SOIL EROSION AND CONTROL TRAINING MANUAL

Prepared By A.M. Manyatsi, Faculty of Agriculture, University of Swaziland, Luyengo, Swaziland

Soil Conservation, Watershed and Dam Management Training Course

Offered By Environmental Consulting Services, Mbabane, Swaziland

2 February - 13 March 1998

Preface

This manual is divided into two sections. Section A deals with the theoretical aspect of soil erosion, and section B addresses the practical steps for soil conservation. It is intended to provide basic information to be applicable in soil conservation, watershed and dam management under Swaziland, or similar conditions. Soil conservation practicals will be done on a dam site with land degradation problems. Prior to the soil conservation exercise the participants in the course would have been introduced to land and site surveying, watershed management, small earth dam maintenance and repair, and they would have also had countryside trips to observe soil conservation activities. It is hoped that with the knowledge and techniques they will acquire from this course, the participants will be in a better position to assist and spearhead soil conservation strategies in their respective areas.

A.M. MANYATSI
JANUARY 1998

Please note that all the figures referred to in this document are currently unavailable.

Theoretical Aspects of Soil Erosion and Control

INTRODUCTION

Land use issues and soil erosion

Shifting cultivation and nomadic pastoralism are simple forms of agriculture where farmers rely on natural soil fertility for food and other products. The practice was widespread in Africa in a variety of forms. In its original form it involved allowing for perennial growth of secondary forests in agricultural lands through fallowing the farm plot for a period of five to six years). Farmers shifted locations to allow exploited sites to replenish their productive capacity. This land management practice involved continuous selection for domestic plants and animals which have adequate tolerance or resistance to diseases and pests, and the farmer also imposed certain measures of control through an appropriate cropping and grazing time-table. Opportunities for shifting locations became restricted mainly due to population growth. In Southern Africa, including Swaziland, one of the major consequences of population growth has been an evolution from shifting cultivation to permanent agriculture where subsistence farming is practised. Under this system crops are taken annually on the same piece of land, usually with short periods of fallow. Consequently, the land is used more intensively compared with shifting cultivation. The other important consequence of population growth has been the expansion of the total area under cultivation and, because the area of land that can be handled by a farmer using simple hand tools is limited, the increase in population has led to a corresponding increase in the number of small subsistence farmers. The transition from shifting to sedentary patterns of cultivation and pastoralism have led to heavier use of land, depletion of soil nutrients, reduced yields, use of marginal lands for crops, poor land management practices, and soil erosion. Hence, there is an urgent need to develop improved systems for the use and management of natural resources in a manner which allows sustainable yields with minimal degradation of soil and vegetation resources.

Factors affecting soil erosion

Soil erosion, which includes those processes which entrain soil material and deposit it elsewhere, is accelerated when the vegetation cover is sparse or absent. Removal of vegetation cover also makes the land prone to soil degradation which may involve the alteration of soil characteristics such as soil salinity, soil structure, soil acidity, and soil productivity. The risk of land degradation is increased in rangelands because of the removal of vegetation cover through overgrazing caused by high livestock densities. Excessive populations of domesticated animals, because of their different behaviours, produce a variety of effects that are more harmful than those caused by wild animals. For example, instead of spreading over the land to feed, the domestic stock tend to remain in groups, sometimes restricted in their movement by fences, and so increase the amount of vegetation removed from the area they occupy. Trampling of the livestock compacts the soil, reducing water infiltration, and increasing surface water run-off with gullies forming along the cattle paths. There is decrease in ground cover and soil organic matter which results in poor soil structure and poor water entry and plant growth properties of the soil which eventually results in less ground cover and increased soil erosion.

• Short-term over-utilisation
• Seasonal over-utilisation
• Medium-term over-utilisation
• Long-term over-utilisation

Figure 1. The sequence of land degradation processes in a natural resource ecosystem continually over-utilised over the long term.

The current major economic use of southern African savannas it cattle ranching. Cattle eat mainly grass leaving the way open for the proliferation of weedy varieties with little or no food value, thorny or toxic species, or annual herbaceous varieties (as opposed to perennials) with limited size, very short growing seasons, and consequently poor nutritional value to the grazing animal.

Soil erosion in Swaziland

In 1953 the King of Swaziland issued an order commanding the Swazi nation to observe the following soil conservation measures in arable land (King’s Order No. 2 of 1953).

• No person was to plough up and down the slope of a hill, but they were always to plough across the slope.
• Land with slope of more than 14% was not to be used for cultivation purposes.
• Grass filter strips were to be left between ploughed land at such intervals as advised by agricultural extension officers.

The King’s Order has been observed for more than 4 decades by the nation and at present all ploughing is done across the slope, and all cultivated land has grass filter strips. The grass filter strips are usually 2 metres wide at intervals of between 5 m and 20 m depending on the slope of the land, being closer on steep slopes. Since 1956 agricultural extension officers have been involved in surveying virgin lands and in setting up the grass filter strips between ploughed land. The chiefs are responsible for making sure that the order is observed and any offender is charged before the traditional community court; if found guilty, the offender may be sentenced to pay a fine which must not exceed one cow or its equivalent in money. As a result, soil erosion related to cultivation has been virtually eliminated in Swaziland, which is unusual for any hilly country; hence the current erosion problem is primarily livestock related.

The common beliefs concerning soil erosion in Swaziland are summarised in the following statements:

• Overgrazing is the main cause of soil erosion in the country;
• Soil erosion is confined almost exclusively to SNL;
• Soil erosion is localised at dip-tanks on steep slopes, watering points and near homesteads.

Principles of soil conservation

The aim of soil conservation is to obtain the maximum sustainable level of production from a given area of land whilst maintaining soil loss below a threshold level which theoretically, permits the natural rate of soil formation to keep pace with the rate of soil erosion. In addition, there may be a need to reduce erosion to control the loss of nutrients from agricultural land to prevent pollution of water bodies; to decrease rates of sedimentation in reservoirs, rivers, canals and ditches In the longer term, erosion has to be controlled to prevent land deteriorating in quality until it has to be abandoned and cannot be reclaimed, thereby limiting options for future landuse. Since erosion is a natural process it cannot be prevented, but can be reduced to an acceptable rate. A decision on what that rate must match the requirements for sustained agricultural production with those of minimising the environmental impacts of erosion.

The strategies for soil conservation must be based on covering the soil to protect it from raindrop impact; increasing the infiltration capacity of the soil to reduce impact; increasing the infiltration capacity of the soil to reduce runoff; improving the aggregate stability of the soil; and increase surface roughness to reduce the velocity of runoff. The purpose and use of various conservation techniques can be described under the widely-accepted headings of agronomic measures, soil management and mechanical methods. Agronomic or biological measures utilise the role of vegetation in helping to minimise erosion. Soil management is concerned with ways of preparing the soil to promote dense vegetation growth and improve its structure so that it is more resistant to erosion. Mechanical or physical methods depend upon manipulating the surface topography, for example, by installing terraces to control the flow of water. When deciding what conservation measures to employ, preferences is always given to agronomic treatments. These are less expensive and deal directly with reducing raindrop impact, increasing infiltration, reducing runoff volumes and decreasing water velocities. Mechanical measures are largely ineffective on their own because they cannot prevent detachment of soil particles. Their main role is in supplementing agronomic measures, being used to control the flow of any excess water that arise.

SOIL EROSION AND CONTROL

The ultimate success of soil conservation schemes depend on how the nature of the erosion problem has been identified and on the suitability of the conservation measures selected to deal with the problem and relate to the agricultural or landuse system so that farmers and others are willing to implement them. A sound landuse plan is important for soil conservation, whereby the land is used for what it is best suited under present or proposed economical and social conditions, land tenure arrangements and production technology. By adopting the land capability classification as the methodology for landuse planning, the distinction will be made between areas where erosion is likely to occur when the land is used in accordance with its capability and that which will arise from misuse of the land. Once the most appropriate landuse has been determined, soil conservation is a matter of good management of the land. Erosion-control measures proposed must be relevant to the farming system. Figure 2 gives a summary of sequence of events in planning a soil conservation strategy.

Figure 2. A sequence of events in planning a soil conservation strategy.

Soil erosion and control in cultivated lands

Background information

A risk of soil erosion exists on cultivated land from the time trees, bushes and grasses are removed. Erosion is exacerbated by attempting to farm slopes that are too steep, cultivating up-and-down hill, continuous use of land for same crop without fallow or rotation, inadequate use of fertilisers, and compaction of the soil through the use of heavy machinery. High standards of agricultural practice are an essential part of soil conservation. Erosion begins when rain falls on to bare or poorly protected soil and then moves over the surface. Anything which helps to increase the absorption of rain water where it falls and prevents the accumulation of water on the surface, helps prevent erosion.

Appropriate soil erosion and control on cultivated lands

Strip Cropping

This is a method by which strips of row crops and closely growing crops, planted on the contour, are alternated. Erosion is largely limited to the row-crop strips and soil removed from these is trapped in the next strip down slope which is generally planted with a leguminous or grass crop. In Swaziland strip cropping is encouraged where maize is grown. The grass strips are about 2-4m wide and the cropped area about 15-45m wide depending on the slope. The size of strip will be determined by the number of passes one would make - meaning that the size of strip will be a function of the machinery to be used.

The slope will also limit the strip size e.g., sloppy lands requires a smaller strip width, yet a rather flat land will necessitate a wider strip.

The following equation can be used to determine the width of strip:

W = 51.2 - (2.1 x S)

Where:
W = strip width (m)
S = slope (%)

For example; if the slope of an area is 8%, then the appropriate stripe width would be:

51.2 - (2.1 x 8) = 35.3 m

This works for slopes of between 3 -18%

The main disadvantage with strip cropping is the fragmentation of the land which limits the efficient use of machinery so is not suitable for highly mechanised systems. Smallholding are better served with strip cropping.

Figure 3. Grass strips which have been in place in most arable land in Swaziland for the last fifty years.

Contour tillage

Contour tillage is carrying out ploughing, planting and cultivation on the contour. This can reduce soil loss from sloping land up to 50% compared with cultivated up-and-down the slope land. The effectiveness of contour farming varies with the slope steepness. Protection against more extreme storms is improved by supplementing contour farming with strip-cropping.

Tied ridging

This consists of covering the whole surface with closely spaced ridges in two directions so that the ground is formed into a series of rectangular depressions. The rainfall is held in place where it falls until it infiltrates into the soil. There will be no run-off and therefore no overland flow erosion. If the soil becomes saturated and the depression fill up and then overflow, the ridges will break. If they fail, the sudden release of run-off is likely to cause more serious damage.

Contour bunds

Contour bunds are earth banks, 1.5 to 2 m wide, thrown across the slope to act as a barrier to runoff, to form a water storage area on their upslope side and to break up a slope into segments shorter in length than is required to generate overland flow. They are suitable for slopes of 2 to 15% are often used as permanent buffers in a strip-cropping system. The banks are spaced at 10 to 20m intervals and are normally hand constructed.

Figure 4. Tied-ridges on maize lands which effectively hold the rain until it can infiltrate.

Figure 5. Diagrammatic sketch of a contour bund.

Waterways

The purpose of waterways in a conservation system is to convey runoff at non-erosive velocity to a suitable disposal point. A waterway must therefore be carefully designed. The most satisfactory location of a waterway is in a well vegetated natural drainage line where the slopes, cross-sections, soil and vegetation have naturally developed to received and carry the runoff - it therefore needs only to be protected against deterioration. If there is no natural waterway than an artificial waterway needs to be constructed. Artificial waterways are normally protected by grass (Paspalum spp, kikuyu, African star grass) and so are referred to a grassed waterways. Grassed waterways are shallow and wide to obtain the maximum spread of water over a wide cross-section. A certain area of land has to be withdrawn form production and dedicated to the protection of the soil. Grassed waterways can be used in areas where there is sufficient moisture available to sustain a good grass cover. Where moisture is not sufficient and irrigation is not feasible, then the waterway may be paved with stone, masonry, concrete or some other durable material.

Figure 6. Typical layout of waterways in a soil conservation scheme.

The cross-section of waterways depends on the slope, soil texture and the area to be drained. Waterways should have a parabolic cross-section and be covered densely with locally adapted grasses. The deepest cut should be between 0.5 and 1.0 m. Generally, grasses which spread by rhizomes (e.g., Cynodon dactylon; ngwengwane) are the best types for the purposes. Once a waterway is in place, it should always be crosses with raised implements, otherwise the vegetation will be destroyed. In the case of implements which cannot be raised, crossing lanes should be provided. Before the onset of the rains, the grass in the waterway must be cut, so that the flow of water can proceed smoothly without causing eddies. Fertilisers should be applied regularly according to the requirements of the grass stand.

Figure 7. Typical cross-section of a grassed waterway.

Soil erosion and control in grazing lands

Grazing occupies about 65% of the lands in the whole of Swaziland, and most of the grazing is restricted to the Swazi Nation Land (86% for SNL). Natural veld comprises about 95% of the land available for grazing in the country (100 for SNL), with the remaining 5% being improved pastures used mainly for dairy production. Soil erosion is exacerbated by overstocking and poor veld management in SNL which is communally grazed. Maintenance of the more productive and palatable perennial grasses should be the objective of sound veld and pasture management. Where this is achieved, pasture and soil stability is assured. Management strategies are also necessary to address the soil erosion problems in Swaziland. Stocking within the carrying capacity of available grazing area is of primary importance in sustainable management of grazing lands. Periodic (rotational) resting of pastures is necessary to allow built-up of root reserves in the plants during autumn when plant nutrients are translocated down from the leaves and stems. Rotational grazing should also be practices. This implies the movement of animals from one grazing camp to another, each being grazed in turn. The problem faced with communal grazed lands in Swaziland is lack of fenced camps to enable rotational resting and rotational grazing. Fenced camps without proper management may lead to more soil erosion, since some camps may end up being overgrazed. The quality of fodder available in natural rangelands can sometimes be improved by introduction of superior grasses or legumes. The introduction of legumes could improve pasture and livestock production in Swaziland, and thus help in controlling soil erosion; but the economics as well as the establishment and management practicalities must be given due consideration. Overgrazed and eroded areas can be fenced to exclude grazing and allow regeneration of vegetation. In some cases trees and grasses can be planted to speed the establishment of a good ground cover to minimise impact of soil erosion.

GULLY EROSION

Gully erosion in Swaziland

An eroded rill, on deepening and widening, becomes a gully. A gully is sufficient deep that it will not be obliterated by normal tillage operations, whereas a rill is of lesser depth and would be smoothed by ordinary tillage. The distribution of gullies in Swaziland as mapped from aerial photographic surveys is shown in Figure 2. Densities were found to reach 20 gullies within an area of 5.0 km2 in parts of central Swaziland where some of the gullies (dongas) cover areas of up to 5.0 ha and more than 25 m deep. In rural areas, the erosion constitutes not only a threat to land availability, consuming pastoral and arable land alike, but it also disrupts roads and railways by undermining their foundations. It is argued that gully erosion alone accounts for the loss of a total of about 2,000 to 3,000 ha of land per annum in Swaziland. Although there is loss of land due to gully erosion, often the eroded soil is relocated to the lower parts of the same catchment.

Causes of gully erosion

Gully erosion is determined by many factors. Some factors determining the potential hazard and others determining the intensity and rate of gully advance. In addition to anthropogenic factors, rainfall, vegetation cover, lithology, land form, and land use are also important physiographic factors affecting gully erosion. It can be argued that livestock (cattle) is the main cause of gully erosion in Swaziland. Uncontrolled overgrazing in SNL has led to denudation of vegetation. Cattle grazing in and around active gullies extend the gullies knick point and dimensions. The destruction of protective vegetative cover and the trampling effect of animals on reduction in infiltration rate are important factors in increasing the rate and volume of runoff flow into the gully. Cattle tracks form waterways where a large concentration of water runoff found. The tracks eventually develop into gullies. This leads to development of new cattle tracks that are also destined to a similar fate.

Figure 8. Distribution of gullies in Swaziland.

Figure 9. Anthropogenic factors responsible for gully.

Gully erosion control

Controlling gully erosion can be an elusive process. The rate of success in such schemes depends on the planning, design, and techniques employed. The ultimate success is governed by the proper diagnosis of the problem, steps taken to eliminate the causes, and the drastic changes in land use to stabilise the ecosystem. The benefit/cost ration of gully control must be carefully assessed. Some gully control measures are extremely expensive and resource-poor farmers cannot afford to invest. Furthermore, subsistence farmers, pre-occupied with food production for hand-to-mouth consumption, are not concerned with the stewardship appeal for preserving the resources for future generations. This means that gully preventive or control measures must produce short-term benefits in terms of increased yield, more land available for cultivation, and reliable crop yields through improved soil-water use. Above all, expensive measures of gully control and/or restoration have not been widely successful. A sequence of steps recommended for restoring land degraded by severe gully erosion is shown in Figure 10. Inventory of land, vegetation, hydrology and drainage pattern, climate, and land use is the first major step towards understanding the present status and potential risks of soil erosion. It is also important to assess the capability and suitability of major landscape units and to evaluate various options and priorities them with due considerations to socio-economic factors.

The first step in controlling gully erosion is fencing of the gully head to protect it from grazing cattle and/or wild animals. Second, diversion ditches or waterways should be installed to divert the surface runoff away from the gully head. The waterways should be properly designed and laid out. The runoff should be properly disposed to avoid erosion. The land use and soil management in the watershed area feeding into the gully should be changed to soil-enhancing practices, eg., planting cover crops and trees. Stabilising the eroding faces and bed of gully is an important reclamative step. Establishing vegetation at the gully bed to provide more biomass is an important factor in decreasing the sediment-carrying capacity of the gully runoff. Engineering structures for gully erosion control have been used in other countries. Thy are however expensive to install and maintain. There are a wide range of engineering structures, e.g., diversion channels, gabions, check dams, rino blankets, and drop structures. Some of these structures will be described further in the practical manual accompanying this handout.

Figure 10. Sequence of steps in restoring land degraded by gully erosion.

CONSTRUCTION AND MAINTENANCE OF EROSION CONTROL STRUCTURE.

Stabilisation structures play an important role in gully reclamation and gully erosion control. Small dams, usually 0.4 to 2.0 m in height, made from locally available materials such as earth, wooden planks, brushwood or loose rock, are build across gullies to trap sediments and thereby reduce channel depth and slope. These structures should be used in association with agronomic treatment of the surrounding land where grasses, trees and shrubs are planted. The dams have to be carefully designed.

Table 1. A guide to spacing of dams of various heights.

Keying a dam into the sides and floor of the gully greatly improves its stability. This entails digging a trench, usually 0.6 m deep and wide, across the channel.

The design and construction of various types of stabilising structures will be discussed in the practical manual to accompany this manual.

CONCLUSION

In this manual land use issues and soil conservation was discussed in general. Factors affecting soil erosion were highlighted, and the status of soil erosion in Swaziland was discussed. The appropriate soil erosion and control on cultivated lands were explained. The basic concepts on soil conservation in grazing lands were highlighted. The theoretical aspect of factors responsible for gully erosion were also discussed, and gully erosion control introduced. This theoretical background on soil erosion and control will be followed by practical sessions on soil conservation. The practical session will be held on a catchment for a dam which is due for rehabilitation in the lowveld of Swaziland. The exercises will include the following activities:

• Identification of erosion problems
• Basic soil classification and land evaluation
• Stone packing (rock packing)
• Stone check dams
• Brushwood dams
• Brush packing
• Gabions
• Rino mattresses (baskets)
• Stone lines
• Wattling
• Rip-rap

Members of the community where the dam is situated will be involved in the exercise with the hope that they will continue with the rehabilitation exercise even after the end of the training session.

Practical One

Identification of erosion problems around the Ndvongeni dam catchment area.

Introduction

The Ndvongeni dam system area is located in the Sigcaweni area in the Lubombo region. There is an existing dam, which breached in 1992. The Ministry of Agriculture and Cooperatives (MOAC) has repaired the embankment. But the embankment is now in a very poor condition and the reservoir has been severely silted up, causing the dam to dry up in winter. The dam has suffered extensive damage to the spillway. It has been decided to fully rehabilitate the dam system. The catchment of the dam covers about 3.5 km2, and downstream areas are entirely Swazi Nation land with extensive grazing and small scale rainfed arable farming as major land uses.

A map has been prepared by the EDF Dams Rehabilitation and Construction Programme of the MOAC (NEDECO. 1996). Showing the actual erosion status of the dam site (Figure 1). The status of erosion has been found to range from slight sheet erosion to extreme gully erosion with gullies being as deep as more than 2 m.

Aim of exercise

The aim of this exercise is to identify the different types of soil erosion in the area in order to come up with a plan of action of stabilise the erosion and rehabilitate the area.

Equipment

Topographic map of site at scale of 1:25,000 (PWD number 19).

Ndvongeni actual erosion status map at scale of 1:10,000 (prepared by EDF earth dams rehabilitation and Construction programme (MOAC, NEDECO 1996).

Transparency with grids of 1 cm by 1 cm

Calculator

Procedure

Study the topographic map of the area and identify the site of the dam.

Indicate the catchment area boundaries on the map.

Estimate the size of catchment area using the transparency with grids of 1 cm by 1 cm. This is done by calculating the number of 1 cm by 1 cm squires within the catchment area, and multiplying them by the area represented by each squire. For example if the scale of the map is 1:25,000, then each 1 cm by 1 cm squire will represent 25,000 cm by 25,000 cm; which is 250 m by 250 m (being 0.25 km by 0.25 km). If there were 100 squires covering the whole catchment area, then the ground area for catchment will be (0.25 km x 0.25 km) x 100. Which would be 6.25 km2.

Identify the different types on soil erosion and compare them with those indicated on the "Status of soil erosion map".

Discuss about the possible measures to control the erosion on several of the soil erosion sites.

Practical Two

BASIC SOIL CLASSIFICATION

Introduction

The soil survey of Swaziland started in the 1950s with the identification of soil series called "the lowest conceptual soil category of the genetic classification". The soil series is a group of soils having soil horizons similar in differentiating characteristics and arrangement in the soil profile and developed from a particular type of parent material". Soil series that are alike in their morphology, and in the practical use that can be made of them, are grouped into "soil survey sets". One hundred and seven soil series have been allocated to 34 soil sets in Swaziland. The soil sets have been designated by letters of alphabets and the names of series within each set begin with the same letter. The majority of soil series are named after places or condition of the soil in relation to its potential use. Series nomenclature has been popularised within Swaziland, and many farmers know and use the series names relevant to their own fields. A handbook for use as a guide in classification of the Swaziland soils is available from the MOAC.

Quantitatively one can distinguish very bad areas of soil erosion as often having poorly structured soils of light texture on steep. Gradient. Another category of soils which are erosive has Two-Deck morphology with a shallow, sandy top. These soils may be quiet gently sloping and yet subject to rilling and topsoil sheet movement because of their poverty in binding agents such as humus and low basal cover caused by a combination of overgrazing and recurrent drought. Table 1. Below shows the soil sets (with examples of soil series) which are considered to be erosive in Swaziland. Most of the conspicuous barren hillsides in Swaziland comprise M set. It has been noted that steepness alone does not seem to be sufficient reason for erosion on the M set. Exhausted, abandoned fields occupy most of the area concerned. The proportion of M set that is eroded is not large, but the bright colour of exposed Malkerns series makes eroded areas catch the eye.

Table 1. Quantitative erodibility of some soils of Swaziland.

The soils are differentiated on the basis of effective depth, soil colour, soil texture, soil structure and parent material. The relative proportions of sand, silt and clay in a soil are used to classify soils into soil texture. Figure 1. shows the soil texture triangle which can be used to determine the texture. Several methods can be used to assess texture of the soil; including laboratory analysis, and simple hand test. For the simple hand test you start with a moist soil sample, free of roots, stones, etc., and follow the flow chart shown in Figure 2. This method is sufficient for estimating texture in the field.

Aim

The aim of this exercise is to determine the different soil textures.

Material

• Auger
• Shovel
• Water bottle (with water)
• Flow chart for determining soil texture

Procedure

Obtain a sample of soil from different eroded sites (eroded and non eroded). Follow the procedure indicated in Figure 2 to determine the texture of the soil sample.

Figure 1. Soil texture triangle.

Properties of soil textures.

Figure 2. Flow chart for determining soil texture by "feel" method.

The texture of a mineral soil can be assessed by simple hand tests. Start with moist sample, free of roots, stones, etc., and follow the flow chart below.

Practical Three

CONSTRUCTION OF STONE CHECK DAM

Introduction

Construction of a stone check dam begins by sloping back the tops of the banks. A trench is then dug across the floor of the gully and into the banks into which the large rocks are placed to form the toe of the structure. Rocks smaller than 100 mm in diameter should not be used because they will be quickly washed out. A dam made of large rocks will leave large voids in the structure through which water jets nay flow, weakening the dam. To avoid these effects, the dam should be made with a graded rock structure. An effective composition is 25% of rocks between 100 and 140 mm diameter, 20% between 150 and 190 mm, 25% between 200 and 300 mm, and 30% between 310 and 450 mm. A second trench should be made to mark the downstream end of the apron and filled with heavy rocks. A 100 mm thick layer of litter, such as leaves and straws is laid on the floor of the apron and covered with a solid pavement of rock. A thick layer of litter is also placed on the upstream face of the dam.

Material

• Pick
• Shovel
• Rocks of varying sizes ( between 450 mm and 100 mm)
• Litter (leaves, straw, fine twigs)

Method

See above notes (introduction) and Figure 3 below)

Figure 3. Construction of a rock check dam.

Practical Four

CONSTRUCTION OF A BRUSHWOOD DAM

When building a single brushwood dam, the gully banks are first sloped back and stout posts are then driven into the floor and banks of the gully to a depth of about 1 m below the surface and about 0.5 m apart. A 150 mm thick layer of litter is placed on the floor of the gully between the posts extending upstream to the proposed base of the dam and downstream to the end of the apron. Green tree branches or brush are laid on top of the litter, the longer ones at the bottom, with the butt ends upstream. The gully is filled with brush which is trampled to compress it into a compact mass. Cross poles are fixed on the structure with galvanised wire. A layer of litter is placed on the upstream face of the dam and packed into the openings between the butt ends of the brush.

Material

• Pick
• Shovel
• Poles (about 2.5 m long)
• 4 Pounds hammer
• Bushknive
• Galvanised wire (flexible)
• Litter (leaves, straw, fine twigs)
• Brushwood wood

Methods

See Figure 4 and notes on introduction above.

Figure 4. Construction of a brushwood dam

Practical Five

INSTALLATION OF WATTLES FOR SLOPE STABILISATION

Introduction

Wattles can be used to control erosion on steep slopes. Stability for a short period until a dense vegetation cover has had a chance to grow can be achieved. Bundles of woody plant stems are placed into bundles, using species which will root easy such as Lusololo into shallow trenches with rows spaced 1 to 6 m apart. The wattling is tied to stakes on the downslope side of the trench and secured by further stakes driven through the material . After installation, the wattling is covered with soil until only 10% of the bundle is exposed. Grasses and trees can be planted between the wattles. This system can be very effective in stabilising the banks of a dam.

Material

• Stakes
• Pick
• Shovel
• Woody steam
• Tree bark (emantfweshu)
• 4 Pounds hammer

Methods

See Figure 5 and notes on introduction above.

Figure 5. Installation of wattles.

Practical Six

CONSTRUCTION OF GABION BASKETS

Box gabions consist of rectangular units, fabricated from a double twist hexagonal mesh of soft annealed, heavy zinc coated wire. The wire quality and the zinc coating meet all international as well as S.A.B.S. specifications. The mesh panels are reinforced at all edges with wires of a longer diameter than that used for manufacturing the mesh, to strengthen them and to facilitate construction. The double twist hexagonal mesh construction of the gabion permits it to tolerate differential settlement without fracture. A gabion basket is a heavy monolithic unit able to withstand earth thrust. It efficiency increases instead of deceasing with age since further consolidation takes place as silt and soil collect in the voids and vegetation establishes itself. There is little maintenance required for gabions. A minimum foundation preparation is required, the surface needs to be only reasonably plane. There is no costly drainage required, as gabions are permeable. Because gabion baskets permit the growth of vegetation and maintain the existing environment, they provide attractive and natural building blocks for decorative landscaping. They can also be used in gully control and reclamation structures. The local suppler of gabion baskets is Swazi Wire (Tel 84010).

Figure 6. Steps involved in construction of gabion baskets.

Practical Seven

CONSTRUCTION OF RENO MATTRESSES

The reno mattress is a special form of gabion with a large plan area. It is fabricated from similar but smaller double-twist hexagonal mesh of that used to manufacture the gabions. The wire characteristics are the same. The diaphragms are spaced usually at 1.0 m, and a continuous panel of mesh forms the base, the side and the walls of the unit to obtain an open topped multi cell container. The same mesh is used for the base, diaphragms and the separate lid. All panel edges are salvaged with a wire of larger diameter than that used for the mesh, so as to strengthen the structure. The reno mattress is often referred to as "the African gabion". An outstanding advantage of the reno mattress is its flexibility. It’s double twist hexagonal mesh construction permits it to tolerate differential settlement without fracture. This property is especially important when a structure is on unstable ground. Reno mattresses can be used to stabilise gully heads of less than 1 m deep. They can also be used to dacipate the kinetic energy below spillway walls to prevent erosion on the spillway.

The local suppler for reno mattress basket is Swazi Wire (Tel 84010).

Figure 7. Steps involved in construction of reno mattress.

Practical Eight

BRUSH PACKING

This method can be used in small gullies of less than 1 m. The following steps are followed:

Cut the lower branches of the tree canopy, as is not recommended to chop the whole tree in conservation.

Pack the branch in the gully with the leaves facing upstream, and put the closely.

The main advantage of brush packing is that there is no technical knowhow required. Local material is used. The branches packed in the gully will act as a barrier to water runoff, reducing its velocity, and encouraging sedimentation. Brush packing works best with a combination of planting vegetation on the deposited sediments.

Material

• Bushknife
• Local trees

Practical Nine

STONE LINING

The first point to be considered in simple and easy measures of soil conservation if is farming on the contour. Structures on the contour are simpler and cheaper than graded channel terraces as there is no need to set them out on a precise gradient. They should be more or less on the contour, but small errors are not as important as in the case of graded channel terraces. A general term for simple structures on the contour is "stop wash lines". The form of such line will depend on what materials are available. On stony ground, using the stones to build stone lines serves the dual purpose of clearing them from the filed as well as building the stop wash line. Where stones are not available, lines can be formed by piling up crop residues, perhaps with a few shovels of soil, and progressively built up later by adding weeds from hand hoeing. Stone lines can be used effectively to control sheet erosion as well as erosion along minor cattle tracks.

Figure 8 Stone line set out on the contour.

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This page was last updated on 12 February 2008