Erosion Controls part 2
by Hans G. Jepson, assistant agricultural engineer, Soil Conservation Service
Originally appeared as PREVENTION AND CONTROL OF GULLIES
Farmer’s Bulletin No. 1813, Issued September 1939, Washington, D.C.
U.S. Department of Agriculture
NOTE: Some common practices and specific procedural and specie recommendations back in 1939 have fallen from favor in recent years. For example, in many parts of the southern U.S., Kudzu is seen as an accursed problem. We present this reprint unedited so that you may pick and choose how it might apply to your understanding and problem solving palette. We trust that you will make pertinent and intelligent inquiries in your region with regard to plant varieties and procedures.
DIVERSION OF RUN-OFF
Where necessary and practical, run-off should be diverted from a gully head before control measures are attempted within the gully. This principle generally applies to gullies of all sizes except those having so small a drainage area that the run-off is negligible, as for example, a gully with a drainage area of less than an acre. In using either terraces or diversion ditches careful consideration should be given to the disposal of the diverted water. If safe disposal cannot be provided, the water should not be diverted. The disposal of concentrated run-off over unprotected areas may cause gullying.
Terraces are very effective in the control of small gullies on cultivated fields or even medium-sized gullies that are not too deep to be crossed with the terracers (fig. 8). Terraces placed above a gully too deep to be terraced across will divert headwaters from the gully, which may then be treated further if necessary. Terrace construction may be difficult and somewhat expensive on gullied areas, but despite this it is frequently the most satisfactory control measure for terraceable slopes, and particularly where numerous parallel gullies are encountered on slopes that are difficult to vegetate. Figure 9 shows a previously gullied field that has been almost completely reclaimed by terracing. The terraces were constructed across the gullies. This diverted the run-off at points where the terraces crossed the gullies and allowed the gullies gradually to fill with silt.
If the slopes above a gully are too steep to terrace or if the drainage area is pasture or woodland, diversion ditches (fig. 10) may be used to keep run-off out of the gully. Diversion ditches are particularly adapted to areas already covered with trees or grass because ditches below these areas are not so likely to receive silt loads from the drainage area. Diversion ditches are not recommended immediately below cultivated fields not fully protected from sheet erosion unless a permanent filter strip of close-growing vegetation is placed above the ditch to catch the silt carried in the run-off from the fields. This filter strip should have a minimum width of 50 feet and adequate cover to filter out and retain the silt in the run-off. This will reduce silt deposition in the ditch channel and eliminate the need for much subsequent maintenance.
Diversion ditches should be made large enough to carry all the run-off that will be discharged from the contributing drainage area during periods of maximum run-off from rains within the design frequency selected. This runoff should be conveyed through the ditch at non-erosive velocities. The velocity of run-off in the ditch can be controlled by regulating the grade and shape of the water channel. Wide, shallow ditches on mild grades produce lower velocities than narrow, deep ditches on steep grades. In channels maintained by a permanent plant cover a higher discharge velocity is recommended than in raw channels. The higher velocity is essential to prevent undue silting.
For shallow earth channels not having a cover of vegetation the maximum grade should seldom exceed 12 inches per hundred feet of length in order to prevent scouring velocities. Where a permanent plant cover is to be maintained in the channel, grades as high as 2 to 3 percent or more may be used. Serious scouring will likely occur with these steeper grades if the vegetative protection is ever destroyed, and they should be used only where adequate vegetative protection can be depended on during all periods that produce runoff.
The terrace-type diversion ditch is extensively used for ordinary locations where the watershed area is about 7 or 8 acres (fig. 11). This type of diversion ditch is constructed from both the upper and lower sides on relatively flat slopes. On steeper slopes (of above 3 or 4 percent) construction is generally from the upper side only. There are, of course, many variations from the indicated cross sections because of the local variations in soil types, available construction equipment, rainfall, and slopes. The ditch should be constructed according to the required capacity. The settled depth of the water channel should seldom be less than 18 inches, and a minimum water cross-sectional area of 7.5 square feet is suggested for drainage areas up to 5 or 6 acres. Drainage areas up to 10 acres require a channel depth of 24 inches and a minimum cross section of 12 square feet.
For larger drainage areas, cross section C, shown in figure 11, is suggested, especially on the steeper slopes. It is recommended that the services of an individual with experience in hydraulic design be secured to compute channel capacities and dimensions for these larger watersheds. A poorly designed diversion ditch is more apt to induce than to prevent gullying.
The diversion ditch should be set back from the head of the gully a minimum distance of three times the height of the gully overfall. Low points in the ridge and high points in the channel should be corrected before the ditch is put to use. If well-protected natural outlet locations can be found, no special outlet revegetation or construction will be necessary. If none are available it is necessary to take certain precautions to spread the concentrated run-off on vegetated areas. Natural outlets should always have preference if they are satisfactory. It is sometimes possible to discharge the diversion ditch into an outlet or drainageway already established for a terrace system. If this is done, provision should be made for the additional water to be carried by the outlet channel.
Sometimes it is necessary to establish sodded outlet areas before constructing the diversion ditch. Contour furrows have also been used to spread and absorb water from diversion ditches where soils are pervious and the amount of run-off is not very large. The furrows should preferably be established at least a year before the water is diverted into them, so that the furrows as well as the intervening areas may become vegetated before additional run-off is discharged onto them.
CONVEYING RUN-OFF THROUGH GULLIES
If it is not possible to keep water out of gullies by retaining it all on the watershed or diverting it from the gullies, the run-off must be conveyed through the gullies. To do this and to check erosion in those gullies at the same time is possible if vegetation can be established in the gullies or mechanical structures built at critical points to supplement vegetation or to give complete control.
It is emphasized, however, that where a gully is located in a natural drainageway that is to form part of the disposal system for surface run-off, it becomes necessary to convey run-off through the gully. Any erosion control applied in the gully must not reduce the capacity of the drainageway below that required to carry the run-off discharged into it.
It is usually much more difficult to establish adequate vegetation in a gully through which run-off must be conveyed during the period of establishment than in one from which run-off can be diverted. Erodible portions of a gully through which water is conducted must usually be protected by transplanting sod, by the use of specially anchored mulches, or by the use of mechanical structures. The mechanical structures need be only temporary if the plant cover, once established, can provide sufficient protection.
If mechanical measure will ultimately be required for satisfactory control, permanent structures, such as masonry check dams, flumes, or earth dams, supplemented by vegetation, should be provided to convey the run-off over critical portions of the gully.
The methods of preventing gullies that have been recommended, as well as the control measures described herein, are for the purpose of reducing or controlling run-off so as to make it easier to establish vegetation. The objective of gully control is, in fact, the establishment of an erosion-resistant cover of plants that not only stabilize the gullies but also produce a usable crop, and hence a supplementary income. Just what kind of plants are to be used and how they are planted will, of course, vary according to locality.
In any part of the country, gullied areas are usually the most difficult places on which to try to grow plants. The topsoil is gone; the subsoil is poor or is gone; and the layers finally exposed in the gully may be hardpan or even rock. The revegetation of such places will be difficult. Yet, if the run-off water can be diverted from its old path through the gully, if the soil is given a little preparation, and if the area is temporarily protected from trampling by livestock, a plant cover can be established (fig. 12).
Bank sloping is expensive as a means of preparing a gullied site for planting. It can usually be justified only if the banks of the gully are so steep that vegetation cannot otherwise be established or if it is desired to partly fill a small gully to facilitate tillage operations. As little bank sloping as possible should be done, for under ordinary conditions the banks will gradually assume a stable slope. The necessary sloping can be done by hand labor, by machinery, or by blasting.
Sloping by hand labor is slow and may be expensive if much of it is to be done. A more practical method is to use a light walking plow and team. Considerable skill is necessary to maneuver the plow and team properly, but with a little practice it is soon possible to “plow down” a gully bank. For the first few rounds it may be necessary to hitch a chain to the plow if it is difficult to bring the team near the edge of the gully bank. Fine brush, straw, manure, or similar material may be used in filling a gully if sufficient loosened dirt is used to anchor and cover it.
On large gullies or in hard soils it may be necessary to use a bulldozer or explosives. Either may prove costly, and they should not be used indiscriminately. On large jobs the bulldozer will usually move soil at a lower cost per unit volume, but it requires considerable overhead investment and equipment that is not available on the ordinary farm.
It should be kept in mind that in site preparation bank sloping is necessary only to the extent of securing ground slopes and a surface condition satisfactory for planting or seeding. Usually planting and seeding can be done and a satisfactory growth produced without any extensive bank sloping. The ground should be left in a roughened condition for either planting or seeding.
Any gully, no matter how large, and regardless of its condition, will usually be reclothed with vegetation, provided it is properly protected and is in a locality where vegetation will grow. If the water that causes the gully is diverted and livestock, fire, or any other cause of disturbance kept from the gullied area, plants begin to come in. At first they come very slowly because it is hard for them to get a foothold. Later, when the pioneer plants have improved the soil somewhat, other plants appear. The whole natural process may take many years in the drier parts of the country, but where there is more moisture, the process is more rapid.
Plants will always come in naturally on protected areas, but on gullied land there are several things that slow down the final healing of the erosion scar. One is the continued loss of soil caused by freezing, thawing, and washing (fig. 2, C). This loss cannot always be stopped, but it can often be reduced by the use of a mulch of boughs, straw, or leaves, which also assists in catching and holding plant seeds. Another thing is the steepness of some gully banks. Until the steep sides cave in and reach a gentler slope (about a 1:1 slope), it is difficult for plants to root themselves. Unless large gullies with steep banks are plowed in, pushed in with a bulldozer, or dynamited, it may take many years for them to become stabilized.
In spite of conditions such as these, hardy, thrifty plants capable of surviving in gullied areas will generally appear naturally. So-called “weeds” will usually come first. They prepare the way for other plants, which always follow them in a year or two. Given sufficient time, this natural process will eventually reclothe the gully with the predominant vegetation of the region, whether that be trees, brush, or grass. Frequently this opportunity to obtain a cheap protective covering is overlooked and unnecessary expenditures are made for structures or plantings. Many previously active gullies have been completely stabilized by a natural growth of vegetation that sprang up after the land was abandoned and livestock excluded (fig. 13). Natural revegetation, however, may be a lengthy process. Where natural growth does not appear to be able to cope with existing erosion or where certain plant species of economic value are desired, it may be necessary to consider ways and means of establishing vegetation artificially.
PLANTING AND SEEDING FOR REVEGETATION
CHOICE OF PLANTS
So far as erosion control alone is concerned, it makes little difference whether trees, shrubs, vines, or grasses are used in a gullied area. Any of these, if well-established, provides good protection for soil. Consequently, the kind of vegetation to use is best chosen on a basis of what the planted area will be used for when it is stabilized.
A stabilized gully can be used as a woodland area, as a wildlife habitat, as a drainageway for water, or as pasture land or hay land. It should not be cultivated, however, nor should it be burned, brushed, or used in a way that will promote erosion. If it is to be used as a drainageway, grass is ordinarily the primary cover (fig. 14). Grass sod can carry more water safely and at higher speeds than can woody plants. A sodded drainageway can be crossed with farm machinery, whereas one with a woody cover cannot. Grassed drainageways produce good yields of hay in some sections. If they are utilized as pasture, grazing should be regulated. It should not be forgotten, however, that good grass growth demands a well-prepared seedbed and reasonably fertile soil, a condition rarely present in gullies.
In certain regions trees and shrubs are easier to establish in gullied areas than grasses, but they should not be used in drainageways unless the amount of water flowing in the channel is relatively small (fig. 15). Trees on a gullied area will produce a crop of fence posts or rough timber for general use about the farm. Shrubs are used particularly to attract wildlife, especially insect-eating birds, if the gully is near cropland, although many trees and grasses are also valuable to wildlife.
Where severely eroded and gullied areas must be retired from crop land or pasture land, woody plants are usually used to help stabilize them, and the areas are then ordinarily reclassed as woodland and wildlife areas.
TREES, SHRUBS, AND VINES
Preparation of area. – Before gullied areas are planted, they should be fenced from livestock unless they are in a field in which livestock does not run. Trampling and grazing by domestic animals in these critically eroding areas will prevent vegetation from forming a good cover, which is essential in preventing washing.
The area to be fenced or otherwise protected should be larger than the width of the gully. For example, if a gully is about 10 feet deep, the distance from the fence to the nearest edge of the gully should be 20 to 25 feet. It is better to allow an even greater distance at the gully head because at the head the erosion hazard is greatest.
Selection and planting of species. – The first choice of trees, shrubs, and vines for gullies should be plants that are native to the locality and grow on similar sites. These plants are already acclimated and thus have the best chance of survival under the harsh growing conditions in gullies. If useful native plants are unsatisfactory, second choice would fall on plants introduced from other areas or countries.
There are a number of native trees, shrubs, and vines that have proved satisfactory in controlling gullies. Among trees may be listed black locust (Robinia pseudoacacia), catalpa (Catalpa sp.), Chinese elm (Ulmus parvifolia), cottonwood (Populus sp.), green ash (Fraxinus lanceolata), hackberry (Celtis occidentalis), honeylocust (Gleditsia triacanthos), mulberry (Morus sp.), northern red oak (Quercus borealis), Osage-orange (Maclura pomifera), pine (Pinus sp., in general), and willow (Salix sp.). Shrubs and vines that have proved effective in gully planting done by the Soil Conservation Service so far include several shrub dogwood and osiers (Cornus asperifolia, C. stolonifera, C. paniculata), viburnums (V. lentago, V. dentatum, and V. trilobum), chokecherry (Prunus virginiana and P. demissa), Russian-olive (Elaeagnus angustifolia), wild plum (P. americana), skunkbush (Rhus trilobata), paloblanco (Forestiera neomexicana), sugar sumac (Rhus ovata), and coralberry (Symphoricarpos orbiculatus), kudzu (Pueraria thunbergiana), and Japanese honeysuckle (Lonicera japonica). It should be mentioned that honeysuckle is of very little value to wildlife.
This list could easily be supplemented by other species, since there are many others that have been used successfully. Additional selections may be made from plants recommended by the local county agent.
In using woody plants it is well to remember that solid plantations of one kind are undesirable because they favor the spread of any insect or disease that may attack them. Neither do they encourage wildlife as much as mixed stands do. In mixtures of shrubs and trees, the shrubs should be placed on the less favorable sites, the trees on the better ones. Also, the whole gullied-area plantation may well be planted around its outer edge with two or more rows of shrubs of particular value to wildlife. These outer-edge rows also serve to protect the tree plantings within.
Vines are ordinarily not planted with trees and shrubs because they prevent the best growth of these woody plants by clambering over them. In some parts of the country, however, vines may be used satisfactorily in a gullied area (fig. 16).
The spacing of plants will depend on local conditions, but 6 feet apart in each direction is the maximum spacing ordinarily recommended for erosion control. Spacings of 3 by 3 feet and 4 by 4 feet are better for shrubs in the gully. On the edge rows along the banks 6 by 6 feet is satisfactory. No plants should be placed on very steep banks in steps cut in the bank. Nor should they be planted under overhangs or too close to edges of banks that will eventually cave in.
It is important to get good planting stock to transplant. Nursery-grown stock has proved much better than wildings in most parts of the country. Because the gullied areas have such poor soil, the stronger and more thrifty the planting stock the greater the chance of establishing the plants the first time. Suitable nursery stock can ordinarily be obtained from local nurserymen or from State nurseries.
Just as with other crops, the use of fertilizer with trees or shrubs placed in gullies assures increased survival and growth.
In small or medium-sized gullies with small drainage areas it is frequently possible to construct checks consisting of shrubs placed across the flow line of the gully (fig 17). The shrubs are placed 4 to 5 inches apart in shallow trenches and are sometimes protected by rows of stakes. The stakes are placed about 1 foot down the channel from the shrubs so that the plants will benefit from silt collected by the stakes. The shrub checks reduce water velocities in the gully channel and induce silting, which gives other vegetation a chance to become established. Shrub checks should be closely spaced if they are to be effective. They should be used only in gullies that have a mild grade.
Another type of check frequently used in gullies with small drainage areas and of moderate slope is constructed of willows. Willows are used only in locations where the soil is naturally moist throughout the year. Cuttings are made from green willow trees and set in rows across the gully at intervals of about 1 foot. Sometimes several rows are placed close together. These cuttings take root and form a living check across the gully.
Further information on the use and planting of trees, shrubs, and vines in the control of gullies is given in various State publications and in Farmers’ Bulletin 1788, Wildlife Conservation Through Erosion Control in the Piedmont.
GRASSES AND GRASS-LEGUME MIXTURES
Grass covers are usually preferable for scattered gullies in good pasture or in cultivated fields where the preparation of an adequate seedbed is not difficult.
Grass covers may be produced by seeding or by transplanting sod. The use of manure or commercial fertilizer is usually required for good growth. Wherever applicable, seeding is the cheaper method. Mulching the seeded areas with light covers of straw, cane, or fine brush conserves moisture and is beneficial in assisting grass seed to become established on these areas.
In seeding, it is generally advisable to use a mixture of adapted grasses and legumes to insure complete stands and early stabilization of the gully. As the less adapted species thin out, the more aggressive ones will spread to replace them and thus perpetuate a complete ground cover.
Among the grasses and legumes that have stabilized gullies, Kentucky and Canada bluegrass, redtop, Bermuda grass, clovers, annual and perennial lespedezas, Kikuyu grass, wheatgrasses, bromegrass, and Napier grass have been commonly used in areas where they are adapted. Many other plants may be used for the purpose, and the local county agent should be consulted before a selection is made.
Small or medium-sized gullies to be seeded in cultivated fields should be plowed in or otherwise partly filled in order to convert them into broad channels, which permit the water to spread. The spreading of water reduces its cutting power. After the channel has been properly shaped, a firm seedbed should be prepared and fertilizers applied if they are needed. Seeding may be done by broadcasting or drilling. If the seed is broadcast, the ground should be harrowed lightly afterwards to cover the seed and then firmed by packing. In the semiarid sections, drilling is much preferred to broadcasting because it places the seed in moist soil and thereby insures more rapid and uniform germination. If the drill is used, it is advisable to run it in a zigzag pattern, crossing the gully as often as conveniently possible. This will keep the drill rows from running with the gully and will reduce the danger of rilling between the rows. After the ground has been drilled, it should be firmed with a packer driven in a zigzag line. In constructing drainageways of this kind it is poor economy to attempt to confine the channel to too small an area. The grasses should be seeded well out over the embankment, and it is desirable to leave the edge of the seeding irregular to prevent the formation of new gullies along the sides.
Grass and leguminous plants are very tender in the seedling stages, and every precaution must be taken to keep run-off water out of the newly seeded drainageway until the plants are well established. In addition to any permanent diversions that may have been installed, it is sometimes advisable to construct a temporary dike along the sides of the drainageway to exclude runoff until a good growth has been obtained. In the larger drainageways where such a dike would not be practical and where it is impossible to exclude all run-off in any other way, it is often advisable to plant a quick-growing annual crop to stabilize the drainageway before the grasses are seeded. Small grains, Sudan grass, lespedezas, and similar cover may be seeded in the spring to hold the soil and produce a residue in which to seed the grasses the following fall. Nurse crops may also be seeded with the grasses to give quick protection, but care must be taken not to seed the nurse crop so thick that it will compete with the grasses for moisture or smother the small grass and leguminous plants.
If an immediate grass cover is required and suitable sod is available, it may be necessary to transplant sod. Sodding is generally too costly for extensive use over large gullied areas. Sod is needed, however, on critical sections at the gully head or at points along the bank or bottom where protection against waterfall erosion is necessary. It is also frequently used in connection with permanent structures. Grass covers can usually be established through sodding on areas exposed to run-off where it would be impractical to secure a cover by seeding. Where the amount of runoff is not too large, and good sod is available, it can be used as a substitute for the more costly masonry and concrete materials. Sod flumes, sod check dams, and sod spillways have all functioned satisfactorily when properly constructed and applied.
There are three general methods of transplanting sod: Solid sodding, broadcast sodding, and spot or strip sodding. Solid sodding consists of cutting the sod in strips and transplanting it as a continuous cover over the area to be sodded. The sod is usually cut in strips about 1 foot wide, 8 to 10 feet long, and 1 ½ to 2 ½ inches thick. The amount of time and labor required to cut the sod can be materially reduced by the use of special horse- or tractor-drawn sod cutters. When transplanted, the strips are continuous and are usually laid on the contour or at approximately right angles to the direction of the water flow. It is desirable to staple the sod down or cover it with wire mesh if there is danger of heavy run-off before the sod can establish itself. Care must be exercised in cutting, placing, and tamping the sod. Solid sodding is the most costly type of sodding, but it provides a cover that is capable of conveying considerable run-off almost immediately after it is placed.
Broadcast sodding requires less labor and time, particularly if suitable equipment is available. The area from which the sod is to be taken is disked until the sod is well cut up and mixed with the topsoil. This mixture of grass roots and topsoil is then collected and transported by scrapers or spreaders and spread in layers 2 to 3 inches thick over the area to be sodded. After it is placed, the area should be disked so as to work the sod into the soil on the new location. The fertile topsoil transplanted with the sod roots promotes a vigorous growth. Unfortunately broadcast sodding can be used in transplanting only grasses that spread by rootstocks, and it does not provide a surface cover capable of conveying run-off so soon as solid sodding. It has been used with good results in transplanting Bermuda grass. This method is relatively inexpensive.
Strip sodding and spot sodding are commonly used in transplanting sod. In strip sodding, the strips of sod are ordinarily laid in trenches of the same dimensions as the sod. Sometimes they are laid at regular intervals across the slope to be sodded, and the intervening areas are filled with topsoil and seeded. In either arrangement the top of the sod strip should be flush with the top of the adjacent ground surface.
Spot sodding consists of transplanting small clumps of grass, rootstocks, or stolons at random over the area to be sodded. These are usually placed in small holes.
Strip sodding and spot sodding give quicker coverage with less sod than solid sodding, but they require a longer time to provide complete protection. They give the most satisfactory results with grasses that spread rapidly by stolons or rootstocks and when used as a means of establishing grass cover on gully bottoms or sides not exposed to excessive run-off.
Sod should be obtained from areas where its removal will not cause serious erosion, or it should be removed in such a manner as to minimize later erosion damage. On sloping land it should be removed only in alternate strips at right angles to the land slopes. Leaving undisturbed contour strips at intervals will usually give sufficient protection to prevent harmful washing until the bare areas are revegetated. For transplanted sod the need and value of liberal applications of farm manure or commercial fertilizer should not be overlooked. The ultimate success of any sodding is dependent on a vigorous growth. If the plant food necessary to assure good growth is not provided on infertile soils, all control efforts may be wasted. The usual procedure is to apply the fertilizer over the area to be sodded before the sod is laid.
Sod flumes may be successfully used to control overfalls in gullies with heads less than 10 feet and drainage areas less than 25 acres. Suitable sod must, of course, be available. In areas of good Bermuda grass or Kentucky bluegrass, sod flumes have been successfully used on larger watersheds in gullies that have low overfalls. A flume merely serves the purpose of preventing further waterfall erosion by providing a protected surface over which the run-off may flow into the gully (fig. 18). It is not intended to fill a gully or to stabilize the gully channel or sides below the flume.
The overfall to be sodded must be cut back to a slope flat enough to make possible the establishment and maintenance of a permanent plant cover. The allowable slope will depend on the soil type, size of watershed, height of the overfall, and the quality and type of sod used. Generally a 4:1 slope should be about the steepest. To avoid destructive velocities through the flume it should be wide in proportion to its depth and should have sufficient crosssectional area to carry the expected run-off at nonerosive velocities. A width of at least 15 inches per acre of watershed is desirable, and the maximum depth of flow expected over the flume should seldom exceed 12 inches.
Unless the anticipated run-off is small, solid sodding is usually necessary for sod flumes. It is important that the gully channel below the flume be on a stable grade. If it is not, overfalls may develop below the flume and undermine the sod. On small watersheds that have erosion-resistant soil, an unprotected gully channel may be reasonably safe for grades of as much as 1 ½ percent. Channels protected by vegetation can generally withstand considerably higher grades. On the more erodible soil types and larger watersheds unprotected channel beds should usually have a fall of less than 1 foot per hundred feet of length. Unstable grades may be reduced by mechanical measures if it is impracticable to provide the necessary protection by vegetation.
Sod checks are frequently used to stabilize gully channels until the intervening areas become vegetated. The several types are merely variations that are made necessary by existing gully conditions and required protection. Two of the more important are the sod strip and the sodded earth fill.
The sod-strip check is best adapted to small gullies (fig. 19) that have small-to-medium-sized watersheds and relatively flat channel grades. The strips are laid across the gully channel so that each strip when set will be flush with or slightly below the bed of the gully. The strip should have a minimum width of 12 inches and should extend up the gully sides at least 6 inches above expected high-water crests. The spacing of the strips will depend on the spreading characteristics of the sod, the drainage area, and the grade of the gully. Spacings of 5 to 7 feet are commonly used. Where it is difficult to establish vegetation on gully sides, contour sod strips may sometimes be used to advantage.
SODDED EARTH FILLS
Low, sodded earth fills (fig. 20) can often be advantageously used as a substitute for ordinary brush or wire check dams in stabilizing gully channels. Where suitable sod is available they can be constructed at less expense; and they have an additional advantage because of the fact that by their use vegetation is immediately established at intervals along the gully. They have been successfully used in small- to medium-sized gullies with drainage areas of less than 25 acres. The earth fills should be located at strategic points or at regular intervals along the gully, as is necessitated by existing conditions. Frequently these fills are spaced so that the top of each will be as high as the base of the next one above. Side slopes steeper than 3:1 on the upstream side and 4:1 on the downstream face should seldom be used. The fill should be well tamped, and heights in excess of 18 inches should be avoided because of the overfalls created. An average height of 10 or 12 inches is commonly used. The top of the fill should be low in the center and should gradually curve upward to meet the gully sides and provide the necessary spillway capacity. The fill should be solid-sodded on the top and on the downstream face, and it should be carried up the gully sides to a height of 6 to 12 inches above maximum water crest expected over the structure.
Structures are used in gullies to facilitate the establishment of vegetation or to provide permanent protection at points that cannot be adequately protected in any other way. They are usually used only in gullies through which run-off must be conveyed. If the run-off that must be conveyed through gullies is not in excess of the amount that can be handled by well-established vegetation, temporary mechanical structures may be used in the gullies until the vegetation becomes established. Such material as brush, poles, wire, and loose rock are usually used for constructing temporary check dams. If the run-off is of sufficient volume to make ultimate control by vegetation impracticable, permanent mechanical control measures will have to be used. These structures, built to give permanent reinforcement to vegetation, should be made of durable materials; reinforced concrete, masonry, metal, or earth. They should be used only where less expensive means are impracticable and should be supplemented by vegetation wherever possible.
The proper use of structures in gullies requires good judgment in determining the need for them and the extent of their use. It is just as great a mistake to attempt to control a gully without structural assistance in areas where structures would ultimately provide the most satisfactory control as it is to use structures in gullies that could be stabilized more economically with vegetation alone. Temporary structures do not require such good materials as permanent structures, nor is so much precision in construction necessary. A permanent structure usually costs more than a temporary structure, and its failure is therefore a greater loss.