2a. Catching surface water run-off
We will accept the maintenance of the top, root zone and above, of sustainable drainage systems or traditional systems that catch 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 catchment
New roads and footpaths designed with sustainable drainage systems (SuDS) to catch and filter surface water using natural processes should be designed specifically to suit the layout of the new road and footpath.
Catchments 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.
The following examples of sustainable catchments shall have specific requirements.
Rain gardens
Rain gardens shall be sized appropriately based on catchment area and soil infiltration rate to prevent over or under-watering of plants.
Topsoil depth shall be at least 200mm and selected based on plant species. Filter medium depth should depend on storage requirements. A geotextile layer should be installed to prevent small particles entering the wider surface water system.
A rodding eye should be installed for pipe maintenance.
The proposed plant types shall be submitted to, and approved by, the County Council.
Example rain garden drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Permeable tree pits
Permeable tree pits should be sized appropriately based on catchment area and soil infiltration rate with perforated pipes wrapped in a geotextile to collect small particles.
Only mature or semi-mature trees shall be used in permeable tree pits. The proposed tree types, age and soil type selection, shall be submitted to, and approved by, the County Council.
Soil depth and type should be determined based on tree species in accordance with BS 3882.
A rodding eye should be installed for pipe maintenance.
Modular units may be considered in areas of heavy loading to maintain root space and structural stability.
Example permeable tree pits drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Grass filter strips
Grass filter strips shall be 35mm below the road channel level to prevent buildup of silt at the road edge which would obstruct the surface water run-off. The filter strip shall provide a smooth and consistent longitudinal slope to encourage surface water flow.
Any obstructions, including still build up, to surface flow, shall be removed prior to adoption.
A newly built filter strip shall be protected from stormwater flows unless grass has been established. If there is less than 70% of growth after four weeks following seeding, grass shall be reseeded to establish more than 90% growth.
Swales
Swales should be designed to be infiltrating or surface conveyance wherever possible. The base of the swale shall be a minimum of 0.5m and shall have a maximum longitudinal fall of 1in16 (6%). The maximum depth of the swale shall be 600mm and shall have a minimum freeboard depth of 100mm.
Example underdrain swale (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Example infiltration conveyance swale (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Filter drains
Filter drains shall not be constructed near to waste storage which may pollute the surface water.
A geotextile with a higher permeability than the surrounding soil shall be used on the inside of the trench to prevent sediment ingress.
Example filter drains drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Permeable paving
Permeable paving which catches surface water within the construction of the new road shall only be used on sites with a minimum 2.5% soaked CBR.
Example permeable paving drilling drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Example permeable paving section drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Vehicle entrances crossing over SuDS
Vehicle entrances crossing over SuDS shall have a maximum gradient of 1 of 60 towards the centre of the SuDS catchment or a maximum gradient of 1 in 25 towards the road.
Geotextiles
All SuDS catchments shall include a fabric layer, known as a geotextile between the base of the SuDS and the subgrade. Geotextiles shall be selected based on the soil particle size confirmed in the ground investigation report. The permeability of the geotextile shall be compatible with the infiltration rate used for the design.
All geotextiles shall have a minimum CBR puncture resistance of three kilonewtons
Woven geotextiles shall have a minimum tensile strength of 30kN/m in each direction and pore sizes shall be 90% finer than 200/µm.
Nonwoven geotextiles shall have a minimum tensile strength of 15kN/m and a pore size 90% finer than 100/um.
Traditional catchment
Where sustainable catchment cannot be achieved, gullies will be accepted with a maximum catchment of 180 square metres for each gully.
Gullies
Gullies shall be made in precast concrete and include a sump below the outlet pipe to catch debris.
Road gullies shall be at least 750mm deep and 450mm in diameter with a 150mm outlet pipe, with a stoppered rodding eye and chain.
Footpath gullies shall be rectangular and made of self-glazed ware or precast concrete. The gullies shall be 250mm square and 475mm deep, and have a 150mm diameter outlet.
Gullies shall be laid on and surrounded with 150mm thick ST4 concrete with sulphate resisting cement.
Junction pipes for gullies which are not connected shall be fitted with temporary plastic seals and the position of all such junctions shall be clearly defined by marked and labelled stakes or tracing wires.
Precast concrete gullies shall be unreinforced and comply with the requirements of BS5911 4+A2:2010 and BSEN1917:2002. Salt glazed ware gullies shall comply with the requirements for round gullies as specified in BS65.
Example gully drawing (PDF) provided as a typical detail for reference only. Scheme-specific versions may be required.
Gully covers
Gully covers shall be hinged, but not removable. In the road, they shall have an opening width of 450mm and in the footpath they shall have an opening of 300mm square.
Gully covers and frames shall be made of 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.
The upper surface of gully gratings shall be flat and slots in gratings or between gratings and frames shall not be orientated parallel to the direction of traffic.
Where gradients are greater than 1 in 50, storm pattern grids shall be provided.