Rain Gardens

With climate change creating increasingly volatile weather conditions, garden and landscaping experts are compelled to be more inventive in the way they manage the effects. German drainage company, ACO offer five considerations for creating a sustainable drainage systems to cope with our changing weather patterns

As climate reports indicate that the UK is becoming wetter, it is understandable that rain gardens are one such method growing in popularity. According to the Met Office, rainfall events exceeding 50 mm have consistently increased over the past decade – pointing to a rise in both rainfall intensity and frequency.

With this in mind, the Royal Botanic Garden of Edinburgh (RBGE) has implemented a rain garden at the southside of its site. The feature will serve to manage heavy rain fall events, as the location had historically suffered from waterlogged lawns and flooded paths.

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 outside space. It is for these reasons that the feature can function as a major component of a sustainable drainage system (SuDS).

While bioretention facilities present an increasingly pertinent drainage solution, it must be kept in mind that such features require careful planning, planting and landscaping. There are five key considerations gardening and landscaping professionals should keep in mind to design a rain garden that aligns with SuDS principles of quantity, quality, amenity and biodiversity.

1. 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.

RBGE carefully considered soil type for the creation of its rain garden. The composition was specially developed with a mixture of compost, sand and fine gravel to allow for sufficient water infiltration, while at the same time providing nutrients to support the plants.

To determine whether conditions are adequate for the feature, a percolation test should be carried out. The test is conducted by firstly digging a 250mmdeep 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 50 mm per hour, soil conditions may be unsuitable.

2. 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% 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 150 mm and 450 mm 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 150 mm wide.Image credit: Kirsty Wilson

Right: MultiDrain MD SuDS Infiltration Outlet SuDS distribution system for rain gardens and bioretention

3. 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.

4. 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. As such, RBGE planted a selection of Scottish native and non-native plants to form a shrub and perennial mix. Flora of this variety is suitable for accommodating diverse wildlife, as the flowers will encourage insects while perennials provide a home for numerous invertebrates.

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.

5. Ongoing maintenance While rain gardens are simple to maintain, it is still important to do so. 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.

Final thoughts With erratic patterns continuing to characterise the UK climate, it is more vital than ever that a landscaped garden can withstand such conditions. Bio-retention facilities hereby offer a practical drainage method that is both sustainable and beneficial to the environment.

For more information on rain gardens, please visit www.aco.com