Permaculture Designers Manual
CHAPTER 5 – CLIMATIC FACTORS IN PERMACULTURE
Section 5.6 –
Wind in Permaculture
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Both wind and water transport can influence (by impedance or reflection) the quantity or light and heat on any site.
Particles or molecules carried in air have a profound effect on available light and heat can easily be transported about our system by air and water or by substances mixed in with them.
Of all of these elements, we have least control of wind in terms of storage or generation, but we can control its behavior on site by excluding, reducing or increasing its force, using windbreaks and wind funnels to do so.
As a resident of a bare, cleared (once-forested) peninsula, when I speak of wind I know of what I write!
The very table on which this book is penned rocks to gusts from the “roaring forties” and when the wind blows from the east, spicules of Silt form on beards, clothes lines and plants, burning oil leaves and killing plant species; some hardy plants that withstand years of normal gales can die in salty summer winds.
When it comes to crops, winds of 8 km/h are harmless. Those of 24 km/h reduce crop production and cause weight loss in animals, and at about 32-40 km/h, sheer mechanical damage to plants exceeds all other effects; in fact, I have seen my zucchini uproot and bowl along like tumbleweed.
Trees are severely wind-pruned by a combination of mechanical damage and salt burn near coasts, and by the additional factor of sandblast in dunes (desert or coasts) and ice blast in cold climates.
Wind transport of sands in deserts and incipient deserts buries fences, buildings, trees and crops.
Although many sites are little affected by wind as a result of fortunate local conditions, or a general low level of wind effect in a region, very few coastal, island, sub tropical or exposed hill sites can afford to ignore this factor.
There are broad categories of wind speed and effect, just as there are for rainfall and temperature. The Beaufort Scale is the normal way to report winds, and equivalents are given in Table 5.5
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More severe than mechanical damage are the minute sulfur and nitrogen particles carried by wind.
In Colorado, Virginia, Utah, and The Urals and in fact anywhere downwind of nuclear waste stores, tests, and accidents we can add plutonium and other radio-actives to the wind factor.
Dry sulfur, falling on leaf and soil, converts to acid in misty rains. On parts of the northeast coast of America and Canada, these rains can burn gardens and forests, or make holes in garments and tents in a few days.
Near acid production factories, even paints and roofing are pitted and holed; this factor has become general in industrial areas and for many miles downwind.
Windbreaks may mean the difference between some crop and a good crop, but in severe wind areas, the difference is more absolute and may mean that susceptible plants will produce no crop at all.
Thus, a list of wind-tolerant (frontline- trees) is a critical list for food production and animal husbandry.
DIRECTIONS AND SEASONS
Winds are fairly predictable and often bi-modal in their directions and effects in local areas.
For the landscape designer, wind-flagging or older trees and wind pruning tell the story; the site itself has summed total wind effects over time.
From latitudes 0° to 35° north and south in oceanic areas, winds will be bi-modal and seasonal, southeast or northwest in the southern hemisphere, southwest or northeast in the northern hemisphere.
Locally, the directions will be modified by landscape, but the phenomena of windward and leeward coasts are almost universal.
From latitude 35° to the limits of occupied coasts, westerly’s will prevail in winter, easterlies more sporadically as highs or lows pass over the site and remain stationary to the east or west.
Cold winds will blow from continental interiors in winter, and warmer but still chilling winter winds blow in from the seas.
Islands and peninsulas from latitudes 0° to 28° experience two main wind modes, those of the winter winds or trade winds (southeast in the southern hemisphere, northeast in the northern hemisphere) and those of the monsoon. In effect, we look for two seasons of winds and two short periods of relative calms or shifting wind systems in these latitudes.
These are the main winds of tropical oceanic islands.
In summer, the cross-equatorial monsoon winds deflect to blow from the northwest in the southern hemisphere and as southwest monsoons in the northern hemisphere.
Southeast Asia and the Pacific or Indian Ocean islands are most predictably affected by these bi-modal systems.
Although many sites are also affected by only two main strong wind directions, these are rarely as strictly seasonal in effect as they are closer to the equator.
WIND LOADS
On warm sea coasts, where onshore winds not only carry salt but also evaporate moisture, salt deposits on vegetation are the limiting factor on species selection and only selected hardy species with fibrous or waxy surfaces can escape death or deformation by salt burn.
As well as inorganic materials, wind may transport organisms ranging from almost impalpable spores of fungi and ferns to very weighty insects such as plague locusts which are swept aloft by heated air columns, and carried as frozen or chilled swarms to downdraught areas.
Here, they thaw out and commence feeding or perish in oceans far from land.
Mosquitoes, fruit flies, wasps, and spiders deliberately spin aerial float lines and also migrate over mountain and oceanic barriers on wind streams.
Flocks of migrating birds also take advantage of wind streams as they circle the globe.
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Flow of air (wind) over leaf surfaces promotes rapid transpiration, as does, high light intensity (Dauben-mire, 1974).
When we have both affects together, shrubs and trees may lose too much water, and trees guard against this combined factor by presenting whitish undersides of leaves to the light as the wind blows, thus carrying on a dynamic balance between the light and wind factors.
Vines and trees may alter leaf angles to reflect light, trap air, or to reduce the area of leaf exposed to light or wind.
Thus, both pigmentation and leaf movement are used to balance the effects of variable incoming energy and leaf pores close down to prevent moisture loss.
WIND HARMONICS
In a radio program on sailing (Australian Broadcasting Corporation, 19 Dec ’84), Frank Bethwaite, a New Zealand-born Australian boat designer, pilot, and sailor outlined some of the characteristics of ground winds.
Such winds do not blow steadily, but vary as gusts and calms in a predictable and locality-specific way; that is, the common winds of any one site have regular pulses.
He states that such regular variations are easily timed; a 49-60 minute frequency of gusting is typical of mid-latitudes, with gusts 40% stronger than lulls.
In lulls, the wind direction also changes, as light crosswinds at about 15° to the main wind direction.
The variation in wind speed and direction is systematic and regular and frequencies, durations, and amplitudes can he obtained by combinations of stopwatches, anemometers, and wind vanes (or all of these recorded on automatic equipment).
Such “waves” of wind are made visible in grasslands or on the surface of waters viewed from a cliff.
They are also, at times, reflected in clouds as “rank and file” systems.
The lulls show as spaces between cloud ranks, and in these spaces, light clouds of different alignment represent the change of direction typical of lulls.
The gusts are ponderous, representing vortices; the lulls are of light crosswinds.
Some periods are short (Bahrain, 5.25 minutes; Sydney, 6-12 minutes; Toronto, 10 minutes).
Wave “fronts” on grasslands may come every 14 seconds, with gusts at longer intervals.
Sea waves themselves have a characteristic periodicity and speed, usually about 5-12 per minute, the period lengthening in storms.
In the westerly wind belts, we can distinguish between the PREVALENT WINDS of from 8-24 km/h. which blow for five out of seven windy days, and the ENERGY WINDS of from 16-40 km/h which blow on the other two days.
The energy winds come from between 15°-20° off the direction of the prevalent winds (Michael Hackleman, Wind and Wind Spinners, Peace Press, California, 1974).
WINDBREAKS
It is the chill factor – the removal of heat from surfaces and evaporation of fluids – that creates cool to cold climates in the tropics at lower altitudes than adiabatic or altitude factors would indicate.
This chill factor retards plant growth and lowers the efficiency of solar devices and insulation.
In cyclonic or hurricane areas, catastrophic winds may become the over-riding design modification, around which all other factors must be arrayed.
We are not much concerned with sheltered and low wind-energy sites, except to choose them for our dwellings in exposed landscapes, but close attention must be paid to shelter strategies in exposed sites.
On sites with predictable wind patterns, revealed either by trees, derived from local knowledge, or indicated by wind records over time, we can plan directional, patterned windbreak of earth bank and trees.
On sites where severe winds and sandblast may come from any direction (as in some deserts), the strategy is to impose a close rectangular or network pattern on windbreak.
However the windbreaks are arranged, buildings, gardens and animal shelters can be arranged to face the sun and benefit from solar impact.
Essentials of a Windbreak
The essentials of windbreak are fairly well known and local lists of species for windbreak are often available from forestry and agricultural advisors or departments.
Essentials are:
Good species selection to be used as pioneers (easily grown);
Initial protection of planting from mechanical or wind damage (bagging, fencing);
Periodic or trickle irrigation to reduce desiccation;
Anchoring by stones or mulch;
Species with 40-50% penetrability in the front line or as dominants.
Many fire-resistant plants are also wind resistant and in addition to these, some drought-resistant but fire-prone species (pines) will withstand wind.
What they have in common are ways of resisting desiccation and sandblast.
Such plants have, as common features:
Fibrous stems (palms).
Fleshy leaves (aloes, agaves, Euphorbias).
Hard, needle-like leaves or stems (pines, tamarisks, Casuarinas, some Acacias).
“Furry” or hairy (tomentose) leaf covers. or waxy leaves (Coprosma, eucalypts, some pines, some Acacias).
Initial protection can come from:
Individual open-ended plastic bags around stakes (a common and effective establishment method).
Earth mounds or side-cast earth bank of greater length than the tree line.
Brush fences, even wire mesh fences or staked fences with 40% wind penetrability.
Tussock or tough un-mown grass to windward (leave if already present).
All of these can be used in combination in very hostile areas.
It is usual for the windward row of trees to be heavily wind-flagged, and for taller species to be placed in their lee.
On coasts and in deserts, it is not until after the fourth or even filth tree row evolves that wind-prone fruit or nut-bearing trees will yield, so windbreak is the first priority for gardens in these situations.
Substantial trellis is a more immediate alternative, but care should be taken to make this sinuous (if made of brick or mud brick) or zigzagged (if made of timber), as it has to withstand persistent and severe forces until shelter grows on either side of it.
Earth mounds can be better streamlined, being less sensitive to wind throw.
The hollow from which the earth is taken to make the mound can be made to hold water or to give protection to young plants.
Tire walls are sometimes feasible, and create great warmth inside the tires, but are scarcely anesthetic unless very regularly arranged and planted.
They have the advantage of being cheap and can be removed once effective.
Mesh fences, if stoutly built with a heavy top rail, can be the basis for ledges (fence-hedges) of thick-leaved vines, which on coasts may completely mound them over with tough semi-succulents such as Rhagodia, Tetragonia, Carpobrotus, or Mesembryanthemum.
Rock walls and tires may be similarly mounded with scramblers or cacti, some of which provide bee forage and berries or edible fruits.
It is rare for tree canopies on dry salt wind coasts to gain more than 46 cm height in 1 m width (18 inches in 3 feet), so considerable width must be given to pioneer windbreaks in these situations, unless those hardy pioneers such as the Norfolk Island pines can be nursed to grow to windward.
However, as this slow climb to height commences from ground level, a fence, building, earth bank or barrier gives it a great start for far less spread. (See Figure 5.15 D)
Even a 46-62 cm (18-24 inch) high “fence” or mound earth will grow a sweet potato, strawberry, or cabbage
In the lee, while hard-pruned canopies need not be barren, as many dwarf fruit, vine, and flower crops will grow below these if mulch and water are provided.
In windbreak forests near coasts, small openings of 6 – 9 m (20 – 30 feet) provide garden shelter and admit light.
There is, in fact, a special charm about those 3 – 3.5 m (10 – 12 feet) high dense coastal shrubberies in which nestle small shacks, through which wander sandy paths and in which people create small patches of scattered garden using wastewater and mulch.
Once shaped, fruit trees in this situation seldom need pruning and at times one wonders if the wind is not an advantage in that it forces compact and careful work, punishes carelessness and promotes wastewater use.
Across the whole of the flattish peninsula of Kalaupapa on Moloka’I, the Hawaiians had built tiny stone fences of 25-50 cm (10 – 20 inches) high and only 4.5 – 5 m (15 – 18 feet) apart, behind which they grew a basic sweet potato crop, and in which grew tough fern for mulch.
All are now abandoned, but on the seaward coast, wild date somehow struggles to 4.5 m (15 feet) or so in the teeth of the trade winds and would have made a grand windbreak had the Hawaiians retained their land against tourism and grazers.
Just to windward, the strong winds bring so much salt spray ashore that it crystallizes out in pinkish ponds, mixed inexorably with the red of the volcanic earth on which it forms.
Even today, it is gathered as “Hawaiian salt“, and is further mixed with the roasted kukui nut and chilies for a delicious raw-fish condiment.
Some Benefits of Shelterbelt
1. Shelterbelt effects on house design
For glazed areas and hot water (flat plate) collectors, wind chill factors remove 60% of heat alone.
Shelterbelt (including thick vine trellis) around a house can affect a 20-30% saving in heating fuels in moderate to severe winters.
Thus, in cold areas earth banks plus a shelterbelt and a sun-facing aspect, is a critical design strategy.
In deserts, where adverted (wind-carried) heat is the most severe effect on human comfort, shelterbelt trees serve to reduce ground temperature up to 15°C.
2. Effects of exposure on livestock
Blizzards will kill livestock and newborn lambs, and even hardy and adapted animals can lose 30% of their bodyweight in 3 days of blizzard.
As well as shelterbelts in fields, we need to be very careful to design fences so that they do not form downwind or down slope traps, as herds escaping blizzards will pile up against them and smother in fenced corners.
All moorland and high plateau fences should allow easy downwind escape to woodlots, sheltered valleys or lower elevations.
In less severe conditions, sheep weight in unsheltered fields in New Zealand is 15% less than that of sheltered areas.
Australia attributes 20% of all lamb losses to wind chill factors and issues regular wind chill warnings at shearing time to prevent adult sheep loss.
Cattle fed winter rations on exposed sites will eat 16% less of this food, so that winter hay and concentrates need to be fed out in shelter for animals to obtain full benefit.
Both heat and cold have similar effects on weight gain and a shelterbelt is one of the most effective ways of increasing livestock production, and conserving rations.
Thus, in designing for livestock, fences, shelter, and access to shelter, feeding and watering points need sensible placement, so that animals are not exposed to extreme temperatures.
In the tropics and subtropics, a ridge planting of pines or Casuarinas with a wind gap left below the crowns affords both shade and an induced breeze that discourages flies and mosquitoes.
Such ridges are also rich mulch sources for lower slopes.
3. Civil Construction
Snowdrift across highways is more effectively and permanently blocked by hedgerow of hardy Caragana and Eleagnus, which are estimated to be 50% cheaper than stout fences and of course outlast them.
Juniper in high country actually grows better in areas of snow drift (below the sharp ridges where snow forms cornices) and swales at such places enable more snow melt, therefore more available root moisture for trees in spring and summer.
Wind shear on exposed highways or at caravan parks can cause casualties and property damage, so that we need to design wind fast median strips and highway shelterbelt in areas of known hazard, but especially on mountain passes and near exposed coasts subject to gales.
4. Shelter in and around croplands and orchards
For croplands, a matrix of shelterbelt species 10 – 16 m in height and 33 – 66 m apart, (Casuarina, poplar, Matsudana willow and trimmed eucalypt) affords wind protection for such crops as kiwifruit and avocado and give the greatest increases in yield while reducing wind damage to fruit and leaf.
For instance citrus culled as damaged is 50% of the crop in unsheltered areas, versus 18.5% in shelterbelt systems, cotton yields are 17.4% higher within five times the height of the shelterbelt and fall off to a 7.9% advantage at ten times the height of the belt.
Effects of shelterbelt are compound and include more melt water from snow, much greater fruit or seed set in bee-pollinated crop, the preservation of good shape in the trees, hence less pruning.
Species selection of shelterbelt trees is essential and a set of factors can be the criterion that assists the farm enterprise sheltered.
These Include:
Nitrogen fixation or good mulch potential from leaves and trimmings;
Hosting of predatory insects or birds that control crop pests;
Least moisture competition with crop (although roots from the shelterbelt can be ripped or trenched at the edge of crops);
Excellent forage yields or concentrated foods for livestock;
Natural barriers to livestock (thorny plants, or woven hedge).
The Shelterbelt is planted as a succession from a tall grass to a taller legume to a long-term, tall, wind fast hedge of e.g. Casuarina, poplar, willow, eucalypt, oak, and chestnut.
This entire complex can be set out at once, and managed as it evolves to maturity.
Quickset (by cuttings) hedges of poplar and Erythrina are popular because of their fast windbreak effect, but species must be chosen to suit a particular climate.
Where space is ample and winds strong, the profile of a windbreak can be carefully streamlined, and up to six rows of tree and tall grass lines established, giving a mixed yield of forage, timber, fuel, mulch, honey, and shelter.
In more constricted areas, a matrix of single tree lines is usual, and effective if close-spaced.
However, there is no such thing as a standard shape or windbreak, and very different configurations are needed for different sites, functions and as accessory species to the enterprise sheltered, the wind strength, and the wind load (salt, sand, dust).
5. Effects of windbreak on soil moisture
A Windbreak is very effective in snowy areas, increasing soil moisture 4% to four times the height of the break, and that to 1.2 m depth in soils.
Obviously, the benefits to trees in cold deserts are as a reserve of soil moisture that is rare in cold, dry climates. The same effect occurs locally in the lee of tussock grasses, and can be used to establish a tree.
In foggy climates or facing sea coasts, we must add the effect of sea air condensation, which can be from 80% – 300% of rainfall as leaf drip. In hot deserts and hot winds, the advected hot winds are the major factor in soil moisture loss.
Such effects are produced over large treeless areas of dry grain crop as well as in deserts.
The effects on grain crop of windburn and seed shattering in hot winds is insignificant for up to 18 times the height of the windbreak.
6. Less soil loss due to windstorms
Very serious soil losses of up to 100 Ton/ha/day in dust storm episodes (usually followed by torrential rain) are prevented by windbreak and soil pitting with tussock grasses.
Approximately 50-70% of dusts settle out of the air 100m into tree clumps, so that tree lines are the essential accompaniment to any pastoral or crop system in arid areas.
On coasts, removal of mangroves and coastal dune vegetation results in a sudden acceleration of wind erosion on beaches and coastal soils, and following deforestation, up to 30% more silt per annum, flows into and reduces the useful life of water storages.
7. Windbreak and hedgerow as accessory to crop and livestock
Quite apart from the above effects, windbreak species can be chosen to provide excellent crop mulch (Prosopis, Acacia. Erythrina, Melia, Canna) and fodders (all the foregoing species plus Leucaena, Fig, Pennisetum), and also to fix or recycle nitrogen and phosphoric fertilizers or to mine trace elements (Casuarina, Banksia, Eucalyptuss camaldulensis).
Dry or cold-deciduous species and monsoon deciduous trees give a natural leaf fall in crop, automatically adding growth elements to the crop.
In every crop and orchard it is advisable to interplant leguminous trees for mulch, soil building and in-crop windbreak or frost cover.
Trees like avocado and crops like papaya can be grown on sub-tropical frosty sites providing there is a high canopy of hardy palms or light-crowned legumes (e.g. Butia palm, Jacaranda, Tipuana tipu).
Such sites do not frost, as there is no bareground radiation at night, and advected frost is impeded.
Finally, forage and firewood from windbreak provides excess fuels to cook crop products, which is an important factor in the third world.
In summary, a well-chosen and designed windbreak can occupy up to 30% of the total area of any site without reducing crop yields, and if windbreak species are chosen that aid the crop itself, there will be an increase in total yield, soil quality and moisture available.
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Hedgerows and Shelterbelts
Shelterbelt species must be carefully selected to give multiple uses, to either ASSIST the crop yield, or ADD TO the end use yield (e.g. forage trees in pasture).
This ensures the area occupied by shelterbelts adds to the total crop yields, rather than deducts from them.
In general, we would gain in crop or pasture yield using nitrogen fixing and browse edible shelterbelt species, lose crop yield by using high water-demand, nonleguminous and inedible shelterbelt species.
However, where we experience severe sea or desert winds, which greatly reduce all yields, we must select salt-resistant or sand-blast-resistant windbreak no matter what the intrinsic yields of the shelterbelt.
It is rare for sea-front trees to bear effectively (e.g. the outer 4-5 rows of coconuts on exposed Islands yield little crop). so that choice of frontline seacoast plants for seed or fruit yields is often irrelevant when considering species for multiple function.
For isolated trees, or trees whose canopy lifts above the general forest level wind of even low speeds may increase the transpiration rate, sometimes doubling water use.
The effect is greatest on water-loving plants, and much less on dry-adapted species which have impermeable leaf cuticle and good control of stomata, or a cover of spines and hairs.
Hot, dry winds, and winds laden with salt have the most damaging effect on plant yields (hence, animal yields), although at high wind speeds mechanical damage can occur, which prevents or reduces yields no matter what the humidity or salt content of the winds.
Damaged crop plants such as corn or bananas suffer photosynthetic inefficiency ranging from 2 % when the leaf laminae are torn or frayed, or the midribs are broken (Chang, 1968).
Plants show different resistances to wind damage:
Wind Tolerant (and wind-fast).
These are the many short or creeping plants at the boundary layer of still air near the ground, or the front-line plants of sea coasts. e.g. Cerastium, Araucaria heterophylla.
Yields are little affected by strong winds.
Exposure Tolerant
e.g. barley, some Brassitas, Casuarina and Coprosma repens.
Yields are reduced in strong winds, but dry matter yield is less affected than in wind-sensitive plants.
Wind Sensitive
These are the many important crops such as citrus, avocado, kiwifruit-vines, many deciduous fruits, corn, sugar cane and bananas.
Both plant height and yields rapidly decrease with increases in wind speed. For these species, very intensive shelterbelt systems are essential.
Problems arise when the plants used for shelterbelt (e.g. poplar) are themselves heavy water-use species with invasive roots.
An annual root-cutting or rip-line may be necessary along such windbreaks to permit the crop sheltered to obtain sufficient water, but it is best to choose more suitable species in the first place
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Windbreak Height and Density
The height density and ability of windbreak trees are the critical shelter-effect factors.
Some configurations of wind belt may causes frost-pockets to develop in the still air of sheltered hollows.
PERMEABILITY is an important factor if we want to reduce frost risk or to extend the windbreak effect (Figure 5.16).
In the establishment of wind-sensitive tree crop we may actually need continuous (interplant) windbreak.
The length of windbreak needs to be greater than the length of the field protected, as wind funnels around the end of windbreaks in a regular flow pattern.
Windbreak Configurations
In general, species chosen for windbreak should permit 40-70% of the wind through, which prevents the formation of a turbulent wind overturn on the leeward side. Windbreak height is ideally one-fifth of the space between windbreaks, but is still effective for low crop at one-thirtieth of the interspace.
Sensible configurations are shown in Figure 5.18. Note that some wind shelter systems are placed throughout or within the crop or fruit area.
A. Dense Windbreak with bare stem area below
Effects: good summer cool shade for livestock; poor to useless winter shelter. Clumps of such trees on knolls allow animals to escape heat, and flies and mosquitoes are much reduced .
Sample species: Cupressus, Pinus, Casuarina.
B. Alternate (zig-zag) planting of very permeable trees
Effects: good “front-line” seafront systems to reduce salt burn and provide shelter for more dense trees on islands and coasts.
Species: Araucaria, Pinus, Casuarina.
C. Compound Windbreak of high density. Effects
The best protection for eroding beaches, lifting the wind smoothly over the beach berm and trapping sand. Also, effective in dust-storm areas as a dust trap.
Species:
Ground: Convulvulus, Phyla (Lippia), Mesembryanthemum.
Low shrubs: Echium fastuosum, wormwood.
Shrubs: Coprosma repens.
Trees: Lycium, Cedrus, Cupressus, some plants.
D. Permeable low hedgerow of Acacia or legumes
Effects: Good effects on grass and crop growth, allows air movement to reduce frosts. Species: Acacia Leucaena Prosopis, Albizia, Glyricidia, tagasaste and like tree legumes.
E. “Incrop” Windbreak
1: Savannah-style configuration of open-spaced light-crowned trees in crop or pasture.
Effects: Excellent forage situation in arid areas, especially if trees provide fodder crop; pasture protected from drying winds.
Species: Several fodder palms, Inga, Acacia, tagasaste, baobab, Prosopis.
2: Complete or almost complete crown cover in tree crop.
Effects: Excellent frost free sub-tropic and tropic lowland configuration where fruit trees (F) are interplanted with leguminous trees (L) as shelter and mulch, with Casuarinas (C) as borders. Suited to humid climates or irrigated areas.
Species: Fruits (F) from palms, avocado, Inga, banana, citrus. Legumes (L) of tagasaste: Acacia, Albizia, Inga, Glyricidia, Leucaena. Borders (B) of Casuarina, low palms (Phoenix canariensis), Leucaena, Prosopis, and other wind-fast trees and tall shrubs.
The partial list of windbreak configurations given above covers only some cases and in every case a designer must select species, study suitable total conformation and allow for evolution or succession.
As with all Permaculture Designs, general known principles are followed but every actual site will modify the design, as will the purposes for which shelter is intended.
Windbreak is essential for many crop yields, particularly in orchards.
As discussed, wind causes mechanical damage, salt-burn, and may transfer (advect) heat and cold into the crop.
Unless conditions are very severe single-line windbreak spaced at 15 times height may have a satisfactory effect on ground conditions and this is recommended for crops and grasslands.
However severe montane and coastal winds need more careful design and a complex windbreak of frontline species able to buffer the first onslaught of damaging winds are needed (Figure 5.19).
For both tree crops and orchards, we have a very different potential strategy in that the windbreak may be composed of trees compatible with the protected forest or orchard system we wish to shelter, and can then be integral with the crop (Figure 5.18 E1 and E2).
Great success with such strategies has been demonstrated both for wind and frost moderation in susceptible crop such as citrus, avocado and macadamia nuts or chestnuts, using a protective interplant of hardy Acacia.
Casuarina, Glyricidia, Tagasaste or Prosopis spaced within the crop.
As all of the windbreak species mentioned fixes nitrogen or phosphates, provide firewood, radiate heat, and shelter crop, it is sensible and beneficial to fully interplant any susceptible tree crops behind barriers of front-line windbreak.
Windbreak in this instance is integral with the crop (as it is in natural forests).
The importance of windbreak extends to SOIL CONSERVATION.
In dry light soils, windbreaks can reduce dust and blown sand to 1/1000th of unsheltered situations (Chang. 1968) within 10 times the height of windbreak.
Thus, in crops in arid or windy areas, it is necessary to plant windbreaks closer together for the sake of soil conservation.
The loss of soil at 20 times windbreak height is 18% of open situations, which is still too much when we can lose 8-40 t/ha in windstorms!
Similarly, WATER EVAPORATION can be halved in strong winds (32 km/h or more) for distances up to 10 times height.
Over 24 km /h, 30% gain in soil water conservation is achieved.
Only in still-air conditions is evaporation loss about the same for sheltered and open field conditions.
SNOW MOISTURE is increased by a windbreak of type A or B (Figure 5.18) when the snow is trapped on fields.
The snow depth in winter bears a close correlation to dry matter yields in spring and summer, so that crop yields are highest downwind from windbreak areas.
Wherever snow blows across the landscape, windbreaks of savannah configurations create spring soil moisture traps. It is also possible to do this by using open swales in snow-drift areas.
Windbreaks in exposed snowfield areas can be better established in the lee of earth bunds or in natural cornice just pole wards of ridges.
CROP YIELDS vary in increase from the 100% increase in such crops as avocado to 45% in corn, 60-70% in alfalfa, 30% in wheat, and lesser gains (7-18%) for low crops such as lettuce.
All these increases follow windbreak establishment on exposed sites. Effects are of course less in naturally sheltered situations or areas of normally low wind speed.
However, almost all normal garden vegetables (cucurbit, tomato, potato) benefit greatly from wind shelter.
For this reason, a ground pattern similar to that in Figure 5.20 is recommended for such crops and wind affected pastures.
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Vortices revolve anti-clockwise in the northern hemisphere, clockwise in the southern and thus coastal areas to the north side have the highest water and wave levels in the northern hemisphere, and to the south side in the southern.
The combined effects of rapidly fluctuating pressures, tidal bulge, wave and sea pile-up, and wave backwash create devastation on coasts.
Although hurricanes cannot persist far inland, as the sea itself generates the vortex, the intense rains generated do reach well inland to flood rivers and estuaries, adding to the general destruction.
With all effects combined in a “worst case” of high tides and prior rains, destructive wave attack can reach 6-9m (20-30 feet) above normal high-tide wave levels.
As wind strength increases at sea, wavelength also increases, so that normal wave fronts arriving at 8 per minute in calm Atlantic conditions slow down to a storm frequency of 5 per minute before great winds.
These wider-spaced waves travel fast, are larger, and create severe backwash undermining of shorelines.
Storm waves may therefore arrive long before a cyclonic depression or hurricane, and the change of wave beat gives warning to the shore crabs, birds, fish, and turtles, which either take shelter inland or go to sea to escape the approaching hurricane.
As modern satellite photographs are used to track the hurricane, there is usually a few days’ warning for coastal areas and evacuation is sometimes ordered.
Well-built towns (such as Darwin, Australia, after its cyclonic devastation in 1972) can withstand cyclones with minimal damage, but such stoutness is usually only built in after an initial (and sometimes total) destruction.
It is possible to strictly regulate and supervise buildings to be safe in hurricanes and in areas where flimsy constructions are normal, to dig refuge trenches and caves for emergency shelter. All such shelter must be in well-drained hillside sites.
TORNADOES
Hurricanes are large, slow phenomena covering hundreds of square miles and mainly confined to coasts facing large stretches of tropical seas, with very large heat cells.
Tornadoes, however, may occur in quite cold inland areas, last only seconds or minutes, and affect only a few square kilometers.
Thus, they usually escape detection by satellite and ground sensors.
Nevertheless, the stresses placed on buildings, civil constructions, chemical or nuclear facilities, airfields and villages can be disastrous.
Wind speeds may reach 120 km/h, at worst 280 km/h; these speeds can exceed hurricane winds.
The conditions for tornadoes are:
Thunderstorms with fast-growing cumulonimbus cloud;
A persistent source of warm moist air to feed the up draught side of the front;
An input of cold dry air entering the system from another direction; and
A vortex formation in the resulting storm; this reaches the ground as a tornado, caused by wind-shear effects at the border of the conflicting system, or as a frontal dust storm in deserts.
Effects: Trees twisted off and broken; people and objects sucked out of cars and buildings; “rains” of soil, fish, frogs may fall out ahead of the disturbance. (See Figure 5.22)
FIRESTORMS
Intense wildfires (urban and rural) fanned by dry winds will create powerful vortices due to conditions very much like that of the tornado.
The mass ignition of large areas of forests and buildings feed a powerful up draught. Colder dry air rushes in to replace the air consumed in burning, and fire tornadoes (firestorms) result, carrying large burning particles aloft on “smoke nimbus” clouds.
Whole house sections pinwheel across the sky to drop out ahead of the main fire front, where they in turn set up secondary firestorm conditions.
The effects on people and property are very much like those of tornadoes, but with the additional danger of intense heat. (See Figure 5.23)
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