Dr Liz Lake: Sustainable Urban Drainage (SUD): The basic principles
Landscape Institute East of England Branch 7th July 2011

The background. The start of the 21st century brought extensive flooding across the UK and encouraged the development of an existing concept first developed in Portland, Oregon, USA: Sustainable Urban Drainage (SUD). This note is intended to give a simple explanation of the principles of SUD to help external works designers negotiate with drainage engineers to create optimum solutions for the disposal of surface and storm water.  

SUD, SUDS or SuDS ? We are aiming to achieve Sustainable Urban Drainage (SUD) by using Sustainable Urban Drainage Solutions or Systems (SUDS).  You will also see SuDS – Sustainable Drainage Systems.

What’s new? SUD is about replicating the water cycle so when rain falls on the ground as much as possible is returned to the ground water system, is absorbed by plants or is recycled and the minimum goes into the public surface drainage water system. The future form of urban drainage will involve less gullies, more ditches, integrated SUD solutions and streets and open areas that act as large scale ‘open’ storm drains. Whereas previously a drainage system was hidden below ground in a closed system, now it will be visible above ground in an open system that allows space for water.

Information. The SUDS Manual produced by Ciria is the ‘bible’ that provides all the detail needed for the drainage engineer to design solutions and calculate volumes, but probably only 20% of this information is relevant to the external works designer.  Local Authority Guidelines provide easier and helpful explanations and Cambridge City Council has an excellent example.  The Flood and Water Management Bill 2010 will change the way SUDS are adopted and there will be a variation in the speed with which the relevant authorities sort out their procedures.  You may expect a difficult and confusing period concerning adoption until the new system settles down.

Water falling on the site. Any SUD solution has to take account of, and deal with, three elements: the sheer volume of water, the pollutants and the sediment. Water is dealt with in three broad areas: source control, infiltration/filtration and attenuation/retention/detention. 

Collecting water off the roof. Dealing with surface water run-off  starts with source control, which involves collecting water reusing it for irrigation, directing it to a Rain Garden, recycling it as grey water before it reaches the ground or letting it fall on a ‘green’ roof. ‘Rain Gardens’ are a more accessible and friendlier name for SUD systems, but they can also be gardens specifically designed to absorb rainwater from roofs and hard surfaces. 

Green/Brown roofs. The terms ‘Green’ and ‘Brown’ roofs are bandied about but, to be more precise, they are Living Roofs and come in three varieties: intensive, semi intensive and extensive.

An intensive roof is a traditional roof garden and has a fair depth of topsoil, from a 300mm minimum to as much as a metre.  It is irrigated and capable of growing a wide range of plants.   (Green)

A semi-intensive roof has a topsoil depth (or substrate) of around 150mm and might be irrigated – this would depend on the finish that was required and, if plants were used that had a low water requirement, it might be avoided.  A ‘meadow’-looking roof would require this kind of semi-intensive approach, but if the visual requirement is for it to look like the meadow on the ground, it would definitely need irrigation and possibly a deeper depth of topsoil. (Green - if irrigated or not) 

A sedum roof comes under the category of an extensive roof, as these are not meant to be walked on except for maintenance. This type of roof has a very shallow substrate between 20mm and 150mm. The sedum is grown on matting rather than a substrate, but some of these are dying out after 10 or 12 years and there is a school of thought that suggests they will grow better on 100mm depth topsoil. Sedum roofs are generally intended for urban areas.  (Brown)

A roof could be a hybrid of semi intensive and extensive, but the flora would vary in the different habitats.  It is in the ‘extensive’ area that ecological experiments are taking place – these use different substrates (eg crushed stone, aggregate, pumice) at different depths which results in a very varied flora. Crushed brick is used to provide a particular habitat for a bird – the black redstart - as this resembles the old bomb sites/ brown field sites. (Brown)

It is clear that a vegetated roof cools the building very effectively in that it warms up slowly and the need for air conditioning arrives at around 5pm when everyone is going home from work rather than earlier in the day. If properly constructed, there is little evidence that roots damage roof membranes (root inhibitor barriers are available) and they protect them from deterioration from the sun. The roof does have to be well constructed to be leak proof and take the additional loading, but research has shown that it can last twice as long and so it pays for itself in the long run.

All such roofs require pedestrian access (if only for the ecologist monitoring it each year!) and even sedum roofs benefit from an annual feed. If a ‘meadow’ is considered desirable, this will need to be cut once a year and the arisings raked off. While irrigation may not be used long term, water may be needed for plant establishment.  There are two alternative strategies - provision of a water point or acceptance that plants that have died due to lack of water will need to be replaced.

The safety of everyone on the roof is paramount; no one should be put at risk so a parapet (minimum 1.1m high) is desirable. These need to be integral to the design of the building and not added later. The use of fixed cables which maintenance personnel clip onto instead of a parapet is no longer acceptable practice.

Source control on the ground is about reflecting the water cycle by slowing down the rate at which water enters a public surface water drainage system. New and interesting ideas are coming forward for designed landscape features that bring benefits for biodiversity.

Planting. In discussions with drainage engineers always remember the basic principle that plants, particularly trees, absorb water from the ground and release it back into the atmosphere by transpiration.  Plants can make a major contribution to SuD solutions.  Proprietary products exist that allow for a tree to be planted in a pit in a hard surface that has a cell underneath to collect surface water and provide water for the tree.

Pervious or permeable pavements fall into all three categories as they can be used to collect water under a surface to be recycled (source), allowed to drain into the substrate (infiltration) or collected in a container for slow release (attenuation).  All types of solutions can be linked by ‘controlled conveyance’ which in itself provides a fourth area. This can make use of pipes of various sizes, open ditches and swales (open, wide, gently sloping grass channels, wet or dry) which reduce peak flows and provide the potential for infiltration and filtration.

Infiltration/filtration. The benefit of infiltration/filtration is that it allows the base flows of water to be recharged. Infiltration includes familiar features like soakaways and ‘French drains’ which ‘attract’ water, but filtration uses more sophisticated devices such as surface and sub-surface sand filters, perimeter sand filters, bio-retention/filter strips and filter trenches. These also contribute to the trapping of sediments but more particularly to the removal of pollutants.

Attenuation/retention/detention. In simple terms, the three solutions of attenuation/retention/detention can be described as:
Attenuation: where surface water collects for slow release. This could be an underground tank or an open grass area where water can collect during periods of heavy rain and then be released slowly.
Retention: Making use of a permanent water body with some flood capacity. Some attenuation ponds are over deepened to provide permanent wet areas for nature conservation benefits. At times of high rainfall, the water level temporarily rises and is then released slowly back down to the permanent level.
Detention: These areas remain dry except in times of flood. These are usually larger areas where water may be retained for some time and are cleared as water evaporates or levels fall.

Water quality. The principle of dealing with the volume of water at source, on site and then allowing it back into the wider environment in a controlled way also applies to water quality treatment.  Collecting water to keep it uncontaminated for recycling will become an important factor in water harvesting. As now, pollutant (petrol) interceptors will be required at the edge of parking areas but reed beds will be a more common sight both on site and in the wider landscape.  This is called the SUD Management Train.

Nature conservation benefits. Landscape architects do not need to be reminded about the nature conservation benefits of wetlands – it is integral to their work but the difference is that the drainage engineer has more of a duty to consider habitat creation. This can include shallow wetland, extended detention wetland, pond/wetland, pocket wetland, submerged gravel wetland and wetland channels.

Exceedance flow. A key principle of SUD is to delay or reduce the flow of water before it enters the public surface water drainage system.  There is an acceptance that not all water can be accommodated within a SUD solution and the excess will need to find its way to the public system.  This excess is called the ‘exceedance flow’. 

Sources of Information

•    PPS25 Practice Guide: Integrated Urban Drainage Management
•    Flood & Water Management Bill 2010
•    Defra Surface Water Management Plan Technical Guidance. Integrated urban drainage (IUD)
•    Approved document H: Drainage and Waste Disposal: 2002 Building Regulations Requirement H3
•    SUDS Manual ciria.org.uk/suds
•    environment-agency.gov.uk
•    Understanding permeable paving.  Interpave, www.paving.org.uk
•    Sudsnet
•    Suds for Roads: Scottish Government Guide 2009
•    Local authority design guides and adoption procedures eg Cambridge City Council
•    Rain Gardens by Nigel Dunnett and Andy Clayden