Evening Republican, Volume 21, Number 145, Rensselaer, Jasper County, 6 July 1917 — AVERT EROSION OF AGRICULTURAL LAND [ARTICLE+ILLUSTRATION]

AVERT EROSION OF AGRICULTURAL LAND

(Prepared by the United States Department of Agriculture.) Field surveys disclosed that fields with graded terraces where the grades varied were in better condition than were any having uniform graded terraces. The profiles of the grade lines of these terraces showed a tendency of the grade to increase toward the outlets, a short distance at the upper end of the terrace being level. This practice possesses much merit. The grade is increased at intervals along the terrace to accommodate the continually augmented discharge from the increasing size of the drainage area. A lesser grade may be used at the lower end of a variable-graded terrace than is required for a uniform-graded terrace of the same length. This is due to._the_fact that a smaller rate of rain-, fall can be used, since with the lesser grade of the variable-graded terrace, the time required for the water to flow the length of the terrace is greater than for the uniform-graded terrace. Studies and calculations show that the lengths of a variable-graded terrace that can be used, for a grade of 0.5 per cent at the lower end, are 1,570, 1,280, and 1,100 feet on slopes of 5, 10 and 15 per cent, respectively, as compared with lengths of 1,210, 970, and 820 feet for terraces with a uniform grade of 0.5 per cent. In laying off a terrace with variable grade, the grade should be increased at intervals of 200 or 300 feet and at all sharp bends where the terrace crosses a gully or depression in a field. For example, if it is desired to lay off a terrace on a 10 per cent slope, 1,200 feet long and with a vertical spacing of 4 feet, and the grade of the terrace is to be changed every 300 feet, then the grades would be as follows:

Station. Grade In feet per From— To— 100 feet 0 300 0.05 300 600 .14 600 900 .27 900 1,200 AF It is seen from the above that the grade for the first 300 feet of terrace is almost negligible. This portion coulcUwell be laid off level. If a terrace with a uniform grade were used, a grade of 0.77 per cent would be required. Both practice and theory show that the variable-graded terrace is superior to the uniform-graded type. Outlets. Wherever’possible terraces should end at natural drainage channels. The absence of a suitable drainage outlet within the~limlls of a field often necessitates ending the terraces at fence lines, depressions or draws. The volume of water which is discharged from the ends of a system of graded terraces often erodes unsightly and objectionable ditches along the ends of the terraces to the foot of the slope. Erosion in such channels can be reduced greatly by lining them with stones or seeding them to grass. The channels and banks of graded terraces should not be cultivated for 20 to 30 feet from the Outlet channel but should be permanently sodded. Breaks commonly occur and erosion is most active near the ends of graded terraces, owing to the usually large volume of water passing. Some sort of •protective covering of stones, boards or other hard material should be employed to prevent this washing. Where a terrace discharges Into a deep ditch a box trough is used sometimes to -give the water a free overfall Into the ditch. This prevents erosion in the 'terrace channel. Sometimes hillside ditches are constructed as outlets for terraces. Such idltches should have a fall two or three times , that of the terraces and should be located so as to cross them and discharge into the nearest available drainage channel. Often wooded strips s os land are left iri fields to afford a place for the discharge of the water 'with a minimum amount of erosion. Mdny of the failures of graded terraces may be attributed, to irregjjlqrlYtes in grade. Breaks occur often with abrupt; reductions in the grade. This 'causes a piling up of tlis water and a •consequent overtopping of the terrace by reason of the inability of a full

channel to carry the same amount of water on a light grade as on a heavy one. With a variable-graded terrace there is less likelihood of overtopping because the grade Is increased at short intervals along the terrace. Again, breaks in graded terraces are very frequent where gullies and depressions are crossed and at abrupt bends. Such breaks are due to sudden changes in the direction of flow or to a change in grade, and often to both. The usual practice of crossing depressions at a low elevation to avoid abrupt bends, results in an increase of grade to the middle of the depression and a decrease beyond the middle. In order to avoid a break due to this diminution in grade it becomes necessary to maintain the top of the terrace at a uniform grade. This necessitates the building of a high and broad embankment across the depression similar to the one described for level terraces. Wherever It can be done without increasing lhe gr/ide to such an extent as to cause serious erosion, it is advisable to make the grade greater for that portion of the terrace Leading away from the middle of the depression than for the portion leading to the middle. Use of Graded Terrace. The graded terrace is adapted particularly for use on Impervious and worn-out soils, and on shallow open soils with an impermeable subsoil foundation —in general, soils that are incapable of absorbing much water., Since the object of terracing is to prevent erosion, and as this is accomplished best by securing the least movement of the surface water, it can be seen readily that, within limits, the efficiency of a graded terrace varies inversely with the amount of fall given to it. The greater the fall, the greater the velocity and, hence, the greater the erosive power of the moving water. The embankment of a graded terrace, being subjected to the erosive action of the water on its upper side, is often washed considerably, particularly at bends. The deposit of soil in the terrace channel reduces both the grade and the cross-sectional area of the channel and renders the terrace extremely susceptible to overtopping during the 'nett rain. Also the finer, lighter, and more fertile particles of soil remain suspended in the moving water and are carried off the field. In such cases, by the use of excessive grades, the .very cream of the soil Is lost. Where erosion of a terrace takes place no attempt should be made to cultivate th e ter race. It should be seeded to grass. The result that should be attained by a system of terraces and proper farming methods has been expressed in this way:

The primary object is conservation of both solid and fluid parts of the soil through a balanced distribution of the water supply. The ideal distribution is attained when all the rainfall or melting snow is absorbed by the ground or its cover, leaving none to run off aver the surface of the field or pasture; in which case the water so assorted is retained Iq the soil and subsoil until utilized largely or wholly in the making of useful' crops, while any excess either remains In the deeper subsoil and rocks as ground water or through seepage feeds the permanent streams. These conditions are fulfilled most nearly by the horizontal bench terrace and the broad-base level-ridge terrace, since the movement of the water Is redbced to a minimum by both. The graded terrace lacks much in meeting the requirements. 1 In general it is recommended that the broad-base level-ridge terrace be used wherever Conditions of and topography will permit —that Is, where the soil absorbs a portion of the rainfall and -the slopes are not tbo steep. The broad-base level-ridge terrace supplemented by efficient tile drains suitably located would afford the most ideal method for preventing soil erosion on any type of soil. Often the yields obtained "and the saving resulting from the absence of soil erosion would justify, in a financial way, the installation of tile.

BRUSH DAMS BUILT FOR CHECKING EROSION.