Discover the Perfect Green Defense that Banishes Flooding for Good!

As the weather becomes increasingly unpredictable and extreme, it’s not just heat waves that are breaking records. This year the UK had its own sixth of July wettest on record and the wettest since 2009, with 70% more rainfall than average, according to Met Office data. Northern Ireland has experienced its wettest July since 1836. Six of the 10 wettest years in the UK since 1862 have occurred since 1998.

According to the Met Office, the climate will become wetter. It is predicted that, compared to 1990, UK winters could be up to 4.5°C warmer with 30% more rain by 2070. Summers could be up to 6°C warmer and 60% drier, but downpours will be up to 20% heavier in summer and up to 25% heavier in winter.

Significantly heavier rainfall is not good for our gardens. In addition to overwhelming our sewer system, floods erode and damage soil structure and drown plants.

Developers are adapting in a variety of ways: reducing runoff by building rain gardens and stormwater planters, and reducing landscaping, including including plantings in courtyards and terraces. On 4 July 2021, the Royal Botanic Garden Edinburgh’s Edinburgh site had a month’s worth of rain in less than an hour. Paths have been eroded, compost has been swept away from flowerbeds and plants have been flattened.

All four RBGE gardens must adapt to climate change. All are replacing paths with porous materials, such as gravel and open aggregate, and have expanded their drainage systems to slow the emptying of runoff into drains. Everyone needs to check for species that thrive in wetter conditions. For example, Logan Botanic Garden in Dumfries and Galloway is seeing the spread of Epilobium brunnescens and Dicksonia antarctica.

The Edinburgh site built an experimental rain garden. A rain garden is a basin-shaped depression that sits below the level of its surroundings, designed to intercept and slow rainfall through a mixture of improved soil and plantings that can cope with both waterlogging and drought. There are two aims: to tackle persistent flooding, particularly on Birch Lawn, and to collect scientific data for a growing number of organisations, including Scottish Water and local councils, who are looking at nature-based ways to prevent flooding.

“You absolutely couldn’t walk on our Birch Lawn in the winter months because it was very wet, and it had been getting significantly worse over the last 10 to 15 years,” says David Knott, curator of living collections at RBGE. “Even by Scottish standards, we are getting wetter.”

RBGE has worked with local Heriot-Watt University to consider the options. They chose a rain garden because its plantings make it more attractive than a well, a hole in the ground filled with stone to allow water to seep into the soil. They also increase biodiversity by attracting pollinators and providing food and shelter for insects and birds, Knott says.

A rain garden typically absorbs 30% more rain than a lawn. The RBGE rain garden measures 20 x 7 meters and has a depth of 450mm at the centre. Runoff pools in the center slowly sink into soil that has been improved to improve drainage. The roots of the plants absorb some of the water and the leaves intercept and slow down the rain. Plantings are a mixture of perennials and grasses that can cope with extreme conditions.

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

Every time we work on it, people come to ask us what we are doing. . . It’s an effective way to communicate with people

In Edinburgh, the planting is stratified, with species able to survive for prolonged periods in pools of water in the center and those that prefer slightly drier conditions at the edges. For the more humid areas, plantations such as Filipendula ulmaria, Cicerbita alpina, Ligularia fischeri and Aruncus gombalanus were selected. For the drier edges, species including Aquilegia formosa and Anthyllis vulneraria were chosen.

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

The rain garden is a success.

“We were seeing a rain infiltration rate of 20 mm per hour in the lawn before the rain garden. With it we get 200mm to 300mm per hour, which is a substantial difference,” says David Kelly, associate professor, public health and environmental engineering group at Heriot-Watt. “Since we built the rain garden, we haven’t had any more flooding.”

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

“Every time we work on it, people come to ask us what we’re doing. They always seem to know someone who has been affected by localized flooding, or have been affected themselves. It’s an effective way of communicating with people,” says Kelly.

The Gloucestershire Wildlife Trust is creating urban rain gardens to combat flooding caused by concrete and asphalt runoff, including from homeowners converting landscaping into driveways. Funders include the Environment Agency, Gloucester City Council, Severn Trent Water and insurer More Than. A rain garden has been built to stop flooding in a paved area of ​​Matson Baptist Church in Gloucester. Measuring 6m x 3m and 2m deep, the downspouts direct roof water into the garden and any excess is emptied via an overflow pipe into a drain.

“But it flows much slower, so it doesn’t flow down there,” says Nicola Simpson, GWT service manager. Water also becomes cleaner when it reaches aquifers and rivers. This is because the plantings and soil mix of a rain garden filter toxic materials from polluted runoff from hard surfaces. According to the Wildfowl & Wetlands Trust, rain gardens remove up to 90% of nutrients and chemicals and up to 80% of sediment from runoff.

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

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

An alternative for a garden with a high water table or limited space is a rainwater planter connected to a drain pipe to collect runoff, Thorne says. A 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 stadium to combat tarmac flooding. The downspouts empty directly into the planters. Plantings include Salvia ‘Hot Lips’ and Tulipa ‘Kingsblood’ to match the club colours.

“We want to show people what they can do at home, at work and at school. It works, we’ve had feedback from people building their own rain gardens,” says Simpson.

Creating green roofs is another way to address runoff, says Simon Rose, experience development manager at WWT. The trust was at the forefront of creating rain gardens with its first project, designed by Neil Dunnett in 2010 at London’s Wetland Centre. Any excess water not absorbed by a green roof flows into connected pools that spill into plantings.

For Kelly, the more of us who dig, the better. “If a few people did this on their street or in their area, it would have a big impact on flooding,” she says. “As an individual it is difficult 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 services incentives property owners to increase stormwater management, including by building rain gardens.

“This is what we need our government to do here,” Wilson says.

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