Drainage types

Highway drainage on new developments should be connected to a drainage system via one or a combination of the following:

  • drained by a piped highway-drainage system (minimum pipe size 225mm) running to an out-fall adopted by a water company or an out-fall to a ditch or watercourse agreed by the Lead Local Flood Authority or Environment Agency. 
  • infiltration into the ground in a safe and environmentally sustainable manner.

Traditional drainage systems should be designed in line with the water company’s specifications and requirements (which should be treated as complementary to this document) and should be adopted by them.

Sustainable drainage systems and assessing site suitability 

The following should be considered when determining which sustainable drainage systems (SuDS) intervention is suitable for a site. There are site conditions and constraints which may prohibit or restrict the use of certain SuDS features for the purposes of adoptable highway drainage, including: 

  • Floodplain 
  • Groundwater; 
  • Soils and geology;
  • Contaminated land;
  • Space constraints; 
  • Maintenance.

These constraints must be considered at the design stage.  The design of drainage systems must ensure ease of maintenance. Depending on the type of intervention suitable gated access and hard standing must be provided. Design proposals must be shared with the environmental operations team.

As the use of SuDS features to drain the highway is relatively untested in Leicestershire, there will be a bedding in period and not all features are currently accepted for adoption. Developers should discuss proposed SuDS during pre-application discussions where they will be updated on the current position on both acceptability and adoption of different SuDS features. 

Generally, ground conditions throughout the County are unsuitable to enable efficient drainage by natural percolation and soakaways are often unsuitable for public highway drainage. There may be areas where infiltration is possible, and if a soakaway is proposed, suitable percolation test results must be provided (to BRE365). 

SuDS must be designed to accepted industry standards including those detailed in Ciria’s SuDS Manual. Designers should consider the potential adverse effects on soil, agricultural land and the geology of the area from the development. ‘Suds Advice Note – Brownfield Sites’ provides useful guidance and references for the purposes of designing SuDS schemes. The council is keen to work with developers and the wider industry in developing its own design standards and specifications as experience of the use of SuDS increases.

SuDS types and acceptability

Table 24 below sets out the adoptability of SuDS interventions and where a commuted sum will be required. The council will not adopt systems that accept non-highway run off. 

Table 24: Acceptability of highway SuDS interventions
  SuDS Intervention Adoptable Early consultation? (1) Commuted sum required?

Ciria

Chamber soakaway

Y

N

N

Infiltration basin

Y

Y

Y

 

Soakaways trench

Y

Y

N

Crate / tank soakaway

Y

Y

N

Proprietary treatment systems (e.g. oil interceptor)

Y

Y

Y

Filter strips (contaminated waste)

N

-

-

Filter drains/trenches

Y

N

N

Swales, channels and rills

Y

Y

Y

 

Bioretention systems

Y

Y

Y

Trees/shrubs

Y

Y

Y

Permeable / Pervious block pavements (option for areas where positive drainage not available and ground conditions allow)

Y

(private drives and shared surfaces with low traffic flows)

Y

Y

Other permeable/pervious surfacing (asphalt or concrete for example)

N

-

-

Attenuation Storage Tanks (crate system preferred, speak to LCC)

Y

N

N

Rain Gardens

Y

Y

Y

Detention Basins

Y

Y

Y

Source control (e.g. Green & Blue Roofs, rainwater harvesting)

N

-

-

Inlets, outlets & control structures

Y

N

N

Ponds & Wetlands

N

-

-

Other

Landscaped area

Y

Y

Y

Ditches /Watercourses

Y

Y

Y

Drainage Grips

Y

N

N

(1) Infrastructure Planning and Maintenance Operations. Any interventions that include tree planting must be discussed with the Forestry team.

Image
Diagram showing an example of  the design of a sustainable drainage system bioretention area

Figure 33 – Example of a bioretention area

A SuDS scheme will only be adopted when the council is satisfied that the scheme is performing satisfactorily and has been built in accordance with the agreed design and specification and is in accordance with the council’s adoption requirements.

 

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