Definition:
These are small reservoirs not filled with rainwater on a daily basis, and their task is to retain rainwater, infiltrate it and purify it through the phenomena of phytoremediation and sedimentation. Water in the rain garden should stay not be longer than 12-24 hours. This allows to reduce the amount of rainwater discharged directly to the storm drain. These reservoirs are planted with vegetation, which allows for the creation of both esthetic urban spaces and habitats for local fauna, especially for insects, amphibians or birds (increasing biodiversity in the city).
For use in private spaces, intimate residential settlements, educational units.
A solution for professionals and local communities.
Environmental benefits: Rainwater retention, reduction of surface runoff from paved areas and roofs, rainwater purification (sedimentation of pollutants, phytoremediation by plant). Increasing the biodiversity in the city, creating new habitats for the fauna in the city.
Social benefits: Educational aspects, decorative aspects.
Economic benefits: Reduced costs of draining rainwater into storm sewer.
They are part of the creation of green networks in the city: together with green roofs, green walls, dry swales, grass channel.
Construction:
– The location of the rain garden is best determined during the rain. The area for the rain garden must be flat (water should accumulate evenly over the rain garden, avoid an area with a slope greater than 12%) and at least 9 meters away from the wall of the building. The facility should be located in a direct sunlight or in a semi-shade.
– The water should drain into a specially formed trough. It is therefore important to determine the garden surface and the level of its recess. These factors will include: the area from which rainwater will be retained, e.g. roof area, streets, parking lots, etc.
The conversion is as below:
100 m2 (impermeable surface) x 0.7 = 70 m2
When there is low permeable ground in the area where you want to install the rain garden, the surface should be even larger. However, when there is not enough space for it, its surface can be reduced and appropriate drainage should be used directly under the garden, so as to accelerate the infiltration of rainwater. The drainage layer should be about 30 cm in the form of aggregate in which we lay the drainage pipe.
It should also be borne in mind that large-area gardens are the most efficient ones, because then we have a larger area both for filtering water into the soil and for evaporation.
– The depth of the garden should be between 15 and 20 cm. The sides of the rain garden should be slightly sloping. The greater the slope of the lawn the depth of the garden should be greater. If we are concerned that water from the garden can spill, it is best to build small embankments around the garden.
– We remove the top layer of soil so that there are no roots at the depth of 30-45 cm in the soil where the rain garden is to be built. When needed, we create a new layer of soil (sample mix: 50% sand, 30% compost, 20% native soil). The thickness should be approximately 15 cm.
– Designing the rain garden. The garden should be located so as not to impede the use of space on flat lawns, but also to interact with the surrounding landscape.
– The best way to green rain gardens is to use native species, which are cheap and easy to care for. Planting according to a previously prepared design. The plants should take root for three months before starting the system.
A sample plant species set:
| Aster novae-angliae | Liatris spicata | Solidago spp. |
| Echinacea purpurea | Lobelia cardinalis | Spartina pectinata |
| Epilodium angustifolium | Physostiegia virginiana | Vernonia fasciuculata |
| Eupatorium purpureum | Rosa spp | |
| Eupatorium maculatum | Rudbeckia subtomentosa |
The rain gardens of Gdańsk
Gdańsk is a city where we observe many initiatives related to the small retention and construction of rain gardens, the largest low retention system in Poland is being developed. They are created in different urban areas, starting with settlements on the so-called suburbs, as well as in underserved areas of worker’s settlements, within city tissue, modernist settlements, public areas, etc. The initiatives organized by the Polskie Wody [Polish Waters] in Gdańsk are also intended to perform integrating and educational functions. The residents are co-forming rain gardens, learning how to set them up – which can also be used for their own initiatives.
The website contains a map of rain garden projects that have been built in Gdańsk so far. http://www.gdmel.pl/
More about the garden under the link: https://www.gdansk.pl/wiadomosci/ogrody-deszczowe-w-gdansku-zobacz-jak-powstawal-czwarty-z-nich,a,129611
Example: Mount Tabor Middle School Rain Garden, Portland, Oregon
The school’s rain garden, which was built in the asphalted parking lot, is also an educational element used by teachers during school activities. The entire garden has the area of approximately 600 m2, but it store water from both parking lots and roofs of nearby buildings with a total area of 30,000 m2. It also reduces the degree of heating of the rooms in the school. This saves $ 100,000 per year.
Sources and additional information:
http://raingardens.info/wp-content/uploads/2012/07/UKRainGarden-Guide.pdf
https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs142p2_011366.pdf
https://www.chicagobotanic.org/downloads/wed/WI_DNR_homeowners.pdf
https://pecpa.org/wp-content/uploads/Water-Resources-Create-Your-Rain-Garden.pdf
https://www.700milliongallons.org/wp-content/uploads/2015/04/Rain-Garden-Handbook-for-Western-Washington.pdf
https://seagrant.oregonstate.edu/sites/seagrant.oregonstate.edu/files/sgpubs/onlinepubs/g11001-lid-rain-gardens.pdf
https://www.nj.gov/dep/seeds/syhart/rgmp1.pdf
http://44mpa.pl/wp-content/uploads/2018/10/OGR%C3%93D_W_5_KROKACH_6_do_druku_samodzielnego.pdf