Rain gardens and swales soak runoff into the ground where it falls instead of channelling it away. A swale is a shallow channel, about 30 cm deep and 1-2 m wide, dug along a contour line so water collects in it and soaks in. A rain garden is a planted depression, 15-30 cm deep and 3-5 square metres, that takes roof and paved-area runoff. Both need soil that drains, so test the ground first.
Part of Flood Control and Drainage
Swales and rain gardens are infiltration features: low places built to hold rainwater briefly and let it sink into the soil, reducing the water that reaches ditches and streams.
Keeping The Water On Site Reduces What You Have To Carry Away
Ditches move water away from a place. Swales and rain gardens do the opposite: they keep it, slow it, and let the ground take it. That cuts the volume reaching your outlet and often feeds plants that need the water. They suit small areas well, such as roof runoff, garden runoff and paved yards, and they do not replace ditches for a big catchment.
Two Features, One Idea
- Swales are broad, shallow channels 30 cm deep and 1-2 m wide, dug along contour lines. Water collects in them and soaks in slowly. Plant them with deep-rooted plants. They suit garden and roof runoff. See Swale Design.
- Rain gardens are shallow depressions, 15-30 cm deep and 3-5 square metres, filled with fast-draining soil and planted with water-tolerant species. Route roof downspouts and paved-area runoff into them. See Rain Garden Construction.
What You Need
- Soil that drains — see Soil Drainage Capacity. Slow clay needs larger, shallower features, or a different answer.
- A level or A-frame — a swale only works if it is level along its length.
- A place away from buildings — water should soak in beside the house, not against it.
- Plants — deep-rooted for swales, water-tolerant for rain gardens.
Method: Decide Which Feature You Need
Step 1: Test the soil. Fast or moderate drainage suits both features. Slow drainage needs more surface drainage as well.
Step 2: Pick the source of water. Roof or paved-area water that arrives from a downspout suits a rain garden. Water running down a slope suits a swale across it.
Step 3: Pick the place. Choose ground that takes water without it reaching a foundation. Swales go along contour lines.
Step 4: Build and plant. Follow the design pages, and keep an overflow route for storms bigger than the feature holds.
Common Mistakes
- Using them in soil that will not drain — water sits for days, drowns plants and breeds mosquitoes. Test first, and see Mosquito Prevention.
- Putting them against a wall — you move the wet to the foundation.
- Digging a swale off the contour — water runs to one end and overflows.
- Expecting them to take a whole catchment — they handle moderate rainfall, not flood flows.
- No overflow path — in a big storm the feature fills and spills uncontrolled.
What’s Next
- Learn next: Swale Design — contour, size and overflow
- Learn next: Rain Garden Construction — dig, fill and plant
- Combine with: Cisterns and Rainwater Storage — keep roof water for use instead