Grass in Farm Waterways

by C. E. RAMSER

A DENSE cover of grass or other close-growing plants is nearly always the best and cheapest protection for farm waterways the draws and other channels through which excess surface water flows off farm lands into streams. Unless these channels are well protected against erosion, the action of flowing water can turn them into gullies.

Any farmer who has vegetation-lined natural waterways on his land will find it profitable to maintain the vegetation within them and keep them from being damaged by farm implements, livestock, or rodents. Whenever necessary, the waterways should be fertilized and reseeded. It is much cheaper to improve and maintain natural waterways than to build and maintain artificial ones.

Artificial waterways should be established on farm land only where adequate natural waterways are not available. If a farmer lays out terraces that cannot drain into a natural waterway on his own land, he may be able to extend them to one on the land of a neighbor.

To be safe from erosion, a waterway must have vegetative or mechanical protection, or both. Also, it must be large enough not to be flooded by the greatest flow of water that usually comes off the land draining into it. The most commonly used mechanical device for protecting a waterway is the check dam a small dam of concrete, stone, wood, or other material. A series of check dams in time transforms the slope of a waterway into a succession of steps with low risers and long, almost flat treads. At times of storm flow, water falls abruptly over each dam but flows much less rapidly than it would have if dams had not been used. Thus the danger of soil erosion is reduced.

Although for many years farm terraces have commonly drained into vegetation-lined natural waterways, until rather recently a farmer who established an artificial terrace outlet or other waterway usually depended on check dams alone to control erosion within it. Research in the use of vegetation for lining artificial waterways began with the establishment in 1929, at Guthrie, Okla., of the first of 10 soil-erosion experiment stations. In a terrace outlet with a 2 1/2-percent slope, concrete check dams 6 inches high were used and Bermuda grass was planted. At the station at Bethany, Mo., an outlet with a 6-percent slope was protected by burying 2- by 12-inch planks on edge at vertical intervals of 21/2 feet and sowing seed of lespedeza, White Dutch clover, Kentucky bluegrass, and redtop.

Both methods proved effective. Later results at Guthrie indicated that in the gently sloping outlet a good growth of Bermuda grass, once it had become established, would have controlled erosion satisfactorily even without the concrete check dams, which served mainly to prevent erosion during the period required for the establishment of an adequate growth of vegetation. Also, it has since been found generally possible on moderate slopes in most localities to dispense with plank checks such as were used in the waterway at Bethany. One way of doing this is to prepare the vegetation-lined outlet before constructing the terraces. Another is to divert the flow from the terrace outlet until vegetation has become well established.

When these waterways were built we did not know much about how fast water could flow through waterways lined with various kinds of vegetation without injuring the vegetation and scouring the soil. In 1935 the Soil Conservation Service established an outdoor hydraulic laboratory near Spartanburg, S. C., primarily for study of the flow of water in vegetation-lined channels. Such study was begun later at McCredie, Mo. At each of these places, test channels were prepared and different plants common in the locality were grown in them and exposed to flows of different velocities. Thus we have found out a good deal about the rates of flow to which different kinds of vegetative channel linings can be exposed without damage. A farmer who is planning to establish a waterway can now learn from a Department technician what sort of vegetative lining he should use, and whether he should use check dams or other mechanical protection. The needs are determined by the slope and size of the waterway, the character of the soil, and other local conditions.

For vegetation-lined channels in which water flows only part of the time, permissible velocities of flow (that is, the highest rates of flow found to be safe) are greater than for those in which water flows constantly. Where flow is intermittent some scour can be permitted; usually any part of the surface that becomes slightly eroded heals over quickly or the damage can be repaired between rains. The velocities listed here as permissible, therefore, include some that may cause slight scour. For waterways lined with most kinds of vegetation, higher velocities are permissible when the plants are green and uncut than when they are dormant or dead, or have been cut short. Dense, tall vegetation retards water flow, and thus reduces the capacity of the waterway.