Work so far.

MapleLodge

drainagestrategy.

2019

Summary 2 Our drainageinvestigations so far 5

Findings so far 7

Our next steps 9

Appendix 10

Contents.

1

* ofwat.gov.uk/publication/drainage-strategy-framework-for-water-and-sewerage-companies-to-prepare-drainage-strategies.

** The estimated delivery timeline is dependent on factors including weather conditions, risks and costs, and is, therefore, open to change.

We’re working on how to reduce the risk of sewer flooding, pollution, blockages and restricted use of toilets and bathrooms in the Maple Lodge area. For us, it’s a priority. We’ll continue to work with our customers and other stakeholders, such as Buckinghamshire County Council, the local Parish Council and private land owners, to take action and make drainage improvements as quickly as possible.

Summary.

2

Our drainage strategy work

The drainage strategy process

From 2018onwards2015 - 2017**

‘Optionsappraisal’

‘Initialise/prepare’

‘Riskassessment’

‘Intervention’Stage 1 Stage 3 Stage 4Stage 2

We'rehere

What this means

Optionsappraisal

Initialise/prepare

Riskassessment

Intervention

Starting the process

including gathering

information and forming relationships

with stakeholders

Identifying the things that could prevent

drainage improvements,

and how to manage

them

Developing different

options for making

drainage improvements that work and are value for

money

Implementing selected drainage

improvements based on their

potential benefits outweighing their

costs and their priority

Drainage strategy framework stages and timescales

We began our drainage strategy work in 2015 by carrying out research and regularly meeting with customers who had been affected by drainage issues, as well as stakeholders in the area.

Since 2016, we’ve carried out some detailed investigations which have helped us to understand more about the drainage problems in the Maple Lodge area, and how they might be improved in the future.

We’ve followed the guidance from our regulators, Ofwat and the Environment Agency, for our drainage strategy work. The good-practice process they recommend has four stages*. We completed the first stage in 2016, which included a range of background research into the drainage issues in the area.

We’ve put together stages two and three of the drainage strategy process as their activities are closely linked. So far in this combined stage we’ve:

• Completed some detailed investigations in the Maple Lodge area and identified the others that need to happen

• Started to assess the risks in the area and how we’ll manage them, such as its growing population and rising groundwater levels.

Unfortunately, we haven’t been able to complete this combined stage for the Maple Lodge area as we’d hoped to do at this time. This is because of the unusually dry winter weather experienced across the majority of our region between 2016 and 2017, and throughout the following winter of 2017 and 2018. During these periods rainfall was mainly at, or below, the yearly average for our region, as illustrated by the maps*.

The dry winter weather stopped us from carrying out a number of the investigations that we needed to fully understand the root causes of the drainage issues. Therefore, we’ve been unable to finalise our proposals at this time, to improve the drainage issues in the Maple Lodge area for the long term.

3

Work so far

Cotswolds-East

Cherwell

Thame

Upper Thames

Ock

Chilterns-West

Loddon

Nth Downs-Hants

Wey-Greensand

Upper Mole

Lower Wey Nth Downs-Sth London

North London

Chilterns-East-Colne

Lee-Chalk

Roding

Lower Lee

South London

Cut

Cotswolds-West

Berkshire Downs

Enborne

Maple Lodge

Cotswolds-East

Cherwell

Thame

Upper Thames

Ramsbury

Ock

Chilterns-West

Loddon

Nth Downs-Hants

Wey-Greensand

Upper Mole

Lower Wey Nth Downs-Sth London

North London

Chilterns-East-Colne

Lee-Chalk

Roding

Lower Lee

South London

Cut

Cotswolds-West

Berkshire Downs

Enborne

Maple Lodge

Cotswolds-East

Cherwell

Thame

Upper Thames

Ock

Chilterns-West

Loddon

Nth Downs-Hants

Wey-Greensand

Upper Mole

Lower Wey Nth Downs-Sth London

North London

Chilterns-East-Colne

Lee-Chalk

Roding

Lower Lee

South London

Cut

Enborne

Cotswolds-West

Berkshire Downs

Maple Lodge

Annual rainfall from April 2016 to March 2017 compared to the yearly average across our region

Annual rainfall from April 2017 toMarch 2018 compared to the yearly average across our region

Annual rainfall from October 2017 to September 2018 compared to the yearly average across our region

Below average rainfall

Average rainfall (approximate)

Above average rainfall

Source: Thames Water, 2018, based on provisional and unverified data from the Environment Agency.

* For more information on current rainfall and groundwater conditions across our region please use this link: thameswater.co.uk/Help-and-Advice/Water-Quality/Where-our-water-comes-from/Reservoir-levels-and-rainfall-figures.

* You can find out more about the 21st Century Drainage Programme and Drainage & Wastewater Management Plans, by using the following link: water.org.uk/policy-topics/managing-sewage-and-drainage/drainage-and-wastewater-management-plans.

4

We’re extending our drainage strategy work in the Maple Lodge area. We’ll continue to assess and respond to the drainage risks that we identify through our ongoing monitoring of rainfall and groundwater levels, together with the flooding trigger levels that we’ve developed to manage the volume of flows within our sewers.

If the weather is much wetter over the coming months, we’ll also be able to finalise our investigations and drainage strategy for improving the drainage issues in the Maple Lodge area for the long term. To develop this our first activity will be to assess all of our proposed long-term drainage interventions. As well as looking at their potential customer benefits and investment costs, we’ll also assess how they support the fundamental aims of the 21st Century Drainage Programme*.

We’ve joined together with more than 40 organisations from across the water industry to support this groundbreaking programme. We all recognise the current and future challenges we face from factors including climate change and population growth, and we’re committed to creating more resilient drainage systems to cope with them.

The 21st Century Drainage Programme has also highlighted the need for water companies to adopt a consistent approach to drainage and wastewater planning. In response, by 2022, and at regular intervals after that, each water company in England and Wales will produce a new document called a Drainage & Wastewater Management Plan (DWMP)*. This will build on the good-practice drainage strategy framework process we’ve been using, and the drainage work we’ve already carried out.

Our DWMP will detail the current and emerging drainage risks across our region, our customers’ drainage issues and needs, the operational requirements we have and the technological and innovative advancements we’ll use. Overall, our DWMP will detail the long-term interventions we propose to achieve safe and reliable drainage and wastewater services within our region, for many years to come.

Therefore, the long-term drainage interventions that we select for implementation in the Maple Lodge area within our drainage strategy, will also feature in detail within our DWMP when it’s introduced. The new planning process and DWMP documentation will then replace the existing drainage strategy process and documentation.

Our ongoing work

Since 2016, we’ve carried out a number of investigations in the Maple Lodge area. They’ve helped us to understand more about the root causes of the area’s drainage problems, and how they might be improved in the future. So far our drainage investigations have included:

Our drainage investigations so far.

Customer and stakeholderconsultations We sent sewer-flooding questionnaires to our customers in the Maple Lodge area. We wanted to hear their drainage views and experiences, and to pinpoint problem locations. We also met with Buckinghamshire County Council and the local Parish Council regarding the planned housing developments in the Maple Lodge area, and how they will affect our existing assets and services.

Flow andasset surveys We checked the performance of our sewer network and the main assets we operate in the Maple Lodge area. We placed four rain gauges and twelve flow monitors within the sewers across the area. We also installed sewer rising main monitors at our sewage pumping stations, carried out manhole inspections and surveyed over 700 metres of the local sewer network using CCTV. This helped us to understand if our assets were contributing to the area’s drainage issues.

Impermeable areasurveys We measured the size of the impermeable area that’s connected to our sewers, including driveways made out of concrete or tarmacadam, and roofs. As rainwater flows from these areas into our network, rather than naturally draining away, it reduces the capacity and performance of our sewers and contributes to the drainage issues in the Maple Lodge area.

The surveys enabled us to identify locations in the area where the reduction or removal of impermeable areas is possible. In such instances sustainable drainage systems (often called SuDS*), could potentially be put in place in customers’ homes. This would help to slow down the flow of rainfall that enters our sewers during wet weather, and reduce the risk of sewer flooding.

5* SuDS are a range of drainage methods that can be used for slowing the speed at which surface water

enters our sewer network, and/or redirecting surface water away from our sewers. Examples of SuDS include using water butts, planters and paving that allows water to pass through it.

Sitewalkovers We investigated beyond our sewer network to find the root causes of other drainage issues in the area, such as river flooding and surface water runoff from agricultural land that may find its way into our sewer manholes. To help us to do this we hosted ‘walkovers’ at known flood sites in the area, with members of the local Parish Council.

Flooding 'trigger-levels' analysis We analysed the groundwater levels and river levels that were recorded during periods when there was flooding and other drainage issues in the area. We wanted to use this information to help us to define groundwater thresholds or ‘trigger levels’.

If these levels are reached in future, it could trigger our local response teams to carry out specific work to help prevent the flooding and drainage issues, such as removing excess water from our sewers using lorries that carry large water tanks, (we call them tankers).

x

x

6

We found Our assets such as sewers, manholes and pumps can be damaged in many ways, from age deterioration and blockages, to roadworks and tree roots. We’ve identified a number of defects that are allowing groundwater to enter our sewers. We’ll continue to inspect our assets, and try to fix any problems we find that are causing significant drainage issues.

Our proposed interventions• Reinforce sewer repairs with watertight lining. • Install new monitors at some of our local sewage pumping

stations, identified with red circles on the map, while also analysing the quality of monitored data at a number of other sewage pumping stations in the area. This will help us to keep a close check on how our sewage pumping stations are working and alert us to any issues.

Benefits of proposed interventions • Increase the capacity of our sewers and reduce the risk of sewer

flooding through repairing damaged pipework, sealing manhole covers and monitoring the operation of our sewage pumping station flows.

• Help us to maintain our wastewater services to customers in the Maple Lodge area during wet weather.

Delivery timeframeProposed for 2018- 2025.

We found Groundwater can enter our sewers when levels are high which reduces their capacity and increases their risk of flooding. There’s a strong link between the rising groundwater levels across the Maple Lodge area, and the drainage issues some of our customers have experienced, including sewer flooding and restricted use of their toilets and bathrooms.

Our proposed interventions• Monitor groundwater levels in our control centre and inform our

response teams if thresholds are passed. The teams will carry out an action plan specifically designed for the area to reduce or remove the flooding issue, such as checking sewer water levels and using tankers to take away excess flows from our sewers.

• Install monitors in our sewage pumping stations to help us to identify when our pumps are operating more than we expect them to. This could mean that groundwater is entering our sewers which shouldn’t be there. We’d investigate further to find out the root cause of the drainage issue affecting our sewage pumping stations.

Benefits of proposed interventions • Give us the capability to predict and prevent some sewer

flooding issues before they occur, through monitoring groundwater levels and sewage pumping station flows.

• Increase the capacity of our sewers and reduce the risk of sewer flooding when tankering is used to remove excess flows from our sewers.

• Help us to maintain our wastewater services to customers in the Maple Lodge area during wet weather.

Delivery timeframeProposed for 2018- 2025.

The drainage investigations that we’ve carried out so far have helped us to understand some of the root causes of the drainage issues in the Maple Lodge area. We propose to carry out further drainage activities (we call these interventions), to improve the drainage issues we’ve found so far, in both the short and long term.

Our findings from the drainage investigations we’ve completed so far include:

Findings so far.

Chalfont St Peter

0 0.55 1.10.275 Miles

Chalfont St Giles

West Hyde SPSNorthoft Road(Chalfont St Peter) SPS

Denham Way(West Hyde) SPS

Seer Green SPS

West Hyde SPSNorthoft Road(Chalfont St Peter) SPS

Denham Way(West Hyde) SPS

Seer Green SPS

Proposed sewage pumping station monitor installation.

7

Maintainingour assets

Rising ground-water levels

For more information on the ‘Lift & Look’ and CCTV surveys that we carried out as part of this work, please see the Appendix.

For more information on the flooding ‘trigger-level’ survey and analysis that we carried out as part of this work, please see the Appendix.

8

We found Over time property roof downpipes may mistakenly become connected to our foul sewers. Misconnected property roofs are contributing significant volumes of rainwater into our sewer network. This is reducing the capacity of our sewers and adding to the area’s drainage and flooding issues.

Our proposed interventionWorking with Buckinghamshire County Council and the local Parish Council to:• Investigate if roof drainage can be separated from our sewer

network. It may be possible to do this jointly through customer campaigns and the installation of SuDS for our customers in affected areas. The type of SuDS we propose are planters, as illustrated, which would be connected to the downpipes of affected property roofs.

Benefits of proposed intervention • Increase the capacity of our sewers and reduce the risk of sewer

flooding, by limiting the volume of water that enters our sewers from misconnected property roofs.

• Help us to maintain our wastewater services to customers in the Maple Lodge area during wet weather.

Delivery timeframeProposed for 2020- 2025.

Exis

ting

dow

npip

e

Rainwater from roofs

For more information on the impermeable area survey that we carried out as part of this work, please see the Appendix.

Proposed SuDS planters for misconnected roof drainage.

Over the coming months our work in the Maple Lodge area will continue to be focused on:

Our next steps.

x

x

Further drainageinvestigations Carrying out further drainage investigations including checking the performance of our local sewage pumping stations. We’ll make sure that they’re working at their best and not more than they should be, which can often mean water is entering our sewers that shouldn’t be there. We’ll investigate further any issues we find to help us to improve drainage in the Maple Lodge area.

Stakeholder engagementAttending the local Parish Council meetings and Buckinghamshire County Council meetings to regularly update on our findings and progress. We’ll also offer advice on how everyone can help improve drainage in the area.

Flooding ‘trigger-levels’ planImproving the accuracy of our flooding warnings or ‘trigger levels’, and planning how we can best respond if they’re triggered. This will help us to manage the flooding in the area and reduce other drainage issues.

Maintaining our sewers Ongoing sewer inspections in the area and trying to fix any problems we find that are causing significant drainage issues.

9

Our next steps are to extend our work in the Maple Lodge area. Over the coming months, if this is a much wetter period, we’ll be able to carry out further detailed investigations and finalise our proposals for improving the area’s drainage issues for the long term.

Our next steps

This will be the core of our drainage strategy, and an essential part of the Drainage & Wastewater Management Plan (DWMP) that we’ll produce for our whole region by 2022.

The long-term drainage interventions that we select for implementation

in the Maple Lodge area within our drainage strategy, will feature in greater detail within our DWMP when it’s introduced.

The new planning process, and DWMP documentation outlining our proposals for achieving safe and

reliable drainage and wastewater services within our region for many years to come, will then replace the existing drainage strategy process and documentation.

Appendix.

The ‘Lift & Look’ survey, together with CCTV footage that we recorded in 700 metres of sewer network, found points where groundwater is entering our network through cracks and other defects. When we find groundwater entering our sewers we class it as a: seeper, runner or gusher, with a gusher being the most severe and creating the biggest problem for our network’s performance.

The main findings from our surveys so far include:

The Broadway We found groundwater entering a section of sewer along The Broadway, in Amersham, as identified in dark green in Figure 1. An example from our CCTV survey findings in this area can be seen in Figure 2. Reducing the number of locations where groundwater enters our sewers in the Maple Lodge area will help to increase capacity in the sewer network. Greater capacity in our sewers means fewer incidents of flooding for our local customers. We’ll make sure that the sewer is repaired as a priority.

The CCTV surveys that we carried out along this section of the sewer network, didn’t identify any further defects or locations where groundwater is entering our sewers. However, they did find locations where there were restrictions in our sewers caused by fat being poured down drains. We’ll jet-clean these sections of sewer to make sure that the wastewater they transport for our customers can continue to be carried safely.

‘Lift & Look’ and CCTV surveys

During our recent investigations we carried out a ‘Lift & Look’ survey in the Maple Lodge area. We went down into our sewer network to inspect the condition of our sewers and to see how they were working.

Figure 1 Location of groundwater entering our network along The Broadway, in Amersham.

Figure 2 Groundwater entering the sewer along The Broadway, in Amersham.

0 0.025 0.050.0125 Miles

10

This additional water reduces the capacity of our sewers, stops them from working properly and contributes to the drainage issues in the local area. Our survey measured the size of the impermeable area in the Maple Lodge area and we investigated its impact on our sewer network.

Our survey in the Maple Lodge area found:

• Examples of rainwater caught from the roofs of local properties flowing directly into our sewer network, rather than draining away naturally

• The majority of the rainwater collected from roofs came from properties located in the Chalfont St Peter area.

We recommend investigating the possibility of removing roof drainage from our sewer network with the local Parish Council and Buckinghamshire County Council. This could increase the capacity in our sewers and reduce the risk of flooding incidents for local residents.

An example of our impermeable area survey results can be found in Figure 3. The colour-coding represents the locations that surface water runoff currently drains to in the Maple Lodge area, the red areas are currently draining into our sewer network.

Cobbins

Campions

Langsett

Whinbrook

Tudor Lodge

Laurel House

Silvanus

Southfield

Ti Soleil

Hambleton

White House

Deep Acre

Kilima

3

W y ndyate

2

Turville

LIND

END

RIVE

Chalfont View

10

63

Brook

9

63a

Salix

The

Clisby

57.9m

65.5m

LOW

ERR

OAD

WILLO W CLO SE

Und

WardBdy

11

5

54

2a

FF

RH

32

12

23

15

11

43

76

H ILL RISE

44

SCHOOL LANE

43

El Sub Sta

40

Craiglea House

Dr ain

Drai n

C R

AME RS

HAMRO

AD

Dra in

Cycle Way

89a

87

89

85

3

Riv erM

is bour ne

65

65 a

69

5

FF

KIN

GS

WA

Y

86

12

6

2

4

36

Side

1

1214

58.2m

40

1

40a

62a

2

62

7

2

74

10

4

18

84

4

Garage

1

Webs

5

41

White

Riv er Misbourne

El Sub Sta

Chiltern Park

61

42

56

1

55

65.2m

2

58

60

C GF

RH

16

CR

7

FF

15

10

8

8

1

15

7

CR

ESC

ENT

26

The Grange

7

GRANGE ROAD

2

12

6

CROFT CLOSE

Old Mead

15

11

HILL RISE

22

29

22

28

80.8m

77.1m

27

RH

31

4

4

1

16

32a

30

32

8

10

Linden Lea

45

AM

ERS

HA

MR

OA

D

Row an House

53

30

46

43

HIT H

ERM

EAD

OW

8

1

LOW

ERR

OAD

37

2

25

3

39

El Sub Sta

1 4

21

House

1

Acacia

15

11

El Sub Sta

4

2

HILL

CROFT ROAD

5

Wa rd

Bd y

17

2

36

Un d

FF

2

14

6

25

R H

Ground

Games Courts

Burial

Chapel

Coppermill

The Dower Barn

12

6

CROFT

FF

Und

Games Courts

Edina

ED&

War

dBd

y

46

17

72

53

50

85.6m

53

46

74

78

Madasa

The Cottage

14

Oakdene

6

Tarn Cottage

LINC

OL N

RO

AD

35

Hawthorns

59.4m

11

GR

ANGE

FIE LDS

School (disused)

Chiltern Cottage

5

Balcomie

Willow Cottage

8

7

Works

27

15

7

59.1m

1to

9

58.2m

64

1

1

1

WOO

DSIDECL O

SE

6

WOODSIDE HILL

L IND

EN

DR

IVE

Mead

Willow Bridge

Swan Ridge

Cycle Way

73

83

El Sub Sta

1

CGF

971 0

11

1

15

LB

LAMBSCR O

FTW

AY

Def

34

Misbourne

R ISE

L OW

ER

RO

AD

2

22

ASHLEA ROAD

20

1

6

11

33

SCHOOL LANE

STM

ARY'SW

AY

24

26

16

Tra ck

GR

ANGE

CLO

SE

U nd

1

20

Holy Cross Convent

27a

7

18

64

33

FF

Maple Lodge IAS ResultsHighway Drain

Soakway

Foul

Permeable

Region

Watercourse

N

Impermeable area surveyImpermeable ground doesn’t allow rainwater to drain through it naturally such as driveways made out of concrete or tarmacadam, and roofs. Instead the rainwater from these areas pools together and often flows into our sewer network.

11

Figure 3 An example of impermeable area survey results for the Maple Lodge area.

Maple Lodgeimpermeable area survey

Highway drain

Soakway

Foul

Permeable

Region

Watercourse

We defined red and amber ‘trigger levels’, with red being the most critical warning sign. A red ‘trigger level’ tells us that sewer flooding is very likely and that rapid action is needed to reduce the drainage issues. An amber ‘trigger level’ warns that sewer flooding is possible.

If an amber level is reached our local team will check for unusually high activity at our sewage pumping stations. This could be a sign of groundwater entering our sewer network which shouldn’t be there. If a red level is reached the team will lift manhole covers in areas known to flood, to make sure that our sewers have a safe level of water in them, and we can reduce their risk of flooding.

Figure 4 is a graphical illustration of our flooding ‘trigger-level’ analysis for the Maple Lodge area. In Figure 4 the blue line illustrates the changing river level over time, the orange dotted line represents the groundwater levels and the grey dots indicate known sewer flooding incidents in the Maple Lodge area. The horizontal red and orange lines indicate our proposed red and amber warning ‘trigger levels’.

Figure 4 demonstrates the relationship between sewer network issues and high groundwater levels, particularly when the red warning ‘trigger-level’ threshold has been reached.

We propose to continually check and refine these flooding ‘trigger levels’ to improve their accuracy, particularly when ‘trigger levels’ have been reached.

Ongoing monitoring will increase our understanding of how our sewer network copes with different weather patterns and groundwater conditions. This will help us to plan how we can further reduce sewer flooding and drainage issues for our customers in the Maple Lodge area, now and in the future.

0.05

0.1

0

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

117.5

118.5

119.5

120.5

121.5

122.5

Groundwater levelRiver level Flooding incident

Riv

er le

vel

Gro

undw

ater

leve

l (m

AOD

)

11/2

6/20

1212

/24/

2012

1/18

/201

32/

12/2

013

3/9/

2013

4/3/

2013

4/28

/201

35/

23/2

013

6/17

/201

37/

12/2

013

8/6/

2013

8/31

/201

39/

25/2

013

10/2

0/20

1311

/14/

2013

12/9

/201

31/

3/20

141/

28/2

014

2/22

/201

43/

19/2

014

4/13

/201

45/

8/20

146/

2/20

146/

27/2

014

7/22

/201

48/

16/2

014

9/10

/201

410

/5/2

014

10/3

0/20

1411

/24/

2014

12/1

9/20

141/

13/2

015

2/7/

2015

3/4/

2015

3/29

/201

54/

25/2

015

5/22

/201

56/

17/2

015

7/13

/201

58/

7/20

159/

3/20

159/

28/2

015

10/2

3/20

1511

/17/

2015

12/1

2/20

151/

6/20

161/

31/2

016

2/25

/201

63/

23/2

016

4/17

/201

65/

12/2

016

6/6/

2016

7/1/

2016

7/26

/201

68/

20/2

016

9/14

/201

610

/9/2

016

Red trigger level

Amber trigger level

12

x

x

Flooding ‘trigger-levels’ analysisWe analysed the groundwater levels and river levels that were recorded during periods when there was flooding and other drainage issues in the Maple Lodge area. We wanted to use this information to set up groundwater warning signs or ‘trigger levels’ so that we could investigate the impact this groundwater has on our sewers.

Groundwater levels and River Misbourne levels Vs Flooding incidents

Figure 4 An example of ‘trigger-level’ analysis for the Maple Lodge area.