Supplementary requirements for surface water drainage proposals

We have a duty to ensure that new developments do not increase flood risk and that surface water from the development is appropriately drained.  We have therefore 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 quickly and efficiently review and evaluate the submission.

This 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:

Guidance

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 order 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

For additional guidance, please refer to the West Sussex County Council (lead local flood authority) Policy for the Management of Surface Water.

Infiltration drainage design

Any infiltration drainage design must include adequate winter groundwater monitoring data to determine the highest winter groundwater table. Residential developments of ten properties or more will require ground water 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/February but is dependent on the weather et cetera up to that point.

Adequate freeboard must be provided between the base of the soakaway structure and the highest recorded groundwater level identified in that location.

Infiltration rates for soakage structures are to be based on percolation tests undertaken at an agreed time during the winter period and at the location and depth of the proposed structures. The percolation 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 percolation tests must be carried out in accordance with BRE 365, CIRIA R156 or a similar approved method. For the purpose of design, the percolation rate must be applied to the sides of the infiltration structure only and the rate for the base must be zero, unless otherwise agreed. In respect to infiltration basins or permeable pavements, the percolation rate is generally applied to the base only.

There must be provision to ensure that there is capacity in the system to contain the 1 in 100 year storm event plus 40% on stored volumes/rainfall intensity, as an allowance for climate change and cater for the 1 in 10 year storm event plus climate change between the invert of the lowest entry pipe into the infiltration structure and the base. The infiltration structure should also drain 50% of its total volume in 24 hours or less for both the 1 in 10 (plus 40%) and 1 in 100 year (plus 40%) storm events, in order 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, using the impermeable area (and including other permeable areas that are positively drained) of the site to be developed as the basis for the calculations, rather than the entire greenfield site area.

For brownfield sites, the above criteria also applies, but if this cannot be achieved, then a minimum 50% reduction in existing peak runoff rate should be considered. 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.

Greenfield runoff rates can be derived from IH124 or a similar approved method. Any storage design must be submitted with winter groundwater monitoring data where applicable 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.

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 (West Sussex County Council), or its agent Arun District Council – 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, connections, headwalls and the installation of trash screens.

Please also refer to the West Sussex County Council ‘culvert policy’ and consent application form

The development layout must take account of any existing watercourses (open or culverted) to ensure that future access for maintenance is not restricted.

See also: Land Drainage Act 1991 and Land Drainage Act 1994

Maintenance and management

Ditches and watercourses (including culverts) should retain a three metre easement with access that allows for its future maintenance.

Details of the maintenance and management of the SuDS system 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

During 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 LPA’s within West Sussex, need to satisfy.
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
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.

Verification Report Author

The verification report must be completed by an independent suitably qualified individual or company. The individual or company completing the report should have suitable professional indemnity insurance and must be independent of the developer/contractor/ subcontractor and had no involvement in the original/proposed scheme, to ensure there is no conflict of interest.
A suitably qualified person or company means an individual or company who has relevant training and experience, e.g. Civil Engineer with at least 10 years’ experience of 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 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
• dates of inspections undertaken by report author
• photographs and details of components inspected. e.g. photographs of excavations, crate installations, sub base construction
• 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
• confirmation that no trees have been located in areas which would be detrimental to the drainage infrastructure
• 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
• details of remedial works that are required or have been carried out
We would strongly recommend that the applicant and their Groundworker 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 files should be supplied to the LPA engineer upon request.
The ‘As Built’ Survey shall include:
• surface water drainage piped network, including pipe diameters, pipe material, pipe gradient, cover levels and invert levels
• road finish levels, including details of road surfacing type
• 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)
This information shall be reviewed against the design by the independent engineer, any variations must be explained within the verification report.