WEST LONDON PIPELINE AND STORAGE LIMITED AND UNITED KINGDOM OIL PIPELINES LIMITED WLPS BUNCEFIELD REBUILD Flood Risk Assessment Green Lane Hemel Hempstead Hertfordshire HP2 7HZ September 2009 Prepared for: British Pipeline Agency Limited as agent for West London Pipeline and Storage Limited and United Kingdom Oil Pipelines Limited 5-7 Alexandra Road Hemel Hempstead Hertfordshire HP2 5BS Prepared by: Arcadis UK Ltd. 1 Manor Court Leeds LS11 8LQ Tel: 0113 239 0494 Fax: 0113 239 0495 www.arcadisgmi.com
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 Client: Site Address: British Pipeline Agency Limited as agent for West London Pipeline and Storage Limited and United Kingdom Oil Pipelines Limited Green Lane, Hemel Hempstead, Hertfordshire, HP2 5BS Report Title: Flood Risk Assessment WLPS Buncefield Rebuild Report Date: September 2009 Document Number: 92802_01 Document Issue Number 01 Date Issue Status Author September 2009 Senior Review First Issue Paul Keighley Kelvin Hughes Authorised Maurice Brown Signature If you have any queries regarding this project, please contact Paul Keighley or Kelvin Hughes.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 TABLE OF CONTENTS 1. INTRODUCTION...1 1.1 Objectives...1 1.2 Scope of Work...1 2. SITE SETTING...2 2.1 Topography and Site Setting (See Appendix A)...2 2.2 Existing Site Drainage...2 2.3 Geology and Hydrogeology...3 2.4 Historical Flooding...3 3. PROPOSED DEVELOPMENT (SEE APPENDIX B)...4 4. PLANNING CONTEXT...5 4.1 PPS25 - Development and Flood Risk...5 4.2 Environment Agency Standing Advice...5 5. SOURCES OF FLOODING...6 5.1 Fluvial Flood Risk...6 5.2 Overland Flow...6 5.3 Groundwater Flooding...6 5.4 Surcharged Sewer Flooding...6 6. DEVELOPMENT AND FLOOD RISK MANAGEMENT...7 6.1 Mitigation of Flood Risk On-Site...7 6.2 Mitigation of Flood Risk Off-Site...7 7. CONCLUSIONS...8 Figures 1 Site Location Plan Appendices A B C Historical Site Layout Proposed Site Layout Environment Agency Flood Zone Mapping
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 1. INTRODUCTION Arcadis UK Limited (ARCADIS) has been commissioned by British Pipeline Agency Limited as agent for West London Pipeline and Storage Limited and United Kingdom Oil Pipelines Limited to undertake a Flood Risk Assessment for a site at Green Lane, Hemel Hempstead ( the Site ). The site is currently developed and is partially operating as an oil product storage facility. The site is to be redeveloped to restore the storage capacity for Jet A1 aviation fuel to the same amount as was stored on-site prior to an incident in 2005 that damaged storage tanks rendering them unusable. 1.1 Objectives The objective of the study is to undertake a Flood Risk Assessment in accordance with Planning Policy Statement 25 (PPS25) Development and Flood Risk. This report also incorporates an outline drainage strategy for the disposal of foul and surface water flows from the development based on currently available information. Mitigation measures have been included to demonstrate that the site is not at risk of flooding and the proposed development will not increase flood risk to properties elsewhere. 1.2 Scope of Work The appraisal process comprises a desk study, data research and site visit to ascertain the extent of information available and identify any constraints with regard to drainage and flood risk that may affect the site. The assessment is a holistic risk based assessment of potential flooding from various sources including fluvial flooding, groundwater and surface water run-off. The assessment also examines residual flood risk and proposes mitigation to offer protection against flooding for the lifetime of the development.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 2. SITE SETTING 2.1 Topography and Site Setting (See Appendix A) The site area is approximately 5 Hectares located within the Buncefield Oil Storage Terminal off Green Lane, Hemel Hempstead around National Grid Coordinates 508690, 208650 and is approximately 2 km to the north east of Hemel Hempstead town centre. The areas immediately adjacent to the site are a mix of retail/commercial and industrial units and agricultural land. The surrounding land uses are summarised in Table 1.1. Table 1.1 Summary of Surrounding Land Uses North East South West Punchbowl Lane and agricultural land is located immediately to the north of the application site with some scattered residences. The closest residence is located approximately 50m from the application site boundary. The application site is bordered by undeveloped grassland and an area owned by Hertfordshire Oil Storage Limited. Beyond this is Green Lane with agricultural land and the M1 motorway located 0.75km further to the east of the application site. To the south of the application site is a road providing access to the surrounding fuel storage and distribution facilities. Beyond this is the BP facility and further facilities for industrial/commercial use. The land to the west of the application site is owned by Hertfordshire Oil Storage Limited. Beyond this is a mixed use industrial estate approximately 500m from the application site. The nearest watercourse is located in Pratt s Dell approximately 1km to the northwest of the site which drains to a surface water-balancing pond adjacent to Redbourn Road. The outlet from the pond discharges to the River Red which flow northwest and ultimately discharges to the River Ver at Redbourne approximately 4 km northeast of the site. The overall topography across the site falls from southwest to northeast. Land to the north of Cherry Tree Lane rises forming a valley. Cherry Tree Lane which bisects the site has an overall fall to the east towards the M1 Motorway although there is a localised low spot to the in the vicinity of the site. 2.2 Existing Site Drainage The site is currently developed as an oil storage depot and is almost entirely covered in hardstandings which are drained by a traditional system of pipes and gullies/drainage channels that discharge to an effluent treatment works (ETW) in the northeast corner of the main site. The outlet from the ETW is pumped to Pratt s Dell watercourse at a restricted rate of 7.5 l/s with surplus flows attenuated in a storage lagoon. Bunded areas surrounding storage tanks are isolated from the drainage system and periodically emptied, draining to the ETW during times of low flow to prevent accumulation of rainwater within the bunds. The main foul drain is located beneath the southern access road (Oil Road), collecting flows from the various terminals and discharging to a foul water pumping chamber located at the main entrance to the site adjacent to Green Lane. Submersible pumps lift the water into a pumping main which discharges to an adopted sewer in Boundary Way further south of the site.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 2.3 Geology and Hydrogeology The BGS geology maps and borehole logs indicate the solid geology beneath the site to comprise Cretateous Upper Chalk. Deposits of the Tertiary Woolwich and Reading Beds are indicated near to the surface close to the western boundary. Drift deposits are indicated as Pleistocene (clay with Flints) of varying thickness. The underlying Upper Chalk is described by the EA as highly permeable strata that generally has a known or probable presence of fracturing and may be highly productive. The site is shown on the Environment Agency groundwater vulnerability maps as being underlain by a Major Aquifer which can reasonably be assumed to be defined as a Principal Aquifer under the new classification scheme which is currently being introduced. The site is close to the boundary of a Total Catchment Source Protection Zone (SPZ111) defined by the EA as the total area needed to support removal of water from a borehole and to support any discharge from the borehole. Groundwater flow in the vicinity of the site is understood to be generally to the east. 2.4 Historical Flooding There is a local hollow on Cheery Tree Lane where it bisects the site. This area is known to have flooded with firewater and foam during the 2005 incident.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 3. PROPOSED DEVELOPMENT (SEE APPENDIX B) The application site comprises two sections which are located either side of Cherry Tree Lane. Cherry Tree Lane is the boundary between Dacorum Borough Council and St Albans City & District Council. The proposed redevelopment will consist of hardstandings and above ground oil storage tanks that will be located within bunded areas together with tertiary containment provision in the form of additional bund walls and a lagoon. The existing Effluent Treatment Works will be retained. A copy of the proposed redevelopment plan is included in Appendix B. The site is the former Buncefield Oil Storage Depot which has continued to operate on a reduced scale following an incident in 2005 that resulted in damage to much of the oil storage facility. The site is crossed by several supply pipelines that facilitate delivery of fuel to and from the site. The previous facility consisted almost entirely of hardstanding. As such the site should be considered as brown field development. The proposed redevelopment will result in an increase in impermeable area.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 4. PLANNING CONTEXT 4.1 PPS25 - Development and Flood Risk The site is located within Flood Zone 1 (low risk) as defined on the Environment Agency Flood Zone Maps. This is land designated as having an annual probability of fluvial flooding from rivers of less than 0.1% (less than a 1 in 1000 annual probability of flooding) in any given year (See Appendix C). In Flood Zone 1, developers and regulatory authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through layout and form while applying sustainable drainage techniques where appropriate and practical. The sequential test within PPS25 aims to steer all new development to areas at the lowest risk of flooding. The site is located in FZ1 and therefore meets the requirements defined in Table 2 of PPS25 and the type of proposed development is appropriate for this location. 4.2 Environment Agency Standing Advice For a site of greater than 1 hectare located in Flood Zone 1 the Environment Agency standing advice is that they should be consulted as part of the planning process. A flood risk assessment, focussing mainly on the management of surface water run-off to demonstrate that the development will not increase flood risk to itself or elsewhere within the catchment should be provided in support of a planning application.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 5. SOURCES OF FLOODING 5.1 Fluvial Flood Risk The site is shown located in Flood Zone 1 (low risk) on the Environment Agency mapping which is land that has a probability of flooding less than 0.1% (1 in 1000 years). There are no watercourses in the immediate vicinity of the site, therefore the risk of fluvial flooding is considered negligible. 5.2 Overland Flow Flooding from overland flow is considered unlikely. The general topography falls from southwest to northeast. The areas to the south and west of the site are developed and drained by a traditional piped system that should deal with surface run-off in all but the most extreme rainfall events. Should overland flow occur this would be expected to be short term and shallow in depth. 5.3 Groundwater Flooding In view of the localised topography and the fall across the site it is unlikely that groundwater will come out of the ground within the operational areas of the site during a rise in water table level. 5.4 Surcharged Sewer Flooding The site is currently served by a private drainage system that discharges to an effluent treatment works in the northeast corner of the site. In the event of surcharge flow due to lack of hydraulic capacity during extreme rainfall events there may be occasional short term surface flooding which would be contained on-site. The rebuild will increase impermeable areas that will discharge to the on-site drainage system.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 6. DEVELOPMENT AND FLOOD RISK MANAGEMENT 6.1 Mitigation of Flood Risk On-Site Surface water drainage systems have the potential to flood under extreme conditions. Current drainage design standards generally require no surface flooding from the piped system for a 1 in 30 year storm. Development of the site will not affect existing overland flow routes. The proposed secondary/tertiary containment bunds and lagoon will be isolated with no direct discharge to the overall site drainage system. This will result in shallow short term surface ponding of rainwater within the containment bunds and storage lagoon during rainfall. The bunds will be emptied by pumping liquid to the Effluent Treatment Works at times of low flow to prevent overloading of the system. Other hardstandings will continue to drain to the existing below ground surface water drainage system which ultimately discharges to the Effluent Treatment Works. 6.2 Mitigation of Flood Risk Off-Site Requirement H3 of the Building Regulations 2000 states that the preferred hierarchy for disposal of surface water is infiltration then watercourse then sewer. In view of the historical/proposed use of the site and the strategic nature of the underlying aquifer it is considered inadvisable to discharge surface water run-off from the proposed development into the ground using infiltration techniques. The nearest suitable water feature with potential for discharge of surface run-off from the proposed development is the watercourse in Pratt s Dell to the northwest of the site. The site currently drains to Pratt s Dell via the Effluent Treatment Works which will continue to discharge to watercourse at the consented rate of 7.5 l/s. The proposed redevelopment will generate only nominal foul flows which will continue to discharge to foul pumping station at the site entrance for discharge to the existing public sewers to the south of the site.
Flood Risk Assessment WLPS Buncefield Rebuild 92802_01 / September 2009 7. CONCLUSIONS The site is shown as lying within Flood Zone 1 (low risk) on the Environment Agency mapping. The nearest watercourse is 1 km to the northwest of the site therefore the risk of Fluvial Flooding is considered to be negligible. It is recommended that finished floor and external pavement levels are set to direct water away from buildings. This would protect against water entering the building due to drainage infrastructure failure, overland flows or groundwater. Surface Water run-off will not impact on the Effluent Treatment Works serving the whole site which will continue to discharge at the restricted rate of 7.5 l/s to the watercourse in Pratt s Dell to the northwest of the site. Foul drainage from the development will discharge to the private sewers within Oil Road and will be pumped to the public sewers in Boundary Way to the south of the site. The proposed development is not at risk of flooding and will not result in an increased flood risk elsewhere in the catchment. This flood risk assessment and outline drainage strategy is based on a review of currently available information and is considered to be appropriate to the scale and nature of the development.
Figure 1 Site Location Plan
Appendix A Existing Site Layout
Appendix B Proposed Site Layout
Appendix C Environment Agency Flood Zone Mapping