Contents - Previous - Next
Mass movement control
Mass movement control tends to be both expensive and far from simple. Unlike sheet or linear erosion control, mass movement control often means preventing rainwater from soaking into the soil, adding to the weight of the soil cover and rapidly reaching the slide bed-plane. The surface is therefore drained to evacuate runoff to less vulnerable zones, generally the convex sections of a slope. The zone over the slide bed-plane can be drained in depth to prevent interstitial pressure from detaching the soil cover from the stable zone beneath the slide bed-plane.
Another method is that of drying the land by increasing plant evapotranspiration, for example by planting eucalyptus or other plants with a high evapotranspiration capacity. However, it is important to prevent such vegetation from becoming overwhelming, so shrubs must be kept on the edges of fields. If trees are introduced they must be coppiced, i.e., the vegetation must be kept young as it will then evapotranspire and produce maximum biomass. Very tall trees should not be kept on slopes where risks of sliding are high. When the slide bed-plane is close to the soil surface, tree roots can oppose strong mechanical resistance to shearing of the soil cover, whereas when the potential slide surface is too deep for the roots to reach, such resistance is no longer operative: overloading slopes with trees may even add to slide risks. Moreover, trees can shake in the wind, transmit vibrations to the soil and produce cracks that favour localized infiltration of runoff water down to the slide bed-plane. Quick-growing species with tap-root systems are preferable, and clear felling is to be avoided, for it destroys the whole root framework in the soil cover at one time. Trees not only increase resistance to shearing through the mechanical action of their roots, they also alter the water content of the soil: evapotranspiration is high in a forest and this keeps the interstitial pressure of water in the soil cover lower - which is why there is a sharp increase in soil humidity after clear felling.
Preventive methods are the most important. Infrastructures should not be built on unstable slopes and, if there is no other choice, the cuts and fills that upset slope equilibrium must be kept to a minimum. If, for example, a slope has to be cut into for a road, the embankment must be strengthened by providing the abutment with a riprapping mask or a supporting wall which counters rotational sliding and improves drainage on the slope. There should be a ditch uphill of the road to intercept runoff and prevent it from infiltrating the traction cracks in the soil cover above the cutting. Drains level with the weathered rock of the threatened zone will reduce hydrostatic pressure.
If cracks are observed on the soil surface, for example between micro-terraces formed by untethered livestock, surface tillage can help infiltration water to spread over the whole soil cover, and thus delay the advance of the wetting front toward the slide bed-plane and improve evaporation of the water mass (Rwanda: Moeyersons 1989a, b). When a road is built on a steep slope, it is a good idea to start stabilizing the road plate by planting and coppicing eucalyptus on the banks above and below it, or planting grass and ensuring it is not removed. A drained wall can also be built, with foundations well-anchored in the rock. Lastly, on very steep rocky slopes in mountainous areas, sheets of wire netting can be thrown down to break the fall of rocks.
In Tanzania, Temple and Rapp (1972) showed that mass sliding in plates is very rare in forest zones (1 %), and that even isolated trees can reduce its occurrence, particularly along roads. However, reforestation is not an infallible solution, or even a method that can be widely used in mid-altitude mountain areas (like Mgeta) with high population densities (170 to 510 inh/km²) and where people depend on rich and well-watered land for their livelihood (staple food crops and vegetables for the towns). At the most, they can be advised that the annual crops grown on small step terraces 1 m wide would be best combined with lines of trees on the ridges (eucalypts), on the banks around fields (fruit trees) and along river-banks (bamboo, eucalyptus or other local species) (Rwehumbiza and Roose 1992).
In Rwanda, zones subject to land-slides on slopes of over 45% are often planted to eucalyptus and left as pasture land. Houses are built on a flat space dug out of the convex side of a stable slope, and a double line of eucalyptus dries the bed-plate along the principal tracks by drawing up water.
Mass movement control must be primarily preventive: e.g., mapping vulnerable zones, drawing up a land use plan, banning building work or any modification of slopes, and protection in the form of coppice forests. However, it is not always possible to avoid cropping in these fragile mountain areas, which are often more densely populated than the surrounding lowlands because the climate is healthier (malaria-free) and the land better-watered.
control calls for expertise and major funding in order to
drain slide bed-planes - and this is beyond the reach of
small farmers. State investment in such measures are only
justified where vital structures are at risk: road
networks, villages, dams, etc. There are, however, some
measures well-known to farmers long familiar with the
region: the use of trees - particularly eucalyptus and
bamboo - to dry out the ground and stabilize the slow
movement of soil cover on steep slopes and along
river-banks. Careful choice of species should make it
possible to transform these inhabited landscapes into a
stable landscape dominated by hedges, as has been done by
the Bamiléké (see Chapter 10).
Lastly, the relative risks of the various erosion processes in each zone must be carefully evaluated before erosion control is undertaken. Sheet erosion control (which tends to improve infiltration) and the digging of diversion ditches on slopes steeper than 25 % (which drain the surface horizons but can lead water more quickly down to slide bed-planes) are often the source of huge and even more catastrophic land-slides. Temple and Rapp (pers. comm.) report that after a single rainstorm of 100 to 186 mm in three days (23-25 February 1973) in Tanzania, the overall damage caused by about a thousand landslides was estimated at 500000 FF (US$ 100000), with six dead, nine houses destroyed, 20 goats drowned, and 500 hectares of crops wiped out; 14% of the farms lost their harvests, roads were cut by floods for six weeks, etc.
PLATE 17: TILLAGE
Hoeing with a Manga hoe. This implement allows surface tillage of the soil after the first rains in order to destroy both the slaking crust and the young weeds. Later, a slight adjustment allows hoeing and ridging of widely spaced crops (cereals, groundnuts, cotton, etc.). Saria Station, Burkina Faso. [Photograph Dugué]
On a gentle slope, tied ridging allows some 50 mm of rainfall to be stored, improving infiltration by this amount. In Sudano-Sahelian zones it allows better crop-water saving in low-rainfall years, whereas in wet years, the crops can become waterlogged and produce less than on the control plot. In the long run, this technique leads to the removal of fine particles (clay, loam, alluvium and organic matter) from the surface horizon. Puni, Burkina Faso.
Ridging with digging in of organic matter. After a short fallow the farmer first rakes over the biomass so that it dries, then collects it along a line and buries it under a thick ridge of earth to form a contour line. Stones are piled on the closest edge of the field. Salagnac, Haiti.
Draining ridges. On steep slopes (20 to 40%), slightly oblique ridging with a spillway every 10 m breaks runoff energy and collects a large amount of well-aerated humus-bearing soil for growing root vegetables (cassava, yam, potato, sweet potato, etc.). However, the simple action of tillage tends to shift soil downwards, thinning it. Additionally, during the heaviest cloudbursts, runoff can overflow and gouge gullies in or around the fields. Mount Okou, Cameroon. [Photograph Bedel]
PLATE 18: AGROFORESTRY
Karité stand in a Sudano-Sahelian savannah area. When farmers prepare the soil (for cereals, cotton, groundnuts, etc.), they keep about forty useful trees per hectare fruit, forage, medicine, timber, litter-improving, etc.). Note also the attempts at water and fertility management (line of stones, grass and branches, corrugation of the soil to trap water and manure). Yatenga, Burkina Faso.
Traditional selective clearance. Fire is an indispensable tool in traditional systems for disposing of woody vegetation progressively and selectively. In the foreground note the soil, which remains covered and retains its root network, and in the background, the forest fallow, which regenerates the soil in the course of 12 years, under a continuously harvested natural palm stand. Fresco, south-western Côte d'Ivoire.
Lakou, agroforestry "Garden A." Around the home on the freehold land, the farmers often plant hedges to protect an intensive household plot + fruit trees, taking advantage of the proximity of the stable and household waste. The positive interaction between trees, livestock and crops is optimal here. Salagnac, Haiti.
Forage trees planted along the risers keep the soil in place and provide large amounts of forage prized for its nutritional qualities. These nitrogen-rich foods are essential to the digestion of dry coarse forage during the dry season. Gulmi District, Nepal. [Photograph Ségala]
PLATE 19: AGROFORESTRY
Alley cropping between Leucaena hedges. The use of live hedges means that soil fertility can be maintained by immobilizing 10 to 20% of the usable area - a partial solution clearly unable to meet the challenge of a doubling of the population in 20 years. Ibadan, Nigeria.
Cropping under cover of 200 Cedrella or Grevillea trees. By removing trees of different ages and pruning low branches and surface roots, cereals and other foodcrops with a staggered growth cycle can be produced under their cover. Ruhandé Arboretum, Butaré, Rwanda.
In the Sahelian zone of Niger, only the valleys are covered with trees (Acacia albida), which send their roots down to tap the groundwater. Their protection is vital to reducing the ill-effects of drying winds on crops. Tahoua Valley, Niger. [Photograph Oumarou]
Fruit and vegetable garden in the Sudanian zone of Côte d'Ivoire. After selective clearance, a good number of karité, locust bean, kapok and other useful species are left on the slopes. The Sénoufo use the bottom lands for rice fields, and traditionally build fruit and vegetable gardens surrounded by an earth bank. Many varieties of fruit tree are grown here, together with some bananas. Korhogo, Côte d'Ivoire.
PLATE 20: HAITI
Contour channels on calcareous bluffs. The hills are covered with shallow, grey rendzinas. The small plots are rented out to "rich city folk," and their edges are marked by contour channels, which quickly fill with sediment, so that they are now of use only as paths on these very steep slopes. Since the whole approach has never increased yields, the local small farmers do not maintain it unless paid to do so. Clearance is general, except around houses. Bouchereau, Jacmel.
Individual cistern. Rural development started in the Jacmel region with the building of cisterns to catch rainwater from roofs or small cemented areas. The water has allowed improvements in family hygiene and reductions in the time and labour entailed in fetching water, and is also used to water livestock and a small vegetable and fruit garden. Jacmel.
Gully garden. Once it becomes "worn-out land, " the slopes are scoured down to the rotten rock and left to fallow (RAK) grazed by livestock. Only the bottom lands are still productive where dry stone walls have been built to trap water and sediment. Jacmel.
PLATE 21: HAITI
Stabilized tracks and communal cisterns. One of the first development activities in mountain areas is the creation of tracks. However, these tracks are often the cause of serious gullying, and they have therefore been paved so as to collect runoff from the slope in a sand trap, from which it flows into a large communal cistern. The water is used for livestock watering, household purposes, and irrigation of a small off-season household vegetable and fruit garden. Salagnac.
Water management on the Salagnac toposequence. A little below the track, the soil is deeper, and cassava is grown on mounds in combination with beans and maize. Lower still, on the shelf where the houses are located, the red soil is much deeper and is intensely cropped (multi-storey gardens). Such plots are in danger of gullying by runoff from the scoured hilltops, so that it is important to catch this runoff on the track. Salagnac.
Plot-bordering hedges. In the foreground, large cuttings, which act as hedges to protect the cultivated plot on the left from passers-by. The project has tried to improve these hedges by introducing forage and fruit species. In the background, the stony surfaces (or "worn-out land") where runoff concentrates. Salagnac.
In the Nippe area, the weathering of basaltic soils has given birth to an undulating landscape covered with fertile vertisols. Traditionally sorghum is sod seeded, and vetiver, which is very resistant to overgrazing but not to gullying of the valley bottoms, is planted on the edges of the fields. Mangoes produce masses of fruit, and are used to feed pigs. Since the ravages of swine fever, the dried foliage has been used as forage, but many mango trees have been sold for timber. Petite River, Nippe.
PLATE 22: ECUADOR
In mountain areas, runoff and linear erosion (E = 100 t/ha/yr) scour the soil down to the cangahua, a hardened layer unsuitable for cultivation. Cayambe Basin, altitude 2800 m. [Photograph De Noni]
Station to measure runoff and erosion risks, soil surface conditions, and yields as a function of natural rainfall on control plots (bare or under traditional crops) and improved plots (1000 m²). Mojanda, altitude 3300 m. [Photograph De Noni]
Low contour walls built of blocks of cangahua or grass clods according to local practices have turned the landscape into gradual terraces, and reduced water erosion to acceptable levels (under 5 t/ha/yr). [Photograph De Noni]
Watershed management by the local rural community. High yields on the experimental plots encourage the farmers to invest in land husbandry; if they sign a contract undertaking to maintain the works, they can receive a loan enabling them to purchase sufficient inputs to double yields. When they repay the loan after a year, another family is granted the same loan, so that a small amount of aid eventually benefits a whole community. Pedro Moncayo, altitude 3300 m. [Photograph G. Noni]
PLATE 23: ALGERIA
Marls and soft rocks are very susceptible to water erosion. Following clearance of steep slopes, extensive cereal cropping and centuries of overgrazing, the hill has lost 1 metre of soil, and sheet and rill erosion are clearly visible. The form of the tree trunk also indicates mass movement.
On the nearby marry hill, the effects of sheet erosion can be seen at the top, and those of rill and gully erosion on the steep slopes, while the wadi eats away at the foot of the hill, causing the banks to slide.
Sheet erosion carries only a few tonnes of sediment down to the bottom of the hill, whereas gullying and wadi streambed displacement carry hundreds or thousands of tonnes of sediment right down to the dam. This should influence the choice of sites and strategies for erosion control intervention at the watershed level.
PLATE 24: LAND HUSBANDRY
With a view to developing mountain farming, international aid projects introduced fruit tree crops, which considerably increase small farmers' income. However, apricot trees lose their leaves in winter, so that these orchards provide very poor protection to the soil during the rainy season. On this plot with a 35% slope, 15 to 30 cm of soil has been lost after 30 years. Ouzera, Algeria.
With a view to reducing runoff and erosion risks and improving income still further, the INRF-ORSTOM research team has established grass buffer strips under the trees, combining this with rotations (beans and cereals) that cover the soil during the rainy season and complete their cycle before summer starts. Without reducing fruit yields, this system assured an additional crop of grain, produced straw useful for animal husbandry, and cut erosion risks. It aroused considerable interest among neighbouring peasant farmers. Algeria.
Half-orange landform in the gneissic regions of Vietnam is perfectly developed in terms of management of water, biomass and fertilizing elements. The top and the steep slopes are protected by a crop of tea. Runoff irrigates sugar cane and a rice field before flowing into a pond that is surrounded by a collection of useful trees. Tilapia provides food for people, hens and pigs, and the latter recycle banana and sugar-cane residues, so that their dung fertilizes both rice field and pond. In this way, nutrients can be recycled several times per year. Bac Thai, Viet Nam.
PLATE 25: LAND HUSBANDRY
On these fields in the Sudano-Sahelian zone of Burkina Faso, there are stone lines to curb the velocity of water, a stand of acacia, and heaps of dung which will be dug into the soil: a mineral supplement is indispensable. The interaction of all these ways of managing water, biomass and nutrients allows hopes of a relatively productive and sustainable agriculture. Burkina Faso. [Photograph Dugué]
Land husbandry in Nepal. The case of the foothills of Nepal illustrates the complexity of traditional production systems which combine sophisticated water management on irrigated terraces on the slopes or in the valley bottoms, agroforestry and animal husbandry in order to propagate fertility on cultivated gradual terraces. Gulmi District, Nepal. [Photograph Ségala]
Multi-storey gully garden. Runoff on the basaltic slopes causes gullies, which can easily be controlled with sills of earth protected by plastic bags. The sediment that collects is immediately planted with a wide range of fruit trees, bananas, cane sugar and various forage shrubs. Such gully gardens are eventually intensively farmed as "linear oases. " Petite Rivière, Nippe, Haiti.
Risers of blocks of rock or tufts of grass have been built in order to treat the steep slopes of the Ecuadorian Andes, reducing water erosion to under 5 t/ha. In order to make the most of this system, a whole series of other inputs were necessary, such as chemical fertilizers, improved seed and pesticides. This technological package made it possible to both stabilize the slopes and intensify farming. Ecuador. [Photograph De Noni]
PLATE 26: WATER MANAGEMENT
Jessours in Tunisia. In semi-arid zones where plants cannot take root on slopes, small dams of earth or gravel can be built to trap runoff and sediment. Cereals are then planted under various fruit trees (palms, olives and figs). Matmata region, Tunisia. [Photograph Chassany]
The authorities forced groups of peasant farmers to dig blind ditches (0.5 × 0.5 × 10 m) to store runoff water. These ditches require a great deal of work (250 days/ha to install + 50 days/ha to maintain) without increasing production. Unmaintained, they fill with sediment, causing gullying or landslides. The majority of these ditches have now vanished, leaving risers and gradual terraces. Central uplands of Rwanda.
Runoff diversion terraces. This method does not reduce soil degradation or increase yields, and requires maintenance. When exceptional rainstorms occur, the water overflows the terraces, causing gullying (on the left in the photograph), whereas a plot protected by clover (on the right) shows no trace of erosion. Biological methods prove much more effective than mechanical terracing. Capetown, South
Grass banks to dissipate runoff energy: runoff cannot cut a gully unless its velocity is over 25 cm/s (Hjülstrom). Rather than concentrating runoff water, it is better to develop techniques that leave a very rough soil surface (rough tillage, mulching) and pervious erosion control structures (grass banks, hedges, stone lines) that can slow down the water and spread it out as a sheet. CVHA Project, Burundi.
PLATE 27: WATER MANAGEMENT
Earth bunds sealing off a tank. In Sudano-Sahelian zones of Burkina Faso, villages are in dire need of water at the end of the dry season. With small earth dams, runoff water from the hills can be trapped to water livestock and a small irrigated garden. Yatenga, Burkina Faso.
Mulching degraded land allows restoration of both infiltration capacity and fertility through the action of termites which redistribute the organic matter in their galleries. Yatenga, Burkina Faso.
Multi-storey garden irrigated by a bouli, a small earth dam. With the runoff wafer collected by a modest dam of this kind, livestock can be watered after the onset of the rainy season, and a small early vegetable garden irrigated. Sabouna, Burkina Faso.
Development of terraced rice fields along the slope is based on the possibility of gravity irrigation. The seasonal availability of water and the altitude then decide whether one, two or three crops should be grown per year. Gulmi District, Nepal. [Photograph Ségala]
PLATE 28: BIOMASS MANAGEMENT
In the zaï method, 3 tonnes of sun-dried faeces or corral soil must be dug into the pan. The concentration of water and available manure restores productivity on this degraded land, even in the first year. The organic matter not only contributes a minimum of mineral elements but also provides the microflora needed for assimilation of the nutrients in the soil.
A fallow of legumes grown as a catch crop under the cereal is another solution, allowing an increase in biomass, bringing nutrients to the surface, and protecting the soil during early storms. However, it is possible only in Sudanian areas where rainfall is over 1000 mm and distributed over more than 5 months of the year.
Compost pit irrigated by runoff water. This system consists of building a field compost pit, thus eliminating the need to transport crop residues and compost. Unfortunately, the time needed for decomposition (18 months) and the quality of the organic product leave much to be desired. In future, efforts will be focused on setting up "compost-dung-rubbish" heaps or pits near the dwellings, which will allow each family to produce 5 m³ of a compost that is well-decomposed and recycled even by the following season. Ziga, Burkina Faso.
PLATE 29: BIOMASS MANAGEMENT
Under coffee trees, a thick mulch (25 t/ha/yr) retains soil moisture in the dry season, protects the surface against erosion, lowers competition from weeds, and concentrates nutrients from all over the farm. The trick is to produce enough biomass without upsetting the whole farm.
Maintaining field fertility by adding dung, a practice that is part of a complex foddering system. There is a real transferral of fertility from uncultivated to cultivated areas. Gulmi District, Nepal. [Photograph Ségala]
After a short fallow, the farmer brings tethered livestock a forage supplement, the residue of which will be recycled directly during tillage. Jacmel, Haiti.
The transport of dung is one of the factors that limit its use to the immediate vicinity of dwellings, i. e. home gardens. [Photograph Ségala]
PLATE 30: ANIMAL HUSBANDRY
Dung contract. Farmers in the African savannah traditionally propose that herdsmen have their livestock graze on crop residues in exchange for leaving them on the fields during the night. This produces localized dung, possibly in considerable quantities, although it is poor in nitrogen, since the faeces are exposed to the sun and are trampled by the animals. Boukere, Burkina Faso.
Night corralling. When the livestock are herded into a corral for the night, they produce so much dung that nothing more will grow there. Powdered faeces crushed by the animals' feet and mixed with varying amounts of soil from the corral are removed. The quality of this product, which is unfermented (and hence full of seeds ready to germinate), can be improved by the addition of a litter of coarse straw. Production of this improved dung can be as high as 1.5 t/ha/cow/yr. Southern Mali.
A movable corral can also be made using barbed wire, thus improving distribution of dung on cultivated fields.
PLATE 31: ANIMAL HUSBANDRY
Dung stable. In farms using animal traction, a pair of oxen are often stabled under a rudimentary roof that allows storage of crop residues. Combined with the urine and faeces, the litter is then taken to the dung pit where it ferments, lowering the content of live weed seeds. When household refuse, ash and other organic waste are added, a farmer can produce up to 5 t/yr of good-quality composted manure, especially if he digs in a mineral supplement (N. P. Ca) with it to compensate for soil deficiencies. Kaniko, southern Mali.
Village compost-dung pit which could be made more efficient if the pit were surrounded by trees. The roots would recover nutrients in solution now carried away by drainage water, the litter would return nutrient-rich biomass, and the shade would maintain an environment favourable to decomposition. Yatenga, Burkina Faso.
The top system is a stable where the livestock are kept permanently on litter. Watered daily and trodden down by the animals, the litter is quickly transformed into good-quality manure. CVHA Project, Bugaramé, Burundi.
Paddock surrounded by contour hedges (Leucaena, eucalyptus, etc.). The stable is joined on to the house. CVHA Project, Bugaramé, Burundi.
PLATE 32: RWANDA
Grass lines and step microterraces on steep slopes. The lines of Pennisetum that can be seen in the foreground do somewhat slow erosion on these 60% slopes. In the middle ground on the right are micro-terraces 50 cm deep dug into the topsoil and protected by grass risers. This network of grass keeps the soil in place while producing foodcrops on slopes of up to 80%. Note also the eucalyptus stands in areas where there is a risk of sliding. Ruhengeri region, Rwanda.
Radical terraces in Rwanda. In order to absorb all the rainfall and maintain the fertility levels needed for intensified cropping, radical terraces were built. This involved building risers with clods of grass from the land, removing the topsoil, building the terrace with a 4% reverse slope, and shifting the topsoil back onto this almost horizontal surface. Unfortunately, since the subsoil is sterile, apart from the necessary investment of I 000 working days per hectare for the terracing, the method requires 10 tonnes of dung, 3 tonnes of lime, and 300 kg of NPK if it is to produce viable results. It is therefore unaffordable for most Rwandan farmers. There are also many hills where it would be dangerous to build such terraces, because the slopes are susceptible to landslides. Rwanda.
Grass bank with bananas planted below it. Some experts hope to reduce the density of bananas between erosion control structures in order to intensify foodcropping. Meanwhile, banana is a cost-effective crop because of the organic residue dug into the planting hole. Burundi.
The grass bank retains the earth pushed by runoff and above all by tillage. Central uplands, Burundi.
Chapter 8. Wind erosion
Forms of wind structures
Effects of wind erosion
Factors affecting the extent of wind erosion
Wind erosion control
There is considerable wind erosion in West Africa in dry tropical zones where annual rainfall is below 600 mm, the dry season lasts more than six months, and steppe-type vegetation leaves large stretches of bare soil. It can also develop elsewhere when the soil is being prepared and large amounts of surface matter are crushed fine.
The wind exercises a pressure on solid particles in repose. This pressure is exerted above the centre of gravity on the surface exposed to wind and is opposed by a friction centred on the base of the particles. The two forces combined tend to rock heavy particles (0.5 to 2 mm) and make them roll.
Moreover, the difference in speed between the top and bottom of particles means that they are drawn upwards. The lighter particles rise vertically until the gradient of velocity is too low to bear them, at which point they fall back, pushed by the wind, following a subhorizontal curve. As they fall, these grains of sand transmit their energy to other grains of sand (as in a game of bowls) or degrade loamy-clay aggregates, releasing dust (Heusch 1988).
The three processes described below can be observed in the field when the wind-speed exceeds 15 to 25 km/in (or 4 to 7 m/s) depending on air turbulence (De Ploey 1980, Mainguet 1983, Heusch 1988) (Figure 58).
Saltation of fine sand (0.1 to 0.5 mm): in this process, sheets of sand raised by violent wind travel several dozen metres over smooth surfaces, leaving sheets of ripplemarked sand on the ground or small mounds of sand trapped by plant tufts. These sand sheets lash at rocks in desert areas, giving them a typical mushroom shape (corrasion), and cause serious crop damage.
Deflation: in this process light particles of soil (clay, loam and organic matter) are carried away in suspension. This dust is sucked up by vortices as high as several thousand metres, and then dispersed as a dry mist, or it may travel several thousand kilometres as a dust cloud. This category covers both wind-borne loam torn from periglacial loess steppes, and the Sahara dust that falls in Montpellier three times a year and once or twice a year in Paris.
Creep: grains of sand 0.5 to 2 mm and too heavy to be sucked very high are thrown off balance by gusts of wind, and rolled and dragged over the soil surface to the tops of dunes, which can advance several metres per hour in this way in strong winds.
FIGURE 58 Three processes of wind erosion: suspension, saltation, traction
dry, fairly loose soil
1. Suspension: cloud of superfine silt (e.g. dust in dry season = dust bowl) rising as high as 10 km and extending over hundreds of km
2. Saltation: fine sand grains, ø ~ 100 µ / major ill-effects: moving dunes - damage to crops and other vegetation
3. Traction along the ground:
coarse sand rolling along the surface of dunes
fine sheets of sand
Forms of wind structures
The form of dunes depends on the prevailing winds.
If the prevailing winds come from only one direction, the dunes can be straight, paralleling the coast (formed by the winds that sweep across the beach at low tide) or crescent-shaped, with the side toward the wind gently sloped. In the second form, the wind pushes grains of sand up to the top of the gentle slope, and they then fall on to the steep slope inside the semicircle. Dunes advance more slowly as they grow in size. According to studies by Bourgoin (1956) along the route of the Mauritanian railway, dunes 3 metres high will advance between 40 and 80 metres per year, dunes 12 metres high will advance between 12 and 35 metres, and those 24 metres high, between 8 and 17 metres.
In order to avoid the risk of sanding-up, lines of communication are not taken through areas with live dunes. A 50-cm bank with a very gentle slope (1/5 to 1/10) is also built so that the wind speeds up as it crosses the road, preventing it from depositing sand. The wind-speed can be increased still further at particularly vulnerable points by setting up 3 × 1 m deflecting panels at a 60° slant, or triangular cross-sectioned sand mounds 8 metres from the road, with the top and sides covered with a 20- to 50-cm layer of gravel.
If the prevailing winds are multidirectional, sand dunes can sometimes stretch several hundred kilometres; lying at a tangent to the wake of an obstacle, the Silk is oblique with respect to what could be termed the annual wind. During storms, sand travels along the dune, parallel to this structure, which continues growing in the same direction (Mainguet 1983). The profile is of two steep slopes of moving sands, meeting in a sharp ridge.
There are also pyramidal dunes (ghourd) with several ridges leading down from the top, as evidence of multidirectional winds.
There are also hollowed dunes - corridors between two dunes where the wind pours through and digs out yardangs. The sheets of sand carried between the dunes in this way will be trapped by tufted plants, gradually forming what are called nebkas, which continue to grow, eventually forming larger and larger dunes.
The material often comes from matter previously removed by water erosion - inland or marine sediment, products of weathering or disintegration of coarse-grained rocks, or else from soil finely powdered by tillage techniques, particularly the ill-advised use of disc ploughs, especially on volcanic soil (for example the basaltic soil of Nicaragua or the loam of the Great Plains in the United States).
Contents - Previous - Next