Sustainable drainage systems (SuDS) provide an alternative to the direct channelling of surface water into the sewerage network. Such an approach feels timely, as growing populations and changes in land uses are putting increasing pressure on existing sewers, and making it harder to prevent flooding (see also pp18-20). Storm overflows in sewers are helping to prevent such a scenario from playing out by serving as relief valves (see www.is.gd/afumiq) but the Environment Agency revealed that 5% of sewers overflowed more than 100 times in 2021. Flood prevention, therefore, calls for a wider approach that relies on other technologies.
The UK government is recognising the potential of SuDS in a recent review for the implementation of Schedule 3 of the Flood and Water Management Act 2010 (see box, p22).
New building developments – which include commercial buildings with large roof areas such as factories and warehouses, as well as their car parks and pavements – add to the risk by replacing natural surfaces that would otherwise assist in dealing with heavy rainfall. SuDS, however, rely on combinations of soakaways, grassed areas, permeable surfaces and wetlands to reduce the overall amount of water that ends up in the sewers and storm overflow discharges. Water collectors such as tanks and water butts also allow for water reuse and reduce pressure on water resources.
Samer Muhandes, a PhD student at Imperial College that has worked alongside the British Geological Survey (BGS) on the CAMELLIA (Community Water Management for a Liveable London) project, see box on p21, says monitoring data shows SuDS are working.
“We monitor the flow reduction and water quality improvements of many SuDS structures, and we have found that our porous pavements, ponds and infiltration basins are operating well. People should believe in SuDS because traditional drainage systems have failed us over and over again.”
So, what are the benefits of installing SuDS for commercial buildings with large roofs? “There is a big opportunity to harvest the water. This benefits both the water supply and drainage spaces because you’re reducing the runoff from that roof while also reducing the water demand in the building.”
Elaborating further, Steven Brown, associate director at Ambiental – a Royal HaskoningDHV company – says rainwater harvesting can be useful for car washing and toilet flushing at commercial buildings with large roof areas.
“Companies have to pay rates for high-quality water from water authorities that are then flushed down the toilet,” he explains. “Rainwater harvesting allows you to use the free water that lands on the site and in doing so, reduce the flood risk to the wider catchment area.”
FITTING AND MAINTAINING
The process involved in retrofitting a SuDS to existing downpipes depend on the presence of a storage tank below ground.
Brown adds: “You basically divert the existing downpipe into the storage tank and you then have a small electrical pump in the storage tank to boost it back to where you need it and the rest then gets throttled and discharged to the sewer.”
He describes the maintenance requirements for SuDS as straightforward; running a lawnmower over a swale (a drainage channel with side slopes) to collect rubbish is simpler than using dedicated drainage jetting equipment to remove blockages from drainage systems.
“With swales, you have this open feature that allows you to see if there is a piece of rubbish that needs to be cleared away. Most people do not carry out inspections on below-ground drainage systems on an annual basis, so therefore an undetected blockage can lead to flooding during a storm event.”
However, there is no one-size-fits-all approach when it comes to SuDS. Ambiental offers a tailored approach for each client based on different features, which will depend on site constraints.
For example, a client that wants to resurface large areas of hard standing such as car parks or service yards could opt for a permeable pavement that helps to clean up rainwater. Describing the performance of this pavement material, Brown explains: “The solution removes brake dust, oils and bits and pieces that drop off cars, and these nasties get broken down in the various layers as it goes through the substrate. It also stores the water, so you can release at restricted and controlled rates to reduce the flood risk.”
The company also offers a ‘detailed drainage design’ service to assess the different SuDS features, such as the size of the basin. This process involves assessing the earth geotechnical conditions, maximum load and maintenance options.
WEIGHING UP THE OPTIONS
What options are available to ensure the water carried by a gutter downpipe does not end up going into the sewer? At a small scale, Brown states that a water butt can be placed at the bottom of a pipe to intercept a large proportion of the average annual rainfall, while the soil held in a rain planter can nurture plants with rainwater, with the overspill being returned into the sewer. Another option is a landscape retrofit in which a pond or wetland can be created to allow the intake of some water that can be diverted from the sewer.
Meanwhile, Muhandes reveals that BGS would initially approach this scenario by assessing whether the down drain can be taken to a tank for harvesting. Another option is infiltration, which diverts water from the sewer by injecting it into the ground.
However, infiltration depends on soil conditions. Infiltrating into clay soils, which can be impermeable, could cause structural problems if the work is being carried out less than 10 metres away from the foundation of a building. Considerations involved in this process include the location of the groundwater, the soil type and California Bearing Ratio, a value used to evaluate the sub-grade strength of a road or pavement. It measures the ratio of a bearing load penetrating into a given material compared to the load with the same penetration value in crushed stone.
“When CBR is less than 8%, I would normally be nervous of infiltrating near a highway, as this might have an adverse effect on the pavement stability,” Muhandes continues. “Groundwater should be a minimum of 1m away from the bottom of the SuDS layer. If the infiltrated water has pollutants, we should be away from any water abstraction zone.”
If infiltration is not possible, BGS would then advise passing the water through either a ‘rain garden’ – a shallow depression planted with flowers to absorb surface runoff – or permeable paving, which could then be connected to the nearest river or water course.
Of course, some urban environments do not have a water course within 500 metres. “In these situations, you would connect to the nearest storage unit, and throttle the flow to the nearest surface-water sewer, which is a pipe that only pumps stormwater,” Muhandes continues. “You can put forward an application to the necessary water company to form a connection with the nearest pipe.”
BOX: BGS INFILTRATION MAP
The British Geological Survey’s infiltration map (www.is.gd/uwujuf) offers a score on the suitability of infiltration SuDS in a particular area.
Head of environmental modelling Dr Christopher Jackson says the map takes into account a number of factors such as the permeability of the ground.
“If you are in an area covered by clay soil and clay rocks, then you are not going to be able to infiltrate much water.
“The map takes into account the depth of the groundwater, so if the groundwater levels are near the surface, you infiltrate water; then you can cause those groundwater levels to rise, which can cause problems of waterlogging of ground subsurface infrastructure.”
BOX SCHEDULE 3 GUIDANCE
Schedule 3 provides a framework for the approval and adoption of drainage systems and national standards on the design, construction, operation and maintenance of SuDs. It also makes the right to connect surface water runoff to public sewers conditional upon the drainage being approved before any construction work can start.
The review (www.is.gd/qekora) recommends that the government must act and implement Schedule 3 with a unitary authority or county council as approving bodies to ensure a consistent approach to using SuDS to help address the impacts of climate change. It also recommends that successful implementation of Schedule 3 will require professionals with skills and knowledge to design, construct and maintain SuDS. A public consultation will be launched in 2023.
BOX: CAMELLIA
CAMELLIA is a five-year programme funded by the Natural Environment Research Council that is bringing together engineering, urban planning and socio-economic experts with governmental and planning authorities to understand the communal perception of the water system and its challenges. Projects include: water quality modelling in Enfield, a water data online portal, online learning tools for secondary schools, smart water tanks for rain harvesting and flood allieviation, developing an integrated model of rivers, water supply and wastewater in London, Moselle Brook and Stonebridge Brook river restoration, Walworth community garden network watering impact study and the co-design of Kipling rooftop garden.