Sizewell B Power Station Dry Fuel Storage Project

Transcription

1 Sizewell B Power Station Dry Fuel Storage Project SURFACE WATER DRAINAGE SCHEME Report prepared for: EDF Energy Author: C.G.Thackeray Approved: Youguang Pan McAlpine Design Group Edmundson House Tatton Street Knutsford Cheshire WA16 6AF Report No.: DG/S/66841/TR/10037, Rev. C QA Grade 4 January 2012 NOT PROTECTIVELY MARKED

2

3 Page iii CONTENTS 1 INTRODUCTION.1 2 SCOPE EXISTING SURFACE WATER DRAINAGE SYSTEM NEW SURFACE WATER DRAINAGE SCHEME General Rainfall Climate Change Ground Conditions Infiltration Tests..3 5 DRY STORE BUILDING EQUIPMENT STORE BUILDING..4 7 WEST CAR PARK EXTENSION 5 8 SUMMARY General Maintenance 6 9 CONCLUSIONS CODES, STANDARDS & DESIGN GUIDANCE.8 APPENDIX A - DRAWINGS APPENDIX B - INFILTRATION TEST RESULTS

4 Page 1 of 8 1 INTRODUCTION Sizewell B (SZB) Nuclear Power Station is located on the east Suffolk coast near to the town of Leiston. The infrastructure element of the Dry Fuel Storage Project consists of a large aluminium clad steel framed Dry Store building together with a dedicated smaller Equipment Store building. The location of the Dry Store building is on the site of the existing south car park. To compensate for the loss of parking spaces resulting from the construction of the Dry Store building the west car park is to be extended. In order to minimise any increased risk of flooding, to improve and protect water quality and ensure future maintenance of the surface water drainage system, it is essential to consider source control within the surface water drainage scheme proposed. There are different techniques which can be employed to manage the surface water runoff, from buildings and hardstandings, at or close to the receiving surface. These techniques range from swales, ponds and soakaways to attenuation tanks. The suitability of each method is dependant largely on ground conditions, available space, interface with other services and cost. The surface water drainage scheme proposed for the Dry Fuel Storage Project will be designed, using Microdrainage WinDes software, to manage rainfall events up to 1 in 100 year return period with appropriate allowance for climate change, in a sustainable manner with due consideration of the quantity of discharge, quality of discharge and any potential impact on local amenity and biodiversity. 2 SCOPE The scope of this document is to respond to the requirements of Planning Condition 29. The document will describe how the design of the surface water drainage system for the Dry Fuel Storage Project achieves the requirements of planning condition, as set out below: (a) Details of how the surface water drainage proposals shall be maintained and managed; (b) Details of how the volumes of water produced by the car park in all rainfall events up to the 1 in 100 year return period event including climate change will be managed; (c) Where appropriate, details of how the discharge rate will be limited to the Greenfield runoff rate for all rainfall events up to the 1 in 100 year return period event including allowances for climate change.

5 Page 2 of 8 3 EXISTING SURFACE WATER DRAINAGE SYSTEM The existing surface water drainage from the South Car Park flows through a local bypass oil interceptor and then by gravity to the surge chamber and to the outfall which discharges to the North Sea. The existing surface water drainage system from the West Car Park flows by gravity pipework to the bypass oil interceptor adjacent the Training Centre and then by a combination of pumped and gravity drainage pipework flows via the surge chamber to the outfall which discharges to the North Sea. Both routes are identified on Fig 1a in Appendix A. The design of the original SZB surface water drainage system was performed in the mid 1980 s in accordance with BS8301 [1], Building Regulations and Central Electricity Generating Board (CEGB) design memorandum number 097/306 [2]. Design rainfall intensity used at the time was 75mm/hr for roofs and 50mm/hr for roads and hardstandings. In both cases the systems were designed for a 1 in 30 year return period rainfall event with no flooding. The CEGB Design memorandum specified an enhanced design rainfall intensity of 110mm/hr which was used for the calculation of rainfall runoff from the roofs of buildings. 4 NEW SURFACE WATER DRAINAGE SYSTEM 4.1 GENERAL This document describes the surface water drainage scheme for the Dry Fuel Storage Project and encompasses the full extent of the new build element of the project. The overall scheme is described under the following sections: Dry Store building Equipment Store building West Car Park Extension Drawings of the scheme are provided in Appendix A. The surface water drainage system for the scheme has been designed using parameters given in sections 4.2, 4.3, 4.4 and 4.5 in accordance with the appropriate British and European Codes and Standards and, where appropriate, using best practice. 4.2 Rainfall The surface water drainage system for the Dry Fuel Storage Project will be designed using Microdrainage WinDes [7] design software. The variables within the software will be set as follows: FSR Rainfall. Return period 100 years Climate Change: +20% M5-60 (mm): Storms: Summer (Cv 0.75) Winter (Cv 0.84)

6 Page 3 of 8 Shortest storm duration: 15 minutes. Longest storm duration: 720 minutes 4.3 Climate Change In accordance with Planning Condition 29 and in the absence of a percentage specified in any code of practice, reference was made to Planning Policy Statement 25 [9] which suggests that by the 2080 s, winter peak rainfall intensity could increase by between 15% and 30%. With the design life of the Dry Store building projected to be 100 years, an allowance of 20% for climate change has been added to rainfall intensity for calculation of the runoff from Dry Store building, Equipment Store building, roads and west car park extension. 4.4 Ground Conditions Located on the south western side of the SZB site, the ground level at the Dry Store building is approximately +9m OD and ground level of the West Car Park extension varies between +6m OD and +4m OD. The Geological Survey map of the area indicates the site to be underlain by undifferentiated sand and gravel which is ultimately underlain by the Crag Formation. A significant thickness of generally medium dense gravelly sand has been encountered during previous site investigations. These granular soils are shown to become dense below a level of around - 3m OD. 4.5 Infiltration Tests Infiltration tests have been carried out on the existing South Car Park, in line with the method set out in Building Research Establishment document BRE 365 [10] and the results are provided in Appendix B. The tests indicate that an average soil infiltration rate of 0.18mm/s was achieved in ground described as light and sandy. This infiltration rate has been used for the design of the soakaway drainage serving the Dry Store building. In the absence of specific infiltration test results for the area of the West Car Park extension, an infiltration rate of 0.09mm/s (a 50% reduction from the infiltration test results in Appendix B) has been used in the outline design of the infiltration trench/soakaway required for the West Car Park extension. This infiltration rate is considered to be conservative as it is expected that the ground conditions and permeability at the West Car Park will be similar to that in the South Car Park. Prior to the detailed design of the West Car Park surface water drainage system, infiltration tests, in line with BRE 365 [10], will be carried out on the proposed soakaway site. 5.0 DRY STORE BUILDING Rainfall onto the duo pitched roof of the Dry Store building is collected by eaves gutters along the north and south elevations. Rainwater outlets set in the gutters are connected to internal vertical rainwater pipes which convey the surface water runoff to a number of soakaways located around the perimeter of the building. To mitigate the risk of water ingress into the building in the event of a blocked roof outlet, overflow gargoyles will be provided, the gargoyles will discharge through the cladding at each end of both eaves gutters, any discharge will permeate into the ground through the permeable paved area surrounding the Dry Store building. The infiltration tests carried out on the South Car Park indicates a sandy soil with excellent infiltration

7 Page 4 of 8 properties. The concept design for the Dry Store building proposes 16 rainwater outlets in the eaves gutters, 8 per side. 12 of the rainwater downpipes discharge individually to mm diameter soakaways. 4 of the roof outlets connect to 2 separate rainwater downpipes which in turn connect to 2700mm diameter soakaways. Microdrainage WinDes design software output summary for 1 in 100 year + 20% CC return storms indicate that: - Peak rainfall: 15 min winter and summer storms: 107 mm/hr Worst Case for soakaway storage capacity design: 60 min duration winter storm Peak rainfall: 44 mm/hr Locations of 1800mm diameter soakaway Maximum required water storage: 8.5 cubic meters Infiltration rate: 3.7 l/s Locations of 2700mm diameter soakaway Maximum required water storage: 16.4 cubic meters Infiltration rate: 6.4 l/s 6.0 EQUIPMENT STORE BUILDING The Equipment Store building has a mono pitch roof with an eaves gutter to the east elevation. Surface water runoff is conveyed through roof outlets to 2 100mm diameter internal vertical rain water pipes. The concept design of the building indicates that the surface water roof runoff will discharge via the existing surface water drainage system. To mitigate the risk of water ingress into the building in the event of a blocked roof outlet, overflow gargoyles are specified, the gargoyles will discharge through the cladding at each end of both eaves gutters. In the event of an overflow the discharge will be onto the car park surfacing and then into the existing surface water drainage system. Microdrainage WinDes design software output summary for 1 in 100 year + 20% CC return storms indicate that: - Peak rainfall: 15 min winter and summer storms: 107 mm/hr No storage or infiltration rates are provided as the proposed scheme is connected to the existing surface water drainage system. During detailed design, the option of installing a local soakaway for the Equipment Store building will be investigated. If the soakaway option was selected an approximate soakaway size of 1.5m diameter concrete ring soakaway, 1.8m effective depth would be required.

8 Page 5 of 8 Assuming local soakaway the following Microdrainage WinDes design software output summary for 1 in 100 year + 20% CC return storms would be apply. Peak rainfall: 15 min winter and summer storms: 107 mm/hr Worst Case for soakaway storage capacity design: 60 min duration winter storm Peak rainfall: 44 mm/hr 1500mm diameter soakaway. Maximum storage: 6.5 cubic meters Infiltration rate: 3.3 l/s 7.0 WEST CAR PARK EXTENSION The surface water runoff from the car park extension will be directed by surface gradients to a system of linear drainage channels. Each run of drainage channel will discharge to a catch pit which removes silt before connecting to a system of underground pipework connected to a class 2 bypass oil interceptor, sized to treat the runoff from the impervious area of the car park, 0.757ha (7,575 square meters). A suitable model selected is the Klargester NSBP015, or similar subject to detailed design, which will be installed together with all necessary vents and high level warning alarms, in accordance with the requirements and recommendations of the manufacturer. The outflow from the oil interceptor will discharge to ground by way of pipework and a soakaway structure, gravel filled or modular crates. Infiltration tests will be conducted in line with BRE 365 [10] on the site of the west car park extension prior to the detailed design of the surface water drainage system. For the basis of the outline design, an infiltration rate of half that found by infiltration tests carried out on the site of the South Car Park will be used to calculate the size of the soakaway/infiltration trench required for the West Car Park extension (see section 4.3). The access road to the west of the car park extension will drain by way of a cross fall to the west where combined kerb linear drainage will collect runoff, discharging it to ground via a number of lengths of perforated pipework located in gravel filled infiltration trenches. There is no parking on this access road and the risk of hydrocarbon pollution is low, however, should the runoff become contaminated, microbiological action within the infiltration trenches will break down the contamination. The surface water runoff is collected by a system of linear drainage channels which discharge through catch pits which are connected via a system of underground pipework to an oil interceptor. The treated discharge from the oil interceptor is infiltrated to ground, local to the car park, using perforated pipework in an infiltration trench or an equivalent system.

9 Page 6 of 8 Surface water runoff from the West Car Park extension. Microdrainage WinDes design software output summary for 1 in 100 year + 20% CC return storm discharging to soakaway. The concept design assumes a 34m x 5m x 1.6m deep soakaway. Peak rainfall: 15 min winter and summer storms 107 mm/hr Worst Case for soakaway storage capacity design: 60 min duration winter storm 44 mm/hr Maximum storage required: cubic meters Infiltration rate of concept design soakaway: l/s The final details are to be determined during detailed design using the actual infiltration rate established from tests. 8 SUMMARY 8.1 General The surface water drainage for the various elements of the new Dry Fuel Storage Project, with the exception of the Equipment Store, will be discharged to ground at or near to source by the use of a sustainable system of drainage - soakaways. The nature of the site does not lend itself to the use of soft SUDS features, such as swales, detention or retention basins (ponds), however by the use of soakaways the overall load on the existing surface water drainage system and consequently the risk of local flooding is reduced. There is a net reduction in the area of hardstanding discharging to the existing surface water drainage system. The surface water runoff from the West Car Park extension is treated for hydrocarbon pollution by the incorporation of a class 2 bypass oil interceptor, the discharge from which will be monitored and percolated to ground by way of a soakaway. 8.2 Maintenance In order that all elements of the surface water drainage system operate at maximum efficiency for the projected life of the project, regular maintenance is essential. The Operating and Maintenance Instructions produced as part of the development s records, will define any plant specific requirements, as determined by manufacturers and a regime will be instigated to ensure that regular checks of the constituent parts of the system, from the roof outlets through to the soakaways are conducted and any remedial work identified is carried out expeditiously.

10 Page 7 of 8 9 CONCLUSIONS This document describes how the surface water runoff from the Dry Store building, Equipment Store building and car park extension will be managed: Surface water runoff from the Dry Store building is discharged direct to ground at source by way of soakaways around the perimeter of the building. Surface water runoff from the Equipment Store building will be connected to the existing surface water drainage system. It is noted that the Equipment Store building is located on an area of existing impermeable car park. Surface water runoff from the West Car Park extension will be treated to remove any hydrocarbon pollution and discharged to ground, adjacent to source, by infiltration trench or modular crate structure. More than 95% of the surface water runoff from the Dry Fuel Storage Project is managed at source by infiltration to ground. There will be a net reduction of 0.82ha (approx. 34%) in the area of hardstanding contributing surface water runoff to the existing surface water drainage. (2.38ha existing 1.56ha proposed = 0.82 ha reduction), see Fig 1a & Fig 1b in Appendix A. This will reduction will reduce the load on the existing surface water drainage system and consequently reduces the flood risk downstream of the SZB site. Infiltration tests were conducted on the site of the Dry Store building in accordance with BRE 365 [10], subsequent to the original Flood Risk Assessment and show that the ground conditions have excellent infiltration properties. The infiltration rate used in the outline design of the infiltration trench for the West Car Park extension is considered conservative at 50% of the rate identified from tests conducted on the South Car Park. Further infiltration tests will be undertaken in the West Car Park to inform the detailed design.

11 Page 8 of 8 10 CODES, STANDARDS & DESIGN GUIDANCE DOCUMENTS This section specifies the codes, standards & design guidance documents that have been used in the preparation of the design of the surface water drainage system for the Dry Storage Building Project. 1) BS 8301:1985 Building Drainage (superseded) 2) C.E.G.B. Design Memorandum 097/306 3) BS EN Roof Drainage, layout and calculations 4) BS EN Drain and Sewer systems outside buildings 5) Building Regulations 2000:2002 edition Part H drainage and waste disposal 6) UK Climate Impact Report No UKCIP09 7) MicroDrainage WinDes computer drainage design software 8) Institute of Plumbing, Engineering Services Design Guide 9) Planning Policy statement 25 (PPS 25) : Development and Flood Risk 10) BRE365 Soakaway Design

12 APPENDIX A DRAWINGS Figure 1a Figure 1b Figure 2 EXISTING ARRANGEMENT OF SURFACE WATER DRAINAGE PROPOSED ARRANGEMENT OF SURFACE WATER DRAINAGE DRY STORE BUILDING DRAINAGE LAYOUT Figure 3 WEST CAR PARK SERVICES LAYOUT SHEET 1 of 2 Figure 4 WEST CAR PARK SERVICES LAYOUT SHEET 2 of 2

13

14

15

16

17

18 APPENDIX B INFILTRATION TEST RESULTS

19 Cont Ref:B0128/INFILT/T1and 2 Contractor Rev: Not Protectively Marked Infiltration test results - Test 1 and 2 Date of test Test No Water placed in hole (Litres) length (L) width (w) Effective Depth Ed (from 1m below ogl to invert) Effective depth form OGL (75% full) Ed 75 effective depth from OGL (25% T p75-25 (seconds) V p75-25 A p50 full) Ed 25 Soil infiltration Rate (m/sec) Infiltration Test 1 28/02/ Ex F 1 = 2.00E-04 28/02/ Ex F 2 = 1.57E-04 28/02/ Ex F 3 = 2.01E-04 01/03/ Ex F 4 = 1.58E-04 Infiltration Test 2 07/03/ Ex F 1 = 1.87E-04 07/03/ Ex F 2 = 1.74E-04 07/03/ Ex F 3 = 1.79E-04 Notes: The following formulas have been used to caluclate the infiltration rate of the ground for the two tests: V p75-25 =L*w*(Ed 75 -Ed 25 ) Where V p75-25 is the volume of te soakaway considered for the test A p50 =(L*(Ed/2)x2)+(W*(Ed/2)x2)+(L*w) Where A p50 is the surface area of the excavation considered for the test T p75-25 =T 2 -T 1 Where Tp75-25 is the time taken for the water to soak away between 75% and 25% of the excavation f=v p75-25 /(A p50 *T p75-25 ) Where f is the calculated infiltration rate