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Sometimes the wind comes from the west bringing heavy rains. So in this breakks you have a constant wind Aisan a Asain wind and one that indicates change. In New Zealand we Asian wind breaks a constant wind that blew almost always from that direction. Windbreaks were orientated for that but once or twice a year flat line storm winds would blow off the coast brsaks have damaging winds to crops. Eind neighbor had half his corn crop flattened after weeding that part earlier wnd the day. Meanwhile Asan I worked in South India in Aurovillethe drying winds were like putting a Asian wind breaks dryer on ones head for what seemed like endless hours of the day.
This parched the landscape and made food production a challenge for sure. Shade structure for vegetable production in India With that, windbreaks should be sighted perpendicular to wind flows you are dealing with. This allows for the wind to be diffused at the most efficient way. Spacing With windbreaks, trees are spaced closer than normal recommended plant spacing of the same species as to tightly pack branches and leaves together. Whether that is in the same row with the same height of vegetation or in between rows of differing species and heights, the trees are packed in tightly.
The windbreaks themselves are spaced based on the ratio that the ultimate height of the tallest tree will afford you times the protection area. This means it allows the wind through, but slows it considerably at the same time. Secondly, the extent of the area which is sheltered by the windbreak is largely determined by the height. A windbreak will typically shelter an area behind it which is 10x to 15x the height of the windbreak. Windbreaks can be constructed either from living materials, i. In the latter case it is very important to use an open framework with plenty of gaps to allow the wind through.
In practice most small orchards and many large ones use living windbreaks, formed from trees which naturally have fairly dense foliage.
When planted close together such trees are highly effective at braking the speed of the wind. Tree varieties for orchard windbreaks Whilst any tree species can be used for the purpose, orchard windbreaks traditionally use Alder species. However, the fluid dynamics of flow through vegetation barriers are based on universal laws of physics, so simulating flow through a coastal shelterbelt is Asian wind breaks different than simulating flow through agricultural shelterbelts, provided that the characteristics of the coastal vegetation see Section 2. Windbreaks consisting of shelterbelts one or two rows of trees and forest belts multiple rows are commonly used at inland locations as natural barriers to reduce windspeed, modify the microclimates of small regions and suppress the movement of snow, pollen, dust, sand and odours.
Therefore, the methods used and results derived from studies of agricultural shelterbelts can be applied to coastal shelterbelts and forests as well. In this section we discuss the knowledge base on shelterbelt design and application as it has been established through research on agricultural shelterbelts. In Section 3 we apply the qualitative results of these agricultural applications to the design of coastal shelterbelts and forests. Windbreaks substantially reduce windspeed on the windward side for a horizontal distance of 2—5 H, where H is the height of the barrier Figure 3. A much larger region of reduced windspeed, typically extending from approximately 10 H downwind to 30 H downwind, is created in the lee of the barrier, with the sheltering effectiveness near the barrier being determined by the incident angle of the wind to the shelterbelt Wang and Takle, a.
Some very limited windspeed reduction as far as 60 H downwind has been reported Caborn, ;but the biological or practical significance is considered to be quite limited Brandle et al. However, as will be shown in Section 2. The solid vertical line marks the downwind edge of the shelter 2. These factors determine the overall size and characteristics of the protected zone. Each will be briefly described. In shelterbelts with species of various heights used throughout the belt, the average height of the tallest species usually is taken to represent H.
In plantings with distinct regions of short and tall species, multiple heights may need to be specified see Section 2. The protected distance is measured from the most leeward row of trees in the windbreak. The fraction of unoccupied volume of a windbreak, or porosity, is sometimes used to describe the shelter. With these definitions, the sum of the density and porosity is 1. Although simple to define, these factors are difficult to specify for a given shelterbelt. Wind flows through the open portion of the shelterbelt, so a shelter of higher density allows less air to flow through and forces more air over its top. The length of the protected region depends on the shelterbelt density.
A solid barrier forces all air to flow up and over its top, creating a downward flow behind the barrier that allows for only a short protected area. On the other hand, a very porous vegetative barrier does little to slow the wind and is not very effective for sheltering purposes. U is windspeed in the lee of the shelter and Uo is the speed at the same location if there were no shelter. This figure shows that very porous shelters are only marginally effective in reducing windspeed. Nearly solid barriers see porosity equalling 10 percentby contrast, sharply reduce the windspeed immediately behind the shelter but allow the windspeed to recover to its undisturbed value more quickly, thereby limiting the leeward extent of the sheltered region.
A shelter of porosity equalling 50 percent reduces the windspeed to 40 percent of its undisturbed value and creates a protected region of greater length than the shelter of porosity equalling 10 percent.
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For numerical models that simulate the detailed characteristics of the flow field Asian wind breaks and around the shelter, a more practical measure is the surface area per unit volume A of the shelter Wang and Takle, a. In a physical sense, it is the drag force created by individual plant parts that extracts momentum and energy from the moving air, so the surface area of these plant parts is highly relevant to determining the mechanical influence of the barrier on the flow. Many tree species consist mostly of open space with a shell of leaves or needles at the outer boundary, in which case A is a function of location within the shelterbelt.
Some shelterbelts using tall tree species have been deliberately pruned to create open space near the ground to allow modest low-level air penetration in exchange for a longer Asian wind breaks distance. Seasonal changes in the vegetation surface area, such as for deciduous trees, will change A and hence, the sheltering function throughout the year. Leaf-area index LAIa biophysical parameter frequently used to describe stands of trees and crops, is sometimes used as a measure of surface area per unit volume of the shelter.
The density of the shelter, and particularly the presence of gaps in forest belts, also influences how well the trees can survive high winds Quine, The processes of gap formation and expansion are related to the spatial and temporal variability in occurrence of strong winds. Once a gap forms it can expand at lower windspeed than that required to form new gaps Quine, For winds oblique to the shelter, the protected zone is restricted to a region close the leeward edge of the shelter see Figure 3. Wind directions oblique to the shelter at high angle of incidence nearly parallel to the shelter can create recirculation vorticies in the lee that can locally increase surface windspeeds over those in unprotected areas Wang and Takle, a.
Wind flow around the lateral edges of a shelterbelt reduces the effectiveness of wind reduction in the protected zone, but increases the lateral influence of the barrier in ways that can increase particle deposition see Section 2. The length of a shelterbelt should be much greater than H. Both observations Read, and numerical models Wang and Takle, b showed that narrow shelterbelts consisting of two to three rows of trees can create nearly as large a protected zone as much wider belts. As can be seen in Figure 3. This is an important factor for applications of shelters in locations such as agricultural areas, where it is desirable to remove only minimal land area for planting shelterbelts.
For the combined purposes of reducing windspeed, increasing salt deposition and reducing storm surge in coastal areas, however, wider shelters may be more advantageous as will be discussed in later sections. Gaps in windbreaks tend to compress the flow and create localized jets in the near lee that can have adverse effects in the protected area. Gaps may have less negative impacts on particle deposition see Section 2. As will be discussed in Section 4, biophysical factors may be more important than sheltering characteristics in determining the species to be used on the windward seaward side of the shelter.
Both mean and turbulent components of the wind are reduced in this region. Large turbulent eddies of the undisturbed flow are broken down into smaller eddies as they pass through the barrier. These smaller eddies dissipate rapidly in the quiet zone. The leeward edge of the shelter is indicated by the solid vertical line The wake zone is characterized by windspeed increasing in downwind distance, first abruptly and finally with very little increase with increasing distance from the shelter. In unprotected areas, the vertical component of turbulence counteracts gravitational fall to keep particles from settling out of the flow field.
Asian wind breaks The elimination of large turbulent eddies in the quiet zone of a shelterbelt allows gravitational settling to dominate over upward turbulent transport, which enhances particle deposition in the lee of the shelter. Tamate discussed Japanese use of shelterbelts to reduce airborne salt movement in the coastal zone. He reported that airborne salt concentrations in the lee of shelterbelts were wijd to be 12 percent lower than winr the windward side. Because there is scant research elsewhere describing the role of Asian wind breaks barriers in reducing onshore movement of spray droplets and sea-salt bresks, an example taken from the dispersion of pollen from a field of maize with and without a protective barrier will be used to illustrate the effect.
Combining the flow model of Wang and Takle a which has been shown to be in broad agreement with measured flows near shelterbelts with the model of Arritt et al. Breams and Y distances are in metres. The colour scale indicates the number of particles more or less than deposited without the shelter Figure courtesy of C. Arritt Particles are assumed to be flowing from left to right, and the change in deposition over an unprotected area is plotted. The plot shows the increase beaks the number of particles deposited over the amount deposited in the absence brexks the shelter. Results breaos that there is substantial deposition yellow and red immediately downwind sAian the fence at the leeward edge of the field.
The influence of the shelter is seen to extend laterally beyond the edge of the field and broaden out with downwind distance. Even in the far lee beyond metres, or 30 H the fence produces enhanced deposition compared Asian wind breaks an unprotected field. Close inspection wine that some enhanced deposition also takes place immediately upwind of the shelter. Only in a narrow region immediately behind the upwind and downwind edges of the field is the deposition behind the shelter less than would occur without the shelter.
For application to the capture of sea spray and salt particles by coastal forests, we can interpret these results as follows. Compared to a shoreline with no vegetation, a coastal region having shelterbelts or forest fragments will provide regions where locally reduced wind allows droplets and particles to settle out by gravitational settling and by capture due to flow through the vegetation. Even a forest block in analogy to the fenced region shown in Figure 3. Because drifting sand creates a hostile environment for developing plantings of woody vegetation, use of grasses for holding the dune intact and preventing saltation launching of sand particles from the dune by wind action is the first goal of such restoration.
Low levels of organic matter and hence water-holding capacity and nutrients particularly phosphorus and nitrogen limit the types of grasses that can be used. Soil amendments in the form of manures, leaves, detritus and so forth provide both nutrients for recycling and a means of enhancing plant-available water. Groundnuts provide crop residues rich in nitrogen, and residues from crops in close proximity have proved to be successful sources of such materials Jerve et al. Local climate considerations must be addressed to cope with the possible loss of nutrients particularly nitrogen due to leaching if heavy rains are a frequent occurrence, and drought-tolerant species if a prolonged dry season is a natural part of the local climate.
Reclaiming such sandy areas has been proved difficult but successful, and may take a few years to accomplish. As there is very little, if any, economic return from these initial labour investments, the use of such areas to develop the local agricultural infrastructure will require financial investments in advance. Land-management policies allowing personal ownership or long-term leases provide incentives for the continuation of reclamation practices through the initial revenue-less period. As soon as the dune is stabilized and moving sand is suppressed, seedlings of woody perennials such as shrubs and trees may be introduced.
Establishing the shelterbelt on dunes or protective dykes serves multiple purposes. The additional elevation allows better inception of high winds and a deeper layer of the atmospheric boundary layer for the capture of sea spray and salt particles see Figure 3. The presence of woody vegetation also helps to protect the dune or dyke from erosion or from being a source of dust or sand moving inland. Such combinations may, however, provide alternatives that protect newly planted trees from excessive damage, increase the density of a shelterbelt in places where available tree species do not provide sufficient vegetative mass, and reduce the area needed to achieve a particular sheltering objective.
Of particular concern for shoreline environments is the tolerance to high salt conditions. If the leading edge of the shoreline shelter is to experience regular immersion in saltwater, the most likely choice is a species of mangroves, owing to their high salt tolerance and tolerance of frequent or constant inundation by seawater. Locations beyond the inundation zone have more options for species and allow for the consideration of characteristics that provide ecological and economic benefits in addition to more effective sheltering properties.
To enhance the sheltering function, tall trees with strong rooting characteristics, rigid branches and dense foliage would be optimum.