Waverley Borough Council Committee System - Committee Document
Meeting of the Environment and Leisure Overview and Scrutiny Committee held on 31/10/2006
DRAFT FLOOD RISK ASSESSMENT FOR THE WAVERLEY BOROUGH COUNCIL CORE STRATEGY
DRAFT FLOOD RISK ASSESSMENT FOR THE WAVERLEY BOROUGH COUNCIL CORE STRATEGY
This Strategic Flood Risk Assessment has been prepared on the basis that it would be a broad-brush approach. It is a strategic assessment in the sense that it provides a general picture of the situation regarding flooding in the Borough. It does not deal with details of particular flood areas, nor does it include proposals for remedial work
Draft “Planning Policy Statement 25: Development and Flood Risk” (2005 Office of the Deputy Prime Minister) (PPS25), published in December 2005 requires that Local Authorities carry out a Strategic Flood Risk Assessment (SFRA) in relation to development planning in their area, as set out in the Core Strategy of the Local Development Framework and that the development is assessed against the sequential test.
This stipulation was issued after the Borough Council had approved its Local Development Framework Core Strategy document in December 2005. Therefore, the SFRA is has been completed after submission of the Core Strategy to the Secretary of State, and will be presented at the Examination.
The level of the appraisal
The guidance in PPS25 states that the SFRA “should be proportionate to the risk and appropriate to the scale nature and location of the development.” (Para E3). It should be fit for purpose.
Waverley is not subject to regular severe flooding, but it has Zone 2 and 3 flood areas along all the branches of the River Wey. (See Appendix 1 for maps and Annexe 4 for a detailed definition of Flood Risk Zones). Zone 1 is not mapped because it covers all the rest of the Borough not in Zones 2 and 3. The SFRA should ensure that it is proportionate to the risk and appropriate to the scale, nature and location of proposed development to make a broad appraisal.
The assessment is not a comprehensive appraisal of all the flooding risks in the Borough. It provides an indication of the severity, type and general locations of flooding incidents recorded in the recent past.
Relationship with the Core Strategy Sustainability Appraisal
Flooding is considered throughout the SA in the following way:
Appendix A Plans and programmes refers to PPS25;
Appendix B gives baseline data on flooding;
Appendix C raises sustainability issues on flooding;
Appendix D looks at sources of objective on flooding;
Appendix E the Sustainability Framework considers the reduction of the vulnerability to flooding;
Appendix F considers the possible conflicts between the Core Strategy objectives and the sustainability objectives;
Appendix G compares the sustainability objectives and the Core Strategy Options in relation to flooding;
Appendix H appraises the Core Strategy Policies in relation to flooding;
Appendix I gives a summary;
Appendix J includes the cumulative effects in relation to water management generally.
The SFRA has been prepared in cooperation with the Environment Agency and with Mole Valley District Council. The appraisal has been prepared using PPS25 as a basis and the EA document “Strategic Flood Risk Assessments: Midland Region Interim Guidance” March 2006, as well as referring to the documents listed below.
The general situation in the Borough regarding the sources of flooding.
In Waverley, the Main Rivers comprise:
The River Wey(south branch) from Frensham to Tilford;
The River Wey (north branch) from Wrecclesham to Tilford via Farnham;
The River Wey from Tilford to Peasmarsh via Elstead and Godalming;
Craleigh Waters- the main river from Ellens Green and Cranleigh to Bramley;
The River Blackwater on the County boundary at Badshot Lea; and
The Loxwood Stream from Chiddingfold to Alfold.
Over two thirds of the Borough is in the catchment of the River Wey, North Branch while the south east corner is in the catchment of the River Wey South Branch These rivers account for the majority of the flooding events in the Borough.
A further 18 strategic watercourses, totalling approximately 60km in length, which were identified as Critical Ordinary Watercourses (COWs) (those with the greatest flood risk) have now been enmained (adopted as Main river) by the Environment Agency. These are listed at Appendix 3.
The different sorts of flooding in the Borough
Many of the flooding risks in Waverley are fluvial relating to the above Main Rivers and their enmained tributaries. There are however numerous flooding risks from Ordinary Watercourses, overland flow and incapacity in fou and surface water sewers and highway drainage, as described later in the assessment.
Geology and topography
The geology and topography of the Borough has a significant influence on the flood risks associated with any particular rainfall event or series of events. The free draining Badshot and Bracklesham Beds in the Farnham area and the Lower Greensand in the central areas of the Borough, for instance, have a capacity to absorb relatively intense rainfall events, whilst the heavily impermeable Weald Clay in the south-east parts of the Borough has little capacity for retention or absorption of intense rainfall. For this reason, the return period (likelihood of recurrence) of storm rainfall events is not a reliable measure of flood risk. The circumstances of each rainfall event or series of events and the condition and nature of the catchment must be considered in assessing flood risk.
The area and proportion of the development plan area that is identified in Zones 2 and 3
The maps in Appendix 1 show that approximately 7% of the Borough is identified as being is Zones 2 and 3.
Selection of development areas - the Sequential Test in PPS25
The aim of the Sequential Test is to steer new development to areas of the lowest probability of flooding, and this can be achieved in Waverley. There are three flood Zones identified by the Environment Agency, and decision makers should aim for Zone 1 (see Appendix 1 for detailed definitions of the Zones.
Zone 1 has a less than 1 in 1000 chance of river flooding in any one year (less than 0.1%). Only a brief FRA is required.
Zone 2 has between a 1 in 100 and 1 in 1000 chance of river flooding in any one year (1% to 0.1%). The highly vulnerable uses (see Appendix 2) are only appropriate in this Zone if the Exceptions Test is passed. A full FRA is required.
Zone 3 has a 1 in 100 or greater chance of river flooding (over 1%). The high vulnerability uses should not be permitted in this zone. The more vulnerable and essential infrastructure should only be permitted if they have passed the Exception Test. Only water compatible uses and essential infrastructure should be permitted on the functional floodplain.
If it is not feasible to direct development to Zone 1, then the selection of Zone 2 or 3 needs to be justified and the Exception Test needs to be passed
The Exception Test
The Exception Test should be applied in certain circumstances only after the Sequential Test has been applied, when more vulnerable development cannot be located in Zone 1 or 2 and when highly vulnerable development cannot be located in Zone 1. (See Appendix 4 for the flood risk vulnerability classification). The Exception Test should be applied to the Core Strategy and used to draft criteria based policies against which to consider planning applications.
The Exceptions Test is that:
a) the development makes a positive contribution to sustainable communities, and to sustainable development objectives of the Core Strategy;
b) the development is on developable brownfield sites;
c) a flood risk assessment demonstrates that the residual risks of flooding are acceptable and can be managed;
d) the development makes a positive contribution to reducing or managing flood risk.
The sequential approach to flood risk in the development allocation and development control process.
The Core Strategy was completed before the need for a SFRA was identified. Therefore the sequential test was not part of the approach to selecting the development areas, nor was it used in the Waverley Borough Council Urban Housing Potential Study 2006. However, PPG 25 on development and Flood Risk was applied and was incorporated into the Core Strategy.
STAGE 1 THE SCREENING STUDY
The approach adopted is based on the guidance in PPS25 and the Environment Agency document “Strategic Flood Risk Assessment-Midlands Region Interim Guidance” (March 2006) on how to carry out a SFRA.
The Core Strategy development proposals
The preferred option for the Core Strategy is for there to be development in the existing urban areas of Farnham, Godalming, Haslemere and Cranleigh, and in villages with community facilities, such as Milford and Bramley. This appraisal therefore focuses on these areas.
There are flood risks in these areas. Map 2 shows the locations of flooding as identified on the Environment Agency flood zone maps (see Appendix 4 for the definitions). Surface water issues impact on all locations. A reduction in runoff back to Greenfield rates should be achieved through the use of sustainable drainage systems (SUDS).
Is flood risk a significant issue in the plan area and where?
The flood risks in Waverley Borough are many and varied. Flooding incidents have occurred right across the Borough relating to Main Rivers, Critical Ordinary Watercourses (now classified as Main Rivers) Ordinary Watercourses, ditches, overland flow, rising ground water, highway runoff and foul and surface water sewage overflow. Flooding has generally been localised, of modest depth and short duration.
The “benchmark” flood, in living memory, was the event of September 1968, when it is understood that several hundred properties across the area, which now constitutes Waverley Borough, were affected. A large part of South East England was severely flooded on this occasion.
Following that event, Waverley ‘s predecessor authorities commissioned various studies of flood risks and in some cases undertook significant flood relief schemes. After the 1974 Local Government reorganisation, Waverley Borough continued the process of managing flood risk through a rolling capital programme of flood relief works across the Borough.
Further severe rainfall events particularly in 1979 and June and August 1981 resulted in further property flooding particularly in Cranleigh. A full engineering assessment of the flooding problems in the developed areas of Cranleigh was subsequently commissioned and a substantial amount of flood relief works progressively constructed, as described later in this report. Other studies were concentrated on the Bourne Stream and Frensham Vale Stream, and schemes were constructed here and at Haslemere.
This successfully reduced the more persistent flooding problems and the period to the end of 1998 was relatively uneventful in this respect. A severe rainfall event in January 1999 resulted in the flooding of over 30 properties in Cranleigh and a few elsewhere.
Exceptional rainfall events occurred on 30th/31st October and 6th/7th November 2000, following the wettest September for 19 years, and one of the highest monthly rainfalls ever recorded in October. The following is a summary of the effects of this extreme rainfall pattern and is an illustration of the range of flood risk extant in Waverley However, it is important to recognise that this particular pattern and volume produced very little flooding in the Cranleigh catchment, which has proved to be particularly vulnerable to high intensity, short duration rainfall, falling on a thoroughly wetted catchment.
In the Autumn 2000 severe rainfall events approximately 434 requests for sand bag protection relating to 336 properties, were responded to. Other severe flooding incidents occurred in parts of the Borough in January 1999, July and September 2000, and July 2001. It is estimated that approximately 100 properties suffered internal flooding in this period.
Main River Flooding
The River Wey (north branch), River Wey (south branch), River Blackwater and Loxwood Stream (southern area EA) all pass through Waverley. Flooding due to out-off-bank conditions in main rivers was reported at:-
Catteshall Road, Godalming (River Wey) – 20 houses surrounded and 12 flooded internally and the highway closed.
Elstead Mill, Elstead (River Wey) – Mill House flooded. Tilford Green, Tilford (River Wey south branch) properties flooded.
Priory Lane, Millbridge (River Wey south branch) properties flooded.
Fisher Rowe Close, Bramley (Cranleigh Waters) properties flooded.
Elmbridge Road, Cranleigh (Cranleigh Waters) property flooded.
Sand bags were supplied to up to 12 other properties in Bramley in fear of flooding from Cranleigh Waters.
2 properties flooded.
Mill Lane and Petworth Road, Chiddingfold (Loxwood Stream, Southern EA) – 14 properties flooded in Autumn 2000.
Tributaries of Cranleigh Waters
Critical Ordinary Watercourse tributaries of Cranleigh Waters are:-
i) Cranleigh Stream (Littlemead Brook) through Cranleigh village;
ii) Bramley Mill Stream (Hascombe Stream), Bramley;
Cranleigh Stream (Littlemead Brook)
Out-off-bank conditions in the Cranleigh Stream have resulted in flooding of properties throughout the history of Cranleigh. Recent events occurred in January 1999, July 2000 and September 2000. No such flooding was reported in the Autumn 2000 events.
Flooding reported in Cranleigh;
16th January 1999 – 13 domestic (internal)
11 trade (internal)
10 patio/drives and gardens
4/5th July 2000 - 18 domestic (internal)
3 trade (internal)
19 drives/patios and gardens
Note: flooding occurred throughout Cranleigh on this occasion. Only half of the number above were related to the main water course overflow.
September 2000 - approximately 10 properties in total (internal)
Bramley Mill Stream (Hascombe Stream)
2 domestic properties in Mill Lane and 2 commercial premises in the High Street have flooded internally on a number of occasions including during the Autumn 2000 events.
Ordinary Watercourse tributaries of Cranleigh Waters at Ellens Green resulted in the flooding of two properties at Pollingfold Bridge and Furzen Lane (severe and repeated internal flooding). Flooding has also occurred of up to 20 further properties in the Cranleigh catchment as a result of run-off from land, highways and foul and surface water sewerage deficiencies.
Tributaries of the River Wey (North Branch)
The one identified Critical Ordinary Watercourse is the Bourne Stream through Shortheath and Middlebourne, Farnham to its junction with the main rivers. There are also some isolated risks associated with the Frensham Vale Stream.
There are several dozens properties at risk from the Bourne stream but flood defence works by Waverley over the last ten years has reduced those risks. Up to 20 properties are protected by raised embankments. The one confirmed repeated flooding from the Bourne stream continues to be the Bat and Ball Public House, but sand bags are regularly issued to up to a dozen properties which fear flooding from this stream.
Tributaries of the River Wey (South Branch)
A large part of the tributaries which drain the built-up part of Haslemere are culverted, the blockage or failure of which has the potential to flood dozens of properties and are therefore identified as Critical Ordinary Watercourses. Few problems were reported during the Autumn 2000 events but an exceptionally intense storm rainfall event in July 2001 resulted in the overload and partial failure of a culvert which resulted in the flooding of 6 properties in St Christopher’s Road and Meadow Vale. 4 domestic and 2 trade premises at Popes Mead, Tanners Lane, and High Street are also reported to a flood repeatedly due to run-off from highways and other land and the result of overflowing sewers.
Tributary of the River Wey
The River Ock (now adopted as Main River) flows from Milford/Witley to Godalming to its confluence with the River Wey. There are isolated risks associated with this watercourse principally at Mill Lane, Godalming where 3 properties needed sand bank protection in the Autumn 2000 events.
Tributary of the River Blackwater
The overflow of the tributary at Badshot Lea Road, Farnham has resulted in the flooding of one commercial premises (Badshot Lea Motors).
ributary of the River Ock
Flooding occurred at 3 properties at Station Lane, Milford.
Rising Ground Waters
Rising ground water levels in the Mare Hill area of Witley has resulted in increased surface water run-off which has led to the flooding of up to 8 houses in Gasden Copse, Gasden Lane and Cramhurst Lane, Witley.
General Flooding and Flooding Risk
The Autumn 2000 conditions resulted in approximately 434 requests for sand bags relating to 336 properties distributed right across the Borough. Many of these related to localised flooding resulting from rural run-off from highways, incapacity or blockage of local ditches, highway drains, and foul and surface water sewers, and rising ground waters. It is estimated that one third of the request related to a real and imminent risk of inundation.
The rainfall events of Autumn 2000, which were largely of lower intensity but of long duration affected only parts of the Borough. The more intense but shorter duration events affect other parts of the Borough eg. the events of January 1999, July and September 2000 caused widespread flooding in Cranleigh but little elsewhere but the events of Autumn 2000 caused widespread flooding across the Borough but very little in Cranleigh. It is estimated therefore that up to 500 properties may be at risk or feel threatened by flooding if the whole spectrum of rainfall events is considered.
Would new development add to that risk?
There are no allocated housing sites in the Local Development Framework in Cranleigh, but any large housing development proposal that may arise would be likely to add to the flooding problem because it may cause increased runoff, even if it was not in the flood plain.
A large housing development upstream of the River Wey in Godalming may increase the risk of flooding onto the Lammas Lands, if there were no SUDS but there is no such housing proposal. There is one site in the Urban Capacity Potential Study in the flood plain Zone 2 It is in Borough Road/Charterhouse Road and would be for the development of 10 houses. This would not create a significant flood risk.
STAGE 2 THE SCOPING STUDY
What information is available?
Planning Policy Statement 25 Development and Flood Risk 2005
“Strategic Flood Risk Assessment, Midland Region Guidance” Environment Agency March 2006
Guildford, Surrey Heath, Waverley, Rushmoor and Hart Strategic Flood Risk Assessment – Information pack provided by the Environment Agency 2006.
Waverley Borough Local Plan 2002 Proposals Map
Flood Zone Map 2 and 3 from the EA
Information from Waverley Borough Council on historic flooding
EA information on flood defences.
“Water Framework Directive and Planning” EA 2006-08-14
ODPM “Preparing for Floods” (2003)
“National Standing Advice to LPAs for Planning Applications development and Flood Risk” EA
“Making Space for Water-Government Flood Risk Management Strategy” DEFRA
The Environment Agency is also in the course of preparing the Wey Strategy for the whole catchment of the river, and this will identify aims and objectives, problems issues and mitigation for flooding.
A qualitative assessment of the likely flood risk to the development areas and to sites
The development areas in the Borough are Farnham, Godalming, Haslemere and Cranleigh, development appropriate to the needs of rural communities villages.
The River Wey passes through Farnham town centre. It is known that significant flooding due to out of bank conditions occurred in the benchmark flood of 1968. Subsequent to that event the river channel was remodelled, straightened, a second stage channel constructed and culverts and bridges reconstructed. No out of bank conditions have been recoded since the reconstruction. The flood risk zones associated with the River Wey are shown on the Environment Agency’s published Flood Map
The Crosby housing development in West Street was designed with the advice of the Environment Agency to be at a level above the maximum anticipated flood level when the role of the flood plain for storage of water is mobilised.
Flooding of individual retail premises, particularly in and adjacent to Downing Street, has been an ongoing issue. This was related to highway runoff and surface water sewer incapacity. The sewerage undertaker, Thames Water Utilities has carried out a number of surface water sewer enhancement schemes in the town, which has reduced the risk. Since completion of these works, the Borough Council has reported no reports of regular flooding in the town.
In the outer Farnham areas in the Bourne valley, Frensham Vale, Weybourne, Heath End and Badshot Lea, there is a range of flood risks from various sources.
There have been about 126 consents given for new development in the town each year, but the majority are in the north hilly part of the town in the Hale area. Even so, this moderate amount of development may have an effect on the runoff into the drainage system though it is difficult to assess because it is not measurable.
The major development that will be partly in the floodplain Zone 2 is East Street, Farnham where 294 dwellings are proposed. The issues on this site will be both fluvial flooding and surface water flooding which must be managed to ensure no detriment to other locations and the management of flood risk on the site itself.
The River Wey and Hell’s Ditch, which also conveys main river flows, run adjacent to the town centre through the Lammas Lands, which comprise an extensive area of flood storage for out of bank conditions. The flood Risk Zones are identified on the Environment Agency Flood Map, (See Appendix 1).
Properties at particular risk of flooding are in Catteshall Road, where out of bank conditions of Hell Ditch, in combination with restricted culvert size under Catteshall Road result in floodwaters crossing the road into adjacent properties. Main River flooding threatens properties in Bridge Road, the Burys and Borough Road.
The River Ock, (now classified as a main river) is a tributary of the River Wey and it threatens property in Mill Lane in out of bank conditions.
There are a number of localised flooding risks in other areas of Godalming and Farncombe, largely associated with incapacity in surface water sewers and highway drainage.
There are no major developments proposed in the town and the annual amount of housing site windfalls is about 67 Therefore it is not anticipated that the flood risk should increase significantly. The potential housing sites amount to 10 dwellings on the site In Borough Road/ Charterhouse Road and as has been noted, it is in the floodplain Zone 2
Haslemere High Street forms a watershed between the Wey and Arun Catchments. The area to the west of the High Street is drained by tributaries of the Wey(Southern branch). A large part of this has been culverted and many properties rely on the integrity and effective maintenance of the culvert for their flood protection. Town centre retail premises and dwellings in the West Street and Tanners Lane areas are at risk from storm rainfall events which exceed the capacity of the highways drainage system.
Frequent flooding of highways at Foster’s Bridge and Tanners Lane results from modest rainfall events. Incapacity in highway drainage systems also results in flood risk in Wey Hill and St Christopher’s Road. A Victorian culvert which carries an enmained tributary of the Wey South is vulnerable to storm rainfall damage and exposes a number of properties in St Christopher’s Road, Meadow Vale and Wey Hill to flooding risk.
The annual housing windfall sites amount to about 69 dwellings which does not have a marked effect on flood risk provided there are SUDS schemes in place. There are 5 potential housing sites and none are in the floodplain.
Cranleigh village and surrounding settlements are drained by an extensive network of newly enmained watercoures, (Classified as a Main River), and Ordinary Watercourses. The Main River watercourses on the Littlemead Brook (which bisects the village), the Nuthurst Steam which drains the Nuthurst and Cranleigh village Estates and the Alderbrook Stream which drains Smithwood Common and Rowley.
Much of the developed area has a history of flooding incidents. Following the engineering assessment of flooding problems in 1981/82, the Council constructed an extensive amount of flood relief works to reduce the flooding risk to many of the more vulnerable areas.
Major works were undertaken on the principal watercourses (the Littlemead Brook) at Ewhurst Road to Mead Road, Horsham Road, Snoxhall Fields, Knowle Lane and Littlemead. Other schemes were constructed at Grove Road, Kings Road, Woodland Avenue, Cranleigh Common, Lashmere and Rowly.
Many of the more frequent flooding incidents were eliminated by this programme, but large parts of Cranleigh by the nature of the topography and impermeable soil, remain vulnerable to flooding risk, particularly for storm events in excess of 1in 100 (1%) return period.
The major part of the schemes comprised the construction of new enlarged culverts and additional relief culverts and the flood protection therefore relies on the effective maintenance of the underground assets. In particular, the 200m length of culverted Littlemead Brook in Ewhurst Road is protected from blockage by a substantial trash screen which requires regular maintenance and almost permanent attendance in extreme storm rainfall events.
The areas of known flooding risk are outlined in Appendix 2 but all proposals for development in Cranleigh will require careful examination of the flooding risks both to and from the development.
There are not large-scale proposals for further new estates and the annual windfall sites has been small at about 12 dwellings a year. However, there has been recent consent for 79 units in Wyphurst Road. There are no potential housing sites proposed for Cranleigh. Therefore the flood risk from additional housing should be very limited if surface water is appropriately managed through the use of SUDS
There are two large raised reservoirs (lakes): Snowdenham Mill Pond and Bramley Mill Pond and associated smaller ponds immediately upstream of Bramley Village, which together with lakes at Thorncombe Park and Winkworth, are drained through Bramley by the newly enmained (classified as a Main River) Hascombe Stream to the Main River of Cranleigh Waters, which runs adjacent to the developed areas of Bramley. Properties in Linersh Wood, and Fisher Rowe Close are at risk from both sources. Properties in Mill Lane and the High Street are at risk should any of the lakes not be maintained or managed adequately.
Frequent flooding of commercial properties in the High Street occurs due to the construction and level of the highway in relation to the properties and insufficient means of its drainage.
Careful consideration of any development proposals in the Hascombe Stream corridor and in the flood risk zones of Cranleigh Waters is required.
There have been localised problems with overland flow and inadequate drainage in the Bramley Park Farm area.
There has been a whole series of new estates built adjacent to the floodplain, but only the gardens lie within it. Surface water runoff must have considerably increased but there do not appear to have been any repercussions. However, in future the cumulative effect of surface water runoff from development has to be assessed
The newly enmained River Ock drains through Milford and is fed by a number of lakes and ponds: Witley Pond, Enton Lakes and Rake Manor Pond. There is also an on-line pond at Milford Golf Course. There is no known internal flooding of property from fluvial sources, but there is a risk to property at Roke Lane and Station Road when out of bank events result from extended periods of exceptionally wet weather.
There are high ground water levels in parts of Milford particularly in the Milford Lodge estate, largely resulting in garden flooding. Excessive infiltration and connection of surface water to the foul sewerage system has resulted in sewage flooding, particularly in the Milford Lodge, Cherry Tree Road and Church road areas. The sewerage undertaker has recently completed the third and final stages of a sewerage improvement programme in Milford and Witley, which has reduced that risk. Sewage capacity, however, remains an issue and the level and drainage of any new development must take into account the sewerage issues.
Like Bramley, there have been new estates in Milford. The cumulative impact of surface water should be considered.
Milford Hospital is due to close and there will be a major housing development of over 120 dwellings. This site will drain into the newly enmained River Ock if not controlled by SUDS The runoff from the new housing could cause flooding. This will need to be considered as part of any new development proposal. Rates of surface water discharge from the site needs to be reduced as much as possible through the use of SUDS in order to minimise the cumulative impact of surface water.
Determination of whether design factors would reduce the flood risk sufficiently
The design of a housing layout can take account of the need to control runoff and contain it from getting into the rivers and streams. On green field sites this is achieved through the rate of runoff being no worse than the original conditions before development. The aim should always be to reduces rates to those of a green field site unless it can be clearly demonstrated that this is not possible, and it should then be reduced to as close to green field rates as can be achieved. Certainly the rate should not increase. This constraint would control the flood risk.
Sustainable drainage systems (SUDS) are an essential part of modern designs of housing and industrial estates and include the following, (as set out in PPS25):
Preventative measures including rainwater recycling, drainage enhancing design and green roofs.
Infiltration devises to allow water to soak into the ground, that can include individual soakaways and communal facilities.
Filter strips and swales which are vegetated features that hold and drain water downhill mimicking natural drainage patterns.
Filter drains and porous pavements to allow rainwater and runoff to infiltrate into permeable material below ground and provide storage is needed.
Basins and ponds to hold excess water after rain and allow controlled discharge that avoids flooding.
Such measures as these need to be incorporated into the design of new development, and if they are included then this would reduce the flood risk substantially.
Determination of whether the land allocation is particularly sensitive to flood risk or the effects of climate change.
Only 2 out of the sites that have been identified as suitable for housing development in the Urban Housing Potential Study are in or partly in the floodplain: part of the East Street development in Farnham and the site on the corner of Borough Road and Charterhouse Road, Godalming.
These sites must be carefully designed to ensure no worsening of flood risk and wherever possible should actively seek to reduce flood risk to the area and catchment as a whole. Only a small part of the East Street site is in the floodplain (Zone 2) and the EA advises that flooding that has occurred on the site was caused by surface water rather than flooding from the River Wey adjacent. All forms of flooding should be considered however, including the impact of local groundwater from the development.
It is not anticipated that the potential housing development of 10 dwellings in Godalming would add significantly to the runoff from the site if surface water is appropriately managed through the use of SUDS. Nor would it be affected by flooding from the adjacent floodplain of the Lammas Lands which is Zone 2.
STAGE 3 THE DETAILED STUDY
The potential flood risk of the development
There are two parts to this study: the flood risk in the broad development areas identified in the Core Strategy and the flood risk to the allocated sites, described above. This study aims to provide more detailed data.
In 2001 a report was made to the Council on the impact of the 2000 floods on the Borough. This provides details of the severity of the flooding in the urban areas identified by the Core Strategy as development areas. An extract from the Report is included in Appendix 2 and it gives a clear picture of the vulnerable areas in the Borough.
The potential impact of the development (areas and sites) on flood risk in associated areas
Cranleigh is the location which is most vulnerable to flooding, and that is the one urban area where there are no potential housing sites. However, windfall proposals continue to come forward, and there remains pressure for development and redevelopment. The potential impact of the development areas on flood risk is limited as has been explained above, if surface water is appropriately managed through the use of SUDS.
How mitigation measures can reduce the flood risks
The Borough Council undertook an ongoing programme of flood prevention schemes in Cranleigh and elsewhere from 1986, but no further schemes have been carried out since about 2000 in the Borough. These works successfully reduced the risk of flooding to many properties across the area.
Determine within flood zone 3 the variation in actual flood possibility and depth, now and in the future, taking into account the effect of flood risk management measures
There is no information available of this aspect.
Consider the effects of a range of flooding events including extreme events
The recent flood events have been 1981,1986,1999/2000 and more recently 2006. The effects on the four urban areas have varied considerably, because in some causes there have been very localised storms and in other instances there has been a very long prolonged period of rain. In 2006, there was surface water flooding from a summer storm in Farnham while Cranleigh did not experience any storms at all. In 1999/2000, there was a period of long rainfall and Cranleigh was seriously affected.
Include the assessment of the remaining residual risk, after measures have been taken into account.
There are no new flood improvement measures that are planned by the Council, so an assessment of the residual risk is based on the existing situation. Based on the event in 1999/2000, it is likely that there will be some future incidence of households being flooded adjacent to the Lammas Lands in Godalming and there could be flooding in Cranleigh , especially if there are blocked culverts and debris causes a problem, as it has done in the past.
Consider how the ability of water to soak into the ground may change with development and the layout of development may affect drainage systems.
The average housing development comprises about half hard surface and half soft surface. The loss of opportunity for water to soak into the ground is considerable, reducing the accumulation of ground water. However the scope for runoff may be increased, and this is beneficial provided the water is not polluted from oil and petrol from parked cars.
Take into account climate change predictions over the lifetime of any proposed development
The Environment Agency publication “The Climate is Changing: Time to Get Ready” 2005 predicts that there will be longer, drier, hotter summers with intense bursts of rain that may cause flooding, and winters are becoming wetter with more rain in heavy downpours. The autumn and winter floods in 2000 were the worst in 270 years in some areas. Half a million people called the national Environment Agency Floodline.
It would appear that flood risk in the Borough is likely to worsen to some degree over the Plan period up to 2018.
FURTHER CONSIDERATIONS REQUIRED IN A SFRA REQUIRED BY PPS25
PPS25 stipulates that a number of aspects need to be considered in a SFRA, which are set out below.
Determination of the variations in flood risk across and from the area
The upper River Wey catchment covers a large part of the Borough and there are flood zones adjacent to the river over all its extent. However the most likely area with future flood risk is the Cranleigh area. Flooding here would not affect other Districts directly. However, the Upper Wey affects Guildford and the lower reaches of the river, and has an impact on those other areas.
Enable LPAs to determine the acceptability of flood risk in relation to emergency planning capability
The Borough Council is preparing a Flood Plan in accordance with the Civil contingencies Act 2004. The |Plan is a multi agency approach, risk assessment based. Measures are in place under the Waverley Civil Emergency Plan and the Surrey Major Incident Plan to deal with flood events. However, there is concern about the problems of evacuation of care homes and other dedicated elderly residential accommodation in Farnham and potentially elsewhere in the Borough. The degree of acceptability of flood risk should always be led by PPG 25 and PPS 25 informed by the sequential test, and in PPS25 by the exceptions test. The SFRA will influence the Flood Plan because it will consider future development on the flood plain and any situation where flooding will create additional problems because of an increase in vulnerability.
Existing flood defences and infrastructure
The existing defences consist of ditches, drains, culverts, dykes, sluices, private sewers and any passage through which water flows. These are too numerous to show on a plan.
There are very few developed areas of Waverley that are defended from flooding by raised embankments or walls and therefore few issues on maintenance. Many properties are protected by culverts and other underground drainage, the condition of which is variable. There are a few remaining lengths of fragile Victorian culverts and pipelines that have been identified for possible reconstruction. A number of the culvert entrances are protected by trash (storm debris) screens, the maintenance of which is crucial to the prevention of out of bank flooding.
The cost of improving inadequate defences
The Environment Agency is producing the Wey Strategy which may well identify proposals and cost for dealing with inadequate defences.
There are no new infrastructure schemes proposed in Waverley in the short term, but a small-scale project has been identified as needing to be implemented in St Chrisopher’s Road in Haslemere.
Opportunities to restore the natural flood plain
There are no opportunities for restoring the natural flood plain in Waverley. Most of what has been lost has been to development and this is irrevocable.
Physical features that could convey flood flow to other areas
There are no examples in Waverley.
The impact of the EA flood risk management capital programme
The EA Wey Strategy has not yet been published.
THE CORE STRATEGY POLICY FOR FLOOD RISK MANAGEMENT
The Core Strategy includes a policy on water management:
Policy CP7 WATER MANAGEMENT
“Developers will take appropriate measures to attenuate runoff through sustainable drainage systems. 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. Appropriate water, flood and drainage management initiatives will be required, particularly to deal with the incidence of flash flooding arising because of climate change. A flood risk assessment appropriate to the scale and nature of the development will be required for any development proposal within areas identified as being at high risk and for any new residential development in areas of low to medium risk.
The disposal of surface water from new development to public sewers will not be permitted.
Arrangements for the provision and maintenance of flood defences and other water management measures that are required as a consequence of development, including the provision of services, will be secured by planning obligations or by conditions attached to a planning application.”
The EA has provided comments on this policy for the Core Strategy submission documents in accordance with the “Tests of Soundness”. It is recommended that it should be renamed “Flood Risk” with a separate policy for Water Management. It is recommended that the policy be rewritten. The new wording is considered as part of the documentation on the Core Strategy.