Rain gardens provide the first green line of flood defence

As weather becomes more volatile and extreme, heatwaves aren’t just breaking records. This year, the UK experienced its sixth wettest July on record, and the wettest since 2009, with 70 per cent more rainfall than average, according to Met Office data. Northern Ireland had its wettest July since 1836. Six of the ten wettest years in the UK since 1862 have occurred since 1998.

According to the Met Office, the weather will become wetter. The study predicts that, compared to 1990, by 2070, winters in the UK could be up to 4.5°C warmer with 30 per cent more rain. Summers may be up to 6°C warmer and 60% drier, but precipitation will be 20% heavier in summer and 25% heavier in winter.

Noticeably heavy rainfall is not good for our gardens. In addition to overwhelming our sewer system, floods erode soil structure, damaging and drowning plants.

Designers are adapting in several ways: reducing runoff by building rain gardens and stormwater farms, and reducing hard landscaping, including incorporating plantings into patios and terraces. On 4 July 2021, the site of the Royal Botanic Garden Edinburgh in Edinburgh saw one month’s worth of rain fall in less than an hour. Paths eroded, manure washed from flower beds and plants flattened.

All four RBGE parks are having to adapt to the changing climate. All are in the process of replacing paths with porous materials, such as gravel and exposed aggregate, and have expanded their drainage systems to slow runoff water from draining into drains. Everyone should control species that thrive in wetter conditions. For example, Logan Botanic Garden in Dumfries and Galloway is witnessing the spread of Epilobium brunnescens and Dicksonia antarctica.

The Edinburgh site has built a pilot rain garden. A rain garden is a basin-shaped depression below the level of its surroundings, designed to intercept and slow rainfall through a combination of improved soil and plantings that can handle both waterlogging and drought. There are two aims: to combat persistent flooding, particularly in Birch Park, and to collect scientific data for a growing number of organisations, including Scottish Water and local councils, who are looking at nature-based approaches to flood prevention.

“You couldn’t walk through our birch park at all in the winter months because it was so wet, and over the last 10 to 15 years things have been getting noticeably worse,” says David Knott, curator of living collections at RBGE. “Even by Scotland’s standards, we are getting wetter.”

RBGE has teamed up with local Heriot-Watt University to look at options. They chose a rain garden because its plantings make it more attractive than a soak, which is a hole in the ground filled with stone to allow water to filter through it into the ground. They also enhance biodiversity by attracting pollinators and providing food and shelter for insects and birds, Knott says.

A rain garden typically absorbs 30 percent more rain than a lawn. The RBGE rain garden measures 20m x 7m and has a depth of 450mm at its centre. The center runoff baths then slowly soak into the soil that has been improved to promote drainage. Plant roots absorb some of the water and leaves intercept and slow down rainfall. The plantings are a mix of perennials and grasses that can handle harsh conditions.

“In the past, gardeners would put plants in bogs, but that’s no longer the case because these plants have to stay there either saturated or dry, so we’re asking a lot more of them,” says Kirsty Wilson, garden manager at the company. RBGE and presenter on the BBC Beechgrove Park.

When we work on it, people come and ask what we do. . . It is an effective way to communicate with people

In Edinburgh, layering is planted, with species able to survive for long periods in the pooled water in the center and those preferring slightly drier conditions at the edge. Plantations such as Filipendula ulmaria, Cicerbita alpina, Ligularia fischeri and Aruncus gombalanus were selected for wetter areas. Species including Aquilegia formosa and Anthyllis vulneraria were chosen for the drier edges.

The team improved the drainage of the mixed clay soil at the site using a formula recommended in the Ciria SuDS (Sustainable Drainage System) manual used by water management professionals. The soil mix consists of 30 percent existing soil, 45 percent fine sand, 10 percent fine gravel, and 15 percent compost.

The rain garden is successful.

“We were getting 20mm per hour of rain seeping into the lawn before the rain garden. We’re getting 200 to 300mm per hour, which is a big difference,” says David Kelly, associate professor in the Public Health and Environmental Engineering Group at Heriot-Watt University. “Since we built the rain garden, we haven’t had any floods.”

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

“Whenever we’re working on it, people come up to ask what we’re doing. They always seem to know someone who’s been affected by local flooding, or has been one themselves. It’s an effective way to connect with people,” Kelly says.

Gloucestershire Wildlife Trust is creating urban rain gardens to combat 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 insurance company More Than. A rain garden was built to stop flooding in the paved area of ​​Matson Baptist Church in Gloucester. Downpipes are 6m x 3m and 2m deep and direct surface water into the garden and any excess is emptied via an overflow pipe into the drain.

“But it’s flowing much slower, so it’s not flowing there,” says Nicola Simpson, head of engagement at GWT. The water is also cleaner when it reaches groundwater and rivers. This is because rain garden plantings and soil mix filter toxic substances from runoff that contaminates hard surfaces. According to the Wildfowl & Wetlands Trust, rain gardens remove up to 90 percent of nutrients and chemicals and up to 80 percent of sediment from runoff.

In a domestic setting, a rain garden should be about 20 per cent the size of the runoff area, whether it is a roof, lawn or hard paving, says Adrian Thorne, a horticultural consultant at RHS Wisley. Building a medium-sized rain garden can take an afternoon, and once the plantings are established, it’s low-maintenance.

‘Bombproof’ plants recommended by the Royal Horticultural Society to tolerate temporary wet as well as dry soil include Sambucus nigra, Cornus sanguinea ‘Midwinter Fire’, Hydrangea ‘Annabelle’, Crocosmia ‘Lucifer’, Iris sibirica, Calamagrostis brachytricha, Deschampsia cespitosa and Miscanthus cultivars. sinensis.

An alternative for a garden with a high water table or limited space is a rainwater planter connected to a drainage pipe to collect runoff water, Thorne says. The planter follows the same principles as a rain garden but on a smaller scale.

In 2022, GWT installed three stormwater planters behind Gloucester Rugby Club’s pitch to combat grandstand flooding. Down tubes empty directly into planters. Plantings include ‘Hot Lips’ salvia and ‘Kingsblood’ tulipa to match the club colors.

“We want to show people what they can do in their homes, workplaces and schools. It’s working, and we’ve had feedback from people who are building their own rain gardens,” says Simpson.

Creating green roofs is another way to deal with water runoff, says Simon Rose, head of experience development at WWT. The Foundation was at the forefront of creating rain gardens with its first project, designed by Neil Dunnett in 2010 at the Wetlands Center in London. Any excess water that is not absorbed by the green roof drains into continuous pools that extend to the plantations.

For Kelly, the more people dig, the better. “If a few people do that in their street or neighborhood, it will have a big impact on flooding,” he says. “As an individual, it’s hard to know what to do about climate change. Creating a rain garden is something you can do.

In the United States, the Philadelphia Water Department offers financial incentives to property owners to increase stormwater management, including building rain gardens.

“That’s what we need our government to do here,” Wilson says.

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