Terracing (NBS)

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Description

Terracing consists in converting a steep slope in a series of step-like structures. This technique was adopted centuries ago for agricultural purposes to create more suitable places to cultivate. It is still a very common technique used in tropical and humid regions in which high rainfall events or monsoon systematically occur. More recently this technique has been applied along both transportation corridors and residential developments in steep terrains to accommodate roadways or building sites. In this last case the cuts should be rounded at tops and sides to avoid the overhangs, to be in accordance with surroundings and should have relatively large benches to provide a better soil availability for plant establishment (Gray & Sotir, 1996). The main functions of terraces are:

• controlling and guiding the surface runoff across the slope converting it into a suitable outlet at low erosive velocity;

• reducing soil erosion by entrapping soil particles into the terraces;

• create more flat and suitable lands for cultivation.
  There are three main types of terraces depending on the aim and the intensity of the erosion process to be addressed. 

Contour (Level) terraces
They are constructed along the slope contours mostly for retaining water and sediments. Since the terrace is formed along contour the runoff flows across but not along the terrace (Figure 1). Usually the terrace edge is planted with trees, shrubs, small plants or grass with trees on the outward facing edge to increase also the soil stability by roots.

 

Channel (Parallell) terraces
They are mainly used in areas with heavy rainfall. Usually they are constructed with constant slope or gradient along their length in order to convey excess runoff at a safe velocity into a grassed waterway or channel.

Bench terraces
The most commonly used, they can be suitable when the availability of water is seasonally variable. They enable crops to grow on a steep slope rather than retaining water or entrapping sediments. They are similar to the contour terraces but in this case the terraces do not strictly follow the contour line, so the runoff can run independently along or across the terrace.

Advantages

• Improve soil fertility with reduced runoff erosion;

• Help water retention, especially during dry season;

• Slow down and drain away of runoff during heavy rainfall events, counteracting the tendency for sliding.

Disadvantages:

• Intensive labor and request of equipment;

• When designed for large slopes, it can be expensive;

• Removal of boulders, rocks or trees are necessary before terracing: in general, any obstacle should be removed working from top to the bottom.

 

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Design methods

Bench terraces (from ICIMOD)

When designing, the desired width, vertical interval and spacing, length, gradient and cross section of the terrace should be selected. However, other agricultural-related factors can influence the use of the terrace: the soil depth, the distribution of the top soil, the land use of slope, the rainfall distribution and the final farming practices (Sharda et al., 2007).

First, the type of terrace to be realized is selected according to the rainfall distribution and soil conditions of the area.  

Bench terraces can be realized in three different layouts (Figure 3):

• Outward sloping: the terrace outwards the original slope level. It is typically constructed in low rainfall areas with permeable soil;

• Level sloping: the terrace is realized by maintaining the same level of the original slope. It is typically constructed on highly permeable soil or medium rainfall areas;

• Inward sloping: the final level of the terrace is under that of the original slope. This is typically constructed in areas with heavy rainfall and with less permeable soils.

 

Then the width of the terraces (W) is determined as function of the soil depth and the slope: normally building very wide terraces is more expensive and requires deep cuttings but it very useful for homogeneous cultivable areas and it results in higher riser slopes (Sharda et al., 2007). The terrace spacing depends on the soil type, surface condition, gradient, depth of cut and in a way by the agricultural use. The length of the terrace depends on the shape and size of the land, as well as the permeability and the erodibility of the soil. Normally the longer is the terrace the more efficient is the agriculture use and less costly is the measure, even if this may increase the runoff velocity and thus the soil erosion. The gradient of the terrace should be determined based on the rainfall intensity, permeability of the soil and width and length of the terrace. Usually for low rainfall areas with high permeability a gradient lower than 0.5% is preferred, whereas in high rainfall areas a gradient of 1% is preferred to reduce the runoff. It can be also determined with a simple approach of 1 m for every 100 m terrace length.

 

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• Gray, D. H., & Sotir, R. B. (1996). Biotechnical and soil bioengineering slope stabilization: a practical guide for erosion control. John Wiley & Sons.

• International Centre for Integrated Mountain Development, ICIMOD. Chapter 5: Physical Methods for Slope Stabilization and Erosion Control.  http://lib.icimod.org/record/27709/files/Chapter%205%20Physical%20Methods.pdf

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