Branksome Care Home Extension

Flood Risk Assessment

June 2014

Document Ref 020PDL-002

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Rev Date Description/Amendment Author Appr’d

1 02/06/2014 Final for issue to client EA JW

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Originator:

Eric Agyemang

Date 27/01/2014

Approved By

John Ward.

Date 02/06/2014

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Contents

1. Introduction ............................................................................................................. 3

2. Development Description, Location and Soil Type................................................... 4

2.1. Development Description ........................................................................................... 4

2.2. Development Location ............................................................................................... 4

2.3. Soil Type .................................................................................................................... 4

3. Sequential and Exception Tests .............................................................................. 5

4. Definition of the Flood Hazard ................................................................................. 6

4.1. Potential Flooding Sources ........................................................................................ 6

4.2. Ground Water ............................................................................................................ 6

4.3. Surface Water ............................................................................................................ 7

4.4. Piped Drainage .......................................................................................................... 8

5. Flooding Probability ................................................................................................. 9

5.1. Flood Risk Map .......................................................................................................... 9

5.2. Strategic Flood Risk Assessment ............................................................................. 10

5.3. Climate Change ....................................................................................................... 11

6. Detailed Development Proposals .......................................................................... 11

7. Flood Risk Management Measures ....................................................................... 11

8. Off Site Impacts ..................................................................................................... 12

9. Residual Risks ...................................................................................................... 12

10. Conclusions ....................................................................................................... 13

11. Recommendations ............................................................................................. 13

12. Report Authors................................................................................................... 13

13. Disclaimer .......................................................................................................... 14

Originator:

Eric Agyemang

Date 27/01/2014

Approved By

John Ward.

Date 02/06/2014

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1. Introduction

Phoenix Adam Limited (PAL) was appointed by Pharaoh Designs (West Midlands) Ltd to

undertake a Flood Risk Assessment (FRA) in support of a full planning application by

Four Seasons Health Care for a proposed extension of Branksome Care Home in

Buxton, Derbyshire.

The proposed redevelopment site lies within the Environment Agency’s (EA) Flood Zone

1, which means that it has a low probability risk of flooding from rivers.

This document details the findings of the FRA undertaken in May 2014. The degree of

detail entered into any FRA is dependent upon the scale and potential impact of the

proposed development. This FRA has been developed based on guidance from the

National Planning Policy Framework (NPPF) March 2012 and its accompanying

technical guide and considers flooding from all sources, not just main rivers.

Originator:

Eric Agyemang

Date 27/01/2014

Approved By

John Ward.

Date 02/06/2014

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2. Development Description, Location and Soil Type 2.1. Development Description

The proposed development is an extension of the existing Branksome Care Home

building, at St Johns Road, Buxton. The 12 bedroom extension will comprise ground

floor and first floor levels. Other external works will comprise landscaping, planting and

additional car parking.

2.2. Development Location

The development site is located in the historic spa town of Buxton in the Peak District,

the Branksome Care Home is situated close to all local amenities. The River Wye lies

approximately 30m to the south.

The town centre lies to the east. The Grid Reference of the site is E404898 N373428.

2.3. Soil Type

The broad classification soil types, or superficial deposits (formerly known as drift

geology), as described by the British Geological Survey for the area are indicated in the

map below.

The site lies within the pale yellow map zone which is described in detail below.

Site Location

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Date 27/01/2014

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Superficial deposits are the youngest geological deposits formed during the most

recent period of geological time, the Quaternary, which extends back about 2.6 million

years from the present. They rest on older deposits or rocks referred to as bedrock.

The superficial deposits for this site are described as;

‘Alluvium - Gravel, Sand, Silt And Clay. Local environment previously dominated by

rivers. These rocks were formed from rivers depositing mainly sand and gravel

detrital material in channels to form river terrace deposits, with fine silt and clay from

overbank floods forming floodplain alluvium. Normally soft to firm consolidated,

compressible silty clay, but can contain layers of silt, sand, peat and basal gravel. A

stronger, desiccated surface zone may be present’.

3. Sequential and Exception Tests

The NPPF Technical Guide paragraphs 3 and 4 describe the FRA procedure. There are

two tests to be applied in the initial assessment; these are called the sequential test and

the exception test.

The aim of the Sequential Test is to direct development away from areas at risk of

flooding. Development should be directed to Flood Zone 1 wherever possible, and then

sequentially to Flood Zones 2 and 3, and to the areas of least flood risk within Flood

Zones 2 and 3.

Site Location

Originator:

Eric Agyemang

Date 27/01/2014

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Date 02/06/2014

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From undertaking the sequential test it is clear that the proposed development lies within

Flood Zone 1 which is assessed as having a less than 1 in 1000 annual probability of

river flooding in any year (<0.1%).

The development is classified as ‘more vulnerable’ according to Table 2 of the NPPF

Technical Guide.

As the proposed development lies within Flood Zone 1 - Low Probability, the Exception

Test is not applicable.

Under the current NPPF Technical Guide the proposed development is considered

appropriate for planning consent.

4. Definition of the Flood Hazard 4.1. Potential Flooding Sources

Flood Source Relevant to Site?

Justification

River / Watercourse No Outside the functional flood plain of the River Wye

Sea / tidal influence No Non coastal and outside the zone of tidal influence

Ground Water Yes Geographic location and nature of the underlying soil

Surface Water Yes Geographic location and nature of the underlying soil

Piped Drainage Yes Both wastewater and storm water systems lie close to the site boundary.

4.2. Ground Water

The SFRA conducted by Halcrow Group in 2008 did not find any specific problems with

flooding from groundwater in this area.

However, as the site is low lying compared to the land immediately to the north east and

is also located adjacent to the river Wye the water table may be relatively close to the

surface. It is possible that natural seasonal variation in water table depth could result in

groundwater flooding at the site. It is also possible, given the geography and the nature

of the underlying soils, noted in section 2.3 above, that rainfall induced changes in

ground water level may also pose a flood risk.

Originator:

Eric Agyemang

Date 27/01/2014

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Ground water flooding is unlikely to reach depths that would cause significant disruption

to the normal use of the site. On balance, the hazard arising from this flooding source is

low.

4.3. Surface Water

The land slopes mildly from an elevation of 343m above sea level at the A5004

Manchester Road (North of the site) to 300m above sea level at the site location. This

equates to a fall of 43m over a distance of around 500m, an average gradient of 1 in 12.

During severe rainfall events there is a risk of surface water runoff being routed by

overland flow paths to the site location. This is known commonly as ‘flash flooding’.

The hazard from flash flooding will be greatest during the summer months during high

intensity, short duration storms may exceed the capacity of the below ground drainage in

the area.

Localised overland flood routing would be along Carlisle Road towards Branksome Care

Home. Overland flows would split at the junction with the Paddock heading west and

south of the site. The site itself is protected by a high stone wall which would divert flow

View north along Carlisle Road with

Branksome Care Home to the left.

View north south towards Branksome

Care Home from Carlisle Road.

Originator:

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Date 27/01/2014

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away from the building. The building itself is low lying compared to the road and there is

some residual risk of road channel flows overtopping the driveway entrance level.

The impact of flash flooding is likely to be of short duration with shallow flows of high

velocity along Carlisle Road. However, any overland flow reaching the site would slow

down and ‘pond’ around the building before finding an exit route to the river wye. Flows

across the site would be shallow and of low velocity.

The hazard from this flooding source is low.

4.4. Piped Drainage

Public sewer records held by Severn Trent Water Ltd indicate both wastewater and

storm water piped drainage systems along The Paddock to the north, Carlisle Road to

the east and St John’s Road to the south, as shown in the plan below.

The 225mm diameter pipelines shown in red are wastewater sewers which also take

some surface water. These are known as combined sewers and can be considered to

be a flood risk for the site during extreme rainfall events.

The 225mm diameter pipeline shown in blue is a surface water sewer which takes roof

and road runoff only. This too can be considered to be a flood risk for the site during

extreme rainfall events.

Flood register from Severn Trent Water does not show any historical flooding at the site

and its immediate vicinity over the past 20 years.

Site Location

Originator:

Eric Agyemang

Date 27/01/2014

Approved By

John Ward.

Date 02/06/2014

Revision No

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The site location is low lying compared to the surrounding drainage network. However,

this is not considered a risk factor as we understand the site is hydraulically separated

from the gravity network in Carlisle Road by a private pumping station.

The hazard from this flooding source is low

5. Flooding Probability 5.1. Flood Risk Map

The EA Flood Risk Maps identify the likelihood and severity of flooding across the

England and Wales. The areas shown in blue on each side of a river channel is Flood

Zone 3 and the extended area in cyan is Flood Zone 2. All other areas on the map are

Flood Zone 1.

At its closest, the River Wye lies just 20m from the southern edge of the proposed

building extension. However, the latest EA flood maps indicate that the development site

is outside the functional flood plain of the river.

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315.944313.468

Nodeground (m AD)

Linkwidth (mm)us inv (m AD)ds inv (m AD)

m AD

303.0

319.0

305.0

307.0

309.0

311.0

313.0

315.0

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m

The Paddock

Carlisle Road

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It is evident from the Flood Risk Map that the proposed development falls within Flood

Zone 1 – Low Probability risk of flooding from rivers or sea. This zone comprises land

assessed as having a less than 1 in 1000 annual probability of river or sea flooding in

any year (<0.1%).

5.2. Strategic Flood Risk Assessment

The SFRA produced for the Peak District Sub Region by Halcrow Group has been used

to inform the relevant sections of this report. This study was completed and the final

report published in September 2008.

A key outcome of the SFRA process is the establishment of the Sequential Test in

accordance with paragraph 3 of the NPPF Technical Guide. The SFRA informs the

planning process, by taking into account flood risk through all sources and the impacts

of climate change. The SFRA categorises the area in terms of the likelihood (or

probability) that flooding will occur.

Site Location

Originator:

Eric Agyemang

Date 27/01/2014

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John Ward.

Date 02/06/2014

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5.3. Climate Change

The UK Climate Impacts Programme (UKCIP) is responsible for promoting use of the

UK Climate Projections, alongside other tools and guidance, to help organisations to

prepare for the unavoidable impacts of climate change. UKCIP co-ordinated the

stakeholder input into the development of the Projections. UKCIP is principally funded by

Defra, and provides its services free of charge. UKCP09 is the short name for UK

Climate Projections. Detailed guidance on the use of the Projections can be found on

the UKCP09 website http://ukcp09.defra.gov.uk alongside information on how to access

further training in using them.

There is an increasing body of scientific evidence that the global climate is changing as

a result of human activity. As a result, sea levels will rise globally. In addition to this

there is a predicted increase in rainfall and stormy weather. This could all have

significant impacts on the likelihood and frequency of flooding.

Within the Derbyshire / Peak District area winter precipitation is predicted to increase by

between 10% and 20% by the 2080s. It seems highly likely that one of the effects of

climate change will be flooding on a larger scale in most winters than has previously

been recorded.

UKCP09 also predicts increases in summer drought followed by heavier rainfall which

could increase the frequency of flash flooding.

6. Detailed Development Proposals

The proposed 12 bedroom extension will comprise ground floor and first floor levels.

Other external works will comprise landscaping, planting and additional car parking. No

absolute levels were available but we understand that the proposed finished level of the

ground floor is to be around 300mm below the existing building floor level.

Based on Table 2 of NPPF Technical Guide, Flood Risk Vulnerability Classification, the

land use for residential care homes is “More Vulnerable”. This means that the Building is

required to be operational even during a flooding event.

7. Flood Risk Management Measures

It is unlikely that groundwater, surface water or piped drainage flooding would breach

the door thresholds. The site slopes towards the river and any surface water would find

natural overland flow paths away from the building.

Originator:

Eric Agyemang

Date 27/01/2014

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John Ward.

Date 02/06/2014

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Based on the assessment of current data, no specific flood risk management measures

are required at this site.

However, some flooding resilience measures are appropriate. The layout and

landscaping of the site should aim to route flood water away from the building and avoid

creating hazards to access and egress routes. The CIRIA publication ‘C635 Designing

for exceedance in urban drainage’ provides design guidance and examples of good

practice.

8. Off Site Impacts

There will be a slight increase in the overall impermeability of the site due to the

development works. The current surface water and wastewater discharges will

marginally increase.

Wastewater flow increase will reduce the capacity of the wastewater network which

could increase flood risk from local piped drainage. We understand that wastewater

from the site is transferred to the public sewer system via a private pumping station. Any

increase in discharge rate would need to be agreed with Severn Trent Water Ltd.

Surface water flow increase, if connected to the surface water pipe network may have a

significant localised impact on flood risk. To mitigate this risk the EA strongly

encourages a Sustainable Drainage Systems (SuDS) approach to the management of

surface water for all new developments. The SuDS principle is to try to replicate natural

drainage systems using cost effective solutions with low environmental impact.

SuDS offer significant advantages over conventional piped drainage systems in reducing

flood risk by reducing the quantity of surface water run-off from a site and the speed at

which it reaches water courses, promoting groundwater recharge, and improving water

quality and amenity. The range of SuDS techniques available means that a SuDS

approach in some form will be applicable to any development.

9. Residual Risks

As flood resilience measures only are considered advised for this development,

flooding from the sources described above remains a residual risk, though the risk is

considered low.

Projected climate change effects may increase the risk of flooding from all sources.

However these effects are not considered significant over the anticipated design life of

the proposed building.

Originator:

Eric Agyemang

Date 27/01/2014

Approved By

John Ward.

Date 02/06/2014

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10. Conclusions

This FRA demonstrates that flood risk issues at this site are manageable and that future

site users can be safeguarded for the lifetime of the development.

This FRA demonstrates that:

• The development site is situated in Flood Zone 1, having a less than 1 in

1000 annual probability of river flooding in any year (<0.1%).

• There is a low risk of flooding at the site from all other sources.

• The development should have minimal off site impact on flood risk to others.

11. Recommendations

Flood resilience measures are recommended. The layout and landscaping of the site

should aim to route flood water away from the building and avoid creating hazards to

access and egress routes. The CIRIA publication ‘C635 Designing for exceedance in

urban drainage’ provides design guidance and examples of good practice.

The use of SuDS to control surface water runoff from the development site is

recommended. SuDS offer significant advantages over conventional piped drainage

systems in reducing flood risk by reducing the quantity of surface water run-off from a

site and the speed at which it reaches water courses, promoting groundwater recharge,

and improving water quality and amenity.

12. Report Authors

Eric Agyemang graduated with an MSc in Water and Environmental

Engineering from the University of Surrey in 2004. Eric has over 8 years

water industry experience specialising in hydraulic and hydrological

modeling of urban drainage networks to alleviate flooding and river water

quality issues.

John Ward is a Chartered Water and Environmental Manager. John has

over 25 years water industry experience in urban drainage network

hydraulic, hydrological and water quality modelling. He has specialised in

urban drainage area planning and strategic options development to meet

EU directives on flooding.

Originator:

Eric Agyemang

Date 27/01/2014

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John Ward.

Date 02/06/2014

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13. Disclaimer This assessment report has been prepared by PAL with all reasonable skill, care and

diligence and taking account of the scale and nature of the development. Information

presented in the report is based on interpretation of the data collected which has been

accepted from third parties in good faith as being current, accurate and valid.

This report is for the exclusive use of Four Seasons Health Care. No warranties or

guarantees are expressed or should be inferred by any third parties. This report may not

be relied upon by other parties without written consent from PAL.

PAL disclaims any responsibility to the client and others in respect of any matters

outside the agreed scope of the assessment.

Phoenix Adam Ltd

Unit 7 Riverside Business Park

Buxton Road

Bakewell

DE45 1GS

Tel: 01629 813330 email: info@phoenixadam.com web: www.phoenixadam.com