Terraces are constructions built mainly in hilly areas to reduce water erosion losses from cultivated erodible soils and for water conservation.

Hilly area cultivated with olive groves which has been terraced for erosion protection and water conservation. (photo by C. Kosmas)

Two major types of terraces can be distinguished. Bench terraces reduce land slope and the broadbase terraces remove or retain water on sloping land. The bench terraces are constructed by laying out strips across the slope and carrying soil from the uphill side so that level steps or benches are formed. Where, because of erosion, soil is shallow, stones are gathered and walls built across the slope, or crescent-shaped to provude roothold for individual olives, chestnuts or fruit trees. Soil usually is transferred from nearby to fill the area above the stone wall. Such terraces have been built in the past in extensive hilly areas in Europe.

The broadbase terraces may be constructed with no grade (level) or with a slope (graded) in a channel in order to intercept runoff and direct it to a protected outlet. The level terraces are primarily designed for soil water storage where rainfall is limited. The graded terraces are mainly used for minimizing erosion by reducing slope length.

In recent decades, the value of such terraces has markedly decline due to: (a) difficulties associated with accessibility and use of machineries, (b) decreasing product price and increasing labor cost for repairs, and (c) abandonment of hilly areas due to lower productivity. Collapse of terraces results in wash out of the protected soil and the rate of land degradation is very high.

The stability of stone-walled terraces is mainly related to the slope gradient, soil type, stone composition and management practice. Stones used for terrace construction present various rates of weathering and disintegration and therefore the rate of collapse is related to the parent rock from which stones are extracted. The terraces constructed by stones derived from igneous rocks, sandstones, shale or schist have usually higher rate of collapse than terraces built with stones derived from limestone or marble.

The coefficient of linear extensibility (swelling-shrinking) of the soil used for filling the terrace greatly affects stone wall stability. If soil extensibility is high, the horizontal pressure after wetting is high especially at the base of the terrace causing inflation, instability, and finally collapse. For example, soils formed in marl and ultrabasic rocks have usually higher coefficient of linear extensibility than soils formed in shale, sandstones, acid igneous rocks greatly affecting terrace stability.

Average coefficient of linear extensibility measured in soils formed in various parent materials in the island of Lesvos (source: C. Kosmas).

Slope gradient greatly affects terrace structure stability. Studies in the island of Lesvos have shown that three critical slope classes can be distinguished: (a) lower than 15%, (b) 15-35% and (c) greater than 35%. Terraces constructed in slopes lower than 15% usually remain undisturbed under certain management practices. The rate of collapse increase almost linearly with increasing slope gradient from 15% to 35%. The rate of collapse expected is very high in slopes greater than 35%. Trampling of animals over a terraced land favours terrace collapse and soil erosion.

Relation of terrace stability and slope gradient measured along transects in hilly areas of Lesvos (source C. Kosmas).

Terraced area as a percentage of total area.

• <20%,
• 20-50%,
• 50-75%,
• >75%

Troeh, H.R., Hobbs, J. A., and Donahue, R. L. 1980. Soil and water conservation for productivity and environmental protection. Prentice-Hall, Inc., Emglewood Cliffs, New Jersey, 320-394 pp.

Spencer J. E. and Hale G. A. 1961. The origin nature and distribution of agricultural terracing, Pacific Viewpoint I, 1-40 p.