Learn how to use keylines and contour to work with the slope of the land.
By Kelda Lorax
Slope and Contour
Working with contour/slope is like seeing the naked shape of the land and appreciating it, rather than ignoring, or worse, misusing it.
When we ignore the contours of the site:
- Gardens and other soil disturbances might be placed in ways that increase erosion and damage to the site
- We might place components of design in the wrong place, with decreased efficiency
- We might cause ourselves unnecessary work
It’s a crazy idea to make land flat without good reason! Sure, we want it flat for building foundations, terraces, tent camping, etc, but if we make land flat just for the heck of it:
- It can all flood out in one rain event, it doesn’t sink water to appropriate areas
- Because it doesn’t sink water, it can also all dry out at once
- Many useful, precious microclimates can be erased
- Is more prone to extremes from pests, weather, disaster, than a site with many different nuances
So, how does one appreciate the natural shape of the land? By building, gardening, working “on contour”. Contour is a line along one elevation measurement. It is a line directly perpendicular to the slope (up and down) of a site. If you think of your standing body as a hill, then contour lines run across your body like a belt, bracelets, or a neck scarf. If someone were to pour water on your head, the water would travel downhill, but when water hits the belt (if you and the belt were made of earth), the water would slow and spread along that line.
We slow and spread water on a site by building on contour. This decreases the speed with which water runs off the site, decreases the amount of erosion that running water can cause, and is stored in the earth (which holds it like a sponge, which leads to massive increases in fertility).
What exactly is “building on contour”? It’s placing a path along one elevation line, or garden beds along an elevation line, or swales along one elevation line. They could all be on different elevation lines than each other, but if a garden bed starts at certain point, it then also ends at the same elevation, above sea level, as it started at. This is much easier to understand in pictures.
There are many ways to find contour on a site in order to design for it. The easiest way to learn is to play around with a carpenter’s level on somewhere cleared, like a lawn or parking lot.
- Lay a carpenter’s level on the ground and place a stone at an end, that’s your first marker
- Looking at the bubble, slide the other end of the level slightly uphill or downhill, on the ground, until the bubble is centered. Place a stone at That spot
- Move the level to the last stone and look for level again with the other end
- Repeat the process to lay a handful of stones out wherever you find level.
- Use chalk or string or some other marker to run a line through all the markers that were level to each other.
- Now stand back, and without any maps, fancy equipment, or even numbers, you have found a contour line. See how that line runs perpendicular to the slope?
- Pick a different spot, either uphill or downhill of that line. Eyeballing is fine, or you could start that line exactly 1 foot (or some other interval) away from the first line.
- Repeat the process of placing stones, finding level, and drawing a line. Now you have two contour lines
- Repeat the process of finding contour lines.
- Now stand back and you see the pattern of the land just like a topographic map. Beautiful isn’t it, like seeing a naked body?
So, how do you find contour on a site if you don’t have a level with you? If you can make a very secure “A” shape of sticks out of any materials, and some kind of string and weight, you can find level. The frame doesn’t need to be perfect, just sturdy (not going to slip out of the shape you make it). Hang a string from the top, making sure it goes past the horizontal stick, and then add a weight to that string. Now stand it up, and somehow mark the spot where both legs touch the ground, and mark on the horizontal stick where the string hangs past it. (Nevermind level! You’ll find it if you do this exactly). Now, turn the A-frame around so the legs are switched and in exactly the same spot as each other just were. Now mark on the horizontal stick where the string now goes past it again. Exactly in between those two points on your horizontal stick is where the weight will hang when the two legs are on exactly level ground with each other.
In the picture below, the carpenter’s level checks what the string and weight are already showing.
These tools do the same thing on a faster/bigger scale, though with less accuracy to tiny undulations:
Article about using the Bunyip Water Level.
What method you use depends on your scale, the degree of exactness you need in measurement, and of course access to different tools/materials.
Here’s a free, cool online contour mapping tool
Keyline design is a step beyond working with contours if you need regular spacing of earthworks of any kind. If you’re working on a scale where you need all your tree rows exactly 10 feet from the next tree row downslope (or garden or terrace or animal fence, the exact component doesn’t matter), you’d quickly be in trouble if you start assuming you can do it on contour. Contour varies too much. At some points your tree rows would be 10 feet apart, and then 8 feet apart, and then 15. This matters if you’re using equipment and those variations will give you a headache. For example: if you want just one pass with a lawnmower or tractor between rows instead of sometimes one, sometimes half, sometimes two. What also happens is that if the initial row is on contour and the following rows are exactly 10 feet (or whatever) downslope, is that the lower rows get more and more off-contour until they are potentially causing all the erosion problems that we’re trying to avoid.
Thus, the keyline system. The basic idea is that the starting row is on contour, but starts at a place called the keypoint on the slope’s valley. First find the inflection point, where the slope goes from concave to convex, and the keypoint is just below that where water coming down a hill would naturally start slowing.
Check here to see how close you were:
And correct, the inflection point from one valley doesn’t necessarily match up on the same contour line as the inflection point from the next valley. They all erode and deposit in different ways.
Going from your map into the field is when you start to see the keypoints just downhill from the inflection points. It is a greener area that is hard to guess exactly from a topo map alone. Once you’ve identified your keypoint, this is the place to put your first marker. Then find your contour line (using any of the tools listed above) and map that line on the landscape. It’s called your Keyline because it’s a contour line starting at the Keypoint. Then, from that keyline you can figure out rows (other lines) at regular intervals upslope or downslope. For example, if you want tree rows 20 feet apart, stand at your first Keypoint marker, walk upslope 20 feet and place a marker for your first upslope row (leave a friend behind on the keyline, holding the dummy end of a tape measure). When you’re making that new row, don’t find the contour, simply walk with your friend and always be 20 feet upslope of her. Repeat for lines further upslope or downslope of the keyline. You can see that keyline also lays out faster in the field, though it may take some time to find the keypoint.
Instead of creating erosion, what happens is that all of the lines (though many of them are ever slightly off-contour) will slow and spread water from the valley to the ridge. This technique is known for sinking water into arid landscapes, thus the original intentions of the designer to hydrate landscapes to avoid wildfires.
Why would you use keyline rather than contour to set up your earthworks?
- For me, only if I needed regular intervals, otherwise the ‘on contour’ option serves me just fine (but remember, contour will never be regular). So if you’re ever wanting each berm (etc) 10 feet apart, you want to do a keyline system
- You’re in a highly brittle or dry landscape and you always want to sink even more water than you can on contour, or to spread it further into the ridges
- You are integrating your earthworks into an extensive design for water catchments like ponds
- Keyline moves more water and does a better job at preventing erosion than building on contour does.