In this article we will discuss about the design and type of windbreak used to control soil erosion due to wind.
Design of Windbreak:
Windbreaks are the barriers, used to reduce and redirect the wind, with the objective to check the soil erosion due to wind. These are constructed with the help of trees and shrubs, but can also be by using perennial or annual crops and grasses, fences, or other local materials. The d/s area of windbreak is called sheltered zone. The reduced wind speed modifies the environmental conditions or microclimate of sheltered zone.
As the wind blows against the windbreak, then there develops air pressure towards windward side; and a large quantity of air get move up, and over the top or around the ends of windbreak. After crossing the top of windbreak, the wind currents strike at few distance away from the break to the ground surface, d/s side of break. The d/s striking distance depends on the height of the break.
The effectiveness of windbreak depends on the height, density, number of rows, species composition, length, orientation and continuity of the structure, described below:
The density of windbreak is defined as the ratio of solid parts (trees) of the barrier to the total area of the barrier. At greater density the tress/plants are very dense; or in other words windbreak becomes more solid as result the passing of wind through them is less. This causes into development of low pressure zone on the leeward side of highly dense windbreak. The low-pressure area behind the windbreak pushes the air which starts blow over the windbreak downward.
This action causes turbulence and reduces the length of area protection, downwind. In contrast, when density of windbreak is less, then the quantum of air passing through windbreak gets increase; and thus moderating the low pressure and turbulence. And there is increase in length of area protection, downward. In this way, by adjusting the windbreak density, a better wind blow pattern and area of protection can be achieved.
A windbreak/shelterbelt with the density of 40 to 60% provides maximum downwind area protection, along with excellent soil erosion control. The windbreak density depends on a number of factors, such as number of rows, the distance between trees, and species composition. The density gets increase at greater number of rows. The combine effect of windbreak/shelterbelt height and density determines the degree of wind speed reduction, and the length of protected area, too.
For a given height, the protected area usually gets increase as the density increases. However, if density is less than 20%, then windbreak does not provide useful wind reduction. If density is more than 80% then excessive leeward turbulence may reduce the windbreak effectiveness beyond 8H.
The cross-sectional shape of windbreak/shelterbelt with equal density develops minimum effect on wind velocity within 10H of the leeward side. Beyond 10H, the straight side provides slightly more protection than the slanted sides, because more wind gets pass through the plantations.
Effect of Orientation:
The orientation of windbreaks at right angles to the prevailing winds is most effective to check the wind erosion. Since, the purpose and design of windbreak is individual, therefore, orientation of individual windbreak depends very much on the design objectives. For example, the farmsteads and feedlots usually require protection from cold winds and blowing dust.
For this condition, the orientation of windbreaks perpendicular to the troublesome winter wind direction provides most effective protection. Similarly, the field crops need protection from the hot & dry summer winds, abrasion, wind-blown soil particles or both. For this specific situation, the most suitable orientation of windbreak is perpendicular to the prevailing winds during critical growing periods.
Also, the field windbreaks should be designed to suit the farming operations likely to be performed under cultivation system. The design should be based on the consideration of reducing wind erosion, providing crop protection, increasing irrigation efficiency and improving wildlife habitat. For controlling soil erosion, the windbreaks should be established to block the prevailing winds during greatest soil exposure-winter and early spring. Similarly, for recharging the soil moisture with drifting snow, the windbreaks should be established perpendicular to the prevailing winter winds.
However, it is not essential that the wind blowing predominantly in one direction in a specific season, would also blow in same direction in other seasons. For this situation, the orientation should be such which could be suitable for average blowing wind direction. However, as the wind changes its direction, and is no longer blowing directly against the windbreak, then protected area gets decrease. For this condition, the multi-leg windbreaks can provide a larger protected area than the single windbreak.
Effect of Length:
Although, the height of windbreak/shelterbelt determines the extent of protected area d/s, but the length of windbreak also determines the total protected area. For achieving maximum protection, the uninterrupted length of windbreak/shelterbelt should be kept more than the height, at least by 10:1 ratio. By this arrangement there is effect on end-turbulence on the total protected area.
The windbreak continuity also affects its efficiency. The gap in windbreak acts as tunnel, from which wind is likely to get concentrate, and creating effects on soil removal from there at high rate. This effect is called tunneling. The gap makes the wind break/shelterbelt ineffective. The design should be such that there should not be tunneling effect.
The reduction in wind speed behind the windbreak leads to develop a micro-climate with different wind velocity within the protected zone.
Various features of this zone are as under:
i. Temperature and humidity levels are normally high.
ii. Evaporation rate is less; and
iii. The plant water loss is also less.
The change in temperature from windbreak depends mainly on the break height, density, orientation and time of the day. Normally, the daily air temperature within 10H leeward of windbreak is several degree higher than the temperature in the open. While beyond 10H, the air temperature near the ground to be cooler during the day.
On most nights in sheltered area (up to 30 H) the temperature near the ground is slightly warmer than in the open. However, on very calm nights, the temperature of sheltered areas may be several degrees cooler than the open areas.
The soil temperature in sheltered areas is slightly greater than the unsheltered areas. In this way, taking the advantage of greater temperature an earlier planting and better germination can be have with short growing duration.
In the area next to east-west windbreak, the soil temperature is significantly higher towards south side due to heat reflection by the windbreak. On the north side of east-west windbreak, the soil temperature especially in the early spring is low because of shading effect developed by the windbreak.
The relative humidity in sheltered area varies from 2 to 4% higher than the open areas, depending on the windbreak density. Since, high humidity decreases the rate of plant water use, therefore, production is more efficient in sheltered area as compared to the unsheltered areas.
However, when windbreak is very dense, then humidity level becomes very high. In this condition there is possibility of development of disease effects in few crops. The heat loss due to wind-chill is reduced towards leeward side of windbreak.
Of a single row windbreak the design is carried out under following points:
i. It is used for field or orchard protection.
ii. Dense plantation for maximum effect.
iii. Maintenance and replanting to avoid the gaps from dead or weak trees.
iv. Use shrubs and dense evergreens that retain lower limbs and foliage like junipers, spruces, and densely branched deciduous trees, preferably with narrow crowns.
The followings are the important features of this type of wind break:
i. It is used for field or orchard protection, outdoor barn, or urban setting; and also for few wildlife-values.
ii. Densely planted as in single row system.
iii. Plants are alternately planted with a space in one row filled by the tree in the other row; rows are close together.
iv. Standard two-row wind break uses two rows of dense evergreens, or one evergreen and one shrub or deciduous tree row; standard between-row spacing.
Its various details are outlined as under:
i. It is for field protection of bigger area, farmstead and feedlot protection; wildlife conservation with the row of food-bearing shrubs.
ii. At least one row of dense evergreens, and other two rows of deciduous trees, shrubs, or pines.
iii. Standard between-row spacing.
It is established for farmstead and feedlot protection; and is excellent for wildlife.
i. Uses mix of shrubs, deciduous trees, and at least one row of dense evergreen trees.
ii. Standard between-row spacing.
iii. Can alternate similar species within rows.
iv. Greater design flexibility; uses a lot of land.
Types of Windbreak:
The windbreaks can be of following three main types:
These windbreaks are used for protecting the buildings, barns, greenhouses etc., from strong winds. Also, they create comfortable living environments, d/s. The effectiveness of farmstead windbreaks depends on the density of plantation. Normally, medium density is more suitable.
Windbreaks with 60 to 80% density provide very good protection over a fairly small area like a farmstead, residential lot, or feedlot. Their design and construction is done by keeping the view that they will provide full protection to the buildings; and also maintain a cool surrounding by allowing some air through windbreak.
The air passing during cool winter cause little rise in temperature, while during summer season develops cooling effect in the surroundings of the windbreak established in north and west sides of building, at the distance of about 30m from the building, the effectiveness gets increase. Fig. 7.7. Illustrates the view of farmstead wind break.
Function of these windbreaks is to control soil erosion and protect the crop against turbulent winds. These windbreaks are of medium density. A windbreak with 40 to 60% density can protect a large area like crop field. Windbreak with density below 20% provides little wind reduction.
A single row of tall variety vegetation planted on north and west sides of the field is the best location for such type of windbreak. However, planting on all sides of the field provides more conducive effect on protection. This type of windbreak also provides a good protection during summer, when plants are full flourish with leaves.
The species such as silver maple, green ash and few poplars are found better for this type of windbreaks. It is suggested, that there should planted one row of shrubs along with hardwood trees to fill the gaps that normally takes place, when the trees become taller, and lose their bottom branches. The plantation of shrubs should be done parallel to the break at the distance of about 3 m from windbreak on the upwind side. The view of field windbreak is shown in Fig. 7.8.
These windbreaks are established for trapping the snow before it drifts onto lane ways or farmyards. These are the high-density windbreaks. A single row of white cedar or 2-rows of spruces is found better for establishment.
In case of double row system the row to row-spacing can be 3 m. Most of the snow gets pile up within 10 to 15m of windbreak. The location of this type of windbreak should be at the distance of about 20m from the nearest building, roadway or farmyard. Fig. 7.9 shows the view of this type of wind-break.
Overall, the selection of trees and shrubs species for establishment of windbreak should be based on the following features:
ii. Good form and foliage
iii. Fairly fast growth
v. Low maintenance
vi. Pest resistance, and
vii. Suitable for planting site and that fit to windbreak design.