Terraces (individual, discontinuous narrow, and continuous bench terraces)

Terraces serve the functions of stopping downslope soil and water movement and also give the advantage of providing a flat surface for the planting of crops, thereby further reducing the possibility of erosion.

Some important points should be noted before deciding to build terraces. Since the formation of terraces requires a maximum of soil disturbance and rearrangement, they are extremely susceptible to erosion if not properly measured, compacted, and maintained with risers(sloping banks) protected by vegetation. Also when considering terracing a steep slope (25-30% +) it should be noted that a much deeper cut and fill will be needed, much more land space will be lost in the sloping terrace walls, and the useful planting space is likely to be extremely narrow unless a very deep soil is present.

Individual terraces are constructed to provide a level platform for the growth of an individual tree. They are always used in combination with another type of conservation structure, such as contour ditches, since the small platforms by themselves do not appreciably control surface water movement. (Fig. 13). When planting a hillside with individual terraces, the layout should be in an equilateral triangular or hexagonal pattern, with the distances varying according to the variety of tree being planted. The terraces generally have a diameter of 1 to 1.5 meters and should be throughly compacted to prevent collapsing.

Fig. 13. Individual terraces.

Discontinouous narrow terraces (orchard berraces/provide both -a flat platform for planting crops and an inverse slope which allows it to serve as a drainage or infiltration ditch.(Figs. 14, 15).

Fig. 14. Discontinuous narrow terrace (orchard terrace)

Fig. 15. Discontinuous narrow terrace (X.S.)

Construction is done, after plowing if possible, with pickaxes and rakes or hoes Some design information is given in Table 4. Construction is most rapid in soft, deep soils.

In these cases the earth from the cut section is merely moved to the fill section, compacted, and raked smooth. If it is judged necessary to remove the topsoil and redistribute it later, then a construction sequence such as sequence "B" given for the bench terraces is recommended (Fig. 18). In order to keep the risers from eroding, grass should be planted on them. The drainage way for this type of structure should be constructed similar to that of the hillside ditch, slightly steeper right before emptying into the protected drainage area. (See 'Waterways for draining excess water from fields").

TABLE 4 DISCONTINUOUS NARROW

TERRACE CONSTRUCTION GUIDE

From Michaelsen, 1980

*Based on 2m platform width, 1.5 wide platforms are also used, especially on steeper slopes with thin soils.

Continuous bench terraces are staircase-like structures which diminish erosion because the reshaping of the Land surface results in the planting of all crops on gently inversely sloping platforms (Fig. 16). The mast feasible application is in intensively worked vegetable plots where each planting bed may be a separate terrace.

Bench terraces can be designed level (0% slope) for water retention or with a slight slope (0-1%) to facilitate water drainage or distribution of irrigation water. (See "Waterways for Draining Excess Water From Fields']

Fig. 16. Continuous bench terraces (X.S.)

There are many different terrace design specifications published in soil conservation books, but only the basic features are described here. Table 5 may be used as a general guide for designing terraces with the total terrace width dependent upon the slope of the hillside and the depth of fertile or tillable soil.

TABLE 5 BENCH TERRACE CONSTRUCTION GUIDE

*"Depth of A Horizon" in original changed to permit use in eroded areas where horizons are often indistinct.

From Suarez Castro, 1980

Bench terraces are constructed using pickaxes, hoes, and rakes. In some areas, large, specially designed hoes are available. Construction is much easier if the section of earth to be removed (cut section) is plowed beforehand to loosen the soil. If construction is being undertaken during the rainy season, it is advisable to begin construction near the drainage area and with the uppermost terrace. In this way, any rainwater will drain off without damaging the terraces.

There are several ideas as to the best method of constructing a series of terraces. One, which may require several years for completion, is the planting of a live barrier grass or the construction of rock walls along the contour. Over time terraces are formed as soil fills in behind them. Two other construction sequences are presented here. (Figs. 17,18)

Fig. 17. Bench terrace construction sequence "A"

Fig. 18. Bench terrace construction sequence ''B"

Fig. 18. Bench terrace construction sequence "B"