What is a Rain Garden?
A rain garden is an attractive gully in a garden used for capturing rainwater from sealed and impermeable surfaces such as roofs, sidewalks, roads and parking lots.
Ongoing development increases the amount of impermeable surfaces with drainage systems for rainwater which negatively impacts the quality of the environment where humans live. Construction practices generally seal the earths surface and limit the absorbing of rainwater into the ground and subsequent evapotranspiration, and with it increase the risk of flooding resulting from excess water runoff. Impermeable surfaces negatively impact our environment by reducing air humidity and with it promote the growth of dust particles and pollen in the atmosphere. Research also shows that a large cause of our polluted streams, rivers and lakes is caused by the run-off of rainwater from our roads, parking lots or yards. A simple way to maintain clean rivers and lakes is to implement rain gardens.
The benefits of rain gardens are diverse and provide the following functions:
• Help maintain clean water in our rivers by filtering rainwater before entering the local stream
• Help maintain a healthy local water basin
• Increase aesthetic appeal of garden architecture in cities and towns
• Provide shelter and sustenance for small animals and birds inhabiting cities
• Refill groundwater sources
• Improve the micro-climate of an area by increasing evaporation
Rain gardens are not only an aesthetically appealing part of revitalization projects in urban centres providing areas of relaxation, but also an important feature for the protection of water sources. Rain gardens are not fishponds. A well-designed rain garden will retain rainwater for up to 48 hours after rainfall. The lifespan of mosquitos is 7-12 days. A well-designed rain garden will not create conditions for standing water so that mosquitos and other insects could finish their lifecycle. Rain gardens also attract dragonflies which are natural predators of mosquitos. Using local vegetation in a rain garden create beautiful flower beds and therefore do not require much maintenance or fertilization.
Ideal Location for a Rainwater Garden
A rain garden should ideally be located on a grass lawn within close proximity of a building collecting rainwater from the roofs of the building and other impermeable surfaces on the property. The largest volume of rainwater comes from roofs, sealed surfaces and compressed soils. A rain garden should not be within 1 metre of a building so that water would not seep underneath the building foundation. Also, a rain garden should not be near an existing sewage system or ditch. The ideal location for a rain garden is a sunny location well integrated into the entire garden layout and preferably not under tree cover or in areas with large tree root structures.
Ideal Size of a Rain Garden
A rain garden’s size is of personal preference. However, an ideal rain garden is the one that is capable of absorbing all the rainwater from the impermeable surfaces under normal conditions. A typical rain garden for a family house is around 10 – 30 m2.
The size of a rain garden depends on the following factors:
• Depth of the garden
• Volume of rainwater run-off from roofs and other sealed surfaces
• Soil type in the garden
Rain gardens were first used in Maryland, US in 1990, as a means of reducing the risk of polluting water bodies. Gardens have been developed to strive for the best practice of rainwater bioremediation, as a tool for collecting water into overflows and depressions thus slowing run-off from heavy rain. All this, in an effort to protect the streams from pollution and at the same time reduce as much as possible household fees for the drainage of rainwater.
There are several ways to bring rainwater from the roof and sealed surfaces into the rain garden. It is possible to disconnect the rain gutters from the rainwater drainage and to direct the water into the rain garden on the paved surfaced by the ditch. Alternatively, it is possible to fit an underground PVC pipe under the surface carrying rain coming from the roof to the rain garden. To protect against strong water flow and erosion from pipes to the rain garden, it is advisable to install the pipeline with geotextile and stones. The rain garden can be combined before the entrance by still fiing a barrel into which the rainwater flows from the roof.
The size of the rain garden affects the depth and incline of slopes. The ideal depth of the rain garden is between 15 - 30 cm. At a depth of 15 cm, the rain garden will have to be quite large to have enough capacity to accumulate the collected rain volume. On the other hand, a rain garden deeper than 30 cm can hold rainwater for too long, depending on the soil substrate. Installation of the rain garden is recommended in the lowest areas of the flat part of the plot. Rain barrels can also be used to collect rainwater from the roof with its subsequent use in the rain garden. The use of the barrel also affects the size of the area of the rain garden.
Designing Rainfall Depth Zoning and calculating rainfall collection area in the garden is another step in determining the area of the rain garden. If you know the roof area from which you will collect rainwater into the rainwater garden, then the next step is to do soil tests.
The soil type determines the speed of any water from the garden. If the soil profile is sandy, it is necessary to improve the soil with compost. Dusty soil is better than clay. Clay soils also need to be improved by compost, in the extreme case, the entire soil substrate is replaced to improve the infiltration of water in the rain garden. The recommended soil mix is 50 - 60% sand, 30 - 40% clay soil. The organic matter should be 5-10%, the most suitable of the compost.
By selecting the size of the garden and the soil substrate, we will design a shape of a rain garden that aesthetically fits into the landscape or public space. There are several principles that go with designing a rain garden. The longer part of the garden should be perpendicular to the water inlet. This maximizes the ability of the rain garden to capture water. It should be wide enough to make water flow evenly in the garden. A good rule is to keep the ratio of the length and width of the garden in a 2: 1 ratio.
If a rain garden has more than 30m2, we recommend splitting the rain garden into several smaller gardens. For well-permeable sandy soil, the ratio of the water collecting area to the rain garden should be 5:1. This means that if we have a collection area of 150m2, the area of the rain garden should be 30m2. If the soil is of inferior quality and less permeable, a 3: 1 ratio is recommended.
The rain supply to the rain garden is gravitational coming directly from the gutter or subsurface. The end of the inflow pipe or stream into the garden must be secured against the inflow of strong currents of rainwater to avoid erosion of the bottom and deformation of the flora in the garden.
Calculating the Accumulation Capacity of a Rain Garden
The volume of potentially retained water in the rain garden is defined as the total sum of surface and subsurface volumes that can be retained below at a safe run-off level. The amount of storage depends on the specific conditions of the site and its design.
The accumulation capacity of the rain garden is made up of two components:
1. Surface Accumulation Volume [m3] = Floor Area of Bed Rain Bed Garden [m2] x Projected Water Depth [m];
2. Volume of soil accumulation [m3] = area of bed [m2] x retention soil depth [m] x retention volume coefficient (usually 10-20%, may be higher if the soil has a high organic content);
Rainfall accumulation capacity = Surface Accumulation Volume + Soil Accumulation Volume.
Properly designed and installed rain gardens require standard maintenance:
• When vegetation is well received, it must be patched and treated.
• When erosion is evident, mulch needs to be re-filled. Once every 2-3 years, the whole area may require mulching.
• At least twice a year, it is necessary to perform sedimentation removal, erosion prevention, planting of new vegetation, etc.
• During long periods of drought, rain gardens can be irrigated.
• The trees and shrubs should be inspected twice a year for their health status.
Please send your questions, comments or pictures of your completed rain gardens to firstname.lastname@example.org