(all buildings)
| Number of credits available | Minimum standards |
|---|---|
| 5 | No |
Aim
To avoid, reduce and delay the discharge of rainfall to public sewers and watercourses, thereby minimising the risk and impact of localised flooding on and off-site, watercourse pollution and other environmental damage.
Assessment criteria
This issue is split into three parts:
- Flood risk (2 credits)
- Surface water run-off (2 credits)
- Minimising watercourse pollution (1 credit)
Up to two credits - Flood resilience
Two credits - Low flood risk
| 1 | Where a site-specific flood risk assessment (FRA) confirms the development is situated in a flood zone that is defined as having a low annual probability of flooding (in accordance with current best practice national planning guidance). The FRA must take all current and future sources of flooding into consideration (see CN3.2). |
One credit - Medium or high flood risk
| 2 | Where a site-specific FRA confirms the development is situated in a flood zone that is defined as having a medium or high annual probability of flooding and is not in a functional floodplain (in accordance with current best practice national planning guidance). The FRA must take all current and future sources of flooding into consideration (see CN3.2). |
| 3 | To increase the resilience and resistance of the development to flooding, one of the following must be achieved: |
| 3.a | The ground level of the building and access to both the building and the site, are designed (or zoned) so they are at least 600mm above the design flood level of the flood zone in which the assessed development is located (see CN3.5) OR |
| 3.b | The final design of the building and the wider site reflects the recommendations made by an appropriate consultant. |
Two credits - Surface water run-off
Prerequisite
| 4 | An appropriate consultant is appointed to carry out, demonstrate or confirm the development's compliance with the following criteria: |
One credit
| 6 | Relevant maintenance agreements for the ownership, long term operation and maintenance of all specified sustainable drainage systems (SuDS) are in place. |
| 7 | Calculations include an allowance for climate change; this should be made in accordance with current best practice planning guidance (see Relevant definitions ). |
One credit
| 8 | Where flooding of property will not occur in the event of local drainage system failure (caused either by extreme rainfall or a lack of maintenance); AND |
EITHER
| 9 | Drainage design measures are specified to ensure that the post-development run-off volume, over the development lifetime, is no greater than it would have been prior to the assessed site's development for the 100-year 6-hour event, including an allowance for climate change (see criterion 14). |
| 10 | Any additional predicted volume of run-off for this event is prevented from leaving the site by using infiltration or other SuDS techniques. |
OR (only where criteria 9 and 10 for this credit cannot be achieved):
| 11 | Justification from the appropriate consultant indicating why the above criteria cannot be achieved, i.e. where infiltration or other SuDS techniques are not technically viable options. |
| 12.a | The pre-development 1-year peak flow rate; OR |
| 12.b | The mean annual flow rate Qbar; OR |
| 12.c | 2L/s/ha. |
Note that for the 1-year peak flow rate the 1-year return period event criterion applies (as described in the peak run-off criteria above).
| 13 | Relevant maintenance agreements for the ownership, long term operation and maintenance of all specified SuDS are in place. |
| 14 | For either option, above calculations must include an allowance for climate change; this should be made in accordance with current best practice planning guidance. |
Two credits - Surface water run-off - Single dwellings only
For single dwellings, the below criteria should be applied in place of the surface water run-off criteria above (please see CN2.1 for more information).
| 15 | Either of the following criteria is met: |
| 15.a | There is a decrease in the impermeable area by 50% or more, from the pre-existing impermeable hard surfaces; OR |
| 15.b | Where all run-off from the roof for rainfall depths up to 5mm from all new and existing parts of the building have been managed on site using source control methods. |
OR
One credit - Surface water run-off - Single dwellings only
| 16 | Either of the following criteria is met: |
| 16.a | There is no increase in the impermeable surfaces as a result of the new construction; OR |
| 16.b | If there is an increase in the impermeable surfaces as a result of the new construction then the following must be met: |
| 16.b.i | Hard standing areas - where there is an extension or increase in the hardstanding areas and hence an increase in the total impermeable area as a result of the new construction, the hardstanding area must be permeable or be provided with on site SuDS to allow full infiltration of the additional volume, to achieve the same end result. The permeable hardstanding must include all pavements and public rights of way, car parks, driveways and non-adoptable roads, but can exclude small garden paths which will drain onto a naturally permeable surface. |
| 16.b.ii | Building (new-build or extension) - where there is an increase in building footprint, extending onto any previously permeable surfaces, the additional run-off caused by the area of the new-build or extension must be managed on site using an appropriate SuDS technique for rainfall depths up to 5 mm. |
One credit - Minimising watercourse pollution
| 17 | There is no discharge from the developed site for rainfall up to 5mm (confirmed by the appropriate consultant). |
| 18 | In areas with a low-risk source of watercourse pollution, an appropriate level of pollution prevention treatment is provided, using appropriate SuDS techniques. |
| 19 | Where there is a high risk of contamination or spillage of substances such as petrol and oil (see CN3.17), separators (or an equivalent system) are installed in surface water drainage systems. |
| 21 | A comprehensive and up-to-date drainage plan of the site will be made available for the building or site occupiers. |
| 22 | Relevant maintenance agreements for the ownership, long term operation and maintenance of all specified SuDS must be in place. |
Checklists and tables
None.
Compliance notes
| Ref |
Terms |
Description |
|---|---|---|
| Shell and core | ||
|
CN1 |
Applicable assessment criteria | Both options: All criteria relevant to the building type and function apply. Refer to Appendix D – Shell and core project assessments for a more detailed description of the shell and core assessment options. |
| Residential - Partially fitted & Fully fitted | ||
|
CN2 |
Applicable assessment criteria - Single dwellings |
Criteria 1 to 3 - Flood resilience Both options: All criteria relevant to the building type and function apply.Criteria 4 to 16 - Surface water run-off Both options: Only single dwelling criteria 15 or 16 apply. Criteria 17 to 16 - Minimising watercourse pollution Both options: All criteria relevant to the building type and function apply.Refer to Appendix E – Applicability of BREEAM New Construction to single and multiple dwellings, partially and fully fitted for a more detailed description of residential assessment options. |
| Applicable assessment criteria - Multiple dwellings |
Criteria 1 to 3 - Flood resilience Both options: All criteria relevant to the building type and function apply.Criteria 4 to 16 - Surface water run-off Both options: Criteria 4 to 14 apply only. Criteria 17 to 16 - Minimising watercourse pollution Both options: All criteria relevant to the building type and function apply.Refer to Appendix E – Applicability of BREEAM New Construction to single and multiple dwellings, partially and fully fitted for a more detailed description of residential assessment options. |
|
| General | ||
|
CN3 |
Alternative standards and recommendations from an appropriate statutory body. See criteria 1 and 2. |
None of the credits can be awarded where the assessed development has proceeded against the recommendation of the statutory body on the basis that the flooding implications are too great (this includes a recommendation given by the statutory body even where such a recommendation cannot be, or is not, statutorily enforced). Where the local authority (or other statutory body) has set more rigorous criteria than those above these must be met in order to achieve the relevant credits. |
|
CN3.1 |
Contaminated sites. |
Drainage designs for sites must take into account legislation relating to contaminated sites; however in many circumstances even on contaminated sites there may be opportunities for the installation of some SuDS techniques. Please see Other information for more details. |
| Flood resilience | ||
|
Sources of flooding. |
The Flood Risk Assessment (FRA) must detail the risk of flooding from the following sources:
Please see Other information section which provides more detail on the above sources of flooding. The content of the FRA should be based on historic trends, but should also account for predicted changes to the climate which may impact on the flood risk to the site in the future. |
|
|
CN3.3 |
Functional flood plain. See criterion 3. |
The BREEAM credit for locating in a flood zone of 'medium or high annual probability' cannot be awarded where the building is located in the functional flood plain. This is defined in the current best practice national planning guidance for each country. |
|
CN3.4 |
Flood defences. |
Third party defences There are many landscape feature defences, owned by third parties, which due to their location act as a flood defence by default, e.g. motorway, railway embankments, walls etc. It can be assumed that such embankments will remain in place for the lifetime of the development, unless the assessor or project team have reason to believe otherwise. For walls, assurance must be sought that the wall is likely to remain for the design life of the building. Pre-existing flood defences In an area protected by existing flood defences (designed to withstand a certain magnitude of flooding) the appropriate number of flood risk credits can be awarded where the defences reduce the risk to 'low' or 'medium' and the following conditions are met:
A statutory body's local or regional office may be able to provide more information on existing defences in the area in which the assessed development is located. |
| 600mm threshold. See criterion 3.a. |
It is accepted that, for buildings located in medium and high risk flood zones, areas of the car park and site access may be allowed to flood and therefore fall below the 600 mm threshold. In such cases the credit is still achievable provided safe access to the site, and the ground floor of the building can be maintained (i.e. they are 600 mm above the design flood level) to ensure the building and site do not become an 'island' in the event of a flood. Where the development has been permitted and the ground levels of the topography or infrastructure immediately adjacent to the site fall below the 600 mm threshold, the credit can still be awarded, provided there are no other practical solutions for access to the site above this level and the assessed building, and access to it, meet the assessment criteria. As much of the external site area as possible (or as required by an appropriate statutory body) should be designed at or above the threshold. For buildings located in medium or high flood risk zones, any areas used to store sensitive, historical, hazardous, valuable and perishable materials, e.g. radioactive materials, microbiological facilities, server rooms, libraries, etc., must be located above the 600 mm threshold. |
|
|
CN3.6 |
Level of detail required in the FRA for smaller sites. |
For smaller sites, e.g. less than 1 ha (10,000 m²), the level of detail required in an acceptable FRA will depend on the size of the site and the arrangement of buildings on that site. For a small site with a relatively simple arrangement of buildings this might consist of a brief report. For larger sites with a higher density of buildings a more detailed assessment would be appropriate. For small simple sites (2000 m² and less), an acceptable FRA could be a brief report carried out by the contractor's engineer confirming the risk of flooding from all sources of flooding, including information obtained from the water company or sewerage undertaker, other relevant statutory authorities, site investigation and local knowledge. |
| Surface water run-off | ||
|
CN3.7 |
Sites with many buildings |
Where the assessed building is part of a larger development of buildings, there are a number of options for assessment of the surface water run-off credits:
Whichever approach is taken to demonstrate compliance, it must be consistent when completing both the rate of run-off and volume of run-off calculations. |
|
CN3.8 |
Discharge to the sea or tidal estuaries |
The peak rate of run-off and volume run-off criteria can be deemed to be met by default if the site discharges rainwater directly to a tidal estuary or the sea. The site must discharge run-off directly into the tidal estuary or the sea, if these criteria are to be awarded by default. Typically, this would mean that drainage pipes would only carry run-off from the site and that they would not need to cross privately owned land outside the boundary of the development before reaching the sea. Please see Relevant definitions section for a definition of tidal estuary. |
|
CN3.9 |
No change in impermeable area |
Where the man-made impermeable area draining to the watercourse (natural or municipal) has decreased or remains unchanged post-development, the peak and volume rate of run-off requirements for the surface water run-off credits will be met by default. Flow rate calculations will not need to be provided. Instead, drawings clearly showing the impermeable areas of the site draining to the watercourse should be provided for the pre-and post-development scenarios. Figures must also be given (ideally on the drawings) to show a comparison between the areas of drained impermeable surfaces pre-development and post-development. In this instance a flood risk assessment must be carried out and any opportunities identified to reduce surface water run-off are implemented. |
|
CN3.10 |
Limiting discharge flow rate. See criterion 12. | For the surface water run-off credits, where the limiting discharge flow rate would require a flow rate of less than 5L/s at a discharge point, a flow rate of up to 5L/s may be used where required to reduce the risk of blockage. |
|
CN3.11 |
Highways and impermeable areas | Where new non-adoptable highways are built, including those for developments with a mixture of buildings, all of the new impermeable surfaces must be included in calculations to demonstrate compliance with the peak rate of run-off and volume of run-off criteria. Where buildings are built beside existing highways or where adoptable highways are built, the impermeable area of the highway does not need to be included in the calculations. |
|
CN3.12 |
Derelict sites. |
If the site has been derelict for over five years, the appropriate consultant must assess the previous drainage network and make reasonable assumptions to establish probable flow rates and volumes. To do this they should use best practice simulation modelling to determine the 1-year and 100-year peak flow rates at the relevant discharge points. To complete the calculations, a site visit prior to development will be required unless accurate data already exist from a previous survey. The resultant professional report can then be used to determine the pre-development volumes and rates of run-off. Without this professional input, the site must be deemed greenfield pre-development, assuming Soil type 5 for the calculation of the pre-development site run-off. |
|
CN3.13 |
National best practice guidance on the design of SuDS and rainwater harvesting systems |
Please refer to the Approved Standards and Weightings List (ASWL)to locate the appropriate national best practice standards in the country of assessment. Alternatively, please demonstrate applicability as follows:
|
| Minimising watercourse pollution | ||
|
CN3.14 |
5 mm discharge for minimising watercourse pollution. See criterion 17. |
In a small number of sites it may not be possible for the first 5mm of rainfall to be prevented from leaving the site completely. Where this is the case, an appropriately qualified professional must design the system to ensure that the intent of this criterion has been met as far as possible and provide justifications to explain why the criterion could not be fully achieved on the site. Where this can be justified, the awarding of the water quality credit would not be affected, provided all other relevant criteria have been achieved. |
|
CN3.15 |
5mm requirement - end-of-pipe solutions. See criterion 17. |
End-of-pipe solutions, such as ponds and basins, will only be deemed to comply with the 5 mm criteria where the principal run-off control to prevent discharge from the first 5 mm of a rainfall event is achieved using source control and site control methods. |
|
CN3.16 |
5mm requirement - green roofs. See criterion 17. |
Green roofs can be deemed to comply with this requirement for the rain that falls onto their surface. However, evidence is still required to demonstrate that the 5mm rainfall from all other hard surfaces on site is being dealt with, to allow this credit to be awarded. |
| Areas that are a source of pollution. See criteria 19 and 20. | For the purpose of assessing the watercourse pollution credit, an area that presents a risk of watercourse pollution includes vehicle manoeuvring areas, car parks, waste disposal facilities, delivery and storage facilities or plant areas. | |
|
CN3.18 |
Extension or infill building on existing site | Where the assessment is of an individual building on an existing site, i.e. infill development, the watercourse pollution criteria apply to areas within the construction zone that present a risk of pollution, as well as any areas external to the construction zone that are affected by the new works, i.e. drainage onto or from the proposed development. |
|
CN3.19 |
Suitable level of treatment. See criteria 15–18. | In all cases the appropriate consultant should use their professional judgment to determine the most appropriate strategy for minimising watercourse pollution. |
|
CN3.20 |
Roof plant. See criteria 20, 21 and 15. | Roof-top plant space must be considered where there is a risk from polluting substances such as petrol or oil. Refrigerants are not assessed under the pollution aspect of this issue, as the main risk of pollution is to air and not the watercourse. |
|
CN3.21 |
Permeable paving system | Where it can be demonstrated that a permeable paving system designed to retain silts and degrade oils has been used, then this will meet the assessment criteria for minimising watercourse pollution for car parks and access roads. |
|
CN3.22 |
Workshop areas in retail buildings | Where workshop areas are specified, they should be assessed against the above requirements (minimising watercourse pollution). This is due to circumstances where there may be some form of vehicle servicing as part of a car showroom or other type of retail space. |
Methodology
Calculating peak rate of run-off
The assessor is not required to perform any calculations. Calculations should be provided by the appropriate consultant to demonstrate that they have sized the drainage facilities appropriately. Further guidance on calculating peak rate run-off for different sites and situations include:
- The SuDS Manual 1.
- Preliminary rainfall run-off management for developments.
- National planning policy guidance or statement for the specific country.
- IH Report 124, Flood estimation for small catchments (Marshall and Bayliss, 1994).
- Flood Estimation Handbook (Centre for Ecology and Hydrology, 1999) 2.
Greenfield sites of less than 50 ha
The calculation of greenfield run-off rates must be in accordance with IH Report 124, Flood estimation for small catchments (Marshall and Bayliss, 1994). The pro-rata method on the size of catchment detailed in Table 4.2 in The SuDS Manual, CIRIA C697 (2007) must be followed.
Greenfield sites of 50 ha to 200 ha
The calculation of greenfield run-off rates must be in accordance with IH Report 124, Flood estimation for small catchments (Marshall and Bayliss, 1994). Flood Estimation Handbook (Centre for Ecology and Hydrology, 1999) can be used for these sites as an alternative, where there is a preference to do so, but only if the catchment is considered to be suitable for its application.
Greenfield sites of more than 200 ha
The calculation of greenfield run-off rates must be in accordance with the Flood Estimation Handbook (Centre for Ecology and Hydrology, 1999) and any subsequent updates. Where the Flood Estimation Handbook is not considered appropriate for the development, IH Report 124 can be used.
Brownfield sites
The calculation of brownfield run-off rates should be as follows:
- If the existing drainage is known then it should be modelled using best practice simulation modelling, to determine the 1-year and 100-year peak flow rates at discharge points (without allowing surcharge of the system above cover levels to drive greater flow rates through the discharge points).
- If the system is not known, then the brownfield run-off should be calculated using the greenfield run-off models described above but with a Soil Type 5.
Limiting discharge rate
The limiting discharge for each discharge point should be calculated as the flow rates from the pre-developed site. The calculation should include the total flow rate from the total area of the site feeding into the discharge point (this should include both BREEAM-assessed and non-BREEAM-assessed parts of the development, if applicable). The discharge point is defined as the point of discharge into the watercourse or sewers (including rivers, streams, ditches, drains, cuts, culverts, dykes, sluices, public sewers and passages through which water flows, see Relevant definitions ). Where this calculation results in a peak flow rate of less than 5L/s, the limiting discharge rate may be increased up to a level of no more than 5L/s at the point of discharge from the site to reduce the risk of blockage.
For example, if the flow rate for the 1-year and 100-year events were 4L/s and 7L/s respectively, then the limiting discharges would be 5L/s and 7L/s. Similarly, if it was calculated to be 2L/s and 4L/s, then a maximum of 5L/s limiting discharge rate could be applied to both discharge points.
Sites should not be subdivided to enable higher overall limiting discharge rates to be claimed. It is, however, recognised that some sites may require more than one discharge point as a result of the local topography or existing surrounding drainage infrastructure, and in such cases, the limiting discharge flow rate may be increased to a level no more than 5L/s at each discharge point. The assessor should seek evidence that the number of discharge points is necessary, either due to topography, infrastructure limitations or both. Evidence may be in the form of a topographical map and an explanation from the appropriate consultant as to why multiple discharge points are required, stating that it is not feasible to have fewer discharge points.
100-year peak rate event: excess volume of run-off
The storage of excess flows from the 100-year event does not necessarily have to be contained within the drainage system or SuDS features (the features designed solely for the purpose of drainage). Where appropriate, storage of some or all of this volume can be achieved using temporary surface flooding of areas such as a playing field. Specific consideration should be given to overland flow routing. Overland flood flows and temporary storage of flood water on the surface must not be so frequent as to unreasonably inconvenience residents and other users.
Evidence
| Criteria | Interim design stage | Final post-construction stage |
|---|---|---|
| Flood Risk | ||
| 1– 3 |
Flood risk assessment. Design drawings. Where appropriate, correspondence from the appropriate statutory body confirming reduced annual probability of flooding due to existing flood defences. |
Flood risk assessment updated as necessary. ‘as-built’ drawings. Confirmation that the basis of the Flood Risk Assessment has not changed where more than five years have passed since the Flood Risk Assessment was carried out. |
| Surface Water Run-off | ||
| 4–-16 | Statement from the appropriate consultant confirming that they are qualified in line with the BREEAM definition. Consultant's report containing all information necessary to demonstrate compliance with the requirements. |
Evidence to confirm that maintenance responsibilities have been defined for any SuDS solutions installed. AND EITHER Written confirmation from the developer or appropriate consultant that the solutions assessed at the design stage have been implemented OR Where the design has changed, the evidence identified for the design stage assessment is provided for post-construction or as-built details. |
| Minimising Watercourse Pollution | ||
|
Design drawings or relevant section or clauses of the building specification or contract indicating:
|
Assessor’s building or site inspection and photographic evidence AND EITHER Written confirmation from the developer or appropriate consultant that the solutions assessed at the design stage have been implemented OR Where the design has changed, the evidence identified for the design stage assessment is provided for post-construction or as-built details. |
|
Additional information
Please note this section will be revised when the National Standards for Sustainable Drainage and associated regulations come into force.
Relevant definitions
- Adoptable highways
- For the purposes of BREEAM, an 'adoptable' highway is a highway that is the responsibility of the highways authority in terms of installation and maintenance of surface water drainage which only carries run-off from the highway itself. This means that to fall under the definition of an 'adoptable highway' the drainage network must not be directly connected to any other upstream drainage network (e.g. from a private development) and only handle run-off from the adoptable highway. Where drainage within the highway will carry run-off from both the highway and housing, it is not regarded as an 'adoptable' highway. In this instance the drainage design must take account of the highway run-off.
- Appropriate consultant
- A consultant with qualifications and experience relevant to designing SuDS and flood prevention measures and completing peak rate of run-off calculations. Where complex flooding calculations and prevention measures are required, this must be a specialist hydrological engineer.
- Appropriate statutory body
- This refers to the statutory organisation, legal organisation or entity whose duty it is to carry out the planning approval function for the project.
- Catchment
- The area contributing surface water flow to a point on a drainage or water course. It can be divided into sub-catchments.
- Current best practice planning guidance
- The document should include independently published figures for an allowance for climate change (based on a minimum period of 100 years) according to the annual probability of flooding in the area.
- Design flood level
- The maximum estimated water level during the design storm event. The design flood level for a site can be determined through either known historical data or modelled for the specific site.
- Design storm event
- Historic or notional weather conditions of a given annual probability, against which the suitability of a proposed development is assessed and mitigation measures, if any, are designed.
- Discharge point
- The discharge point is the point at which the run-off from the site leaves the site boundary and enters a watercourse.
- Flood defences
- Flood defences do not completely remove the risk of flooding, but they do reduce it. Building in areas where flood defences are present (and appropriately designed to withstand a certain magnitude of flooding) is therefore preferable to building in medium or high risk areas without defences. However, for the purpose of this issue, it is still preferable to build in areas of low risk than encourage development of new flood defences in areas with a higher risk of flooding purely for the sake of new development.
- Flood event
- A flooding incident characterised by its peak level or flow, or by its level or flow hydrograph.
- Flood probability
- The estimated probability of a flood of given magnitude occurring or being exceeded in any specified time period. For example, the 100-year flood has a 1% chance of occurring in any given year.
- Flood risk
- The combination of the flood probability and the magnitude of the potential consequences of the flood event.
- Flood risk assessment
- A study to assess the risk of a site flooding, and to assess the impact that any changes or development on the site will have on flood risk to the site and elsewhere. A Flood Risk Assessment (FRA) should be prepared according to relevant planning policy and technical guidance documents. Confirmation must be based on historical, geological and geomorphic data (e.g. altitude) and take all sources of flooding into consideration. The FRA must account for future climate change and detail any necessary adaptation measures if required. Where more than five years have passed since the FRA was carried out, evidence would be required to demonstrate that the basis of the FRA has not changed in that time.
- Flood storage
- The temporary storage of excess run-off or river flow in ponds, basins, reservoirs or on the flood plain during a flood event.
- Greenfield
- A site which has either never been built on, or one which has remained undisturbed for five years or more.
- Greenfield run-off rate
- The rate of run-off that would occur from the site in its undeveloped and therefore undisturbed state.
- Hard surfaces
- These include roofs, car parks, access roads, pavements, delivery and service yards and external hard landscaping. Footpaths less than 1.5m wide which have free drainage to soft landscaped areas on both sides may be excluded.
- Infiltration
- The passage of water into a permeable surface, such as soil, permeable paving and soakaways.
- Limiting discharge
- The limiting discharge is based upon the calculated pre-development flow rate at a discharge point.
- Level of pollution prevention treatment
- When used in the context of one, two or three levels of treatment for surface water, the treatment level should be regarded as the number of SuDS components in series through which run-off passes from the originating surface on which rainfall fell to the site discharge point. Where a SuDS component has more than one treatment process, it might be considered to provide more than one level of treatment. In these circumstances advice should be sought from the BREEAM office.
- Low-risk areas (with respect to watercourse pollution)
- Low-risk areas can be defined as areas where the risk of contamination or spillage of substances such as petrol and oil is reduced. For the purpose of this issue, roofs and small car parks may be considered as low-risk areas.
- Peak run-off rate (referred to as Qp [m 3/sec])
- This is the highest rate of flow from a defined catchment area assuming that rainfall is uniformly distributed over the drainage area, considering the entire drainage area as a single unit and estimation of flow at the most downstream point only.
- Pre-development
- The state of the site under assessment immediately prior to purchase of the site by the client or developer (or, where the client has owned or occupied the site for a number of years, its current state).
- Qbar
- An estimation of the mean annual flood flow rate from a catchment (see Report IH124 Flood estimations for small catchments).
- Rainwater discharge
- Rainwater discharge is the rainwater which flows from the development site to watercourses and sewers. It is also referred to as run-off.
- Run-off
- This is usually rainwater, but can also be groundwater or overspill from sewers and other sources.
- Sewerage undertaker
- This is a Body, typically a water company, with statutory responsibility for sewerage and sewerage disposal and also surface water from roofs and yards of premises.
- Soakaways
- A subsurface structure designed to promote the infiltration of surface water into the ground. As a general point, soakaways may be shallow and broad – as in a blanket under permeable paving, or deeper structures. Deeper, point source soakaways should be avoided for road and car park drainage; shallow structures providing infiltration in an extensive way (infiltration trenches and permeable paving) do not need oil separators.
- SuDS management train
- An approach to drainage design that combines a sequence of appropriate surface water drainage structures using SuDS systems for management of the run-off to treat the flow, reduce run-off volume and restrain the run-off rate in order to minimise man's impact on the environment. Additional benefits associated with operation and maintenance, ecology and amenity are aspects which are considered when designing a management system. The management train incorporates a hierarchy of techniques:
- Source control. Examples of SuDS techniques include:
- Soakaways
- Porous or pervious paving
- Roof water directed to garden (rather than piped drains)
- Rainwater reuse or harvesting
- Green roofs
- Other surface infiltration, attenuation and conveyance techniques that deal with run-off at source.
- Site or local control. Examples of SuDS techniques include:
- Swales
- Pond
- Infiltration basins
- Detention basin
- Larger soakaways
- Pervious (porous or permeable) paving.
- Regional control. Examples of techniques include:
- Balancing ponds
- Wetlands
- Large detention basin.
- Source control. Examples of SuDS techniques include:
- SuDS techniques
- One or more components built to manage surface water run-off to prevent flooding and pollution, including for example: wet ponds, infiltration basins, detention basins, swales, reed beds, pervious (porous or permeable) paving, soakaways, rainwater harvesting, filter strips, filter drains and trenches with or without perforates pipes, green roofs and underground attenuation storage. For more information refer to The SuDS manual.
- Surface water run-off
- Water flow over the ground surface to a drainage system. This occurs if the ground is impermeable, is saturated or if the rainfall is particularly intense.
- Tidal estuary
- A tidal estuary is defined as a semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater is measurably diluted with fresh water derived from land drainage. An estuary should be unconstrained tidal waters, i.e. there should be no barriers or constricted shorelines that would restrict the free flow of water into the open sea in any conditions. The impact on the total volume of run-off from the site (and other sites which may in future discharge into the estuary) should be insignificant in terms of the overall water levels in the estuary. Tidal rivers (i.e. where no or limited measurable seawater content is present during normal tidal movements) cannot be included as part of the estuary for the purposes of BREEAM.
- Treatment
- Improving the quality of water by physical, chemical or biological means.
- Types of oil separator
- Class 1 Separators: These are designed to achieve a concentration of less than 5mg/l oil under standard test conditions. They should be used when the separator is required to remove very small oil droplets, such as those arising from car park run-off.
- Class 2 Separators: These are designed to achieve a concentration of less than 100mg/l oil under standard test conditions. They are suitable for dealing with discharges where a lower quality requirement applies or for trapping large spillages. Both classes can be produced as 'full retention' or 'bypass' separators:
- Full retention separators: These treat the flow that can be delivered by the drainage system, which is normally equivalent to the flow generated by a rainfall intensity of 50mm/hr.
- Bypass separators: These fully treat all flows generated by rainfall rates of up to 5mm/hr. Flows above this rate are allowed to bypass the separator. These separators are used when it is an acceptable risk not to provide full treatment for high flows. Pollution Prevention Guideline 3 contains more detailed guidance on the selection and sizing of an appropriate type of separator.
- Volume of run-off
- The volume of run-off that is generated by rainfall occurring on the site. This is typically measured in cubic metres. Additional predicted volume of run-off is the difference between the volumes of run-off pre-development and post-development.
- Watercourses and sewers
- A term that includes rivers, streams, ditches, drains, culverts, dykes, sluices, sewers and passages through which water flows.
Other information
Sources of flooding and flood risk
- Streams and Rivers: Flooding that can take place from flows that are not contained within the channel due to high levels of rainfall in the catchment.
- Coastal or Estuarine: Flooding that can occur from the sea due to a particularly high tide or surge, or a combination of both.
- Groundwater: Where the water table rises to such a height where flooding occurs. Most common in low-lying areas underlain by permeable rock (aquifers), usually due to extended periods of wet weather.
- Sewers and highway drains: Combined, foul or surface water sewers and highway drains that are temporarily over-loaded due to excessive rainfall or due to blockage.
- Surface water: The net rainfall falling on a surface (on or off the site) which acts as run-off which has not infiltrated into the ground or entered into a drainage system.
- Infrastructure failure: Canals, reservoirs, industrial processes, burst water mains, blocked sewers or failed pumping stations.
Contaminated sites
Where the site risk assessment confirms that infiltration SuDS techniques are not appropriate, SuDS techniques that do not allow infiltration, such as swales lined with an impermeable membrane, can be used. It may be the case that only some areas of the site are contaminated and therefore infiltration SuDS techniques can be used elsewhere on the site. There may also be a requirement to remediate the contaminated soils, creating opportunities for the use of infiltration SuDS post-remediation.