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 too little.
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 inhibits growth.
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 areas.
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.