| French Drains
The French drain is a time-honored system
for eliminating excess water from low points and other areas
prone to saturated soil.
French drains are quite simple: in essence they are just trenches filled
with gravel, with sand on top of that. Often, you'll see French
drains defined to include a drain pipe as well, though the
traditional design is simply the gravel-filled trench. The
advantages of French drains are low cost and easy installation.
In addition, they can be covered over with turf after
installation, making them less conspicuous.
1. Dig the
trench
A French drain starts with digging a trench. The
depth and width of the trench can vary, but 5 to 6 inches wide
and 8 to 12 inches deep are common sizes and usually satisfy
most needs.
2. Ensure proper slope
Grading is a
critical consideration � you must ensure that enough slope
exists for the water to actually flow, and flow in the right
direction. It might be adequate to check very short stretches of
drain with a level to ensure that a slope exists to carry water
in the desired direction. However, you should take whatever
measures are necessary, including a survey and grading, if
needed, to ensure that you have at least a 0.5 percent slope. A
1 or 2 percent grade is better.
3. Fill with gravel
Add gravel to the trench to within a few inches of the
surface. Gravel for this use is typically 0.5 to 1 inch in size.
4. Top off with sand
On top of the gravel, lay at
least 3 or 4 inches of coarse sand. This provides a medium in
which turf can grow so that the trench will not be visible. But
remember that the sand must be coarse or it won't allow water to
properly drain through.
5. Turf the trench
Turf
may be seeded into the sand or simply allowed to grow in from
the adjacent stand, if the turf is a spreading type. Or, you can
lay sod over the sand. However, if you do this, be sure to wash
the soil from the sod roots before laying it so that you don't
contaminate the sand with finer soil.
Adding a
drain pipe
If you wish to use drain pipe, the process
is similar to that just described, except you will first lay an
inch or two of gravel in the bottom of the trench, on which a
perforated drain pipe will rest. The drain pipe should be
wrapped in a filter fabric. Then, finish filling in with gravel
around the pipe, ensuring at least 1 inch of gravel surrounds
the pipe on all sides. Four- or 6-inch drain pipes are commonly
used, and are easily accommodated by 6- to 8-inch trenches.
Be sure to run the upper end of the drain pipe to the
surface and cap it so that you can, if necessary, access the
pipe to unclog it.
Don't get in over your head
Consider hiring a site engineer or similar
professional for complex systems that cover large areas and
require multiple lines. Proper grading, pipe-sizing and other
design factors can make or break a complex drain system.
A 1-percent slope equals a drop of 1 foot over a distance of 100
feet. For shorter distances, the drop can be quite small. For
example, a 20-foot run needs a drop of just 0.2 foot (a little
more than 2 inches) for a 1-percent grade. Shorter drains may be
easy to grade with the help of a good level. However, when in
doubt, hire a surveyor.
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Thank You. |
