Drainage design standards

Developments must provide adequate drainage for surface water.

Norfolk County Council will resist any development which involves surface water flowing onto the public highway from private accesses or non-highway areas.

Standing water must be drained away or it can pose a hazard and carry debris etc onto the highway to the detriment of highway safety.

Disposal of surface water from public highway

Disposal of surface water run-off from new highways within residential or commercial developments must be through a sustainable drainage system, which incorporates adequate water quality treatment measures where possible.

Previously run-off water was drained from highways directly to a watercourse, such as a river, through a network of pipes. This means that the water enters the watercourse very quickly. In an urbanised area with a lot of run-off this can lead to flood conditions or pollution when a lot of water enters the watercourse at once.

Urban drainage is changing to balance the impact of drainage on cumulative impacts of flooding.

Norfolk County Council seek to reduce the rate of surface water run-off through the use of Sustainable Urban Drainage Systems (SuDS), which may incorporate filter strips and swales (which can be under-drained to allow for crossing points and maintain connectivity), filter drains, permeable surfaces, bio-retention areas, infiltration devices and basins or ponds.

These systems are more sustainable than conventional drainage methods as they:

• Manage run-off flow rates, reducing the impact of urbanisation on flooding
• Protect or enhance water quality
• Are sympathetic to the environmental setting and the needs of the local community
• Provide a habitat for wildlife in urban watercourses
• Encourage natural groundwater recharge (where appropriate)

They do this by dealing with run-off close to where the rain falls, managing potential pollution at the source, and protecting water resources from point pollution (such as accidental spills).

SuDS that drain Highways, should be approved by the adopting authority, including Anglian Water, in consultation with Norfolk County Council as Local Highway Authority.  The scheme should be offered solely to the Highway Authority if the drainage system only drains surface water from an adoptable Highway.  Early engagement is encouraged to discuss options and understand constraints such as buried services, sensitive receiving watercourses / groundwater or potential for vehicle overrun of verges. Norfolk County Council is open to footpaths being set back from the carriageway to enable over the edge SuDS to be implemented between a footpath and carriageway.

A commuted sum for future maintenance may be required.

Design Standards

All schemes should attenuate runoff to pre-development Greenfield runoff rate and volumes for all rainfall events up to and including the 1% annual probability (1:100 year) plus 40% climate change.

Designs for S278 and S38 will be in accordance with the SuDS Manual C753 (2015) Chapter 9 and the following Highway Authority standards:

• Drainage will not surcharge at 100% AEP (1:1) rainfall event
• Minimum diameter of 225mm pipe drains will be provided
• There will be a free-flowing outfall at 10% AEP (1:10)
• There should be no flooding above ground at the 3.33% (1:30) plus climate change rainfall event (unless in a drainage system designed to convey water (eg a swale)
• Flooding at the 1% AEP (1:100) plus 40% climate change rainfall event be kept within the development boundary and / or within the Highway boundary if works are only within the Highway boundary. Any area of flooding should be kept to areas designed or expected to accommodate it. It may not be appropriate to hold water within the carriageway.

Climate change allowances must be in accordance with the following table:

Water catchment areas are embedded in the interactive map which is available online. 

There should always be a presumption that the new scheme will provide brownfield betterment to existing drainage and a robust justification provided to explain why little or no betterment can be achieved, evidencing that runoff does not exceed the existing runoff rates / volumes. There is unlikely to be a consistent standard of design for many brownfield schemes and a site by site assessment should be undertaken to determine what protection the current infrastructure provides e.g. 50% (1:2) or 20% (1:5) AEP rainfall event. This assessment should use up to date data i.e. FEH 2013 online rainfall / catchment characteristics.

Exceedance of the drainage system should also be considered, so to minimise flood problems downstream, avoid being a hazard to vehicles, pedestrians or property.

Water quality should be reviewed at an early stage to include SuDS features where possible. It is unlikely that vortex separators will be accepted in greenfield locations, especially to support new development of housing.  It is expected that master planning of a development considers the wider use of SuDS for all Highways works.

It should be noted that Norfolk County Council undertakes new development of roads and applies the above standards. Where existing roads are requiring improvement works, Norfolk County Council considers the possibilities of brownfield betterment.

The following information should be submitted as a minimum to enable checks to be carried out on the design:

• Site investigation details including; topographic surveys, ground investigation surveys, infiltration testing, seasonally high groundwater levels, assessment of connection to wider watercourse networks and any CCTV survey of existing drainage networks
• Drainage design details including; discharge location, catchment area of the drainage system (pre and post development), officers may request long sections and cross sections as necessary, hydraulic calculations and modelling results, flow paths/extent/depth of expected water above ground, pipe schedule, maintenance plan and inspection access proposals with appropriate easements showing distance from adjacent dwellings, structures, utilities and carriageway
• Evidence of any consents including permits from Environment Agency, IDB, LLFA or Anglian Water. Agreements from adjacent landowners may also be requested.

Private soakaways

The design and installation of private soakaways used to drain surface water from properties or private hard-standings must take account of the needs of the Highway Authority.

They must be positioned to not interfere with the stability or use of the public highway.

Drainage apparatus - distance from buildings/structures/landscaping

In order to maintain the drainage infrastructure, Norfolk County Council requires:

• A minimum of 3m width easement strip free from any obstructions from the extremity of the drainage feature (including SuDS) should be proposed, this will depend on the size or type of feature and the maintenance equipment required to maintain it. Larger easements may be required depending on the size of plant that will be required to maintain it.
• A minimum of 3m easement from the root protection zone (existing or predicted) of any landscaping
• Maintenance laybys may also be required in some cases

Exceptions will be considered on a case by case basis where the form of development proposed makes achieving this impractical, eg a high-density urban development. In these cases, special measures may need to be agreed to protect the apparatus and allow future maintenance.

Unhindered access shall be provided to the Adopting Authority to enable access to all adoptable drainage at all times.