French
Drains
Introduction
The term “french drain” is commonly used to refer to shallow
subsurface drainage in landscapes, parks, golf courses and athletic
fields. People use the term with a wide variety of intents. The exact
meaning varies from one geographic region to another and the connotation
has shifted with the passage of time. In 2004 alone, nearly 600,000
people googled the term “French drain!” Generally speaking,
the term has followed this course of development. 
Evolution
of the French drain
No, the French drain is not some exotic European drainage method.
Although the method, as well as the term, is widely used in Europe,
it originated in 19th Century New England. It derives its name from
Judge Henry French from Concord, Massachusetts, who published Farm
Drainage in 1859.
Stage
1 French drain
Early French drains, and some still today, were merely egress trenches
cut through low- lying wet areas and filled with gravel, crushed rock,
or fragments of brick. Trenches ranged from 6 inches in width to two
feet. They were dug to the depth that desaturation was desired. This
method has certainly provided for drier basements, less soggy gardens,
and more usable athletic facilities for many years. However, this
type of French drain had two chief shortcomings:
1)
Low flow rates: These gravel filled trenches are certainly more
porous than the surrounding soil and clay, but water oozes through
them at a trickle at best. While eventually they can desaturate
soil quite effectively, they take a long time to do so.
2)
Short lifespan: These unprotected trenches can wash full of fines
in a surprisingly short time. Particles of clay and silt are carried
along with the water and deposited in the empty spaces until these
are packed full. Drainage in these trenches eventually slows and
stops.
Stage
2 French drain
Placing a clay drain tile or a slotted or perforated plastic pipe
at the bottom of the gravel filled trench dramatically increases the
flow rate. The water saturated aggregates readily transfer water to
the drainage pipes, which in turn provide a speedy escape path. Although
the flow rates differ considerably depending on the size and type
of drain tile, all provide flow rates far superior to a simple rock
filled trench.
Unfortunately,
this improvement addressed only the first shortcoming of the Stage
1 French drain. These trenches were as prone to blockage as were their
predecessors. Sometimes, especially following dry periods, even the
drainage tile became blocked. Placing a drain tile in the trench did
very little to extend the life of the system.
Stage
3 French drain
To address the issue of life expectancy, installers began the practice
of lining the trenches with a geotextile filter fabric. These drains
were also generally more selective in their choice of backfill, usually
requiring uniform crushed rock.
Unfortunately,
these trench liners extend the life of the French drain only marginally.
They succeeded in keeping the rock and pipe clean but the fines that
the filter held away from the rock tended to cake up on the fabric
eventually restricting and blocking the water entry.
Stage 4: Narrow Trench Installation Technology (NTIT)
A Multi-Flow drainage system addresses all
of the inadequacies of previous French drain systems employing a methodology
known as Narrow Trench Installation Technology
Multi-Flow has pioneered this revolution in drainage technology.
Multi-Flow is comprised of multiple, interconnected, high density
polyethylene flow channels wrapped in a geotextile fabric. Multi-Flow
is typically installed in a 4 inch wide trench and embedded in clean coarse sand.
Narrow Trench Installation Technology™ has several significant
advantages over traditional French drains:
1)
High flow rates: Not only does Multi-Flow carry water away at a
rapid rate, due to its shape, but
it also collects the water very efficiently. The net result is that
more water leaves the saturated area in less time.
2)
Long lifespan: Multi-Flow’s high quality filter wrap keeps
its flow ways clear. A second level of protection comes from the
sand backfill which protects the geotextile from blockage. An inch
to an inch and a half of filter sand provides substantial protection
from filter blinding and consequently it dramatically extends the
life of the drain system.
3)
Simpler and less costly installation:
Traditional French drains usually employ a trench that is a minimum
of 12 inches in width. A back hoe is generally used to dig the trench.
A large amount of spoil is produced and a similar amount of backfill
is required. NTIT systems are installed in a 4 inch wide trench
created by a chain trencher. The result is less surface disruption,
less spoil to remove, less backfill to bring in, and of course,
less effort and cost.
