Sometimes we have limited options for where we can put a rain garden, and that may mean having to put the rain garden on the side of a hill.
Although this location isn’t necessarily ideal, it also isn’t very hard to do as long as a couple of design changes are made to accommodate for the conditions created by the slope.
There are two techniques that I employ to address and remedy these conditions created by the slope, and I cover them below.
The first issue we have to accommodate for is seepage.
Seepage can be seen as a wet area in the location just below the rain garden and is caused by the water actually traveling from inside your rain garden basin, down into the soil, and then laterally through the soil until it re-emerges from the lower side of the hill causing a seep just below the rain garden.
Because your rain garden will be in essence “leaking” water, you will need to account for this loss of water in your Design Volume.
To do this, I multiply the original Design Volume by the percent slope, and then add that number back onto the original Design Volume.
Let’s say we have a hillside that rises vertically one foot for every four feet horizontally, this would be a 25% slope (1/4 = 25%).
Let’s also say our Design Volume is 40 Cubic Feet.
The math would look like this:
40 x .25 = 10
10 + 40 = 50
50 Cubic Feet is our new Hillside Design Volume.
The second issue we need to address comes from the nature of the slope making it difficult or impossible to make one large flat rain garden because of the large elevation changes at play.
To get around this, I spread my Hillside Design Volume across multiple tiered rain garden basins, preferable three because odd numbers seem to look better in Nature than even numbers of things.
You can cut and fill the soil on the hillside, meaning you pull the soil from the upper part of the hill and move that soil to build up the lower side of the hill, creating a level basin on a hillside capable of holding water.
If you are so inclined, and the conditions exist, it is good practice to move the top soil aside and then use the heavier soils below to build up the berm on the low side of the hill; thus creates a slow permeability berm that holds in the water longer, while also saving the better soils for our plants in the rain garden.