Mechanical methods are usually adapted to support agronomic and soil management strategies, tte methods adapted depend on whether the objectives for controlling movement of water over soil is to reduce the velocity of runoff, increase the surface water storage capacity or safely dispose excess water. We will briefly review only control strategies that help minimize runoff and soil loss. Construction of contour bunds and terraces are the most common strategies of mechanical methods for erosion control. Contour bunds are soil banks 1.5 to 2 m wide constructed across the landscapes with slopes of 1-7° as a barrier for runoff and form water storage areas and to break up the slope into shorter segments, tte banks are placed at 10 to 20 m intervals depending on the slope and extent of the landscape.
Soil characteristics such as texture can be considered in the design of the counter bunds and there are no precise specifications for their design.
Numerous studies have demonstrated a range of effectiveness in the construction of contour bunds (Hurni 1987; Clark et al. 1999).
Terraces are an earth embankment, or a combination ridge and channel, constructed across the field slope that intercepts, detains and safely conveys runoff to an outlet. Terraces are used to shorten the length of long slopes and serve as small dams to catch water and guide it to an inlet, ttey also serve as a guide for a contour row pattern in the field and help to improve water and soil quality. Terraces can be classified into three main types: diversion, retention and bench, tte primary purpose of a diversion terrace is to intercept runoff and channel it across the slope to a suitable outlet, ttese diversion terraces can be as narrow as 3-4 m wide and cannot be cultivated with machinery. Also for cultivation to be possible, the banks should not exceed 14 0 slope if small machinery is used or 8 0 if larger farming equipment is used. In general diversion terraces are not suitable for agriculture if ground slopes exceed 7° because of the expense of construction and close spacing required.
Retention terraces are used where water storage is necessary on the hillside and generally designed with a capacity to store the expected runoff volume, ttey are recommended only for permeable soils on slopes that do not exceed 4.5 Bench terraces are generally used where landscape with slopes up to 30° needs to be cultivated. A series of shelves and risers are constructed and should be properly constructed and maintained to prevent erosion. Vegetation cover or stones are usually used to minimize erosion and unprotected risers can be the source of most of the erosion in terraced systems (Critchley and Bruijnzeel 1995). Bench terraces are constructed in various modifications to suit the landscape, soil properties and rainfall patterns or water supply.
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