Supplementary requirements for surface water drainage proposals

National Planning Policy requires Local Planning Authorities to ensure that flood risk is not increased due to development. To ensure this requirement is met and that developments are adequately drained, we have adopted the West Sussex County Council (WSCC) Policy for the Management of Surface Water [pdf]. We have also developed a surface water drainage proposals checklist [pdf] 495KB  for planning applicants and their consultants.

This checklist clearly outlines our expectations and requirements for surface water drainage proposals.  When submitted with a surface water drainage proposal it will enable our drainage engineers to review and evaluate the submission quickly and efficiently.

The checklist should be used when completing discharge of conditions applications or if the applicant wishes to avoid pre-commencement conditions relating to surface water drainage.

Before completing this checklist please read the guidance notes below.

In order to satisfy the surface water drainage verification planning condition, please read the  Surface Water Drainage Verification Condition Guidance v1.3.pdf [pdf] 198KB  (surface water drainage verification report [HTML]) for information.

Any proposed surface water scheme must consider sustainable drainage system (SuDS) principles.

SuDS selection

The following destinations must be considered for surface runoff in order of preference:

  1. Discharge into the ground (infiltration).
  2. Controlled discharge to a surface water body.
  3. Controlled discharge to a surface water sewer.

Notes:

  • surface water must not be discharged into the foul sewer system 
  • infiltration structures include soakaways, basins, swales and permeable paving 
  • open SuDS features are encouraged
  • structures that span individual property boundaries are discouraged
  • discharge to surface water bodies and surface water sewers must be restricted to an agreed rate

SuDS selection hierarchy is based on:

  • CIRIA C753 - The SuDS Manual
  • BS8582:2013 – Code of Practice for Surface Water Management for Development Sites
  • Approved Document H of the Building Regulations

Infiltration drainage design

Any infiltration drainage design must be supported by adequate winter groundwater monitoring data to determine the highest winter groundwater table.  Residential developments of five properties or more will require groundwater monitoring to be carried out between October and March inclusive.  The extent of monitoring required for smaller developments will be subject to agreement with our engineers but will need to capture likely peak groundwater levels during the winter period. This is likely to be during January or February but is dependent on certain factors, including the weather up to that point.

Freeboard is to be provided between the base of the infiltration structure and the highest recorded groundwater level identified in that location. Ideally this should be 1 metre where possible, as stated in the CIRIA Suds Manual guidance. However, on the coastal plain in particular, where geology dictates and where shallow perched/tidally influenced water tables are often present, this is unlikely to be achievable Irrespective of this, infiltration must still be fully considered. Therefore, to maximise this potential and avoid utilising other less favourable methods of surface water disposal, the bases of infiltration structures are permitted to be immediately above the peak recorded groundwater levels where it is deemed necessary. 

In areas where an aquifer is to be protected (subject to guidance from the Environment Agency) then a minimum 1 metre freeboard must be provided.

Suitable water treatment is required upstream to the point of discharge in all circumstances to minimise any groundwater pollution risk or detriment to the drainage network.

Infiltration rates for soakage structures are to be based on infiltration tests undertaken at an agreed time during the winter period and at the location and depth of the proposed structures. The infiltration test depth is also dependent on the peak groundwater levels recorded at that location, ensuring that the test depth does not exceed the depth to the peak groundwater level recorded. The infiltration tests must be carried out in accordance with BRE 365, CIRIA R156 or a similar approved method.

For the design, the infiltration rate must be applied to the sides of the infiltration structure only and the rate for the base must be zero, unless otherwise agreed. For infiltration basins or permeable pavements, the percolation rate is generally applied to the base only. 

All design storms must include a climate change allowance, as per https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances, on stored volumes or rainfall intensity.  All major applications must also include a 10% allowance for urban creep applied to the design.  Infiltration structures must cater for the critical 1 in 10 year storm event, (plus 40%) between the invert of the entry pipe to the soakaway and the base of the structure. The design must also have provision to ensure that there is capacity in the system to contain the critical 1 in 100 year storm event (plus 45%) on site.

The infiltration design should also drain 50% of its total volume in 24 hours or less for the 1 in 10 (plus 40%) critical storm event and also the 1 in 100 year (plus 45%) critical storm event if possible, to provide spare capacity for subsequent storms.

Restricted discharge

Discharge to a watercourse or surface water sewer must be restricted to the estimated mean greenfield runoff rate (Qbar) for all design storm events.  The calculations must be based on the positively drained area, rather than the entire greenfield site area. Runoff rates can be derived from IH124 or a similar approved method. 

For brownfield sites, the same criteria applies. If it is deemed that this is not achievable, evidence must be provided and flow should be restricted to as close to Qbar as possible, with a minimum requirement of 50% betterment.

Flow restriction is to be achieved using a suitable controlled outflow with a minimum outflow of 2 l/s, unless otherwise agreed, with satisfactory blockage mitigation measures specified. 

The storage design must include a climate change allowance, similar to that specified in the ‘Infiltration Drainage Design’ above.

Any storage design must be submitted with winter groundwater monitoring data and where applicable, floatation calculations, to ensure there will be no detrimental effect on the structure or storage. 

Flow exceedance routes

The drainage design should show flow routes through the proposed development, demonstrating where surface water will be conveyed for three types of flow:

1. Low flow routes

Regular flow from source control features such as permeable pavements should travel in low flow channels through the development in a controlled way contributing to landscape quality.

2. Overflows

In the event of local blockages or surcharge a simple overflow arrangement should allow water to bypass the obstruction and return to the management train sequence until conditions return to normal.

3. Exceedance routes

When SuDS are overwhelmed by exceptional rainfall, then exceedance routes are required to protect people and property.  These provide unobstructed overland flow routes from the development and should be considered for all drainage schemes.  Exceedance routes should also be protected from future changes in land use.

Landscaping and drainage

All sites must demonstrate that root potential areas of existing and proposed trees do not conflict with the proposed surface water drainage network. This is to ensure no future detriment to the infrastructure in terms of its structure and functioning.

Water quality

All developments must demonstrate provision of adequate treatment of surface water prior to discharge. Treatment at source and via a treatment train should be provided. Please refer to CIRIA C753 Chapter 26 for further details. For example, permeable paving is encouraged wherever possible.

Culverting a watercourse

Culverting (piping) a watercourse is not advised unless there is no alternative.  The resulting reduction in storage volume, flow capacity and habitat potential would be unacceptable.  Culverted watercourses are also more difficult to maintain due to the limited accessibility.

Land Drainage Consent must be sought from the Lead Local Flood Authority (WSCC), or us acting as their agent (land.drainage@arun.gov.uk), prior to starting any works (temporary or permanent) that affect the flow of water in the watercourse.  Such works may include culverting, channel diversion, discharge of flows, bank reinforcement, connections, headwalls and the installation of trash screens.

Please also refer to the ‘culvert policy’ and consent application form available on the WSCC website; https://www.westsussex.gov.uk/fire-emergencies-and-crime/dealing-with-extreme-weather/dealing-with-flooding/flood-risk-management/ordinary-watercourse-land-drainage-consent/

The development layout must take account of any existing watercourses (open or culverted) to ensure that future access for maintenance is not restricted. A minimum 3 metre access easement is normally considered adequate.

See also:

Land Drainage Act 1991 (amended 1994)

 

Maintenance and management

Details of the maintenance and management of the SuDS system, including any watercourses for which the landowner is responsible, are to be set out in writing in a site-specific maintenance manual.  This manual shall include details of the financial management and arrangements for the replacement of components at the end of the manufacturers recommended design life.  This document is then to be submitted as part of the planning process.

References

The Building Regulations 2000
Drainage and waste disposal
Approved document H
ISBN: 1-859462-08-1

 

Building Research Establishment,
Soakaway Design – Digest 365 (BRE 365)
ISBN: 978 1 84806 918 6, 2016

 

Centre for Ecology and Hydrology
Flood Estimation for Small Catchments - IH Report 124
Marshall, D.C.W. & Bayliss, A.C.
ISBN: 0948540621, 1994

 

CIRIA
C753 The SUDs Manual
Woods-Ballard, B.; Kellagher, R. et al
ISBN: 978-0-86017-760-9, 2015

 

CIRIA
R156 Infiltration Drainage – Manual of Good Practice
Bettess, R.
ISBN: 0 86017 457 3

 

British Standards Institution
BS8582:2013 – Code of Practice for Surface Water Management for Development Sites
ISBN: 978 0 580 76700 5

Surface water drainage verification report

As part of the planning process for major applications, applicants are often required to submit a verification report for appraisal by the Local Planning Authority’s (LPA’s) Drainage Engineers. The verification condition is typically required to be discharged prior to occupation/use. This ‘Surface Water Drainage Verification Condition Guidance Note’ sets out to clearly define the minimum expectations and requirements that verification reports, submitted to LPAs within West Sussex, need to satisfy. You will be informed about the need for a verification report as part of any Conditions applied to a Planning Application approval.

Applicants should be aware that:

  • It is in an applicant’s interest to provide all the information requested by the guidance note, to enable the LPA’s Engineers to quickly and efficiently appraise their submission.
  • The omission of information may lead to delays in the planning process.
  • The verification/completion report refers to the SuDS scheme which must consider all components of the surface water drainage system ie. pipes, manholes, gullies, channel drains, ponds, basins, soakaways, headwalls, culverts, ditches, swales, flow controls, permeable pavements, storage tanks, land drains, etc. As constructed floor levels should also be verified/considered to ensure no increase in flood risk.

However, if this guidance note requests information that an applicant does not consider to be relevant to their application:

  • In the first instance the applicant should discuss this with the LPA’s Drainage Engineer.  (The Planning Officer dealing with the application will be able to provide applicants with the appraising Engineer’s contact details).

It is recommended that applicants take time, at the outset of the planning process, to familiarise themselves with the requirements set out in this guidance note. It is particularly important that the Applicant engages the services of an independent Engineer at the earliest opportunity and that inspections are undertaken by the independent engineer from the start of, and throughout construction, in order that sufficient evidence of the detailed construction is gathered.

Verification Report Author

The verification report must be completed by a suitably qualified civil engineer. The individual completing the report should be covered by a suitable professional indemnity insurance and must be independent of the contractor/subcontractor to ensure there is no conflict of interest. Where a developer uses ‘in-house’ engineers, impartial reporting must be provided.  

A suitably qualified person or company means an individual or company who has relevant training and experience, e.g. Civil Engineer experienced in surface water drainage design/construction.

Verification Report Contents

The verification report should include details of all key drainage components, evidencing that the construction of these elements has been completed fully in accordance with the approved design (as cited in the Planning Decision Notice).

As a minimum, the verification report should contain the following information:

  • Details of author, including independence, qualifications, and experience.
  • Clear statement on whether the author believes the constructed surface water drainage is adequate and whether any identified issues will affect the operation, maintenance and performance of the drainage system, now or in the future.
  • Dates and appropriate records of inspections undertaken by the report author. Photographs and details of components inspected. e.g. excavations, crate installations, sub base construction, etc
  • Confirmation of how key materials were verified, e.g. inspection of goods receipts for key components such as tanks, geomembranes, sub-base materials, flow control devices; inspections and tests of welded membrane joints; etc.
  • Commentary on ‘As Built’ survey, and details of any variations from approved design. This includes any discrepancies in ground levels and pipe/structure levels.
  • Provide CCTV survey video footage (including reports) of pipelines.  
  • Confirmation that no trees have been located in areas which would be detrimental to the drainage infrastructure.
  • Confirmation that there has been no unauthorised land raising or watercourse infilling/culverting/alterations.
  • Where variations have occurred the report shall include supporting evidence to show that the as constructed drainage will meet policy requirements and that these variations have been reviewed and approved by the designer.  This may require new calculations to be completed and be provided with the verification report. Any significant deviation from the approved design may require a ‘Discharge of Condition’ application to be re-submitted.
  • Details of remedial works that are required or have been carried out.

We would strongly recommend that the; independent inspecting engineer, the applicant, and their groundworker all keep clear records of their inspections and verification of each surface water drainage element during construction in case further detail is required to support the verification report condition.

‘As Built’ Drawings

The verification condition requires the supply of ‘As Built’ Survey Drawings. These should be referenced within the verification report and supplied with it in pdf format. Autocad .dwg (or similar approved) files should be supplied to the LPA engineer upon request.

The ‘as built’ drawings may be based upon the Applicant’s drawings but all relevant levels, dimensions, etc, should be confirmed or revised to the ‘as built’ information and clearly noted and certified as such.

The ‘As Built’ Survey shall include:

  • Surface water drainage piped network, including pipe diameters, pipe material, pipe gradient, cover levels and invert levels.
  • Roads, footpaths, driveways and parking areas with finished levels, including details of surfacing type, noting whether permeable or impermeable
  • Soakaway (where applicable) base level, height of soakaway, cover level and plan dimensions.
  • Attenuation crate (where applicable) base level, height of crate, cover level and plan dimensions.
  • Ponds/ basins (where applicable) incoming pipe invert levels, outgoing pipe invert levels, base level, crest level, embankment (where applicable) height and width, side slope.
  • Swale (where applicable) incoming pipe invert levels, outgoing pipe invert levels, base levels, crest level, side slope.
  • Permeable Paving (where applicable) base levels, finished surface levels, depth of sub base material.
  • Flow control device (where applicable) invert levels, cover level, overflow (where applicable) level.
  • Any other surface water drainage elements, e.g. land drains, filter trenches, rain gardens, gullies, channel drains, etc. (where applicable).
  • Finished floor levels

This information shall be reviewed against the original approved design and any variations must be explained within the verification report.

Certification

The report author must provide a statement clearly confirming their name, company, experience and qualifications. They must also confirm that the contents of the report are correct and a true reflection of what happened on site. This statement should also be signed and dated by the author.