With more frequent periods of extreme rainfall occurring, designers are looking for new solutions to manage surface water runoff in our urban locations. The cost of flooding to the UK economy in 2019 alone has been estimated at £270 million. This has put increased focus on some vital questions about rainwater management:
- Where will rainwater land and where will it collect?
- Where will it migrate to (i.e. what is its easiest pathway)?
- Where will the rainwater discharge?
For this reason, Sustainable Urban Drainage Systems (SUDS) are becoming of paramount importance within the design of our green infrastructure. These SUDS utilise natural water systems such as trees, ponds and swales to minimise the risk of flash floods after sudden heavy rainfall. This approach gives designers the possibility of using the natural landscape and its benefits for water management, as opposed to hidden networks of pipes.
Trees intercept rainfall, holding it within their canopy and leaves. This allows it to slowly trickle down the branches and into the ground over a prolonged period of time. One single tree with a large canopy size, such as Tilia cordata, can hold over 4,500 litres of rainfall in an average year. A percentage of this rain will then evaporate directly from the leaves, significantly reducing the speed of storm water runoff.
Urban flooding usually occurs once the canopy storage of trees has been exceeded. Green open spaces densely planted with trees can be utilised as storm water retention areas. This allows the soil to take the excessive stormwater and for the trees to disperse it back into the atmosphere through transpiration, increasing the soil permeability. For this, swells are created for accumulated stormwater to migrate. Trees planted here are species that thrive within periods of waterlogging and are able in these situations to uptake the water from the soil and transpire it back into the atmosphere. Due to their location, these trees must also be able to tolerate dry periods and urban pollution, which collects within the water.
A number of trees have been noted for their use with SUDS. These include trees with large canopies to intercept and hold rainfall along with trees that thrive in locations prone to sudden flooding. These are ideal varieties to specify within areas designed to collect stormwater runoff.
Recommended SUDS Trees
Alongside a canopy of large leaves, this tree has corky, crocodile-like bark that also helps hold rainwater. It enjoys wet locations and periods of waterlogging. For tight, condensed areas where storm water migrates, try mass planting of Liquidambar styraciflua ‘Slender Silhouette’. View Liquidambar styraciflua
This large evergreen tree captures rainfall throughout its dense canopy all year round. It has a tolerance for flash flooding and periods of dry weather. View Pinus sylvestris
Ulmus ‘New Horizon’
This is a full canopied tree that excels in both prolonged waterlogged and dry periods. It makes an excellent tree for inner city areas and will develop a full, dense canopy of lush leaves even within very poor locations. View Ulmus ‘New Horizon’
Hippophae salicifolia ‘Streetwise’
The sea buckthorn can thrive in even the most difficult of inner-city locations. Its leaves are willow-like, capturing rainfall nine months of the year whilst also tolerating salt and gritters. View Hippophae salicifolia ‘Streetwise’
Platanus x hispanica
This large, spreading tree has large leaves that can grow to 15cm across. This makes it ideal for capturing rainfall, with its long limbs slowing the process of water entering the soil or urban locations. View Platanus x hispanica
The Italian Alder is able to withstand periods of water logging. It is also extremely tolerant of poor soil conditions and, unlike most alders, it tolerates dry sites well and will grow swiftly in difficult exposed locations with strong winds. View Alnus cordata
This large canopied oak thrives within wet locations. Its large form quickly transpires water, relieving the soil, making it an ideal tree for capturing rainfall and transpiring storm water runoff. View Quercus palustris
This is a perfect tree for difficult locations that are prone to waterlogging. The bark offers all season interest, while also being well-suited for areas of urban stormwater runoff. View Betula nigra
This imposing pyramidal tree has a horizontal branch structure that captures and holds large quantities of rainfall. It succeeds well in waterlogged locations, successfully holding and dispersing stormwater. View Metasequoia glytostroboides