«Bench Terrace Design Made Simple Ted C. Sheng Department of Earth Resources Colorado State University Fort Collins, CO 80523, USA E-mail Address: ...»
12th ISCO Conference Beijing 2002
Bench Terrace Design Made Simple
Ted C. Sheng
Department of Earth Resources Colorado State University
Fort Collins, CO 80523, USA
E-mail Address: email@example.com
Abstract: Bench terraces are effective soil conservation measures used on slopelands for crop
production. When world’s population increases rapidly, with a rate of 77 million a year and
mostly in developing countries, many slopelands are brought into cultivation where land pressures are high. In many instances in the past, however, bench terraces were built without proper design, resulting in either high construction and maintenance costs or limited use. On the other hand, some design criteria are over-complicated which hampers their application.
This paper introduces a simple but scientific design for bench terraces. The design is based on many decades of field experience in the countries of Asia and Central America. It employs a step by step approach using basic arithmetic that can be understood by field technicians, extension officers, or farmers. Based on this design, a realistic estimate of construction cost and a land use plan can be easily produced.
Keywords: bench terraces, terrace design, land treatment, conservation measures 1 Foreword For cultivation of slopelands, bench terraces are one of the most effective measures for erosion control and crop production. With the world’s population rapidly approaches 9 billion in next five decades, many of the slopelands in the developing countries will be brought into food production. In many instances in the past, bench terraces were built without scientific design, resulting in unusually high cost, maintenance difficulties, or limited use. On the other hand, some over-complicated designs have discouraged people from using them.
The present paper concentrates on the design of two major types of bench terraces: Level Bench Terraces for dry land environment and Reverse Sloped Terraces for humid regions. The principles, however, can be applicable to the other types. For instance, level bench terraces can easily be converted to paddies by simply adding a small dike on the edge for impounding water. Outward sloped terraces can be designed using similar approaches and calculations.
Using land slope and the width of the bench (flat part) as two starting points, the design proceeds step by step with basic arithmetic that can be easily understood by field workers, land users, or farmers.
2 Design basics Use simple arithmetic and a step-by-step approach to design.
Design bench terraces such that the volumes of cut and fill are to be equal for minimizing construction cost.
Design terraces according to the needs of farmers, crops, climate, and tools to be used for farming.
3 Design procedures and criteria
appropriate widths whereas for mechanization 3. 5 m to 8 m are proper where depth of soil does not constitute a limit.
(2) Slopes Slopes can be measured by using a hand level or a clinometer. In design of terraces, a representative slope or a mode slope should be obtained from the field.
If the farmer will build terraces by hand, the appropriate slope range is from 7 degrees to 25 degrees (or 12.3% to 46.6%). If machines will be used for construction, the range is from 7 degrees to 20 degrees (or 12.3% to 36.4% ) according to past experience. Slopes gentler than 7 degrees may best use simple conservation measures or agronomic measures. Using machines on a slope over 20 degrees is unsafe.
(3) Vertical Intervals After the slope and the width are determined, the Vertical Interval (VI) can be calculated by a simple equation. VI is the elevation difference between two succeeding terraces. It is essential to calculate the VI;
it not only shows roughly the height of future terraces but also provides the basis for further designing.
The simple equation using slope and the width of the bench as the main inputs is as follows:
VI= (S Wb) / (100 – S U) (1) Where S is land slope in percent (%), Wb is the width of the bench, and U is the slope of the terrace riser or side slope. Use a horizontal to vertical ratio to put into the equation such as 1 for machine built terraces,
0.75 for hand-made terraces, and 0.5 for stone terraces.
Example: The VI of 4m-wide bench, machine-built, on 15 degree (26.8%) slope is as follows:
VI = (26.8 4) / (100 – 26.8 1) = 107.2 / 73.2 = 1.46 m To verify the validity of this simple equation, an equation used in Peru (Low & Paulet, 1967) was tested and the results were found to be the same. However, their equation is rather complicated, requiring two steps to get an answer and tangent values of two slope angles.
VI = 2d / (1 – tan α tan β ) (2) d = (Wb / 2) tan α α is the slope of the land in degrees and β is the top slope angle of the riser, in degrees, in relation to a vertical line. Wb is the width of the bench.
Another complicated equation using trigonometric functions was also tested and the same answer was obtained (Sheng, 1981). Consequently, it is appropriate to us Equation (1) which is simpler and easier to apply.
In Taiwan, three different equations are used for design of level, reverse sloped, and outward sloped bench terraces. (Chinese Soil and Water Conservation Society,1987). In fact, when width of bench and slope are fixed, VI does not change regardless of the type of bench terraces. VI is measured from center to center of the succeeding terraces which also marks the non-cut and non-fill point.
Fig.1 Same vertical interval (VI) between level and reverse sloped bench terraces 502
4 Final remarks The above-mentioned equations and calculations should be considered practical and sufficient in terrace design, bearing in mind that we are dealing with earth moving and farm work in the field. The survey, layout and construction of bench terraces are discussed in several publications (FAO, 1977; FAO, 1988; Sheng,1986).
Land slopes are usually not even and smooth. When we design terraces we need to use the mode slope or the representative slope of the site as the basis for calculation so that future terraces will produce an even width. It is important to have an even width to benefit farming operations, especially if machines are to be employed.
To design bench terraces for lasting uses, accessibility roads, waterways or irrigation installations should be considered integrally and early in the planning process.
Finally, bench terraces can be costly; they should not be built everywhere on the slope. For gentler slopes, and for semi-permanent and permanent crops, other simple conservation measures and inexpensive terrace systems can be applied (FAO, 1989; Sheng, 2000).
FAO. 1977. Guidelines for Watershed Management. Conservation Guide 1: pp. 147-179. FAO, Rome.
FAO. 1988. Slope Treatment Measures and Practices. Conservation Guide 13/3: pp. 33- 44. FAO, Rome.
FAO. 1989. Soil Conservation for Small Farmers in the Humid Tropics. Soils Bulletin 60: FAO, Rome.
Low, F., M. Paulet. 1967. Conservacion de Suelos. Lecture Notes. Universidad Agraria, Lima, Peru.
Sheng, T. C. 1981. Technical Notes on Terraces. UNDP/FAO JAM/78/006 Project Paper, Kingston, Jamaica.
Sheng, T. C. 1986. Watershed Conservation: A Collection of Papers for Developing Countries: pp. 17-22.
Colorado State University, Fort Collins, Colorado.
Sheng, T. C. 2000. Terrace System Design and Application Using Computers. In Laflen et al., ed: Soil Erosion and Dryland Farming: pp.381-390. CRC Press. Boca Raton, Florida.
The Chinese Soil and Water Conservation Society. 1987. Soil Conservation Handbook: pp. 11-18. Taibei, Taiwan.