Use of Small Swales – a Case Study
You might have seen Geoff Lawton’s wonderful ‘Greening the Desert’, and his ‘Establishing a Food Forest’ DVD where he wades through a swale metres wide. It’s not commonly discussed, but swales can be quite small too. It depends on the space you have available, the magnitude and intermittency of the rain events, how fast it will soak in and the capacity of your soil to hold it. As always, observing and interacting will yield good results, and you’ll learn as you make mistakes.
The partially completed swale is about to be extended.
The drain is near my right foot.
My latest project is a 25 square metre vegetable patch in subtropical Narangba, South East Queensland, Australia. I could choose the site, and although it wasn’t exactly ‘zone 1’, I settled on an area adjacent to the neighbour’s fence. The soil and solar access are good, and the adjacent fence reduced the amount of additional fencing needed to keep out the dogs, the most expensive purchase for the garden. Being out of the way was an advantage because the owners were not completely sold on having a vegetable patch. I was also hoping that the activity would encourage the neighbour to resurrect their neglected vegetable garden, and they did come out to enquire while the garden was being installed. There is hope.
These are all great qualities, but the main attraction was a downpipe from the house that drained onto the lawn. I could catch the water and distribute it along the length of the garden. Even a light shower would contribute. I have observed that apparently heavy showers can fail to penetrate more than a couple of inches of mulch, so I believe getting the water into the soil is important. I marked out the contour, used that as the upper boundary, and dug the swale trough about 20-25cm wide. I also put a few pavers in front of the drain to prevent erosion of the mound if the water came out in a gush. On the first day I did not get around to installing an overflow, and as it happened there was a storm the day after. The swale filled as predicted, but the water overflowed and flooded part of the garden, partly washing out the path.
The drain is at the bottom left, obscured by grass and pigeon pea.
The pavers for erosion prevention and flow restriction are visible.
The next task was to extend the swale away from the garden with a level sill spillway, as done in Geoff’s Harvesting Water DVD, so that the garden wouldn’t be flooded and it would release water gently onto the lawn. It took some time for me to observe this happening. In the meanwhile I heard that the extension was a little lower than the garden section – it was getting small downpours instead of the garden. Even if I leveled it properly the water could be wasted on the lawn. I placed another paver as a dam to the extension. On Christmas day I got to see it in action. The water didn’t gush, but the drain has since been cleaned so it could happen yet. The original garden section of the swale filled up and the paver slowed the water enough to direct it to the garden first. The mulch and absorption in the swale slowed the advance of the water as it moved through, so even with the paver the extension was getting some early water. While the swale was filling up, the water level on the garden side was higher. The spillway worked as designed when full, letting the water cascade down hill.
The swale is full and overflowing via the level sill spillway.
Further observation revealed that during heavy downpours, there was perhaps too much water in the garden. I had not noticed that the driveway and the neighbour’s driveway could feed the swale too. This explains the good soil, as the area is like a fertile valley, collecting water and sediment which is slowed by the grass and absorbed. After extended rains, the water was springing out of the garden and onto the path! A hole made for planting would fill up with water! To make use of this excess water, I’m considering installing another swale further up the hill, wide and shallow to prevent impeding vehicle access to the back yard. This should charge the soil above the first swale, providing a reserve for when the weather dries out.
The garden was a popular attraction on Christmas day. On the left you can see
the bean trellis, prayer flags and escaping pumpkin.
The garden, while it has some gaps in the planting, has so far been a success with only a few disappointments. There has been a constant supply of lettuce, zucchinis, cucumbers, and now some corn. The garden has pests but the predators seem to be keeping them in check after only 2 months. A 1kg zucchini was just harvested, there is a sunflower I can’t reach the top of, and the owners are talking about expansion!
Much more beautiful than lawn!