Swales, bioretention planters, and rain gardens are all types of landscaping features with and engineering element, that are designed to manage stormwater runoff and improve water quality. However, they are different in their design and purpose.
Swale
A swale is a shallow depression in the landscape that is designed to channel and temporarily store stormwater runoff. It is usually wide and shallow, with gently sloping sides. Swales are often used to slow down the flow of water and reduce erosion, and they can also help to recharge groundwater. They are typically located in low-lying areas where water naturally collects, such as alongside roads or in drainage areas.
One of the key characteristics of a swale is its shape. Swales are typically designed as gentle, sloping channels that allow water to flow slowly and infiltrate into the ground. The slope of the swale may vary depending on the site’s conditions and the amount of water expected to flow through it.
Another important characteristic of a swale is its vegetation. Swales are typically planted with a variety of native species, including grasses, shrubs, and trees. These plants help to slow down the flow of water and absorb nutrients and pollutants, which helps to improve water quality. The vegetation in a swale can also provide habitat for wildlife and enhance the aesthetic value of the landscape.
In terms of construction, swales are typically designed as open systems that allow water to infiltrate into the ground. However, some swales may be designed as closed systems, which involve the use of a drainage layer to collect and transport runoff away from the site. Closed swales are typically used in areas with limited space or where there is a risk of flooding.
The soil mix used in a swale also is important. It should be well-draining and permeable, with a high infiltration rate and also be able to retain moisture without becoming waterlogged. Swales are typically lined with a layer of sand or gravel, which helps to improve infiltration and drainage.
Bioretention planter
A bioretention planter is a type of landscape feature that combines the functions of a planter and a bioretention area. They can be constructed as a raised planter box or container but also as a shallow depression in the ground filled with soil, compost, and plants that is designed to capture and treat stormwater runoff. They can be used to filter pollutants from the runoff and they also help to reduce runoff volume and peak flows.
One of the key characteristics of a bioretention planter is its location. Like a rain garden, a bioretention planter is typically located in a low-lying area where runoff from impervious surfaces can naturally collect, or it may be located near downspouts or other sources of runoff. Bioretention planters may also be used in urban areas where space is limited, as they can be constructed on rooftops, sidewalks, or other areas where there is no available ground space.
Another important characteristic of bioretention planters is their design. They typically include a variety of layers that help to capture, treat, and infiltrate stormwater runoff. These layers may include a gravel layer for drainage, a sand layer for filtration, a soil layer for plant growth, and an organic layer for nutrient removal.
The vegetation used in a bioretention planter is also important. Like rain gardens, the plants used in a bioretention planter are typically native species that are well adapted to local growing conditions. However, because bioretention planters are typically constructed in raised planter boxes or containers, the plants used may be different from those used in a rain garden. For example, plants with deeper root systems may be used in bioretention planters to help enhance infiltration and drainage.
In terms of construction, bioretention planters are typically designed as open systems, which means that water is allowed to infiltrate the ground below. However, as with rain gardens, a closed system may be used in some cases, which involves the use of a drainage layer to collect and transport runoff away from the site.
The soil mix used in a bioretention planter is also similar to that used in a rain garden. It should be well-draining and permeable, and it should be able to retain moisture without becoming waterlogged. The soil mix in a bioretention planter may also include perlite or vermiculite to improve drainage and aeration.
Rain garden
A rain garden is a shallow depression in the landscape that is designed to collect and temporarily store stormwater runoff from impervious surfaces, such as rooftops, driveways, and parking lots. The main purpose of a rain garden is to reduce the volume and velocity of stormwater runoff, as well as to filter out pollutants and nutrients from the runoff.
Like bioretention planters, rain gardens are filled with a mixture of soil and plants. However, rain gardens are typically larger than bioretention planters and they are often used to manage runoff from large impervious surfaces, such as parking lots and roofs.
One of the key characteristics of a rain garden is its location. Rain gardens are typically located in low-lying areas where runoff from impervious surfaces naturally collects, or they may be strategically placed to capture runoff from downspouts or other sources. The shape of a rain garden is typically a shallow depression with slightly raised edges, which allows water to pool and infiltrate into the ground slowly.
Another important characteristic of a rain garden is its vegetation. The plants used in a rain garden are typically native species that are well adapted to local growing conditions, and they are chosen for their ability to withstand both periods of drought and heavy rainfall. The vegetation in a rain garden not only helps to slow down runoff and enhance infiltration, but also provides important ecological benefits, such as habitat for birds and insects.
In terms of construction, a rain garden is typically designed as an open system, which means that water is allowed to infiltrate the ground below. However, in some cases, a closed system may be used, which involves the use of a drainage layer to collect and transport runoff away from the site. The choice of open or closed system will depend on a variety of factors, such as soil conditions, site hydrology, and project goals.
The soil mix used in a rain garden should be well-draining and permeable, and it should be able to retain moisture without becoming waterlogged. In addition to compost, sand, and topsoil, perlite or vermiculite may also be added to improve drainage and aeration.
Here’s a table that summarises some of the typical features of swales, bioretention planters, and rain gardens:
| Feature | Swale | Bioretention Planter | Rain Garden |
|---|---|---|---|
| Shape | Shallow depression with gently sloping sides | Shallow basin | Shallow depression with slightly raised edges |
| Purpose | Slow down and temporarily store stormwater runoff | Filter pollutants from runoff | Filter pollutants from runoff and reduce runoff volume |
| Size | Can be large and spread out over a large area | Typically smaller than rain gardens | Typically larger than bioretention planters |
| Material | Typically grass or gravel with native vegetation on sides | Soil, compost, and plants | Soil, compost, and plants |
| Runoff type | Used to manage runoff from small to large areas | Used to manage runoff from smaller areas | Used to manage runoff from large impervious surfaces |
| Drainage | Open; water infiltrates the ground below | Open or closed; can be designed with a drainage layer to collect and take water away, or it can be designed to allow water to penetrate the ground below | Open or closed; can be designed with a drainage layer to collect and take water away, or it can be designed to allow water to penetrate the ground below |
| Drainage layer (if closed system) | N/A | Typically 200-300 mm of gravel or crushed stone, covered with a permeable geotextile fabric | Typically 200-300 mm of gravel or crushed stone, covered with a permeable geotextile fabric |
In closed system bioretention planters and rain gardens, a permeable geotextile fabric is typically placed on top of the drainage layer (gravel or crushed stone) to separate the drainage layer from the soil and compost mixture. The permeable geotextile fabric allows water to flow through it and into the drainage layer, but prevents soil from clogging the drainage layer. The permeable geotextile fabric is typically made of a material that is specifically designed to allow water to pass through while still providing structural stability and protection against clogging.
Soil mix
The soil mix used in bioretention planters and rain gardens typically contains a mixture of compost, sand, and topsoil, with the exact proportions depending on local conditions and design specifications. Here are some general requirements for the soil mix used in these types of systems:
- Compost: A high-quality compost that is rich in organic matter is typically used as a growing medium in bioretention planters and rain gardens. It helps to improve soil structure, retain moisture, and provide essential nutrients for plant growth.
- Sand: Sand is typically used in the soil mix to improve drainage and help prevent soil compaction. A coarse sand with a particle size of 2-3 mm is often used, as it allows water to flow freely through the soil mixture.
- Topsoil: Topsoil is typically used to provide a suitable growing medium for plants and to help improve soil structure. It is important to choose a topsoil that is well-draining, as plants in bioretention planters and rain gardens need to be able to access adequate amounts of oxygen in the soil.
In addition to compost, sand, and topsoil, grit is another commonly used material in these types of systems. Grit, which is made up of small, sharp fragments of crushed rock, is added to the soil mix to improve drainage and prevent soil compaction.
The use of grit is particularly important in areas with heavy clay soils, which can become compacted and limit the movement of water and air through the soil. Grit helps to create spaces between soil particles, allowing water to move more freely and ensuring that plant roots have access to oxygen and other essential nutrients.
However, it is important to choose the right type of grit for your bioretention planter or rain garden. The grit should be clean, free of contaminants, and have a particle size of around 1-2 mm. Grit that is too fine or too large can impede drainage or make it difficult for plants to establish roots.
In general, the soil mix used in bioretention planters and rain gardens should have good permeability and should be able to retain moisture without becoming waterlogged. The pH of the soil mix should also be suitable for the types of plants that will be grown in the system, as different plants have different soil pH requirements.
It’s important to note that these are general guidelines, and the specific soil type requirements for bioretention planters and rain gardens can vary depending on local conditions and design specifications. For example, in some cases, an engineered soil mix may be required to meet specific performance criteria, such as enhanced nutrient and pollutant removal or to provide better growth conditions for certain types of plants. In these cases, the soil mix may need to be custom-made to meet specific project requirements.