Sensationalist title, yes, but unfortunately swales can cause damage, or even loss of life. When designing and building earthworks, “Don’t kill people” should be your first rule. I’m going to highlight 3 situations in which these water-harvesting earthworks could be dangerous and perhaps even fatal.
The swale is a water-harvesting ditch on contour.
You’ve Heard of Quicksand. How About Quick Clay?
When I get a call from a client who is east of me inquiring about earthworks, I immediately head for a soil map. The reason is that much of Eastern Ontario was covered by the Sea of Champlain around 10,000 years ago. Here silt, clay and organic matter were deposited in the sea’s saline environment, leaving a deposit of a particular kind of clay known as Leda clay, quickclay, or Sea of Champlain clay. The salt in the water acted as a cohesive agent. When the sea disappeared, this clay was then exposed to fresh water from rainfall, washing away much of the bonding properties of the salt, leaving the very unstable structure of the Leda clay.
Under pressure, or sometimes when highly saturated with water, Leda clay can liquefy – something which has triggered a number of landslides in Eastern Ontario. If you place a swale somewhere, you are going to make the ground downhill of the swale wetter. This creates conditions that could make the ground unstable, triggering a landslide.
So what do you do in this situation? I don’t have any set rules. I look at the need for swales and if other approaches would suffice. I also look at the slope and the likelihood that the land might slide. (Keep in mind that these slides can cover a lot of area and might start a long distance off site.)
In some situations, swales just might help prevent damage. Leda clay contracts a lot when dry, which can lead to foundation damage. If sliding is not a danger, you might help a building keeping the ground hydrated with swales. Personally, I’d look to preserve moisture with mulch and ground cover in those situations, just to be safe.
In short, Leda clay makes me nervous. I’ve yet to install or recommend any swales in these situations. If you’ve had any experience dealing with quick clay, I’d love to hear about it in the comments, or via the contact form.
Video by Christian Olsen.
Trouble in Paradise
Swales can trigger landslides in tropical highlands. These hilly areas get tremendous volumes of rainfall. Forcing more water into hillside soils in these regions can trigger a slide, even when they are forested. Consider the following passage from Jared Diamond’s Collapse:
Image of tropical highland landslide by Radhakrishnansk
[O]ne European agricultural advisor was horrified to notice that a New Guinean sweet potato garden on a steep slope in a wet area had vertical drainage ditches running straight down the slope. He convinced the villagers to correct their awful mistake, and instead to put in drains running horizontally along contours, according to good European practices. Awed by him, the villages reoriented their drains, with the result that water built up behind the drains, and in the next heavy rains, a landslide carried the entire garden down the slope in the river below.
These are areas of 3 or more metres of rainfall a year. There’s no problem with available water in these regions. Don’t risks potentially deadly landslides by floating mountainsides with swales.
Swales to Stop a Leaky Basement? No
As mentioned above, swales make the area downhill of the swale wetter. I had someone near the waterfront in Toronto contact me a few years back about doing some work on her property. Her house was at the bottom of a bluff, and she was wondering about swales above her home to keep the water out of her basement in the spring. In this situation, there was the increased risk of sliding, as well as a good guarantee that spring flooding would be made much worse. Sometimes the best advice is not to do anything. I was happy not to ruin her home with swales and I suggested some drainage to carry water away from the uphill side of the home.
pietro says
hi, i m pietro from south italy. thank you for your suggestions about the raising of the water table as a consequence of swales. i think i m lucky because the calculator doesn t answer, it gives “error”; and my eyes are fallen on your introduction and suggestion link. now i m reflecting about the water table. we have a very shallow soil ( 30-40 cm) on a bedrock (travertin tufa, it s porous, calcareous ….. it means water!!); the lowest point of the ground is always a puddle!!! i ll keep studying. really thank you!!!! PS i always notice that wild trees around (even at few metres from the “puddle zone”, are high and healthy, ulmus family, oak family)……my neverending question is: how can grow 15 metres tall with so shallow soil??? and wheat and mais are cultivated at 4 metres far from their trunk (so their roots aren t a problem for these plant). hoping in some wise words from you. bye pietro
Douglas Barnes says
Hi Pietro,
I’m glad the information was helpful. For the calculator, If you can give the the figures for your site, I can test to see if the error is expected or not.
As for the trees, sufficient calcium is one of the requirements for healthy growth, so that certainly doesn’t help. It would also help to counteract acid rain. With the shallow soils, the Precambrian Shield covers much of Canada. Soils there are very then – often 15 cm or less. Despite these shallow soils, trees such as pines can reach more than 20 metres in the old growth virgin forests. The tallest tree in Ontario, Canada, which stands at 47 metres, is in the Precambrian Shield.
Chris Przybyla says
Thank you, sound advice and needed warnings. Slumping and debris flows can absolutely happen on forested slopes (although trees, of course, help minimize risk). Another risky scenario besides quick clays are earthworks that infiltrate excessive water on slopes with impervious layers under shallow soils: the top layer of soil can just slide away. And, of course, loose/sandy slopes, if saturated, and recently burned slopes. An unusually heavy rain at the start of the wet season, record rain events, and hot days in early spring that rapidly melt snowpack, are all risk factors.
Zia Parker says
Hello, It seems to me that much of your caution about swales can be resolved by modifying the volume of the swale and the slope within the swale, which then makes it a “diversion swale”, or you could say a modified drainage canal. The video of the “quick clay” is very interesting, however again pointing out that it is so much about language and the specifics of a place. Simply suggesting that on steeper ground that potentially has heavy rain events, swales should always be “diversion swales” with smaller volume and more slope within the swale seems much more useful and educational that a dramatic title like “swales can kill”. Here in Ecuador on slopes as steep as 70%, small swales with 4-5% gradient slope within the swale and only 20 cm x 20cm, stabilized with vetiver grass planted below the swale has been very effective in drawing water away from steep ravines that threatened the entire hillside to slide.
April says
Thank you for this! While we have a different type of clay here we live on a very steep hill & I was extremely leary when the consultant we hired began talking about swales & bulge ponds above our home especially with how much water we get in winter/spring.
I feel like keyline plowing & a healthier ground cover will be less likely to destroy our home/kill us in let’s say an earthquake situation with beyond water logged clay.
Davide Foresti says
Thank you for the Article! Very interesting. We are in Northeast Thailand 200 m a.s.l. Thinking if it would be the case to Swale our Northern Slope around 1% first 150 m and 2 % last 100 m, we have about 1100 mm of rain in a 5/6 months period. The area is a rectangle of 250m x 30 m.
We will plant the area as an Integrated Orchard. Until 2006 it was covered by local Dipterocarpacae Forest.
Do you think it is the case to swale? Or would it be enough to make a pond at the end of the slope to catch the runoff?
Thank you!