Kudzu is suitable for use in large natural waterways or gullies where reduction of cross section by its heavy growth is not objectionable.

Centipede grass, a heavy sod-forming grass adapted to the Southeast, competes successfully with other vegetation but can be killed easily by plowing. Therefore it offers promise as a channel lining to be used by farmers who object to the vigorous spreading characteristics of Bermuda grass. No tests were made on cut centipede grass, because this grass does not grow tall and is seldom cut. The permissible velocity of flow for channels lined with centipede grass and having slopes of less than 10 percent was 9 feet a second when the grass was green and 8 feet a second when it was dormant. Centipede grass cannot be grown successfully farther north than South Carolina.

Sudan grass, a fast-growing annual, was tested as a temporary cover for waterways from which flow cannot be diverted while perennial plants are becoming established. For a Sudan grass-lined waterway having a 3-percent slope the permissible velocity of flow is about 4 feet a second when the grass is full grown and green and about 3 feet a second when the grass is dead.

An objection to using Sudan grass as a channel lining is that during dry years it crowds out any permanent vegetation seeded with it, so that the channel has very little protection after the grass dries or is harvested, and it is difficult to establish permanent vegetation before the following spring.

Sudan grass is grown successfully as far north as the northern boundary of South Dakota.

Dallis grass and crabgrass offered rather poor protection in comparison with other grass mixtures. For a channel lined with these grasses and having a 6-percent slope, permissible velocity of flow is about 3.5 feet a second. Dallis grass is adapted to southern conditions only.

At McCredie, Mo., tests have been made on two kinds of channel lining, Kentucky bluegrass and a mixture of timothy and redtop, in channels in Putnam silt loam.

Kentucky bluegrass occurs throughout the northern half of the United States, except where the climate is too dry. The results of the tests showed that the permissible velocity of flow for a channel of 4-percent slope lined with a poor 1-year-old stand of Kentucky bluegrass was only 3 feet a second. After another year, however, permissible velocity for this channel had increased to 7 feet a second. In any locality, the permissible velocity for a channel lined with Kentucky bluegrass is governed by the density of the stand of grass that the soil can produce and maintain. It may be as low as 1 foot a second or as high as 8 feet a second. Kentucky bluegrass that is thick and long enough to shingle the channel surface completely can withstand velocities much higher than 8 feet a second, but the possibility that such a stand will deteriorate makes it unjustifiable to use this grass where velocity will be high.

Timothy and redtop make a good grass mixture for our purpose in northern climates. A channel lining of timothy and redtop withstood a velocity of 7 feet a second at the end of a year and about the same velocity after 2 and 3 years. Only moderate scour resulted when this lining was exposed to a flow of 8 feet a second.

For waterways on soils that are sandier and more easily erodible than the Cecil sandy clay loam at Spartanburg or the Putnam silt loam at McCredie, permissible velocities of flow are lower, of course. Channel vegetation must be carefully maintained if it is to be dense, uniform, and free from weeds. Any damage such as may be caused by rodents, livestock, or farm implements should be repaired immediately. Long-continued heat or drought often reduces the density of vegetation in waterways, with the result that parts of the soil surface are left thinly covered or entirely bare, and thus are likely to be eroded even when velocities of flow are less than those classed here as permissible.

In considering the recommendations I have made, the farmer should bear in mind that the test channels were carefully constructed and were free from irregularities and that nearly all the linings were in uniformly good condition. For waterways of irregular cross section or waterways in which the vegetative lining is expected to become less dense as it grows older, the permissible velocities are lower.

THE AUTHOR

G. E. Ramsey, research specialist in hydrology of the Soil Conservation Service, grew up on an Illinois farm, was graduated from the University of Illinois, and has devoted the past 33 years to engineering research in the Department of Agriculture. His contributions to agriculture won him the 1944 award of the John Deere medal of the American Society of Agricultural Engineers.