London Borough Hillingdon. Ruislip Lido Improvement Programme Flood Risk Assessment September 2011. Halcrow Group Limited

London Borough Hillingdon Ruislip Lido Improvement Programme Flood Risk Assessment September 2011 Halcrow Group Limited

London Borough Hillingdon Ruislip Lido Improvement Programme Flood Risk Assessment September 2011 Halcrow Group Limited Halcrow Group Limited Burderop Park Swindon Wiltshire SN4 0QD Tel +44 (0)1793 812479 Fax +44 (0)1793 812089 www.halcrow.com Halcrow Group Limited has prepared this report in accordance with the instructions of their client, London Borough of Hillingdon, for their sole and specific use. Any other persons who use any information contained herein do so at their own risk. Halcrow Group Limited 2012

London Borough Hillingdon Ruislip Lido Improvement Programme Flood Risk Assessment Contents Amendment Record This report has been issued and amended as follows: Issue Revision Description Date Signed 1 2 3 Draft Final Final* For agreement 10 February 2011 16 June 2011 23 September 2011 CJ Weeks R Bird PS Rayner * minor update only to include statements on baseline operating level for Ruislip Lido and sustainable drainage systems for planned improvements

Contents 1 Introduction 1 1.1 Scope of FRA 1 1.2 Ruislip Lido 1 2 Background 2 2.1 Ruislip Lido 2 2.2 Flood risk downstream of Ruislip Lido 2 2.3 Flood risk impact of operating level at Ruislip Lido and downstream 3 3 Planned improvements for Ruislip Lido 4 4 Flood risk assessment (FRA) requirements 6 5 Flood risk and development considerations 7 6 Updating the Flood Zones for Ruislip Lido 8 6.1 Approach 8 6.2 Previous Pinn flood mapping study 8 6.3 Technical studies supporting this FRA 8 6.4 FRA Model Report 9 7 Revised Flood Zones for Ruislip Lido 11 8 Flood risk implications of planned improvements for Ruislip Lido 13 8.1 Planned improvements implications that follow from PPS25 13 8.2 Overflow car park proposal compensatory floodplain storage 15 8.3 Overflow car park proposal depth of flooding and flood warning time 15 8.4 Overflow car park proposal surface water drainage and SUDS 16 8.5 Other planned improvements surface water drainage and SUDS 16 9 Flood risk implications of alternative normal operating water level for Ruislip Lido 17 9.1 Option under consideration 17 9.2 Basis for normal operating water level at 1m below datum 17 9.3 Review of the 1991 calculation by LB Hillingdon 17 9.4 Current normal operating level at 0.65m below datum 18 9.5 Flood risks downstream of Ruislip Lido Cannon Brook/River Pinn 19 9.6 Conditions on the Cannon Brook 24 9.7 Recommendations 25 10 Concluding remarks 27 Appendix A Photographic appraisal of study area 28 Appendix B Review of LB Hillingdon records on Ruislip Lido water level management 29 Appendix C Overflow car park proposal surface water drainage and SUDS calculations 36

Tables Table 1: Appropriate development types Table 2: Flood risk implications for planned improvements for Ruislip Lido current water level management scenario Table 3: Modelled peak flood levels on Cannon Brook at Ladygate Lane Table 4: Modelled peak flood levels on Cannon Brook at Ladygate Lane for climate change scenario Figures Figure 1: Plan showing location of the site Figure 2: Flood risk map Figure 3: Ruislip Lido facilities / proposals Figure 4: Flood Zone classification Figure 5: Revised Flood Zones current water level management scenario Figure 6: Overflow car park proposal Figure 7: River system Figure 8: Environment Agency Flood Map Figure 9: Model results to illustrate the flood flow regime in Cannons Brook for 1:100-year climate change event Figure 10: Model results to illustrate the flood flow regime in River Pinn for 1:100-year flood event

1 Introduction 1.1 Scope of FRA This Flood Risk Assessment (FRA) report has been prepared on behalf of London Borough of Hillingdon for the planned improvements and new and enhanced recreational use at Ruislip Lido. This FRA considers these improvements including the overflow car park proposal and supports the planning application recently submitted for this. This FRA also considers a possible change in operating regime for lake levels for recreational purposes and to improve environmental conditions and Ruislip Lido facilities of benefit to the local community. The FRA follows the guidance of Planning Policy Statement 25: Development and Flood Risk (PPS25) and its accompanying practice guide, and is required by the Environment Agency as statutory consultee in the planning process with a lead role in providing advice on flood issues in relation to planning applications. 1.2 Ruislip Lido Ruislip Lido is located in the London Borough (LB) of Hillingdon in green belt land in western Greater London, centred on Ordnance Survey Grid Ref. TQ 088 894 (Figure 1, see below). Ruislip Lido is owned by LB Hillingdon. It includes a large man-made reservoir surrounded by semi-natural woodland, scrub and grassland habitat. It is managed as a recreational and educational facility for the community, with visitor attractions including a miniature (narrow gauge) railway, a café, a pub and a visitors centre, plus associated toilet facilities. Ruislip Lido is largely surrounded by Ruislip Woods National Nature Reserve (NNR) and Site of Special Scientific Interest (SSSI), which it directly borders to the east, north and west. The site occupies a small (approximately 0.5 hectares) area to the west of the Lido, located between the reservoir to the east and the miniature railway to the west. A photographic appraisal of the study area is included in Appendix A. This appraisal shows the main spillway area for overtopping events, including the main overflow culvert, the auxiliary overflow pipe for low level flows and the outlet area downstream of the Lido. Figure 1: Plan showing location of the site Site location 1

2 Background 2.1 Ruislip Lido Ruislip Lido is a man-made reservoir dating back to the nineteenth century. It was alleged that in the late 1980s water discharged from the Lido was partly responsible for flooding downstream along the Cannon Brook (receiving watercourse for outflows from Ruislip Lido); to prevent any reoccurrence the water level has since been lowered (by up to a meter). The water level at Ruislip Lido is carefully monitored and managed in view of past flooding problems to residential properties downstream on the Cannon Brook. Under the current operating regime, the outlet is manually activated to maintain water level between 0.65-1.0m below the auxiliary overflow pipe invert (referred to as the datum level), according to information advised by the LB Hillingdon. Over the past year the lake level was allowed to exceed this normal operating level on occasion, but there are no plans to repeat this. The concrete apron in-front of the overflow pipe provides an additional 0.3m freeboard (approximately). Thus to hold the lake level at 1m below the datum level effectively provides 1.3m freeboard for flood storage. For this flood risk assessment the baseline operating level is taken as as 1m below the datum as this was adopted by LB Hillingdon over ten years ago. Under the current operating regime, no water is discharged from the Lido during rainfall events, except on the rare occasion when the lake levels rises above the level of the concrete apron in-front of the auxiliary overflow pipe. At other times any outflow is restricted to not more than 25mm (drop in water level) per day, by manually operating by a system of valves that control a scour outlet pipe (its inlet is permanently under water). During dry periods the lake level will fluctuate below the normal operating level. 2.2 Flood risk downstream of Ruislip Lido A strategic flood study by the Environment Agency indicates that the flood risk on the Cannon Brook affects two residential areas. One area is located near the junction of Ducks Hill Road (A4180) and Breakspear Road, and further downstream in the vicinity of Ladygate Lane (Figure 2) based on the current Environment Agency flood map. Flood Zones do not take account of the presence of defences since there is a risk that defences may fail during a flood event, however, as the flood map shows no areas benefiting from defences then it can be assumed that the area is undefended. Based on the Pinn flood mapping study (2008) the flooding on the Cannon Brook, in the vicinity of Ladygate Lane, results from a combination of flows (e.g. 7.6m 3 /s for 1:100-year design flood event) derived from the Ruislip Lido outflow (60%), Mad Bess Brook that joins the Cannon Brook at this point (30%) and a local inflow on Ladygate Lane (10%). The more detailed modelling of Ruislip Lido completed for this FRA indicates that the Ruislip Lido outflow (60%) is overstated in the previous flood mapping study, given the reservoir outlet arrangement. This means the flood risks are overstated, and should be further considered by the Environment Agency in any future updates of their Flood Map in this area. 2

The FRA finds that the Ruislip Lido is not such a significant factor in the flooding problems in the Cannon Brook, and its storage provides considerable flood alleviation benefit to the local community. Without the reservoir the flood risk in Cannon Brook and further downstream would be significantly greater, and flooding of properties would be predicted on a regular basis. Figure 2: Flood risk map (source: Environment Agency website) Flood risk to residential properties indicated at these two locations downstream of Ruislip Lido 2.3 Flood risk impact of operating level at Ruislip Lido and downstream Assessment of the impact of changes in operating level at Ruislip Lido shows that to hold the initial lake level at either 1m below the original datum (i.e. 1.3m freeboard for flood storage) or 0.65m below (0.95m freeboard for flood storage) has no impact downstream at Ladygate Lane or at the confluence of Cannon Brook with the River Pinn based on the current flood risk. This is because as the Ruislip Lido holds back flood flow the flood risk on Cannon Brook results primarily from flows on Mad Bess Brook. The above operating levels can be compared with the 1992 recommendation for...keeping the water level at -1.5 metres. (LB Hillingdon Engineer's Information Report, Utilisation of Ruislip Lido as a flooding prevention facility, ref: ESH.259). For the purposes of this FRA the modelling and mapping of strategic Flood Zones assumes the normal operating level at Ruislip Lido is held at 0.65m below datum. Further consideration of the impact of the operating level is included in Section 9. 3

3 Planned improvements for Ruislip Lido The proposals for planned improvements and new recreational use include: new toilets, new changing room facilities, improved catering facilities, nature area enhancement, beach swimming area, angling, parking, children s paddleboat area, woodland centre, etc. The planned improvements, some of which are improvements to existing facilities, are located in Figure 3. These improvements are categorised below in terms of new buildings, replacing existing facilities and other (non-structural) improvements. A. New or replacement buildings / facilities Changing room facilities: beach side changing room facilities. Toilets: new toilet block next to overflow car park Rain shelters. Parking: overflow car park (160 spaces, tarmac) proposal within the Lido grounds on area previously used for this purpose. B. Replacing existing facilities Woodlands centre / catering facilities (as replacement of existing building within same footprint) to help inform / educate visitors about the local flora and fauna; and improved catering facilities: propose greater choice; something for everyone. Toilets: new beach side toilet blocks already installed as one of the first improvements. Model railway: plan to invest further in this much enjoyed and valued feature. C. Other (non-structural) improvements Parking: in addition to overflow car park (as above) to investigate arrangements for car parking outside and options for resident parking. Nature area: proposals for enhancement of this man made feature will still respect and provide quiet natural space for wildlife to flourish, subject to an appropriate EIA. Beach swimming area: plan to reintroduce swimming subject to regular water quality testing, with costs of a suitable enclosed swimming area near the beach under review. Angling: plan to offer angling opportunities in designated areas. Children s paddle boat area: plan to recreate the enclosed children s boating area on the Lido Boating: LB Hillingdon have been asked to consider reintroducing boating for families and older children but not motor boating. Site security - full review of all aspects of the security of the site is underway, including the Council officers and the Police Safer Neighbourhood Team discussing ways to address any concerns from residents regarding anti social behaviour. As part of this improvement programme, LB Hillingdon is considering a possible change in operating regime for lake levels for recreational purposes (swimming and possibly boating) and to improve environmental conditions and Ruislip Lido facilities of benefit to the local community. A change in operating level (to 0.5m below datum) is being considered and relevant to this is the assessment in Section 9 (see also final paragraph in Section 2). 4

Figure 3: Ruislip Lido facilities / proposals Ruislip Lido facilities (source: LB Hillingdon) Note: LB Hillingdon confirmed the changing rooms and toilets as permitted development, with only the overflow car park and woodland centre/catering facility proposals subject to formal planning applications Ruislip Lido proposals (source: LB Hillingdon) Overflow car park Toilets Rain shelters Woodland centre with catering facility Changing room Toilet blocks Rain shelters 5

4 Flood risk assessment (FRA) requirements In view of the flood risks at Ruislip Lido and downstream on the Cannon Brook, the Environment Agency require a formal FRA in accordance with Planning Policy Statement 25: Development and Flood Risk (PPS25) and its accompanying practice guide, which takes into account climate change effects The site specific FRA requirements were established following discussion and guidance from the Environment Agency (development and flood risk team). The main purpose of the FRA is to identify the extent of the floodplain and to confirm no adverse impact on flood risk from the Ruislip Lido improvement programme. The specific requirements for the FRA can be summarised as: Identify the extent of the floodplain during the 1 in 100-year event and whether or not the site or proposed buildings are outside the flood envelope, in accordance with PPS25, which defines the types of development appropriate in different Flood Zones. Investigate the impact of climate change as this may generate a significantly greater flood risk, with the current climate change guidance of Defra (2006). Based on this guidance, PPS25 states that in the Thames catchment, increases in peak flow of up to 20% for a given return period could be experienced within 50 years due to climate change. The latest research suggests that this increase may come later, from 2080, but new guidance based on this research is not yet available. Seek to ensure that storage in the floodplain is not reduced and flow conveyance is not interrupted, for any severity of flood up to the 1 in 100 year flood outline (including an allowance for the increase in peak flows as a result of climate change). Practically, this may entail not constructing any permanent buildings and not raising ground levels within the floodplain, without providing adequate compensatory storage. The Environment Agency advised that as part of the FRA the model prepared for the Pinn flood mapping study, which stops just downstream of the Lido at Duck Hill Road (A4180), should be extended further upstream to include the Lido and the flows that enter the Pinn model carefully considered. This extended model can then be used to investigate more accurately the flood risks at Ruislip Lido and downstream. 6

5 Flood risk and development considerations From PPS25 the appropriate types of development according to Flood Zones are listed below in Table 1. The Flood Zones, as currently defined, are shown in Figure 2 (Flood Zone 3a is indicated by the darker blue and Flood Zone 2 the lighter blue). Many of the Ruislip Lido proposals (Figure 3) fall within the Water Compatible Development category, with development permitted as they are appropriate for any Flood Zone. Restrictions would apply for any buildings, such as the Woodland centre proposal. In the case of the overflow car park proposal the Environment Agency advised that car parks located in areas that flood must be designed to mitigate risk to parked cars. Table 1: Appropriate development types (only types that may be relevant to Ruislip Lido) Source of Flooding Flood Zone 1 Flood Zone 2 Flood Zone 3a Flood Zone 3b Essential Water Compatible Infrastructure Development Permitted Development Highly Vulnerable No restrictions on development other than managing surface water runoff Essential transport infrastructure which has to cross the area at risk, and strategic utility infrastructure Development should be avoided. flood control infrastructure water transmission infrastructure and pumping stations water-based recreation (excluding sleeping accommodation) lifeguard station amenity open space, nature conservation and biodiversity 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 flood warning and evacuation plan. caravans, mobile homes and park homes intended for permanent residential use Development should not be permitted More Vulnerable Less Vulnerable Buildings used as: dwelling houses drinking establishment educational establishment sites used for holiday or short-let caravans and camping Subject to a specific warning and evacuation plan Development should be avoided Development should not be permitted Buildings used as: shops professional and other services restaurants and cafes, hot food takeaways offices non-residential institutions not included in more vulnerable, assembly and leisure land and buildings used for agriculture and forestry Development should not be permitted 7

6 Updating the Flood Zones for Ruislip Lido 6.1 Approach Updating the Flood Zones as part of this FRA has involved detailed hydrological analysis and hydraulic modelling, following the site-specific advice from the Environment Agency (Section 4) and their guidance: Requirements for completing computer river modelling for Flood Risk Assessments Guidance for developers (February 2009). As stated in this guidance: This document provides a comprehensive guide to the information and approach expected of a complete FRA. It is intended to give our best practice guidance on the standards that should be used when carrying out computer modelling of watercourses when completing an FRA, and to comply with our local requirements. The site specific requirements for modelling were established in discussion with the Environment Agency (flood risk and data management team) with knowledge of the downstream Pinn model, and based on Halcrow s extensive knowledge of flood mapping to the Environment Agency specification (the Strategic Flood Risk Management specification). 6.2 Previous Pinn flood mapping study The Pinn flood mapping study commissioned by the Environment Agency (2008) developed a model using a channel X-section survey from the 1990's and built in 1D ISIS river modelling software package (developed by Halcrow, see details at this link: http://www.halcrow.com/software/). The model results were used to generate the Environment Agency published flood map (Figure 2), which confirms the flood risk to residential properties downstream of the Lido. Anecdotal evidence about flooding in 1987 and the reasons for maintaining the Lido lake level at a low level confirms this 6.3 Technical studies supporting this FRA The studies completed for this FRA can be summarised as: Data gathering: including topographic data (aerial LiDAR survey at 2m grid resolution); hydraulic model that covers the Main River limit of the River Pinn with its upstream limit at Duck Hill Road last updated in 2008; hydrological (inflow) model based on the Environment Agency FRQSIM model; data specific to Ruislip Lido including the reservoir inspection reports and previous flood studies. Topographic survey (to the Environment Agency SFRM specification): including channel survey sections for the Cannon Brook immediately downstream of the Ruislip Lido, bridge sections for Duck Hill road bridge, outfall structure details of the Ruislip Lido and spot levels around the Lido to confirm the accuracy of the LiDAR. Flood flow estimation: deriving design inflows to Ruislip Lido following Environment Agency guidelines that recommend the Flood Estimation Handbook (FEH) methodology, with flow predictions reconciled with the original (FRQSIM) estimates from the previous Pinn modelling study (2008) approach agreed with the Environment Agency technical specialist (D Rylands). 8

Model extension: include Ruislip Lido (new survey and outflow structures/culverts) then calibrate/validate to confirm model performs as per original model, and run model for known flood events to verify new schematisation. Design model runs: including design events to update the Flood Zones for Ruislip Lido for baseline conditions and for climate change scenarios as per PPS25 (following Defra guidance for +20% increase in flows, October 2006). Propose compensatory storage measures for the car park proposal if the car park footprint falls within the (1 in 100-year return period) floodplain taking account of climate change. 6.4 FRA Model Report The technical studies are presented in a separate FRA Model Report, set out in accordance with the Model Report Requirements section of the Environment Agency guidance on requirements for completing computer river modelling (in Section 5). The Ruislip Lido Model Report is structured as follows: Introduction Data sources Hydrological model Hydraulic model Hydraulic model parameters Model calibration/verification Model results Sensitivity analysis Audit trail Model limitations Concluding remarks References An addendum section on modelling of Cannon Brook is also included, which assesses the flood risk downstream of Ruislip Lido as far as Ladygate Lane, which last experienced damaging flooding in the 1980s. The FRA Model Report, supporting this main FRA report, was previously submitted to the Environment Agency technical specialists to confirm the reported results and revised Flood Zones. The model results are discussed in the following section of this FRA report. 9

7 Revised Flood Zones for Ruislip Lido The Ruislip Lido Model Report presents the modelling work carried out to prepare revised Flood Zones for Ruislip Lido (as discussed in Section 6 above) these Flood Zones revise the currently published Environment Agency Flood Zones that are based on very broadscale modelling and for this reason required updating as part of this site-specific FRA. In line with PPS25 the Flood Zones (FZ) are defined as follows: FZ1 (Low probability) This zone comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). FZ2 (Medium probability) This zone comprises land assessed as having between a 1 in 100 and 1 in 1000 annual probability of river flooding (1% 0.1%) in any year. Figure 4: Flood Zone classification FZ 3a (High probability) This zone comprises land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%) in any year. Flood Zone 3b (Functional Floodplain) This zone comprises land where water has to flow or be stored in times of flood (land which would flood with an annual probability of 1 in 20 (5%) or greater in any year, or is designed to flood in an extreme (0.1%) flood, including water conveyance routes). The assumptions used to model the impacts of climate change are based on the following precautionary predictions (to the end of this century) as advised by Annex B of PPS25: increase in fluvial flows by 20%. This is applicable to the planning horizon for the planned improvements at Ruislip Lido. Figure 5 shows the revised Flood Zones, illustrated as flood outlines and depths for FZ3b (assumed as 1:20-year return period), FZ3a (1:100-year return period) and FZ2 (1:1000-year return period). These Flood Zones are undefended, which are equivalent to the defended case as no formal flood defences are identified. The FZ2 (0.1% AEP) event is an indicator of an extreme situation. For all the modelled Flood Zones, the normal (pre-flood) water level in the Ruislip Lido is set to that agreed between LB Hillingdon and the Environment Agency as the baseline when preparing this FRA (set 0.65m below the original datum level to provide flood storage). The model outputs, in the form of the revised Flood Zones, are intended to update the currently published Flood Zones, and will need to be accepted by the Environment Agency based on the information reported as part of this FRA Ruislip Lido FRA Report and supporting FRA Model Report. This is in line with the Environment Agency guidance for developers How to challenge (Section 6.2 of the guidance document). Commentary on the implications of these revised Flood Zones for the planned improvements at the Ruislip Lido is given in Section 8 below. 11

Figure 5: Revised Flood Zones current water level management scenario Flood Zone 3b (1:20-year return period) Current flood risk flood depths (m) Flood Zone 3a (1:100-year return period) Flood Zone 2 (1:1000-year return period) Future flood risk based on climate change (+20% flows) flood depths (m) 12

8 Flood risk implications of planned improvements for Ruislip Lido 8.1 Planned improvements implications that follow from PPS25 In line with PPS25: any development must be safe, without increasing flood risk elsewhere, and where possible reduce flood risk overall. Flood mitigation measures may be appropriate. Flood risk must be assessed over the lifetime of the development and therefore account for the impacts of climate change. Access and egress routes also need to take account of climate change. A failure or breach of flood defences is a scenario that must be considered, but is not applicable in the case of Ruislip Lido. The Ruislip Lido FRA has evaluated the fluvial flood risk implications for the existing facilities and planned improvements (Figure 3) advised at this stage by LB Hillingdon. The results are presented in Table 2 (next page) that considers the Flood Zones (Figure 5) and implications that follow from PPS25. This is based on the current water level management scenario, i.e. based on normal lake level held at 0.65m below the original maintained level for reasons previously discussed. For the existing buildings at Ruislip Lido the revised Flood Zones indicate that the: Water s Edge public house is just outside the future FZ3a (1:100-year with climate change event)* and is shown to be inundated for the current and future FZ2 (1:1000- year without and with climate change). Miniature railway track / ticket office is vulnerable to the current FZ3a (1:100-year). *a survey check on the building threshold (air brick level) indicates that the public house is above the 1:100-year with climate change flood level (by 0.25m approximately), with the flood threshold of the building was surveyed at 51.10 maod (air bricks, all at same level) and door frame level at 51.14m AOD. For the planned improvements at Ruislip Lido the revised Flood Zones indicate that the: Facilities associated with the changing room, rain shelters, toilet block, model railway, nature area, beach swimming area, angling, children s paddle area and rowing boat area are acceptable in terms of flood risk these are categorised as a Water Compatible development type under PPS25, subject to a specific flood warning and evacuation plan. Overflow car park: categorised as Flood Compatible for managed public parking (not defined in Table 1), in this highest risk area that partly falls within FZ3b (1:20-year). The acceptability of parking use is dependant on the depth and the ability to manage parking during potential flood events, and the modelling indicates this depth to be between 0 to 0.5m for the current FZ3 (1:100-year) mitigation measures including compensatory floodplain storage are therefore proposed as discussed below. This similarly applies to the new toilets proposed adjacent to the car park (see Figure 3). Improved catering facilities: again acceptable as a Less Vulnerable development type, and it would be feasible to design the floor levels above the 1:100-year levels (future FZ3a) with appropriate freeboard recommend say 0.6m. Importantly, there is safe access/egress via the main road into the site and perimeter tracks at the Ruislip Lido. 13

Table 2: Flood risk implications for planned improvements for Ruislip Lido current water level management scenario Planned improvements Applicable development type from Table 1 classification Applicable Flood Zone based on latest detailed flood modelling in line with EA guidance to developers Current Future (climate change) Flood risk implications with reference to PPS25 on Development and Flood Risk. A. New Woodlands centre with catering facilities Less Vulnerable (4) FZ3a FZ3a Needs careful design buildings Changing room facilities: beach side Water Compatible (3) FZ1 FZ1 Acceptable (replacement building within same footprint) Rain shelters Water Compatible (3) FZ1 FZ1 Acceptable Toilets: new toilet block next to overflow car park Water Compatible (3) FZ3a FZ3a Needs careful design B. Replacing Toilets: new beach side toilet blocks Water Compatible (3) FZ1 FZ1 Acceptable existing Model railway: plan to invest further Water Compatible (3) FZ3a FZ3a Acceptable facilities Parking: improve the previous overflow car park Flood Compatible* part FZ3b part FZ3b Needs careful design C. Other Nature area: proposals for enhancement subject to EIA Water Compatible (2) n/a n/a Planned improvements (more nonstructural) Beach swimming area: subject to water quality testing Water Compatible (1) n/a n/a acceptable in terms of Angling: plan to offer opportunities in designated areas Water Compatible (1) n/a n/a flood risk, subject to a improvements Children s paddle boat area Water Compatible (1) n/a n/a specific flood warning Families and older children rowing boat area Water Compatible (1) n/a n/a and evacuation plan Site security - full review of all aspects of site security n/a n/a n/a Review access/egress With reference to Table 1 on appropriate development types: (1)water-based recreation (excluding sleeping accommodation) (2)amenity open space, nature conservation and biodiversity (3)outdoor sports and recreation and essential facilities such as changing rooms (4)restaurants and cafes, hot food takeaways *categorised as Flood Compatible for managed public parking (not defined in Table 1) 14

8.2 Overflow car park proposal compensatory floodplain storage The car park site is within a flood risk area and its construction will involve some landfill within floodplain, which is unavoidable in order to provide 1:40 cross fall for effective drainage and to limit the flood depth. To offset any adverse impact of lost floodplain due to landfill, compensatory floodplain storage is required as advised by the Environment Agency. This is normally provided by recontouring of land to create new floodplain (on a level-for-level basis). For this purpose it was initially planned to utilise the available land at the north end of the lake, however, this is not feasible as a buried gas/water mains crosses it. An alternative form of compensatory storage is therefore proposed, which involves reducing slightly the lake outflow through the auxiliary overflow pipe to increase flood storage within the lake. This can easily be done by placing a collar across the pipe at its inlet to reduce the pipe diameter and limit the outflow from the pipe up to say 50% of its current capacity. Modelling demonstrates that a loss of 1000m 3 of floodplain at the car park level results in not more than a 1cm rise in flood level (1:100-year climate change). It only requires a collar that restricts the pipe inlet area by 20% to counteract this, by increasing the lake level by an additional 1cm. The impact on peak outflow is minimal (reduces the peak outflow from 2.01m 3 /s to 1.98m 3 /s). The Environment Agency advised that this (collar) proposal as compensation is acceptable provided that reducing the flow in the outlet pipe and holding water back in the Lido would not effect flooding elsewhere or cause an increase upstream. Modelling demonstrates that this is the case. 8.3 Overflow car park proposal depth of flooding and flood warning time The Environment Agency advised that consideration needs to given to the depth of the flood water and the ability of people to move their cars within the flood warning time. Car parks located in areas that flood must be designed to prevent vehicles floating out away. A flood depth at the car park up to 0.45m is proposed, which the Environment Agency agreed as possible. The Environment Agency also advise on signage (FD2320) and LB Hillingdon will consider this at a later stage in the design. Based on a flood level of 50.85m AOD (1:100-year climate change), the car park design is based on a minimum level set at 50.40m AOD. At 0.45m depth vehicles can float but without danger of reaching the lake. This is because along the lake side of the car park the adjacent land is higher (flood depth <0.3m), which acts as a natural barrier. There is one stretch, near the entrance to the car park, where this is not the case and here an existing tree line with some additional tree planting completes the barrier. This planting of new trees or bollarding concealed by semi-mature shrubs (i.e. barrier in place immediately) to form part of the planting proposals for native planting, without affecting bank stability or interrupting flood flows these two latter points not considered an issue given the setting. The Environment Agency advised that their Flood Incident Management team aim for a target flood warning lead time of 2 hours, in the absence of actual lead times for Ruislip Lido. It should be noted that the critical flood for Ruislip Lido is over 40 hours duration and in such an event the lead time would be significantly longer than 2 hours. For shorter 15

duration events the flood depth is less than 0.45m (e.g. <0.2m depth for 12 hour storm and <0.1m for 6 hour storm). LB Hillingdon staff responsible for Ruislip Lido can act upon any flood warning to clear the car park in advance of flooding even out of hours there is always an emergency contact. 8.4 Overflow car park proposal surface water drainage and SUDS Surface water drainage systems developed in line with the ideals of sustainable development are collectively referred to as sustainable drainage systems (SUDS). These systems are designed both to manage the environmental risks resulting from urban runoff and to contribute wherever possible to environmental enhancement. PPS25 requires that the volumes and peak flow rates of surface water leaving a developed site are no greater than the rates prior to the proposed development (PPS25 Annex F paragraph F10) for storm events up to and including those with a 1 in 100 year return period. The Environment Agency advised that as the site is over 1 hectare, the FRA must include an assessment of the surface water drainage on the site. Their criteria is that surface water discharge from the developed site should mimic that of an undeveloped greenfield site, up to and including a 1 in 100 year critical duration storm event. Greenfield run off rates are generally between 2-8 l/s/ha for storm events up to the critical 1 in 100 year return period event. A sustainable approach to the surface water drainage system is required. The Environment Agency expect the use of SUDS hierarchy to be maximised, with any barriers to their use clearly detailed. The design of the overflow car park incorporates two petrol/oil interceptors and sustainable drainage in the form of a swale along its northern edge, as illustrated in this design drawing. The swale is open (no rock fill) and will be planted with native species, in line with advice from the Environment Agency. Outline of swale area (blue shading) In view of the SUDS proposed the surface water drainage from the new car park will not contribute any additional rainfall-runoff, i.e. avoids any adverse impact on flood risk. The swale is designed to accommodate a 1 in 100-year critical duration storm event and supporting calculations are included in Appendix C. Figure 6: Overflow car park proposal (schematic only to illustrate position of swale area) 8.5 Other planned improvements surface water drainage and SUDS The LB Hillingdon designer advised that surface water drainage and SUDS proposals for the planned improvements will be considered as they are progressed through the design stage. 16

9 Flood risk implications of alternative normal operating water level for Ruislip Lido 9.1 Option under consideration LB Hillingdon are considering the option to hold the normal operating water level of Ruislip Lido at 0.5m below original datum instead of its currently agreed operating level of 0.65m below datum, i.e. 0.15m higher. This would provide 0.8m freeboard of flood storage, taking into account the 0.3m high concrete apron immediately in-front of the auxiliary overflow pipe outlet. Initially LB Hillingdon had considered the option to hold the operating level at the original datum level (i.e. 0.65m higher), which is assumed to be the level that the reservoir and its outlet works were originally designed for. This option was dropped in view of concern that with only 0.3m freeboard it may increase outflow from the reservoir to a point that affects the flood risk to property downstream. 9.2 Basis for normal operating water level at 1m below datum Previous flooding affected properties particularly between Howletts Lane and Ladygate Lane in the 1980 s. In view of the known flood risks on Cannon Brook and the downstream River Pinn any increase in flood flows at times of overtopping and flooding of properties is unacceptable. For this reason the Ruislip Lido is drawn down to prevent or limit any outflow from Ruislip Lido that would otherwise exacerbate flooding problems. Following the floods experienced in the 1980s it is understood that the decision to change the operating level of Ruislip Lido, by drawing it down to 1m below the original design datum level, was first made in 1993 by the management advisory group and after discussions with the National Rivers Authority (now Environment Agency). From records held by LB Hillingdon, it appears that the technical justification for the change in normal operating level is limited to few page calculation dated 26 September 1991 and subsequent letter dated 22 October 1991 from the National Rivers Authority (now Environment Agency) these records are included in Appendix B. The letter from the National Rivers Authority advises that a comprehensive appraisal study will be necessary to assess the impact of the proposed change of use of the Lido from amenity to flood defence. This should take account of the environmental impact of the proposed change of use. A search has not revealed any records relating to this appraisal study. 9.3 Review of the 1991 calculation by LB Hillingdon The NRA letter (that post dates the calculation) advises: it to be essential that the Lido is operated in the future in such a way that all run-off from the 1-in-100-year rainfall event in the Northwood sub-catchment is stored until levels in Cannon Brook and the River Pinn have dropped. This would require the normal level in the Lido to be maintained some way below the overflow level by releasing water through the penstocks (scour pipe valves) in times of low flow in the downstream river channels. The 1991 calculation attempts to determine the normal level to achieve this. However, this FRA reveals that outflow from Ruislip Lido will occur in events less severe than 1 in 100- year, and also that the impact downstream is far less a threat than originally perceived. 17

The 1991 calculation is a simple water balance to estimate the rise in lake level in response to a rainfall event. A fixed water surface area of 170,000m 2 is assumed for a water level of 49.8m AOD and a rainfall-runoff volume of 222,000 m 3 entering the lake in a 100-year event (12 hour storm). The freeboard required, including provision for a 0.15m allowance for wave action, was calculated to be 1.61m for a 12 hour storm (and 1.43m for a 6 hour storm). This FRA estimates equivalent figures to be: surface water area of 136,200 m 2 at datum level (49.56m AOD) reducing to 133,355m 2 at 49.43 m AOD based on MasterMap data and 109,550m 2 at 1m below datum (48.56 maod); flood volumes entering the Lido of 223,000m 3 for 6.75 hour storm increasing to 259,000m 3 for 12.75 hour storm and 371,000m 3 for the critical 48 hour storm. These volumes increase by 20% for climate change. A 1.3 m drawdown in water level corresponds to 221,000m 3 of flood storage based on the 1991 calculation, which compares with 123,000 m 3 estimated in this FRA (for 1.3m freeboard with draw down to 1m below datum). Clearly there are significant differences. This FRA estimates 44% less flood storage to be available and up to 68% additional flood volume (48 hour 1:100-year storm) compared with the 1991 calculation. With climate change the additional flood volume increases to about 100%. The implication is that less flood storage is available within the Lido, and for longer duration storms there can be significantly more runoff entering the Lido than previously predicted by the 1991 calculation. This helps explain why outflow on two occasions has occurred despite dropping the normal operating water level to 1m below datum (see below). Another factor, as recognised in the 1991 calculation, is that the restricted operating regime that controls the outflow from Ruislip Lido (2.5cm drop in level per day only during dry periods, i.e. no rainfall) means there could be times after successive periods of rainfall when the water level cannot be fully drawn down below datum in advance of a storm. Again this helps explain why outflow from the lake has occurred. 9.4 Current normal operating level at 0.65m below datum In February 2008, at the request of the Friends of Ruislip Lido a compromise of 0.65m below datum was agreed for environmental reasons with the proviso that the situation will be monitored. LB Hillingdon advised that water level records for the Lido cover the period from November 1998 to date. Over this period uncontrolled overflow occurred on two occasions when the water level came high enough to discharge into the Cannon Brook via the auxiliary overflow pipe (0.6m diameter): in 2000/ 2001 when operating at a draw down level of 1m below datum for the period 1998 to February 2008. in February 2009 when operating at a draw down level of 0.65m below datum for the period from February 2008 to date. Thus even by drawing down the water level there are times when outflow can occur. There is one option to partially limit this, by restricting the diameter of the auxiliary overflow pipe at its inlet (a simple collar can achieve this), without impact on the safety of the impounding dam structure confirmed by Halcrow s All Reservoirs Panel Engineer. 18

Before this FRA the potential impact of uncontrolled overflows on the flood risk areas downstream on Cannon Brook had not been assessed in detail. The modelling appraisal discussed below now considers this. 9.5 Flood risks downstream of Ruislip Lido Cannon Brook/River Pinn The Environment Agency study (2008) modelled the River Pinn including Cannon Brook (Figure 7, next page) in order to define strategic Flood Zones as currently published (reproduced in Figure 8). For this FRA to better understand the flood risk on the Cannon Brook the Lido model (used to determine the revised Flood Zones as detailed in previous chapters) was extended downstream to include a section from the Pinn model to just beyond Ladygate Lane. To be consistent with the Lido model the Pinn model flood inflows were also updated for Mad Bess Brook and lateral inflows into the Cannon Brook for the intervening drainage area between Lido and Ladygate Lane. Modelling results confirm the significant flood attenuation provided by Ruislip Lido, even when operated at datum level, and demonstrates that the flood risk on Cannon Brook, mainly at Ladygate Lane, results primarily from flows on Mad Bess Brook (Figure 7). Results also indicate a much reduced flood risk on Cannon Brook compared with the currently published Flood Zones (Figure 8), e.g. the 1:100-year flood level at Ladygate Lane is predicted to be about 0.3m lower than previously assessed. The modelling demonstrates that the attenuating effect of the Lido means that effectively very limited outflow can occur on Cannon Brook from the Lido at the time of peak flow on Mad Bess Brook. However, it does not prevent outflow over the full duration of the flood event on the River Pinn target advised by the National Rivers Authority in 1991. The modelling demonstrates that even if outflow occurs from Ruislip Lido the impact is minimal as illustrated by the results in Table 3. The flood levels for various storm events quoted in Table 3 are modelled at Ladygate Lane, immediately upstream of the road culvert. The initial water level refers to the level either at the original design datum (0m) or below this datum (-0.5m / -0.65m / -1m). In Table 3, red shading indicates that the flood level exceeds the critical threshold level at which overtopping would occur into the residential area - approximately 41.7. In most cases the initial water level at the Lido does not impact on the peak flood level at Ladygate Lane. But there are exceptions for the 48 hour storm (indicated in bold in Table 3): 1:100-year event with initial water level held at datum the peak level increases to above the critical threshold. 1:100-year climate change event the peak level exceeds the critical threshold and slightly increases for higher initial water levels in the Lido, e.g. 0.1m increase for the initial water level held at 0.5m below datum compared with 1m below (41.86 41.76). Overall the modelling predicts the standard of protection at Ladygate Lane to be as low as 1:20-year return period. In a residential area such as this, the target standard is typically much higher, at 1:50-year (minimum). Additionally, as indicated by the comments against Figure 8, this FRA finds that the study over-estimates the design outflows from Ruislip Lido and therefore the flood risks on Cannon Brook. 19

Figure 7: River system Source : Environment Agency River Pinn study report Ruislip Lido Flood risk at A4180 considered over-stated due to unrealistic flows estimated in EA study Flood risk at Ladygate Road (see below) due to very limited channel and road culvert capacity (about 3m 3 /s) Figure 8: Environment Agency Flood Map (http://maps.environment-agency.gov.uk/wiyby/) 20

Table 3: Modelled peak flood levels on Cannon Brook at Ladygate Lane Ruislip Lido initial Storm duration water level* 6 hours 12 hours 48 hours Design event: 5-year return period 0m 41.20 41.17 41.11-0.65m 41.19 not modelled 40.94-1m 41.19 not modelled 40.94 Design event: 20-year return period 0m 41.80 41.74 41.29-0.5m 41.80 41.72 41.12-0.65m 41.80 41.72 41.12-1m 41.79 41.71 41.12 Design event: 100-year return period 0m 41.99 41.97 41.84-0.5m 41.98 41.95 41.52-0.65m 41.98 41.95 41.47-1m 41.98 41.95 41.40 Design event: 100-year climate change (+20% flows predicted from 2025) 0m 42.03 42.02 41.95-0.5m 42.02 42.00 41.86-0.65m 42.02 42.00 41.83-1m 42.02 42.00 41.76 *levels quoted relative to the original design datum level for Ruislip Lido To illustrate the flood mechanism at Ladygate Lane and understand why the Lido initial water level slightly increases flood levels for the longer duration event the 1:100-year climate change model results are presented as flow-time plots in Figure 9 (next page). The plots show the flows just upstream of Ladygate Lane: (a) Ruislip Lido outflow; (b) Mad Bess Brook; and (c) the two lateral inflows between the Lido and Ladygate Lane. From the plots it is evident that the highest flows at Mad Bess Brook are the main problem. For the shorter duration storms (6.75 hour and 12.75 hour) the plots show that the Lido outflow varies from about 0.5 to 2 m 3 /s depending on the initial water level, with a peak that comes after the much higher peak on Mad Bess Brook. The Lido outflow only coincides with the higher flows on Mad Bess Brook for the longer duration (48 hour) storm, and this explains the small differences in Table 3. One further flow-time plot in Figure 10 illustrates that there is no impact on the River Pinn, by showing the Cannon Brook flow at the confluence with Pinn Brook, and the Pinn Brook flow at this point for the 1:100-year event. A final set of model runs examined the beneficial effect of placing a flow restricting collar across the Ruislip Lido auxiliary overflow pipe at its inlet to reduce the pipe diameter, as introduced above to provide compensatory storage for the car park. It is feasible to restrict the inlet by as much as 50% in order to marginally increase the storm runoff held in flood storage. For the 1:100-year climate change event a 50% overflow pipe area increases the Lido level by 2.3cm and reduces the outflow by 0.05m 3 /s (from 1.98m 3 /s), assuming an initial water level at 0.5m below datum. The impact is not significant but in addition to compensatory storage it also more than offsets any very marginal increases in flood level indicated in the above table. 21

Mad Bess tributary at Ladygate Lane 6.75 hour storm event Lateral inflows between Ruislip Lido/Ladygate Lane 0m -0.5m -0.65m -1m Ruislip Lido outflows (blue lines) for four different initial WL below datum scenarios as indicated. Disjointed lines such as this indicate minor model instability Mad Bess tributary at Ladygate Lane 12.75 hour storm event Lateral inflows between Ruislip Lido/Ladygate Lane 0m -0.65m -1m -0.5m 48 hour storm event Lateral inflows between Ruislip Lido/Ladygate Lane Mad Bess tributary at Ladygate Lane 0m -0.5m -0.65m -1m Figure 9: Model results to illustrate the flood flow regime in Cannons Brook for 1:100-year climate change event (+20% flow increase) Cannon Brook inflow to River Pinn taken from original EA Pinn model River Pinn flows immediately upstream confluence with Cannon Brook taken from original EA Pinn model -1m (red line) 0m -0.65m (green line) Cannon Brook inflow to River Pinn modelled in this FRA (2.75 hour storm) for different Ruislip Lido initial WL scenarios as indicated Figure 10: Model results to illustrate the flood flow regime in River Pinn for :100-year flood event (note: no climate change event in Pinn study) 22

Further assessment of climate change effects The further assessment follows the current Defra PAG appraisal guidance (2006) on climate change, which is to allow for a 10% increase in fluvial flows now up to 2025 and 20% increase in fluvial flows from 2025. However, recent climate change research predicts such flows (20% increase) are unlikely to be experienced until 2050/80. Following discussion with the Environment Agency (April 2011) the normal operating level for Ruislip Lido to take forward for consideration is 0.65m below datum, in preference to 0.5m below datum. Model testing therefore considered the -0.65m operating level as a preferred case, with the overflow car park in place and outflow from the overflow pipe restricted by about 50% (i.e. more restricted than considered previously). The model test results for the preferred case (-0.65m) are compared with two existing cases with the normal operating level held at 1m below datum (1.3m freeboard) and 1.2m below datum (1.5m freeboard) this 1.2m case replicates an early recommendation in 1992 (information report titled Utilisation of Ruislip Lido as a Flooding Prevention Facility ). The modelling results in Table 4 demonstrate the minimal impact on flood levels for the critical storm (48 hours) at Ladygate Lane, immediately upstream of the road culvert. The red shading indicates that the flood level exceeds the critical threshold level at which overtopping would occur into the residential area - approximately 41.7. For the climate change scenario predicted from now until 2025 (+10% flow increase) the peak flood level is modelled below the critical threshold level. Only for the climate change scenario from 2025 is this threshold exceeded, and there is minimal difference for the three operating levels tested. Table 4: Modelled peak flood levels on Cannon Brook at Ladygate Lane for climate change scenario 100-year climate change event Ruislip Lido initial water level* Storm duration 48 hours +10% flows -0.65m 41.58 predicted -1m 41.58 up to 2025-1.2m 41.55 +20% flows -0.65m 41.75 predicted -1m 41.76 from 2025 on -1.2m 41.73 How the compensatory storage measure provides mitigation By reducing the outflow (from collar that restricts the outflow from the overflow pipe by about 50%) more effectively utilises the flood storage in the Lido by further attenuating the inflows from Cannon Brook. Over the full duration of the event the amount held in storage is unchanged as the lake will fill above the initial lake level and then empty back to the initial lake level. The reduced outflow rate will mean it takes longer to empty the lake.. Modelling tests the impact of the car park landfill (average of approximately 0.3m landfill over 3.6 ha), which is a dynamic calculation as it depends both on the storage available within the Ruislip Lido (as the level increases so the plan area of the lake increases) and the outflow for a given lake level. The model results above illustrate the small change in peak lake levels due to the change in initial lake level, the car park landfill and reduced outflow. 23

Modelling of landfill also indicates less than a 1cm rise in lake level due to the car park landfill and 4cm rise from the reduced outflow. Thus the modelling demonstrates the minimal impact. The reason for minimal impact resulting from the car park landfill (less than 1cm) is because the car park area (0.36ha) is small compared with the lake area (over 15ha). Downstream flood risks In helping to address the flooding problems at Ladygate Lane and upstream at Howletts Lane, local improvements are proposed for the Cannon Brook as discussed below. 9.6 Conditions on the Cannon Brook Initially the scope of the FRA focussed on the planned improvements for Ruislip Lido and the need to avoid any impact on flood risk downstream in view of the known flooding problems. Modelling results (Section 9.5), confirm the significant flood attenuation provided by Ruislip Lido, even when operated at datum level, and demonstrates that the current flood risk on Cannon Brook, mainly at Ladygate Lane, results primarily from flows on Mad Bess Brook (Figure 7). In view of the above, during the course of the FRA and from discussions with the Environment Agency the concerns about the condition of Cannon Brook (as detailed below) came to light and for this reason improvements outside the Ruislip Lido were investigated. The following information was advised by the Environment Agency (operations team with local knowledge of Cannon Brook): Trash screens across the entrance to the road culverts at Ladygate Lane and upstream at Howletts Lane can suffer from debris blockage problems (mainly leaves/twigs). Trash screen at Ladygate Lane is unsafe to operate due to its position/layout and therefore cannot be cleared by the Environment Agency at times of high flows. Road culvert capacities are known to be a bottleneck to flood conveyance (e.g. 0.3m head loss modelled at the culvert inlet) and therefore any blockage could quickly trigger overtopping from the Cannons Brook at times of high flows. In-channel siltation along the Cannon Brook restricts flow conveyance both in the channel and at the road culverts the full extent of this problem is not known. According to the Environment Agency, a recent inspection of Howletts Lane culvert, upstream of Ladygate Lane, found a significant depth of silt at the outlet that appeared to extend along the channel. For this reason the Environment Agency recommend a siltation survey to establish the extent of the problem and then possibly at some stage de-silting of the channel funding must first be justified, i.e. the benefits of flood damage avoidance should outweigh the de-silting costs. Clearly there is a strong case for improvements on Cannon Brook in view of the flood risk to the area (not caused by the operating regime at Ruislip Lido). From the above information and from the knowledge gained through modelling and site inspection the recommended improvements include: De-silting the channel/culverts to restore the flow capacity. Reinforcing the river banks and infilling any eroded low spots (possibly the dredgings can be utilised for this). 24

Replacing the trash screens at Ladygate Lane and Howletts Lane to minimise any blockage, ensure safe access at times of flood and reduce backwater effects caused by the inlet by adopting modern trash screen layout designs. Installing water level monitoring equipment on Cannon Brook, possibly near Ladygate Lane for flood warning purposes; such a measure can also support the operating rules when discharging flows from Ruislip Lido (controlled flow via the scour outlet pipe). Potentially these measures could locally raise the standard of protection to 1:50-year, if channel silting is restricting the channel flow capacity to the degree suspected. This combined with the risk of blockage of the trash screens only makes the problem more acute. In view of the above, LB Hillingdon are consulting with the Environment Agency and other potential partners in order to promote and secure funding for improvements on the Cannon Brook, which will form part of the overall planned improvements for Ruislip Lido. 9.7 Recommendations The condition of Cannon Brook (Section 9.6), has raised concerns and there is a strong case for an improvement scheme in view of the flood risk to the area (not caused by the operating regime at Ruislip Lido). In view of these concerns, LB Hillingdon has already approached the Environment Agency with a view to working in partnership on the required improvements needed to ensure the river system operates effectively The modelling (Section 9.5) confirms the reported claims that the reservoir spillway was surcharged to a depth of at least 2/3 metre and high volumes of water from the reservoir contributed greatly towards the damage flooding. in 1977 and 1988 (from report titled Utilisation of Ruislip Lido as a Flooding Prevention Facility ). At that time the Lido level was not in a drawn down state before these events, i.e. no flood attenuation. The current operating regime that holds the water level below the spillway means flood storage is now available to prevent the Lido outflow contributing in the same way as in previous flood events. An important point to highlight is that the current operating regime makes no use of the additional outflow capacity via the scour outlet pipe (controlled flow operated by a system of valves) during high flow periods. To change this could potentially exacerbate flooding problems on the Cannon Brook. In light of comments made by the National Rivers Authority (Appendix B), quite possibly this was also a contributing factor to flooding during the 1980s. No records have been found to confirm this. The current operating regime only permits outflow via the scour outlet pipe to draw down the water level below datum only in dry and low flow periods this should not be changed. The operating rules could be tied to water level monitoring on the Cannon Brook. It is recommended that LB Hillingdon consult with the Environment Agency about this. In deciding the initial water level operating rules for the Lido a balance needs to be struck between amenity/recreational use and flood defence in line with the basic requirement set by the National Rivers Authority in 1991 (Appendix B). At that time a comprehensive appraisal study was called for, but there is no record of this being carried out. The results presented herein therefore provide a current and more thorough understanding of the flood risk implications of the operating regime at Ruislip Lido. 25

The National Rivers Authority stated in their letter (1991): we consider it to be essential that the Lido is operated in the future in such a way that all run-off from the 1-in-100-year rainfall event. is stored until levels in Cannon Brook and River Pinn have dropped. This assessment finds that not all runoff can be stored in this way, though by drawing down the water level additional flood storage can be provided to help reduce peak flows downstream. Initially as part of the planned improvements for Ruislip Lido, LB Hillingdon considered an option to return to the original operating regime, with the normal water level at zero datum (0.3m freeboard). However, it was rejected as this limits the flood defence benefit. The normal operating water level is currently held at 0.65m below datum (0.95m freeboard), which for environmental reasons is a compromise on the previous arrangement at 1m below datum (1.3m freeboard). The preferred option of LB Hillingdon is to continue to hold the level at 0.65m below datum (0.95m freeboard), which is considered acceptable without adverse impact on the current flood risk predicted downstream on Cannon Brook and the River Pinn. The option to limit the outflow by placing a flow restricting collar across the auxiliary overflow pipe will reduce outflow into the Cannon Brook and this is recommended. The situation should be reassessed in the longer term as there is a small impact only for the 1:100-year climate change event, which defines the future flood risk based on the predicted flow increase from 2025. If climate change effects become evident or new information comes to light then the initial water level can be adjusted as necessary. Thus it is important to review the operating rules on a regular basis, including a check on the future flood risk associated with climate change (say every 5 years) and also following any significant storm event. 26

10 Concluding remarks The Ruislip Lido FRA is compliant with PPS25 and advice of the Environment Agency. It fulfils the requirement to undertake a site-specific FRA to update the Flood Zones and improve the understanding of the flood risks for the site. The FRA will be submitted as part of the planning applications for the planned improvements, and will be used to inform the subsequent more detailed planning and design studies. The FRA provides an overview of flood risk, presenting the revised Flood Zones from technical studies separately reported (in the FRA Model Report), and considers the planned improvements at Ruislip Lido. The model outputs, in the form of the revised Flood Zones, are intended to update the currently published Flood Zones, and will need to be accepted by the Environment Agency based on the information reported as part of this FRA. The FRA provides important information for interested parties and the general public about the flood risks at the site and the implications of the planned improvements, including the technical studies (in the FRA Model Report) to demonstrate that the planned improvements at the Ruislip Lido site will not adversely impact the flood regime downstream. The Waters Edge pub is identified as not at risk in the 1 in 100-year plus climate change event for either the current risk or any of the scenarios assessed by the FRA. The highest peak levels are below the flood threshold of the building (51.10 maod). The detailed information on flood risk for the Ruislip Lido site, included in this FRA report, confirms that in line with PPS25 the planned improvements are acceptable in terms of flood risk and that the flood risk can be sustainably managed (i.e. in response to increasing flood risk due to climate change). The FRA considers the impact of alternative normal operating water levels for Ruislip Lido and concludes that to operate up to 0.65m below datum (0.95m freeboard) is acceptable without adverse impact on flooding problems downstream on Cannon Brook and the River Pinn. As with any flood risk management measure, it will be important to review the operating rules on a regular basis. Local improvements on the Cannon Brook are recommended in view of the past flooding problems and site conditions. These improvements might include de-silting the channel to restore the flow capacity, replacing the trash screens and installing water level monitoring equipment. No further FRA site-specific studies are required, as the planned improvements are acceptable in flood risk terms, subject to meeting the above requirements in respect of the improved catering facilities, overflow car park and flood warning and evacuation plan. 27

Appendix A Photographic appraisal of study area Spilways Outfall Downstream Ruislip Lido 28

Appendix B Review of LB Hillingdon records on Ruislip Lido water level management The review presented in this appendix reproduces the original records provided by LB Hillingdon, as listed below, and alongside these records comments are annotated to explain the review findings of this FRA. LB Hillingdon calculation: Ruislip Lido flood protection, 26 September 1991 Letter: National Rivers Authority to LB Hillingdon, 22 October 1991 The calculations prepared in September 1991 precede the letter dated 22 October 1991. There is reference in NRAs letter to the need for a comprehensive appraisal study to assess the impact of the proposed change in use of the Lido from amenity to flood defence. This should take account of the environmental impact of the proposed change of use. No records of such a study have been uncovered. 29

Halcrow comment (as part of FRA) Clearly NRA were unaware of overtopping from Ruislip Lido (assume uncontrolled via auxiliary overflow pipe) combined with outflow from operation of the scour outlet pipe during flood events. Under the current operating regime the scour outlet pipe is only operated during dry periods or when agreed with the Environment Agency The outcome of this feasibility study and any subsequent scheme to improve flood protection on the River Pinn has not been investigated as this FRA focuses on Ruislip Lido and Cannon Brook (stated here as outside the scope of the Pinn study). The important point here is that NRA advised The control of flows into the Cannon Brook is essential to minimise the impact on the River Pinn downstream and for this reason...consider it essential that the Lido is operated in the future in such a way that all run-off from the 1-in-100-year rainfall event in the Northwood sub-catchment is stored levels in Cannon Brook and the River Pinn have dropped. To achieve this the NRA propose releasing water through the penstocks (scour outlet pipe) in times of low flow in the downstream river channels. Based on a number of assumptions the calculation by LB Hillingdon (pre-dates this letter) estimates the draw down in normal operating (water) level at Ruislip Lido to prevent the runoff, in line with NRA advice. However, this is not possible for all design events based on the more detailed assessment completed as part of this FRA (see Halcrow comment against the calculation). 30

Halcrow comment (as part of FRA) No record found of any comprehensive appraisal study. This statement infers that no formal decision or agreement on the primary use of Ruislip Lido (for amenity or flood storage) had been taken. Clearly the NRA recognised the benefit of the flood storage and the need to limit the release of floodwater due to the potential impact on the Pinn catchment. 31

Halcrow comment (as part of FRA) (2) auxiliary overflow level taken to be 49.86m AOD as explained in the model report accompanying this FRA - no bearing on flood storage calculation that follows. (3) surface area of 133,355 m 2 based on Master Map as explained in the model report accompanying this FRA, which is 20% less than assumed here. At lower levels the surface area reduces. It means the flood storage volumes quoted here are overstated. (4) not clear where this capacity estimate comes from. (5) and (6) not relevant to this review (7) volume figures quoted for total runoff entering Ruislip Lido during storm events compares with the following based on FEH analysis as part of this FRA only for 100-year (add 20% to factor in climate change). - 223,000 m 3 for 6 hour storm - 259,000 m 3 for 12 hour storm - 371,000 m 3 for 48 hour storm The above can be compared with 250,000m 3 assumed in the calculation. Note that 48 hours is adopted as the critical storm duration for Ruislip Lido, this long duration event results in a higher flood volume. But shorter duration events are more critical downstream on Cannon Brook and River Pinn, e.g. the Environment Agency River Pinn study examined only short storm durations. 32

Halcrow comment (as part of FRA) - calculation against present water level not relevant to this review - based on FRA estimates of surface water area estimates and 1:100-year flood volume entering Ruislip Lido (see above) the flood storage is less than 123,000m 3 for 1.2m freeboard - valid point relating to draw down operating regime for Ruislip Lido 33

Halcrow comment (as part of FRA) Not assessed 34

Halcrow comment (as part of FRA) - compares with 223,000 m 3 for 6.75 hour storm, 259,000 m 3 for 12 hour storm and 371,000 m 3 for 48 hour storm from this FRA, i.e. up to 68% additional flood volume, which increase to 100% for a climate change scenario (+20% flows based on current guidance of Defra, 2006). - freeboard of 1.3m (i.e. 1m below datum) estimated to provide flood storage of only 123,000 m 3, which is 44% less than the 1991 calculation, and it is not considered feasible to draw down Ruislip Lido to offset the above additional flood volume estimates. 35

Appendix C Overflow car park proposal surface water drainage and SUDS calculations An analysis has been carried out to assess the potential increase in runoff from the proposed car park extension to be mitigated to Environment Agency standards. A swale is proposed along the northern edge of the car park. The volume requirement is calculated as the additional runoff, i.e. the difference between the runoff from the undeveloped and developed site, as explained below. A conservative approach to the calculation of storage volume is given in Drainage of Development Sites A Guide by HR Wallingford: The Environment Agency advised on typical greenfield runoff rates varying between 2-8 l/s, and an average of 5 l/s (equivalent to 6.5 m3/h) is taken for Ruislip Lido. Additional volume of runoff = Paved areas draining to the network + Pervious runoff draining to the network or river - Pervious area of the Greenfield site Volxs = RD.A.10(0.8PIMP/100 PIMP/100.SOIL) Where: Volxs is the extra runoff (m³) RD is the rainfall depths (mm) see below PIMP is percentage impermeable area = 100% A is area of the site = 0.36 ha SOIL is the soil index (or HOST index) = 0.5 (taken from flood estimation calculation record for Ruislip Lido) It should be noted that FEH uses HOST for soil classes, but the same concept of a standard percentage runoff exists for each HOST class and the formula is equally applicable using HOST or SOIL. FEH rainfall depths (RD) are used: RD for 1 hour RD for 2 hours RD for 3 hours RD for 6 hours RD for 12 hours RD for 24 hours 51.8 mm 58.8 mm 63.4 mm 72.0 mm 81.7 mm 94.9 mm Volxs for 1 hour 143 m 3 Volxs for 2 hours 156 m 3 Volxs for 3 hours 163 m 3 Volxs for 6 hours 168 m 3 Volxs for 12 hours 158 m 3 Volxs for 24 hours 118 m 3 The critical volume requirement is therefore 168 m 3 and on this basis the swale design (with 1:2 side slopes) provides 173 m 3 of storage, i.e. an additional 5 m 3. A simple pipe outlet will limit the peak outflow into Ruislip Lido to 5 l/s (6.5 m 3 /h). The above calculation ignores infiltration into the ground from the swale. 36