Designing, constructing, operating and maintaining drainage for surface runoff
December 2011© Crown copyright 2011
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http://www.defra.gov.uk/consult/
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SuDS Team
Defra
Area 2A, Ergon House
London SW1P 2AL
Email: suds@defra.gsi.gov.ukDepartment for Environment, Food and Rural Affairs
Contents
Executive Summary .................................................................................................... 4
Introduction ................................................................................................................. 5
Part I – Principles........................................................................................................ 6
Affordability...............................................................................................................................6
Functionality .............................................................................................................................6
Part II .......................................................................................................................... 7
Section A – Runoff destination .................................................................................................7
Discharge into the ground.....................................................................................................7
Discharge to a surface water body .......................................................................................7
Discharge to a surface water sewer or local highway drain ..................................................8
Discharge to a combined sewer............................................................................................8
Section B – Peak flow rate and volume....................................................................................8
Exceptions ............................................................................................................................8
Low rainfall............................................................................................................................8
High rainfall...........................................................................................................................8
Approach 1: Restricting both the peak flow rate and volume of runoff..................................8
Approach 2: Restricting the peak flow rate ...........................................................................9
Previously developed land....................................................................................................9
Section C – Water Quality ........................................................................................................9
Exceptions ............................................................................................................................9
Effective Treatment.............................................................................................................10
Groundwater.......................................................................................................................10
Infiltration ............................................................................................................................11
Surface Water Body............................................................................................................11
Section D - Function...............................................................................................................11
Design.................................................................................................................................11
Flood Risk...........................................................................................................................12
Operation & Maintenance ...................................................................................................12
Information..........................................................................................................................124
Executive Summary
0.1 This document presents the National Standards for Sustainable Drainage Systems
(SuDS) for consultation.
0.2 SuDS are an approach to managing rainwater falling on roofs and other surfaces
through a sequence of actions. The key objectives are to manage the flow rate and
volume of surface runoff to reduce the risk of flooding and water pollution. SuDS
also reduce pressure on the sewerage network and can improve biodiversity and
local amenity.
0.3 The Department for Environment, Food and Rural Affairs (Defra) has developed
National Standards to be used in England in order to manage surface runoff in
accordance with Schedule 3 to the Flood Water and Management Act 2010.
0.4 The National Standards set out what to design and construct in order to obtain
approval from the SuDS Approving Body (SAB) and for operating and maintaining
SuDS which the SAB adopts.
0.5 Drainage for approval by the SAB must be designed to comply with the National
Standards. The Sustainable Drainage (Approval and Adoption) (England) Order
2012 defines the exemptions to the requirement for approval.
0.6 There are two parts to the National Standards:
Principles that:
i. Must be taken into account for the design of SuDS; and
ii. Set the criteria for governing the judgement of SABs on the functionality
of drainage they adopt; and
iii. Exempt development from complying with certain aspects of the
standards on the grounds of disproportionate cost.
Standards with design, construction and maintenance requirements for SuDS.
0.7 In addition, the Local Planning Authority may set local requirements for planning
permission that have the effect of more stringent requirements than these National
Standards.
0.8 Defra invites comments on the National Standards. An online questionnaire can be
found at http://www.defra.gov.uk/consult/. The deadline for comments is 13 March
2012.
Responses can be sent by email to: SuDS@defra.gsi.gov.uk or
By post to: Defra, Ergon House, Area 2A, Horseferry Road, London, SW1P 2AL5
Introduction
1.1 The Flood and Water Management Act 2010 requires new developments and
redevelopments in England and Wales to have drainage plans for surface runoff
approved by the SAB where the construction work would have drainage
implications. The Sustainable Drainage (Approval and Adoption) (England) Order
2012 defines the exemptions to the requirement for approval.
1.2 The SAB is responsible for adopting and maintaining new SuDS that serve more
than one property and have been constructed as approved and function as
designed.
1.3 These National Standards set out the requirements for the design, construction,
operation and maintenance of SuDS. There are two parts to the Standards:
Principles that:
i. must be taken into account for the design of SuDS; and
ii. set the criteria for governing the judgement of SABs on the functionality of
drainage they adopt; and
iii. exempt development from complying with certain aspects of the
standards on the grounds of disproportionate cost.
Standards with design, construction and maintenance requirements for SuDS.
1.4 The Local Planning Authority could set local requirements for planning permission
that have the effect of more stringent requirements than these National Standards.
1.5 The National Standards are part of Governments policy for the construction of
drainage for new developments and redevelopments, which includes the National
Planning Policy Framework, Building Regulations and the Code for Sustainable
Homes.
1.6 Guidance will be prepared to help interpret the National Standards and recommend
approaches to comply with them.6
Part I – Principles
2.1 A proposed drainage system does not comply with these National Standards unless
it is designed so that:
a. Surface runoff is managed at its source where it is reasonably practicable to do
so;
b. Surface runoff is managed on the surface where it is reasonably practicable to
do so;
c. Public space is used and integrated with the drainage system, where it serves
more than one property and it is reasonably practicable to do so;
d. Design is cost-effective to operate and maintain over the design life of the
development, in order to reduce the risk of the drainage system not functioning;
e. Design of the drainage system accounts for the likely impacts of:
climate change; and
changes in impermeable area;
over the design life of the development, where it is reasonably practicable to do
so.
2.2 Where arrangements have been made before seeking SAB approval, which do not
take these Standards into account, this is not a reason to regard compliance as not
reasonably practicable.
Affordability
2.3 If full compliance with the Standards would necessitate the construction of a
drainage system that is more expensive than an equivalent conventional design
then full compliance is not required, and instead the drainage system must comply
with the standards to the greatest extent possible, without exceeding the cost of the
equivalent conventional design.
Functionality
2.4 For the purposes of its duty to adopt a drainage system, a SAB:
a. may carry out inspections in accordance with the conditions of approval and
then may further consider the functionality of a drainage system for up to 8
weeks before it makes a judgement as to whether it is satisfied that the system
is functioning; and
b. must presume that a drainage system is functioning in accordance with the
approved proposals unless there is evidence that it is not.7
Part II
Section A – Runoff destination
A1. Subject to complying with these National Standards, different destinations may
apply for different parts of the site.
Discharge into the ground
A2. Surface runoff must be discharged to the ground except where one or more of the
following criteria can be demonstrated:
a) The rate of surface runoff is greater than the rate at which water can infiltrate
into the ground. In this case as much of the water as reasonably practicable
must be discharged by infiltration; or
b) There is an unacceptable risk
1
of ground instability or subsidence; or
c) There is an unacceptable risk of pollution from mobilising existing contaminants
on the site; or
d) Infiltration is not compliant with the water quality requirements (C7, C8); or
e) There is an unacceptable risk of groundwater flooding (D5); or
f) The infiltration system would create a high risk of groundwater leakage into the
combined sewer.
Discharge to a surface water body
A3. Surface runoff not discharged into the ground must be discharged to a surface
water body except where it can be demonstrated that:
a) It is not reasonably practicable to convey the runoff to a surface water body; or
1
The effects of subsidence will be unacceptably adverse or cannot be minimised through appropriate layout, ground treatment or structural design.
The following destinations must be considered for surface runoff in order of
preference:
1. Discharge into the ground
2. Discharge to a surface water body
3. Discharge to a surface water sewer
4. Discharge to a combined sewer 8
b) Pumping of the surface runoff, either on site or further downstream, would be
required and there is a reasonably practicable alternative; or
c) Discharge would result in an unacceptable risk of flooding from the surface
water body. Discharge to a surface water sewer or local highway drain
A4. Surface runoff that cannot be discharged into the ground or to a surface water body
must be discharged to a surface water sewer or local highway drain, except where it
can be demonstrated that it is not reasonably practicable to do so.
Discharge to a combined sewer
A5. Surface runoff that cannot be discharged into the ground, a surface water body or a
surface water sewer or local highway drain must be discharged to a public,
combined sewer system.
A6. Surface runoff must not be discharged to a separate foul sewer.
Section B – Peak flow rate and volume
Exceptions
B1. B3 onwards does not apply to any surface runoff that is discharged:
a) By infiltration; or
b) To a coastal or estuarial water body; or
c) To an alternative water body where the SAB considers it appropriate to do so.
Low rainfall
B2. There must be no discharge to a surface water body or sewer that results from the
first 5mm of any rainfall event.
High rainfall
B3. Either of the two approaches below must be used to manage the surface runoff
discharge, unless B9 applies:
Approach 1: Restricting both the peak flow rate and volume of runoff
B4. The peak flow rates for the:
a) 1 in 1 year rainfall event; and9
b) 1 in 100 year rainfall event;
must not be greater than the equivalent greenfield runoff rates for these events.
The critical duration rainfall event must be used to calculate the required storage
volume for the 1 in 100 year rainfall event.
B5. The volume of runoff must not be greater than the greenfield runoff volume from
the site for the 1 in 100 year, 6 hour rainfall event.
Approach 2: Restricting the peak flow rate
B6. The critical duration rainfall event must be used to calculate the required storage
volume for the 1 in 100 year rainfall event. The flow rate discharged:
a) For the 1 in 1 year event, must not be greater than either:
the greenfield runoff rate from the site for the 1 in 1 year event, or
2 litres per second per hectare (l/s/ha);
And b) for the 1 in 100 year event, must not be greater than either:
the greenfield mean annual flood for the site, or
2 litres per second per hectare (l/s/ha).
Previously developed land
B7. Where the site is on previously developed land and neither Approach 1 nor 2 is
reasonably practicable then:
a) An approach as close to Approach 1 as is reasonably practicable must be used;
b) The flow rate discharged from the site must not exceed that prior to the proposed
development for:
the 1 in 1 year event; and
the 1 in 100 year event.
c) The volume of runoff may only exceed that prior to the proposed development
where the peak flow rate is restricted to 2 l/s/ha.
Section C – Water Quality
Exceptions
C1. C2 onwards does not apply to any surface runoff that is drained to a combined
sewer provided that appropriate measures are included to manage the risk of discharging
oil and sediments to the sewer.10
Effective Treatment
C2. Water quality treatment components must be designed to ensure that they function
effectively and the design of the drain must make use of an environment that incorporates
and supports plants where reasonably practicable.
C3. The minimum number of treatment stages depends on the potential hazards on the
site and are characterised in Table C1; together with the sensitivity of the receiving water
body to pollution as categorised in Table C2 and C3.
C4. Where the sensitivity of the potential hazards and sensitivity of the receiving water
body requires a level of treatment which is not reasonably practicable, then a lesser level
of treatment must be considered.
C5. The discharge of surface runoff from a site categorised as high hazard as set out in
Table C1 may not be permitted. It is advisable to consult the Environment Agency about
environmental permitting to determine whether a permit is required. If a site is categorised
as high hazard but a permit is not required then it is treated as a med hazard.
Hazard Table C1: Level of hazard
Low Roof drainage
Med
Residential, amenity, commercial, industrial uses includes car parking and
roads
High
Areas used for handling and storage of chemicals and fuels, handling and
storage of waste. This includes scrap-yards.
Lorry, bus or coach parking or turning areas
Groundwater
C6. A series of treatment stages must be included before the surface runoff reaches the
infiltration device. The minimum number of treatment stages must be in accordance
with Table C2.
Table C2. Minimum number of treatment stages low med high
G1
Source Protection Zone I, within 50m of a well, spring or
borehole that supplies potable water.
1 3
Consult the EA (C6)
G2
Into or immediately adjacent to a sensitive receptor that could
be influenced by infiltrated water. Includes designated nature
conservation, heritage and landscape sites – including
Biodiversity Action Plan (BAP) habitats and Protected Species.
1 3
G3 Source Protection Zone II or III or Principal Aquifer 1 3
G4 Secondary Aquifer 1 211
G5 Unproductive strata 1 2
Infiltration
C7. Surface runoff from roof drainage must be isolated from others sources and must
not be significantly contaminated where it is discharged to G1 and G2 in
accordance with Table C2.
C8. Infiltration may only be used to discharge to G1 and G2 in accordance with Table
C2 where a risk assessment has been undertaken and the design effectively
addresses the risk(s) identified.
Surface Water Body
C9. The minimum number of treatment stages required in series prior to the discharge
of surface runoff to a surface water body or surface water sewer must be in
accordance with Table C3. The table stipulates where discharge to a sensitive
surface water body
2
then one extra treatment stage must be added.
Table C3: Minimum number of treatment stages to surface water
Hazard Normal surface water Sensitive surface water
low 0 1
med 2 3
high Consult the EA (C6)
Section D - Function
Design
D1. The drainage system and associated structures must be designed to ensure they
function under anticipated loading conditions over the design life of the
development.
D2. The conveyance of surface runoff must minimise erosion of soil.
D3. As far as reasonably practicable the design of the drainage system must minimise
the use of energy over its design life. Pumping must only be used to facilitate
drainage for those parts of the site where it is not reasonably practicable to drain
water by gravity.
2
Any catchment smaller than 50km2; any catchment with less than 20% urbanisation; any catchment with an environmental designation of national or
international recognition, or any catchment where good ecological status is at risk.12
Flood Risk
D4. The design of the drainage system must take into account the impact of rainfall
falling on any part of the site and also any estimated surface runoff flowing onto the
site from adjacent areas.
D5. Drainage systems must be designed so that, unless an area is designated for flood
management in the Local Flood Risk Management Strategy, flooding from the
drainage system does not occur:
a) On any part of the site for a 1 in 30 year rainfall event; and
b) During a 1 in 100 year rainfall event in any part of:
a building (including a basement); or
utility plant susceptible to water (e.g. pumping station or electricity substation); or
c) On neighbouring sites during a 1 in 100 year rainfall event.
D6. Flows that exceed the criteria in Standard D5 must be managed in flood
conveyance routes that minimise the risks to people and property both on and off
the site.
Operation & Maintenance
D7. The approved drainage plan must include the safe operation and maintenance of
SuDS. Where appropriate this must make provision for a warning system and
contingency arrangements.
D8. Maintenance must be carried out safely in accordance with the approved drainage
plan to maintain the designed function of SuDS.
Information
D9. The approved plan must be provided to responsible parties, which identifies the
measures to maintain the designed function.
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