To Swale or Terrace or Both and Why
- David Spicer
- Oct 11, 2024
- 4 min read
Updated: 7 days ago
Introductions to
As most readers know, planting the rain is becoming more and more important to mitigate extreme weather events. Water is the ultimate thermal mass, hence why deserts are cold at night and hot during the day. We know retaining the green is critical to lower daytime temperatures; like standing in a parking lot or on bare soil compared to standing in pasture, let alone under a tree, there's a vast difference—we can all feel that with our bodies.
There are many ways to do the repair work for water cycle restoration work in our local landscapes. For me, as a designer primarily working in the restoration of broad landscapes, I choose to use mostly two techniques: the well-known swale documented in Bill Mollison’s global gardener series, and the lesser-known terrace, adapted to become a water conduit. The terrace performs the same functions as a swale, but for me, it allows greater flexibility for the user. There is a catch, and I’d like to point out why, and why I choose to use both approaches in the work I do.
Why I chose to install a swale over a terrace? Is free-board, in earthworks language,
means the earthen bank above the high water mark. In swales, terraces, or dams, these structures all need free-board to survive a major rain event. Below is a clear example of a free-board: you can see water overflowing from the spillway and clearly see the mound above the water—that's your free-board.
the swale
A swale has a greater free-board height and thickness of mound, so I like to term them as the shock absorber. This means that the form of a swale, if built right, can take a far greater volume of runoff rainwater coming off the landscape above the swale and/or connect to a valley, directing excess water from one valley to another or from valley to ridge. However, in order to have security and not have the mound blow out in large rain events, we need to have a hefty mound and a good amount of free-board. One of my colleagues, Dan Lawton, states he aims for a 60% free-board. So, if for example you have a swale mound of 1m high, when the spillway starts to overflow, there is still 600mm of mound above water, AKA free-board, ensuring the mound has adequate strength to hold the water back as long as the spillway size is calculated correctly.
The Terrace
The terrace has a lot less material in the mound, so we need to be careful, as we cannot achieve the same free-board with terraces. The terrace has a much broader bottom, allowing more percolation of water. The terrace is more user-friendly and easier to maintain; often, gentler batters are created depending on slope percentage. It's a less intrusive structure and can be placed on steeper slopes than a swale; it allows greater accessibility than the classic swale form. My inspiration for moving to terraces was from the folks at Water Stories
With terraces or swales, I achieve 50% free-board to 50% storage capacity, meaning when the work is done, the structure will hold a certain amount of water in the floor or base of the structure before it overflows. With terraces, I’m generally aiming for a 300mm tilt on the floor, so the volume of water held back is 150mm. With swales, we often beef up the storage capacity because of the free-board and overall volume of the mound holding back the water.
Particle Examples of Why I Will Choose a Swale or Terrace
On a project in the High Atlas of Morocco, with 250mm annual rainfall, it is pretty dry; although people still get killed in flood events here. I chose to build a swale as the primary shock absorber for this site, which then, once the water is pacified, overflows onto terraces, allowing for maximum infiltration into the soil because of the much broader floor in the terrace compared to the narrower floor in a swale.
Another example is here in Australia, in a little place called Jingellic, NSW. It’s quite an extreme country, with 600m from ridge top to valley floor, primarily shale soils and varying rock strata—some vertical, which catch a lot of runoff and have built quite a deep soil profile, and some running horizontal, shedding most of the rain it receives. So, the need for some serious shock absorbers is definitely required. In this case, I chose to install just swales.
This has been a journey of working out what's best for the landscape, rain event volumes, and client brief. So, in this article, I hope I have helped folks make a better decision on what forms of water harvesting earthworks to apply. Of course, there's the option of doing nothing and just building soils with good livestock management. But at times, we do need to intervene, like in the project above, building the shock absorbers to slow water down to percolate into soils, also catching organics, as both swales and terraces are deposition systems. An example of this is below, clearly highlighted by the charcoal deposition from a fire a month before this swale was installed.
