Windbreaks and Shelterbelts

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Windbreaks can be defined as any type of barrier for protection from wind. Commonly, they are associated with mechanical or vegetative barriers for buildings, gardens, orchards, and feedlots. Living windbreaks are known as shelterbelts. These works consist of shrubs and trees that form barriers longer than windbreaks. In addition to reducing wind speed, shelterbelts result in lower evapotranspiration, higher soil temperatures in winter and lower in summer, and higher soil moisture; in many cases, these effects can lead to increases in crop yield. A shelterbelt is designed so that it rises abruptly on the windward side and provides both a barrier and a filter to wind movement. Such structures can reduce wind speed to less than half of that in the open, as show in Fig. 4.36 [39].

A complete belt can vary from a single line of trees, to two or three tree rows and up to three shrub rows, one of which normally is placed on the windward side. Belt widths may vary from about 9 m for a two-row tree belt, to about 3 m for a single-row hedge belt. Shelterbelts should be moderately dense from ground level to tree tops if they are

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Figure 4.36. Wind velocity distribution (40 cm above ground level) in the vicinity of a windbreak as a percentage of what it would be without the structure. Source: [39].

to be effective in filtering the wind and lifting it from the surface. The correct density is equivalent to a porosity of 40% to 50%. Less-dense barriers do not reduce wind velocity sufficiently. Field investigations have shown that, with the increase of the density, there is a greater reduction in wind speed initially. However, because the velocity increases more rapidly with distance downwind than in the case for more porous barriers, these works are effective only for very short distances [11]. Moreover, taking into account that the wind velocity at the ends of the belts is as much as 20% greater than velocities in the open, long shelterbelts are more effective than short ones [31]. Belt lengths should be a minimum of 12 times the belt height (H), which depends on the plant species selected. To allow for deviations in wind directions, a longer length is desirable and a length of 24 times the height is generally recommended [11]. In establishing the direction of shelterbelts, records of wind direction and velocity, particularly during vulnerable seasons, should be considered, and the barriers should be oriented at right angles to the prevailing direction of winds. Where erosive winds come from several directions, grid or herringbone layouts may be necessary. The requirement is to provide maximum protection, averaged over all wind directions and all wind velocities, above the threshold level.

Wind-tunnel tests have shown that the effectiveness of these works depends on porous break. Hagen [59] determined the relative wind speed and erosion for 20% and 40% porous windbreaks. According to Hagen's experiments, the velocity and erosion were reduced from 6 H windward to 24 H leeward with the 20% porous windbreak and from 6H windward to 32Hleeward with the 40% porous break. Erosion in the protected area was less than 50% of that in the open from 2 H windward to 16 H leeward, and from 2 H windward to 17H leeward, respectively.

The plant species selected should be rapid growing and resistant to wind, light, and frost. The branches should be pliable and the roots should provide a firm anchorage to the soil. In any case, preference should be given to local rather than imported species. These works need to be protected in their early years against livestock and spray drift.

Layout Wind Breaks And Shelter Belts

Figure 4.36. Wind velocity distribution (40 cm above ground level) in the vicinity of a windbreak as a percentage of what it would be without the structure. Source: [39].

Distance downwind in windbreak height units (meters)

Distance downwind in windbreak height units (meters)

At intervals of every three or four years, mechanical cutting is required to maintain the sharp rise from the ground on the windward side.

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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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