Contour Barriers 291
How to ready an A-frame by marking its center
➊ 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.
➋ 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.
➌ Stretch a string between the 2 marks and fold the string in half to find the middle.
Make a third mark there.
➍ 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.
Steep
Moderate
10 meters
Easy
20 meters
15 meters
You can measure
with an A-frame,
a tape measure,
or your steps.
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.
A Community Guide to Environmental Health 2012