Diversions versus Terraces
John E. Vavroch, Engineering Technician (Civil)
Natural Resources Conservation Service
One of the first things learned as a Natural Resources Conservation Service (NRCS) field technician is that diversions are not placed in fields in lieu of terraces.
Let’s first define the difference between a diversion and a terrace. A terrace is designed to hold the amount of water coming down the slope from the terrace above it. A diversion is designed to catch and hold all the water flowing into it, which usually includes a substantial amount of foreign drainage.
When federal or state cost-share dollars are involved, diversions are paid for by the cubic yard. Terraces are paid for by the linear foot. With large amounts of foreign drainage, level- or storage-type diversions can quickly get very large in height and very expensive to construct. The required storage is based on drainage acres and the quantity of cover that is on those drainage acres. With the amount of needed storage, the diversion can be long in length with less height or short in length with a large constructed height.
Land slope also plays a key role in the required height of the level diversion. Flat slopes of 0.5 to 1.0 percent can accommodate wide channels from 70 to 100 feet while steeper slopes require a narrower channel to keep the excavation at the upper edge of the channel from getting excessive. All these factors influence the design, height, and cost of the level diversion.
If the size and cost of the level diversion becomes an issue, the easiest way to cure this problem is the establishment of a safe outlet to drain the structure. Grassed waterways or underground outlets can be used in conjunction with gradient diversions to convey runoff to a suitable outlet. Since the diversion is graded to a safe outlet, it is a more normal-sized structure with a smaller price tag.
Sometimes producers are tempted to install two or three big diversions on a field instead of a multiple-terrace system. This is not a good idea. They may get away with this for a few years in today’s high residue environment, but sooner or later, along comes a large, high intensity rainfall event. These events cause maintenance nightmares. Channels fill with silt due to excessive horizontal space. Water breaks over the ridges and causes even more substantial ephemeral gullies, and the longevity of the structure can be severely diminished.
For more information on this, or to discuss other resource concerns, please contact your local NRCS office or conservation district office located at your local county U.S. Department of Agriculture (USDA) Service center (listed in the telephone book under United States Government or on the internet at offices.usda.gov).
More information is also available on the Kansas Web site at www.ks.nrcs.usda.gov. Follow us on Twitter @NRCS_Kansas. USDA is an equal opportunity provider and employer.