Let’s consider a “common-sense” question: Would you rather your drainage waterway take up more or less space on your farmland?
The answer seems simple. The more land you use for purposes other than growing crops, such as diverting drainage, the less output you get. This is why many farmers opt for drainage waterways with a “concentrated” profile. To put it simply, drainage waterways with “concentrated profiles” have steeper walls. The steeper the walls, the less space needed. Also, steeper walls result in quicker drainage.
Quicker drainage…less space…why on Earth would anyone do anything besides a concentrated profile?
If it sounds too good to be true, it usually is. In this case, the speed at which a concentrated-profile waterway drains water comes at a cost. Ironically, that cost comes in the form of land.
The Grand Canyon is a good metaphor for the dangers of concentrated profiles. The Colorado River, which winds through the canyon, features some of the roughest waters in the United States. Those waters were so strong, historically, that they actually carved the Grand Canyon out of the rock across 5 million years. If water can make something as big as the Great Canyon, just think what it can do to the dirt that forms the walls of your drainage waterway.
The steep grade of concentrated-profile waterways makes them less structurally stable. As the dirt is eaten away by flowing water, the soil above it will also fall into the waterway. Over time, your waterway widens…cancelling out the space you saved by making a waterway with a concentrated profile.
Now, if you opt for a “dispersed waterway,” or one with a more gradual slope, you’ll be starting with a larger space occupied. So why bother?
There are two advantages that dispersed waterways ultimately provide, which set them apart.
The first is that grass waterways, the most popular form of dispersed waterways, are far more effective for filtering harmful nitrates that would flow into your drainage channel. Grass is able to grow on these gradual slopes, and its roots take in the excess fertilizer that flows off of your fields. In the process, these strong root systems prevent further soil from eroding during the drainage process.
Another benefit that dispersed waterways offer farmers is logistical. You can drive a tractor across them! The steep slopes of concentrated profiles mean you need bring farm equipment around, or build a bridge, to get past them. The easy slopes of the dispersed profile allow you to cross anywhere.
No one wants to sacrifice planting space when constructing a drainage waterway. But it’s ultimately a better idea to install a waterway that allows better movement on the farm, protects the environment, and won’t require future maintenance.