Terraces

Terraces are earth embankments designed and constructed to intercept runoff and convey it to a protected outlet. They reduce soil erosion by shortening slope length, slowing the velocity of the running water and improving infiltration. Terraces can be classified into the following main types:

The characteristics and functions are given in Table 4.10 and Fig. 4.30.

Diversion terraces are designed primarily to intercept runoff and divert it to a suitable outlet. Broad-based diversion terraces (Nichols and Magnum), with the embankment and channel occupying a width of about 15 m, are wide enough to be cultivated if bank slope does not exceed 14°. Narrow-based terraces do not exceed 4 m in width. Diversion terraces are suited mostly for moderate ground slopes (< 7°) that allow a sufficient spacing and reasonable expense of construction. Modest slopes (<4.5°) and permeable soils are recommended also for the use of retention terraces, which are aimed at the conservation of water by storing it on the hillsides. They are generally level and the design storm has about a 10-year return period. Bench terraces are suited for cultivation of steep slopes (up to 30°). They consist of an alternation of shelves and risers. The latter are protected from erosion by vegetation cover or by concrete or stone facing. For several thousand years, bench terraces have been widely adopted over the world, particularly in Europe, Australia, and Asia. A special type of bench terrace is the orchard terrace. Orchard terraces are narrow bench terraces built on very steep slopes, from 25° to 30°, and their spaces are determined by the planting distance of the fruit or food trees. Because of steepness, the spaces between should be kept under permanent grass.

Figure 4.30. Types and main characteristics of terraces. Source: [11].

Another practice used on steep slopes is hillside ditches. The hillside ditch is a discontinuous kind of narrow, reverse-sloped terrace built across the land in order to break long slopes into a number of short slopes so that the runoff will be safely intercepted and drained before causing erosion. The cross section of this kind of narrow bench is more convenient for maintenance than the conventional type of ditch. They also can be used simultaneously as roads. The distance between two ditches is determined by the degree or percentage of the corresponding slope. The cultivable strip between two ditches should be supplemented with agronomic conservation measures.

As technology has advanced, terrace design (spacing, length, location of outlets) has been scientifically adapted to the hydrologic and erosion control needs of the treated areas. Channel cross sections have been modified to become more nearly compatible with modern mechanization. Table 4.11 gives some criteria that can help in decision making about slopes and dimensions for terraces [1]. Many formulas also have been

Table 4.11. Design criteria for terraces

Criterion

Dimension

Maximum length

Normal Absolute

Maximum grade

First 100 m Second 100 m Third 100 m Fourth 100 m Constant grade

Ground slopes

Diversion terrace

Retention terraces Bench terraces

250 m (sandy soils) to 400 m (clay soils) 400 m (sandy soils) to 400 m (clay soils)

1:1000

1:500

1:330

1:250

1:250

Usable on slopes up to 7°; on steeper slopes the cost of construction is too great and the spacing too close to allow mechanized farming Recommended only on slopes up to 4.5° Recommended on slopes of 7° to 30°

developed for determining the so-called vertical interval (VI), that is, the difference in height between two consecutive terraces [11]. Several empirical formulas have been calibrated on a national basis [11, 12]. To know what formula is most suitable in a given situation, one should find out which design criteria are successfully used in an similar situation (if possible in the same area). The most important design parameters are the width of the terrace and the height and the slope of the riser [1, 15]. From a theoretical point of view, terraces can be built on any slope with deep soil but, in practice, on very steep slopes the riser becomes too high and therefore difficult to maintain, and the terrace becomes too narrow. Practical limits are about 20° for terraces built by machine, and 25° to 30° when handmade. The width of the bench depends on three factors:

• what is desirable for ease of cultivation,

• what is practical for construction.

For a given slope, increasing the width of the terrace increases the amount of both earthwork and fill. Narrow terraces are therefore cheaper than wide terraces. For terraces cultivated by hand or walking tractor, bench width of 2 to 5 m is feasible. For animal draft or four-wheel tractor cultivation, a minimum width of 3 or 4 m is desirable. It is not practical to excavate down into the subsoil or bedrock, because of both the increased cost of excavation and the lower fertility of the exposed subsoil on the inner edge of the terrace. This may be avoided by building a terrace and then putting the topsoil removed from the next terrace on it [1].

A variety of equipment is available for terrace construction. Terracing machines include the bulldozer, pan or rotary scraper, motor patrol, and elevating grader. Smaller equipment, such as moldboard and disk plows are suitable for slopes of less than 8%.

The rate of construction is affected chiefly by the following factors:

• crops and crop residues,

• degree of regularity of land slope,

• gullies and other obstructions,

• terrace cross section,

• experience and skill of the operator.

Proper maintenance plays an important role in the terrace conservation practices.

The procedures must not be expensive; normal farming operations usually will suffice. The work should be watched more carefully during the first years after construction because tillage tends to increase the base width of the terrace. Therefore, tillage practices such as stubble-mulch operations, disking and harrowing, and planting have to be performed parallel to the plow furrows.

On the whole, the main features required for terrace conservation are [1]:

• A deep soil is required, both to provide sufficient soil moisture storage and to lessen the effect of cutting down during the construction process. Good permeability is required so that the contained runoff can be absorbed quickly.

• Smooth slopes are an advantage where mechanized farming can be made more convenient by constructing all of the terraces parallel to each other and of equal width.

• Precise leveling of the terrace is important to ensure uniform infiltration.

• If there is a risk that runoff from the catchment area will be greater than can be absorbed and stored on the terrace, there must be outlets at the end of the terrace.

• Typical ratios of catchment area to terrace area are from 1:1 to 2:1.

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