Erosion Controls part 3
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.
TEMPORARY CHECK DAMS
Temporary check dams are used to collect and hold soil and moisture in the bottom of sterile gullies so that vegetation can be established. They may also be used to check temporarily erosion in the gully head or in the channel of a gully until a protective cover of vegetation can be produced. If run-off from the contributing drainage area is diverted from a gully or retained on the watershed or if a gully has advanced to the drainage divide, any check dams used in it would act primarily as obstructions to hold silt and the precipitation occurring on the immediate area. As they would not be exposed to any appreciable amount of run-off, construction could be simplified considerably. Piles of closely compacted rock and brush are all that is necessary across the bottom of the gully if run-off is negligible.
Where temporary structures have been used to control gullies, it has been found that several low check dams are more desirable than one large dam of equivalent height. Low dams are less likely to fail, and after they silt up and rot away, the vegetation can protect low overfalls at these sites much easier than high ones. A temporary dam should seldom exceed 15 inches in overfall height, and an average effective height of about 10 to 12 inches will be better. By effective height is meant the vertical distance from the original gully bed to the spillway crest of the structure. It requires considerable field judgment to determine the most satisfactory location and spacing for temporary check dams. They may be located on definite intervals and spacings, but usually they are constructed at strategic locations so as to protect and facilitate plant growth at critical points. In this way effective results can be produced with fewer dams.
Temporary dams are used most successfully in gullies that have small drainage areas. The total amount of soil collected by these dams is relatively small, but they hold enough to promote the growth of some vegetation, which in turn forms a barrier that collects more soil. The dams should be constructed far enough into the bottom and sides of the gully to prevent wash-outs underneath or around the ends of the checks. They should also have sufficient spillway capacity to convey run-off at the maximum rate that can be expected during the life of the structure. An apron will generally be necessary to protect the structure from the undermining action of the run-off as it is discharged from the spillway.
The life of temporary structures will, of course, depend on the locality, the quality of the materials, and the construction procedure used, but under ordinary field conditions these structures should last from 3 to 8 years. Spillway capacities are usually designed for rainfall intensities having a frequency of 5 to 10 years. To determine the notch size required, the size and nature of the contributing drainage area can be determined in the field and the probable amount of run-off selected from table 1. The size of the rectangular spillway notch required to handle this run-off can be taken from table 3. As a margin of safety, the depth of the notch should generally be made about 5 to 6 inches deeper than the actual dimensions necessary to conduct the run-off without overtopping. The notches should be made as wide as practical in proportion to the depth in order to avoid unnecessary concentration of the water as it passes over the structure. It is desirable to have the notch length at least several times as great as the depth. With certain types of check dams a curved or a triangular notch is often the most practical.
Suppose it is desired to know the size of a rectangular spillway that will carry a discharge of 32 cubic feet per second. From table 3 it will be noted that it is possible to use several different combinations of depth and length to obtain this discharge. For example, any one of the following notches would be large enough to carry an approximate discharge of 32 cubic feet per second: 1 foot deep by 10 feet long; 1.5 feet deep by 6 feet long; 3.0 feet deep by 2 feet long.
Since it is desirable to spread the water as much as possible in order to avoid undue concentration of water downstream from the structure and to keep the depth of the water that goes over the dam as low as possible, the best notch size, if the channel width permits, is the one first listed, namely, 1 foot deep by 10 feet long. If a 6-inch margin of safety is added to this depth, the size would be 18 inches deep by 10 feet long.
The temporary check dams most commonly used in gullies are the woven-wire dam, the brush dam, the loose-rock dam, and the plank or slab dam. Each name indicates the principal material used in the dam. There are numerous variations in the way each of these check dams can be constructed. Satisfactory dams may also be constructed from materials other than those listed or from a combination of those materials. The type selected for use on any particular job depends primarily on the materials available. If several types of suitable materials are available the size and characteristics of the gully will be important considerations in selecting the structure. Often the builder prefers to construct a certain type of dam.
Woven-wire dams (fig. 21) are widely used because of the ease with which they can be constructed, the availability of material, and the possibility of using them in different types of gullies. They are used in gullies of moderate slope and small drainage area. The dam is commonly built in a somewhat crescent or half-moon shape, with the open end upstream. This allows for a longer spillway than is possible on a straight dam and at the same time protects the ends of the dam. The amount of curvature is not fixed by rule. An offset equal to about one-sixth of the width of the gully at the dam site will generally provide sufficient curvature. For example, if the gully is 24 feet wide, the spillway of the dam would be about 4 feet downstream from the abutments. A row of posts is set along the curve of the proposed dam at about 4-foot intervals and 2 to 3 feet deep (fig. 22). One interval should fall near the center of the gully to form the central portion of the spillway. A trench about 6 inches deep and of about the same width is dug along the upstream side of the row of posts. Heavygage woven wire is placed against the posts, with the lower part set in the ditch so that 10 to 12 inches projects above the ground surface along the spillway interval. It is desirable to place the coarse mesh on the bottom. The wire should be securely stapled to the posts. If the top of the wire is kept approximately level along the central interval used for the crest of the spillway a much better spreading of water will be obtained as it passes over the structure.
If plenty of rocks are available they may be used for the apron; otherwise brush or sod is generally used. The brush is anchored by pulling the butt ends through the wire mesh, where both the fill and projecting branches will help to hold it. Enough brush is laid to make an apron at least 4 feet long and extending at least 2 feet on each side of the posts that form the level portion of the spillway. Sometimes a tie pole anchored to stakes is placed across the center of the apron to compress the brush. It is desirable to countersink the apron and to use shorter bush near the upper end to produce a shingle effect. A layer of fine mulch underneath the apron is recommended so that a closer bond with the earth can be secured.
To promote rapid filling and to seal the structure, straw, fine brush, or similar material should be packed against the wire on the upstream side to the height of the spillway crest. This should be backed with a well-tamped earth fill of at least a 2:1 slope. Sodding along the spillway crest has proved helpful in preventing channeling along the lip. For spillway dimensions, see Table 3. The minimum value of d (fig. 22) should be 18 inches.
Brush dams are best suited for gullies with small drainage areas and with soil conditions that permit the driving of necessary anchoring stakes. These dams are cheap and are easy to build. For this reason they are used in most sections of the United States where brush is available. Many kinds of brush dams are in use; the kind chosen for a particular site depends on the amount of brush available and the size of the gully to be controlled. Whatever type is used, it is important that the center of the dam be kept lower than the ends to allow water to flow over the dam rather than around it. The minimum center height for spillway capacity is indicated in figure 23.
The sides and bottom of the gully for a distance of 10 to 15 feet along the site of the structure are covered with a thin layer of straw or similar fine mulch, which is slightly countersunk in order to form a bond between the structure and the soil. Brush with butts pointing upstream is next packed closely together over the mulch to about one-half of the proposed height of the dam. Several rows of stakes are then driven crosswise of the gully, with the rows about 2 feet apart and the stakes 1 to 2 feet apart in the rows (fig. 25). The stakes should extend up the sides of the gully and should be driven only partly in at first. The brush fill is then completed and heavy galvanized wire stretched along the rows of stakes and fastened to them. When this has been done, the stakes are driven down until the wire firmly compresses the brush in place. Several large rocks are sometimes placed on top of the brush in order to keep it compressed and in close contact with the bottom of the gully. The center of the dam should be made low enough to provide the necessary spillway capacity.
Loose-rock dams (fig. 24) are desirable where plenty of suitable rock is available. They are used in gullies of moderate slope that have small to medium-sized drainage areas. A well-constructed loose-rock dam can be made more durable than the other type of temporary dams because rock will outlast brush, wire, or slabs. It also has an advantage because its flexibility and weight constantly hold it in contact with the bottom of the gully. The best structures can be built from flat stones, which are commonly known as flagstone. With practice these can be so laid that the entire structure will be keyed together. If round or irregularly shaped rock must be used, the structure is usually encased in woven wire so as to prevent the outside rock from being washed away. Irregularly shaped rocks should preferably be placed in such a way that voids will be reduced to a minimum. If considerable time must be spent to fit the rocks in laying up the dam, it is cheaper and more effective to construct a rubble masonry dam or a concrete dam.
For the loose-rock dam an excavation is made across the gully to a depth of about 12 inches (fig. 24). This forms the base of the dam, on which the laying of the rocks is begun. The rocks are laid in rows across the gully with sufficient overlap to produce a shingle effect and are brought to the required height. The center of the dam is kept lower than the sides to form the spillway. Several large flat rocks countersunk below the spillway so as to be flush with the channel surface will serve as an apron. These should extend at last 3 feet downstream from the base of the dam. For the size of the spillway notch, see Table 3.
PLANK OR SLAB DAMS
Plank dams or slab dams (fig. 26) can generally be used in gullies with somewhat larger drainage areas than can the woven-wire or brush dams. They are constructed of planks, heavy boards, slabs, or railroad ties. These dams can generally be constructed with less labor than other types of temporary dams and are particularly applicable in areas where there is appropriate material. If a group of these dams is to be placed in a gully the head wall can readily be constructed to required dimension at the farm shop or some other convenient location and later dropped, as a complete unit, into a suitable trench across the gully.
In building the dam, posts are set in a row across the gully to a depth of about 3 feet and about 4 feet apart. The posts should be so arranged that one of the intervals between posts will occur in the approximate center of the gully to form the spillway. After the posts are set, a narrow trench is dug along their upstream side. The trench is made about 1 foot deep and just wide enough to permit placing of the head wall and a thin layer of straw or grass as a seal. If the planks are close fitting or the slabs overlapped, the seal material may not be necessary. The required spillway notch is cut out after the dam is constructed, or a suitable opening can be left at the top of the head wall during construction. Spillway dimensions are specified in table 3. A well-tamped earth fill is next made against the upstream face of the head wall and brought to the height of the spillway crest. This fill should have a slope not steeper than 2:1 and should extend well up on the gully banks.
The final stage in construction is the apron. If suitable rock is available a rock apron is easy to construct and will give satisfactory results. A brush apron might also be used. Where a plank apron is necessary, an area extending at least 18 inches on each side of the spillway notch and downstream at least twice the effective height of the dam, is excavated to a depth of 2 to 3 inches. A thin layer of mulch or gravel is spread over this area, on which the apron planks are then placed. One end of the planks is butted against the downstream face of the head wall and the other fastened to a toe log. A baseboard should be placed above the juncture of the apron and the head wall as a protection from water running over the spillway. The side walls of the apron are formed by laying up additional planks parallel to the apron floor and conforming with the slope of the gully sides. If properly laid, the top of the apron floor at the lower end will be flush with the surface of the gully channel. Sod is sometimes used for the apron if the drainage area is small and sod of good quality can be obtained. The life of this type of dam may be greatly increased if the wood used is first creosoted.
Log and pole dams are sometimes used, but it has been found that the construction cost of these dams is generally higher than that of woven-wire, brush, loose-rock, plank, or slab dams. Some difficulty has also been experienced in getting a good bond between the log or pole structures and bottom and sides of the gully. If proper care is exercised, however, such dams can be used satisfactorily. Poles or logs are in some localities the only material available for the construction of temporary check dams. On favorable sites dams constructed from willow poles have proved satisfactory. The poles should be cut from green willow trees. This dam would be similar to the check constructed from willow cuttings that is described under shrub checks. However, the willow-pole dam is built with a regular spillway notch formed from the poles.
PERMANENT SOIL-SAVING DAMS
Permanent structures are those that are constructed from more durable materials, such as reinforced concrete, masonry, and earth. These structures are used in gullies where temporary dams are either inadequate or impractical. In certain areas rock-filled crib dams and dams constructed from railroad rails have been used. Such structures are usually classified as semi-permanent.
Permanent dams are frequently necessary in gullies that must be retained as permanent waterways. Bullies from which the water cannot be diverted are generally very active if their drainage areas are large and they also are hard to control without some permanent structure. Even a small amount of water going over a 15- or 20-foot overfall will produce enough energy to make the possibility of control by vegetation alone questionable. A gully just beginning to cut back into a large drainage area (fig. 27) should usually be controlled by permanent structures in order to protect the area above from further destruction.
The variety of conditions in gullies necessitates the use of several types of permanent structures. Dams with regular notch spillways are usually built in locations requiring low structures. Where the run-off must be brought down from high overfalls, multiple-drop dams, flumes, or drop-inlet dams are usually used. Generally, neither the masonry nor the concrete notch spillway dam should be used to control overfalls in excess of 8 feet.
The selection of the structure to be used for any particular job is largely a problem of determining the type that will provide the necessary capacity and meet other requirements and yet give the most economical construction costs. Large dams represent a sufficient investment to warrant special precautions. Considerable skill is required to build permanent structures. It is recommended that for the building of concrete or masonry dams with an effective height of over 5 feet the services of a person skilled in design and construction be obtained. If this is not practicable, the plans should at least be reviewed by him. The construction of small earth dams usually does not present the foundation problems encountered with concrete or masonry dams. For this reason earth dams can ordinarily be built to heights of 12 to 15 feet without serious difficulty.
Most States have laws governing the design and construction of dams that exceed certain heights or that impound water beyond a fixed amount. Anyone planning to construct a dam should familiarize himself with all local and State regulations affecting such an undertaking.
To check the advancement of gullies effectively, permanent control structures must be located near enough to the gully head so that the grade from the spillway crest to the ultimate lip of the gully will not exceed the silting grade that can be expected for the particular soil type and cover (fig. 28). This grade may range from ½ to 3 percent or more, depending on the extent to which the future growth of vegetation in the channel above the structure may tend to increase the natural soil gradient. Permanent structures should also be located so that the line of discharge from the spillway will be parallel to the center line of the gully immediately below the dam. This will prevent side cutting of the drainage channel below the structure.
With permanent dams the discharge quantities and velocities are relatively high, so channel grades below or between permanent structures should be almost flat, seldom exceeding a slope of 1 percent unless the channel bed contains sufficient rock to prevent harmful scouring. The stability of a structure is dependent upon stabilized grades in the channel below. Excessive grades lead to the development of channel overfalls, which gradually advance and undermine structures. Steep grades maintained by a plant cover are generally not dependable enough to be relied upon below permanent structures. Excessive downstream grades can be reduced by changing the location of the structure, by excavating and constructing the spillway apron to a lower elevation, or by constructing one or more permanent check dams in the channel below.
It is important that a firm foundation be secured for permanent structures. Generally, wet foundations should be avoided, especially with earth dams. Surface soil and organic matter should be removed from the dam site to get a good bond between the structure and the impervious foundation material. Cut-offs should be extended into the bottom and sides of the gully to provide a more complete seal. An apron of sufficient length, width, and depth to prevent undermining is necessary in the bottom of the gully below the dam. Care must be exercised in selecting and planing the construction materials so that an impervious structure can be assured. Carelessness in construction will result in a weak dam and may lead to failure of the entire structure.
The size of structures to be used is largely determined by the run-off capacity required, the height of overfall to be protected, and the width of gully at the selected site. Since there is generally less danger of failure with low structures and since for low structures somewhat more simple design and construction procedure can be used, it may sometimes be advisable to construct two or more low structures instead of one high structure. The practicability of this substitution is largely dependent on local terrain and conditions in the gully.
The small permanent dams should be designed to withstand intensities of rainfall that may be expected to occur once in 10 to 15 years. As the larger permanent dams require greater expenditures, they should be designed for intensities that have frequencies of 25 to 50 years or more. The type of structure used will also have some influence on the selection of the rainfall frequency. Local conditions and the judgment of the individual in charge of the construction work enter into the selection of design specifications for the various structures. The run-off that may be expected from drainage areas of various sizes under intensities of rainfall of a 10-year frequency is given in Table 1. The sizes of rectangular notches required for structures with this type of spillway are given in Table 3. In estimating the acreage of a drainage area the entire contributing area above a dam must be considered.
Good materials and safe designs are essential for satisfactory permanent structures. It is impossible to cover adequately in this bulletin the detailed designs required for all the types and sizes of structures that may be employed to control erosion in gullies. Suitable specifications for only the more common type and smaller sizes have been included.
Masonry dams are used in areas where good stone is available for masonry work. If only poor rock can be had or if it must be hauled long distances, concrete structures will be more economical. In the neighborhood of good rock quarries, cutstone masonry may be cheaper than rubble masonry because the latter usually requires more mortar and the masons spend more time in fitting the rock. With a little care, even inexperienced laborers can make a good masonry dam. Precast blocks, bricks, or tile are frequently used for these dams. Any rock used should be hard and durable. Figure 29 shows a rubble masonry dam of standard design. In general the minimum dimension for thickness of side walls, cut-off walls, and apron should be 12 inches. The thickness of the head wall from the spillway crest to the top of the dam should also be a minimum of 12 inches. Below the spillway crest a slope of ½:1 is recommended on the downstream face. In areas subject to severe cold the upstream face of the head wall should have a slope of about 1:10 to diminish ice thrust, and should be made as smooth as practical. In such areas the apron, cut-off wall, and toe wall are often constructed of reinforced concrete as added protection against heaving by frost. An apron length not less than one and one-half times the total height from the apron floor to the crest of the spillway is recommended.
It is important that the cut-off walls at the heel and toe of the dam be carried at least to the depth indicated in figure 29. The head wall should extend well into the banks of the gully. If a thin head wall has been used, it is necessary to add buttresses on the downstream side of the spillway if the spillway length (l) exceeds 6 feet or if the height (h) exceeds 5 feet. An earth fill should be made against the upstream face of the dam to the height of the spillway. A large drainage area or unstable soil conditions make it desirable to place riprap below the apron for a distance of 5 to 6 feet. A good quality of mortar of about 1 to 3 mix is suggested for masonry dams. Table 3 gives the required dimensions of the spillway notch.
The use of concrete dams is recommended where there is not adequate material for masonry structures. The same general specifications given for masonry dams can be used for concrete dams. Detailed specifications are indicated in figure 30. A good grade of concrete should be used. It is important that reinforcing steel be used in concrete structures. At least 3/8-inch round bars, spaced 12 inches center to center both ways, should be used. The specifications indicated in the drawing are for dams under 5 feet in effective height. For structures exceeding 5 feet in height, a special design is necessary. In case the spillway length (l) exceeds 6 feet, buttresses should be used to brace the head wall. Table 3 gives the dimensions of the notch required for the spillway.
Earth dams are used in areas where suitable locations and material for the fill can be secured and where a permanent structure is desired. Besides being used strictly for gully control, they are also often used for the dual purpose of controlling gullies and providing a roadway across deep gullies; and they may even provide a farm reservoir or stock pond if the location is suitable and there is need for the water supply. Two types of earth dams are commonly used to impound water for farm ponds. In one the excess run-off is carried around the dam by means of a side spillway (fig. 31), and in the other the excess run-off is carried through the dam by a pipe or culvert. If this culvert has a vertical intake and a nearly horizontal outlet section the structure is commonly called the drop-inlet, soil-saving dam (fig. 32). In many dams it is desirable to use a combination of conduit through the earth fill and side spillway to bypass the excess run-off. Where this combination spillway is used the conduit through the dam is provided with sufficient capacity to carry only the more common rates of overflow. The side spillway is constructed with its inlet at a slightly higher elevation than that of the conduit and with sufficient capacity to carry the overflow that cannot be carried through the fill during periods of high run-off. This combination enables the use of cheaper protection, such as vegetation, for the side spillway, because the spillway will be used only occasionally. Using a side spillway is considerably cheaper than carrying all the water through the fill.
The use of side spillways in conjunction with earth dams is limited to locations that have suitable terrain. Usually these spillways are most applicable to low dams, from which the overflow can be lowered to a stabilized grade below the dam without undue expense. Where drainage areas are small and good sod is available, side spillways covered with grass have been used. Where drainage areas are large and spillway is frequently the more satisfactory, especially if overfalls are high. This is particularly true in areas where features of the terrain discourage the use of side spillways or where the earth fill is also to be used as a roadway.
The same general fill specifications ordinarily apply to all types of small earth dams. The best site for the dam is usually at a narrow neck in the channel, if foundation and spillway conditions are suitable and if the gully floor downstream is on a stable grade. The site should be stripped of sod and other foreign material and plowed or disked crosswise of the gully. It is desirable to place a core wall in a dam to act as a barrier against seepage, and all pipes or tubes through the fill should be provided with ample cut-off collars to prevent seepage. If a pervious foundation (sand, gravel, or shale) is encountered it is usually better to change the location of the dam; otherwise considerable expense may be incurred in providing the necessary cut-offs. The earthen fill should be spread in well-packed layers of not over 6 inches each. It should preferably consist of a mixture of sandy soil and clay, well compacted, and not too dry. No fill should be made during freezing weather. The side slopes of the settled fill should be not steeper than 2:1 downstream and 3:1 upstream. For ordinary gully-control work a minimum crown width of 4 feet is recommended unless the dam is also to serve as a roadway of reservoir, in which case, top widths of 10 to 20 feet may be required.
The top of the earth fill should be maintained 2 to 3 feet above the maximum height of flow expected through the spillway in order to provide the necessary protection against overtopping. On large earth dams a larger margin of safety should be provided. The fill slopes above the water line and the crown should be seeded to grass. Advantage should be taken of any natural or artificial vegetated spillway available, if only to the extent of using it as an auxiliary spillway in the event of heavy run-off.
Figure 33 shows standard specifications for a soil-saving drop-inlet dam with concrete barrel and riser (reinforcing steel details not shown). The barrel and at least the lower part of the riser must be built before any fill is made. The barrel should rest on a firm foundation, which can usually be obtained if an excavation in firm soil is made for the barrel. After the barrel and riser are in place, the fill can be made. The fill around the barrel and riser must be tamped carefully. If the outside walls of the barrel taper slightly from the bottom to the top, the earth will make a tighter contact with them in settling. It is desirable to locate the barrel as near the center of the gully as possible.
On small or medium-sized drainage areas, tile or corrugated metal pipes are frequently used for bypassing the overflow through the fill. Rounded elbow connections between the vertical and horizontal sections of the tube should be used. Concrete culverts less than 2 feet square are difficult to construct because of the difficulty of removing the forms; so pipe culverts are recommended for small sizes. Table 4 gives culvert sizes recommended in drop-inlet soil-saving dams for various discharges.
It will be noted that round-culvert dimensions are given in table 4 as inside diameter in inches and those for square culverts are given as inside width in feet. For example, it is desired to know the culvert size required for a discharge of 250 cubic feet per second to be carried through a total drop (z) of 20 feet; either a 42-inch pipe culvert or a culvert 3 ½ feet square will carry the discharge. Usually a reinforced-concrete structure will be more durable and is recommended because of the high total drop here involved. The choice would thus be a reinforced-concrete culvert 3 ½ feet square.
It is again emphasized that it requires considerable skill to construct a drop-inlet dam, and it is therefore safer to engage the services of a competent engineer for the construction of at least the larger structure.
Flumes (sometimes called chutes) are used to convey run-off down steep banks or overfalls to a base grade. The steep slopes on which flumes are usually constructed produce high discharge velocities, and care must be exercised to provide necessary apron and grade protection below the structure. Permanent flumes are usually constructed from concrete or masonry. Wood or metal flumes have been used, and sod flumes (discussed under Vegetation) are commonly used in smaller watersheds. Available material and the degree of permanency desired will determine the most suitable construction material. If concrete is used, the slope of the flume should be not steeper than 2:1 to facilitate pouring. A masonry flume may have a slope as steep as 1:1. Concrete and masonry ordinarily make more desirable flumes than wood or metal unless permanency is not essential. Where applicable, sod should be used in preference to any other material because it is the most economical.
Figure 34 shows a standard masonry head flume and figure 35 a flume constructed of concrete. The concrete flume should be reinforced with 3/8-inch round steel bars spaced on 9-inch centers both ways. For either structure it is recommended that both the upstream and the downstream cut-off walls be dug to a minimum depth of 3 feet and that the wing walls extend a minimum of 6 feet in to the lead-in dikes on either side of the flume. Side walls should be high enough to confine completely the maximum flows anticipated during the design frequency. The length of the apron should not be less than the total height from spillway crest to apron floor. The gully channel for some distance below the flume should be on a stable grade; otherwise the structure may undermine. A flume is intended to control waterfall erosion and not channel erosion. In order to assure enough spillway capacity for the flumes, the length l indicated in figures 34 and 35 should be a minimum of 1 foot for every 15 cubic feet per second of discharge expected and should have a total length of not less than 4.5 feet.
CULVERT DROP INLETS
Drop-inlet highway culverts are used to combat gully erosion along highways. Excavation for highway culverts is usually below the natural profile of the ground, and when water flows down the drainageway it has to drop in order to pass through the culvert. This drop sometimes is the beginning of a gully of the waterfall type, which, in eating its way back, may destroy many acres of valuable land on farms adjacent to the highway. If conditions below a culvert are unstable, gullies often eat their way slowly up a watercourse toward the highway and finally undermine the culvert if its substructure is not deep enough to stop further advancement of the gully.
A drop inlet can readily be obtained by constructing a drop box on the upstream end of the existing culvert. The drop box should be built to a height sufficient to prevent further recession of the gully and yet low enough to allow plenty of freeboard for the highway embankment. Before a drop inlet is constructed on an existing culvert it is well to examine the road fill to determine its suitability for the impounding of water. Where it is not desired to impound quantities of water, the drop inlet may be built in stages by adding height as the gully silts up. Wherever possible in suitable locations it is preferable to construct a regular drop-inlet structure at the time the road is being built rather than to wait until a gully is formed and then place a drop inlet on an existing culvert. A drop box on an existing culvert will not be as efficient as a hydraulically proportioned barrel and riser initially placed.
The drop box may be considered as a broad-crested spillway and the required length determined by selecting the length in Table 3 for anticipated run-off and flow depth and then increasing this length by one-fourth. If the entire structure has been designed in accordance with the recommendations for a drop-inlet dam, the size required for different discharges may be taken from Table 4. It is often necessary also to place protection on the lower end of a culvert in order to prevent gullying. This can be done by using either a suitable drop outlet or by constructing small permanent dams to obtain stabilized grades.
Culvert drop inlets should not be constructed without the approval of the highway officials.
BANK PROTECTION ON SMALL STREAMS
Stream-bank erosion is frequently associated with gully erosion because it is essentially a lateral type of ditch or channel erosion. Gullies often begin at the banks of natural watercourses, especially where the subsurface material is soft and easily worn away, and by waterfall erosion moved back into adjacent fields. Portions of valuable bottom lands are frequently damaged by bank cutting that occurs along certain streams, particularly at the bends of winding channels. This cutting may continue even though the stream is on a stable grade. Bank cutting on one side of the stream is usually accompanied by the formation of sand bars or silt deposits on the other, and there is a gradual lateral movement of the main channel. As the process continues, the bends in the channel become more abrupt and the damage to adjacent land more severe.
The numerous factors to be considered and the lack of adequate information makes it impossible to recommend definite control procedure. The success or failure of controlling erosion on stream banks is dependent to a large extent on the judgment of the person in charge. He must have a knowledge of the size of the stream and an appreciation of its destructive force and its flow characteristics. Changing stream flow in one section of the channel will often have a decided adverse effect on the flow for some distance downstream. These resultant effects must be considered, especially when the channel is forced into a new location.
Stream-bank cutting on farms is usually confined to small areas on small streams, some of which flow only intermittently. To control bank cutting on these streams it is generally not necessary to use heavy timber, concrete, or masonry structures, which are ordinarily required for adequate control on large streams and rivers. Suitable trees, shrubs, and grasses are usually sufficient. Before adequate vegetation can be established it is often necessary to use temporary jetties, wing dams, or other obstructions along the eroded bank to check water velocities and start silting. After the bank cutting is checked and sufficient sediment deposited, the necessary vegetation can be planted if sufficient cover is not provided by natural growth.
An important factor in both the prevention and control of streambank erosion is the protection of a border strip immediately adjacent to the stream. Instead of plowing to the water’s edge or allowing livestock to raze off the protective vegetation, which is found along most stream banks, these areas should be protected. Livestock can be given access to water by lanes fenced to the edge of the stream.
JETTIES AND VEGETATION
If jetties are to be used, the point at which the flow line of the stream intersects the eroding bank is about where the first jetty should be located. Preliminary model tests conducted at the Ohio State University indicated that the intersection point of the eroding bank with a line drawn through the toe of the jetty and parallel to the flow line of the stream should be approximately the midpoint between the first and second jetties. These tests also indicated that the approximate location of each succeeding jetty around the bend would be at a point where a line projected across the toes of the two upper jetties intersects the bank. This procedure of locating jetties is shown in figure 37.
Field observations, as well as the Ohio model tests, indicate that the jetties should point downstream and form about a 45 degree angle with the bank. The distance that the jetty should project into the stream will depend on how far it is necessary to divert the flow away from the eroding bank in order to secure a normal channel condition. A projection of from one-fourth to one-third the width of the stream at flood flow should not be exceeded. If the jetties are extended too far the undesirable channel restriction produced endangers their stability. The top of the jetty should slope downward toward the stream on about the angle of repose desired for the ultimate bank. It has been found advantageous to use low jetties.
The jetties may be constructed of various materials. On shallow, slow-flowing streams, piles of alternate layers of rock and brush have given good results. On faster-flowing streams, the jetties must be well-anchored. Wirebound, loose rock, which forms a sausage-shaped jetty (fig. 38), has been used extensively. In locations that require heavier structures, rock-filled log cribs or timber piling have been used. Crib jetties should be keyed well into the bank and should extend below the channel bed so as to be protected from undercutting. Those that have undercut at the toe can often be repaired by the use of wire-bound rock.
Willow and cottonwood are desirable trees to plant for streambank control. Under certain conditions dogwoods, alder, and other species may also be advantageously planted. The entire bank should be planted, and the plantings should extend as close to the water’s edge as possible, provided there is 1 to 2 feet of silt or mud above the low-water level in which they can be established (figs. 39 and 40).
CRIBBING OF RIPRAP
The sharper the bank curvature, the closer the jetties should be spaced. The curvature may be so great, however, that it will require more material to construct jetties than to construct revetment or riprap, which would provide a continuous protection. In making the selection of the most suitable type of protection one must take into consideration the material to be used and the most economical employment of labor in construction. Where only temporary continuous protection is desired, brush matting, loose rock that is bound or anchored with wire, and rock-filled timber cribbing have been used. Where more permanent protection is desired, stone riprap laid in cement mortar on a slope of about 1:1 may be necessary. Continuous mechanical protection should also be supplemented insofar as possible with vegetation.
The cost of protecting the eroding banks of a winding stream can sometimes be reduced and necessary protective measures minimized by cutting a new channel across ox bows. Careful preliminary investigations should be undertaken when channel straightening is contemplated. The new channel may induce eddy currents that will aggravate the bank cutting and ultimately lead to conditions even worse than those being treated. Furthermore, channel straightening increases channel grades and velocities. Sufficient investigations should therefore be made to determine whether the straightening will lead to the development of harmful channel scouring and overfalls. It is only rarely that it can be justified.
When a new channel is provided it is sometimes necessary to construct a cut-off structure across the old channel in order to force the stream through the new channel. Full-height structures (average flood stage) should be used, which cut off the old channel entirely by projecting from bank to bank. A log crib filled with rock or a crib made of piling and woven wire and filled with rock have been successfully used.
MAINTENANCE OF EROSION CONTROLS
An important and frequently neglected practice in the control of gullies is systematic inspection, repair, and maintenance. Too often erosion controls are installed and then neglected until they become so badly damaged that they are no longer effective. When this happens, practically all expenditures and efforts have been wasted. Installations for control of gullies should be inspected periodically, especially after heavy rains, to determine whether they are functioning properly or need minor repairs. This is particularly true of vegetative controls during the period when the vegetation is becoming established; at this time it is in its most critical period. The attention given to minor adjustments or repairs during this period often determines whether or not the vegetation will successfully control the gully. Mechanical structures are also more subject to failure when first installed because they do not become thoroughly settled, compacted, or sealed for some time.
It is especially important that all types of erosion controls be projected from livestock (fig. 41). To protect them adequately from damage by grazing and trampling of livestock it is necessary to fence the gullied areas. Hogs particularly should be excluded, for they root up vegetation and damage structures. Vegetation or structures of combustible materials should also be protected from fires. Burrowing rodents occasionally cause failure of structures by digging through or around them.
Even after erosion controls in gullies have been established, some maintenance is necessary in order to assure proper functioning. Diversion ditches may need occasional cleaning out or rebuilding of damaged sections to renew their capacities. In some gullies, vegetation will require cutting in order that water channels may be maintained to full capacity. Debris that collects in the spillways of structures must also be removed at regular intervals, or it may clog the spillways to such an extent that the structure will be overtopped.
Once a gully has been definitely stabilized, it should be able to carry the normal flow of water without danger of further gullying. There is always the chance, however, that excessive concentrations of flow may cause renewed cutting in the old channel. Gullies that have been stabilized for many years may become active again either because the quantity of water they must carry has been considerably increased or because the original erosion controls have failed to function properly. An increase in rate or amount of run-off may result from changes in land use, cropping, or tillage practices or from an unusually severe storm.
Failure of erosion controls may thus result from improper application, from lack of maintenance, or from some extreme conditions not ordinarily encountered. These factors, potentially capable of disrupting at any time the stability of the gully, should therefore be guarded against to whatever extent is practicable. Any damage incurred should be repaired before it leads to further trouble.