It consists of an earth dam having a spillway of concrete or metal pipe. The spillway has a vertical section on the upstream side of the dam, called the riser, which is connected to a culvert or barrel passing through the earth dam. The crest of the riser is set at the elevation required to stabilize the grade upstream or protect the gully head.

Usually an earth emergency spillway is built around one end of the dam to take the infrequent or high flood flows. The use of an emergency spillway incorporating provisions for temporary flood storage where possible will reduce the size and consequent cost of the drop inlet structure. The cost of passing the total flood flow through a drop inlet may result in costs that may make the structure uneconomical.

The emergency spillway should have a good cover of vegetation, a uniform cross section, a gradual slope to the channel downstream, and no abrupt turns. It is primarily for the purpose of safeguarding the structure under extreme flow conditions at a reasonable cost. The elevation of the two spillways should be established to permit the drop inlet to discharge at its rated capacity before the emergency spillway functions.

The outlet of the drop inlet is placed at or slightly above the elevation of the grade established below the structure.

A propped outlet is generally used. The pipe is extended about 8 feet below the toe of the dam and is supported by piling or a concrete pier. The support is placed 6 to 8 feet from the end of the pipe to protect it from the scour hole that will form. The advantage of the propped outlet is its low cost and its ability to discharge water far enough downstream so as not to endanger the safety of the structure. If the grade lowers excessively downstream from the outlet, the outlet can be extended and lowered at a reasonable cost. A propped outlet permits installation of structures at locations where the grade downstream is not considered completely stable.

As a general rule, a drop inlet with a propped outlet should not be built where the grade downstream averages more than 1 percent. In places where grades are steeper, supporting structures should be built downstream to a point where the resultant grade is less than 1 percent.

The preparation of the foundation and placement of earth fill for drop inlet structures is carried out in accord with standards covering the construction of earthen embankments. These standards, which are based on long experience, can be obtained from the local office of the Soil Conservation Service.

THE DROP SPILLWAY, or notch spillway, is built of reinforced concrete, masonry, or steel sheet piling. It is like the old mill dams built in small streams to serve as a source of power. Because dams in dry draws are not subject to continuous flows, the drop spillway is simpler in design and of lighter construction. It can be designed for almost any height, but it is best adapted to drops of 10 feet or less. The total storm runoff usually passes over the crest of the drop spillway. Emergency earth spillways are rarely used in connection with drop spillways.

Drop spillways require careful design of the apron or floor, on which the water falls, to dissipate the energy before passing to the channel below. Improper apron designs cause scour of the channel immediately below the structure. Excessive scour immediately downstream endangers the safety of the structure. Drop spillways therefore are used only at places where the grade downstream from the structure has been carefully studied and determined stable. Earth fills are used to connect the structure with the earth abutments.

CHUTES ARE USED in combination with earth dams to drop water farther than is ordinarily feasible with drop structures. They are built of reinforced concrete.

Chutes are constructed on foundations on original ground or on fill that has been carefully compacted under controlled conditions. The chute is susceptible to movement because of frost action or other causes. Closely spaced expansion joints are required to relieve the structure of stress that would cause cracking. Chutes are individually designed to fit specific site conditions.

STRUCTURAL IMPROVEMENT of gullies usually requires the service of an engineer to insure an economical, safe job.

The sizes and costs of the structures required to control a gully or system of gullies vary widely. Structural improvement of waterways generally requires a sizable outlay of money.

Standard plans have been developed and can be used successfully when properly adapted to site conditions. The plans are usually confined to drop spillways that have a height of 5 feet or less and to small drop inlets that have a total height of fill less than 15 feet.

These small structures can usually be built by the farmer with technical assistance. Whenever structural or vegetative control of a gully is considered, it is advisable to get in touch with the Soil Conservation Service or the Extension Service to ascertain the technical services that are available.

Most vegetative control measures can be wholly or largely carried out by the individual farmer. For structures, an engineering survey must be made, detailed plans must be prepared, and construction must be carried out by skilled individuals under careful supervision. Good materials and safe designs are essential for satisfactory permanent structures.

Uncontrolled gullies might continue to advance to the point where they cut up fields and whole farms so they must be abandoned. The sediment from eroding gullies is deposited on bottom lands and may destroy their value. The sediment trapped in reservoirs and deposited in stream channels creates an economic loss. Gullies lower the water table and may drain areas to the extent that crop production is reduced. The cost of maintenance of highways, railroads, pipelines, and other public utilities is increased by gully erosion and resultant sedimentation.

Gullies occur in every State. The extent to which they create damage depends on climate, topography, geology, soil type, and land use. Over the ages gullies have developed through the process of natural or geologic erosion. Their formation and resultant effect on the general configuration of the land is considered a normal part of the processes of erosion.