ALTHOUGH control oj erosion by mechanical means usually involves a certain amount oj preliminary technical calculation ij it is to he effective, the measures actually employed can be put into effect by farmers. This article tells what these measures are. First it describes methods that may be applied to cultivated land—ditches Jor the interception of water, absorp- tion and drainage terraces, basin listing, contour furrowing, bench terraces, and special measures for irrigated land. Measures adaptable to pasture and range land are considered next. The building of channels adequate to carry run-off is then dealt withf followed by a discussion of the control of destructive gullies. The article concludes with an account of the mechanical measures that may he used to reduce wind erosion.

Mechanical Measures of Erosion Control

B\' M. L. NICHOLS and T. B. CHAMBERS ^

A LL MEASURES of erosion control in the hist analysis nre ZA mechanical in action. Frequently, however, artificial stnie-

XJL tures such as dams, terraces, and diversion ditches, as well as certain field practices, such as contouring', ridging, subsoiling, etc., are classified as mechanical measures of conti^ol to distinguish them from vegetative controls. In general, these so-called mechanical measures are most practical and economical when used to supplement vegeta- tive controls and wdiere their construction and maintenance may be interwoven with good field cultnral practices. Water concentrated on steep slopes may have great erosive force, however, and an under- standing of the principles of hydraulics is necessary for their practical application.

When rain falls upon the soil, a series of effects is started the integral parts of w^hich, considered by themselves, may be small, but which in the aggregate represent forces of great magnitude. The impact of the raindrops themselves represents a considerable force beating upon the soil. This force nsually may be counteracted by vegetative covering. Where vegetation does not exist, another series of physico- chemical forces is brought into effect by the action of the w^ater on dry lumps of soil. Part of the w^ater that falls upon the soil is drawn into the fine spaces or capillaries. Air is trapped and compressed, the material that causes the soil to cohere in lumps is softened, and the hnnp breaks dow^n in a series of small ^^explosions." Soil mate- rial is dispersed and scattered in a form easily moved by surface water.

The unabsorbed water then collects into small pools, and with 1 M. L. Nichols is Assistant Chiof of the Division of Research, and T. B. Chambers is in charee of the

Section of Ensineerinpf, Division of C^mservation ()])eralions, Soil í\)nsenaíion Service.

G4.G

Mechanical Ivleasures of Erosion Control *î^ 647

increasing accimiLilatiou overilows tlie depressions under the action of gravity to form rivulets and rills, moving tlie loosened soil witli it. Some of the soil particles provide abrasive material, which cuts out channels, forming washes of varying proportions; other particles arc carried into the soil pores, partially or entirel^y clogging them and thus preventing absorption and increasing run-off.

The amount of energy involved in all this may be realized when it is considered tliat 1 inch of rain from 1 acre running down a hill 50 feet high dissipates over 11,000,000 foot-pounds of energy. The force at any point depends upon the concentration and velocity of the water. This total energy, of course, is small compared with the total energy of impact and the total of the physicochemical forces involved in the wetting of dry soil.

From these simple facts it may be readily seen that the fundamental principles of control necessitate practices that assist in—

(1) Surface protection by vegetative covering. (2) Absorption of the maxi muni quantity of watc^i". (3) Tiie movement of large concentrations of water from steep areas tlirough

protected channels.

In general, vegetation furnishes the best surface protection. This has been dealt with in other articles in this Yearbook and need only be mentioned here. However, in cultivated areas this protection is impossible except insofar as proper rotations are followed. When, in a rotation, considerable quantities of organic matter are plowed under, this residue furnishes cementing material and food for numerous molds that tie the soil together with their numberless mycelia. This effect is apparent for a considerable period of time.

Experiments show that various structures, both clods and surface shapes such as are produced by contour furrowing and listing, and such practices as deep tillage, subsoiling, mole drainage, or subsurface tillage, have appreciable effects on the surface movement of water. Any practice that increases the total absorption, such as basin listing, decreases the quantity of run-off to be handled mechanically.

The most common means of preventing large concentrations of water is the use of low-velocity channels or obstructions built across the slopes, which are thus cut into short sections. Mound- or channel- type terraces or hillside ditches are commonly used for this purpose. In general, these provide for the flow of excess water through broad, shallow channels, which offer considerable frictional resistance so that the flowing water has low velocity and little or no erosive power.

The idea that the construction or development and the maintenance of these mechanical features must be a part of regular farming practice is gradually gaining ground, and adaptable practices are being incor- porated into regular soil-management programs. The development of mechaTÛcal erosion-control measures, the determination of the quan- tities of water that can be most practically handled by one outlet channel, the dissipation of the energy generated by flowing water on steep slopes arid erodible soils with reasonable economy—these involve many technical problems, which can be solved only by trained special- ists. Almost invariably, however, the solution resolves itself into a simple, common-sense farm practice, which generally may be applied by the average farm operator with a little guidance from the technician

648 ^ Yearbook, 1938

in estimating the quantities of water to be haiidled and the forces involved.

CONTROL IN CULTIVATED FIELDS

In considering an erosion-control plan, attention is first given to proper management of the cultivated vegetal cover to simulate natural con- ditions as nearly as possible. In addition, mechanical measures must often be used in a supplementary or complementary manner, in order to reduce erosion to a point where losses are practicall}^ equaled by soil ''building" practices. While mechanical measures will ordinarily be beneficial throughout all seasons and are indispensable under some cropping practices, particularly wehere there are clean-cultivated row crops, they are most necessary and most effective where there is no protective cover, as during the dormant season, at the time of seedbed preparation, or immediately after a crop is harvested.

The season of cultural operations may greatly affect the eflSciency of erosion-control practices under any plan of conservation. Plowing for seedbed preparation that destroys surface cover or stubble from pre- ceding crops should not be done further in advance of planting time than is necessary to produce a satisfactory seedbed. Likewise, seed- ing, plowing under cover crops, and harvesting annual crops should be so managed that the land surface will be exposed for the shortest possible time.

Tillage methods and the action of some^ tillage implements may render certain soils more susceptible to erosion. Machines that pul- verize the soil into a fine dust mulch should not be used except where absolutely essential to the production of a crop. All tillage should be along the contour of slopes in order that furrows or marks left by the machines may act as detention dams for the storage and increased absorption of water.

Mechanical methods of control must be correlated with available farm power and the general crop-rotation system. The importance of mechanical measures is often proportionate to the length of time row crops are used or unprotected conditions exist in the rotation cycle. As an example, a 5-year rotation of corn, corn, wheat, oats, and sweet- clover might bo assumed under many conditions to offer complete pro- tection for 3 of the 5 years, while a 3-year corn-cotton-tobacco rotation would offer none. Mechanical measures would be relatively more important under the latter rotation, other conditions being the same.

INTERCEPTION

Interception and diversion ditches to conduct water away from cul- tivated fields are practical on steep or unusually long slopes (fig. 1). The diversion channels are placed above unprotected fields to inter- cept headwaters from higher slopes, or at intervals across the fields to prevent concentration on the lower portions.

The use of diversion channels is dependent on locations affording suitable places for outlets, as water discharged from the channels into unsuitable outlets may do more damage than if allowed to flow across the slope. Broad, flat channels are constructed to reduce velocity and facilitate crossing with farm implements. The gradient of the chaji- nels should be such that nonerosive velocities are maintained under

Mechanical Measures of Erosion Control 4- 649

maximum flow conditions, and the capacity should be suflicient to accommodate the run-off from the heaviest rains to be expected under normal conditions. To avoid blocking of the diver.sion channels by deposition of eroded material from the area above, it is desirable that each channel be located immediately below a well-vegetated area. The upper channel should be at the lower edge of a pasture, meadow, or woodland, while successive channels down the slope must have per- manent buffer strips of close-growing vegetation immediately above them.

TERRACING

The basic function of a terrace ^ is interception of water, which is either absorbed or conducted slowly from the field, depending on

FIGURE 1.—A wcU-constructod diversion chaiiiit'l with protective strip of vegetation above.

the particular requirements of the locality. The terrace and diversion ditch may be identical in function, but the principles of construction and use are different. The terrace most commonly used is of a standard size and shape, regularly spaced, and of a cross section permitting cultivation over the entire surface. In addition, the terrace may also serve as a guide to contouring when listing, laying out row crops, or performing other tillage operations.

There are two different types of terraces—the absorption type and Î The term "terrace" as used in this article refers to the agricultural terrace, which is an earth ridge with

chncnel above placed approximately on the contour of a sloi«. A "drainage-tyi» terrace," often called simply a terrace, is constructed with a slicht gradient toward one end. It is used in the humid parts of the country for the i)uri)ose of intercepting and diverting water away from cultivated slopes. A "level terrace" is constructed through points of the same elevation for the purpose of impounding water above the terrace ridge. The impoundecl water is absorbed on the field. This tyjie of terrace is ordinarily used in semiarid sections where dry farming is practiced. A "bench terrace" is the true terrace defined by Webster as "a raised level space, bench, or platform of earth."

650 ^ Yearbook, 1938

the drainage type. The absorption terrace is a ridge with Httle or no grade designed to hold a large part of the water in the field until absorbed. This is constructed by moving earth from both sides to form a ridge well above the elevation of the slope surface. The drainage-type terrace consists primarily of a channel, the earth from which is moved downhill to form a low flat ridge. The grade of this drainage channel is variable, being level or nearly level at the upper end and increasing little by little along the length of the terrace to afford increased capacity without change in width. The grade and shape of the channel are proportioned so as not to produce an erosive velocity. As a matter of fact, the velocity should be low enough to allow deposition of soil material washed from the interval above (fig. 2).

FiGUKE 2.—Typical draiuage-type terrace. ...

The cross section of the completed terrace must be such that available tillage equipment is readily adaptable to working on the side slopes of both ridge and channel. The capacity of a drainage terrace must be ample to conduct safely from the field the ma.ximura run-off from a rainfall of the ma.ximum intensity to be expected during a 10-year period or less; likewise, the impounding capacity of a level terrace (i. e., one having no slope or grade in the channel) should accommodate run-off from a rainfall of the same intensity. Kains of higher intensity may occasionally cause considerable damage to a new terrace system, but the cost of construction prohibits the building of all terraces to large enough dimensions to take care of any possible rain.

Mechanical Measures of Erosion Control ^ 651

A wide variety of machinery is adaptable to terrace construction, including the turning plow, disk plow, blade grader, V-drag, and other machines. Within the last few years, the blade grader has been notably improved for terracing purposes by adaptation to heavy tractors and other modifications of design. Two other machines liave been developed, the rotary-type pulverizing^ plow and the elevating grader. These have proved, practical in constructing ridge-type terraces imder favorable cojiditions. Under some con- ditions anyone can use the plow, disk, and small blades to construct satisfactory terraces witli farm power, but the process is laborious and few good terraces have resulted from the use of this equipment on the heavier soils. The larger, power-operated, reversible-blade machine is generally the most economical for constructing channel- type terraces. The formation of cooperative associations in several States to purchase this equipment and rent it to farmers has resulted in demonstrating a method of building good terraces at reasonable cost.

While the spacing or location of terraces on the field surface is one of the most important considerations in designing an effective and practical system, very little research data has been developed on this point. The sj^acings most often recommended at present arc largely the result of experience. Numerous terrace systems liave been studied, and engineers have used the average spacings of those giving the most satisfactory results as a basis for spacing tables.

The spacing unit most often used is the vertical interval, that is, the difference in elevation of a point on one terrace to a corresi)onding point on the next. The interval varies with different slope gradients, being greater on the steeper slopes. The increase is not as great as the increase in slope, however, and on flatter slopes the horizontal distance between terraces will therefore be greater than it w^ould be on steeper slopes. ^ Numerous factors such as slope, farming practice, and soil characteristics influence spacing, but the limiting factors are chamiel capacity, erosion between terraces, and interference with tillage operations.

Terraces may be spaced close together on steep slopes to prevent concentration of water washing rills between them, even though they may have capacity for a wider spacing. Interference with tillage operations, which is aggravated by close spacing, encourages wider spacing. In considering these factors it frequently becomes necessary for financial reasons to compromise on spacing or dimensions of mound or channel which govern capacity. Usually, spacing and dimensions are used that will give reasonable immediate security, and plans are made to develop tJie strLictures to larger and safer dimensions as a part of the regular farming operations.

Soil characteristics luidoubtedly influence the amount of erosion between terraces more than any other factor under the same condi- tions of rainfall and cover. The absorptive characteristics of a soil will, of course, afl'ect the total amount of run-oft* and erosion. It seems a reasonable assumption that terraces should have wider spacing on the more absorptive soils, but results from field experieni^e indicate it is not always tenable. Soils with high absorptive charac- teristics may have physical qualities that make them unsuitable for retaining large quantities of water behind the terrace ridge. Under

652 ^ Yearbook, 1938

these conditions, closer spacings with smaller terraces seem necessary. While croppinoj practices and the types of vegetation to be grown

will influence spacings, the decision to increase terrace intervals because an erosion-resisting crop is being used should be made with caution. Economic or other conditions may cause a later change in cropping plans that will produce excessive erosion on the wader inter- vals. Once the terraces are built it is not feasible to change them to conform wûtli changed cropping plans. They should be constructed initially with a spacing that will make the terrace reasonably safe, even under the more adverse cycle of a difterent cropping plan.

Terracing must be correlated with land-use practices, and future uses of the land should be considered. Land to be retired to pasture or other close-growing crops will seldom require the additional protec- tion of mechanical control. Only where tiie soil has been damaged by erosion to such an extent that it \vill not support a protective vegetal cover, or on exceptionally erodible soils that require mechani- cal protection during the interim necessary to produce a cover, will such measures be necessary. Under the latter conditions a smaller terrace cross section than would be used on cultivated fields is generally sufficient.

Population density and economic conditions will sometimes dic- tate land-use practices and produce situations where cultivation is necessary on severely eroded land that should normally be retired, or on slopes too steep for safe tillage. In these situations, construc- tion of terraces may be necessary even though tlie cost will be high and their eftectiveness wall be impaired.

The cost of terracing increases with tlie degree of erosion of tlie land. In other words, it costs more to protect the less valuable lands. This indicates that terraces should be constructed as soon as possible after a field is ])Iaced in cultivation. The virgin conditions of the soil may successfully withstand erosion forces for the time being, but continued unfavorable cropping practices w411 alter these characteristics so as to permit cumulative rates of erosion in the future.

Maintenance is necessary if the terrace system is to continue functioning properly. Depositions of eroded material in the terrace channel raise the flow line and decrease capacity while tiflage wiU wear down the ridge to some extent. Settlement after construction reduces capacity until the terrace may not be able to handle the water of lieavy rains. AMien the low jjlaces and breaks have been properly repaired the terrace can be successfully maintained by the regular plowing operations. A disk or turning plow is entirely satis- factory if used so as to throw earth to the ridge on the absorj^tive terrace and to increase the size of the channel on the drainage type.

When maintaining the chamiel the first plow furrow is started at the outside and turned uphill leaving the water furrow, or joining cuts of the plow, at the channel flow line. One or two plo^\'ings hi this manner will keep the channel clear and if necessary increase its size by widening aiul deepening (fig. 3). In maintaining the ridge or absorptive-type terrace a back furrow^ is made at the ridge top and successive furrows are thrown tow^ard it until the entire width of the ridge has been covered, thus increasing the ridge height and, if desired, the width at the same time.

Mechanical Measures of Erosion Control 4- 653

ABSORPTION

Increasing; absorption by mechanical methods should be an integral part of the erosion-control plan, whether used independently or in combination with other mechanical measures. Probably the method most applicable to all conditions is contour cultivation, which provides numerous small depressions or reservoirs in plow furrows, m wheel marks, or behind lister ridges to retain water and promote absorption. Deep plowing in some soil types, particularlj'^ those subject to excessive packing or other impervious conditions, provides an absorptive mantle of greater depth. Chiseling or subsoiling to break up intractible strata of hard pan, or to shatter the subsoil to permit deep penetration, is an effective measure on adaptable soil types. Subsoding terrace channels on some of the soils of the Piedmont section of tiic Southeast

FIGURE 3.—Maintaining tlie cliannel-type terrace by plowing.

with deep-cutting chisels to a depth of 24 inches or more has increased absorption to such an extent that in some cases there was no discharge at the terrace end during the first year after the operation.

Level terracing in the semiarid dry-farming sections is one of the most effective measures for increasing absorption and decreasing erosion. Drought conditions frequently make such measures neces- sarj' to insure establishment of a controlling vegetal cover. Data from the Stillwell, Okla., Soil Conservation Experiment Station indicate that crop failures from drought are reduced from si.x to three in a 10-year period by impounding all run-off with level terraces. Caution must be exercised here also to see that the practice is cor- related with soil type and other factors, since impounding on imper- vious soils will result in drowning crops or control cover.

Before any mechanical measures for promoting absorption can be made fidly effective, the soil itself must be in condition for optimum absorption and percolation. The porosity of a soil that was originally

654 ^ Yearbook, 1938

highly absorptive may have been seriously reduced as the result of depletion of organic matter and improper tillage. The addition of humus is essential to preserve or restore absorptive characteristics. Field tests on basin listing under identical soil and slope conditions showed 40 to 60 percent more absorption where leguminous cover crops had becîi grown and turned under each season for 10 years, as compared with a portion of the same field where no cover crops were grown and ]io organic matter returned to the soil. The correlation of vegetative and mechanical measures is nowhere more important than where absorptive methods are behig considered. Tillage performed under the proper moisture conditions promotes the breaking up of hard, impervious lumps to smaller particles and provides more voids in the soil mass for increased infiltration. Plowing when the soil is too wet or dry often produces the opposite results, and the trampling of livestock on muddy field surfaces may result in a puddled condition of soil that makes it practiciilly impervious.

Basin listhig is listing with a machine having an attachment that forms an earth dam across the lister furrow at intervals of 15 to 25 feet. The practice is superior to ordinary listing under some condi- tions, especially when used on fallow to store and absorb all run-off. It has little if any advantage when used in preparation for planting row crops, since the dams are soon destroyed by cultivation; but in preparing wheatland for planting it has proved practical when used in connection with a special drill that seeds a row on each side of the lister ridge. It has the further advantage that strict adherence to the contour is not essential. With this practice, rows varying from the contour by 3 or 4 percent are still effective because each dam retains its water in place. Also, in case of a break-over in low places only the water in the broken sections is allowed to escape.

An important principle to be recognized in the laying out of contour rows is to avoid concentration of water on critical spots in the field. Contour rows arc generally laid out parallel to terraces, and the rows have a slight grade corresponding to that of the terrace. While the grade will produce ojily a low velocity, the rows can be used efi*ective]y to divert normal run-off laterally across the field. A method holding some promise has been developed by Soil Conservation Service engineers in iVIississippi whereby the interval between terraces is divided hito two or three approximately equal areas by guide rows located between terraces on grades such that the water will be drained from the steeper parts to the gentler slopes of the land lying between terraces.

SPECIAL MECHANICAL METHODS

Bench terraces arc one of the oldest mechanical methods of erosion control, and have been used for many centuries in thickly populated countries where economic conditions or even the maintenance of a livelihood necessitated the cultivation and preservation of steep slopes. Population density and other factors do not as yet demand the cultivation of excessively steep slopes in the L^nited States except under special conditions. Cultivation of field crops on steep bench- terraced slopes has been practiced in some sections of the South for several generations. In the highly productive citrus lands in Cali-

Mechanical Measures of Erosion Control ^ 655

fornia, bench terraces have been used, to a limited extent, on steep valleyside slopes. Tliere are other places where berich terraces used m connection with production of sach valuable specialized crops as vineyards and orchards are practical in restricted areas.

The ordinary method of ¡producing tlie bench terrace in the Southern States was to construct a series of small ridges on. terrace intervals across the slope, usually on the contour or to a slight grade. Erosion on the strip between two ridges lowered the elevation of the upper side, and the eroded material was caught by the ridge on the lower side to form the bench. As deposition took place the ridge was raised successively higher and was protected with field stones, rubbish, or a natural growth of vegetation, which was allowed to remain in place. The process was hastened by plowing downhill. After several years the residt was fairly level benches witli steep protected risers between. Several methods of handling surface run-oiï were practiced. In some cases it was allowed to flow across the bench over the edge of the steep riser, and to the next bench below; in others the ridge at the lower edge of the bench was maintained at a height sufficient to divert the w^ater and discharge it at the end of the terrace. Some farmers plowed a shallow water channel in the surface of the bench several feet uphill from the lower edge to conduct w^ater to the end of the bench and away from the field.

The California, or Rcddick, terrace is produced in much the same way except that no ridge is used to start benching action. Instead, rows of trees are planted on an irrigation grade across the slope. Cultivation between the tree rows leaves a strip of vegetation in the row that acts as a balk to promote soil deposition to form the bench above the tree row. Cultivation and the plow ing of irrigation furrows between the tree rows fm-ther promotes benching action. When the bench is completed the trees are about half way down the steep side of the riser, wliicli is amply protected with vegetation.

Experiments are now being conducted with bench ten^aces in Puerto Rico, where it is frequently necessary to cultivate slopes up to 40- or even 50-percent gradients in order to support the dense population.

Irrigation water frequently produces serious erosion, particularly where irrigated orchards occupy steep slopes. In several locations in California and Utah, orchards are irrigated by water discharged from head ditches. In such locations it is the usual practice to allow the water to flow for long periods in order that all parts of the slope will receive sufficient moisture. By the time water has sufficiently irrigated the lower portions of the slope it has usually been flowing across the upper portions for several hours or longer, depending on tlie length of the slope. During this period excessive erosion may have occurred on. the upper portions. Erosion control in such situations becomes a problem in the proper application of irrigation water.

At Placerville, Calif., where slo])es ranging up to 30- or 40-percent gradients are occupied by pear orchards, several successful and prac- tical demonstrations of the underground distribution of water to a point near the place of application have been installed. Various methods of application may be used, depending upon the situation. For instance, water may be delivered from the pipes to head ditches

656 -^ Yearbook, 1938

at intervals down the slope where it has a much shorter rim in reach- ing all parts of the irrigated area extending down to the next head ditch below. In other instances, the water may be discharged at intervals along the pipe line to flow laterally, at a low gradient, across the slope without danger of causing erosion. Any practical method to reduce the length of run of the water over unprotected surfaces, or to reduce the gradient of flow, is effective in reducing erosion. \Miere there is enough water to grow a cover crop and at the same time supply sufiicient moisture for the trees, the erosion hazard can be eliminated in this way, but as a usual thing the orchard acreage has been so extended that there is not sufficient water available for both purposes. Serious erosion is also taking place in a number of

■^dtài

FIGURE 4.—Contour furrows on i)a.sture slopes.

western localities on steep slopes used for potatoes antl other crops, grown in rows running downhill, where irrigation water follows the interi'ow furrows.

CONTROL IN PASTURES Mechanical erosion-control measures in pastures generally consist of structures or practices that will aid the establishment and growth of vegetation. The end sought most often is the retention and dis- tribution of moisture, although diversion away from critical places may sometimes be efl'ective. Tillage and fertilization to improve the condition of the soil are practiced under some conditions, particularly where grasses tend to become sod-bound. Structures such as terraces for protection while vegetation is being established may also be feasible for new pastures (fig. 4).

Contour furrowing is often used for retention and distribution of

Mechanical Measures of Erosion Control ^ 657

moisture. Furrows or ridges are constructed across the slope on the contour and act as small impounding basins to promote additional infiltration. The size and spacings of furrows must be dictated in many places by judgment since there is very little experimental data on the subject. Several factors will, of course, influence spacings. Tliese include the effect of soil characteristics on the rate of iniiltra- tion, as w^ell as on the depth of ])eiu>tration and the lateral movemcTit of waiter; the cost of construction; the importance of additional moisture to last over drought periods; and the a/mount of sod that w^oidd be destroyed by construction.

Furrow^s may be made any size that is convenient where it is not desirable to retain all run-oif on the pastnre area. Water in excess of their impounding capacity will flow from the lower side of the furrows without excessive damage to the structure. When better distribution is required a system of spreaders may be used in con- nection with the furrow^s. The spreader is a small diversion cliannel or earth dike so placed that concentrated run-off in naturnl channels is diverted at intervals and discharged through openings onto the more sparsely watered areas. Contour furrows placed between the spreaders insure equal distribution of moisture over the entire slope.

In some locations it may be desirable to retain all nm-ofî except that from the most intense rains. Ridges are generally used under these conditions in order that the area covered by impounded w^ater may be as great as possible. It is obvious that ridge height must be in proportion to spacing intervals, slope, and run-off in order to provide a definite capacity. To avoid the possibility of losing alJ impounded w^ater should a series of breaks occur in the ridges, small earth chuns are thrown up above and at right angles to the îidge line at intervals of 20 to 100 feet to divide the impounded water iîito separate portions. By inaking the dams knver than the ridge, ordinarily about two-thirds its height, they are made to serve also as equalizers, ])ermitting equal distribution to adjoining reservoirs should one be overloaded by concentration in the flow from above.

Terraces or diversion channels may be used to protect critical areas, such as Yery steep slopes, bare or eroded places, or the heads of active gullies. Complete terrace systems may sometimes be necessary to to protect severely eroded pasture areas or to provide protection wdiile new pastures are being established. In either case a small modified terrace cross section is ordinarily suflicient to meet the requirements. Treatment of an entire watershed in this manner may reduce run- off sufficiently to have considerable flood-control value.

Conditioning the soil by mechanical means may often be advanta- geous wdiere a large part of the topsoil has been lost by erosion, and in impervious or closely packed soils. Deep tillage by chiseling 24 to 30 inches deep at intervals has proved efl'ectivo in some places when performed on the contour. Spacings depend on a number of factors. Complete seedbed preparation may be necessary when new pastures are planted. A method that generally gives satisfaction is to plow in such manner as to form small, flat, parallel ridges on the contour, 8 to 12 feet wade, wdth slight channels or w^atcr furrow^s between for the storage of moisture.

]\Iowing, in addition to destroying w^eeds and other undesirable

658 ^ Yearbook, 1938

plants, provides a mulcli and promotes the right kind of vegetation if performed at the proper season. In establishing vegetation on severely eroded, galled spots, nothing appears to be quite so suc- cessful under all conditions as a mulcJi cover, by which velocity of run-off is reduced, more moisture is i^etained in the soil, seeds are held in place, and protection against grazing and evaporation is provided for young plants.

RANGE CONTROL

The depletion of native cover by overgrazing on range land is a common cause of erosion. In many places, regulated grazing is the only control measure justified and it should be included as a necessary and integral part of any erosion-control plan. Fencing to control graz- ing, or to exclude livestock from critical areas, will prove economical under many conditions. Some State laws permit free grazing of any unfenced range whether held in private or public ownership and fencing is necessary to control grazing where such laws exist.

The development of well-distributed watering places is also essential to the control of grazing. Reservoirs, or stock tanks, formed by im- pounding dams on small contributing watersheds may, in addition to furnisliing water for stock, promote moisture conservation, furnish water for irrigating supplemental feed crops, and decrease run-oiï.

Water spreading for better distribution and increased absorption by contour furrows, ridges, and dike spreaders is adaptable to range land in the same manner as described for pastures. Since most of the present range lands are located in the more arid sections of the country, water spreading and mechanical conservation measures assume importance when soil, topographic, and economic conditions permit their use.

Flood irrigation by damming large gullies in wide, flat valleys and diverting the water to the valley floor produces good results where topographical conditions permit economic installation. Run-ofl' from freshets that occur onh^ ouce or twice a year is diverted laterally across the valley behind earth dikes whicli have openings for discharge at suitable places. Flow across the valley is regulated and distributed by means of dikes constructed of earth, stone, or brush. Remarkable increases in native vegetal growth have resulted on many such irri- gated areas, which provide, in addition to gully control and stock water, grazing for increased numbers of livestock, with consequent roihiction in numbers on adjoining overgrazed areas.

Gully control by means of check dams and structures for prevention of overfall cutting are also used on range lands under special condi- tions, but their application is primarily for gully control and their nature and use are described under that heading.

CONTROL OF EROSION IN CHANNELS Run-ofl" must be expected from all agricultural land regardless of the erosion-control measures used. Provision must be made to conduct the excess water to a stabilized channel. Surface water naturally becomes concentrated in channels in such quantities that special attention must be given to them if they are to be protected from erosion. Channels may be considered as minor and major.

Mechanical Measures of Erosion Control *î^ 659

Minor Channels

Minor channels are those that cany the water from a small area such as a terrace system. They can be protected from erosion by vegetation or by simple structures. In the preparation of these chan- nels, advantage is taken of all natural depressions that have a good vegetative covering, or on which cover can easily be established. If natural channels arc not available or are not suitable, artificial chan- nels are created where they will interfere least with the farm opera- tions. Run-off is often diverted from natural channels that are unsafe to other channels that have a good cover. If possible, the diverted water is emptied in wood lots or pastures, and an effort is made to avoid concentration. Whenever natural channels or vegetated areas are used, the grade over which the water will flow must not exceed the safe limit for the existing condition of soil and cover.

The artificially created minor channels are of two types, high- velocity and low-velocity channels.

Low- Velocity Channels

In designing a low-velocity channel an efl'ort is made to limit the velocity of flow in the channel to an established safe maximum for the type of protective cover to be used. This is accomplished by making a wide, shallow channel so that the flowing water will be in contact with a large area and its velocity will be limited by friction. The type of channel used depends upon, the value of the land, the condition of the soil, the type of vegetation that can bo used, and the degree of efl'ectiveness desired.

Meadow strips, a natural drainage protected by grass or other close- growing perennial vegetation, are used as channels wherever possible because of the ease and economy with which they may be established and also because they produce a crop of hay. In establisliing a meadow strip, a slight natural depression is selected that may, if nec- essary, be straightened and reshaped. It is seeded to a permanent hay crop. Aleadow strips may be crossed with farm implements pro- vided tillage implements are raised and the cover is not destroyed. They must be given time to become establislied before outside run-oíl' is turned on them. Where adaptable they are almost universally used for terrace-outlet channels if they can be established prior to the construction of the terraces. Sometimes it is possible to divert the water from a terrace system into a temporary outlet until a meadow strip is established.

When a meadow strip cannot be used, a narrower channel is care- fully constructed to insure accurate control of the velocit}^. A cover more resistant to erosion than that in the meadow strip may be neces- sary. The time required to provide a safe channel is one of the im- portant factors in selecting the protective cover in tliis type. Close- growhig grasses that form a dense turf, such as Kentucky bluegrass and Bermuda grass, are commonly used. Bluegrass sod is usually cut out in narrow strips with which the ditch is either wholly or par- tially lined. Bernmda grass may be establislied by sprigging, sodding, or seeding. If the soil is poor, it should bo fertilized to aid in the rapid development of the grass. Plantings of low wild shrubs may

660 4- Yearbook, 1938

be utilized in some locations. Another method of establishing vege- tation is to seed the channels and protect the seed with a mulch of straw or other litter, but the conditions under which this method may be used are rather limited. Small breaks in old channels are repaired by filling with burlap bags containing chunks of sod. By the time the bag rots away the sod is well established.

High-Velocity Channels

'NMien the grade of the chaimel becomes too steep to permit the practical use of a low-velocity channel, a high-velocity channel is used. In this case an effort is made to produce as high a velocity as possible in order that the channel may be as small as possible and require a minimum of the expensive materials that must be used when the safe limit for vegetation is exceeded. Concrete has been used extensively, and asphalt has been tried experimentally. This type of channel is often necessary at the lower end of a low-velocity channel if a steep

FIGURE 5.—A masonry dam protecting a gully head that was rapidly approaching the highway and cultivated field above.

slope occurs on the bank of a stream, or if there is an abrupt change of slope. In some cases, such a channel may be substituted economi- cally for a low-velocity channel if the topography is sucli that tlie channel can be sufficiently shortened by leadhig it down the steepest part of the slope.

Major Channels

Major channels may be divided into those having intermittent flow and those having continuous flow.

Channels With Intermittent Flow

In the type having intermittent flow the most serious problem occurs at the head where a vertical drop is present. In order to

Mechanical Measures of Erosion Control 4- 661

arrest headward cutting these overfalls are protected by dams when- ever the value of the land endangered justifies the expenditure. The dams may be located a short distance below the channel head and constructed to a height that will pond water over the overfalls, or the dam may be located adjacent to the overfall for mechanical protec- tion. Other structures downstream from the overfall may be neces- sary to raise the bed of the channel in order to prevent side overfalls from developing, and to protect upstream structures from under- cutting (fig. 5).

Permanent dams that pond water either in the channel or at the overfall are called soil-saving dams, since soil collects until the pond

FIGURE 6.—liOW wire and brush dams constructed for temporary protection to the gully while vegetation is being established.

is filled. Two types of dams are commonly used, the notch spillway and the drop inlet. The notch spillway is usually built of rubble masonry or reinforced concrete. The drop inlet is an earth dam with a tube of reinforced concrete through it. A riser on the tube extends nearly to the top of the dam, so the pond must be nearly full before any water can flow out.

Temporary dams are sometimes used as barriers to stabilize the flow hne of gullies while vegetation is being established (fig. 6). Since their need is hmited to a period of 2 or 3 years, such native materials as brush, straw logs, or loose rock are genei'ally used. While expen- sive permanent construction is not usually necessary inider these conditions, such structures must be reasonably secure and carefully designed to insure that they remain in place until vegetation becomes established. Overfalls on temporary dams must be kept extremely

662 ^ Yearbook, 1938

low to avoid the cuttiiig out of deposited material after the dam fails. Such a temporary dam for gully control is sometimes referred to as a check dam, a term that to some people implies the function of storing water. Temporary dams are not used for water storage in soil con- servation work, however, since they would be uneconomical and dangerous where large numbers of dams are used on a watershed.

Permanent dams of the low overfall type are sometimes necessary to establish and maintain a flow line for the channel.

Channels With Continuous Flow

In the case of a stream having continuous flow, most of the damage is done by bank cutting or meandering of the stream, which often destroys valuable bottom land. On raw banks it is usually necessary to construct jetties, wing dams, or riprapping before vegetation can be established. Streams are often straiglitened to cut off critical bends, if such straightening will not create other hazards further downstream. In many cases small storage dams are incorporated into the control measures used on the smaller streams if the topography is such that they can be used. These dams are valuable as a w^ater- conservation measure and for recreational and stock-watering purposes.

GULLY CONTROL

Gullies are channels formed by erosion that either have lost their vegetative cover or never had one and therefore oft'er little or no protection against erosion. Actively eroding gullies usually require some mechanical means of protection before they can. be controllecL The control of run-ofl: water in the narrow channel of a gully is usually both difficult and expensive. The practice of intercepting run-oft' with diversion ditches or terraces, before it enters the gully is widely followed wherever possible in order to lead the water to some place that can be protected more easity than the original gulh^ In many places the diversion of the excess water from the gully is the only control measure needed, and in all cases the control of the gully is very much simplified by this method.

When the water cannot be diverted out of a critical guU}^ it is necessary to build structiu^es to protect it from further cutting. The structures and the method of using them have been explained in con- nection with the control of major channels having an intermittent flow.

The use of any mechanical measures for gully control must be carefully justified from an economic standpoint. If the land is already ruined by gullies there is little need to spend money for their control and stabilization. Protection from grazing and fire damage and assisting native growth may effectively control the area within a few 3^ears. On the other hand, deep gullies cutting headward through fertile land or endangering improvements, such, as highways and buildings, may justify considerable expenditure for control purposes.

COMBATING WIND EROSION Pressure by wind is directly proportional to the square of its velocity, which indicates why so nmcli force can be exerted by strong winds.

Mechanical Measures of Erosion Control ^ 663

It is readily conceivable that wind is capable of transporting or moving vast quantities of material when it is considered that a speed of only 20 miles an hour produces a pressm-e of approximately 1 pound per square foot. A small reduction in velocity, therefore, will cause a proportionately greater decrease in pressure. Thus, if the velocity of a wind that is carrying materials can be reduced by obstructions to its sweep, it will be forced to deposit some of the material. Similarly, reducing the velocity reduces the power of the wind to remove material.

It is also obvious that if the size of the material can bo increased, much less will be carried at a given wind velocity. This is an impor- tant principle utilized in combating wand erosion. It is not possible to reduce wind velocities appreciably over large areas, but it is possible to put cultivated soil in such a condition that the clods or lumps exposed to the full sweep of the wind will have sufficient size and weight to resist being transported.

Any obstruction to the wind's sweep such as lister ridges, clods, or stubble will break the velocity at the ground surface and cause eddying and back drafts that prevent small particles being moved. Such ob- structions also promote drifting on the leeward side, which soon fills the depressions. When the surface becomes smooth, removal by wind action begins anew and the listing or tillage operation nuist be re- peated. An high obstructions to the wind's action may become haz- ards that cause drifting in sections where large wind movement of soil material is in progress. Fences, buildings, idle farm machinery, or trees may promote objectionable and dangerous drifting. There are numerous examples of roads completely covered by drifted material along fence rows, of buildings submerged, and of trees killed.

The fundamental causes of soil movement by wind have not as yet received much scientific study, and causative factors are still largely a matter of speculation. The elementary cause, of course, is the action of wind on smooth surfaces of loose, dry soil unprotected by vegeta- tion. It has been noticed that very light and very heavy soils, such as sand and clay, are the most susceptible to blowing; also that drift- ing is not a spontaneous action over a wide area but originates at localized spots or focal points and spreads rapidly. ^ Suggested control measures are based on the principle of checldng soil movement in its early stages.

Eiffective permanent efforts toward wind-erosion control must center around the maintenance of a good vegetative cover, continuous on range land and during critical periods on croplands. In tlie absence of an adequate cover of vegetation or its residue, mechanical measures must be used.

Tillage Methods

Listing is considered one of the most effective types of tillage for establishing a ridged and cloddy surface that will ofl'er a maximum resistance to wind. It may be either open or basin listing, depending on requirements. Basin listing is more efl"cctive on fallow land but has less advantage where cultivated row crops are used because the pockets are soon destroyed by cultivation. Listing may be either straight or on the contour. Basin listing is well adapted to straight farming where moisture conservation is desired and strips have been

664 ^ Yearbook, 1938

laid out crosswise to the prevailing wind. Under this condition ordi- nary open lister rows woidd serve as channels to carry away water from certain sections of the field instead of conserving it. Under contour tillage open listing would, of course, serve as a means of storing water as well as aiding in the control of erosion by both v/ind and water.

Other tillage machines used to produce a cloddy trashy surface are the one-way disk and the duckfoot cultivator. The one-way disk is of value in that it mixes surface soil with any vegetative cover that may be present and leaves the soil in a somewhat roughened condition. It is particularly useful on heavy stubble. The duckfoot cultivator leaves the soil in small ridges mixed with trash and clods that protect the fine earth from blowing and drifting. It is, of course, essential that the stubble should not be burned if a trashy cultivation is desired.

Deep tillage is often necessary in order to produce a cloddy, lumpy surface. It is also important to limit tillage operations as much as possible to periods when conditions are optimum for the control of soil drifting and blowing. For example, it is usually far better to allow a stubble field to stand over winter than to plow it in the fall and have it **blow'' during critical periods.

Wind Strips

An important practice recommended for controlling soil blowing is strip cropping, division of fields into ah:.ernate strips of fallow and grain. The strips ordinarily average about 10 rods or less in width. When the seeded parts have made sufficient growth to protect the soil, the intervening fallow strips are cultivated b}^ tillage methods previously described to produce a lumpy surface. To be most eflective wind stripping should be practiced on a community basis; otherwise soil movement from unprotected fields will continue and may destroy the control measures on other fields. It is important that the best cultural methods be adopted for summer-fallow strips to insure as much trash cover as possible and to maintain the soil in lumpy condition. The strips should be laid at right angles to the prevailing wind if wind-erosion control is the primary objective and be placed on the contour if water-erosion control is also of importance.

The level or absorptive-type terrace used for the conservation of moisture indirectly aids in wind-erosion control. The actual im- pediment of the terrace ridge to the wind is negligible, but the addi- tional water retained in the soil will aid in producing a vegetative cover that acts as a mechanical control measure for wind-erosion.

Dunes The formation of dunes or drifts through soil blowing may do tre-

mendous damage in addition to the actual removal of soil from the fields. ^ The drifts are formed in the same manner as the sand dunes found in desert regions, or along particularly sandy and wind-swept shores. Dunes are constantly shifting or reforming and thus en- croach on more and more territory. They frequently hamper traffic and by filling highway ditches they destroy necessary drainage.

Careful study and observation coupled with field experience have demonstrated tliat the same forces responsible for forming dunes may be used to dispose of them. Of primary importance, of course, is

Mechanical Measures of Erosion Control ^ 665

prevention of tlicir formation, which can be done by x)reventing soil drifting. Where some drifting still persists in spite of preventive measures, it may become necessary to construct such obstacles as fences or weed rows at strategic points to prevent dune formation at undesirable places. Such obstacles cause a sudden reduction in wind velocity, which induces deposition of some of the material being carried, and ultimately the formation of a drift or dune. The loca- tion of the obstacle with respect to the prevailing wind will determine the location of the dune. Snow fences contribute to the formation of snowdrifts in a similar manner.

Where it is desired to remove an existing dune there are several methods that have been successfully used. One of these is dependent on the fact that increased velocity enables the wind to carry a greater load. Sandbags or similar barriers are placed along the crest of a dune in such a way as to leave an open space between bags. The wdnd passes through these narrovv^ openings at a higher velocity, picking up material from the crest as it passes. The material will be dropped later on but usually will not reconcéntrate unless it meets some barrier. The sandbags or other obstacles may be shifted from time to time so that the dune will be evenly removed.

Another method takes advantage of the fact that when the lip, w^hich usually forms on the leeward side of a drift, is destroyed or broken certain eddies that contribute to the building up of the dune are eliminated and the dune will gradually disintegrate. Any ma- chinery such as a harrow or blade can be used to break up the lip. It is, of course, necessary to continue the operation at intervals, be- cause the lip is continually reforming.