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Cost effective gully treatment
[Plates 14 and 15]
The very high cost of torrent control and gully treatment (between US$ 20000 and 100000/km), is only justified where there are dams, housing, roads or major constructions to be protected downstream.
FIGURE 55 Various types of flexible, fairly inexpensive, pervious sills that are easy to build with local materials
Prevention being better than cure, it is best not to wait until soils are deeply degraded, but rather to reduce runoff and erosion in the fields by developing production systems that provide good soil cover.
However, both small and medium gullies can be treated with simplified and fairly inexpensive sills (20% of the cost of gabions), using unsophisticated materials and the labour of local farmers who can quickly be trained (Figure 55).
The originality of this approach is that it not only stops the linear erosion that produces gullies and traps several dozen cubic metres of sediment behind the sills, it also maximizes the moisture stored in the trapped sediments so that green forage and trees will grow in the dry season, thus motivating farmers to manage their land and treat gullies correctly. This last point is vital in order to advance from an omnipresent State to a new stage of fostering the farmer initiative indispensable for sustainability in the rural sector.
The next stage consists of treating secondary gullies and then building small dams that can provide water for mountain farmers who are chronically short of water for their livestock and for the irrigation of small, highly-productive gardens.
TEN COMMANDMENTS FOR GULLY TREATMENT
1. Until infiltration on the catchment area has been improved, no attempt should be made to block a gully (otherwise it will simply find another bed); a stable channel should be planned that can evacuate the peak flows that occur every ten years (or even less frequently).
2. Mechanical and biological treatment of a gully may be done gradually from one to six years, but must take account of the whole watershed from the outset. Biological fixation of a gully will consolidate the sides and bottom once they have been stabilized by various types of sill: if this order is reversed the plants will be stripped away along with the soil by floods.
3. Sills must be positioned carefully, according to the objective. If the purpose is simply that of raising the gully bottom so that the sides achieve the natural equilibrium, a key narrow section should be chosen where a number of light sills can rest on solid sides. If the purpose is to save as much sediment as possible or to rehabilitate arable land, the best choice is gently sloping places, the confluence of secondary gullies, or sections with sloping banks, and a solid, heavy structure that gradually builds up.
4. The spacing of the sills depends on the slope of the land. The downstream spillway must be on a level with the base of the sill above, according to the on-site equilibrium bed slope (1 to 10% depending on the nature of the gully bottom, in a stable area with neither removal nor sedimentation). Initially, the spacing can be doubled, with intermediate sills being built when the first generation is filled with sediment: it is important to stabilize the trapped sediment immediately with low plants in the centre of the flow and trees on the sides.
5. Hydrostatic pressure should be offset by providing drainage for the sills in the form of grids, baffles, or loose stones.
6. The sills must be anchored in the bottom and sides of the gully by a foundation trench to avoid piping and circumvention. A sand and gravel filter at the point of contact between loamy-clay soil and the stone sill will be needed to prevent uplift from carrying away fine particles and piping the structure.
7. The wings of the sill - which should be higher than the spillway - should keep the watercourse centred along the axis of the gully. The spillway itself must be reinforced with large flat stones with or without cement, or with scrap iron, so that it resists the tearing force of the sand, gravel and stones that tumble down gully bottoms at considerable speed.
8. The energy of the water as it falls from the spillway must be broken by a cushion (riprapping, gabion, metal grid + tufts of grass) or by a subsidiary dam to avoid piping under or actual overturning of the sill.
9. Livestock must be kept away from the treated section, for they would quickly destroy sills and degrade plant cover. However, fruit and forage - and in due course wood - can be harvested in exchange for upkeep of the structure.
10. Mechanical treatment is not complete until the source of the sediment has been tamed, and the gully heads and sides stabilized. Vegetation should then establish itself naturally if the equilibrium profile has been reached, although nature can be assisted by quickly covering the sediment with grass and fixing it with ecologically appropriate, productive trees. Simply managing sediment must be followed by getting the most out of the system thus treated.
Gullies can become "linear oases".
Chapter 7. Mass movement
Forms of mass movement
Causes and processes of mass movements
Mass movement control
While sheet erosion attacks the soil surface and gullying affects drainage lines on a slope, mass movement involves massive erosion of a large volume of the soil cover. This chapter will outline the general principles for means of preventing and controlling mass movement that are within the reach of peasant farmers. The view is that only the State has sufficient technical, financial and legal means to control sloughing and landslides (which can often cause real disasters), and to impose restrictions on the use of land under the greatest threat from mass movement.
Forms of mass movement
The many forms of mass movement can be divided into six main groups (Figure 56):
This is a relatively slow sliding of the surface layers of the soil cover, generally without detachment, and is widely observed on steep slopes where young forest saplings are bent and the base of adult trees crooked. In agroforestry areas, livestock treading on slopes can also lead to a stepped formation flanked by a network of fissures (Moeyersons 1989a, b).
Another form of creep - that of dry mechanical erosion - is caused by cropping techniques, and has been treated separately. Like sheet and rill erosion, these various processes eventually scour the hill-tops and clog slope bottoms.
• Rapid sliding
Sliding or sloughing of plates of earth is the sliding of a thick or thin layer of overlying material over a more compact horizon (often weathered rock) which acts as a slide bed-plane. This is a very common occurrence on schist where the dip parallels the slope of the land, on gneiss, and on marl in process of weathering.
• Sheepback slopes
These are soft forms appearing in wet conditions when the surface horizons pass the point of plasticity and move slowly, like toothpaste, between the root networks holding the topsoil in place, and the compact, impervious horizon made up of a material such as weathered marl or clay.
Forms of mass movement
4) Mechanical or dry erosion
• Mudflows (or torrential washes)
These are high-density mixtures of water and soil which have passed the liquidity point and can carry away large masses of mud and impressively sized rocks at high speed. When such flows occur, they start as a channel and end as an outwash fan (or debris cone) of material with a wide variety of textures. Fine material is subsequently removed by water erosion, either sheet or rill, leaving behind a mass of gravel and blocks of rock of very varied sizes. Such phenomena often appear following a plate slide or in a gully when an exceptional rainstorm clears away weathering debris that has been collecting for some years (Temple and Rapp 1972).
FIGURE 57 Rotational spoon-shaped slide (cf. Neboit 1991)
• Rotational spoon-shaped slides (Figure 57)
This is a slide in which the soil surface and part of the mass rotate as they slide, so that a counterslope appears on the slope. There is often a whole series of such slides, giving the landscape a sheepback appearance. There is usually a moist area in the hollow of the spoon where moisture-loving plants grow (Carex). After very wet periods, runoff often sets in on the sides of the counterslope, gradually gullying it out of existence and leaving only a dip in the slope which is difficult to distinguish from an ordinary gully.
• Local forms
This category covers rock slides, the undermining of banks, and slope subsidence leading to localized sliding. These are very frequent at gully heads: they cause the upper part of the lips of the gully to slide, and move the gully upwards through headward cutting. They are also found in wadis, particularly in the concave sections of meanders.
Causes and processes of mass movements
The cause of both slow and rapid mass movements is an imbalance between the soil cover, stored water and plant cover, and the friction they exert on the sloping substratum of weathered rock on which they rest (maximum slope of 30° to 40° = 65%). This imbalance can manifest itself progressively on one or more slide planes following wetting, or when the soil cover goes beyond the point of plasticity (creep with deformation though without actual breaks) or liquidity (mudflows).
Such imbalance most often occurs suddenly in answer to one or both of two kinds of event: earthquakes and very heavy cloudbursts (over 75 mm in 2-3 hours) (Temple and Rapp 1972). As water races through fissures or megapores (tunnelling) and down to rotten rock, hydrostatic pressure builds up at a certain distance from the crest (5 to 95 m at Mgeta in Tanzania) or at the confluence of underground trickles of water. This pressure can push away the formed soil mass, detaching it from a fragile level of rotten rock; hence the very frequent plate slides on schist, gneiss and porous volcanic material deposits over impervious rock (for example, volcanic ash on granite domes in Rwanda).
This imbalance can be created by earthquakes, cracking as frost and thaw alternate, the dessication of swelling clay, rock weathering, wetting of the soil cover to the point of saturation, wetting of the slide bed-plane so that it becomes slippery (as in the case of silt from weathered mica), the presence of rocks with preferential fracture planes (argilite, clay, marl, schist, micaceous rocks, gneiss). Human beings can cause more such mass movements by altering the external geometry of a slope (by terracing, cutting into it to build roads or houses, overloading it with landfill, altering natural flows, or by the erosion that occurs at the foot of a slope after deviation of a watercourse, etc.). Vegetation also has an effect. In their study of a site subject to debris-slide and mudflow, Temple and Rapp (1972) showed that 47% of cutting occurs on cropped fields (maize + millet + beans), 47% on fallow and grazing land, and less than 1 % on the highest-rainfall forest zones. Even free-standing trees seem to have an effect, for only trails where no trees were planted show signs of sliding. One row of trees would be enough to avoid the process. However, some major slides occurred in the highest-rainfall forest zone (R ~ 2000 mm) which received 185 mm in 72 hours on 23 February 1970. Reforestation is therefore not a sure-fire defence against landslides, and the type of (forest) probably also plays a part. Convex slopes (weathering in orange-half forms) and deeply scoured valleys will also be susceptible to landslides (Temple and Rapp 1972; Avenard 1989; Moeyersons 1989a, b).
According to Ferry (1987), the decisive factors in soil cover resistance to sliding are expressed in Coulomb's equation:
s = c + (p - u) tangent f
s represents resistance to shearing
c soil cohesion
p normal pressure at the surface of the movement due to gravity
u pressure of interstitial water in the soil
f internal friction angle
tangent of f, the friction coefficient.
Sliding takes place when the shearing constraint exceeds soil resistance or when the plasticity or liquidity point is reached. Creep is often seen when the soil cover is thick, the slope steep, and the climate very wet. Plate sliding is more likely in the presence of gneiss, schist or volcanic ash deposits on convex schist or granite slopes where a dip follows the direction of the slope, when soil cover is not very deep, on steep slopes (> 60%), or again when there is an impervious level or a steeply sloping and excessively lubricated contact plane.
Slopes with small ridges, bunds or sheepbacks are generally found in moist, marry sites, as are rotational landslides. Undercutting of riverbanks and the banks of gully-heads or sides is generally linked to flows which undercut soil cover to such an extent that they cause rock slides. Tunnels formed as gypsum or salt dissolves, or dug into the soil cover by rodents, can also collapse when water pours into them. Undermined banks are frequent at turns in the river and in meanders.
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