Rain gardens are modern hydrological tools that support the process of managing rainwater in the landscape. The primary task of rain gardens is to intercept excess rainwater, remove pollutants from it with the help of hydrophytes and a suitable substrate structure, and gradually release the treated water into the ecosystem. Their size and complexity can be tailored to the volume of water draining from a given surface. For small impervious surfaces, such as small roof areas from which water drains through a single gutter, rain gardens usually take the form of living container filters. In the case of large impervious surfaces from which rainwater drains in a less controlled manner, natural or artificial depressions in the terrain are most often used for rain gardens. We call such solutions ground-based rain gardens. Each of the types of rain gardens described above has different performance and design requirements.

Rain gardens in containers

This category of rain gardens is based on the use of aesthetically pleasing containers of various types, made of different materials (e.g. wood, stone, concrete), equipped with a system of drainage pipes placed in a porous, layered filter medium (e.g. gravel, dolomite gravel, crushed expanded clay, wood chips, sand, garden soil, etc.) in which hydrophytes – permanent plants – are planted. The system of drainage pipes is placed in a porous, stratified filter medium (e.g. gravel, crushed dolomite, expanded clay, wood chips, sand, garden soil, etc.) in which hydrophytes are planted – plants that tolerate permanent or periodic flooding (e.g. bulrush, sedges, irises, broad-leaved clubmoss, or even trees and shrubs such as black alder, willow). Rainwater collected from a small impervious area is fed into such a container via a guttering system. Container rain gardens can be set up individually under gutter drains or can be assembled into larger systems by placing them in series or cascade. This type of solution works best in highly urbanised areas where there are very limited or no biologically active areas (covered by vegetation). Thanks to the use of appropriately selected container-filling materials, such gardens are able to capture considerable amounts of rainwater, which, when in contact with various impermeable surfaces, flushes away all kinds of pollutants. The porous structure of the aggregates used makes it possible to store such water, and the presence of plants and micro-organisms developing on their roots makes it possible to carry out biological pretreatment of polluted rainwater. It is estimated that polluted rainwater flowing into aquatic ecosystems via storm sewers accounts for around 70% of the total number of pollutants entering the environment. A well-designed and constructed rain garden can remove up to 90 per cent of eutrophic compounds (nutrients/nutrients such as nitrates and phosphates) and chemicals from the rainwater stream and reduce suspended matter (dust and other particulate matter insoluble in water) by 80 per cent.

How do you choose the right place for a container rain garden?

When it comes to choosing the best place to install a container rain garden, there are several important factors to consider. The first is the ability to feed rainwater into the container (usually via an existing gutter or attachments that modify the outflow of water from an existing gutter). An example of such a modification is the so-called ‘first flush’ system – this allows the first stream of the most polluted water – mainly particulate matter such as leaves and fine solids – to be captured in a separate filter, which we can access and, once the rainfall has stopped, we can easily remove the accumulated pollutants from it. In other words, the rain garden is best placed under the outlet of the existing gutter, adjusting the height of the drain to the height of the container in which the garden will be established. Secondly, it is important to ensure that the container is at least 30 cm away from the facade of the building (this will protect the facade from flooding and ensure adequate air circulation around the building, protecting it from dampness). Another factor is the volume of water we will have to manage, and this parameter is determined by the area from which the rainwater will run off into the gutter system. There are calculation formulas for this that take into account the average annual rainfall that occurs in the area, but to simplify, it is assumed that for lowland areas the area of the rain garden should be 3% of the roof area from which the water will be drained. This indicator takes into account an adequate provision for heavy rainfall (for a 100 m2 roof, the rain garden area should be 3 m2). Care should also be taken to ensure that the location of the rain garden does not block access to existing utilities and installations (e.g. meter box), manholes, drain grates, irrigation systems, etc. It is good if there is a possibility to further manage the treated water flowing out of the rain garden, e.g. for filling a pond or irrigating green areas. It is also worth considering the type of roofing or paving from which the water will be drained (e.g. copper roofs preclude the use of rain gardens due to the toxic effects of copper on plants).

What elements is a container rain garden made of?

The container should be stable and leak-proof. When designing and implementing rain gardens, I always pay attention to aesthetics, the materials used (I try to avoid plastics – microplastics – as much as possible) and safety. Taking into account the full life cycle of such a product, I strongly prefer wooden containers, impregnated by properly firing, smoking and oiling the wood with natural oils. You only need to master basic carpentry skills to build such a container yourself. The rain garden tank should also be equipped with a so-called storm overflow to allow safe and controlled drainage of excess water in the event of very heavy rainfall. Inside the container, there should be a drainage layer made of aggregate of the appropriate granulation, in which we place a system of pipes to drain the filtered and purified water. Most of the rain gardens implemented in Poland are based on PVC pipes, personally I prefer copper pipes because of their positive effect on water quality. Another important element is to fill the container with a layer of coarse sand, sand mixed with organic compost, wood chips and coarse gravel or ballast. Sand mixed with compost should be poor in nutrients, its main task is to inoculate the right groups of micro-organisms to speed up water filtration (I use it mainly to cover the roots of planted plants. Woodchips are of similar importance – into this layer we inoculate a suitably selected mycelium, which also removes a number of contaminants from the water stream (e.g. coliform bacteria that can enter the water from animal faeces deposited on the roof). Woodchips may dry out periodically. They then become light and could be subject to leaching when the containers are refilled with rainwater. It is therefore necessary to lay a layer of woodchips underneath a layer of heavier gravel or ballast, which further breaks up the flow of water coming from the gutter into smaller streams and allows the water to be evenly distributed in the container. The last, but perhaps most essential of the components, are the appropriately selected plants. When selecting them, I always prioritise native and locally occurring species, making sure they are adapted to periodic root flooding. These should primarily be perennial plant species or those that self-sow from seed and do not require annual replanting. It should also be remembered that the plants require periodic maintenance, which mainly focuses on removing dead and withered aboveground fragments, mainly at the end of each growing season. The biomass collected, can successfully supplement the compost heap.

Rain gardens established in the ground

This category includes two main types of ground rain gardens: infiltration gardens and basin waterproofing gardens. I have a strong preference for introducing solutions that allow treated water to soak into deep soil structures, allowing the aquifers below to be recharged. Unfortunately, it is not possible to implement such solutions in all locations. In cases where it is necessary to insulate the basin in which the rain garden will be set up, I always use insulating materials of natural origin (clay, bentonite clay) in order to avoid introducing unnecessary plastic materials into the environment (looking at the long-term functioning of rain gardens, it is always the case that sooner or later the membrane is punctured by plant roots, rodents or insects in the underground tunnels, which necessitates the replacement of the damaged membrane). When locating a ground-based rain garden, the most important factor – apart from distances from buildings and the absence of clashes with underground technical installations – is the local groundwater level. When carrying out field reconnaissance, I always check the ground water level with a series of soil borings extending to a depth of 1.5 m from ground level. If, after drilling the boreholes (preferably in spring after snowmelt), you find that they are filling with water, you should think about opting for a rain garden in a container that can be planted on the ground. The excavation for the establishment of the rain garden should be made at a safe distance from existing buildings (at least 5m), preferably in a location with a gentle slope to allow for the possible drainage of excess rainwater during heavy rainfall. Once the excavation has been sealed (a layer of rammed clay or bentonite at least 30 cm thick) or an infiltration layer has been made, the resulting basin can be filled with porous filter material. Typically, sand of different granulations is used for this purpose. Around newly planted plants, it is a good idea to use sand mixed with a small amount of organic compost. Wood chips inoculated with suitable fungal strains will also work well as an additional filter layer. When using them, it is important to remember to lay such a layer under a layer of heavier gravel or ballast. The choice of plants is similar to that of container gardens, but if gardens with large areas and deep basins are being realised, trees and shrubs (e.g. black alder, willow wilt) can also be planted. When planting trees and shrubs, it is a good idea to apply mycorrhizal vaccines to promote plant growth.

Examples of interesting implementations of ground infiltrating rain gardens can be found in: Jelcz-Laskowice and Tarnowskie Góry. Oba projekty zostały zrealizowane przez Fundację TRANSFORMACJA).

Author: Łukasz Nowacki

The text was written as part of the project „Hydrozagadka – how to win against drought?”