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CLOSE THIS BOOKHandbook for Agrohydrology (NRI)
Chapter 3: Erosion and sedimentation data
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENT3.1 Soil erosion
VIEW THE DOCUMENT3.2 Field measurement of sediments (eroded material)
VIEW THE DOCUMENT3.3 Laboratory analysis
VIEW THE DOCUMENTEquipment costs
VIEW THE DOCUMENTAppendix B: Erosion and sedimentation data

Handbook for Agrohydrology (NRI)

Chapter 3: Erosion and sedimentation data

This chapter covers five main topics:

- The estimation of soil loss by the application of empirically derived equations.

- Methods of measuring soil loss by the collection of the total amount of eroded material from runoff/soil erosion plots.

- Methods of sampling runoff that are carried in suspension to determine overall soil losses.

- Methods of laboratory analysis that determine the quantities of suspended soil material.

- Methods of laboratory analysis that determine soil particle size.

For practical purposes, soil erosion is regarded as the detachment of soil materials from their previous location. Sedimentation is the transport and deposition of eroded soil and although erosion due to wind occurs, the main transporting agent in almost all environments is water. Thus the theoretical calculation of soil losses are covered by estimates of erosion, whereas the actual measurement of material lost from a catchment is regarded as the measurement of sedimentation.

The erosion that leads to a wide range of environmental problems is usually the result of human activity; such as deforestation, cultivation and over-grazing, and is intimately linked with the runoff process. Agriculture is often a powerfu1 agent in promoting soil erosion and water harvesting frequently exploits the conditions that promote runoff and which can intensify rates of soil removal. The processes of sedimentation are frequently attendant.

Soil loss data are expressed in terms of weight per unit area, for example tonnes per hectare, per season. The measurement of sedimentation, the quantification of eroded soil materials, is likely to be of importance to projects that engage in the activities of agrohydrology and methods of reducing soil erosion are an important aspect of this book. A background is given to the main methods of erosion control, below, but the mechanical and constructional aspects of these controls are discussed in detail in chapter 7, Water Harvesting.

3.1 Soil erosion

The climatic factors that influence erosion and sedimentation processes are chiefly rainfall amount and intensity, which largely determine rainfall energy and runoff quantity, though the relation between them and soil loss is very complex.. Vegetation cover reduces rainfall energy and retards surface water flow, encourages infiltration through the physical perforation of soils and the reduction of the soil moisture reserve. Topography determines land slope and the length of flow of surface runoff, while the character of soils themselves, in terms of texture, structure, density, etc., influences the rate at which soil loss and sedimentation will take place.

Erosion can be summarised as taking five characteristic forms:

Rain splash erosion is the local movement of soil particles under the influence of raindrop impact. Soil particles are detached from the soil surface, elevated by the action of splash and return to the surface somewhat lower down the land slope. Areas with high slopes suffer from such erosion to a much greater extent than flat land. Large quantities of soil are removed from their original ground location by the action of raindrops. The energy equation that relates rainfall energy to intensity, developed by Wischmeier and Smith is of the form:

(kinetic) Energy, E 12.1+8.9 i where (3.1)

E is in m-Mg/ha-mm (metre-metric tonnes per hectare-millimetre) and rainfall intensity "i" is in mm h-1

Clearly such factors as wind direction, drop size, velocity and the nature of the soil will also affect the quantity of erosion that occurs and although small, clayey particles are more easily transported than larger sandy ones, they are not so easily detached from the soil surface.

Sheet erosion is a simplified term for the formation of extremely small channels or rills. These are created under the influence of rain splash and microscopic topographical variability and their wandering causes the eventual erosion of soil in the manner of a sheet. For a given soil and vegetation cover, the erosive power of the overland flow will be related to its velocity and depth.

When runoff concentrates into small streamlets Rill erosion takes place, forming small channels. These channels are clearly visible, but their size is such that they can be obliterated by ploughing. The concentration of flow into rills is particularly important because it leads to the concentration of runoff, increasing its velocity and erosive power. On high slopes and shallow soils such erosion can be destructive.

Gully erosion occurs when channel flow is sufficient to overcome tillage practices and is yet another stage of the increasing concentration of runoff, even though it may take place on an ephemeral basis. Gullies extend by the cutting back of their head and by progressive channel erosion, but stabilisation may occur naturally as the channel becomes harmonious with its slope and vegetation growth is established

Stream channel erosion is dissimilar to gully erosion in that it represents a process that occurs in channels with lower gradients, in which streams often flow continuously. Suspended material is carried along without contact with the bed, while material moved by the process of saltation bounces or skips along. The bed load is rolled or pushed along the channel bottom.

Soil erosion is a natural recycling process that has continued throughout geological time and has been responsible for the formation of sedimentary deposits that are now the major components of the continental land masses and sea floor. Tolerable rates of erosion have been suggested as being between 5 and 10 tonnes per hectare per year, but extremely different local circumstances of soil depth, formation and productivity make such values rather meaningless.

Influences on Sedimentation

Catchment Size. Generally, an increase in catchment size reduces the proportion of material removed (the "sediment delivery ratio"). This is due the increased opportunities for the entrapment of sediment within the catchment area. The presence of flood plain areas adjacent to large streams and rivers provides an environment for deposition that does not exist with smaller, steeper channels.

Topography and Channel Density. Important topographic factors that influence sediment removal are channel slope and the channel density of a catchment. The sediment delivery ratio is highest for steep channels with well defined courses, rather than low slope streams with ill defined channels. The use of channel density factors (see chapter 6) is common in the regression analysis of sedimentation data.

Precipitation and Runoff Regimes are critical factors in determining the removal of sedimentary material. Streams of a flashy nature are effective at its removal. High intensity storms not only give large runoff amounts, but also increase soil erosion by rain splash from high energy drops. Catchments that suffer from such storms display high delivery ratios.

Loss of Vegetation Cover and Agricultural Activity play a major part in creasing soil erosion.

These influences are built in to the theoretical models of soil erosion that are discussed below.

3.1.1 Theoretical Estimates of Soil Erosion

a. Universal Soil Loss Equation (USLE)

The Universal Soil Loss Equation (USLE) is widely known and was developed in many locations of the US by Wischmeier and Smith. In the 1978 publication (USDA Handbook 537), site data from US locations are given. For example, the full set of data of which Table 3.3 is a sample, covers 160 crop to fallow conversion ratios of soil loss (see Appendix B). The USLE is used to determine the value of conservation measures in farm planning and predicts non-point sediment losses. As its name suggests, it is the most widely accepted method of estimating soil loss and has generated variations that are adapted to various local conditions. Special note is made of the difficulty of applying sod-based rotations in semi-arid areas as are soil and moisture conservation opportunities of residue/mulch management (Table 3.3). However, it is important to point out that despite the simplification of the variables involved in the erosion process by the USLE, its use is often limited because the evaluation of these variables has not been achieved in many regions of the world. Those wishing to apply the USLE are recommended to obtain Handbook 537, but an outline of procedures is presented below. The average annual soil loss is given by:

A = 2.24 RKLSCP where (3.2)

A = average annual loss of soil in Mg ha-1 (tonnes ha-1)
R = rainfall and runoff erosivity index by geographical location
K = soil erodibility factor
LS = topographic factor
C = crop management factor
P = conservation practice factor

The factor R was found (under fallow conditions) to be related to the maximum 30 minute rainfall intensity and the kinetic energy of storms. The factor K in t ha-1 was assessed by measurements of actual soil loss for a series of soils with a range of physical and chemical properties. The factor LS converts soil losses from the experimental plot length and slope (22m and 9%, respectively). The conversion formulae for different slopes and values of L are:

L =(1/22)x and (3.3)
S=(0.43 + 0.30s + 0.043s2)/ 6.574 where (3.4)

x - a constant, 0.5 for slopes > 4%, 0.4 for slopes 4% and 0.3 for slopes < 3%
I = slope length in m
s = field slope in %.

C, the cropping management factor includes the effects of cover, crop sequence, length of season, tillage and storm time distribution. Conversions may be made from cropping to continuous fallow. P, the conservation practice value, discriminates between contouring, strips and terraces. Terraces alter the value of the slope length L, which becomes the terrace interval for losses from the terrace, whereas if losses from the terrace channel are required, the contour factor is applied.


Table 3.1: Values of R for Different Return Periods, Annual and Single Storms, USA

Estimates of soil losses can be made and if found to be unacceptable, the manipulation of farm management and conservation practices can be planned for its reduction: for example contour and terrace size and intervals. Although soil loss in terms of t ha-1 (with account taken of economic productivity) is the usual criterion by which the acceptability of erosion is judged, the effects of sedimentation may demand that smaller soil losses should be aimed for.

Tables 3.1 to 3.3 and Figure 3.1 give typical values for the variables in equation 3.1.


Table 3.2: Soil Erodibility Factor, K, by Soil Texture in Mg ha-1 (t ha-1) *


Table 3.3: Ratio of Soil Losses from Crops to Corresponding Loss from Continuous Fallow

Crop stages:

0 Turnploughing to seabed preparation: 1 Seedbed to first month after seeding: 2 Establishment to second month: 3 Growing cover from 2 months: 4 stubble or residue to new seedbed.

RdL = Crop residues left and incorporated by ploughing: RdR = residues removed


Figure 3.1: Factor LS soil loss for Length and Slope

Some success was obtained using the USLE at Patancheru, India by ICRISAT, though generally the slope factor was not seen to influence soil loss strongly. In South Africa, research in the Orange Veldt region, using rainfall simulator tests to validate USLE information derived largely from the USA, has shown that the use of these inputs was acceptable.
Table 3.4 below gives values for recommended conservation practices. These are greatly simplified from the complex indices given by Wischmeier and Smith (1978) which include values for rangeland, pasture, crops, woodland mulches, etc. Slope length limits are derived from work by the US SCS.


Table 3.4: Recommended Conservation Practices

Worked Example

Calculate the annual soil loss for a location with R= 310, loamy sand soil with K= 0.10 (Table 3.2), C = 0.32 (Table 3.3) length L= 60 m and slope S =12%.

From Figure 3.1, LS=2.5 and the field is to be contoured, therefore from Table 3.4, PC=0.5.

Annualsoilloss = 2.24 × 310 ×x 0.1 × 2.5 × 0.32 × 0.6 = 33.3 Mg ha-1

A reduction of these levels of soil loss would appear desirable. This must be undertaken by changing the cropping factor C and/or the conservation practice P and thereby reducing the value of these indices.

b. Modified USLE

The replacement in the USLE, of the rainfall energy factor with a runoff energy factor, has been undertaken in the USA. The equation of prediction is:

Y = 11.8 (Qqp) 0.56 KCPSL (3.5)

All components of the equation are as for USLE, except the energy factor, 11.8 (Qqp) 0.56 where Q = runoff volume in m³ and qp is the peak flow in m³ s-1.

A combination of measurements from a variety of catchments were tested against this equation and the results encouraging, however the limitations in this model must be regarded as being similar to the original USLE model.

c. Soil Loss Estimation for Model for Southern Africa (SLEMSA)

This model was developed in Zimbabwe, following disappointing results using the USLE. It is based on defined agroecological zones, their physical environments and soils. In particular, the concentration of the USLE on cropped areas and cropping systems was regarded as unsuitable in a region where rangeland conditions are very important. The structure of the model is shown in Figure 3.2 which also provides details of the model components. However, values of many of these components have not yet been determined outside Zimbabwe. Some extrapolation of the model to other countries in the region has been made, but this work has largely depended upon the direct translocation of experimentally-derived values from Zimbabwe, in particular the regression coefficients for the bare soil sub model K, and the soil erodibility factor F. Other components can be measured or determined at a location. Compared to USLE, this model may become more widely used because of the relative simplicity in obtaining empirical values.


Figure 3.2: Structure and Components of the SLEMSA Model

Empirical studies have defined the values of the parameters K, C, and X in the following terms.

E = Seasonal rainfall energy

Joules m²

Energy of mean annual rainfall

F = Soil erodibility

-

Index of soil characteristics related to known erodibilities

i - Rainfall energy intercepted

%

Vegetation cover

S = Slope steepness

%

From contours

L = Slope length

metres


Sub-models



K = Bare soil condition

tonnes ha-1 linking E and F

C = Canopy cover

-

Soil loss ratio related to i

X = Topography



Main Model



Z = Predicted soil loss

tonnes ha-1 = K·C·X

The value of index K, the bare soil submodel, is obtained by estimating rainfall energy. For example in Zimbabwe and Botswana, research has indicated that with an average annual rainfall of 550 mm, the energy received = 10, 000 joules m-² . Luvisol and Regosols were given an erosion index of 4 (the F value). The equation combining rainfall energy and erosivity was found to be:

K= exp (0.461 + 0.7663 F) ln E + 2.884 - (8.1209)F (3.6)

The canopy cover submodel converts the bare soil submodel K soil loss prediction, to a prediction for an area with vegetation, in the form:

C = exp(-0.06) i where (3.7)

i is the intercepted energy and = mean cover.

The topographic submodel, X, takes account of slope with the equation:

X= L0.5 ( 0.76 + 0.53 S + 0.076 S2) / 25.65 where (3.8)

L = slope length in m
S = slope in %

Calculations of soil loss were undertaken in grid cells of 1 km².

d. Other Models

The inability of general models to give good results under different geographical conditions has led many researchers to develop their own for particular localities. The majority of these have been regression models, whereby the dependent variable, soil loss, is regressed against a combination of independent variables. In most cases, erosivity equations giving the highest statistical correlations have been developed from the use of a rainfall /energy factor.

Some examples of rainfall/energy relations that have been examined and found to be significantly related to soil loss are given below, but the problems of site specificity are equally as important with these examples as with more general types. Variations in soils, slope and vegetation cover, as well as the characteristics of rainfall, render such equations subject to misuse. However, they do illustrate the main direction of research into soil erosion studies and the main factors in soil loss processes.

Some examples are given below:

Index

Localities

1. KE > 25 index

Nigeria, Zimbabwe

2. EI30

index USA, Kenya

3. EI5

index Zimbabwe, Kenya

4. E15

index Zimbabwe, Kenya

5. AI30

index Nigeria

6. p2/P

index Kenya

7. SUM of pi2/P index

West Africa (related to EI30)

In case I, the energy of all rainfall intensities greater than 25 mm hr-1 are summed for example:

(a)

(b)

(c)

(b)

x ( c)




Intensity (mm hr-1)

Amount (mm)

Energy (J m-2 mm-1)

Total

0-25

24

-

-

25-50

25

25.2

630

50-75

20

27.8

417

> 75

10

28.3

290




1337

Energy values for column (c) are obtained from E = 30 - 125/I

In the cases of 2, 3 and 4 (EI indices) the subscript refers to the intensity period in minutes. The energy values are obtained from E = 11.9 + 8.8 log I, then total energy is multiplied by the intensity to give an erosivity value.

In example 5, the index is obtained from the total storm rainfall (A) and peak intensity, I30. In 6 and 7, p is mean rainfall for the wettest month and P is mean annual rainfall.

Examples of EI regression equations from Kenya are:

Soil loss in tonnes per hectare

= 0.026 (EI15) - 1.18 with

R2 = 0.71 and standard

error = 4.19



Soil loss

= 0.35 (EI30) - 1 11

R2 = 0.69 se = 4.26

Soil Loss

= 0.0054 (AI15) -1.35

R2 = 0.73 se = 4.07

3.1.2 Soil Erosion Control Practices

Soil erosion practices are widely known in agricultural practice and a brief description is given of those most widely implemented.

a. Rotations

Rotations assist in erosion control by ensuring that soils are not exposed to the same risks each season. Differing crop plant covers, growing period and rooting densities, especially when periods of grass cover are incorporated, help to reduce erosion and increase the binding organic matter content of soils.

b. Tillage

Tillage helps to increase infiltration and reduce runoff and soil loss, at least for a short time. However, excessive tillage destroys the natural structure of soils and exposes organic matter to oxidation. Thus a balance must be made between sufficient tillage to achieve a good growing environment for crops and too much tillage which can lead to soil crust development and enhanced runoff. Ploughing depths should be varied to reduce the risk of a hard plough pan forming beneath the soil surface.

c. Minimum and Mulch Tillage

Minimum tillage can give better erosion control and reduce costs compared to conventional methods. Mulch tillage involves covering the soil surface with suitable residues and can reduce runoff and soil losses considerably. It tends to even out temperature differences and helps to protect the soil when plants are small and cannot do so themselves.

d. Grazing Control

The control of stocking rates can be an extremely important factor in preventing soil erosion by maintaining sufficient vegetation cover, preventing soil compaction and thereby reducing runoff.

e. Water Conservation

Water control and conservation, which covers a wide range of practices: tillage, cropping systems, farm planning and physical conservation techniques, is the most effective way to reduce soil erosion. It amounts to an integrated approach to land management.

Contouring: any farm operation carried out on the contour, ploughing, planting, weeding, etc. may be regarded as contouring. Surface runoff is reduced by the physical barriers thus formed which restrict the movement of water to small distances and low velocities. Ridging increases its effectiveness, but field with microtopography, drainage and gullies may be unsuitable, especially on high slopes. Breakage of the features formed by contouring concentrates runoff and increases soil erosion.

Strip cropping: this practice may be regarded as a type of contouring, but different crops, grass or fallow are placed alternately on the contour to increase infiltration and reduce runoff.

The design and construction of physical conservation practices (ridges, bunds, terraces and flow channels) is covered in detail in chapter 7.

3.2 Field measurement of sediments (eroded material)

3.2.1 Total Sediment Collection

Equipment and Collection of Data

3.2.2 Suspended Sediment Samplers

Equal Transit Rate Method
Depth Integrated Sampling
Point Integrated Sampling

3.2.3 Pumping Samplers

Considering the site specific limitations of soil erosion models, it is perhaps not surprising that erosion losses are still widely quantified by direct measurement. The practical aspects of the measurement of eroded material are discussed below.

As with runoff studies, the selection of site locations will be largely determined by their suitability to project objectives. From the agricultural point of view, sedimentation and the loss of soil is the main focus of attention and in many ways it is sensible to measure runoff and sediment at the same site and, where possible, on the same experimental plots. Runoff plots are usually used to measure basic erosion rates, or total sediment load, under specified soil/cover/ slope conditions, and replication will probably be necessary. Total sediment loss measurements obtained from small plots may not reflect real catchment conditions, because the natural conditions of runoff loss and redistribution are imperfectly represented. Natural catchment stream-gauging location points are usually suitable for suspended sediment sampling, as are artificial controls emplaced on catchment outlets. Little extra investment is needed to obtain sediment samples manually, though laboratory facilities must be available for soil loss determination. Where the site is remote and the cost can be justified, pumping sediment samplers are sometimes used. A wide range of sampling devices are available and may be adapted to suit local conditions. A number are described below.

3.2.1 Total Sediment Collection

Total sediment collection is obtained from the kind of runoff plot described in the section on volumetric runoff data collection systems, in chapter 2. These systems are constructed and operated exactly as described in the Runoff chapter, and the design criteria of the tanks, peak flows and total volume are calculated in the same way. The limitations of very small catchment size and regular site visits are also the same. The data from these plots are important in agrohydrological research and such erosion plots are commonly found on agricultural research stations. Arrangements for ploughing, cropping will be made and bare or uncultivated plots may be necessary to provide a controlled comparison. Natural slopes are best suited to the collection of useful data, as reshaped areas will not contain normal soil nor slope profiles.

Unlike most plots used for runoff measurement, the surfaces of plots which suffer from high levels of erosion may become lower over several seasons and it is worth considering whether or not to build in facilities that allow the collector gutter at the downslope end of the plot to be lowered accordingly. The gradient of the pipes leading to the collector tanks must be retained to prevent sediment collecting in them (a velocity of 0.6 m s-1 is adequate). This may mean lowering the tanks themselves, and during installation they should not be set in concrete, but put on stands suitable to accommodate the lowering. Over-deep excavation of their position may be necessary and this is often a laborious process necessitating the use of heavy earth moving equipment. Chapter 2 gives details of construction, installation and maintenance.

Samples taken from multislot and rotary dividers are expected to be representative of the total runoff and the following equations may be used in the calculation of sediment discharge.

GS =(QxC)/(Ax 103) where (3.9)
GS = Sediment discharge in kilograms ha-1
Q = Discharge in m³
C = Storm weighted concentration in ppm
A =Area of plot in hectares

Instantaneous sediment discharge rates can only be found if records of runoff rates are available, that is if a control section and water level recorder are installed on the catchment. If these data are available, then the formula to calculate instantaneous sediment discharge 'g' is:

g = (q × C) / 103 where (3.10)
q = Instantaneous discharge rate in m³ s-1
C = Concentration in ppm by weight for runoff rate q

Sediments collected in the conduit and tanks of the runoff measuring equipment are weighed in the field. Samples are taken as the material is weighed and the percent of dry material is determined in the laboratory. Dry weight of the deposits is given by:

Wsd = Wsw × Pdm where (3.11)

Wsd = weight of dry sediment
Wsw = weight of wet sediment
Pdm = percent dry material

Equipment and Collection of data

a. Multislot Dividers

For details of manufacture, installation and use of this equipment, see chapter 2. It is likely that some runoff events will provide no soil material, perhaps because the runoff plot is heavily vegetated or the rainfall is small. In these instances, sedimentation data will not be available.

In cases where runoff provides sediment that can be suspended by stirring the following procedure is followed:

- Agitation should be done energetically using flat paddles. It will probably take two people.

- Fill three 1 litre sample containers, using a smaller container to fill them. It is best to use 2 or 3 samples to fill each litre container so that examples from the mix are selected.

- The containers should be pre-labelled to avoid accidental confusion with date, time, plot, tank and sample no.

- The total measurement of runoff can then be taken, adding the 3 × 1 litre samples to the total runoff amount.

- It is important to ensure agitation of the mix is continuous, so in total at least three people will be necessary.

- This should be done for all tanks.

- As samples will have to be sent for analysis, it is important to check with the laboratory that the required number of samples can be dealt with conveniently. If not, it is probably best to reduce the overall number but increase sampling dips, so as not to become involved with problems of storage and the possibility that results will be late, samples lost or accidentally destroyed. To a large extent this will depend on individual circumstances, but it is an important point to note.

In cases where sediment is too heavy to be stirred totally into suspension, the following procedure should be followed to collect samples:

- In the divider system described in chapter 2, the sludge will be trapped in the first container in line.

- The supernatant (water and suspended) material should be removed to within about 2 cm of the top of the sludge.

- This should be done carefully, with no disturbance of the deposited material. It is likely that a syphon will have to be used rather than a pump, which could suck up the deposited material.

- Allow more time for this procedure than would be needed for runoff measurement alone.

- The sample bottles can be filled as the supernatant is siphoned and measured for runoff volume

- Stir the sludge aggressively until it is liquid enough to find its own level.

- Measure its depth. It is necessary to ensure that the form of the tank/container is known. This can be achieved by calibrating the volume of the container for a given depth (using water), after installation. The precision of calibration depends on the size of the tank, the advantage being with smaller tanks, for which greater imprecision of depth measurement gives smaller inaccuracies.

- Mix the sludge once more and during the process take the samples.

- Tanks and site should be left clean and free of debris.

b. Rotary Samplers

For details of manufacture, installation and use of this equipment, see chapter 2. - Heavy sediment should be collected and weighed from the collection conduit and transfer pipes. Tanks samples are taken as outlined above for multislot dividers.

3.2.2 Suspended Sediment Samplers

Suspended sediment samples are taken from water bodies such as streams and reservoirs and do not involve the wholesale collection of runoff. However, knowledge of the location of the source of material (other than it being somewhere within the catchment), is difficult to obtain.

Suspended sediment samplers are designed to sample the sediment/water mix and should ideally have the following features:

- They should sample with the intake at the same velocity as that of the stream.
- They should sample away from the disturbance they cause.
- They should be able to sample close to the stream bed.
- They should be rugged and inexpensive.

In general it is probably best to purchase specially manufactured equipment, though one set of such equipment could be used to calibrate the results of any locally made equipment capable of fulfilling the criteria listed above.

There are two main types of samplers: depth and point integrating samplers.

The former instrument takes a continuous sample as it is lowered and raised from and back to the water surface. Point samplers are equipped with an electrically-operated valve which can be opened at desired depths and samples taken. If the valve is left open, it will operate as a depth integrating device. These samplers are streamlined bomb-shaped instruments, housing a glass bottle type sampling container. The samplers can be used by wading in streams, but are often used on rivers that necessitate the suspension of the instrument from bridges or cable ways. Several sizes of sampler may be necessary where variations in flow volume are present. The USDA gives recommendations on sampler types according to speed of flow and depth (USDA handbook 224), but these relate only to equipment obtained from within the US. Suspended sediment samplers are especially important where runoff from river catchment areas is under study. Figure 3.2 shows a typical suspended sediment sampler


Figure 3.2: Suspended Sediment Sampler

The concentration of sediment within any flow is not only a function of the physical characteristics listed earlier in the chapter, but also of time of flow. In general, the rising limb of a hydrograph is prone to greater changes in sedimentation as the runoff process gets underway and therefore requires more frequent sampling. Similarly, small streams undergo a more rapid change in flow stage than large streams and these too require more frequent sampling.

Table 3.5 gives guidelines for sampling frequencies, but it is important to determine precise rates of sampling according to experience:


Table 3.5: Approximate Frequency of Sediment Sampling

Figure 3.3 shows the theoretical distribution of suspended sediment concentration in a stream section, compared to the velocity distribution. Coarse, sand-sized particles count for most of the variation in concentration, fine particles are usually fairly evenly distributed throughout the stream section. As in many cases actual concentrations vary with stage and turbulence, samples must be collected systematically throughout the stream section.


Figure 3.3: Sediment concentration and Stream Velocity

a. Equal Transit Rate (ETR) Method

This method is most commonly used on small streams on agricultural land and gives a discharge-weighted sample for the stream cross-section. Six to twelve sampling verticals spaced equally across the stream usually give sufficient accuracy. Stream discharge measurement at the time of sampling is not necessary, total flow may be estimated from gauge height and a rating curve or rating table. The collection and computation of data are relatively simple using this method, though an integrating sampler is needed and the transit (lowering/highering ) rate of the instrument must be uniform. The composite sample that is collected represents the mean, discharge-weighted concentration. Suspended sediment discharge is calculated from the mean concentration and the total water discharge. Because it is commonly used, this method is covered in detail below.

Figure 3.4 shows the path of a sampler during a typical sampling procedure:


Figure 3.4

Path of Sampler During Equal Rate Sampling Procedure

- Select a straight section of stream, with as uniform a cross section as possible, near the gauging station. Avoid shallow sections.

- Lay out a tape or line across the stream, standing downstream of the line if wading.

- Determine the position of about 6 verticals (sufficient for a wadeable stream of 10 m width)

- Record the stage of the stream.

- Rinse the sampler bottle, check for a clear orifice and stand about 1 m down stream of the line at the first position to take the sample.

- Hold the sampler rod and move the sampler down at a constant speed, touching the stream bed at each vertical.

- Sampling from surface to bed is recommended to avoid sampling disturbed bed material at higher sampling points.

- The sampling velocity should not exceed 0.4 times the stream velocity.

- When a bottle becomes almost full, replace it and mark the sequence of verticals and replacement .

- Usually between 1 and 6 bottles are needed.

- Record water temperature.

- Record stage if stream has fallen or risen significantly.

- It is convenient to put as much information on the bottle cap as possible for ease of future sorting, but in any case ETR; the stream/catchment name; date and time; stage; bottle sequence numbers; temperature and signature should be written on the bottle label.

Where sandy bed streams are being sampled, a second sample run will reduce sampling errors. Deeper streams can be sampled in a similar way, but a sampler for use with line and winch equipment will be necessary. Different nozzles will probably be available, so that the rate of collection of samples can be adjusted to such prevailing stream conditions as depth and velocity. Charts are available that indicate the speed of sampler transit and whether a single or two way transit is recommended for particular nozzles, but the details will depend on manufacturer. Errors that may be caused because of different flow velocities close to the stream banks are not serious.

Where panicle-size analysis is to be undertaken, a second sample must be taken. For all other methods, discharge must be measured at time of sampling.

b. Depth Integrated Sampling

A relatively large number of depth integrated samples must be taken on verticals at the midpoint of equal sections of the width of the stream. Usually 6 to 12 are sufficient, each located within cross-sections of equal discharge. Mean sediment concentration is found by weighting the mean concentration in each sampling vertical with respect to the discharge in the vertical.

Total suspended sediment is found by mean cross section concentration and total water discharge. Discharge must be measured at the time of sampling. Variations in sediment concentration across the stream may be obtained. Alternatively, the collection of depth integrated samples at verticals that represent the middle of sections of equal discharge may be undertaken.

c. Point Integrated Sampling

Samplers used in this method are equipped with an electrically operated valve which takes samples on command. With the valve continuously open, they perform in a manner similar to depth integrating samplers. Point samples are taken in stream verticals which represent equal or known discharge. Mean values are weighted accordingly, but the number of points depend on the physical character of stream flow.

In general, the ETR method (a.) is most widely used.

3.2.3 Pumping Samplers

These samplers are complex pieces of equipment and relatively costly to obtain and install. It is probable that their cost can only be justified if soil erosion studies are a major activity of a project and if suspended sediment sampling from streams is an important component of this activity. Pumped samples do not represent discharge weighted samples as they are point measurements. Therefore, calibration curves that plot pumped samples against discharge weighted samples (taken simultaneously) must be compiled until a known relation is established. These must be revised should the relation change with time.

They are located in a shelter at the side of the stream and take samples of the flow, a portion of which is retained. They are useful for remote locations where site staff cannot be stationed. Samples are collected and stored in bottles. An intake is placed in the stream and typically a float activated, battery powered system comes into operation at a predetermined stream level. Samples are pumped into bottles at selected, predetermined time intervals until the stream level falls or the containers are full. Most of the pumped water goes to waste, to remove any debris drawn in by the previous sampling process, samples only being taken at the end of the pumping period. It is usual to site the equipment at a gauging station so that an indicating mark can be made on the water level recorder chart, when sampling takes place. Records of stage and sampling are thus linked. Some systems rely on a gravity feed sampling and are usually sited at such locations as reservoirs and weir installations.

Bed samples may be collected from perennial streams using special sampling dredgers, sampling cores and spuds but in most cases the costs of such equipment will not be justified by the information returned. Deposits of bed sediment may be sampled by soil sampling cores for the determination of bulk density and chemical analysis, during the dry season.

The mapping and surveying of channels, changes in gullies and the upslope movement of gully scarps can be important aspects of erosion and sedimentation studies. Mapping is required in great detail

3.3 Laboratory analysis

3.3.1 Sediment Concentration

Evaporation Method
Filtration Method
Separating Fines and Sands
Particle Size Analysis
Pipette Method
Hydrometer (Bouyoucos) Method
Wet Sieving
Dry Sieving

The analysis of samples will probably be undertaken at a specialist laboratory, but this is an expensive procedure and may even involve the dispatch of samples to another country. Therefore descriptions of common analysis techniques are given below. In some cases analyses can be undertaken with relatively simple equipment and is possible even where orthodox laboratory facilities are not available, if costs can be met. Evaporation and filtration are the two usual methods of determining sediment concentration. As it is more convenient to work with weights rather than volumes concentrations are usually determined as a ratio of dry sediment weight to sediment/water mixture. Conversion to units of milligrams per litre or parts per million is undertaken afterwards.

Packing and Transport of Soil and Water Samples

The most suitable container for a soil sample is a thick polythene bag which can be sealed with tape. This can then be put into a second paper or cloth bag for extra protection. For all analyses except bulk density, there is no problem if the sample is disturbed. Samples of about 1 kg are suitable, large stones having been removed. Where gravel content is of interest the sample may be 2-3 kg. Very wet samples may be dried, but any mixture of samples or contamination should be avoided. Samples should be placed in small wooden or cardboard boxes for transport as soon as possible to the laboratory; the addition of tuline to kill organisms may be necessary if samples are to be tested for nitrate and cannot be delivered promptly.

Water samples should be carried in screw-top polythene bottles and placed in boxes that are fitted with sections to separate each bottle from the next. Any empty space in packing cases or boxes should be packed with wadding to prevent movement of the samples during transport. Samples of 1 litre are adequate and soda glass containers should be used if an analysis for boron is to be undertaken.

Water and soil sample containers should be clearly marked, at least twice, with a water proof pen. So should any outer container. Details of samples should be recorded in a sample book and generally the less detail on the sample the better, to avoid confusion, however a detailed packing note should accompany any container of samples. It may be necessary to check with the laboratory in case it has a preferred system of labelling.

3.3.1 Sediment Concentration

a. Evaporation Method

Basic equipment is as follows:

- Graduated containers 0.5 -1.0 litre capacity.
- Large container, 5 litres or more
- Distilled water
- Vacuum source
- Evaporation dishes of several sizes
- Convection drying oven
- Pipette
- Desiccator
- Flocculating agent
- Balances accurate to 0.1 gram and 0.1 milligram

The procedure is as follows:

- Prepare a worksheet (see Figure 3.5)

- Transfer all sample details from the bottles to the work sheet.

- Weigh the total sample, less the container(s) weight(s) and record.

- If the colloidal material is in suspension, flocculate adding 0.40 millilitre of 0.2 molar solution of alum (90.7 g l-1).

- Allow the sample to settle overnight or for at least 12 hours.

- Note that flocculant is not usually used for concentrations < 1000 milligrams per litre. If this is done then the introduced error can be calculated for example: 50 milligram per litre solution added to a concentration of 1000 milligrams per litre will give an error of 5%, if all the flocculant is sorbed by the sediment.

- If samples have relatively little colloidal clay, they can be allowed to settle for a few hours and no flocculant is needed.

- Using the vacuum source and appropriate tubing, remove all water except 30 millilitres from the sample. This amount can be approximate, so long as it is the same amount for all the samples. All the effluent from the samples from one sampling site can be combined in one large container.

- Wash the remaining sample into a numbered evaporation dish with distilled water.

- Place in the oven and dry overnight at 105 - 110°C. It is best to avoid vigorous boiling and splashing of the sample. The sample container can be used where possible.

- Remove from the oven and place in the desiccator, weigh as soon as possible after removal.

- Enter gross and tare weights and obtain net weight of sample on the worksheet.

- Mix effluent thoroughly and withdraw 100 millilitres. Oven dry at 105 - 110°C, weigh sample enter on the worksheet.

- Compute the correction factor for the effluent by dividing the net amount of dissolved solids (the residue) by the aliquot volume (in this case 100 millilitres) and multiply the volume of water left in the large sample container × 1 million. (0.3888 g / 178.1 g) × 106 = 2183 ppm

To compute the concentration in milligrams per litre:

Concentration = B × ( weight of sediment × 106/ weight of water-sediment mixture) in mg l-1.

The value of the factor 'B' can be obtained from Table 3.6 which is based on specific weights for water and sediment of 1.000 and 2.65 g cm-3.


Figure 3.5: Example Worksheet for Evaporation Method of Sediment Concentration

Compute sediment concentration (in parts per million) as follows. Figure 3.5, the worksheet, provides the example:

Subtract correction factor from the net sediment weight. In Figure 3.5 for example.

0.3930 g - 0.0042 g = 0.3888 g

Divide oven dry weight of the sample by the net sample weight of the sediment plus water and multiply by one million.


Table 3.6: Values of Factor 'B' for the Computation of Sedimentation Concentration in mg l-1 When Used with Ratio (x 106) of Weight of Sediment to Weight of Water/Sediment Mixture (0-29°C)

b. Filtration Method

The filtration method works well with low sediment concentrations and obviates the need for the dissolved solids correction. Compile a work sheet as shown in Figure 3.6

Equipment is as follows:

- Got crucibles, at least 25 millilitre capacity with perforated bottom, suitable to be fined to a vacuum system.

- Filters. Commercial glass fibre or cellulose are satisfactory for most sediments.

- Distilled water

- Vacuum system

- Evaporation dishes of several sizes

- Convection drying oven

- Desiccator

- Flocculating agent

- Balances accurate to 0.1 gram and 0.1 milligram

Set up as follows:

- Determine the weight of the sediment/water mixture (the sample).

- Allow to settle until clear then decant the excess liquid into another beaker.

- Install suitable filter into crucible and determine tare weight.

- Connect crucible to vacuum system and transfer sample.

- When filtration is complete place crucible into oven and dry at 105 -110 °C.

- Remove and place in desiccator. Remove and weigh and compute concentration. Other methods of filtration can be used.

- A simple glass funnel fitted with a filter paper, the sample draining under gravity can be used when samples have a relatively high colloidal content and if care is taken.

Procedure:

- Follow the first three steps of the evaporation method.

- Wet sieve the material using sample water.

- Remove the material < 0.062 mm.

- Dry and weigh this material.

- Remove the material > 0.062 mm (sands) and put in a tared evaporation dish.

- Oven dry and weigh.

- Adjust to pH 3 - 5 with HCl, using pH paper.

- Add about 1 millilitre of 30 percent H2O2 (hydrogen peroxide) per gram of dry sample, in 40 millilitres of water.

- Allow to stand to oxidise the organic material (this will take a few hours) and remove any floating organic material.

- Destroy any remaining organic material and hydrogen peroxide by bringing the sample to a boil.

- Oven dry sand sample and weigh.

- Determine the organic content by subtracting the gross weight of sands from gross weight of sands before peroxide treatment and record.

- Record gross and tare sand weights on worksheet, compute net sand weight and record.

- Subtract weight of organic matter from weight of sand and record. No correction for dissolved solids is necessary as the effluent was washed into the fines portion during sieving.

- Compute sands and fine concentrations as described in the Evaporation method. Total concentration is equal to sum of fines and sands.


Figure 3.6 Example Worksheet Filtration Method

c. Separating Fines and Sands

The separation of fines and sands is frequently used in analysis and is normally made at 0.062 mm, though different preferences can be catered for. The equipment is the same as the Evaporation method, with the addition of a 0.062 mm (or 0.053 mm) meshed sieve. Figure 3.7 gives an example worksheet.

Procedures:

For the sands:

- Follow the first three steps of the Evaporation method
- Wet sieve on the 0.062 mm sieve using sample water.
- Remove material < 0.062 mm. Dry and weigh to the nearest 0.1 ma.
- Adjust to pH 3 -5 with HCl using pH paper.
- Add about 1 millilitre of 30 percent H2O2 per gram of dry sample in about 40 ml of water.

Allow to stand to oxidise the organic matter.

- Destroy hydrogen peroxide by boiling
- Oven dry sample and weigh to nearest 0.1 ma.
- Determine organic content by deducting weight of sample after from before hydrogen peroxide treatment.
- Record gross and tare sand weights, compute net sand weight.
- Subtract organic matter weight from sand weight and record.
- Compute sand and fines concentrations as for Evaporation method.
- Total concentration is found by totalling concentrations of sands and fines.


Figure 3.7: Example Worksheet Sands and Fines


For the fines:

- Continue with the steps of the Evaporation method

3.3.2 Particle Size Analysis

Particle size analysis is undertaken not only for sedimentation work, but also to determine soil textural type. Several methods may be employed because of the wide range of particle sizes frequently present in samples. Table 3.7 gives size range, analysis concentration quantity of sediment and methods of analysis, recommended by the USDA SCS. Table 3.8 gives a grade scale of sediment particle sizes.


Table 3.7 Recommended Particle Size Analyses


Table 3.8 Soil Textural Classes and Particle Sizes

The most commonly used methods of analysis are:

Fine sediments: Pipette, Hydrometer and Bottom Withdrawal (B W) Tube Methods. The former two methods are most commonly used and are detailed below.

Coarse sediments: Sieving and the Visual Accumulation Tube Methods. Details of these methods are given below.

In many cases, suspended sediment samples do not contain sufficient material for accurate analysis and thus procedures must necessarily be limited to the separation of fines and sands, though the possibility of combining samples from the same runoff event could be considered where practicable. Soil samples, by comparison, usually require splitting to the required sample amount before analysis. Dry samples can be split by a commercial sample splitter; moist samples by quartering or by extracting two or three samples from a thin tube inserted to the bottom of the material.

Fine Samples

a Pipette Method

This method is based on the principal that thoroughly dispersed particles of a given size will settle below a withdrawal point in a given time, according to Stokes law which is defined by:

2(D1-D2)GR2/9p where (3.12)

D1 = density of particle in g cm-3
D2 = density of the liquid in g cm-3
G = acceleration due to gravity in cm s-2
R = radius of the particle in cm
P = viscosity of the liquid in g cm-1 s-1

Equipment is as follows:

- 25 millilitre pipette apparatus (see Figure 3. 8 below)

- A vacuum source.

- Sedimentation cylinders of 1,000 millilitre capacity with rubber bungs.

- Stirring rods, brass of 6.4 mm diameter by 61 cm with a perforated plastic disc 5 cm in diameter, attached to end.

- Thermometers, Evaporating dishes.

- Desiccators, Stopwatch.

- Worksheets, 0.062 mm sieve.

Predetermined depths and times of withdrawal are given below in Table 3.9, based on the assumptions that particles have a spherical shape and a specific gravity of 2.65. The viscosity of the fluid is assumed to vary from 0.010087 cm²s-1 at 20 ° C and 0.008004 cm²s-1 at 30 °C. The gravitational constant is 9.80 m s-2.

Procedure:

If an organic matter and soluble salts content is required, the supernate is removed, the sample dried and weighed before these items are removed.

Otherwise the first step is to remove any organic material by oxidation.

- Use HC1 to adjust the sample pH to 3 - 5 for oxidation.

- Add about 1 millilitre of 30% H2O2 for each gram of dry sample. Stir and allow to stand for several hours.

- Any floating material can be removed.

- Usually samples need to be heated (less than 70°C) and more H2O2 should be added.
- When the reaction has stopped, the sample is boiled or washed with distilled water to remove the H2O2.

The second step is to remove soluble salt material.

- Effervescence, evident when a little dilute HCl is added to the sample indicates the presence of carbonates.

- To remove them, add 50 millilitres of a slightly acid sodium acetate (Na O Ac) buffer solution ( Na C2 H3 O2 3H2O,1N, 136 g l-1 adjusted to pH 5 with acetic acid) to each 5 grams of sample.

- Bring to suspension by stirring with a rubber tipped glass rod. Digestion is helped by heating the beaker in a water or sand both at near boiling temperature.

- After 30 minutes the suspension is washed by filtering with a filter candle (= to Pasteur-Chamberian or Selas type FP porcelain candle, 02 or 03 porosity). Some samples may need two or more treatments.

- After salts and organic matter have been removed, use the filter candle to remove excess liquid.

- When the candle is coated with soil, reverse the stopcock and add pressure with the rubber bulb.

- Touch the candle with inner surface of the beaker to remove any soil. Repeat the filtration and soil removal process.

- When free water has been removed, mix the sample with a jet of distilled water.

- Repeat the filtering and mixing process several times and when complete, add pressure as before to dislodge as much soil as possible and wash the soil back into the beaker. The rubber tipped glass rod can be used as an aid to this.


Table 3.9: Time of Pipette Withdrawal for Given Temperature, Depth of Withdrawal and Diameter of Particle


Figure 3.8: Schematic Diagram of Pipette Method Equipment

Thereafter analytical procedures are as follows:

- If both concentration and particle size are needed, weigh the water/sediment to the nearest 0.1 gram before proceeding.

- Remove excess water with a filter candle and place sample in a tared evaporation dish, dry at 100 -110° C and weigh to the nearest milligram.

- Deflocculate by adding a dispersing agent (40g sodium hexametaphosphate (Na P03)6 and 8 grams sodium carbonate in distilled water to 1 litre) for each 5 to 10 grams of sample.

- Transfer to a 250 millilitre shaker beaker adding distilled water to bring the volume to 180 millilitres, shake overnight. More convenient is the use of a mechanical analysis stirrer which will complete the mixing in 2 -5 minutes.


Figure 3.9: Worksheet for Pipette Method

In some cases, for example where the concentration of suspended sediments is very low, dispersion may not be necessary. Where it is, the dissolved solids correction must be determined for each new solution of the dispersing agent as follows:

- Add 10 millilitres of dispersing agent to a calibrated sedimentation cylinder, dilute to volume with distilled water, mix thoroughly and remove 25 millilitres, transfer to an evaporation dish and dry overnight at 105 - 110 ° C, weigh the dish and contents to the nearest 0.1 milligram.

- Perform in triplicate and use the average. The net weight of the dissolved solids is subtracted from the net weight of each pipette withdrawal.

Continue as follows:

- Weigh each sedimentation cylinder while empty then fill to between 500 and 1000 millilitres with distilled water. Weigh again several times and use the average. Do the same for the 25 millilitre pipettes. The ratio of mean weight of water in the cylinder: mean weight of water in the pipette is used as a volume ratio in comparing the results of the withdrawals.

- Select particle size determinations and using Table 3.9 set up a schedule for time and depth of withdrawals.

- Use distilled water to wet sieve (0.062mm) the dispersed sample, passing material into a sedimentation cylinder. Place the sands into a tared evaporation dish and dry, weigh to the nearest 0.1 milligram for the net weight.

- When ready to pipette, bung the cylinder and shake vigorously while turning end over end then plunge with the brass stirring rod.

- Immediately lower the pipette 10 cm into the sample and take a "zero time" withdrawal. Take the temperature and plunge again for 1 minute. After this make withdrawals according to the depth/time schedule, always measuring the sampling depth from the existing surface of the suspension.

- Each time, the pipette is flushed with distilled water and with the withdrawn sample, this is put into numbered and tared evaporating dishes. A rubber bulb may be used to blow out remaining droplets.

- Oven dry the withdrawals overnight (100 - 110 ° C) cool in a desiccator and weigh to the nearest milligram to determine net weight. Results may be tabulated as in Figure 3.9.

Calculations are carried out as follows:

- From the "zero time" withdrawal determine the net weight of fines in the sample and record. Make a dissolved solids correction if a dispersing agent was used. Compute the total weight of fines by multiplying the weight of fines in the suspension by the volume ratio.

- Determine the net dry weight of the sediment in subsequent withdrawals and multiply by the volume ratio. Note: this gives the weight of sediment in suspension finer than the size corresponding to the time and depth of withdrawal.

- To obtain the fraction of total sediment finer than the indicated size, divide the weight of sediment in the sample finer than the size corresponding to the time and depth of withdrawal by the dry weight of the total sediment in the sample.

- To obtain the concentration of the fines, sands and total sample in parts per million, divide the total net weight of each by the weight of the total sample (water/sediment mixture) by one million. Record on the form.

b. Hydrometer (Bouyoucos) Method

The density of the soil suspension is measured with a special hydrometer which is marked with percentage calibrations, calibrated at 20°C. The technique as proposed by Bonyoucos does not remove organic or calcium carbonate material and therefore gives approximate results where these are present in large quantities.

A second hydrometer (ASTM 152 H) was developed by Day with a more rigid adherence to Stokes Law.

The original hydrometer method was devised to provide a quick and easy method, the accuracy of which could be established by comparison with pipette analyses and thereafter be used with confidence. The modifications introduced by Day increase accuracy, but the analysis is no longer rapid (it takes about as long as the pipette method) and it is essential that organic matter and calcium be removed by pretreatment.

Bouyoucos Method

Equipment:

Apparatus and reagents as for pre-treatments and separation of sand
Mixing plunger
Thermometer including the 15 - 25 °C range
Accurate clock or stop watch
Bouyoucos or ASTM hydrometer
Hydrometer jars marked at 1 litre

Procedure:

- Estimate whether sample is sandy (silt and clay < 15%) or not sandy. In first case transfer 100 g oven dry sample to 250 ml beaker and add 100 ml 5 percent solution of hexametaphosphate-sodium carbonate. In second use 50 g.

- Rest overnight, transfer to mechanical stirrer, washing out the beaker and making up the volume to 500 ml with water.

- Stir for 2-3 minutes

(- If sample contains much carbonate or organic material, remove by treatments described previously. Dry and weigh.)

- Transfer suspension to hydrometer jar, wash out stirrer cup and adjust volume to 1 litre. Mix with glass rod.

- Take temperature, which should be 15 - 25 °C. Mix with plunger.

- At selected times lower Bouyoucos hydrometer carefully into centre of solution. Read the scale to nearest 0.5 unit.

The times advocated by Bouyoucos are:

Particles less than

50 microns

40 seconds


20 microns

4 minutes


5 microns

1 hour


2 microns

2 hours

In the case of particles < 2 microns, 6.5 hours may be more accurate for samples with organic matter removed. - Prepare a 0.5 percent solution of sodium hexaphosphate-sodium carbonate, transfer to a hydrometer jar and adjust to 20°C. Insert hydrometer and take reading as a blank. Discard solution. - If the temperature of hydrometer readings was not at 20 °C, the following corrections apply:

Temperature (°C)

Correction (g per litre)

Temperature (°C)

Correction (g per litre)

15

-2.0

21

+0.5

16

- 1.5

22, 23

+ 1.0

17, 18

- 1.0

24

+ 1.5

19

- 0.5

25

+ 2.0

- Subtract the blank reading obtained with the dispersing solution at 20 °C.

The values obtained are direct percentages of clay (< 5 or 2 micron according to time) or silt + clay (< 50 micron or 20 micron according to time) if 100 g were used. Values to be × 2 if 50 g were used.

Coarse Sediments

Both wet and dry sieving procedures may be followed for material greater than 0.062 mm. In each case it is essential not to overload the sieve, the USDA recommends 25 - 50 g for 8 inch diameter sieves for medium - fine sands (3 -7 g for 3 inch sieves) and 10 -20 g for fine sands ( 1.5 - 3.0 g for 3 inch).

Wet Sieving

- Immerse the sieve having the coarsest screen in a ceramic dish containing distilled water until the water surface is about 5mm above the sieve.

- Wash the sample on to the sieve and shake until all small particles have passed through.

- Pass the material and washing water onto the next smallest sieve and continue to repeat the process until the smallest sieve is reached.

- Transfer the material on to a tared container, dry and weigh each fraction. Any material passing through the 0.062 sieve is to be analysed by other methods

Dry sieving

- Set a nest of sieves on a mechanical shaker, coarsest on top, proceeding to the finest.
- Weights for each fraction are determined after about 10 minutes of shaking.

Equipment costs

All costs of locally made equipment are approximate. The costs of raw materials and especially labour are highly variable from country to country, but a good idea of cost magnitude can be gained from the figures quoted below. The costs of manufactured equipment are based on 1993 prices. Shipping costs, agents fees and fluctuations in exchange rate cannot be taken into account.


Figure

Appendix B: Erosion and sedimentation data

Appendix B1: Ratio of soil loss from crop land to corresponding loss from continuous fallow


Appendix B1 part I


Appendix B1 part II

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