There’s a new rain garden in town! With funding from TD Friends of the Environment, the Coastal and Water Team at the EAC was able to partner with Dalhousie’s Office of Sustainability to build a rain garden on the Dalhousie Campus. Our site is a challenging one, in that it receives high flows of stormwater during heavy rains, enough to cause basement flooding of several buildings down the hill toward the Northwest Arm. The garden is located at the intersection of Coburg Rd and Oxford Rd, at the School of Social Work ‘house-turned-office’. While our goal was initially to build a rain garden that demonstrates that this is an accessible DIY project for homeowners, this site and the design it required moved the project out of the DIY category! However, there are many aspects of the design and build process that still apply to a homeowner excited about a rain garden sized for capturing rainwater from a roof via a downspout or two.
Our Process & Design
First, we assessed the site, which means that we looked at a variety of factors that will influence how water moves over the site. This includes soil type and drainage properties, slope, and the size of the rain catchment area. Check out previous rain garden how-to posts here for details on these steps.
Our calculations showed that we needed a very large garden to absorb all the stormwater which we calculated flowed onto the site. In fact, the calculated area was larger than the area we had available! In addition, we found the slope of the site to be steeper than the suggested 12% for rain gardens in even the flattest area. This meant that we had to increase the capacity of the garden not by surface area, but by depth. The garden is 24 inches deep, whereas most rain gardens are 6-8 inches deep. To increase the porous volume in the garden, we filled the depression to a depth of 14 inches of clear stone, which would hold water in the bottom of the garden, while also holding soil on top. The cross-sectional design looked much like this diagram, from the Fairfax County Virginia website (Figure 1).
Figure 1. Approximate cross-sectional view of the rain garden at Dalhousie.
We used drainage tile (underdrain in the diagram) to move the stormwater that flows from the road and sidewalk over the steep hillside into the rain garden. See the photo below (Figure 2).
Figure 2. In the middle of this photo, the drainage tile is sticking out of the clear stone in the middle of the garden. The tile extends back to where the volunteers are sitting on the slope to catch water flowing down the slope and direct it into the rain garden.
In this photo (Figure 2) you can see how deep the garden is and just how much clear stone is at the bottom. And another key design feature is starting to form: the berm! The berm is essential in a garden of this size, on this slope, and so close to the house. The berm will help retain the water in the garden while it pools and infiltrates. Below (Figure 3) you can see how the berm has been planted with larger native shrubs such as sweet fern and bayberry, as well as a serviceberry tree. As these plants establish, they will help anchor the berm in place. To increase the strength of the berm, it was constructed out of sod mats from the beginning of excavation. Each layer was tamped down, and then new sod was staked into the backside of the berm all the way around the edge of the garden.
Figure 3. The depth of the garden, the volume of clear stone and the berm are all clear in this photo.
Another unique feature of the garden can be seen from the photo (Figure 4) of the completed garden below: the centre of the garden is actually filled with clear stone and then beach stone all the way to the surface. The goal of this design is to allow water which rushes down the slope from Coburg Rd to first hit the beach stone, which will slow down the flow. Then it will make its way to this holding pond in the centre, and as that space fills, the water will first infiltrate horizontally into the soil and plant roots, and then only when it is really full will the water flow over the surface of the mulch and soil, up to the height of the berm. The water will be slowed to the point where it can infiltrate into the groundwater.
Figure 4. The back portion of the completed garden contains stone all the way to the surface, with native shrubs planted in the berm.
Stay tuned to the blog over the coming weeks to learn more about the plants we used and how we chose them, as well as thoughts about rain gardens functioning as marine protective areas, coastal erosion prevention tools, and social activities.