A rain garden is a specially designed and constructed garden bed that takes rainwater directly from the roof – the downpipe goes straight into it – or indeed any large, outdoor surface area that produces ‘runoff’ water. If there is an excessive amount of water running into the raingarden bed, an overflow pipe at the top directs water into the stormwater system to prevent water backing up and flooding. At the bottom, aggy pipes slowly take the percolated water away into the stormwater system.
Rain gardens serve two purposes. They help the home gardener by keeping rainwater on site longer, so it’s available to plants and to revitalise the soil. The most important aspect, however, is their effect on the health of urban waterways. Rain gardens slow stormwater entering the stormwater system (which in turn empties into waterways). By the time the rainwater does enter the stormwater system, it has been slowed down dramatically (which reduces flooding events and erosion of banks etc) and the water is also cleaner, because the sand in the rain garden filters out a lot of pollutants (plants and microorganisms also take up some of these pollutants).
Even though rain gardens do help to recharge the soil locally, it’s more about the health of waterways on a larger scale. If there is no rain, the rain garden won’t get any water either, which is why drought tolerant plants that can tolerate temporary inundation (waterlogging) must be planted in a rain garden. If a regular supply of water for your garden is required, a rainwater tank would serve the purpose much better.
Researchers are investigating the efficiency of rain gardens in various areas. Theoretically, any area, regardless of soil type and slope, should be suitable. Appropriate construction is essential though, as rain gardens placed close to a building must not have any potential to undermine foundations etc. They can be built above ground as well as in ground, so those people with heavy clays may find above ground ones easier to install.
It’s important to get good advice on construction. If the rain garden becomes a bog garden, then it’s not working properly. They should be fairly free draining, so even major rain events should drain away within a day.
When designing and constructing overflow rain gardens, it's crucial to ensure they function properly and don't cause any damage to your home or neighboring properties. Here are some important considerations to keep in mind:
Site Selection: Choose a suitable location for the rain garden that allows for proper drainage and doesn't pose any risks to the foundation or structure of your house or neighboring properties.
Professional Assessment: Consult with a professional, such as a landscape architect or civil engineer, to assess the site conditions, including soil type, slope, and proximity to structures, to determine if a rain garden is suitable and to ensure proper design and construction.
Sizing and Capacity: Calculate the appropriate size and capacity of the rain garden based on the surface area it needs to drain and the expected rainfall intensity. Oversized or undersized rain gardens can lead to inefficient drainage or overflow issues.
Overflow Design: Incorporate an effective overflow system to handle excessive rainfall and prevent water from backing up or overflowing into unwanted areas. This can include overflow pipes, swales, or diversion channels to redirect excess water safely away from the property.
Grading and Slope: Ensure proper grading and slope around the rain garden to facilitate natural drainage and prevent water from pooling or flowing towards structures. The grading should direct water away from the house and neighboring properties.
Protective Barriers: Install protective barriers, such as retaining walls or edging, to prevent erosion and maintain the integrity of the rain garden. These barriers should be properly constructed and securely anchored.
Permeable Soil and Plant Selection: Use permeable soil or amend the existing soil to enhance drainage capabilities within the rain garden. Select appropriate plants that can tolerate both wet and dry conditions and have deep root systems to promote water absorption.
Maintenance Access: Ensure easy access to the rain garden for regular maintenance and cleaning. This includes having enough space to maneuver equipment and remove debris that may accumulate over time.
Downspout Diversion: Direct downspouts from roofs towards the rain garden to capture and utilize rainwater effectively. Properly connect downspouts to prevent water from seeping into the foundation or causing erosion.
Consideration of Property Boundaries: Take into account the proximity of neighboring properties when designing the rain garden to prevent water runoff from causing damage or flooding to adjacent land.
Proper Gravel and Mulch Installation: Use appropriate materials, such as gravel or mulch, to cover the rain garden's surface, promoting water infiltration and reducing erosion. Ensure these materials are properly installed to prevent them from being washed away during heavy rainfall.
Regular Maintenance: Implement a maintenance schedule to keep the rain garden functioning optimally. This includes removing debris, controlling weeds, and ensuring proper drainage.
Local Regulations and Permits: Familiarize yourself with local regulations and obtain any necessary permits before constructing a rain garden. Compliance with regulations helps ensure the proper design and construction of the rain garden.
Monitoring and Adaptation: Regularly monitor the performance of the rain garden and make necessary adjustments if issues arise. This may involve modifying the overflow system or adjusting the planting scheme to optimize functionality.
Remember, it is essential to consult with professionals experienced in rain garden design and construction to ensure that your rain garden operates efficiently, avoids damage to your property, and complies with local regulations.