WINDLESHAM PARISH FLOODING INFORMATION Villages of Bagshot, Lightwater & Windlesham This is a general report on the flooding that occurred on 13th August 2006 and 20th July 2007 after heavy rainfall in Surrey Heath. Further information is available on the Surrey Heath Web Site (www.surreyheath.gov.uk-flooding) in respect of the neighbouring Parish’s and general information with links to other agencies. The Rural area – Natural Drainage In the Surrey Heath area flooding is either pluvial (direct result of local heavy rainfall) or fluvial (overflowing of river banks). The village of Chobham suffers initially from pluvial flooding which may cause the rivers to run high and then several hours later from fluvial water as a result of water draining from the surrounding higher ground to the north, west and south. The Chobham ridges (approximately on the line of The Maultway B3015) is the dividing line between the urban area of Surrey Heath (Camberley Town, Frimley etc) and the rural village areas of Bagshot, Lightwater and Windlesham (jointly represented by Windlesham Parish Council), Bisley, Chobham, West End (under sole represented Parish Council areas). The Maultway carriageway runs along the top of the ridges to the western side of West End, Lightwater and Bagshot forming the Parish boundaries. The ridges are approximately 100m above Chobham to the east and 60m above the Blackwater River to the west. To the east of the Chobham ridges, the Blackwater River (Loddon catchment) drains most of the urban area of Surrey Heath via small water courses and piped systems generally under the highways. There are also various retention ponds with controlled outlets to help reduce flood risk. To the west of the Chobham ridges various springs, ditches and streams feed into the two main rivers in Chobham village centre (Mill Bourne and Bourne – Bourne catchments) and they confluence to the east of Emmetts Mill (Philpot Lane, Chobham) on the Surrey Heath boundary near to Fairoaks Airport. These two rivers drain the rural area through Chobham into Runnymede and on to the Thames. The majority of surface water in West End and Bisley drains into the Addlestone Bourne at the Bisley and West End Parish boundary on the A322 at Newbridge. Flows continue towards Chobham under Grants Bridge (Castle Grove Road) past the Sewage Treatment works to the south of Mimbridge (Station Road A3046) towards its confluence with the Hale Bourne near to Fairoaks Airport. The Windlebrook/Hale Bourne river catchment is from the Lightwater part of the ridges (High Curley, Lightwater Bog and Bagshot Heath). High level lakes and springs to the north of Bagshot, on the Surrey Heath/Bracknell borders, feed water through Bagshot Parks and Pennyhill Park into the Windlebrook towards Bagshot village centre. The A30 London Road, Jenkins Hill collects through Notcutts Nurseries (southwest of Bagshot Village) feeding under the railway embankment and on to the Windlebrook. The Windlebrook passes south under the M3 motorway through a series of retention ponds to the Lightwater sewage treatment works where it merges with flows from Lightwater (via connection under A322) to become the Halebourne (near Blackstroud Lane East). The Halebourne flows Eastwards on the north side of the A319 Bagshot Road in West End, past the Chobham Fire Station area into Chobham village where it becomes the Mill Bourne. The river then continues eastwards to its confluence with the Addlestone Bourne near to Fairoaks Airport. The rain falling on the M3 in this part of the Borough goes via two retention ponds within Lightwater Country Park (Hammonds pond) and Windlesham Arboretum (Highway Agency constructed retention pond) then onwards into the Halebourne. Windlesham Village Surface water in Windlesham village area generally flows south along the Chertsey Road (B836) into Thorndown Lane and Broadway Road (under M3) before joining with the Halebourne. Surface Water System last 50 years Over the last 50 years there has been continuous redevelopment of properties and construction of new estates which has significantly reduced areas of green space. This, coupled with the developments installation of new roads and services, decreases the available land for infiltration and increases the speed of surface water run off. Surrey Heath (and also its predecessors) policy has been to attenuate the water flows during heavy rainfall. Generally a dwelling drains its roof and hard surface areas into soakaways or ditches. The ditches (often originally constructed to be beneficial for farming or other land use) are not necessarily part of a watercourse network but can be open soakaways with no connection to a river and only providing water storage and infiltration. Many original rural roads drain surface water into roadside ditches adjacent to the adopted highways, the ownership of which can often be disputed. New developments typically have piped systems which can discharge into large open detention basins (balancing ponds) which slowly drain excess water away; or retention ponds which retain a base level of water throughout the year. These basins/ponds have controlled outflows to balance the amount of water flowing into the pipework and rivers although their design can be overcome in extreme rainfall events. The rainfall events in 2007 and 2006 have been described as a 1 in 50 years and 1 in 100 years respectively, albeit only 11 months apart. The surface water management systems were never designed to cope with these exceptional rainfalls. Surrey Heath Borough Council Drainage Engineers have always reviewed proposed development surface water management systems for adequacy, ease of maintenance, amenity value etc as part of planning permission process. Highways The Highways are either drained by gullies/pipework or via grips into roadside ditches. Gullies have silt traps to catch loose materials carried by the flow of water which, if not regularly cleaned, can fill up and block the connecting branch pipework. In heavy rainfall (albeit some cleared gullies/pipework could still be overloaded) excess water is kept above ground making the highways dangerous and possibly flooding into properties. In 2006 and 2007 some gullies in Surrey Heath were blocked and the older areas of the rural area suffered most from surface water run off when large amounts of debris were carried by high water flows. Surrey County Council Highways has introduced new cleaning schedules and these will be monitored for areas most at risk. Over the years surface dressing of the carriageway has raised some levels allowing water to enter properties via driveway crossovers. Residents need to review their own driveway and crossover to reduce risks of surface water shed from the highway and consult Surrey County with any concerns as appropriate. Private Residencies The Windlesham Parish comprises of four main settlement areas, surrounded by undulating open spaces which feed surface water southwards to West End via the Windlebrook and Halebourne rivers. Where houses have blocked or inadequate guttering or soakaways, excess water can travel overland through neighbours properties or onto the highway contributing to flooding of properties locally or in the other villages especially Chobham. Any extensions, conservatories, patios or hard paved areas such as driveways should all have their own water storage and filtration systems to reduce pressure on the surface water network. Water butts can be used to store water which can be used as a supply to water gardens avoiding use of valuable drinking water. These work in high water table areas. Gardens can be designed with low areas and bunds (raised flower beds etc) to trap water away from dwellings and boundaries. Generally water should not be drained either deliberately or through lack of maintenance into the foul water system as this causes overload of, what should be, a sealed system. Often, several neighbouring houses are connected together on foul drainage systems and any draining of surface water (from conservatories, patios etc) into the foul drainage system can easily flood gardens and houses (including internally) with sewage. Overloaded foul drainage systems can also cause problems at the sewage treatment works (located near Lightwater bypass and in Chobham) as the increase in waste water inundates the treatment plants capability. Private Watercourses (ditches, streams, rivers etc) The majority of watercourses in the area are old field boundary markers where the centre line is the official boundary (not always depicted by the property fence line). Responsibility for the maintenance of these watercourses often lies with the adjacent land owner (unless otherwise stated in written documents) under the Land Drainage Act (1991). Surrey Heath Borough Council is the Land Drainage Authority and has permissive power of enforcement to all minor watercourses, as does the Environment Agency who undertake a similar role for main rivers and watercourses. Any alteration works (including culverting and diversion) to any watercourse requires the written permission from the Land Drainage Authority (Surrey Heath) prior to undertaking any works. Piping of ditches can seriously affect the capacity and flow characteristics of a system, can drastically reduce infiltration in the area and can cause flooding for which the landowner will always be liable. General Duty on Residents There is a general duty (and sometimes a legal obligation) for residents to properly manage surface water systems on their property. Many old style rubble filled soakaways built within the last 40 years are likely to need replacement as debris fills the voids to make them ineffective. New soakaways should be constructed as porous open void chambers with access covers to remove debris and maintain a full working capacity. Water harvesting systems (using rainwater for flushing toilets etc) is now encouraged to save drinking water supplies and as a general move towards more sustainable living. Water harvesting systems can also be placed underground within a sealed chamber and pumped to its point of use. Bagshot Area As stated earlier, Bagshot takes large volumes of water from the open spaces to the north.