Sustainable Drainage Systems, or SuDS, are drainage systems that are designed to mimic the way that water would behave on a site if it were not developed, and it were still a green field. The rainfall should soak into the ground wherever it falls if possible so that it can nurture soils and plants and replenish aquifers. It can be captured in ponds and wetlands too, when there is space. On all sites, rainwater should be captured and reused in the buildings and the gardens, so that it doesn’t rush off the site and cause flooding downstream.

The 4 Pillars Of SuDS

A good SuDS scheme will have four primary benefits, known as the four ‘Pillars of SuDS’. They are – managing the Water Quantity to reduce flood risk; managing the Water Quality to reduce pollution; delivering habitats for wildlife or Biodiversity; and creating green and pleasing places for people or Amenity.

Water Quantity – this involves keeping as much water on site as practical and/or store it for a controlled release, this is commonly known as attenuation.

Water Quality – arguably the most important as the world is in crisis over clean water so treating water to the highest level is critical.

Biodiversity – design SuDS in harmony and sympathy with wildlife and plant life, its deliverability varies from project to project.

Amenity – designing SuDS to deliver benefits to the community such as a quality water feature, this often connects with biodiversity.

Flood & Water Management Act (Schedule 3)

SuDS are now the preferred drainage design method for all sites and many Planning Authorities will expect developers to include them in their projects. With DEFRA’s proposed enactment of Schedule 3 of the Flood and Water Management Act 2010, the inclusion of SuDS on all developments will become mandatory in England; this is already mandatory in Wales. The current focus on sewer overflows and the impacts of climate change mean that all developments should include SuDS, not just residential projects. SuDS are also being retro-fitted into our towns and cities when other construction projects are in progress and many local authorities are looking for opportunities to deliver them alongside schemes such as highway projects.

Water Quality

The Water Quality pillar of SuDS is SPEL’s specialty, and we are passionate about protecting our water resources across the globe. Fact: ‘only about 1.2% of earth’s water can be used as drinking water’ (National Geographic, 2023), so we must look after this precious asset. It is vital that SuDS schemes deliver good pollution control and the Simple Index Approach in the CIRIA SuDS Manual C753 will help us to do this using the Mitigation Index and designing the treatment train to this standard as a minimum. It’s key to understand how much pollution is in the runoff from the site, some sites where for example oil is stored or moved around the site, it will be necessary to manage the spill risk and this must be done first as its at the ‘top’ of the SuDS Management Train.

Other devices include ponds, basins and wetlands but care must be taken as these nature-based devices cannot be certain to deal with all pollution loads and most SuDS Treatment Trains will need to include a manufactured device. The CIRIA SuDS Manual C753 states that these devices must be installed upstream of any natural feature or attenuation tank to capture specific pollutants such as Total Suspended Solids (TSS), Metals, and Hydrocarbons while allowing the vegetation to thrive.

The SuDS hierarchy points to re-use and infiltration as the first stage of a good design with the use of rainwater harvesting, bio-remediation zones, tree pits and swales, allowing the water into the soil where pollutants are captured and broken down and the plants can take up some of the nutrients.

Whole Life Issues and the Importance of Maintenance

Delivering good SuDS isn’t the end of the story. It is essential that they are maintained in the long-term to keep them healthy, and to make sure that they are delivering what they were designed to do. If sediment is captured in the system, it must be removed periodically to ensure that it doesn’t pass through the system and pollute the watercourse. The function of the SuDS system must be protected by keeping inlets, outlets and probes clean and operational, there is likely a need for some vegetation management to keep natural systems functioning correctly. Manufactured Devices also need maintenance so it’s important to identify who is responsible for carrying this out and to have an asset/service log.

Sustainable Urban Drainage

The Pollution Hazard Index and the Mitigation Index are central to the Simple Index Approach which are the tools used for evaluating water quality and pollution control in SuDS design (CIRIA SuDS Manual C753 – 26.2 & 26.3 below).

To use this approach, we first need to evaluate the risk (Low, Medium & High) of each surface and then build a treatment train that mitigates the risk to at least the same or a greater value. For example, a residential car park is classed Low Risk so we can use a SPEL Smartceptor (0.5, 0.4, 0.5) which more than covers the risk, or we could use the SPEL ESR system (0.8, 0.6, 0.9) which provides a much higher mitigation level but is still acceptable.

For High Risk sites, we need to design a treatment train that not only delivers the highest level of mitigation, we need to consider capturing the dissolved metals in order to achieve the required 0.8 on this pollutant. According to the British Water ‘How To Guide’ the particulate content of metals will not exceed a pollution hazard index greater than 0.63, the balance is made up of dissolved metals, these can only be treated with the use of a suitable treatment device such as the SPELFilter.

Where the SPELFitler is used downstream of the SPEL ESR unit, the treatment train will exceed the ‘High Risk’ Mitigation Index (see simplified SuDS Treatment Train concepts opposite). It is worth noting that retention devices such as an oil separator or pond will not capture dissolved metals.

26.2 Pollution hazard indices for different land use classifications
Land usePollution hazard levelTotal suspended solids (TSS)MetalsHydrocarbons
Residential roofsVery low0.20.20.05
Other roofs (typically commercial/industrial roofs)Low0.30.2 (up to 0.8 where there is potential for metals to leach from the roof)0.05
Individual property driveways, residential car parks, low traffic roads (e.g., cul de sacs, homezones and general access roads) and non-residential car parking with infrequent change (e.g., schools, offices) i.e., < 300 traffic movements/dayLow0.50.40.4
Commercial yard and delivery areas, non-residential car parking with frequent change (e.g., hospitals, retail), all roads and trunk roads/motorways1Medium0.70.60.7
Sites with heavy pollution (e.g., haulage yards, lorry parks, highly frequented lorry approaches to industrial estates, waste sites), sites where chemicals and fuels (other than domestic fuel oil) are to be delivered, handled, stored, used or manufactured; industrial sites; trunk roads and motorways1High0.820.820.92
26.3 Indicative SuDS mitigation indices for discharges to surface waters
 Mitigation Indices
Type of SuDS componentTSSMetalsHydrocarbons
Filter strip0.40.40.5
Filter drain0.420.40.4
Bioretention system0.80.80.8
Permeable pavement0.70.60.7
Detention basin0.50.50.6
Proprietary treatment systems5,6These must demonstrate that they can address each of the contaminant types to acceptable
levels for frequent events up to approximately the 1 in 1 year return period event, for
inflow concentrations relevant to the contributing drainage area.

Tables from The SuDS Manual (C753), p568-569
For reference notes, please see the full manual:

Sustainable Urban Drainage