Drain Tile and Wetlands
Two of the most often repeated comments about wetlands and flooding
are: 1) Tile and ditches installed to rapidly drain farm fields
increase chances of flooding. 2) A wetland acts like a sponge to absorb
potential flood waters.
In fact, these views are so pervasive that I'm beginning to think
the people saying them actually believe this drivel. Perhaps, if they
understood what they are talking about, they might change their tune.
All growing plants need water, but only in the proper amount. Excess
ground water takes up space which should be occupied by air and will
cause roots to drown. Too much water is as harmful to plant growth as
Artificial drainage systems usually consist of tile lines buried in
the ground. Open ditches carry water from the tile, and from the ground
surface, to streams and rivers which make up the natural drainage.
Water may be present in the soil in three forms, Hygroscopic, capillary, and gravitational or free water.
Hygroscopic water is held so tightly to the soil particles that plants cannot use it.
Capillary water forms a thin film around each soil particle. This
film of water carries plant nutrients and is available to the plant
when no gravitational water is present.
Free water is that which is in excess of capillary water and is in
the pores between soil particles. It is harmful to plant growth and
will collect as visible water in an open hole.
The purpose of farm drainage is to control soil moisture by removing
the free water in the upper three or four feet of the soil. Artificial
drainage does not disturb the usable capillary water so essential to
plant growth. It only removes the harmful gravitational water which
A completely saturated, heavy loam soil may contain the equivalent
of about six inches of water per foot of soil. Of this, only one half
to one inch is free water that can be removed by drainage. Saying this
another way, a half-inch rainfall, falling on heavy soil loaded with
capillary water, may cause the water table to rise about a foot. If the
soil is dry and drain tile is located four feet deep, nearly two feet
of rain would have to fall before any appreciable amount is carried
away by the tile. If the soil is saturated when a heavy rain hits, the
water will flow across the surface of the ground much quicker than it
can percolate down through the soil. Surface run-off is much more
likely to cause flooding, and undrained land is more likely to be
saturated when a deluge hits.
Ditches, designed to collect the artificially drained water, also
collect surface run-off. These ditches have minimal slope in order to
prevent erosion. Most of the time, water slowly flows along the ditch.
When heavy rains hit, they do carry stormwaters to streams and rivers,
but the velocity is still relatively slow and no more water is
transported than would be without ditches. Certainly not as quickly or
in the volumes transported by city storm sewers.
Now, let us look at an undrained wetland, also known as a sponge.
By definition, the ground of a wetland is normally saturated with
water and, most of the time, contains a pool of surface water. This is
because water flows downhill and wetlands are located in low-lying
Proponents of flood control via wetlands say that a wetland absorbs
snowmelt and rainfall that otherwise would flow directly to a stream or
river. What they don't say is that when the depression is filled to
capacity, the water flows freely from the wetland into a nearby stream
or river. If the wetland does truly act like a sponge, then there is a
simple experiment you can do at home to prove to yourself the accuracy
of this simplistic analogy.
What you do is get two dry sponges, two dinner plates, and two cups
of water. Take one of the sponges and get it as full of water as you
can. Then put it on one of the plates. Take the other sponge, still
dry, and place it on the other plate. Now take one of the cups and
slowly pour the water onto the wet sponge until the plate can hold no
more. Do the same with the other cup and the dry sponge. When you're
finished, measure how much water is left in each cup.
If you did this correctly, the cup you used to inundate the dry
sponge should now have less water in it than the other cup. That is
because the wet sponge could not absorb as much as the dry one.
If a wetland is like a sponge, a dry depression can absorb more new
water than a wet one. What if the wetland has surface water on top of
the saturated soil? Well, what if you changed the experiment so you
start with a plate that has more water on it than the sponge can
absorb. Obviously, you will be able to put even less water on the plate
than you could before.
Knowledgeable wetlands fans know that draining does not exacerbate
flooding. They like wetlands primarily because they provide habitat for
ducks, muskrats and other critters. When the subject of wetlands and
flooding comes up, they deride the artificial filling, not the
draining, of wetlands because this does create less storage capacity
and less opportunity for the wetland to be restored.
Rural wetlands are rarely filled. Most farmers can't afford it.
Urban wetlands are rarely drained. They are filled and destroyed to
allow development. When an urban wetland is filled and a replacement
wetland is set aside to meet no-net-loss requirements, the replacement
is usually out in the country and does nothing to mitigate flooding in
the urban drainage area. Furthermore, urban areas have a lot of hard
surface area that can't absorb water. To handle this water, efficient
storm sewers are installed to take this water quickly to the nearest
stream or river. Urban development clearly does more to exacerbate
floods than any agricultural drainage program. Draining farm fields
actually creates more of a storage buffer than if left alone.
I don't know why some people are so intent on restoring wetlands. If
they truly believe that wetlands should be restored, for whatever
reasons, then perhaps they'd have more credibility if they recognize
the truth and begin calling for the dismantling of our cities instead
of trying to place the blame on people who devote their lives to
keeping us fed.