3.6 Surface water drainage
Climate change is expected to result in more rain in the future and it is essential that this is taken into account in buildings. It is essential that the surface water from buildings is removed safely without damage to the building, danger to people around the building and does not pose a risk to the environment by flooding or pollution.
Development of building sites generally leads to increased impermeable areas that can significantly increase the amount of surface water run-off to be dealt with. The approach to the disposal of surface water from buildings and hard surfaces clearly needs to be considered at the earliest stage in the design and development process (see also the Scottish Executive Development Department's Planning Advice Note No. PAN 61 - Planning and Sustainable Urban Drainage Systems).
For safety reasons it is essential that surface water is not permitted to collect or remain on all access routes to buildings, particularly with elderly and disabled people in mind. Ponding in winter can cause slippery surfaces that can be a hazard to pedestrians.
Controlled activities - the discharge of surface water is a controlled activity under The Water Environment (Controlled Activities) (Scotland) Regulations 2005. Under these regulations surface water discharges to ground or water (wetlands, surface waters or groundwater) must be by means of a sustainable urban drainage system authorised by The Scottish Environment Protection Agency (SEPA). Surface water discharge from a single dwelling or its curtilage is permitted to be by other means.
In some localities there is growing pressure on water resources due to changes in the climate and increasing expectations of personal hygiene. The introduction of conservation measures, such as the collection of surface water for its reuse is strongly encouraged to reduce the reliance on mains water.
Explanation of terms: Paved surface – means any constructed hard surface more than 50m2.
Every building should be provided with a drainage system to remove rainwater from the roof, or other areas where rainwater might accumulate, without causing damage to the structure or endangering the health and safety of people in and around the building. Where gutters and rainwater pipes are used, they should be constructed and installed in accordance with the recommendations described in BS EN 12056-3: 2000.
Eaves drop systems - methods other than gutters and rainwater pipes may be utilised to remove rainwater from roofs. An eaves drop system will allow rainwater to drop freely to the ground. Where these are used, they should be designed taking into account the following:
the existing groundwater level and ground infiltration capacity
the protection of the fabric of the dwelling from ingress of water caused by water splashing on the wall
the need to prevent water from entering doorways and windows
the need to protect persons from falling water when around the dwelling
the need to protect persons and the building fabric from rainwater splashing on the ground or forming ice on access routes. The provision of a gravel layer or angled concrete apron or such like may be acceptable
Gutters and rainwater pipes may be omitted from a roof at any height provided it has an area of not more than 8m2 and no other area drains onto it.
Every domestic building should be provided with a drainage system to remove surface water from paved surfaces, such as an access route that is suitable for disabled people, without endangering the building or the health and safety of people in and around the building. The paved surface should be so laid as to ensure rainwater run-off is not close to the building. Paved surface drainage systems should be designed, constructed and installed, either:
a. incorporating SUD system techniques as in clause 3.6.4, or
b. using a traditional piped drainage system as in clause 3.6.8.
Surface water discharged from a domestic building and a hard surface within the curtilage of a domestic building should be carried to a point of disposal that will not endanger the building, environment or the health and safety of people around the building.
Surface water discharge should be to:
a storage container with an overflow discharging to any of the 4 following options, or
a SUD system designed and constructed in accordance with clause 3.6.4, or
a soakaway constructed in accordance with:
clause 3.6.5, or
the guidance in BRE Digest 365, ‘Soakaway Design’, or
National Annex NA 4 of BS EN 752: 2008, or
a public sewer provided under the Sewerage (Scotland) Act 1968, or
an outfall to a watercourse, such as a river, stream or loch or coastal waters, that complies with any notice and/or consent by SEPA.
Discharge from a soakaway should not endanger the stability of the building. Damage to the foundations is likely to occur where discharge is too close to the building and it is sensible to ensure that any water bearing strata directs water away from the building.
Location of soakaway - to prevent such damage therefore, every part of a soakaway should be located at least 5m from a building and from a boundary in order that an adjoining plot is not inhibited from its full development potential. However the volume of surface water run-off, ground strata or permeability of the soil may influence this dimension and it may be reduced, or indeed may need to be increased, to preserve the structural integrity of the building.
Sustainable urban drainage (SUD) is a concept that focuses decisions about drainage on the environment and people. The concept takes account of the quantity and quality of surface water run-off and the amenity value of surface water in the urban environment.
The variety of design options available allows designers and planners to consider local land use, land take, future management and the needs of local people. SUD systems often stretch beyond the confines of the curtilage of individual buildings but need to be considered as a whole.
Fundamental to a successful SUD system is a management train that allows for a range of components to be incorporated for control or management of surface water, such as:
Source Control – control of run-off at or very near its source by components including soakaways, other infiltration methods, green roofs or permeable surfaces.
SUD systems range from the use of basic components such as permeable materials that allow surface water to infiltrate to ground in a way that can mimic natural drainage to more complex engineered components including filter strips, swales, or wet ponds that will convey or store surface water. The CIRIA document C753 ‘The SUDS Manual’ 2015 http://www.ciria.org provides comprehensive advice on initial drainage design assessments and best practice guidance on the planning, design, construction, operation and maintenance of SUD systems.
Careful consideration should be given to the design of surface water drainage from brownfield land, particularly where contamination might be expected. SEPA provides guidance in their SUDS Advice Note – ‘Brownfield Sites’ http://www.sepa.org.uk/. Generally SUD systems are designed to utilise natural processes and regular monitoring and maintenance will be needed to ensure the system as conceived is operating as intended.
Soakaways have been the traditional method of disposal of surface water from buildings and paved areas where no mains drainage exists. A soakaway serving a single dwelling or an extension should be designed and constructed in accordance with the following guidance:
test the suitability of the ground in accordance with the percolation test method in clause 3.9.1. The trial holes and the finished soakaways should be a minimum of 5m from the dwelling and the boundary. However this dimension may be reduced slightly on small sites where ground conditions allow, such as very well draining soil
there should be individual soakaways for each dwelling.
The soakaways may be sized using the following simplified formulae derived from BRE Digest 365:
(A x 0.0145) – (a x f x 900) = S
Where - A is the area to be drained in m2.
a - is the internal surface area of the soakaway to 50% effective depth, excluding the base in m2. This has to be assumed for initial calculation purposes.
f - is the soil infiltration rate, in m/s, determined in accordance with clause 3.9.1. This calculation produces Vp in secs/mm [conversion = (1/Vp) /1000].
S - is the required storage in m3.
Free draining surface water run-off may be appropriate for small hard surface areas, such as access paths. Free draining run-off can be achieved by laying the surface to a fall, sufficient to avoid ponding, that allows the water to drain to a pervious area, such as grassland, provided the infiltration capacity of the ground is not overloaded. Also the discharge should not be adjacent to the building where it could damage the foundations.
Rainwater harvesting systems allow surface water run-off from dwellings or hardstanding areas to be collected, stored and distributed thereby reducing the demand for potable water, the load on drainage systems and surface water run-off that can lead to incidents of flooding. The re-use of surface water can produce benefits to the home owner and the environment and therefore is recommended.
Rain, as it falls on buildings, is soft, clear and largely free of contaminants. During collection and storage however there is potential for contamination. For this reason it is recommended that recycled surface water is used only for flushing water closets, car washing and garden taps as it is not possible to guarantee the necessary maintenance of complex filters that would be essential if it were used for consumption or personal hygiene.
Manufacturers of proprietary systems or guidance documents such as CIRIA C539 or ‘Harvesting Rainwater for domestic use: an information guide’ published by the Environment Agency http://www.environment-agency.gov.uk/ provides helpful guidance on design considerations.
Where a rainwater harvesting system is to be installed it should be designed, constructed and installed in accordance with the Water Byelaws 2004, the recommendations of the Water Regulations Advisory Scheme (WRAS) Information and Guidance Notes No. 9-02-04 and 9-02-05, 1999 and the CIRIA publication CIRIA C539; ‘Rainwater and greywater use in buildings’, provide good practice installation, design and maintenance advice. The approval of Scottish Water is needed before any such scheme is installed.
Filtration - prior to the storage of water in a tank the rainwater should be filtered to remove leaves and other organic matter and dust or grit. Disinfection may be required if the catchment area is likely to be contaminated with animal faeces, extensive bird droppings, oils or soil.
Water storage tanks should be constructed of materials such as GRP, high-density polyethylene, steel or concrete and sealed and protected against the corrosive effects of the stored water and to prevent the ingress of ground water if located underground.
Water for use in the dwelling should be extracted from just below the water surface in the tank to provide optimum water quality. All pipework carrying rainwater for use in the dwelling should be identified as such in accordance with the WRAS guidance notes and great care should be taken to avoid cross-connecting reclaimed water and mains water. Tanks should be accessible to allow for internal cleaning and the maintenance of inlet valves, sensors, filters or submersible pumps. An overflow should discharge to a soakaway (see guidance to Standard 3.6) or to mains drainage where it is not reasonably practicable to discharge to a soakaway. Backflow prevention devices should be incorporated to prevent contaminated water from entering the system.
There can be substantial advantages from the use of SUD systems, but where a traditional piped system is required it should be designed and constructed in accordance with the guidance in National Annex of BS EN 752: 2008.
Where a discharge into a traditional drainage system contains silt or grit, for example from a hard standing with car wash facilities, there should be facilities for the separation of such substances. Removable grit interceptors should be incorporated into the surface water gully pots to trap the silt or grit.
A surface water drainage system should be tested to ensure the system is laid and is functioning correctly. Testing should be carried out in accordance with the guidance in BS EN 1610: 1998.