2b. Draining surface water run-off
We will accept the maintenance of surface water sewers or sustainable drainage systems that drain only public road and footpath surface water run-off.
Existing water courses and sewers
Existing water courses and sewers that are impacted by the new road shall be assessed, diverted or removed with the relevant consent from the appropriate authority.
Sustainable drainage
New roads and footpaths designed with sustainable drainage systems (SuDS) shall drain surface water in a way that is similar to natural processes or to an appropriate outfall that should be designed specifically to suit the layout of the new road and footpath.
Drainage shall be designed for a 1-in-100-year storm plus 40% for climate change and hydraulic calculations shall be submitted with the design and confirming all dimensions and soil characteristics. The known ground water level shall be a minimum of one metre below the underside of SuDS.
Consideration shall be made with existing infrastructure such as underground services or building foundations and details submitted to, and approved by, us.
The following examples of sustainable drainage shall have specific requirements.
SuDS soakaways
SuDS soakaways shall use new material to backfill and reinstate with a greater infiltration rate than the lowest result from the soakaway testing.
SuDS underdrains
SuDS underdrains shall carry surface water to the wider drainage system with perforated pipes of a minimum 150 diameter laid at a self-cleansing gradient of a minimum two litres per second per hectare with necessary approvals obtained.
Vehicle entrances crossing over SuDS
Vehicle entrances crossing over SuDS shall have a surface water pipe of a minimum 225mm diameter installed under the vehicle crossing. This should have a minimum cover level of 600mm and a Type Z ST4 100mm concrete surround.
Traditional drainage
Where sustainable drainage cannot be achieved, surface water pipes will be accepted where only road and footpath surface water is drained.
Surface water pipes
This section relates to pipes with a diameter less than 600mm.
The minimum depth for pipes below the road shall be 600mm and pipes less than 1200mm below the road shall be protected with concrete type ST4 and reinforced 150mm above and below the pipe across the width of the pipe trench.
Example concrete pipe protection drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Pipes should be black in colour, have a smooth internal bore and laid as a single pipe within the trench.
Gully pipes should be designed to allow for easy maintenance may be a maximum 12m in length.
Vitrified clay pipes
Vitrified clay pipes and fittings shall have flexible mechanical joints. Pipes shall comply with the relevant requirements of BS EN 295 and BS 65 (surface water pipes only).
Unreinforced and reinforced concrete
Unreinforced and reinforced concrete pipes and fittings with flexible joints shall comply with the relevant provisions of BS EN 1916:2002 and BS5911-5:2004+A1:2010.
Thermoplastic structured twin wall pipes
Thermoplastic structured twin wall pipes and fittings shall comply with the relevant provisions of BS EN 13476-1, WIS 4-35-01 and BS EN 13476-2:2018 or BS EN 13476-3:2018+A1:2020. Pipes shall be BSi Kitemarked or have an equivalent third party certification.
Pipes less than 500mm in diameter shall have a minimum of eight kilonewtons per square metre.
Pipes greater than 500mm in diameter shall have a minimum of two kilonewtons per square metre.
When using pipes with flexible joints, the concrete protection shall be interrupted at each joint by a compressible filler.
The compressible filler should be made of bitumen-impregnated insulating board conforming to BS EN 120 and BS EN 317, or another equally compressible material. The thickness of the compressible filler shall be:
- 18mm for pipes less than 450mm in diameter
- 36mm for pipes from 450 to 1200mm in diameter
Compressible packing for use between pipes and precast concrete setting blocks should consist of bitumen damp-proof sheeting complying with BS 743.
Pipes and fittings should be examined for damage and the joint surfaces and components should be cleaned immediately before laying. The maximum length of pipe for laying is shall be three metres or 10 times the diameter of the pipe, whichever is the greater, unless welded joints are used.
An independent soil contamination and resistivity data shall be provided to determine the required external anti corrosion coating for the pipes and type of pipe seal.
All pipes and fittings should have gasket-type joints of spigot and socket or rebated form. Where a fitting is installed on a sewer length, it shall have the same internal bore as the sewer.
Cut ends of pipes should be formed to the tapers and chamfers suitable for the type of joint to be used. Where concrete pipes are cut, any exposed reinforcement shall be sealed with an epoxy resin mortar.
Pipes shall be laid so that each one is in contact with the bed throughout its length. Where socketed pipes are required to be laid on a granular or sand bed, or directly on a trench bottom, joint holes should be formed in the bedding material or formation to ensure that each pipe is uniformly supported.
Pipes should be laid on setting blocks only where a concrete bed or cradle is used and not for temporary support. Precast concrete setting blocks should have rectangular faces, with sufficient plan area to prevent punching of the blinding concrete or formation and an adequate seating for the pipes. The top face of each block being covered with two layers of compressible packing. Blocks shall be manufactured from ST4 concrete, and be cast in an approved mould. Blocks shall not be used until they have achieved a cube strength of 13.5 newtons per square millimetre.
The pipe should be marked by a detectable warning mesh located immediately on top of the pipe surrounding material. This should be colour coded as recommended in Volume 1 of the National Joint Utilities Group guidelines on the Positioning and Colour Coding of Underground Utilities.
Flexible couplings
Flexible couplings shall be in accordance with the provisions of WIS 4-41-01 and BS EN 295-4.
Flow Control Devices
Flow control devices shall be self-cleaning and require no power input or have any moving parts. A bypass door shall be fitted which could be operated at cover level. The minimum aperture shall be 75mm and have a design flow greater than four litres per second.
A site specific design and a manufacturer specification sheet of the flow control device clearly indicating the suitability of the flow control device for intended purpose, shall be submitted to, and approved by, us.
Flow control devices and fixings shall be constructed of grade 316S31 stainless steel in accordance with BS EN ISO 683.
Flow control devices should have a free discharge. Where it is not practicable to achieve a free discharge at all times, attenuation calculations showing both the free discharge and surcharge conditions shall be submitted to, and approved by, us.
Flow Control Chambers
Precast chambers are preferred however in-situ chambers could be permitted where the design of the chamber directs the incoming flow to the flow control device intake with minimal turbulent flow. The flow control device chamber shall contain a sump to act as a catchpit.
Access openings into chambers shall be of sufficient size to allow the removal and replacement of the device.
Orifice Plates
The minimum aperture shall be 75mm and designed and installed so as to prevent detritus and silt build up.
Orifice plates shall be proprietary products made from Grade 304 stainless steel with an EPDM or equivalent seal and installed in accordance with manufactures recommendations.
Orifice plates shall not be accepted as the final outlet from any drainage system.
Throttle Pipes
The minimum pipe size acceptable is 150mm.
Throttle pipes shall not be accepted as the final outlet from any drainage system.
Ancillary Devices (Penstocks, Flap valves)
Ancillary devices shall require no power input or have any mechanical parts.
Ancillary devices shall comply with the relevant European or British Standard, or have third party accreditation. All fixings shall be grade 316S31 stainless steel and in accordance with BS EN ISO 683.
Warning signs for flow control and ancillary devices
Warning signs should be located in the upstream, downstream and manholes containing ancillary devices and flow control devices. The signs should be fixed within the manhole where they could be clearly read without entering the chamber.
The signs should be designed in accordance with BS ISO 3864-1:2011 utilising sign reference BS EN ISO 7010 + A5 with supplementary text specific to the hazard.
The signs should be manufactured from materials that are not adversely affected by effluent typically found in surface water sewer systems. The signs should be clearly legible for a minimum of 10 years.
The fixings shall be Grade 316S31 stainless steel in accordance with BS970 Part 1, BS EN 10084, 10085, 10087, 10088, 10095 and 10250-4.
Surface water chambers
Manholes shall be no greater than 100 metres apart, should not be positioned within 500mm of the kerb line and should be designed and constructed in accordance with BS EN 752-3:2017.
Chambers should be built at the following positions:
- Each change of alignment or gradient
- The head of all pipe runs
- Each junction of two or more pipes (other than gully connections).
- Each change in the size of pipe
Minimum internal manhole diameters (Type A and B only) shall be in accordance with below:
- 1200mm diameter with the largest pipe less than 375mm in diameter
- 1500mm diameter with the largest pipe 375 to 700mm in diameter
- 1800mm diameter with the largest pipe 700 to 900mm in diameter
Where two or more pipes enter the manhole, the internal diameter may have to be increased to accommodate the minimum width of benching. Pipes of different diameters entering manholes should be installed with soffits at the same level.
The height of a Type A manhole (benching to slab soffit) should be in excess of 2000mm but when this is impracticable, Type B manholes could be provided subject to a minimum height of 90mm.
Precast concrete manhole units of circular cross-section for manholes shall comply with the relevant provisions of BS EN 1917 and BS 5911-3:2022.
Units which bed into bases shall be manufactured so that imposed vertical loads are transmitted directly via the full wall thickness of the unit. For joints between units and the underside of slabs, joint profiles shall be capable of withstanding applied loadings from such slabs, and spigot-ended sections shall only be used where the soffit of the slab is recessed to receive them.
The jointing material for precast units should be mortar, or a proprietary bitumen or resin mastic sealant. The jointing material should fill the joint cavity any surplus should be trimmed off and joints should be pointed on completion.
Concrete surrounds to manholes and chambers shall be C16/20 and the height of each concrete pour should not exceed 2m. Each construction joint shall break joint with the precast sections by at least 150mm.
Step rungs
Step rungs should be in accordance with BS EN 13101:2002 and provided in all manholes (except Type D) at 300mm intervals. They should be of galvanised mild steel or plastic encapsulated type.
Ladders
Ladders made of mild steel for vertical fixing should comply with the relevant provision of BS 4211, Class A and PD970. After fabrication, mild steel ladders should be hot dip galvanised in accordance with BS EN ISO 1460. Stainless steel ladders for vertical fixing should be fabricated from Grade 316S31 steel in accordance with BS EN ISO 683.
Safety chains
Safety chains shall be provided in manholes where the outgoing pipe exceeds 600mm in diameter. Safety chains should be made of mild or stainless steel.
After manufacture, mild steel safety chains should be hot dip galvanised in accordance with BS EN ISO 1460 and BS EN ISO 1461:2022. They should be eight millimetres in size in accordance with BS EN 818-1. Stainless steel safety chains should be manufactured from Grade 316S31 steel in accordance with BS 970: Part 1. When tested in accordance with BS EN 818-1:1996+A1:2008, each chain shall withstand a breaking force of 30 kilonewtons and a proof force of 15 kilonewtons.
Benching and inverts
Benching and inverts in manholes shall have a screeded, ridged finish and shall have a smooth, high strength concrete topping applied.
High strength concrete topping shall be produced, laid and finished in accordance with BS 8204-2 and the following approximate mix proportions by weight shall be used: one part cement, one part natural sand and two parts single sized coarse aggregate. Aggregates for high strength concrete topping (granolithic finish) shall be in accordance with BS EN 12620, 10mm in size and graded in accordance with Table 2 of that standard.
A rocker pipe
A rocker pipe shall be provided as close as is feasible to the outside face of any chamber into which a pipe is built. The design of the flexible joints shall be compatible with any subsequent movement.
Stub pipes
Stub pipes into manholes shall be of rigid material.
Manhole covers and frames
Manhole covers and frames shall be made in cast or ductile iron in accordance with BS EN 124:2015 and DMRB CD534, and shall have a minimum depth of 150mm.
Class D400 covers and frames shall be used in the road and shall have a minimum polished skid resistance value of 50.
Only flexible binder course and surface course materials contained within the Road build design section shall be used to re-instate following installation or adjustment.
Frames shall be set to level, bedded and haunched externally over the base and sides of the frame on an approved bedding material. Frames shall be bedded on a non-shrink proprietary bedding material and the compressive strength should be greater than 30 newtons per square millimetre and its tensile strength should be greater than five newtons per square millimetre. A 1:2 mix should achieve this.
Frames shall be entirely seated on precast concrete cover frame seating rings to regulate the distance between the top of the cover and the top rung to no greater than 675mm. A mortar fillet shall be provided where the corners to an opening in a slab are chamfered and the brickwork is not flush with the edges of the opening.
Manhole covers and frames shall be of a non-rocking design which does not rely on the use of cushion inserts.
Covers and frames shall be ergonomically designed to ensure individual lift loads in accordance with manual handling legislation and the Management of Health and Safety at Work Regulations.
Covers shall be hinged to the frame and when open they shall rest at an angle greater than 100 degrees and locked in position to prevent accidental closing. The sections of cover shall be capable of being removed from the frame to facilitate better access.
Backfill of pipe trenches
Pipe bedding and haunching shall consist of natural and/or recycled aggregate including recycled concrete aggregate complying with BS EN 13242 for coarse or fine aggregate.
Filter drains including those forming the base of swales shall be backfilled with Type B filter material which should consist of natural or recycled coarse aggregate or recycled concrete aggregate complying with BS EN 13242 and the following:
- for Type B, geometrical requirements in accordance with BS EN 13242
- a resistance to fragmentation in Category LA50 in accordance with BS EN 13242
- a water-soluble sulfate content of less than 1.9 grams of sulfate (as SO3) per litre when tested in accordance with BS EB 1711-1
- be non-plastic when tested in accordance with BS 1377:Part 2.
Maintenance on completion
All internal surfaces of pipelines, gullies, linear drainage systems and manholes shall be thoroughly cleansed to remove all detritus, without this being passed forward into existing public sewers, highway drains, or water courses.