There are a couple steps you should keep in mind before you start. If you have currently have grass in that area, it would be easier to kill it first. You can either break it apart manually, or cover the space until the grass dies. The other important thing to consider is whether there are utilities running through that sector of the yard. It is much easier to call the municipality and check that there are no lines running through your property than pay for damages.
To construct the garden, you will need the following: a tape measurer, shovels, rakes, trowels, carpenter’s level, wooden stakes at least 2 feet long, string, river stones. You will also need topsoil, mulch, and possibly grasses for the berm. From there you can add ornaments, decorative stones, or whatever you wish to improve your garden’s aesthetics. Residential gardens typically cost 3-5$ per square foot to install if you are not paying for labour. The plants chosen to occupy the garden are the most expensive portion.
The Digging Process
Outline the perimeter of your rain garden with string. Now put stakes along the uphill and the downhill side of your garden approximately 1.5 meters apart (5 feet). Now tie a piece of string to the stake pairs and make sure that it is level, that way you can get a consistent depth measurement for the rain garden.
Start to dig at the uphill stake until you reach the targeted depth you calculated before. Then move down the slope to the downhill side, digging and filling to reach the targeted depth. If you do the one section at a time, it is easier to use the displaced soil to level out the lower end of the garden and start outlining the berm with the surplus. Try to keep the base of the garden as level as possible.
If you want to line the base with compost, add one or two inches to your target depth. Once you have the garden base dug, mix the compost with the soil at the bottom. If you want to add sand pockets to help filtration, do so now.
Depending on the distance from your downspout to the garden, you may need to dig a shallow funnel to direct the flow. The side where the water enters the garden is the inlet. Inlets are susceptible to erosion because of the fast-moving water, so you should add larger, rough-edged river rocks to break the flow.
The BermHeap soil around the garden, leaving the entryway bare and ensuring that the highest wall is at the downhill edge of the garden. Make the berm about 0.3 m (1 foot) across and stomp on it hard to make it compact. The berm should have very gently sloping sides to slow erosion and allow water to soak into the ground. To help the water to soak in, you can cover the slopes with mulch, grasses, or dry-tolerant plant species. If you expect to collect large volumes of water at a time, you can include a notch (a low point) in the berm that will allow water to pass during heavy rain events. Line the notch in landscape fabric and add river stones to slow the exit of the water. Angle the outlet away from structures and towards vegetation if possible.