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. Residencies and properties which have been flooded need to take their precautions against flood. Surrey Heath will be approaching large landowners to seek some improvements in their attenuation systems to benefit residents. Although under the Land Drainage Act (1991) Surrey Heath Borough Council can enforce maintenance of systems, the act does not offer powers to enforce improvements outside of the maintenance requirement (i.e. increased storage capacity, adjustable outfalls etc).

The Bagshot High Street area is affected by the high volumes of water, in periods of heavy rainfall, from the northwest open spaces.

The current planning process for the new housing development on the Notcutts site contains attenuation of both Highway and property drainage within the development area. Additionally, a residents committee (or management company) is to be set up to maintain the open watercourses throughout the site. This scheme will also facilitate improvement works under the railway bridge (dry arch to Bagshot Green area) and condition check the piped watercourse through to the Windlebrook close to Freemantle Road.

The Environment Agency is advising residents abutting or affected by flows through the Windlebrook. Lightwater

Surrey County Council constructed the M3 on behalf of the Department of Transport, as is normal for such main motorway projects. The road levels and gradients were established to suit the relevant speeds of vehicular traffic and to re-use excavated subsoil (via cut and fill method) or to deposit surplus materials locally.

The Engineers adjusted levels either side of a motorway to take into account the differing strata and watercourses. Excess excavated material was then deposited in the Lightwater Country Park to infill a depression. There has been some debate over the years with Bagshot Rural District Council, and continuing with Surrey Heath Borough Council, from some local residents as to the affect of this construction upon the local drainage. Bagshot Rural DC, and subsequently Surrey Heath BC, have improved water attenuation within the park over the years.

Water generally drains from West to East within the park and the main outfall is via Clearsprings through a piped watercourse. Flows then continue through various piped and open watercourse sections before exiting Lightwater under the A322 (Lightwater by-pass) into the Windlebrook near Blackstroud Lane East.

Water from the M3 motorway enters Hammonds Pond and Windlesham Arboretum retention pond. Flows from Hammonds Pond travel south into Clearsprings piped watercourse and flows from Windlesham Arboretum also travel south into the Windlebrook where they conjoin just east of the A322 by the sewage treatment works. The watercourse hydraulic capacity has been exceeded in heavy rainfall and Surrey Heath is continuing its provision of improvements to attenuate these flows.

Water levels in Hammonds Pond have been revised to increase its capacity and to have its outflow adjusted in order to reduce the initial flows (surge) through the system. Further works are planned for 2008.

To reduce excess water shed from the High Curley area and the land filled area of the Park, a detention cascade has been constructed down to the southern boundary (near the Leisure Centre). Downstream of this, two detention ponds are to be constructed as part of the 2008 programme of works to further attenuate flows. Surrey Heath Borough Council is restricted to working on its own land.

Colony Bog (near Clearsprings and Maple Drive) is privately owned and Natural England will not allow modification or additional drainage in this area. Equally under Land Drainage Act (1991) houses built adjoining this land must accept the natural overland flow of water. Houses in this area which have been affected by flood will need to take their own measures to prevent ingress of water into their homes. A longer term solution is being sought.

These additional improvements by Surrey Heath will reduce risks locally and also the downstream effect through to Chobham.

Surrey Heath and Surrey County are also reviewing some kerb levels in the Clearsprings area in order to retain more water on the highway. The Surrey Heath Drainage Engineer has been advising some residents in Clearsprings, Ambleside Road and Riverside Avenue area to reduce risk of flooded property.

Flooding on the Guildford Road, Riverside Avenue, Blackstroud Lane and A322 by- pass area are a culmination of surface water from the west and an overload of the existing pipework.

The public foul sewers (Thames Water responsibility) also seem to flood in this area. The cause, whether through back up from treatment works, hydraulic under capacity or overload due to ingress of surface water, is yet to be determined.

Thames water is committed to carrying out improvements in this area. If all residents ensure they do not allow surface water into the foul system on their property it will reduce the flood risk.

Surrey Heath has improved the detention bund and ditches on Turf Hill Park (West End) with a possibility of carrying out further attenuation works in this area to reduce risk of excess water overflowing into the Guildford Road.

General areas of flooding in Lightwater, away from the watercourses, are pluvial and resultant upon a combination of causes as referred to earlier (namely residents draining their property onto a highway, kerb alignments, blocked gullies, filled watercourses, etc).

Surrey Heath Borough Council Programme of Works

Surrey Heath carries out an annual maintenance review of surface water systems on its own land across the Borough and undertakes any works on watercourses for which it is responsible.

Often the maintenance will include clearance of debris and other minor improvements. Generally in the 2006 and 2007 floods all the Surrey Heath systems worked well, although Surrey Heath has used the opportunity to undertake additional improvements highlighted by the volumes of water encountered.

Environment Agency flood warnings are monitored daily by officers and, when heavy rainfall is predicted, drainage contractors are deployed to check the vulnerable watercourse grilles (trash) gratings. This includes removing natural debris (i.e. branches, leaves, etc) and unnatural (fly-tips, rubbish and grass cuttings dumped by residents etc).

It is a criminal offence to move, cover, block or change the nature of any watercourse and Surrey Heath will act on any information provided by the public.

The Surrey Heath Drainage Engineer reports any non-watercourse problem seen (or notified of) to the other agencies as appropriate (including SCC Highways, Environment Agency, Thames Water, private landowners etc). The Surrey Heath program of improvement works, proposed and previously undertaken, is reviewed by the Executive at least annually.

Absent or unknown land owners can be a problem to trace and Surrey Heath Borough Council is not obliged to carry out work on their land. However, as Land Drainage Authority, Surrey Heath may undertake works at its discretion for the betterment of the area.

Surrey Heath Flood Mitigation Strategy

During 07 Surrey Heath outlined its flood mitigation strategy for the Borough based on greater attenuation of surface water flows (see Executive meeting minutes). This has been discussed with all other agencies and authorities. Following the Pitt Review, the Government is now promoting a similar strategy to that already being followed by Surrey Heath. Indeed some proposals have been Surrey Heath policy for many years.

Surrey Heath Head of Built Environment and Executive Portfolio Holder have met with representatives of the four parish councils and outlined the strategy. The four parish Clerks also attend Emergency Planning and Flood Forum information meetings at Surrey Heath House as part of local communication strategy.

General Conclusion

No one can accurately forecast the intensity of rainfall, 50/100mm steadily over 24 hours is generally accommodated but the same level in 2-3 hours can cause problems. Just 3-4 hours of flood inconvenience to Windlesham Parish, West End and Bisley can be 10-15 hours of related problem flooding in Chobham.

It needs residents, landowners, authorities and agencies all to act responsibility to reduce the risks of flooding in Surrey Heath which has knock-on effects of high insurance premiums/excess, ruined personal property, personal distress and lower house values.