Hesperian Health Guides
If you could make a path across a slope that would let you travel from one end to the other while always staying at the same level, you would be following the slope’s contour line. Barriers built to follow contours, such as walls, mounds, lines of grass or brush, or trenches, prevent soil from being carried away by wind and rain. They also help slow the downhill movement of water, spread it over the soil, and sink it into the ground. Plowing along contours, rather than up and down the slope, slows surface runoff and directs water toward your crops. A tool called an A-frame level can help you find your land’s contour lines so you can build contour barriers.
How to make an A-frame level
An A-frame level is a tool that can help you find contours. Use these materials:
- 2 sturdy sticks about 2 meters long and 2 cm thick to form the legs, and 1 stick about 1 meter long for the crossbar.
- 3 nails long enough to go through 2 sticks with a little sticking out.
- A bottle with a twist cap or cork, or a stone to use as a weight (about ½ kilo, or 1 pound).
- A string 2 meters long with a knot tied at one end.
- A pencil or pen, a hammer or stone, a machete or saw, and a tape measure.
- Fasten the 2 legs together in a triangle shape with about 2 meters between the feet. If you nail them together, leave the head of the nail sticking out because you will use it later.
- Fasten the crossbar to the legs.
- Attach the weight (bottle or stone) to the string. Tie the other end of the string to the head of the nail so the weight hangs about 2 cm below the crossbar. If the bottle is plastic, fill it with water, sand, or soil and put on the cap or cork. The string with a weight on the end is called a plumb line.
How to ready an A-frame by marking its center
- Set the A-frame on a nearly flat piece of land. Mark where each leg stands. Make sure the plumb line can move freely, and then hold it still. Once the string stops moving, make a mark where the string touches the crossbar.
- Turn the A-frame so the first leg is where the second leg was and the second leg is where the first leg was. Mark where the string crosses the crossbar. You will now have 2 marks on the crossbar.
- Stretch a string between the 2 marks and fold the string in half to find the middle. Make a third mark there.
- Set the A-frame on a flat place where the plumb line hangs right over the center mark on the crossbar. When the plumb line hangs at the center mark, the 2 feet of the A-frame are level (at the same height). Turn the A-frame and put each leg where the other was. It should still hang over the center mark. If the string does not hang over the center mark, repeat this process until it does.
Decide where to place each barrier
Once the A-frame is built, decide roughly how close together to place your barriers going down the slope. Your first barrier should be near the top of your field, to stop water from the fields above. Where you place the other barriers depends on the slope. For steep slopes, barriers should be about 10 meters apart. For moderate slopes, they should be 15 meters apart. For easy slopes, they can be 20 meters apart. If you must work on a very steep hill, it is best to make individual terraces for trees, or individual planting holes or small terraces for crops, rather than plowing or digging trenches.
Also, consider the soil. Clay soil will not absorb water easily, so barriers should be a little closer together. If the soil is sandy or has a lot of organic matter, it will absorb water easily and barriers can be farther apart. When you have an idea of the distance you want between barriers, put stakes in the ground to mark them.
How to mark contour lines
The next step in preparing to build contour barriers is finding the contour lines.
- At the top of the slope where you want the first barrier, place your A-frame so it goes across the hill, not uphill or downhill. Put one leg of the A-frame where you want the contour line to start. Move the other leg of the A-frame until the plumb line hangs over the center mark. The contour is where the feet of the A-frame are when the plumb line is on the center mark.
- Put a stake next to the second leg of the A-frame.
- Turn the A-frame to find the next level place across the hill, and repeat the first step. Continue to the end of the field or slope, marking every 2 meters with a stake.
- Move to the next place you want a barrier (10 to 20 meters downhill) and repeat the process.
- When you finish marking every contour line, stand at one end of each line and look at the line of stakes. Notice if each contour line is an even curve or not. You may need to move some of the stakes a little to make an even curve.
|Contour lines marked with stakes|
Guidelines for building contour barriers
Once contour lines are measured and marked, and as you decide what kind of barriers are best for your land, keep these general guidelines in mind:
- Preserve or plant trees and plants. If the slope is very steep, the trees already growing there or trees you plant will protect it from collapsing. Grasses and plants with strong roots will help to hold soil and water.
- Slow down water, but keep it moving. It is important to keep water moving, whether down the slope or into the soil. Poorly planned barriers can lead to standing water, which lets mosquitoes breed and spread malaria and other illnesses.
- Fix problems as soon as they happen. Heavy storms may cause a contour trench to collapse or a wall to break. Fix it right away to prevent further erosion.
- Start from the top. Water runs downhill. By starting at the top, you protect everything below, and can use many small barriers.
Different types of contour barriers
Use the contour barrier that is easiest to build and works best for your land.
Live barriers made of trees, shrubs, grasses, or other plants grown on contour lines hold water and soil.
Check dams of brush, rock, or straw bales placed across gullies and eroded areas where water flows let the water through, but slow it down.
Walls 30 cm wide and at least 25 cm high, made of stone, earth, straw bales, or other materials will slow the water and help it sink into the ground.
Trenches direct the flow of water to a certain area. To help water sink into the soil, make small barriers every 8 to 10 meters
inside the trench.
Gabions are wire cages fixed to the sides of a gully and filled with stones that catch and hold soil.
Swales are small earthen barriers with a trench on the uphill side. Soil dug out to make the trench is piled below it
to make the swale. Make the trench 3 times as wide as it is deep so the
sides will not collapse. Trees or
shrubs may be planted in the
trench to take advantage of the
water, and on the swale to hold
it in place.