Rain gardens offer a first green line of flood defence

As our weather becomes increasingly erratic and extreme, it’s not just heatwaves that are breaking records. The UK experienced its sixth wettest July on record this year and the wettest since 2009, with rainfall 70% above average, according to the Met Office. Northern Ireland experienced its wettest July since 1836. Six of the UK’s 10 wettest years since 1862 have occurred since 1998.

According to the Met Office, the weather is set to become wetter. It predicts that UK winter temperatures could rise by 4.5 degrees Celsius by 2070 compared with 1990, with rainfall increasing by 30%. Summer temperatures may rise by 6 degrees Celsius and be 60% drier, but rainfall will increase by 20% in summer and 25% in winter.

A significant increase in rainfall is not good for our gardens. In addition to flooding our sewage systems, flooding erodes and damages soil structure and inundates plants.

Designers are adapting in a number of ways: by creating rain gardens and rain planters to reduce runoff, and by reducing hardscaping, including planting on decks and terraces. On 4 July 2021, a month’s worth of rain fell in less than an hour at the Royal Botanic Gardens in Edinburgh. Roads are eroded, compost is washed away from flower beds and plants are flattened.

All four RBGE gardens must adapt to the changing climate. All are replacing roads with porous materials like gravel and open aggregate and expanding drainage systems to slow runoff into drains. All must control species that thrive in wet conditions. For example, the Logan Arboretum in Dumfries and Galloway has seen the spread of A. truncatula and A. antarctica .

An experimental rain garden has been built at an Edinburgh factory. A rain garden is a basin-shaped depression below the level of the surrounding environment designed to intercept and slow rainfall through improved soil and plants that can cope with waterlogging and drought. There are two aims: to combat ongoing flooding, particularly in birch lawns, and to collect scientific data for the growing number of organizations, including Scottish Water and local councils, who are looking at nature-based approaches to flood prevention.

“You can’t walk on our birch lawn at all in the winter because it’s so wet, and it’s gotten significantly worse over the last 10 to 15 years,” David Knott, curator of living collections at RBGE explain. “Even by Scottish standards we are getting wetter.”

RBGE is working with local Heriot-Watt University to consider these options. They chose a rain garden because its plants were more attractive than a soakaway, a hole in the ground filled with rocks that allows water to seep through it into the ground. They also promote biodiversity by attracting pollinators and providing food and shelter for insects and birds, Knott said.

Rain gardens typically absorb 30% more rainwater than lawns. The RBGE rain garden measures 20m x 7m with a center depth of 450mm. Intermediate runoff basins then slowly seep into the soil that has been amended to enhance drainage. Plant roots absorb some of the water, and leaves intercept and slow down rainfall. What is planted is a mixture of perennials and grasses that can handle extreme conditions.

“In the past, gardeners would just grow bog plants, but that’s different now because the plants have to sit there, either saturated or dry, so we’re asking for a lot more,” garden manager Kirsty Wilson said. RBGE and BBC presenter Beech Grove Gardens.

Whenever we were doing it, people would come up and ask us what we were doing. . .This is an effective way to communicate with people

In Edinburgh, the planting is tiered, with species that can survive in pool water for long periods of time in the center and those that prefer a slightly drier environment at the edges. Plants such as Filipendula ulmaria, Cicerbita alpina, Ligularia fischeri and Aruncus gombalanus were selected for the wettest areas. Species including columbine (Aquilegia formosa) and wound ivy (Anthyllis vulneraria) are harvested on dry edges.

The team improved the drainage of the site’s clay soil mix using a formula recommended by the Ciria SuDS (Sustainable Drainage Systems) manual used by water management professionals. The soil mix is ​​30% existing soil, 45% fine sand, 10% fine gravel and 15% compost.

Rain gardens are successful.

“The lawn in front of the rain garden has a rainwater infiltration rate of 20mm per hour. With that we can go 200mm to 300mm per hour which is a huge difference,” said Associate Professor in the Public Health and Environmental Engineering Group at Heriot-Watt University David Kelly said. “We haven’t had flooding since we built the rain gardens.”

The garden also shows RBGE visitors what they can do at home.

“Whenever we’re doing this, people come up and ask us what we’re doing. They always seem to know someone who’s been affected by localized flooding, or themselves. It’s an effective way to connect with people,” Kelly said .

Gloucestershire Wildlife Trust is building urban rain gardens to deal with flooding caused by runoff from concrete and tarmac, including from homeowners converting green spaces into driveways. Financial backers include the Environment Agency, Gloucester City Council, Severn Trent Water and insurer More Than. A rain garden was created in a paved area at Mattson Baptist Church in Gloucester to stem flooding. The downspouts measure 6m x 3m and are 2m deep, directing roof water directly into the garden, with excess water being discharged into the gutters via overflow pipes.

“But it flows much more slowly, so it doesn’t gush out there,” said Nicola Simpson, GWT’s director of engagement. The water also becomes cleaner when it reaches groundwater and rivers. That’s because a rain garden’s plant and soil mix can filter toxic materials from runoff contaminated by hard surfaces. According to the Wildfowl and Wetlands Trust, rain gardens can remove up to 90% of nutrients and chemicals and up to 80% of sediment in runoff.

Adrian Thorne, horticulture consultant at RHS Wisley, says that in a domestic setting, a rain garden should cover approximately 20 per cent of the runoff area, whether it is a roof, lawn or hard paving. Building an average-sized rain garden takes an afternoon and is low maintenance once planted.

“Bulletproof” plants recommended by the Royal Horticultural Society that are resistant to temporary wetness and dry soil include elderberry varieties, dogwood ‘Midwinter Fire’, hydrangea ‘Annabelle’, crocus ‘Lucifer’, Siberian iris, short-haired whiskergrass, German iris cespitosa and miscanthus cultivars.

Another option for gardens with high water tables or limited space are rain planters that connect to downspouts to collect runoff, Thorne says. Planters follow the same principles as rain gardens, but on a smaller scale.

In 2022, GWT installed three rainwater planters behind Gloucester Rugby Club Stadium to deal with tarmac flooding. Downspouts drain directly into planters. Plants planted include sage ‘Hot Lips’ and tulip ‘Kingsblood’ to match the club’s colours.

“We wanted to show people what they can do at home, at work and at school. It worked and we got feedback from people who built their own rain gardens,” Simpson said.

Simon Rose, director of experience development at WWT, said creating green roofs is another way to deal with runoff. The trust is a pioneer in the creation of rain gardens, with its first project designed by Neil Dunnett in 2010 at its London Wetland Centre. Any excess water not absorbed by the green roof drains into an attached basin and overflows into the plants.

For Kelly, the more of us who dig, the better. “If a small number of people do this in their street or area, it can have a big impact on flooding,” he said. “As an individual, it’s hard to know what to do about climate change. Creating rain gardens is something you can do.”

In the United States, the Philadelphia Water Department provides financial services Incentives Property owners are required to improve stormwater management, including building rain gardens.

“That’s what we need the government to do here,” Wilson said.

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