I'm currently working on a design involving a new-to-me stormwater retention device called StormTech and it made me realize, again, that there are so many methods out there depending on all the combinations of water quantity, soil type and aesthetics. What is a rain garden vs. a dry stream? When would you use a french drain vs. a dry well? Let's start with the basics...
Whether you call it a rain garden, dry stream, french drain, soakage trench or stormwater retention facility, the intention is the same: manage stormwater during heavy downpours. We need to figure out where the rain water can go that *isn't* the basement floor and doesn't cause erosion. This water almost always comes from impervious surfaces like the roof or a driveway.
Stormwater management is particularly important in the Portland area because a) our wet season is so wet and our dry season is so dry and b) the way our stormwater combines with our sewage in some parts of Portland can cause major issues during big storm events.
The classic rain garden can be seen all around Portland front yards. They generally take the rain water from a gutter downspout or driveway, allow it to pool in a depression in the ground, and then quickly soak into the sub-soil. The term rain garden is the most commonly used around here, but the more official term may be bioretention facility.
The first example, below, takes stormwater from one downspout (the black pipe is visible on the right) and holds it in a depression in the soil filled with river rock. The depression is planted with Juncus and Cornus (Redtwig Dogwood) while Iris and Carex thrive around the edges.
The next rain garden is in Lake Oswego, a very different kind of soil, and is not connected to a gutter downspout. Instead, this depression collects stormwater off the driveway and allows it to sink into the soil. Please excuse the dirt mounds, the moles like this rain garden, too. See more of this project here.
The third rain garden example is barely visible because the plants are so happy. In this case, the gutter downspouts were permanently piped under the walkways prior to pouring the concrete. The plants are doing an excellent job of filtering rain water and preventing erosion. See more of this project here and here.
The next example could be called a rain garden or bioretention facility, but we refer to as a Dry Pond: it fills with stormwater during heavy downpours and looks like a pond, then drains just as quickly to look like you see below. If you use just the right kind of rock and grade the soil correctly, a dry pond can be used as a patio in the summer. I have an example of this going in soon, I will add the photos when it's completed.
A dry stream looks like a little creek bed and often I do not put plants in the depression. It is more linear than a rain garden and it's main job is to move water in one direction instead of just retaining it. The rocks in the depression prevent erosion and slow the water a little bit, but ultimately encourages the water to flow away, in this case, from the house.
A swale or bioswale are terms often used in Permaculture and for bigger sites, but it doesn't need to be exclusive to these applications. A swale intercepts sheet flow of water (rather than concentrated water through a gutter or pipe). Below is close-up picture of a large bioswale on a 1-acre site. The swale captures sheet flow from about 5-acres of lawn above and is planted with grasses, rushes, iris and camas. The picture captures a fleeting moment between when the rain ends and the drainage begins. Within hours, this will look like a ditch again.
I have used ribbon driveways a few times when there are water issues around the existing driveway. In some cases, just filling the center strip with the correct type of rock can be enough to sink the stormwater, but in the project below there is a hidden french drain under the river rock, which helps hold a larger quantity of water until it has time to drain into the sub-soil.
A french drain is essentially a pipe with holes that is installed underground or behind a retaining wall and intended to be entirely hidden from view. Some say that the french drain is overused, but I believe it is very effective when used for the job it's intended. A dry well is another example of an underground product that does an incredible job in the right contexts. It is essentially a barrel with holes that can be used like an underground rain garden when we want the water to stay and soak into the sub-soil. In fact, I've been known to put a dry well under a rain garden if we need to hold a larger quantity of water!
There are many more complex stormwater retention facilities used around Portland that are used especially with new construction and require engineering, like the StormTech product I mentioned at the beginning. From a landscape design point of view, it's often a question of how to minimize the visual impact of these styles. In the example below, the plantings will eventually obscure the concrete rectangle while the path and patio to the right will draw your attention instead.
If you're still with me, you can probably tell I love this stuff. Stormwater management is a real challenge here in Portland with some beautiful, effective solutions. Check out the following resources from the City of Portland here, and the Oregon Rain Garden Guide here. Or, of course, give me a call and we'll design it together to work with the rest of your landscape!