FIGURE 24.—Natural watercourse seeded to grass to serve as terrace outlet channel.
EROSION Protection by Terracing Necessitates Run-off Water
Provision for the proper disposal of the run-off water at the ends of terraces Disposal is one of the most important and difficult problems encountered in terracing work. Pasture or timber areas sometimes make very satisfactory outlets, but careful attention must be given to maintaining the cover and to preventing the development of gullies at the foot of the slope where the water leaves the pasture or timber area. The water must be spread somewhat over the ground surface so as to prevent the concentration of sufficient water to cause gully erosion which may occur even on pasture or timber land.
Natural watercourses protected by vegetation on comparatively gentle slopes make the best outlets. Erosion in a channel on moderate slopes ordinarily can be prevented by a dense growth of vegetation, but on steeper slopes it is often necessary to provide additional protection such as is described later in this article. In figure 24 is shown a broad shallow draw serving as a terrace outlet and protected by a thick growth of grass. It is important that the draw be protected by grass as far up its sides as the run-off water will reach, to prevent the possibility of the water washing a gully down the slope on each side of the grass strip parallel to the watercourse.
Natural watercourses are not always available because the water generally cannot be carried beyond the field being terraced. In order to make the best use of natural drainage outlets, it is sometimes advisable for neighboring farmers to cooperate in terracing adjoining fields by running the terraces across property lines. If this cannot be done then it becomes necessary to take the water from the ends of the terraces directly down the slope along a fence or property line. Broad shallow ditches should be constructed to carry the run-off water from the terraces down the slope generally at a comparatively low velocity. Where narrow deep ditches are used high velocities occur and serious cutting or erosion results.
The upper end of the broad shallow ditches on moderate slopes can be protected by vegetation alone provided a good dense cover of grass is established. However, where the ditch is to carry the discharge from more than three terraces of moderate length, some other protection against erosion is likely to be needed in addition to the vegetation. Usually checks of nonerodible material are installed at intervals down the slope. Ordinarily one check is located at the end of each terrace and another between each two terraces, on moderate slopes. On steeper slopes the checks should be spaced at closer intervals. These checks serve the double purpose of checking the development of small gullies in the bottom of the channel and of spreading the water uniformly over the bottom of the channel which reduces the velocity and thereby the erosive power of the water.
Checks are sometimes built of sod or sod bags, which are effective for small drainage areas and for ditches on moderate slopes. The sod strips should be not less than 30 inches wide. They should be watered occasionally when first set out to obtain the best results. When sod bags are used they should be buried in the channel with the upper sides at the same height and even with the bottom of the channel. The bags should be laid end to end across the channel without leaving gaps between them which may be done more easily if the bags are not filled quite full.
One of the simplest checks consists of a 2- by 12-inch plank across the ditch buried with the upper edge even with the bottom of the ditch. Short planks are spiked at each end to form a protection to the side slopes of the ditch. A lumber check being set is shown in figure 25. Where dry weather is apt to shrink the soil away from the plank, it is recommended that a strip of sod about 12 inches wide be set across the ditch against the upper and the lower sides of the plank. These checks have been found to be effective on moderate slopes up to about 8 percent, for limited drainage areas.
In the installation of all checks it is important that grass be established on the bottom of the channel as soon as possible after the checks are built. Bermuda, bluegrass, and buffalo grass are very effective in controlling erosion in outlet ditches, and different grasses can be employed to advantage in mixtures suited to the different localities. Tall grasses and weeds should be avoided as much as possible, and where used should be kept cut down so that the discharge capacity of the ditch will not be materially reduced. If tall growth is permitted in the channel, overflowing of the ditch banks will result which may start the development of gullies down the slope outside the ditch.
Another type of check that has been found effective is built of small loose rock or stone about the size of an apple. A trench 18 inches deep and 18 inches wide is dug across the bottom and side slopes of the outlet ditch and is filled with stone or rock carefully placed so as to make the volume of voids as small as possible. Usually one rain will fill the voids in the rocks with silt, which tends to form a bond between the pieces.
Erosion in ditches with large drainage areas or on steep slopes cannot be effectively controlled by the above-described method. Also, it is not always practicable to build a broad shallow ditch and in some sections of the country it is not possible to obtain a satisfactory growth of grass in the ditches. Under these circumstances control of the erosion is usually accomplished by means of check dams built of permanent material and so spaced in the ditch that the crest of one dam is at about the same elevation as the foot of the next dam above. The object of spacing the dams in this manner is to reduce the fall of the ditch between dams and thereby the velocity and erosive power of the water. Figure 26 shows a broad shallow terrace outlet ditch in which erosion is controlled by low concrete dams, spaced as described above. Bermuda grass is growing on the bottom and sides of the ditch between the dams.
