With climate change creating increasingly volatile weather conditions, garden and landscaping profesionals are compelled to be more inventive in the way they manage the effects, suggests ACO.
Also known as bioretention facilities, rain gardens are shallow, landscaped depressions that reduce stormwater runoff to mitigate pollution and prevent local sewers being overwhelmed. As well as offering an environmentally friendly way to manage stormwater runoff, a rain garden can at the same time add both beauty and biodiversity to any outside space. It is for these reasons that the feature can function as a major component of a sustainable drainage system (SuDS). There are five key considerations landscaping professionals should keep in mind to design a rain garden that aligns with SuDS principles:
Soil percolation
Soil suitability is an essential element to factor into rain garden design, as water must be able to drain away at a sufficient rate to manage quantity. Water is unlikely to drain away quickly enough if the water table is very close to the surface, or if the soil consists of heavy clay.
To determine whether conditions are adequate for the feature, a percolation test should be carried out. The test is conducted by firstly digging a 250mm-deep hole and filling it with water. Once drained, the pit should be filled with water again and the drainage speed timed. If the rate exceeds 50mm per hour, soil conditions may be unsuitable.
Spatial proportions
If soil drainage meets the required parameters, the next course of action is to consider the size of the area to accommodate water runoff. If the source is a roof for instance, the surface area of the roof that is providing rainwater must be measured to gauge anticipated quantities. As a rule of thumb, a rain garden should be 20 per cent of this area in order to intercept sufficient volume of surface runoff.
Suitable measurements must also be kept in mind when excavating the site for a bioretention facility. The feature will typically need to be between 150mm and 450mm below original ground level. When digging, removed soil may then be compacted around the lip to create a berm. Acting as an aid for containing rainwater, a berm should be around six inches high and a foot wide. At the same time, a gravel-filled channel will need to be implemented for excess water to flow out into an overflow system. In this instance, the channel should be approximately 150mm wide.
Drainage ancillaries
As mentioned, a rain garden is the main component in a SuDS. With this in mind, it’s important to also consider auxiliary elements for optimum performance. If the water source for the feature is ground surface runoff (such as a driveway, patio or similar impermeable surface), its best to implement a sump unit between the channel and the rain garden. This component will help to mitigate pollutants and improve water quality in line with SuDS principles.
A gulley is also recommended as an ancillary in rain garden design. This will ensure any overflow is captured, which is particularly important if the feature is going to be sited in close proximity to a boundary.
Plant diversity
Chosen plants should consist of both non-indigenous and indigenous species for a densely vegetated bed that will attract native birds and insects. When planting, height is a factor that must not be overlooked. Taller species should be positioned in deeper soil around centre of the rain garden in order for the roots to be fully embedded. At the same time, planting shorter plants around the edges allows for a clear view of all flora.
Typically, bio-retention facilities will be comprised of about 10 species planted two or three clumps per square metre. Including a variety of species ensures that the feature will succeed even if one or two do not prosper.
For best results, it’s vital to establish plants that thrive in both dry and wet soil, as rain gardens are unlikely to be damp all year round. That being said, the feature will be subject to occasional flooding. It is for this reason that dry Mediterranean style gardens should be avoided.
Ongoing maintenance
During the first couple years of life, occasional weeding may be required to ensure the plants will thrive. Once established, the beds will fill any gaps and suppress weed growth. Seasonal occurrences should also be kept in mind. During hot spells, the rain garden may need topping up from a water butt to maintain sufficient moisture. In the winter, dead plants will need removing and any signs of erosion should be counteracted through additional compacting of the berm.
For more information on rain gardens visit https://www.aco.co.uk/news/article/creating-a-rain-garden
