The Biggest Contribution of Raingardens – Article One of Two

Author – Ben Howard Engineering Designer

So what is a Raingarden? A Raingarden is a structure used to absorb and redirect stormwater toward appropriate waterways. It utilises plants to maintain natural habitats for wildlife, planted in a composition of materials such as fine gravel and rocks which easily absorb and filter the water. This enables moderate amounts of rainfall to be absorbed and then slowly released via pipes into natural waterways such as streams, rivers and eventually the ocean. So why are Raingardens so important? In nature, soil can absorb rainwater a lot more effectively than in a city. In a city, there are a lot of impervious areas like roads, footpaths and roofs where water cannot be absorbed into the ground. Thankfully some clever environmental engineers came up with the concept of Raingardens to resolve this problem.

In article one of two we are going to look at two things.

• Location and selection of a Raingarden
• A typical cross-section of a Raingarden

Location and Selection

When designing a Raingarden the most important thing is to assess the area for your new Raingarden. Areas that aren’t practical for a Raingarden include industrial or hilly areas and one should also consider if there are any services in the area such as power cables or other pipes that may interfere with the Raingarden. New subdivisions need to have Raingardens as part of their local council approved design in New Zealand.

Design of a Raingarden

When designing a Raingarden it’s important to have the right materials and understand how water can affect plant life and water quality. It is important that the rain garden is not too deep otherwise groundwater will be channelled toward the ground surface and may flow directly into the stormwater pipes damaging local ecosystems by waterlogging plants and trees. Alternatively, if the filter material isn’t permeable that could cause flooding and inundation of a road. Fortunately, we have a list of approved materials that can be used in our design, you can find the list by clicking here.

The layers of a watergarden are as per the below drawing.

1. Surface mulch 50mm
2. 50mm degradable weed mat
3. Filter media 600mm ideal
4. Drainage media layer 150mm

If you found this article useful and want to check out part two where I can directly help you with your rain garden ideas then subscribe out my Patreon by clicking here. As always thank you.

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