Login

Planning Applications

Flooding Report

Flooding Report & Comment on the Applicants Flood Risk Analysis

Planning Application No. 12/P/01973

Land South of Ash Lodge Drive, Ash

Author: Mr. Anthony Pugh
Consultant Engineer


Topography of area

The area proposed for the development consists of a number of fields which form the bottom of a valley and has a low point of 72.5m above sea level. It is bordered to the north by residential properties, which are built on land rising to an elevation of 85m over a distance of 400m, and rising further to 118m just north of Ash Hill Road. To the south, the land is bordered directly by a disused railway line banking which is over 2m higher than the proposed development area. The land on the other side of the railway banking then rises over a distance of 1500m to the Hogs Back A31 road, which reaches to over 130m. To the east, the land rises gently to 85m over a distance of 1000m, and to the west the land is bordered by Manor Road and Ash Manor School which is at an elevation of 77-79m. The proposed development is therefore surrounded by higher land apart from a small area to the north-west corner which opens to the lowest point in Manor Road.


Soil type

The soil is predominantly London clay to a depth in excess of 2.5 metres and therefore very poor for drainage. Water collects and remains on the surface for weeks due to the impermeable nature of the ground.


Rainfall Rates

The standard rainfall rates, for this area, for calculating drainage requirements are as table 1

Table 1

Storm EventRain mm/hr 100 year return periodRain mm/hr 100 year return period + 30%
15 min winter 97 126
30 min winter 64 83
60 min winter 40 52
120 min winter 25 32
180 min winter 18.5 24
240 min winter 14.6 19

Drainage

The railway embankment at the southern border is penetrated by an 18" transit pipe, which allows the surface water from the higher land to the south to pass under the embankment into the development areas main drainage watercourse, and then into culvert A14/1 which connects into the main storm drain. This then conveys the water into the River Blackwater just north of the A331/A323 junction. The size of this watercourse is approximately 2m wide x 1m deep.

Another much smaller watercourse borders the west of the land and travels south to north, eventually connecting into the drainage system near Manor Road. Another watercourse runs east to west joining the main ditch into the main storm water drain adjacent to Avon Close. These watercourses have bunds of between 200 - 400mm high formed by the soil excavated during the digging of the ditch and so the fields do not drain naturally into the ditches.


Occurrence of Flooding

The area is noted for flooding during periods of high rainfall and there are Guildford Borough Council records showing more extensive flooding on a number of occasions, most recently in 2000, 2006, 2007 & 2008. The Capita Symonds report, commissioned by Guildford Borough Council and dated January 2009, shows total flooding of the area right up to the southern border of the railway embankment, in 2000 and 2006. Following this report serious flooding occurred again in July 2007 and August 2008 when properties in Southlands Close, Southlands Road, Lea Close, Wandle Close, Loddon Way, and Colne Way, and Ash Lodge Drive were flooded.

The fields, as they stand, prevented far more extensive flooding to the local properties as they acted as a floodplain, allowing thousands of cubic metres of water to flood the fields before the water rises to the level of the properties mentioned. The writer's property escaped flooding by 25 mm (just below the doorstep) although the garage was flooded to a depth of 125mm.

The field area floods often, particularly during the autumn/winter period, and large lakes form in the fields which are then populated by geese and Mallard ducks.

Due to the low drainage capability of the soil, the area was classified in the Surrey County Council flooding assessment of 2011 as an area subject to surface water flooding.


Reasons for flooding

Assumptions

To illustrate the effects of surface water flooding, a storm rainfall rate for 1 hour of 24 mm has been shown. Records for 20th July 2007 show that 110 mm fell in 24 hours with a peak of 24mm in 1 hour.

Flooding from the north

During heavy rain storms, rivers of water from the Ash Street and Ash Church Road area flow rapidly downhill through residents’ gardens, down roads and alleyways towards the fields. Water bursts up through drain covers, out of road drains, and raw sewage is also thrust up into the torrent of water. The storm drain is rapidly overwhelmed, and so the water flows to the lowest point which is the fields south of Ash Lodge Drive and Loddon Way. Once the fields are flooded the water then continues to rise and starts flooding the homes. An estimation of the quantity of water arriving from this direction could be as much as 8,400 m3 per hour for a rainfall of only 24mm/hr. The 100year+30% rate of 52mm/hr would see around 18,200 m3 per hour crossing Ash Lodge Drive and Loddon Way into the fields.

View from Ash Street looking down the slope of Ash Lodge Drive towards the development site (behind the houses facing). Storm water rushes down this and other adjacent roads in torrents to flood the site.

Particularly bad is Ash Church Road which rises to a high point near St Peter’s Church and then descends to Ash railway station.

For example on Monday 11th October 2012 at 09.00 the writer was travelling to Woking for a meeting with Woking Borough Council. It was raining fairly heavily but not storm conditions. Considerable flooding was noted in Ash Street outside the police station, and as the writer travelled up Ash Church Road there were torrents of water gushing out of the drains. Rivers of water were running down each side of the road towards the junction with Grange Road and round into Ash Street by the police station. Passing over the brow of the hill by St Peters Church there was a similar situation with the water gushing out of the drains forming rivers running down both sides of the road towards the Ash railway station crossing.

The police station yard has indeed been flooded on a number of occasions and in one particularly bad event in 2006, the flow south from Ash Street, through the yard, was so great that after bursting through the rear fence of the police station yard, the floodwater then flowed rapidly across the properties in Southlands Close and eventually adding to the flooding in Southlands Road. Such was the ferocity of the flow that some properties in Southlands Close suffered under-floor damage to their suspended floors, which included electrical cabling being torn out. One owner still has the invoices for the repairs necessary totalling well over £1,000.

Southlands Road – South Ash (July 2007) – houses on the right all flooded internally

Southlands Road near Lea Close– South Ash (July 2007)

Map showing water courses and main directions of flooding into development site.

Map showing main areas of flooding following a storm event

Flooding from the South

The high ground to the south of the proposed development area has the potential to shed huge quantities of water during heavy rain storms. The water is bunded by the railway embankment, causing the southern side of the embankment to flood and the water rises rapidly in the area near the 18" transit pipe to depths often up to 1.5 metres.

Water from Hogs Back starting to flood fields bunded by disused railway embankment. – (1.2m deep near the embankment) - This picture April 2012 but it is a regular occurrence. This is the area to be used as the Bewley Homes SANG

The transit pipe acts as a restriction orifice and allows water through at a rate of approximately 3600m3/hr, depending on the head of water at the pipe entry. The writer has noted on many occasions the northern exit of this pipe completely flooded to a height of 300-400 mm above the top of the pipe. On some occasions it rises so high that it reaches the bottom step of the adjacent stile.

Northern exit of transit pipe (top of pipe submerged) – picture taken April 2012

The water flows rapidly along the large watercourse towards culvert A14/1. Approximately half way along it reaches a soil bridge area between 2 fields which is penetrated by 2 x 12 " pipes. These cannot cope with the flow and the water floods out of the ditch and into the fields. The water also floods over the top of the bridge area into the downstream side of the watercourse. This water flows at an alarming rate (in excess of the EA recommended maximum) and the bridge area cannot be safely crossed to get from one field to the other.

Water has overwhelmed twin 12” transit pipes and is now rapidly flowing across soil bridge. – picture taken April 2012 (similar pictures are available for January 2012 and December 2012)

The volume of water then travels rapidly along the open watercourse, and then enters culvert A14/1 and then into the storm drain which copes in normal rainfall conditions. However, as previously noted, in heavy storm conditions the Ash Lodge Drive storm drain is unable to cope with the water from the residential area north of the fields, and so flow into the drain is blocked (and may even reverse). The water in culvert A14/1 cannot then enter the storm drain and the level rapidly rises adding to the flooding in the fields, and also to the rear gardens, garages and drives of the properties on the south side of Ash Lodge Drive. It should be noted that this flow through the transit pipe continues for many days after the rain has stopped, due to the large volume of water held back by the bund effect of the railway embankment. Releasing this water at a greater rate would inevitably cause flooding downstream both locally and in the Blackwater floodplain between Tongham and Frimley Green.

Water flowing towards A14/1 – This water course is fast flowing, deep with steep sides and would be a source of considerable danger of drowning to children and adults.

Flooding of garden from excess flow into culvert A14/1

Flooding from the West

The greater part of Manor Road and Ash Manor School is also between 2 and 5m higher than the development fields and surface water also floods down into the area during storm conditions. The watercourse bordering the western side of the field is narrow and shallow and rapidly fills, allowing water to then flood into the field.


Greenfield Runoff Rates

The developer has recognised that this area is subject to extensive flooding, as he has proposed raising the development site by at least 300mm in an attempt to provide some protection to the new homes.

There are however serious problems with the Flood Risk Assessment and also several of the documents cross referring to this.

Firstly the developer has used the desk based calculation on the highly inaccurate IH124 procedure to obtain a Greenfield runoff figure. The IH 124 Report is dedicated to small catchment hydrology but nevertheless its formula for QBAR, which is in widespread use for SUDS applications, is based on data of 71 catchments < 25 km2, only one of which is

The fields are low lying and have lips around the perimeter of each field between 200 – 400mm high mainly caused by the soil dug to form the watercourses being heaped along the side. Also the root structure of the trees and hedgerows has caused considerable raising of the perimeters, so each field resembles a tray. There is therefore virtually no surface water runoff from any of the fields. Bewley Homes own soil infiltration tests have also shown there to be no measurable infiltration so the only way flood water levels reduce is by evaporation. This is born out by the fact that after a period of rain the fields remain in a state of flood for months.

View along Footpath 350 (watercourse behind hedgerow) note raised bank and no runoff.

View alongside field adjoining Ash Lodge Drive note raised bank and no runoff. FP350 underwater in foreground.

View alongside field adjoining Ash Manor School (watercourse to right) note raised bank and no runoff

The existing greenfield runoff figures (desk based calculation) of between 44 and 48% claimed in section 5.11 table 2 of the developer’s FRA are simply absurd. The true value is probably around 1%. Having studied the runoff in all accessible fields during a period of rain, one can find only 2 places where any actual runoff occurs. One is at the junction of FP349 and FP351, where a deep pond area discharges a trickle through a very small gulley into a ditch. This has been scraped in the soil in a futile attempt to keep FP349 passable , which is still 200mm deep in water The other is halfway along FP351, where another very small gulley 100mm wide drains a small puddle area into another ditch at a very low trickle rate.

The QBAR figures therefore claimed in the developer’s FRA are totally invalid.

Junction of FP349 and 351. FP349 is under water, note yellow GBC notice of planning application in background – picture taken 20/12/2012

This gross overstating of the existing greenfield runoff renders their whole flood risk assessment worthless.

It would require a depth of flooding of at least 200mm in the fields before any surface water runoff into the watercourses occurred. As there is virtually no greenfield runoff on the undeveloped site any runoff from the new development would be a net gain into the surface water drains causing an increase in the flood risk to the River Blackwater and the existing properties in Ash Lodge Drive, Loddon Way, Southlands Road and Colne Way.

View back across field from FP349. – FP351 is on the raised portion to the left – picture taken 20/12/2012

Aerial view from Bing maps of typical flooding after some rain. Ash Lodge Drive is at bottom of picture.


River Blackwater Flooding

Telemetry values (from over 9000 points) supplied by the EA for Tongham and Farnborough monitoring stations show that the River Blackwater floods with flows exceeding 1.25m3/sec. This affects properties along the floodplain from Tongham to Frimley both in Hampshire and in Surrey. The FRA in 4.12 agrees that the flooding occurs on the River Blackwater on a frequent basis.

From records provided by the Environment Agency the most recent flood alerts that were issued for the River Blackwater were at 22.48 on 22/11/2012 and again at 23.06 on 14/12/2012. The closest to a full flood in 2012 occurred at 13.30 on 28/04/2012 when the Tongham telemetry was showing only millimetres below the critical level for flooding.

Tongham metering station chart for the River Blackwater on 28/04/2012


Transit of water from fields to the River Blackwater

The storm drain into which the culvert A14/1 connects, travels a distance of 650m west and discharges into the river Blackwater at a point some 100 yards north of the A331/A323 road junction, by the Holder trading estate. The pipe drops in height 4.5m over that distance and flow is by gravity. It is impossible to carry out a full hydraulic analysis without full details of all the entries and pipe diameters, but it is evident from past occurrences that this drain is inadequate, as evidenced by the water flowing out of the drains into the roads and residents properties during a heavy storm. Note also that the throughput capability of the pipe will diminish as the water level in the river rises due to the increase in discharge head.


Flooding of other areas from the River Blackwater

The river is narrow and shallow and floods easily, particularly in the area adjoining Lakeside Road and the housing adjacent to the river between Tongham and Frimley Green. The large volume of water currently discharged into the river has on a number of occasions caused serious flooding of properties in and around Lakeside Road and further downstream. Reports by Guildford Borough Council indicate at least 20 properties on the last occasion (ref Guildford SFRA Capita Symonds CS20946 Appendix G-3).

In early 2009 Michael Gove wrote to Thames Water expressing his concern that the improvements carried out to the downstream River Blackwater route had done nothing to alleviate the serious flooding problem at Lakeside. Basically too much water enters the river upstream and the shallow route is overwhelmed by the volume and flooding occurs.


Effects of draining the surface water from the fields into the river

Note the Environment Agency records its flow values in m3/sec so previous m3/hr values have been divided by 3600 for uniformity.

The River Blackwater has a number of monitoring points along its route designed to give early warning of flooding to residents and businesses. The two local monitoring points are at Tongham and at Farnborough.

As stated earlier the developer’s FRA claims that the existing greenfield runoff figures (desk based calculation) are between 44 and 48% (claimed in section 5.11 table 2 of the developer’s FRA). This represents at the 100 year return period rate a total of 8,851m3/hr (2.46m3/sec) being runoff (at 48% runoff) from the total 46.1ha site in 1 hour.

To illustrate the effect of disposing of this additional water, telemetry values from both stations have kindly been provided by the Environment Agency for 15 minute intervals for the July 2007 period. These are available to view on a separate spreadsheet.

Tongham gauging station has a normal level for the river of 0.56 – 1.45 metres above which flooding is likely. (Highest was 1.79 on 20th July 2007).

Farnborough gauging station has a normal level for the river of 0.36 – 1.30 metres above which flooding is likely. (Highest was 1.94 on 14th August 2006).

From the telemetry data for 20th July 2007 it shows that the flow rate at Farnborough at the start of flooding was 1.25m3/sec at 09.45 am with a peak of 2.26m3/sec being reached.

The telemetry shows that an increase of flow during the initial flooding stage from 1.3m3/sec to 2.23m3/sec caused an increase in level of 0.32metres at Farnborough and 0.34 metres at Tongham.

Thus an increase in flow of only 0.93m3/sec caused an increase of the flood depth of over 300mm.

The developers claimed runoff of 2.46m3/sec can now clearly be seen to be false as this rate by itself is virtually double the River Blackwater critical flowrate, without taking into account all the other flows into the river from the Thames Water surface water drainage system.


Existing Surface Water Drain System based on Thames Water Drg 5136-1

The existing surface water drainage system consists of a 1,220 mm dia main drain, fed from a series of smaller diameter pipes. There is a second main drain of 900 mm dia which joins into the 1,220 mm dia drain along Ash Lodge Drive at Avon Close. The 1,220 mm dia drain then continues on its route to the Blackwater taking in further smaller diameter feeds on the way.

Culvert A14/1 joins the 1,220 mm drain at a point near manhole hole 1151 conveying the surface water from the Hogs Back area. It has been noted that this large 1,200mm x 800mm feed has been omitted from the Thames Water Waste water drawing 5136-1

In fact there are at least 4 water courses that enter this 1,220 drain system that are not shown on this drawing. Thames Water do not therefore take these figures into their calculations.

An assessment has been carried out to calculate how much water is carried in each main pipe during varying levels of rainfall.

Each area that feeds into the drains has been assessed based on the following basis:

Percentage amount of rainfall transferred to drains

Road and pavement surfacesRoofs and garage areasDrives and flagged areasGrassy Runoffs Class 4 Soil
100% 100% 100% 50%

Note that the 50% surface water value from the grassy runoff will rapidly increase to saturation excess (100%) due to the impermeable nature of the clay. Calculations for this study however have been based on 50%.

The areas calculated for each drain are as follows. (Note that the park area that slopes down from Ash Church Road to Collins Gardens has been included as this water runs down into the Collins Gardens drains).

Main Drain 1,220mm dia Areas in metres2

Road and pavement surfacesRoofs and garage areasDrives and flagged areasGrassy Runoffs x 50%Total
32,750 50,720 48,600 40,696 172,766

Secondary Drain 900mm dia Areas in metres2

Road and pavement surfacesRoofs and garage areasDrives and flagged areasGrassy Runoffs x 50%Total
16,700 21,160 22,300 25,940 86,100

The detail calculations for the above values are available on an Excel spreadsheet.

Based on the above results it is possible to calculate the flow via the drains into the River Blackwater resulting from various rates of rainfall.

As previously stated culvert A14/1 enters the main drain approximately halfway along Ash Lodge Drive. The culvert carries water from a ditch that drains the large amount of water that is held back by the natural bund formed by the disused railway embankment. The flow is fairly constant at 3600m3/hr (1m3/s). The table below therefore include a constant value add-in of 1m3/s for all the 1,220 pipe values from 20mm/hr upwards. A lower value of 0.5m3/s has been included in the 10mm/hr flow rate as this rainfall rate does not usually result in a full culvert.

Table of flow against rainfall

Rainfall10mm/hr20mm/hr30mm/hr40mm/hr50mm/hr
Flow rate in 1,200mm m3/s 0.98 1.96 2.44 2.92 3.40
Flow rate in 900mm m3/s 0.24 0.48 0.72 0.96 1.20
Combined flow after junction 1.22 2.44 3.16 3.88 4.60

The system is rapidly overwhelmed by rainfall in excess of 20mm/hr. This is confirmed during the torrential rain of August 2008 when a 30 minute rate of approximately 40mm/hr was noted with subsequent flooding of properties.

From the values noted in the flooding study the River Blackwater can take a flow of up to 1.25m3/s before flooding occurs. It can be seen from this table that the combined flow from these pipes could cause the river to flood at even the 20mm/hr rainfall rate.

What actually happens is that much of the excess surface flood water flows in torrents into the fields to the south of Ash Lodge Drive which serves as a flood plain thus saving many properties from flooding. Residents in the area will confirm the large volumes of water that sweep across their gardens and into the fields.

These fields have been 100% flooded right across to the disused railway embankment on a number of occasions, the extent being included in the Capita Symons report of January 2009 commissioned by GBC. Partial flooding occurs every year.

The continued allowing of new developments by GBC in the locality has steadily made the flooding worse. Areas that were natural flood plains have been turned over to developers with the consequential loss of natural water holding capacity and the inevitable increase of more surface water into the drains thus increasing the flood risk locally, and in areas of Surrey, Hampshire and Berkshire along the River Blackwater. Recent developments have included, Bateman Grove (20 homes), Parish Close (61 homes), Oaktrees (45 homes), Collins Gardens (13 homes), Queen Elizabeth Close (10 homes) and The Briars (53 homes). A total of 202 homes that have been added to this area.

The proposal to build on this land is not feasible even with the most sophisticated SUDS system as not only will it take away the flood plain facility, it has been shown that the development would add substantially to an already unacceptable flow into the River Blackwater causing flooding in Hampshire and as far as Frimley.


Omission of flows into the surface Water Drainage System

Due to the concern over the omission of a number of substantial flow inputs into the Thames Water drainage system shown on drawing 5136-1 an enquiry was sent to Thames Water to clarify the situation.

Extract from enquiry to Thames Water 14/02/2012

"I have been asked to contact you by Ash Residents Association with respect to the assessment of storm drain capacities and current applications for development in this area.

Because the area floods frequently and also because of the critical situation with the capacity of the River Blackwater (into which the local drains flow) I was asked to do a drains study. It is evident from past occurrences in 2007 and 2008 that the storm drains concerned have insufficient capacity during periods of very heavy rain, with storm water coming up out of the drains and raw sewage floating on top.

Having compiled the inputs and expected flows into the storm drains I have noticed that a number of inputs into the drains are not shown. One in particular that causes concern is GBC culvert A14/1 which enters the 1,220mm drain beneath Ash Lodge Drive at a point near manhole 1151. This culvert is 1,200mm x 800mm and carries flow from the region of the Hogs Back. Flow is held back by a natural bund but the water is allowed though a restricting pipe at a rate of up to 1m3/sec. Because of the large quantity of water held back by the bund this flow continues, for many days at that rate, following a storm.

"The reason I am given for their omission, is that these inputs are unadopted."

"Are the flows from these unadopted inputs taken into account when calculating the capacity of the storm drains? I am particularly concerned with the flows in the 1,220mm pipe and the 900mm pipe which run parallel with Ash Lodge Drive and join at Avon Close near manhole 9051. The drains are already capable of putting more into the River Blackwater than it can handle and hence the several recent floodings of properties alongside its course."

Response from Thames Water 23/02/2012

Extracts from Thames Water reply:

"I have to admit to having very little knowledge of the catchment to which you refer and I have had to consult with our catchment planning team for their take on the queries you raise."

" It is common that private or unadopted assets are not show on the Map of Sewers. Where we have sufficient data we represent them as unadopted sewers, but we often do not have correct location and asset data in order to map them."

"Thames Water completes various levels of hydraulic assessments on the public sewer networks under its control. This is dependant on the size of the sewer, the proposed increase in capacity as well as issues that have affected our existing customers in the vicinity (i.e. reported flooding events). In many cases we expect developers to fund investigations that we undertake to determine the impact of their proposals on the existing sewer system, as well as to identify any infrastructure enhancements that could be implemented to mitigate against a deterioration in the performance of the sewer system due to proposed development. We have not recently undertaken any detailed modeling assessments in the area of Ash that you are interested in."

"As this is a surface water system, there will be rainfall events that will exceed the design capacity of any surface water sewer system and the hydraulic capacity of the local watercourse. This has the potential to flood the surface water outfall and thereby restrict the hydraulic capacity of the public sewer system."

Comments on reply from Thames Water

It is intimated from Thames Water Authority reply that they do not have sufficient data on the unadopted inlets to enable them to carry out any detailed modeling of the area we are concerned with, and would expect the developer to fund such investigations. It is evident from recent applications for development in the area that the Flood Risk Analysis carried out by the consultants contains none of this data - the consultants are probably totally unaware of the existing drains structure and in particular the high flow rate from A14/1. In consequence the whole process and subsequent planning approvals would be based on highly flawed information.


Raising the Site Level by 300mm

The developer has recognised that this area is subject to extensive flooding, as he has proposed raising the development site by at least 300mm to attempt to provide some protection to the new homes.

The developer has also proposed a flood relief channel, quote “To minimise the flood risk of the culverts on the northern boundary of the Site a flood relief channel is proposed, near to the northern boundary. The developer has managed to quote 3 different sizes for this channel in 3 different documents.

Design and Access Statement : size:- 3.5m wide with no depth or side slope.
Environmental Statement: size:- 4.0m wide x 0.75m deep with 1 in 2 side slope.
Flood Risk Assessment: size:- 4.0m wide x 0.75m deep with 1 in 4 side slope – a mathematical impossibility.

Taking the largest case, the total capacity of this channel along the northern boundary is less than 1,000m3. This would fill within 7 minutes with the floodwater from Ash Lodge Drive/Loddon Way and in possibly only 3 minutes with the 100 year +30% event.

If the developer raises the site level by at least 300mm the surface flooding from the north would not be able to overflow into the fields and properties along Ash Lodge Drive, Kennet Close, Loddon Way, Colne Way, Lea Close, Wandle Close, Southlands Close and Southlands Road will be extensively flooded. This is in direct contravention of the requirements of NPPF TG clause 2 which states that any development must not increase the flood risk elsewhere.

Raising the development site by 300mm would actually raise most of the site to a higher level than Ash Lodge Drive and Loddon Way. Using the developers topography survey the addition of 300mm would mean the site would be 200mm higher near Loddon Way, 50mm higher near the entrance to FP350 and 40mm higher near the Beverley Close end. This is a sure recipe for increasing the flood risk, and existing properties along Ash Lodge Drive, Kennet Close, Loddon Way, Colne Way, Lea Close, Wandle Close, Southlands Close and Southlands Road will be extensively flooded.

Depending on the rainfall it is likely that an additional 100 existing residential properties would be subject to internal flooding as a direct result of raising this development site by 300mm. Note that in all the flooding incidents so far the rainfall has only been a fraction of the 100year +30% event.

Flooding from the North (Ash Lodge Drive has overflowed into field parcel 1)

Another view of Ash Lodge Drive overflow into field


Other Causes of Flooding

Immediately to the south of the proposed development area there are a series of underground main water pipes of up to 14" in diameter belonging to South East Water. These pipes are buried direct in clay and are mainly concrete coated steel. There have been 6 ruptures within the last 4 years of which 4 caused extensive flooding of the fields. The 2 minor ruptures were alongside the disused railway embankment whilst the 4 serious rupture were all in Ash Green Lane West, 2 of which were adjacent to the corner of Ash Manor School playing fields. A long section of damaged (split open) 14" pipe lies nearby as evidence.

One of these pipes burst again at 2pm on 4th January 2012 flooding a large portion of the field adjacent to Ash Manor School playing fields.


Records of Flooding

Serious flooding occurred in Ash in Sept 1968, February 1990, October 1993, November 2000, August 2006 and July 2007. Flooding also took place at Ash Green in November 2000, and October 2006. The source for this information is Guildford Borough Council’s SFRA of January 2009. Further extensive flooding of the development area and houses to the north occurred in August 2008.

There is also record of flooding in June 1981 and August 1981,. 16th June 1999. and 7th July 2001

It is essential that the proposed development is not permitted to proceed as the value of this land in holding back floodwater cannot be under-estimated in its ability to prevent further residents property being flooded.


Summary of Key Points

  1. The Developer has failed to recognise the serious surface water flooding from the north of parcel 1, caused by the rapid overwhelming of the storm drains in heavy rainfall conditions.
  2. The Developer’s Flood Risk Assessment has grossly overstated the Greenfield runoff rate and has thus rendered information in that document invalid.
  3. The raising of the site by 300mm will block the escape of surface water flooding from the north and greatly increase the flood risk to existing properties.
  4. The application to use the field labelled Cardinal Field, south of the disused railway line as a SANG is totally unsuitable as this field acts as a functional floodplain for a series of watercourses from the south and east. As a result it rapidly floods many times a year to depths up to 1.5m and should be classed as zone 3b.
  5. The Developer’s site survey has failed to appreciate the topography of the fields in parcel 1 with their raised borders preventing Greenfield runoff.
  6. As virtually no pre-development water from parcel 1 currently enters the storm drains, the Developer cannot be allowed to put post development water into the storm drains as it would all be a net gain into the system and accelerate flooding of the River Blackwater. This would affect properties from Tongham to Frimley Green in both Guildford and Rushmoor Boroughs.
  7. The watercourse running south to north alongside FP350 is deep and can flow very fast and would be a source of considerable danger of drowning to children and adults

Documents related to flooding in the area

    1. "Surrey County Council’s Flooding and Wet-spots" lists Southlands Road – Ref GUI094 as a flood area.

    2. Guildford Borough Council’s SFRA dated Jan 2009
      Vol 1 Page 11 – "The most intensive urbanised areas are Guildford and Ash – here any further development could potentially increase surface water.

      Vol 2 Para 3.15 "Sandbag requests in 2006 were concentrated of to the west of the study area in particular around Ash Green, Ash and Ash Vale which have recorded problems with surface water runoff.

      Vol 2 Para 6.27 "There are however some areas of moderate to high development intensity within Flood Zone 3, most notably an area through Guildford town centre and through Ash and Ash Vale. The suitability of redevelopment within these areas will require careful consideration given the potentially high risk of flooding".

      Vol 2 Para 12.3 "Surface water and sewer flooding have been considered using recorded incidents of flooding provided by the EA and GBC. There are some notable “hot spots” for surface water flooding, primarily in Ash, Ash Vale and Pirbright. Areas of the catchment underlain by clay geology and clay soils are more likely to be impacted by surface water flooding however maintenance of existing small watercourses and structures such as safety screens and culverts has an important influence on local flooding mechanisms".

      Guildford SFRA Appendix G: Other sources of flooding Drawing Nos CS20946/Appendix G-3 and G-7 showing the extent of flooding covering the proposed development area.

      Local Government Planning Policy Statement 25 – Stating the considerations and requirements of both developers and planning authorities with respect to "Development and Flood Risk"

    3. Core Strategy Policy CP7 WATER MANAGEMENT extract:
      “New development on undeveloped flood plains will be avoided. Development on land at risk from flooding will only be permitted where it can be demonstrated that it will not impede the flow of floodwaters, increase the risk of flooding elsewhere or reduce the capacity of a floodplain.” ……………
      “The disposal of surface water from new development to public sewers will not be permitted.”

    4. National Planning Policy Framework TG
      As set out in the National Planning Policy Framework, inappropriate development in areas at risk of flooding should be avoided by directing development away from areas at highest risk, but where development is necessary, making it safe without increasing flood risk elsewhere

Reference Documents:-

Capita Symonds – Guildford Strategic Flood Risk Assessment / January 2009 (for Guildford Borough Council)

Surrey County Council - List of Wet-spots in Surrey / March 2011

Attached Documents:-

Environmental Agency Telemetry Values Spread-sheet / January 201


Circulation List

Tara Taylor, Case Officer, GBC Planning Department

GBC Planning Committee 

Michael Gove, MP

Nick Sutcliffe, Local Councillor

Environment Agency

Thames Water

South East Water

Surrey County Council Highways     

Surrey Wildlife Trust

John Edwards, SCC Countryside Management & Biodiversity Manager  

Campaign for the Protection of Rural England    

Rushmoor Borough Council (Ref: 12/00928/ADJ)  

Ash Parish Council

Tongham Parish Council

Ash Green Residents Association      

Ashley Bowes, Solicitor 

James Watkins, Trinity Mirror        

Newsletter Subscription

We will, from time to time, want to communicate information to residents without it appearing on our website. Therefore, if you have not given us your e-mail address in the past, can you please click this button and supply it now. All e-mail addresses will be kept securely and will not be disclosed to anyone else. Due to a large amount of fake subscriptions we are now asking for your road name and postcode in order to validate you are a real person. This information will never be made public and will only be used to validate your subscription.
Name: Email:
Road Name: PostCode: