Hydrology Improvements Detailed Evaluation Process (HiDEP): Hydrology and Structure Hydraulics and Recommendations

Transcription

1 , Suite 1, Deer Park Business Centre, Eckington, Pershore, Worcestershire, WR10 3DN t +44 (0) e enquiries@haycock-associates.co.uk Hydrology Improvements Detailed Evaluation Process (HiDEP): Hydrology and Structure Hydraulics and Recommendations Client: City of London Authors: Amy Evans, Alison Pepper, Emily Day, Drew Carthew, Nick Haycock Final Issued: 6th January 2011 Clients comments incorporated: March 2011 Version: 3 rivers soils hydrology landscapes 1 Haycock Associates Limited. Registered in England No VAT No. GB Registered office: Red Roof, Wick Road, Little Comberton, Pershore, Worcestershire WR10 3EG.

2 Table of Contents Executive Summary [Removed] 11 Project Aims 11 Legal Responsibility 11 Current Situation 12 Option Appraisal 12 Recommended schemes 13 Flood Envelope Analysis 13 Other Option Discussed and Discounted 15 Quantity Surveyor Cost Review 16 Visualisations and landscape architect comments 19 Water Quality Improvements 20 Emergency Action Plan and rainfall and pond level telemetry system 21 Aims and Objectives 24 Introduction 25 Flood Risk 26 Reservoir Undertaker and City of London Legal Obligations 30 Statutory Supervising Panel Engineer Role Hydrology 32 Defining catchment areas for the individual lakes 32 Baseline and Extreme Event Hydrology 32 2

3 Qpmf Standard Methodologies Flood Studies Binnie and Partners 34 Hampstead Scientific Society rainfall data 36 Historical storm events 38 14th August th August rd May Revised Rainfall : Runoff Model 41 Distributed Hydrological Model 41 Distributed hydrological model - Metrological Data 41 Hydrology analysis with Dr. Margaretta Ayoung (Atkins WS) 42 Hydrology Summary Hydraulic Assessment 44 Overflow Structure Performance Review Structure Hydraulics Options Appraisal 47 Design principles 47 Option A 47 Option B 47 Option C 48 Reservoir Routing and Option Analysis 48 Justification for using the STELLA isee systems 48 Model Inflows 51 Model structure input information 53 Model flow calculations 53 Modelling the wave effect 54 3

4 Model assumptions 54 Highgate Chain 56 Option A results 56 Option B results 57 Option B and C results 59 Highgate Chain recommended option 60 Highgate Chain recommended option during a FSR calculated1:100 year event 62 Highgate Chain recommended option during during the 1:88 year rainfall event 63 Justification for recommended scheme and constraints 65 Stock Pond 65 Ladies Bathing 66 Bird Sanctuary 66 Model Boating 67 Men s Bathing 67 Highgate No Hampstead Chain 69 Current Option and Option A results 69 Options B results 70 Option B and C results 72 Hampstead Recommended Option 73 Hampstead Chain recommended option during a 1:100 year event 75 Justification for recommended scheme and constraints 76 Vale of Health 76 Viaduct 77 Catchpit 77 4

5 Mixed Bathing 78 Hampstead No Hampstead No Golders Chain 79 Option A Results 79 Option B results 80 Golders Option C and Recommended Option 80 Golders Chain recommended option during a 1:100 year event 81 Justification for recommended scheme and constraints 82 Golders Hill Chain 82 Leg of Mutton Velocities and depths on the spillway and downstream of the toe 84 Velocity of water on the spillway 84 Depth of water at the toe of the spillway 84 Option A velocities 85 Recommended Option dynamic velocities 86 Stock Pond 86 Ladies Bathing Pond 87 Bird Sanctuary Pond 88 Model Boating Pond 89 Men s Bathing Pond 90 Highgate No. 1 Pond 91 Vale of Health Pond 92 Viaduct Pond 93 Mixed Bathing Pond 94 5

6 5. Materials used in Option Designs Discussion and amendments to the Recommended Scheme Other options 101 Lowering of static water levels in all ponds 101 Highgate Chain 101 Hampstead Chain 102 Emptying all ponds but maintaining the structures 104 Highgate Chain 104 Hampstead Chain 105 Removing all dams 106 Engineering only one dam 106 Only upgrading the current Statutory Reservoirs 106 Underground storage Visualisations 109 Highgate Chain Visualisations 109 Bird Sanctuary visualisations 109 Model Boating visualisations 111 Highgate No. 1 visualisations 112 Hampstead Chain Visualisations 118 Hampstead No. 2 visualisations 118 Hampstead No. 1 visualisations Flood Envelope Modelling and impacts 123 Description of Reservoir, Dam and Downstream Valley 123 Hampstead No. 1 Pond : Reservoir, dam and downstream valley 123 Highgate No. 1 Pond : Reservoir, dam and downstream valley 124 6

7 Inundation model input hydrology 124 Model Simulations for Dams 125 Current scheme (Option A) - failure during Qpmf event 125 Current Situation - Hampstead Chain - dam burst timings 125 Current Situation - Highgate Chain - dam burst timings 126 Recommended scheme - non failure during a Qpmf event 126 Model Construction 126 Model Construction 126 Digital Surface Model (DSM) 126 Roughness Assumptions 126 Boundary Conditions 126 Infiltration 126 Model Calibration 127 Model Results 128 Hampstead Chain, Current Situation (Option A) : Failure Under Qpmf Event 128 Hampstead Chain, Recommended Scheme: Non Failure Under Qpmf Event 129 Highgate Chain, Current Situation (Option A) : Failure Under Qpmf Event 130 Highgate Chain, Recommended Scheme: Non Failure Under Qpmf Event 131 Potential Impacts 132 CARES Ltd Assessment 132 Hampstead inundation impacts during a Qpmf event 132 Highgate inundation impacts during a Qpmf event 134 Kings Cross Main Line Terminus 136 Environment Agency RIMs 136 Hampstead No. 1 Pond RIM comparisons 136 7

8 Highgate No. 1 Pond RIM comparisons 138 Flood Envelope Modelling Conclusions 139 Hampstead Chain conclusions 139 Highgate Chain conclusions Quantity Surveyor Cost Review Water Quality Improvements Telemetry and Emergency Action Plan 147 Location 147 Description of Works 149 Predictive Rainfall Radar Warning System 149 Rainfall Early Warning System 149 Reservoir Early Warning System 149 Design Criteria 152 User Interface 152 Training 152 Service Level Agreement Project Programme Project risks 155 References 156 Glossary of Terms 157 Appendix A - Additional STELLA Result Tables and Log Files 159 Highgate chain 159 Current Option / Option A Log File and Results Interface (Qpmf) 159 Option B Log File and Results Interface (0.5 Qpmf) 186 Option B and C (storage only) Log File and Results Interface (Qpmf) 213 8

9 Option B and C (All) Log File and Results Interface (Qpmf) 240 Preferred Scheme Log File and Results Interface (Qpmf) 267 Hampstead Chain 294 Current Option / Option A Log File and Results Interface (Qpmf) 294 Option B Log File and Results Interface (0.5 Qpmf) 323 Option B and C (Storage only) Log File and Results Interface (Qpmf) 352 Option B and C (All) Log File and Results Interface (Qpmf) 381 Recommended Option Log File and Results Interface (Qpmf) 410 Golders Hill Chain 439 Current Option / Option A Log File and Results Interface (Qpmf) 439 Option B Log File and Results Interface (0.5 Qpmf) 447 Recommended / Option C Log File and Results Interface (Qpmf) 455 Appendix B - Performance Thresholds for Overflow Structures 463 HiDEP : Structure Performance Spreadsheet Notes 463 Summary of information in the spreadsheets 464 Functional crests 473 Appendix C - Highgate Chain Option Designs 480 Appendix D - Hampstead Chain Option Designs 507 Appendix E - Golders Chain Option Designs 531 Appendix F - Risk Register 537 Appendix G - Project Timetable 539 Appendix H - Detailed Cost Plan 541 Appendix I - Jonathan Newman (CEH) Water Quality Report 574 Appendix J - NetWeather rain RADAR Quote and method statement 597 Appendix K - Telemetry Tender Analysis 604 9

10 Appendix M - Flood Envelope Modelling Details 647 Appendix N - Landscape architect visualisation comments 651 Appendix O - RSHydro Telemetry Method Statement

11 Aims and Objectives In February 2010, the City of London commissioned Haycock Associates to undertake a review of the hydrology and hydraulics of the dams within Hampstead Heath. The aim of the study was to determine the current operation of these structures and their compliance with the Reservoir Act (1975) and the Flood and Water Management Act (2010). This review builds on the reservoir Panel Engineer inspections of the three designated dams in 1987, 1997 and 2007 plus a catchment hydrology report undertaken by Haycock Associates in In 2009 a review of the operational risk and safety of the dams was undertaken by CARES Limited (supervising panel engineers for the City of London until March 2010) and as part of this review the potential flood risk to downstream properties, if the dams were to structurally fail, was considered. As a result of this 2009 review the current designated dams were classified as high risk structures (A1) and in the event of failure would present a substantial risk to downstream properties and the general public. But critically, the time to issue a warning to these properties and the public would be less than 2 hours, thus the A1 classification of the designated dams. In reviewing the current operation and potential options to ensure the operational safety of the dams, consideration has been given to enhance the time gap between a rainfall event and the movement of this floodwater into the land below the dams. Haycock Associates worked with the City of London and Dr. Andy Hughes (the Hampstead Heath Panel Engineer) to create outline designs for the reservoirs and ponds on Hampstead Heath including draft visualisation for use during the consultation stage. This report also provides a prediction of hydrological behaviour on the Heath and details of a telemetry system currently being installed which in turn will be useful for emergency planning purposes. As part of the review there was a requirement to ensure that the current designated dams can be operated safely and the other dam structures do not compromise the functioning of the designated dams. This review has also taken into consideration that new Flood and Water Management Act (2010) which will extend the number of designated dams on the Heath and therefore the number of structures that need to meet the engineering safety standards as set out in the 1975 Act. The key requirement of this review is to ensure that in the event of an extreme rainfall event (>1: years), that the dam structures are able to cope with this floodwater and that the dam structures are not exposed to structural failure. The review therefore aims to document the current operation of the site and how water is passed from dam to dam. Where water cannot be passed safely the report makes recommendations on how the current structures on the dam could be upgraded to pass water safely from dam to dam. The key concept to recognise is that the options being developed do not aim, and cannot aim, to prevent floodwater from passing from dam to dam and therefore downstream into the land, property and urban environments below the dams. The options being explored centrally aim to ensure that the dams remain intact after the passage of an extreme flood and that these dams do not fail catastrophically and add surges of floodwater to the land / urban areas below the dams. In developing this report and reviewing the current hydrology/hydraulics of the dams and the technical options, Haycock Associates have worked closely with the City of London engineers and the appointed Supervising Panel Engineer, Dr. Andy Hughes (WS Atkins). Within the report, the costs of developing the options have been explored and these are aimed at guiding the discussion with the City of London and users of Hampstead Heath over the coming months. 24

12 Introduction Figure 1 Map showing Hampstead Heath pond chains Hampstead Heath Extension chain: Seven Sisters 1-7 Turners Pond Highgate chain: Golders Hill chain: Sandy Heath Pond Wood Pond (Lily Pond) Thousand Pound Pond (Concert Pond) Stock Pond Swan Pond Lily Pond Kenwood Ladies Bathing Pond Bird Sanctuary Pond Water Garden Pond Leg Of Mutton Hampstead chain: Viaduct Pond Model Boating Lake Mens Bathing Pond (Highgate No.2 Pond) Highgate No.1 Pond Ironstone Bog Vale Of Health Catch Pit Mixed Bathing Pond (Hampstead No.3 Pond) Hampstead No.2 Pond Hampstead No.1 Pond City of London (c) Crown copyright (2011) As shown in Figure 1 above, there are three main chains of ponds on Hampstead Heath; the Hampstead chain, the Highgate chain and the Golders Hill chain. The Seven Sisters is a fourth chain made up of seven smaller ponds; this chain will not be covered in this report. Originally the ponds on Hampstead Heath were constructed by the Hampstead Heath Water Company in the late 1700s, for the supply of water to north London s suburbs. Initially ponds were created by damming the Hampstead Brook, one of the sources of the Fleet. Similarly, damming also created the Highgate and Golders Hill chains. The Highgate chain consists of Wood Pond, Thousand Pounds Pond (both owned by English Heritage in Kenwood Park), Stock Pond, Kenwood Ladies Bathing Pond, Bird Sanctuary Pond, Model Boating Pond (also known as Highgate No 3), Highgate Men s Bathing Pond (also known as Highgate No 2) and Highgate No 1 Pond. Both Model Boating Pond and Highgate Men s Bathing Pond are classified as reservoirs. Swimming is permitted all year round at Kenwood Ladies Bathing Pond and Highgate Men s Bathing Pond. The Hampstead chain consists of Vale of Health Pond, Viaduct Pond, Catchpit, Mixed Bathing Pond (also known as Hampstead No 3), Hampstead No 2 Pond and Hampstead No 1 Pond. Hampstead No 1 Pond lies at the bottom of the chain and due to the volume of water stored above natural ground level, is classed as a reservoir under the Reservoirs Act Swimming is permitted all year round in the Mixed Bathing Pond. 25

13 The Golders Hill chain consists of Leg of Mutton Pond, Water Garden Pond, Lily Pond and Swan Pond. Following a detailed analysis of the soil condition of the Heath by Haycock Associates in 2006, the Haycock Associates were commissioned by the City of London to provide a detailed evaluation of the Heath s hydrology and structure hydraulics. This evaluation includes the following processes: conduct an assessment of the wider hydrology of Hampstead Heath in extreme events; evaluate the current dam structures and built flow paths for efficiency and safety in extreme events; and scope options for improving the layout of the structures to increase the effectiveness, safety and longevity of the built structures in extreme events. An earlier report titled Hydrology Improvements Detailed Evaluation Process: Data Consolidation discusses and explains the collation of data which supports the current report. All items first mentioned in underlined italics can be found in the glossary at the end of this report (page 83). Flood Risk In 2007 Haycock Associates undertook a modelling exercise to document the extent of flooding in the event of structural failure of the designated dams within Hampstead Heath. This flood extent modelling has been revisited in 2010 for the current report (please see section 9). The flood risk maps submitted in 2007 sought to model the extent of floodwater from the dams under two scenarios, firstly a failure when the dam held water under normal conditions and secondly when the dam was full or held water at dam crest level. These flood extent maps were produced in 2007 with a further flood extent map for Golders Chain produced in Figure 2, 3 and 4 below show the extent of these flood areas. In 2009 this modeling work was reviewed by the City of London Supervising Panel Engineer as part of a review of the safety of the dams. Their work in 2009 documented the risk to property and lives downstream of the dams if the designated dams should fail. The summary of this analysis is contained in Table 1. It can be seen that a failure of the dams on the Hampstead and Highgate chains would put many lives at risk, although a warning of >2 hours would reduce the likely loss of life significantly. Although Golders Hill chain contains no designed dams, namely dams that contain >25,000 cubic metres of water (Reservoir Act 1975) or > cubic metres of water (Flood and Water Management Act 2010) this chain has still been considered in the review of flood risk in the CARES 2009 report and in this report. 26

14 Figure 2 Highgate No. 1 Flood envelope for 41,000m3 based on breach of crest water level (CL) 27

15 Figure 3 Highgate No 1 and Men s Bathing Pond Flood envelope for 122,000m3 based on breach of crest water level of Men s and Highgate No 1 and there subsequent failure (CL). 28

16 Figure 4 Hampstead No. 1 Flood envelope for 47,000m3 based on breach of crest water level (CL). 29

17 Table 1 PAR (persons at risk), LLOL (likely loss of life) and estimated damage costs for the dam breach scenarios Structure PAR LLOL Total estimated damage costs No warning Warning & evacuation Hampstead No ,732, Highgate No ,355, Swan Pond ,888, The review by CARES limited in 2009 plus additional concerns about the current operation of the dams has therefore lead to the commissioning of this systematic review of the dams on Hampstead Heath with the aim of better defining their current operation and options to ensure their safe operation. The CARES Limited review will been revisited in Section 8 of this report to assess how the hydrological assessments and recommended designs included in this report change the risk quantification. As this review was being developed, a number of additional requirements had to be absorbed, firstly the change in the legislative environment with the passage of the Flood and Water Management Act in April Secondly a small, but significant flood event in early May 2010 emphasized the current operational issues surrounding the dams and the risk of overtopping flows over the dam structures. Please see section 9 which discussed further modelling undertaken with regard to a Qpmf event. Reservoir Undertaker and City of London Legal Obligations The ponds and reservoirs on Hampstead Heath are subject to two key statutory acts; The Reservoirs Act 1975 and the Floods and Water Management Act The Floods and Water Management Act 2010 is expected to be made into law over the coming months and this will mean major changes for reservoir and pond undertakers. All of the ponds in the Highgate and Hampstead chains will be covered under the Reservoirs Act 1975 when the Floods and Water Management Act 2010 is passed. The size of a designated statutory reservoir will be reduced from >25,000 m cu to >10,000 m cu and all ponds in a chain that have a combined volume of over 10,000 m cu will also be designated. Currently, only Men s Bathing, Model Boating and Hampstead No. 1 Ponds are statutory reservoirs. The Floods and Water Management Act 2010 will categorise reservoirs by a risk approach based on consequence and probability. This will not change the categorisation of the dams on the Heath. The ponds designated under the Reservoirs Act 1975 are subject to strict Health and Safety requirements and annual inspections. As Hampstead Heath is an important area for recreation and relaxation in north London and is surrounded by dense residential areas, the potential risk to the population downstream of the ponds is extremely high. The City of London is legally required to ensure that the dams are not at risk from failure if a Qpmf (Probable Maximum Flood) flood occurred. The upgrading of the three current designated statutory reservoirs are an existing legal requirement and the need for the upgrade to meet the requirements of the Reservoirs Act 1975 have been stated by the appointed Supervising Panel Engineer. The City of London have been recommended to also upgrade the other ponds in the Hampstead and Highgate chains as part of a wider scheme to ensure that the smaller ponds on the chains will be compliant when the new Floods and Water Management Act is passed. The benefits of upgrading all of the ponds at the same time are that the chains are designed as a whole and any attenuation created upstream during an extreme event can reduce the amount of engineering works downstream. Also, the construction of all of the upgrades in one go will result in lower preliminary costs. Hampstead Heath is also covered by a conservation act, The Hampstead Heath Act 1871, which states that the management should preserve the natural aspect and state of the Heath for future generations to enjoy for recreation and relaxation. However, Health and Safety requirements to comply with the Reservoirs Act 1975 may overrule the Hampstead Heath Act. It is desirable to meet safety compliance without compromising the recreational and aesthetic nature of the Heath. 30

18 Statutory Supervising Panel Engineer Role All engineering work, including design and construction, taking place on a statutory reservoir must be completed under the supervision of a currently registered Panel Engineer, one of a group of highly specialist Civil Engineers appointed by the Secretary of State. It is the responsibility of the reservoir undertaker to appoint a Supervising Panel Engineer. It is the responsibility of the Supervising Panel Engineer to approve the likely design flows into and out of a reservoir, and thus the quantity of the flows the spillways and dams are required to be designed to. Any modifications to a dam, for example raising the crest, or changing the materials on the dam must be approved by the registered Supervising Panel Engineer for that reservoir. Additionally, all statutory reservoirs under The Reservoirs Act 1975, must be inspected and supervised by a Panel Engineer at least once a year. Panel Engineers use the 1:10,000 year and Qpmf design event standards when assessing risk on reservoirs. This standard appears in table 1 of the Flood and Reservoirs Safety Guidance, a document that has been developed and revised over the last 30 years. Although not mandatory, this standard is well established and no panel engineer generally goes against these accepted standards. The panel engineer can make a judgement on the level of freeboard or the importance of wave effect in the design solution of the reservoir structures but not on the design event itself. 31

19 1. Hydrology The first section identifies the flows that will act as the standards that the resultant upgraded engineered structures will be designed to cope with. Defining catchment areas for the individual lakes Paramount to the calculation of design flows within a particular catchment it is necessary to determine the catchment size upstream of the point in question. Ponds in a chain will have increasing catchment sizes due to the additional contributing area that drains into the ponds as we move downstream. The Thames Water drainage maps fro the site were acquired but did not provide any indication on the size of the downstream pipes after flow leaves the Hampstead and Highgate Chains. It is likely that the restricting point in the current layout is the outflow pipe itself and not the sewers further downstream. The Reservoirs Act does not require the prevention of flooding downstream of the dam, but just the prevention of structural failure of the dam. The sewers would likely be inundated, thus irrelevant, during the extreme events we are designing for so the downstream capacities of the sewers downstream have not been incorporated into appraisal. The City of London acquired from Infoterra plc. a digital terrain model for the Hampstead area in January This data consists of high resolution ground levels measurements, recorded by an airborne light detection and ranging scanner (LiDAR system). The data was processed by Infoterra into a 1m by 1m ground resolution model of the land surface, with a vertical accuracy of m. All data is recorded relative to the OS grid. This data is then further processed by Infoterra into a surface model, showing the height of the buildings and tree canopies, and a land elevation model showing calculated land levels below the canopy after deleting the buildings from the data. This last dataset, namely the digital land elevation model, was the key data used in this product for the hydrological analysis of the lakes catchments. The digital elevation data was processed in IDL-RT and all the lake catchment areas were calculated. In addition to the catchment areas, additional catchment statistics were created in this process, critically all the flow routes within these catchments. Initially these catchment areas were used within the standard methodology for calculating Qpmf and later the IDL-RT software was used to determine the 1:10,000 flow using a rainfall runoff simulation. Baseline and Extreme Event Hydrology Following a review of the documentation and reports written regarding the Hampstead Heath ponds in the past, it was concluded that the baseline hydrology had not been agreed in any of the historical reports and that this has been an ongoing issue since In the 2007 Section 10 report, the 1987 Flood Studies report is discussed with reference to 10,000 year flows through Hampstead No. 1 and Highgate No. 2 and 3. However, when Haycock analysed these values they came across some uncertainties. The results from the 1987 Flood Studies report (Binnie and Partners) were compared with Haycock s analysis from Table 2 shows a comparison of the two different design flood event estimates (note that Haycock s analysis is for a 1:100 year event and the 1987 Flood Study values are for a 1:10,000 year event) for Hampstead No 1 Pond. The key value for concern is the runoff percentage. The 1987 report suggests a runoff percentage of 27% which means that it assumes that 116mm are infiltrated into the soil during this event as opposed to only 34mm during the 1:100 year event. This value is extremely optimistic and a realistic percentage of runoff during this event would be expected to be approximately 70%. The runoff percentage for a 1:10,000 event should most definitely be higher than that for a 1:100 year event as nearly all the soil infiltration potential will already be utilised during the 1:100 year event. Based on the 1987 flood studies report, time to peak analysis indicated that the unified event for this catchment was a 4.4 hour duration storm for Highgate No. 1. This was checked and accepted as an approximation. No other shorter or longer storms have been simulated. Originally the 1987 report cited 159 mm over 4.4 hours as the 1:10,000 year event. A review of the Hampstead Scientific Society rainfall data over 100 years showed that this was excessive. Equally, the 1975 storm of 170 mm was rated as a >1:50,000 year event, thus there was an inconsistency in the 1987 analysis which we sought to resolve. 32

20 Table 2 Hampstead Heath comparisons for Hampstead No 1 Pond from the 1987 Flood Studies report and 2006 Haycock report Design event description 1987 Flood Studies 1:10,000 year event Haycock :100 year event Area (km 2 ) Event 159 mm in 4.4 hours 70 mm in 4 hours Rainfall volume during event (m-cu) 114,480 50,400 Peak flow in reservoir 3.9 cumecs 1.8 cumecs Total volume through the pond over whole event (m-cu) 30,888 25,920 Runoff percentage (total volume through pond/rainfall volume during event) 27.0% 51.4% Comments Baseflow runoff ratio Expected ratio for higher events Following discussions with the Supervising Panel Engineer for the site (Dr. Andy Hughes, WS Atkins) in April and May 2010, it was decided that Haycock would work to calculate the Probable Maximum Flow (Qpmf) for all the ponds of interest using the standard methodology adopted by the Reservoirs Act 1975 initially. For the purpose of the upgrading of the dam structures and flow paths through the ponds it was important to calculate the 1:10,000 year event flow as well as the more extreme Qpmf. It is standard practice to multiply the 1:10,000 year event peak flow by two to estimate the Qpmf peak flow, as discussed with Dr Andy Hughes. For this reason the 1:10,000 year event is often referred to as the 0.5 Qpmf. Given the designation of the dams as A1 risk, it was agreed that a structure was required to route the 1:10,000 year flow on all designated reservoirs, but the dam as a whole should be designed to allow the passage of a Qpmf flow safely, possibly over the crest of the dam in a way that the downstream face of the dam remains intact and would not be subject to erosion in this extreme event. Qpmf Standard Methodologies There are various standard equations for calculating the Qpmf of a catchment and these are adopted in the Reservoirs Act 1975 and more recent reports by the Institute of Hydrology. These variations take different variables into consideration. Table 3 shows various Qpmf methods used. To obtain the S0100 (average slope from 0km to 100%km down the catchment) figure to calculate the Qpmf for the ponds, 1:25,000 Ordinance Survey blue line data was used to identify the start of the fluvial chain for the Highgate sequence of ponds. The average slope from the start of the blue line to the outfall of the ponds was calculated using elevations from LiDAR data. It was decided that using S0100 would be more suitable than using S1085 (average slope from 10%km and 85%km) due to the small size of the catchments. The last two columns in Table 3 are equations that were published in the Institute of Hydrology Report No. 114 Reservoir Flood Estimation: Another Look (1992). These equations include an urban element and various descriptors of the soil conditions. Equation 4.3 produced the highest values for Qpmf which we believe best represent the Qpmf of the Heath. However, these values still appear to be relatively low when the highly compact nature of the soil at Hampstead Heath is taken into account. These compaction values were determined during the work Haycock Associates did in To assess the hydrology through the Heath, aerial photography was analysed to enable land use to be classified. This included mapping the Heath footpaths which critically influences the hydrology and water movement in the pond catchments. In addition to this analysis, fieldwork on the Heath s soil hydrology was further undertaken to generate key hydrology parameter that informed the runoff and flood modelling work. In 2006, Haycock Associates work concluded that the compaction of the soils on, and around, the footpaths was high and presented special concerns. The compaction is due to the sheer number of visitors (9 million) and dogs that use the Heath 33

21 every year. Compaction in mown grass areas was also high with resultant impacts on soil hydrology and the runoff regime of the soil. Hampstead Heath has a dense network of footpaths amounting to 105 kilometres in length, around 20% of which are within 20m of streams or flow routes. The compaction of these paths means that runoff quickly flows into the streams and ponds with little infiltration occurring. A more detailed analysis of the compaction on Hampstead Heath is discussed in Haycock Associates 2006 report. In Figure 5 below the compaction status of the Heath is shown in relation to land cover classification. Figure 5 Spatial distribution of compaction data for Hampstead Heath applied to land cover classification N N E E E E E E N N Compaction Classes (PSI) v id wat r b di s (OS+LiDAR) rr rs < 50 PSI PSI PSI PSI PSI PSI PSI Buildings N N N N N N E E E E E E M tr s 1987 Flood Studies Binnie and Partners The 1987 Binnie and Partner calculations (1987 Flood Studies) of Qpmf for the ponds were compared with Haycock derived values using the standard methodology in Table 4. The catchment areas that Binnie and Partners use are slightly different than those derived by Haycock in The maximum difference in catchment size is 3.48% which is not a significant enough value to be of concern. 34

22 Table 3 Summary of various methods used to calculate the peak Qpmf Haycock * Equations published in the Institute of Hydrology Report No. 114 Reservoir Flood Estimation: Another Look (1992) 35

23 Table 4 Comparisons of Qpmf from the 1987 Binnie & Partners report and Haycock derived values using standard method Feature 1987 Binnie & Partners report 0.5 summer Qpmf (mcu/s) 1987 Binnie & Partners report 10,000 year flood Qpmf Thus Qpmf from 1987 should be (derived from Haycock) (mcu/s) Qpmf calculated using Binnie & Partners Areas, S1085s & SAARs using Eqn 1 Haycock Qpmf (using S0100) Eqn 1 Haycoc kqpmf equation 4.2* (RSMD) Haycock Qpmf equation 4.3* (SAAR) Model Boating Men s Bathing Pond Hampstead No The calculated S1085 are all greater in the 1987 calculations when compared to the calculations performed by Haycock Associates. The S1085 produced in 1987 are all approximately 8% greater than the S0100 s produced by Haycocks. However, the Qpmf values produced by Binnie & Partners are very similar to those produced by Haycock using the standard methodology (Columns 6-8). Hampstead Scientific Society rainfall data In reviewing the standard methods for estimating extreme runoff, assumptions are made on the nature of the extreme rainfall events, and these number are based on regional average estimates. Given that there is 100 years of meteorological data gathered at the Hampstead Scientific Society, it was decided to examine the extreme rainfall statistics for this station relative to the regional averages used in the 1987 analysis. The Hampstead Scientific Society was established in 1899 to encourage interest in all aspects of Science. The weather station is located at approximately OS 52626,186200, close to the southwest corner of Hampstead Heath and about 1 km away from Hampstead No. 1 Pond. The society has been collecting daily meteorological data for over 100 years. Haycock have digitised this rich data and analysed the data to derive various precipitation return periods. Figure 6 below shows the statistical return periods of a number of rainfall events with varying durations. The sub 24 hour totals were based on the statistical rating curve of 24hr, 48hr, 120hr, 240hr and 600hr totals from which their corresponding 1:10,000 year equivalents were defined. Extrapolation to the 1:10,000 event for the 24hr, 48hr, 120hr, 240hr and 600hr was explored using log:log, normal:log and probability scales. Based on discussions with Tim Burt, a normal log scale was applied, which was made easier after the 1975 event was removed from the data set on the advice of Professor Burt. Using the rating curve for the 1:10,000 year events of varying durations, the sub 24hr totals were defined. This method was discussed with Professor Tim Burt at Durham University in order to achieve a sensible interpretation of long term metrological data, on which he is the UK expert. Haycock Associates are grateful to the Society for allowing access to the data through the offices of Philip Eden, the current operator of the station. 36

24 Figure 6 Magnitude of year events using Gringorten formula for The Hampstead Heath Society daily meterological data Table 5 below shows the results of the weather station analysis. The values in the second column are calculated using the 100 year long daily rainfall dataset. The third column show more intense rainstorms with the 1:10,000 year return period as derived using the resulting equation produced which relates return period with precipitation as shown in Figure 6 above. This shows that a 1:10, hour rainfall event equals to mm of precipitation. This value derived from observed data is slightly lower than that used by Binnie and Partners in the 1987 Flood Studies report (159 mm in 4.4 hours). The lower depth of rainfall from the Hampstead Scientific Society for the 1: year rainfall event was discussed with the project technical group and the Supervising Panel Engineer and their hydrological specialist. Based on these discussions, it was decided that the original estimate of the 1:10,000 rainfall event was too high at 159mm and that the local data would be adopted, namely 134.8mm for the critical 4.4 hour storm. This lower value has been adopted in the hydrological and runoff assessment below. Table 5 1:10,000 year rainfall event analysis using The Hampstead Scientific Society (HSS) daily meteorological data Event duration - hrs 1:10,000 year ppt estimated from HSS (mm) 1:10,000 year ppt calculated using equation derived from HSS data (mm)

25 Event duration - hrs 1:10,000 year ppt estimated from HSS (mm) 1:10,000 year ppt calculated using equation derived from HSS data (mm) Historical storm events Haycock analysed the observed data from the weather station and identified key storm events reported in the media. The following events are worth a mention in this report and are useful in confirming the hydrological behaviour of the Heath. 14th August 1975 Tim Burt at Durham University is currently conducting an analysis of the 14th August 1975 storm event at Hampstead Heath which will be commented on once it has been completed. M.J Miller from Imperial College, London, published a paper regarding this storm in 1977 (The Hampstead storm: A numerical simulation of a quasi-stationary cumulonimbus system). The paper suggested that this storm was a result of the heat island and pollution from London creating a cumulonimbus system and the prevailing wind blowing the storm north over Hampstead. The paper suggested that this event was not uncommon. However, the pollution and heat island effect is thought to be less apparent today than in the 1970s. 7th August 2002 The 7th August 2002 event was used in the Haycock 2006 report as a focus for the distributed hydrology model. The 70 mm of rainfall was calculated to be a 1:88 year precipitation event. 1-3rd May 2010 This event was a result of 46.6 mm of rain falling on the Heath (as measured by the Hampstead Scientific Society met. station). The rainfall graph for this event is shown below, in Figure 7. It was confirmed that following analysis of the rainfall data from the Hampstead Heath Scientific Society, the May 1-2nd event could be defined as a 1:8 year precipitation event with a peak flow at Highgate No 1 Pond of 2 cumecs. It is understood that no flow or water levels measurements were recorded for this event. Figure 8 shows the response of flow at Highgate No 1 Pond as modelled by the IDL-RT software to the rainfall event. The delay between peak rainfall and peak runoff at Highgate No 1 Pond is 2 hours and 30 minutes, which may be useful in the development of the emergency action plan. There is a sustained flow post peak of 0.7 to 0.6 cumecs. 38

26 Figure 7 Rainfall graph of the 1-3rd May 2010 event using The Hampstead Heath Scientific Society 5 minute interval meteorological data Figure 8 Comparing the cumulative rainfall with the resulting flow at Highgate No 1 Pond for the 1-3rd May 2010 event 39

27 During the 1-3rd May 2010 event, overtopping of the dams occurred at Hampstead No. 2, Hampstead No. 3 and Stock Pond. Hampstead No. 2 experienced erosion to the dam structure. The Highgate No. 1 dewatering valve was opened by approximately 1 due to the perceived risk posed on the structure. From Figure 8, it can be seen that the flow peak would have passed through the system at around midnight, thus the photos and water levels observed on the next morning may be much lower than the peaks that occurred several hours earlier. Figure 9 and Figure 10 display photographs taken between 11 am and 1 pm the next morning, 3rd May Figure 9 Photograph of Stock Pond crest, taken between 11 am and 1 pm on 3rd May 2010 by Justin Walsh, City of London Figure 10 Photograph of the spillway at Model Boating Pond, taken between 11 am and 1 pm on 3rd May 2010 by Justin Walsh, City of London 40

28 Revised Rainfall : Runoff Model Distributed Hydrological Model A major weakness of the Reservoirs Act standard method for calculating Qpmf is that it does not allow any manipulation of the catchment factors to explore how land cover manipulation would impact the flow regimes of the catchments and ultimately, in this case, flows into the key ponds and lakes. In order to explore how land cover, in this case the nature of land cover and the path network, would impact the flow regime during very extreme events we deployed a distributed hydrological model for the three main catchments which was first used by Haycock in A distributed hydrological model seeks to route rainfall through or over the soil, portion flows into groundwater, account for groundwater discharges and then route surface flows through the drainage network. The distributed Topoflow model represents all land and water surfaces, so rainfall onto the ponds is represented directly in the model. Due to the high compaction and low sorptivity characteristics of the soils, it is likely that even small preceding storms would lead to saturation. Therefore, saturated soils were used as an antecedent condition for the Topoflow model. The distributed hydrology model (IDL-RT-TF) undertakes these calculations at a 10m by 10m grid for the whole landscape. In the section below we outline the data sets defined in this process. Distributed hydrological model - Metrological Data The distributed hydrology model (IDL-RT-TF) is not a statistical model, it simulates a real event and enables scenarios to be built around that real rainfall event. Hampstead Heath has been impacted by a number of high rainfall events in the last 40 years, many of these events have been recorded by the Hampstead Scientific Society as explained in above. The Society gave this project access to 100 years of rainfall data so we could assess the return periods of key rainfall events and statistically estimate extreme rainfall events. This analysis led this project to adopt a 4.4 hour rainfall event totaling mm of precipitation for the 1:10,000 return period to enable flows resulting from this rainfall event to be determined Haycock have simulated several other scenarios using the rainfall runoff model developed in 2006 as well as the 1:10,000 year rainfall even. The other scenarios that have been simulated are the 1:8 year rainfall event (1-3rd May 2010) and the 1:88 year rainfall event (7th August 2002). The results are illustrated in Figure 11 below. The 1:88 event shows a runoff ratio of 67% and the 1:10,000 year event (0.5 Qpmf) shows a runoff ratio of 87%. These runoff ratios are more realistic than those suggested by the Qpmf standard methodology calculated values. A runoff ratio of between 80-90% should be expected for a Qpmf event, especially where the soils are highly compacted. The 1:10,000 year event simulated here predicts a peak flow of 36 cumecs at Highgate No. 1 which is much larger than the flow values predicted using the standard calculated methodology. The simulations discussed here uses Hortonian Stream Order 5 for the pruning threshold in the rainfall runoff model. 41

29 Figure 11 Flow hydrographs produced of four simulated events at Highgate No 1 Pond. x-axis is time in minutes Hydrology analysis with Dr. Margaretta Ayoung (Atkins WS) Following the various methods of hydrology analysis undertaken and the vast variance of peak flow numbers, Haycock met with Dr. Margaretta Ayoung at Aktins on the recommendation of Prof. Andy Hughes. Highgate No. 1 was used as the comparison pond to assess various methods of calculating the peaks for high magnitude events. The results are shown in Table 6. Table 6 Highgate No. 1: Analysis of various methods of gaining Qpmf and 1:10,000 year flow information Source Return Period Peak Flow (mcu/s) Institute of Hydrology Report No. 114 Reservoir Flood Estimation Qpmf Equation 4.3: QPMF = A S SOIL (1 + URBAN) 2.04 SAAR FSR - Summer storm method Qpmf Topoflow rainfall runoff method 1:10,000 year rainfall event ReFH - Summer storm method 2 x 1:10,000 year flow event The standard Qpmf calculation methodology results in a maximum Qpmf peak flow of cumecs at Highgate No. 1. However, as stated above, this does not represent the highly compacted condition of the soil and the impact of this on the runoff ratio. The summer storm FSR (Flood Studies Report 1975) method for the Qpmf produces a flow of 33 cumecs for Highgate No. 1 and the ReFH (Revitalised Flood Hydrograph) method produces 32 cumecs for the Qpmf peak at Highgate No. 1 when using the summer storm method. (This value has been doubled from the 1:10,000 year peak flow to represent Qpmf.) It was found that both with the FSR and ReFH method the SPRHOST (percentage runoff derived by hydrology of soil types) value 42

30 did not seem to represent the soil (SPRHOST value = 0.3) and both methods use an urban percentage of 12% which is similar to the standard Qpmf calculated methodology. Because SPRHOST at 0.3 is high, runoff estimates are likely to be low. Thus FSR and ReFH peak values represent minimum values. It was decided that increasing the urban proportion of the catchment from 12% to 50%, which would better reflect the highly compacted soil present, by doing this the Qpmf values will increase to cumecs for Highgate No. 1. However, the Qpmf standard methodology and equations are a rapid assessment and are unlikely to be appropriate for small catchments such as those at Hampstead Heath (<2 km). Hydrology Summary Following a collation of all available data: theoretical calculations; rainfall runoff modelling; and historical event data, it was decided that the standard methodology of calculating Qpmf was severely underestimating the flow that the structures on Hampstead Heath should be able to cope with. Based on the ambiguity of the standard Qpmf methodology, it was agreed that Haycock would design spillways on each pond to the 1:10,000 year rainfall runoff event (e.g cumecs at Highgate No. 1). The dam structure as a whole should also be designed and armoured to safely pass double this flow for short periods of time (i.e. Qpmf = cumecs for Highgate No. 1). These values are displayed in Table 6, below. Note that these flows will be attenuated by the lake chain and these values thus represent the boundary conditions of the lake model. The Q100 values in this table were produced using the standard FSR method of calculating Q100 using an equation. Table 7 Summary of the peak: FSR Q100; Q 1:10,000 (0.5 Qpmf); and Qpmf for all ponds Pond Q100 FSR Q 1:10,000 (0.5 Qpmf) (mcu/s) Qpmf (2 x Q 1:10,000) (mcu/s) Stock Kenwood Ladies Bathing Bird Sanctuary Model Boating Men s Bathing Highgate No Vale of Health Viaduct Mixed Bathing Hampstead No Hampstead No Leg of Mutton Swan

31 2. Hydraulic Assessment Overflow Structure Performance Review The capacities of the current overflow structures on the dams shown in Table 8 below have been assessed. The maximum discharge possible through the overflow pipes and, where relevant, auxiliary spillways have been calculated using a water level equivalent to that of the crest. This ensures that the maximum head is applied to the structures, without water passing over the top of the dam crest. Further details regarding these calculations can be found in Appendix B. Table 8 Dams on Hampstead Heath where the current performance of the overflow structures has been calculated Chain Pond Hampstead No. 1 Hampstead No. 2 Hampstead Mixed Bathing Pond Viaduct Pond Vale of Health Pond Highgate No. 1 Men s Bathing Pond Highgate Model Boating Pond Bird Sanctuary Pond Ladies Bathing Pond Stock Pond Golders Hill Swan Pond Leg of Mutton A summary of the current structure performances are shown in Tables 9 and 10 below. The pink cells indicate where the structures are underperforming and require updating. The capacity of the overflow pipes should be sufficient to convey a 1:100 year rainfall event. The current capacity of the normal outflows on Hampstead Heath range from 1:5-1:25 year flows approximately depending on which structure you look at. At Kenwood it was decided to design the outflows to achieve 1: % capacity (2005 English Heritage documents). In 2010 Haycock Associates suggested adopting this approach at Hampstead Heath in the feasibility stage to illustrate that the current pipes and culverts needed to be upgraded. The final decision on the design capacities of outflows is part of the detailed design stage, however, the recommendations seek to delete most of the pipes and to replace these with over-crest spillways. Given that the capacity of the overflow pipes should be sufficient to convey a 1:100 year rainfall event, with the exception of Swan Pond on the Golders HiIll chain, it is evident in Table 9 below that all the overflow pipes are under capacity and are thus highlighted in pink. The spillway on Highgate Number 3 (Model Boating) should be great enough to cope with a 1:10,000 year rainfall event; clearly this is not the case as can be seen in Table 10 below. The design standards for the current Model Boating spillway is not known. It is believed that no design standards were formalised during its design and construction. The structure does not have the capacity to take even the Q100 flow sufficiently, based on our calculations. In addition, whilst it is not 44

32 mandatory for the crest to remain dry in a Qpmf event, the depths of water over the crest are large (0.28m during a 1:10,000 year event and 0.5m during a Qpmf event) and the crests on all ponds are not properly armoured to deal with these flows without risking structural failure. Table 9 Summary of current overflow structure performance Chain Pond FSR Q100 (m 3 /s) 1 Overflow diameter (m) Overflow capacity (m 3 /s) Highgate No Men s Bathing Pond Highgate Model Boating Pond Bird Sanctuary Ladies Bathing Pond Stock Pond Hampstead No Hampstead Number Hampstead Mixed Bathing Pond Viaduct Pond Vale of Health Pond Golders Hill Leg of Mutton Swan Pond Q100 has been calculated using the FSR method as calculated for the 2006 report: 2 Q10,000 and Qpmf have been calculated using the rainfall runoff model (as described in the Section 1, Hydrology above) Table 10 Summary of current spillway and crest performance Chain Pond Q10,000 Spillway (m 3 /s) 2 capacity (m 3 /s) Qpmf Crest Level (m 3 /s) 2 Water depth over crest during Qpmf flow (m) Highgate No Men s Bathing Pond Highgate Model Boating Pond Bird Sanctuary Ladies Bathing Pond