMP (EAW, 2009) which states that groundwater flooding is not considered to be a significant issue within the catchment. The EAW map showing Areas Susceptible to Groundwater Flooding has been used in conjunction with the future Surface Water Flood Map (as this identifies potential areas of ponding due to topographical depressions) to assess the consequences of future groundwater flooding. The results of these are outlined in Section Other Sources of Flooding No local information on other sources of future flooding has been obtained for the Cardiff area. 5.5 Local Drainage Capacity Cardiff has both separate and combined foul and surface drainage systems. No specific information has been obtained on the capacity of highway drainage within the Cardiff area. In addition, although DCWW historical flood records are given a risk category based on a prescribed methodology (a combination of frequency of flooding and storm return periods), no specific information has been obtained on the capacity of sewer systems within the Cardiff area In line with national design standards and information on past floods in Cardiff, the capacity (surcharged conditions) of highway drainage networks and sewers is likely to be exceeded in rainfall events with an annual probability in the order of 3.33%. 5.6 Consequences of Future Floods Detailed records of future floods and their possible consequences are provided in Annex Human Health Table 5-2 summarises the consequences of flooding on human health using the indicators of the number of people and critical services affected. Table 5-2 Summary of the consequences of flooding on human health Criteria Surface Runoff Ordinary Watercourses Number of People Groundwater 4,287 2, Number of Critical Services The number of people at risk of flooding has been calculated by multiplying the number of properties at risk by There are different methods for calculating the number of properties and in this study the address point data rather than the Flood Map for Surface Water Property Hyder Consulting (UK) Limited Page 14

28 Count Method has been used. In the early stages of the project the use of the Flood Map for Surface Water Property Count Method was not feasible due to the combination of the large amount of reformatting of the OS building layer and the study deadline. It is likely that the use of the address point data will slightly under predict the number of people and critical services affected. However, given the high level screening nature of the PFRA and the high level of uncertainty associated with the flood data sets the use of the address point data is considered to be acceptable Economic Activity Table 5-3 summarises the consequences of flooding on economic activity in Cardiff. The predicted consequences of flooding of road infrastructure, although included in Table 5-3, is not considered to be adverse as alternative routes are available due to the dense road network throughout Cardiff. In addition the surface water flood maps are believed to over predict the flood risk to this infrastructure. Table 5-3 Summary of the consequences of flooding on economic activity Criteria Surface Runoff Ordinary Watercourses Groundwater Number of nonresidential properties Infrastructure network (e.g. length of roads and rail flooded) M4-0.9km A48 1.3km A km A km No adverse consequences identified No adverse consequences identified Environment Table 5-4 summarises the consequences of flooding on the environment in Cardiff. The information has been obtained by interrogating NRD data against the associated flood extent. The consequences of the predicted flooding of environmental assets is not considered to be adverse. Hyder Consulting (UK) Limited Page 15

29 Table 5-4 Summary of the consequences of flooding on environmental criteria Criteria Surface Runoff Ordinary Watercourses Pollution Prevention and Control (PPC) sites Groundwater Control of Major Accident Hazard (COMAH) sites No Data Provided Special Areas of Conservation (SAC) Special Protection Areas (SPA) Ramsar Sites Sites of Special Scientific Interest (SSSI) World Heritage Sites Scheduled Monuments Listed Buildings Registered parks and gardens Climate Change and Long Term Developments The Evidence There is clear scientific evidence that global climate change is happening now. It cannot be ignored. Over the past century around the UK we have seen sea level rise and more of our winter rain falling in intense wet spells. Seasonal rainfall is highly variable. It seems to have decreased in summer and increased in winter, although winter amounts changed little in the last 50 years. Some of the changes might reflect natural variation, however the broad trends are in line with projections from climate models. Greenhouse gas (GHG) levels in the atmosphere are likely to cause higher winter rainfall in future. Past GHG emissions mean some climate change is inevitable in the next years. Lower emissions could reduce the amount of climate change further into the future, but changes are still projected at least as far ahead as the 2080s. Hyder Consulting (UK) Limited Page 16

30 We have enough confidence in large scale climate models to say that we must plan for change. There is more uncertainty at a local scale but model results can still help us plan to adapt. For example we understand rain storms may become more intense, even if we can t be sure about exactly where or when. By the 2080s, the latest UK climate projections (UKCP09) are that there could be around three times as many days in winter with heavy rainfall (defined as more than 25mm in a day). It is plausible that the amount of rain in extreme storms (with a 1 in 5 annual chance, or rarer) could increase locally by 40% Key Projections for Severn River Basin District If emissions follow a medium future scenario, UKCP09 projected changes by the 2050s relative to the recent past are Winter precipitation increases of around 12% (very likely to be between 2 and 26%) Precipitation on the wettest day in winter up by around 9% (very unlikely to be more than 22%) Relative sea level at Bristol very likely to be up between 10 and 40cm from 1990 levels (not including extra potential rises from polar ice sheet loss) Peak river flows in a typical catchment likely to increase between 9 and 18% Increases in rain are projected to be greater at the coast and in the south of the district Implications for Flood Risk Climate changes can affect local flood risk in several ways. Impacts will depend on local conditions and vulnerability. Wetter winters and more of this rain falling in wet spells may increase river flooding along the Severn and its tributaries. More intense rainfall causes more surface runoff, increasing localised flooding and erosion. In turn, this may increase pressure on drains, sewers and water quality. Storm intensity in summer could increase even in drier summers, so we need to be prepared for the unexpected. Drainage systems in the district have been modified to manage water levels and could help in adapting locally to some impacts of future climate on flooding, but may also need to be managed differently. Rising sea or river levels may also increase local flood risk inland or away from major rivers because of interactions with drains, sewers and smaller watercourses. Where appropriate, we need local studies to understand climate impacts in detail, including effects from other factors like land use. Sustainable development and drainage will help us adapt to climate change and manage the risk of damaging floods in future Adapting to Change Past emission means some climate change is inevitable. It is essential we respond by planning ahead. We can prepare by understanding our current and future vulnerability to flooding, developing plans for increased resilience and building the capacity to adapt. Regular review and adherence to these plans is key to achieving long-term, sustainable benefits. Although the broad climate change picture is clear, we have to make local decisions against deeper uncertainty. We will therefore consider a range of measures and retain flexibility to adapt. This approach, embodied within flood risk appraisal guidance, will help to ensure that we do not increase our vulnerability to flooding. Hyder Consulting (UK) Limited Page 17

31 5.7.5 Long Term Developments It is possible that long term developments might affect the occurrence and significance of flooding. However current planning policy aims to prevent new development from increasing flood risk. In Wales, Technical Advice Note 15 (TAN15) on development and flood risk sets out a precautionary framework to guide planning decisions. The overarching aim of the precautionary framework is "to direct new development away from those areas which are at high risk of flooding." Adherence to Government policy ensures that new development does not increase local flood risk. However, in exceptional circumstances the Local Planning Authority may accept that flood risk can be increased contrary to Government policy, usually because of the wider benefits of a new or proposed major development. Any exceptions would not be expected to increase risk to levels which are "significant" (in terms of the Government's criteria). Consultation with Cardiff Strategic Planning department has confirmed that there are no new or proposed major developments which may increase local flood risk within Cardiff Local Information Concerning Climate Change Impacts on Flood Risk No local information has been made available regarding climate change impacts on future flood risk. Hyder Consulting (UK) Limited Page 18