I recently put in my first swale on a rural block. It wasn’t as big a deal for me to do as I thought it might have been. I have some experience with excavators and earthworks machinery, but wasn’t sure how that would translate into my new world of permaculture. Even with plenty of experience, and no shortage of enthusiasm, it didn’t all run smoothly. There were some things that I did that I shouldn’t have, and others that I should have that I didn’t. But it was such a gratifying experience seeing it in place after a long period of inaction on our site. It is a fine sight, the swale full of water, intriguingly curving its way across our gently sloping block.
So buoyed with enthusiasm from my success (and wanting to help others avoid some of the little mistakes I made on the way), I thought it would be worth writing a few things down that others may find relevant.
WHAT ARE THEY AGAIN?
Swales are such iconic permaculture features; they are always one of the first things I look for when I am scanning the landscape of permaculture properties. They provide many benefits to a permaculture site, and are not difficult to install. It is often a good place to start for people who want to convert over-grazed pasture into something much more useful, provide some water security to a food forest they are putting in, or are just keen to cut their earthworks teeth.
MORE THAN JUST A DITCH.
Swales are level drains that catch water and let it soak into the ground rather than running off into the nearest river. The water harvested in them is put to good use by trees that are planted downhill (and occasionally uphill) from them, which in turn provide shade and windbreak that helps reduce evaporation. Eventually, with the correct placement of swales, the lenses of sub-surface groundwater they produce turn in to a full aquifer that benefits the whole site.
They also form a natural delineation between different micro climates. The south side of a swale with well-established trees will be quite different to the north. One of the first things that I do when I see a contour plan of a site is work out where the best swale sites are, which in turn flows on to the architecture of the site. Because they are based on the shape of the landform the site is on, they help to visually define the character of a site, and delineate the structural layout.
WHAT ARE THE OTHER BENEFITS?
Putting swales in provides water security for your site. A well thought out design with interlinked swales and dams provides a more constant humidifying effect to the soil, mitigating the extremes.
Swales provide much more than that. During drier times (or if you don’t mind getting your feet a little wet) they allow you to walk around a sloping property with a minimum of effort. As they are on contour you are not walking uphill or downhill, providing easy hauling for wheelbarrows and other gear.
One of the benefits I didn’t take into consideration was the information an excavation can provide. The soil, clay and rock that was exposed in the fresh swale helped me get an idea of where the better dam sites were, what the water table might be doing, and the topsoil depth could be easily seen across the whole swale. As a geologist I was able to obtain quite a bit of information by mapping my swale. I was also lucky enough to have access to information on the previous history of the site (my Dad!). Some charcoal was exposed in an area where the topsoil was a fantastic looking earthy dark brown. “I think Grandfather used to have an old pigsty down here.” Dad said nonchalantly. I went giddy at the idea of 50+ years of well composted nutrient sitting there, waiting for a food forest to be plonked on top of it. I almost rubbed my hands with glee!
Unfortunately the old pigsty burned down in 1967, which links to another benefit of swales. A green, soggy site is also less prone to fire threat. Increased availability of water (In dams) reduces the fire risk on a property, particularly if it is set up with the appropriate infrastructure for fire fighting. Development applications submitted to council now require bushfire assessment and bushfire management plans, so if you are planning to develop a site then a few well-placed dams and swales will work in your favour.
HOW DO I PUT IN A SWALE?
So what are the steps that are involved in designing and installing a successful swale? Getting the design right in the first place is critical. It is possible to fix mistakes or over-sights later, but this adds to the expense (in time, energy and money) so careful planning is key. However there is a proviso here, at least in my case. Spending too long analysing, re-analysing, redesigning, researching and so on can lead to analysis paralysis. Seasons come and go, and they can quickly change while you are trying to cram some research in around your life, and doing a bit more redesign or data collection. All of a sudden you’ve missed a particular planting season, or it’s too wet to get the excavator in. Set a deadline for kick off, and stick to it. Don’t be afraid to get stuck in.
I put a fair bit of thought into the design, and collected quite a lot of data. We had a weather station set up on our site for over a year, but really the nearest bureau weather station was probably adequate for rainfall data. Plus it’s free, validated and comprehensive. I downloaded about 100 years’ worth of rainfall data so was able to look at some pretty interesting long term trends. An example of data downloaded from the bureau can be seen below.
Up there with rainfall data, topography data was important. Our longest swale was about 200m long, and a detailed topographic map helped us refine the overall site. We had our site surveyed by a professional surveyor, who validated our boundary fences (which were a fair bit out in some cases) and picked up the existing features such as dams and contours. As we were going to be building on this site, we figured that it would need surveying sooner or later anyway, so we opted for sooner to allow us to use the info. However it wasn’t cheap. If I was simply planning on doing earthworks and not building the house, I’d recommend a different approach.
Before we had surveyed the site we started off by hiring a laser level and buying some star pickets. We set the laser level up (lucky for me my wife was a surveyor in a former life, so did this step in about a tenth of the time it would have taken me!) and looked for the “highest, longest contour.” This contour would be the prime swale on the site that would catch the most water and provide the most benefit. We pegged this contour with star pickets, and locked it in.
The entire site architecture then cascaded into place from that contour. Swales were designed above and below it, with their spacing determined by slope, tree height and rainfall. I tried to take keylines into account, but there was only one on our particular site. So you don’t need a detailed survey. If you are going to get one done anyway (i.e. you are planning on building) then there certainly is benefit to doing it sooner, but simply observing the lie of the land, and pegging a few key contours will provide you with all you need to get going. A good design stems from the land that is there, not the other way around.
These are some of the design criteria that we had.
Swales were designed to be the width of our tractor. We have a Yeoman’s chisel plough, and I was keen to be able to condition the swale when it was drier to make sure that the subsoil in it was aerated and uncompacted. This was quite important in our case, because the heavy dolerite clay that we were dealing with tends to get saturated in winter and settle out. It needs to be re-aerated after every decent rain event to allow the microbiology and roots to do their work on it. Because of this, the width of the floor of the swale was over two metres, which allows us to run the plough in the swale over summer, allowing it to be uncompacted.
We wanted to maximise the amount of sunlight hitting our trees planted on the swale bunds. I calculated the approximate spacing of the swales based on the winter sun angle and the estimated tree height. This was the minimum spacing that I would use. In practicality, the spacing ended up being constrained by existing features (our one keyline, the existing dam and my longest contour) but it was still good to know, and be able to work out.
I did a fair bit of reading on how big to make my spillways, but in the end a personal comment from Geoff Lawton at his PRI summed it up best. For most swales you want your spillway to be about five times the width of the swale. My swales were 2m wide, so 10m of spillway was required.
As for the number of spillways, I decided to throw in two per swale. I based this on the catchment area and the amount of rainfall we were likely to see in a big event. I also added in a factor of safety to allow for “global weirding” rainfall events.
We weren’t putting in any dams for our first swale, so we didn’t need anything too big. Most of the excavators that you see driving around in the back of landscaping trucks are about 3 to 5 tonnes. These are fine for putting in swales of the size that we were doing. I rang around for some different quotes and asked some detailed questions on machinery type, availability and tried to get a feel for how experienced the operators were.
In the end we just went with a nice bloke who was local. He had plenty of experience, but had never heard of a swale. Because of this there was a bit of communicating that had to be done at the start (see below to see how this panned out) but overall we were happy with the result.
Chances are you will be dealing with an operator who hasn’t heard of a swale. This isn’t a big issue, but there will be some moments where they don’t get what you are doing. Finding someone that you can communicate with is important. Finding someone you can communicate with that has put in swales before is better!
We did our initial set-out with a laser level and star-pickets. I placed the pickets about one every thirty metres, and then went back later and marked the remainder with sticks, rocks, etc. This allowed the operator to see exactly where he was digging at all times.
I decided not to mark in the spillways at the time, opting to add them in on the fly. This approach was fine, but I did hamper myself with my equipment selection. I opted for a homemade, and entirely untested, water level to help me out with my final set-out. Constructed from clear hose, a couple of silver wattle sticks I had to cut down, and some zippy ties I was proud as punch of my inexpensive solution. However it had some fundamental design flaws (a leaking fitting and not enough clear hose for what we were doing) which meant I had to think fast for a while there. With hindsight, for a job this big I would probably just hire the laser level for the excavation. Or even better, get a water level that works up and running! Test your gear beforehand!
Dig-Day arrives! The excavator arriving on site was a very exciting time for me. All of the data collection, analysis and planning was put into action!
We covered about twenty linear metres of swale per hour, which was a pretty good rate for our first go. By the end I suspect it was more like thirty metres, so I think that we did pretty well. I think I had budgeted on about thirty metres per hour.
One of the shortcomings in our equipment selection was the fact that the excavator that we used for the swale job didn’t have a tilting head on the bucket. The lack of this specialised piece of expensive gear meant that the bucket couldn’t tilt from side to side. My Experience at other sites has been that with a tilting head the excavator sits at the end of the swale, and can tilt the head so that the bucket sits flat on the base of the swale even though the excavator isn’t level. This means that the base of the swale is flat, and the overall contour of the swale is better. The material that is excavated is simply side-cast on the downhill side of the excavation as it is dug.
Without the tilting head the excavator is forced to sit below the swale as it digs, pulling the material down towards him. This not only makes it hard for the operator to cut the top of the swale to the right depth, but it also means that it’s more difficult for the operator to give the swale a flat base. Ours ended up with a concave base, with the middle slightly deeper than the edges.
Depending on what you are after, you can design the base of the swale to preferentially put water in your dam, or extra water in your trees, by altering the profile of your swale base. Our “slightly spoon drain swales” are doing something in between.
One other consideration that we had was working around existing elements. As this swale was located in fairly open pasture there wasn’t a lot that we had to contend with. We had a few mature trees that I was keen to keep, and we were able to make some minor adjustments in the field to allow things to fit.
We had a 25mm rainfall event not long after the swale was complete. It is hard for me to explain how gratifying it was to arrive on site and see the swale full of water, with the spillways flowing!
I did some careful monitoring of the swale at this time (it only fills up for the first time once!), noting where water was flowing in, where in the spillway it was lowest, were there any leaks, low points or areas that needed correcting. I found a spring that was flowing in, so I’m hoping that over time, with some more earthworks uphill, we are able to turn that into a permanent feature.
One of the spillways was slightly higher than the other (probably by about 4cm), which given that they are well over 100m apart isn’t too bad. But water can be unforgiving, and I will need to cut down the high one. The low one has a small low point too, where the water preferentially drains from. This will need to be fixed or it will erode. But the loss of a swale or level sill isn’t as critical as say, a dam wall for example. The dam wall failure requires some major rebuilding, while the swale merely requires a patch up at the appropriate time of year.
Due to boring financial reality, I didn’t have access to a Yeoman’s plough before I put the swale in. If I had it, I would have pre-conditioned the subsoil before excavating the swale. The intent here is to reduce the amount of compaction under the bund wall on the lower side of the swale. This in turn promotes root growth, moisture infiltration and aeration. It’s not the end of the world, but I will certainly be pre-conditioning my future swales before excavation. Over time I will also keyline below the swales. This will add to the overall water efficiency of the site; and improve the crops that I will be growing between swales.
Given the savage nature of the local fauna (I’m talking about you, Mr Brushtail) we avoided planting trees on the swale at this stage. Our intent is to set it up as a food forest, however a lack of suitable fencing at the time we excavated meant that this has been delayed. The unfortunate side effect is that the grass will be re-established prior to putting in the food forest. We will just have to manage our way around that one, and possibly apply a bit more elbow grease than I initially allowed for!
So there you have it. Using an excavator on a rural site, or even a big urban site, is a great way to get things done fast. Hopefully you have learned a little about what is required, and can consider putting a swale in at your project site. As always, don’t be afraid to call on your network of permaculture associates for a helping hand.