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Foul Water Management Strategy

Land at Barty Farm

Roundwell

Bearsted

ME14 4HL

RMB Consultants (Civil Engineering) Ltd

December 2014

RMB Consultants (Civil Engineering) Ltd Land at Barty Farm, Roundwell, Bearsted, ME14 4HL Foul Water Management Strategy

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RMB Consultants (Civil Engineering) Ltd

39 Cossington Road

Canterbury

Kent

CT1 3HU

Tel 01227 472128

www.rmbconsultants.co.uk

This report has been prepared by RMB Consultants (Civil Engineering) Ltd in accordance with the

instructions of their client for their sole and specific use. Any other persons who use any information

contained herein do so at their own risk.

© RMB Consultants (Civil Engineering) Ltd 2014


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CONTENTS

1. Background and Introduction 3

2. Development Description and Location 4

Development Location

Development Proposals

3. Policy Background 6

Maidstone Borough Council Strategic Flood Risk Assessment

Local Plan Regulation 18 Consultation 2014

Water Cycle Study 2010

Maidstone Borough Council Infrastructure Delivery Plan - March 2014

4. Site Characteristics 11

5. Foul Drainage Strategy 12

Existing Infrastructure

Strategy Approach

6. Conclusion 15

Appendix A Southern Water Capacity Check

Appendix B WinDes Model Details


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1. Background and Introduction

The following Foul Water Management Strategy accompanies a planning application submitted to

Maidstone Borough Council. The planning application is for residential development on land at

Barty Farm, Roundwell, Bearsted, ME14 4HL.


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2. Development Description and Location

Development Location

The site is located at Barty Farm, Roundwell, Bearsted, Figure 1. The site is situated to the north

of Roundwell. It is a greenfield site that covers 4ha.

Figure 1. Site location plan.

The site has been accepted as a potentially suitable site for development in both the 2009 and

2014 Maidstone Borough Council Strategic Housing Land Availability Assessments (SHLAAs).

Development Proposals

An outline planning application is proposed, with all matters except access reserved for future

determination. An illustrative layout has been produced showing 100 dwellings, Figure 2.


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Figure 2. Proposed development.


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3. Policy Background

The management of foul water across the development has to comply with a number of local

policy documents adopted by Maidstone Borough Council.

Maidstone Borough Council Strategic Flood Risk Assessment

Maidstone Borough Council published a Strategic Flood Risk Assessment (SFRA) in 2008. It

identifies localised sewer flooding incidents reported in Bearsted, Figure 3.

The SFRA states that increases in development will also require an increase in the capacity of

the sewer system.

Figure 3. Bearsted sewer flooding incidents circled blue with the site circled red. (© Maidstone Borough

Council)

Local Plan Regulation 18 Consultation 2014

One of the key local issues identified within the Local Plan Consultation is:

Ensuring that applications for development adequately address:

i. the impact of climate change,

ii. the issues of flooding and water supply, and

iii. the need for dependable infrastructure for the removal of sewage and waste water.


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The following proposed policies are relevant to this foul water strategy:

Policy DM4 - Principles of good design

xii. In areas at risk of flooding, provide mitigation measures integral to the design of

buildings will be required;

xiii. Avoid inappropriate new development within areas at risk from flooding or mitigate any

potential impacts of new development within such areas;

Policy ID1 - Infrastructure Delivery states that:

1. Where development creates a requirement for new or improved infrastructure beyond

existing provision, developers will be expected to provide or contribute towards the

additional requirement being provided to an agreed delivery programme.

2. Detailed specifications of the site specific contributions required are included in the site

allocation policies. Development proposals should make provision for all the land

required to accommodate any additional infrastructure arising from that development.

Dedicated Planning Agreements (S.106 of the Town and Country Planning Act,1990) will

be used to provide the range of site specific facilities which will normally be provided on

site but may in exceptional circumstances be provided in an off site location or as a last

resort via an in-lieu financial contribution. In some cases, separate agreements with

utility providers may be required.

3. Where developers consider that providing or contributing towards the infrastructure

requirement would have serious implications for the viability of a development, the

council will require an "open book" approach and, where necessary, will operate the

policy flexibly.

4. Where there are competing demands for developer contributions towards the delivery of

infrastructure for new development proposals, the council will prioritise these demands in

the manner listed below, which ranks infrastructure types in order of importance:

Infrastructure priorities for residential development:

i. Affordable housing

ii. Transport

iii. Open space

iv. Public realm

v. Health

vi. Education

vii. Social services


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viii. Utilities

ix. Libraries

x. Emergency Services

Southern Water’s consultation response to the Local Plan Consultation states:

Southern Water is the statutory sewerage undertaker in Maidstone Borough. We have carried out

a site-by-site assessment of the sites identified in Policy H1 to determine whether capacity in the

local sewerage network is sufficient to meet the anticipated demand.

The assessment indicates that capacity is insufficient in the immediate vicinity of some of the

sites [including this site]. This is not a fundamental constraint to development. However, new or

improved infrastructure would need to be provided in parallel with the development.

Ofwat, the water industry’s economic regulator, takes the view that local infrastructure specifically

required to serve a development site should be paid for by the development. This ensures that

the cost is passed to those who directly benefit from it, and protects existing customers who

would otherwise have to pay through increased general charges. Southern Water will take future

income from customers into account, so that the development will only need to fund a proportion

of the total cost.

Connection to the sewerage network at the nearest point of adequate capacity is the mechanism

by which the development can provide the local infrastructure required to service the sites.

However, Southern Water has limited powers to enforce such connection. We therefore look to

the planning authority to support this approach in planning policies.

To this end we seek additional text to the site specific development criteria.

For this site the additional text is:

H1(17) Barty Farm, Roundwell Thurnham

Additional development criterion to be met:

Utility infrastructure

A connection is provided to the local sewerage system at the nearest point of adequate

capacity, as advised by Southern Water.


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Water Cycle Study 2010

The Water Cycle Study identifies sewerage as a key issue which, without additional investment, is

likely to limit development in and around Maidstone town.

It states:

There is limited spare capacity in some of the key existing sewers running through the centre of

Maidstone town, which has the potential to restrict the number of new homes which can be

provided to the south and east of Maidstone town. There is also restricted capacity at Allington

Sewage Pumping Station, which pumps sewage from Maidstone town for treatment at Aylesford

Wastewater Treatment Plant. These constraints can be overcome through investment in the

sewerage system; potential solutions to these problems, could involve either increasing the

capacity of the existing sewers, or providing new pumping stations and rising mains which could

bypass existing sewers. However, the need for such investment will have cost and timing

implications for potential new development, particularly in the south-east area adjacent to

Maidstone town. A more detailed technical study would be needed to assess the feasibility, cost,

and timing of these solutions.

Providing that the sewerage network problems can be overcome, wastewater capacity is not

expected to be a significant issue for the town of Maidstone, as Aylesford Wastewater Treatment

Plant has capacity of expansion if required.

Southern Water identified three ways in which the planning authority can assist in achieving

efficient and sustainable wastewater infrastructure:

a coordinated whole-site approach to large and/or mixed ownership sites will promote

sustainable sewerage networks, and prevent the proliferation of smaller, less efficient

networks

separation of surface water to separate sewers or drainage systems provides more

efficient use of the foul sewer, and reduces the risk of foul water flooding

on-site and off-site sewers serving new developments of ten or more dwellings should

be constructed to adoptable standards, to ensure that they operate effectively in the long

term.

Maidstone Borough Council Infrastructure Delivery Plan - March 2014

The Maidstone Borough Council Infrastructure Delivery Plan - March 2014 states that:

Significant off-site sewerage infrastructure will be required to serve the strategic locations in the

Maidstone Urban Area and at the RSCs. Southern Water has advised that the overall draft

strategy can be accommodated in infrastructure terms. The next step will be for Southern Water


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to assess the sites in more detail at public consultation to determine more detailed infrastructure

requirements.


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4. Site Characteristics

Topographical Survey - A detailed topographical survey has been carried out. The site slopes

from a high point in the west of 53mAOD (Above Ordnance Datum), west to a level of 47mAOD,

Figure 4.

Figure 4. Local topography.

The site is a greenfield site.


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5. Foul Water Management Strategy

Existing Infrastructure

Southern Water is responsible for the adopted foul drainage infrastructure in Bearsted.

The public sewer record has been obtained from Southern Water, Figure 5.

Figure 5. Public sewer record with site edged red. (© Southern Water)

There are public foul sewers running southeast to northwest along Roundwell. Pipe diameters

vary from 150mm to 175mm. The foul sewers in Roundwell are shallow at depths of less than 1m

in places.

A Level 2 Design Capacity Check was submitted to Southern Water in September 2014. The

capacity check was for 99 dwellings, less than the stated site capacity in the Local Plan. Sewers

for Adoption 7th Edition states that design flow rates for dwellings should be 4000 litres per

dwelling per day. The capacity check was for a flow of 4.6 l/s.

Southern Water’s response dated 14 October 2014, Appendix A, confirms that there is insufficient

capacity within the local network to accommodate a flow of 4.6 l/s from the proposed

development. Southern Water has identified the nearest point to the site where capacity is

currently available as Leeds WTW (Wastewater Treatment Works), approximately 2,600m to the

southeast.


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It also identifies off-site improvements to existing sewers to provide sufficient capacity, Figure 6.

Figure 6. Foul drainage network upgrades identified by Southern Water, marked in blue.

The upgrade works are extensive and would benefit the wider sewerage network as the capacity

of the Yeoman Way, Bearsted WPS is upgraded from 161 l/s to 188 l/s, significantly more than

the 4.6 l/s added to the system by the proposed development. The developer could only be

expected to contribute a proportion of any network improvement costs. This is in line with the

proposed Policy ID1 - Infrastructure Delivery.

The developer proposes to requisition foul sewers under Section 98 of the Water Industry Act

1991 to ascertain the most efficient mechanism for providing foul sewerage infrastructure which

could be either a new rising main to Leeds WTW or upgrading existing infrastructure to the west

of the site.

Strategy Approach

This Strategy provides a solution for the on-site foul drainage in line with Sewers for Adoption 7th

Edition.

The topography allows foul water to be drained by gravity to sewers within Roundwell. Depending

on the outcome of the sewer requisition a pumping station may be required on site to discharge

foul water to Leeds WTW. An illustrative foul drainage layout is shown in Figure 7.


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Figure 7. Illustrative foul drainage layout.

The strategy has been modelled using MicroDrainage WinDes software to ensure that the

strategy meets Sewers for Adoption 7th Edition requirements. Model output details can be found

in Appendix B. Specific points used for the design include;

all foul sewers with 10 or less connecting properties are modelled as 150mm diameter with a

gradient no flatter than 1:80;

sewers with more than 10 properties connected are 150mm at a slope no flatter than 1:150;

any sewer backdrops within the strategy are no less than 1m high.


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6. Conclusion

This Foul Water Management Strategy accompanies a planning application submitted to

Maidstone Borough Council. The planning application is for residential development on land at

Barty Farm, Roundwell, Bearsted, ME14 4HL.

The site is allocated for housing under the draft Local Plan.

Bearsted is served by public sewers. These drain to Maidstone to the west.

Southern Water’s response to new development proposed within the Local Plan states that:

The assessment indicates that capacity is insufficient in the immediate vicinity of some of the

sites [including this site]. This is not a fundamental constraint to development. However, new or

improved infrastructure would need to be provided in parallel with the development.

A capacity check has been submitted to Southern Water. The response indicates that foul water

capacity is available providing off-site improvement works are carried out. Southern Water has

identified two options to provide wastewater disposal:

i. Connection to Leeds WTW, 2,600m to the east.

ii. Improvements to the foul drainage network west of the site.

The developer proposes to requisition foul sewers under Section 98 of the Water Industry Act

1991 to ascertain the most efficient mechanism for providing foul sewerage infrastructure.

The upgrade works would benefit the wider sewerage network and the developer could only be

expected to contribute a proportion of any network improvement costs. This is in line with the

proposed Policy ID1 - Infrastructure Delivery.

This Strategy provides a solution for the on-site foul drainage in line with Sewers for Adoption 7th

Edition. The topography allows foul water to be drained by gravity to sewers within Roundwell.

Depending on the outcome of the sewer requisition a pumping station may be required on site to

discharge foul water to Leeds WTW.

This Strategy shows that that the development can be adequately served by foul sewers and that

sufficient capacity is available within the local network for the disposal of wastewater, providing

off-site improvement works are carried out. The proposals are considered to be acceptable from a

foul drainage perspective.


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Appendix A - Southern Water Capacity Check


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Appendix B - WinDes Model Details


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