Proposed Construction of Basement Flood Risk Assessment. 35 Edwardes Square London W8 6HH

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1 Proposed Construction of Basement Flood Risk Assessment 35 Edwardes Square London W8 6HH 1

2 Introduction This FRA has been carried out in accordance with the National Planning Policy Statement (NPPF), the Royal Boroughs of Kensington & Chelsea, along with advice and guidance from the Environment Agency (EA) and CIRIA. The works within this Statement refer to the following operations Construction of a basement below rear garden. Proposed Development The proposals are as described above to construct a single storey basement under the rear garden of 35 Edwardes Square, London, W8 6HH. 2

3 Site Description The site occupies an area of approximately HA and is located on the western side of Edwardes Square. The site is within an urban area of London and is bounded by existing properties. The property is a four storey house with lower ground floor level and front / rear gardens. The site is located within flood zone 1. Proposed Lower Ground Floor Proposed Basement Layout 3

4 Geology of the Area The geology of the area consists of sand which overlies a London Clay Formation. Above Bedrock of the area Below Superficial deposits of the area 4

5 Flood Risk Assessment from Watercourses / Sea There is not a risk of flooding from watercourses and the sea as identified on the Environment Agency (EA) indicative flood outline map. The map shows that the site is within flood zone 1. Strategic Flood Risk Assessment (SFRA) Royal Borough of Kensington and Chelsea s SFRA, prepared by JBA in August 2009, concludes that the significant sources of flood risk within the Royal Borough of Kensington and Chelsea and London Borough of Hammersmith and Fulham are surface water and sewer flooding, and the residual risk which arises from a possible failure in the Thames tidal defences. While failure of the defences will have no impact to the proposed development as it is located outside the flood zones, sewer and surface water flooding must be considered in the design. Flood Risk from Groundwater The SFRA states that no records of historical groundwater flooding in this area have been found. The bedrock geology of the area consists of London Clay overlying a chalk aquifer; this impermeable cap to the aquifer should prevent incidents of deep groundwater flooding. The London Clay itself is overlain with superficial gravels in the lower part of the borough. These permeable gravels outline the historic floodplain of the River Thames and may contain a perched water table. If following rainfall, the water table within this gravel layer rises then localised groundwater flooding can result in excavations and basements. The proposed development above ground level is not at flood risk from groundwater considering that it is located well above the water table. The proposed basement will be constructed above the recorded groundwater level. However, engineering techniques such as cavity drainage could be required in order to eliminate the flood risk from groundwater at basement level. 5

6 Flood Risk from Surface Water and Sewers The SFRA provides data from a modelling exercise that was undertaken in the borough in order to identify surface water flow routes and locations where surface water may accumulate and cause flooding. The map below shows the affected areas from surface water flooding in the 100 year storm event as they were modelled for the SFRA. In addition to the modelling results shows the locations of properties which suffered flooding as a result of the 20th July 2007 event. There is a visible correlation between the modelling results (specifically the ponded areas) and the observed incidents. The map shows that the site is located within an area where the surface water flood depth is 0m in the 100 year storm event. The proposed basement is below this level and consequently at flood risk from surface water. The rear lightwell would be at risk if no flood protection measures are provided. 6

7 It is recommended a 200mm step to the external area be provided this will eliminate the flood risk to the basement from overland flows. Thames Water also provided details of sewer flooding on a postal area basis (Map 18). The site is within an area where flood incidents from sewers were reported within the last 10 years. Thames Water have stated that areas which have in the past been affected by such flooding should not be seen as areas to avoid future development and that the reverse is also true, that areas with no known flooding incidents should not always be viewed as the best place to accommodate new development. Surface and foul water from the basement is likely to be pumped into the ground floor drainage on site and drain to the public sewers by gravity. Non return valves will be provided at the pumping stations preventing backflows from the public sewers from flooding the proposed development. 7

8 Impact on Surface Waters The Hydrogeological Assessment that was prepared for the site in July 2014 by CGL confirms the majority of the proposed basement will be below the rear garden, which is currently covered by a paved area and lawn. The basement extension below the rear garden will be beneath a 1m thick attenuation layer immediately in the rear of the property and a 1.5m attenuation layer at the rear of the garden. The attenuation layer is typically formed of a granular drainage layer directly above the roof of the basement overlain by topsoil. This has been designed to allow infiltration and attenuation of surface water prior to discharge to the local drainage system. The proposed drainage measures will continue the current surface water storage and attenuation provided by the property. Impact on Ground Water The Hydrogeological Assessment that was prepared for the site in July 2014 by CGL confirms the ground investigation data of the site and surrounding area indicated that groundwater is likely to be located within the KPG and flowing in a south easterly direction towards the River Thames. The proposed basement formation is likely to be at approximately the same level as the groundwater table at the base of the KPG. However, there is likely to remain some 300mm of KPG beneath the basement formation and therefore, movement of groundwater within the KPG is unlikely to be adversely affected and will continue to flow beneath the proposed basement. 8

9 Scope for Mitigation The following flood risk reduction measures will be applied to the development: As a precaution it has been agreed with the Building Owner they would subscribe to the Environment Agency s Flood Warning service and ensure that this services is made available to all occupants of the proposed building. The agency operates a 24 hour telephone service on that provides frequently updated flood warnings and associated floodplain information. In addition, this information can also be found at agency.gov.uk/floodline along with recommendations on what steps should be taken to prepare for floods, what to do when warnings are issued, and how best to cope with the aftermath of floods. All utility services such as fuse boxes, metres, main cables, gas pipes, phone lines and sockets will be positioned as high as practical. Central heating pipe work shall be easily accessible to allow easy maintenance in event of possible flood. Sewage Backflow Prevention Basements can be damaged by the backflow of sewage. The causes can include severe storms, breakdown of pumping plant or blocked pipelines. Anti flood (or non return) valves have been designed to prevent backflow from surcharging sewers. Wherever there is danger from rising water backing up into a property through the drainage system the valve will close off the backflow automatically to prevent flood damage. An anti flood valve is to be installed as a simple and effective way to eliminate backflow through drainage systems. The flap(s) in the valve opens to allow discharge. In a potential flood situation, the rising water levels seals the flap(s) shut. Damp proofing of Basement Floor The damp proofing of the Basement Level where this is immediately below the surrounding ground level will be by a patent membrane system similar to that provided by DELTA. This allows any surface water to drain down behind the membrane and is then pumped out by sump pumps located within the basement floor. The proposed development will have a non return valve installed at the valves sewer connections to prevent backing up within the pipes should the system become overwhelmed with flood water. All utility services such as fuse boxes, metres, mains cables, gas pipes, phone lines and sockets will be positioned as high as practical. Central heating pipe work shall be easily accessible to allow for easy maintenance in the event of a possible flood. 9

10 Conclusions The site falls within flood zone 1; Areas with low potential risk of flooding from rivers (annual probability lower than 0.1% for fluvial flooding), which are already developed. These areas will generally be suitable for residential developments. Engineering techniques such as cavity drainage will prevent groundwater from flooding the basement. Non return valves will be provided preventing backflows from the public sewers flooding. Flood mitigation measures will be used at the new light well preventing flood water from entering the basement. The proposed basement as confirmed in the Hydrology Report will not affect the local hydrogeology and therefore will not increase the flood risk on site and the surrounding areas. The proposed basement as confirmed in the Hydrology Report will have negligible impact on surface and groundwater flow and local ground stability. Therefore, the proposed development has an acceptable flood risk within the terms and requirements of PPS25. From the evidence seen in the flood maps, it is reasonable to conclude that any development to create a single storey basement would not be damaged by any flooding. 10

11 Product Datasheet June 2012 Sewage backflow prevention There are 2 million properties at risk from flooding in the UK. Many will be directly affected by river or coastal flooding, but more homes are damaged indirectly by the backflow of sewage. The causes can include severe storms, breakdown of pumping plant or blocked pipelines. The Marley anti-flood (or non-return) valves have been designed to prevent backflow from surcharging sewers. Wherever there is danger from rising water backing up into a property through the drainage system the valve will close off the backflow automatically to prevent flood damage. The financial losses caused by sewage flooding events can be in the region of 30,000 to 50,000 per property. In less than 30 minutes a house or basement flat can flood with untreated sewage to a depth greater than 1 metre. An anti-flood valve is particularly suitable for the following places: Low-lying coastal, lake or valley areas Properties connected to a high flow rate sewage system Private discharge systems connected to public sewers supplied with pumping stations Where non-pressure flow control is needed Simple installation, effective operation The anti-flood valves are produced in both PVCu and ABS Plastic and are corrosion resistant. As well as preventing back flow in the system, the anti-flood valve also stops rodent penetration. Installation The Marley anti-flood valve is easy to install within the drainage system. When installing, ensure the valve is set horizontally, as a fall is already built in. Also check that the direction of the flow matches that indicated by the arrow on the cover of the valve. The valve should be installed in a chamber to allow access for maintenance. Emergency closure device The valve can be locked in the closed position for added security (no through drainage allowed) for holiday periods, if required. Operation When backflow occurs in the system the non-return valve closes automatically. For the system to function correctly ensure the handle is on open. Regular flow rate is regained when the backflow ceases. Maintenance The emergency closure device mentioned above should be tested once every six months. In case of a blockage, remove the cover, clean the constituent parts, check the gaskets and replace any damaged parts. It is essential that the valve is accessible at all times. The Marley anti-flood valve should be serviced once every twelve months. marley.co.uk

12 The Marley double-flap valve meets the requirements of Building Regulations, Document H1 Marley anti-flood valves An anti-flood valve is a simple and effective way to eliminate backflow through drainage systems. The flap(s) in the valve opens to allow discharge. In a potential flood situation, the rising water levels seals the flap(s) shut. Normal use Anti-flooding situation The Marley anti-flood valve range: Size Colour Code Width L1 L2 L3 L4 110 Orange USW Single flap Size Colour Code Width L1 L2 L3 L4 110 Black USW Black USW Double flap All of the Marley anti-flood valves conform to BS EN All sizes are in mm USW120 USW130/140 Top Inspection Cover Tested with 5 metres head of water. Slide-Inflap Support All parts are removable for periodic servicing. Closing Lever Locking device for long periods of non-use (holidays) Main Body Rigid PVC injection moulded with structural ribs. Socket inlet with elastomeric ring seal. Spigot outlet. Removable lock Inspection cover Waste flow direction arrow Co-moulded seal Internal Shape Built-in invert. No reduced crossed flow section. Flap Gasket Twin-lip profile to maintain integrity. Plastic Flap Rigid PVC injection moulded. Removable for servicing. Inspection Chamber Full access for pipe cleaning or rodding. Design enables uninterrupted flow. Low offset (7mm) suitable for installing in an existing drainage system Removable double flap, prevents rat ingress Stable base Co-moulded seal Closing handle No screws Lenham, Maidstone, Kent ME17 2DE Tel: marketing@marleypd.com marley.co.uk