Rain gardens as tools for climate change adaptation

In an age of rapidly changing climate conditions, the most sensible approach is naturalistic design. Modern construction focuses on ecology, functionality, minimalism, climate change adaptation, and responsible building practices. One of modern urban and architectural solutions is blue-green infrastructure (BGI). Greenery in private, residential and public spaces may not only serve an aesthetic purpose, but also may fulfil an ecosystem function. While ornamental value is often a priority in private gardens, it’s crucial to consider the usefulness of greenery even in such projects. However, contemporary form of urban areas is mostly shaped by public spaces and property development investments, both in terms of volume and architectural design, as well as nature aspects. It is highly important to implement good practices, especially in these spaces, as they have the greatest impact on a large scale.

Retention greenery instead of lawns

Establishing a lawn as a biologically active surface is an anti-environmental and anti-economic solution. Much higher quality offers functional greenery, which is increasingly common around office buildings certified under LEED or BREEAM systems. It is essential, though, for retention greenery to become a standard also in residential areas and public spaces. Investors are slowly being persuaded to use greenery as an infrastructure element, especially thanks to the low prices of some retentive plants such as currant and willow and the low maintenance costs of naturalistic rain gardens. Small retention imitates nature in collecting, purifying and evapotranspiration rainwater. Meadow, shrub and forest swamps, which, along with bogs, are key natural resources that store large amounts of water and carbon, are models for retention greenery. In urban spaces, small retention includes reservoirs, retention parks and garden squares, wildflower meadows, bioretention basins, rain gardens, swales, and green roofs. A guarantee of a successful project using green infrastructure is cooperation of the investor with a landscape architect.

Rain gardens

Rain gardens are part of blue-green infrastructure (BGI) solutions. They serve as small-scale water retention tools, beneficial for the environment alongside large and medium-scale retention structures. They are used in public spaces, residential areas, as well as in private properties. They involve planting water-resistant flora on specially prepared permeable and water-absorbent soil, allowing them to absorb water up to 40% better than a regular lawn. Over 50% of such a garden consists of hydrophytic plants, well-adapted to wet environments, resistant to drought and flooding, which additionally purify water. Rain gardens store and utilize rainwater from surrounding impermeable surfaces. There are single rain gardens located in a small field depression or in a container, as well as larger systems of artificial wetlands periodically flooded, built – for instance – in housing estates, parks, squares or along roads. It is useful to know why to use a particular green solution, so as not to use them for trendiness and to prevent greenwashing.

Water resources in Poland, according to the Water Exploitation Index plus, are among the smallest in Europe. Our country loses significant amounts of freshwater that flows into rivers and end up in the sea. Further losses are caused by rapid climate change, exacerbating the problem of drought and storms. Rain gardens aim to retain as much water in the landscape as possible and release a larger portion directly into the atmosphere instead of into stormwater drains. The best source for such gardens is water from roofs, pavements, promenades, drives, and car parks. Among the main benefits of rain gardens are:

• protection against drought, flooding, and floods,
• improvement of microclimate and biodiversity increase,
• increase of humidity and air quality,
• soil regeneration,
• natural filtration of rainwater,
• facilitation of storm water drainage,
• lower maintenance requirements and thereby financial benefits.

The establishment of rain gardens brings positive ecological, economic and health changes, but also visual and acoustic ones by attracting birds that enjoy being in the presence of water and naturalistic greenery. Other positive effects include the prevention of unaesthetic and troublesome puddles.

Home rain garden

To improve water conditions in immediate surroundings, a rain garden can be created by anyone, even in their own backyard, either in the ground (preferably subsoil) or in a container, or even on a balcony. Furthermore, a rain garden can be infiltrative or isolated. The cost of a small home rain garden ranges from two to five thousand zloty. The area of the rain garden should be about 3% of the drainage area. Establishing a home rain garden can be commissioned to a landscape architect, but it can also be done by oneself. Substantive information on plants dedicated to such gardens is worth looking for on websites of building and gardening stores.

A CROSS SECTION THROUGH AN ISOLATED RAIN GARDEN IN A CONTAINER

source: EMSWCD, https://emswcd.org/in-your-yard/rain-gardens/stormwater-planters/

Soil and plants in rain gardens

The rain garden’s soil consists of coarse sand and porous rocks, such as limestone, gravel, stone, or volcanic rocks. Rain gardens in containers are particularly suitable for highly urbanized areas lacking green spaces. Hardened surfaces are not an obstacle to improving water conditions. Recommended hydrophytic plants for rain gardens include, among others, iris, marsh gladiolus, loosestrife, valerian, great manna grass, reed canary grass, rush, dwarf bulrush and common reed. A beautiful effect can be achieved with meadowsweet and a combination of irises with loosestrifes. Plants with increased water requirements, such as hydrangeas, ferns, and most other shade plants, are especially good on more naturally permeable soils. On sandy soils, flower meadows work best. Shrubs and trees can also serve as retention greenery. Recommended shrubs include viburnum and currant, while ideal trees for rain gardens are white willow, brittle willow, black and white poplar, black and grey alder, pink oak, and red maple. It is crucial for planted trees to be young and unaccustomed to other environmental conditions.

Local government actions for the idea of sponge city

In recent years, a positive change in green management by many local governments has been observed. Naturalistic greenery and small retention facilities are increasingly being introduced in public spaces, and such solutions used to be difficult to implement due to insufficient public knowledge of the unquantifiable benefits of their use, both ecological, aesthetic, and financial. The number of rain gardens is rapidly growing in Pomerania – an area particularly vulnerable to floods due to its specific geographical location and intensification of weather anomalies. Gdansk is the first city in Poland where territorial rain gardens were created. In 2016, the first citizen panel on flood risks was held in Gdansk. The local community have opted for natural solutions, and the municipal board has appointed a small retention specialist at the city’s municipal water company Gdanskie Wody. By 2020, several municipal rain gardens were established in the city, preventing many local flooding incidents. Throughout Tricity, positive competition between cities on water retention has begun, including through programs to subsidize small retention facilities. In Tricity a positive competition between cities on water retention has begun thanks to programs to subsidize small retention facilities, among others. In the following years, more and more cities have convinced to the idea of a sponge city. There were established a lot of green-blue infrastructure investment projects, conferences (e.g., “Green retention – how to avoid flooding and droughts in cities,” organized by incl. Gdanskie Wody), and competitions for local communities.

Weronika Przypaśniak
Junior Specialist in Research and Market Service