Conservation: Diversions versus terraces
One of the first things I learned as a Natural Resources Conservation Service (NRCS) field technician — diversions are not installed in fields in place of terraces.
When deciding to install structural practices, the question often asked is, “should I terrace the field or install a diversion?” Every field is different, but all fields need foreign drainage control and the work must begin at the top of the field.
A terrace is designed to reduce the slope length for erosion control and/or to provide moisture conservation. A terrace handles the runoff water from the area below the next terrace up the hill, or from the top of the field when that area does not exceed the area associated with normal terrace spacing.
A diversion may also be used for erosion control and moisture conservation, but is designed to divert runoff water from a gully, a farmstead, a terrace system, or other improvement. A diversion handles the runoff water from a larger or more irregular area, often including substantial “foreign” drainage beyond field boundaries, and typically with a longer watershed slope than what a single terrace could treat.
Terraces are typically paid for by the linear foot. Diversions are typically paid for by the cubic yard. With large amounts of foreign drainage, diversions can get very large in height and expensive to construct compared to terraces. The required storage or flow area is based on drainage acres and the quality of cover that is on those drainage acres. To accommodate the required storage or flow area, diversions can vary greatly in length, height and channel width.
Land slope also plays a key role in the required height of a level diversion. Flat slopes of 0.5 to 1.0 percent can accommodate wide channels from 70 to 100 feet for level diversions that temporarily store water. Steeper slopes require a narrower channel to minimize the excavation at the upper edge of the channel. All these factors contribute to the design, height, channel width, and cost of a level diversion.
If the size and costs of a level (or storage type) diversion are excessive, the system design can include 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. If such a diversion is graded to a safe outlet, it is a smaller structure with lower construction costs.
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. This may work for a few years in today’s high residue environment, but sooner or later, a large, high intensity rainfall event will occur. Those events cause maintenance nightmares. Channels will fill with silt due to excessive horizontal spacing. Water breaks over the ridges causing even more substantial ephemeral gullies and the longevity of the structure can be severely diminished.
For more information, or to discuss other resource concerns, please contact your local Natural Resources Conservation Service (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) for assistance. More information is also available on the Kansas NRCS Web site at www.ks.nrcs.usda.gov. Follow us on Twitter @NRCS_Kansas.