The watershed is the system component for originating the sediment yield through the processes of soil erosion. The eroded materials entering into the fluvial system and ultimately getting deposited in the water storage bodies such as ponds, reservoirs etc., is called sedimentation. For storage structures concerned, the sedimentation is always destructive phenomena.
In this article the details of sedimentation are explained as under:
Meaning of Sediment:
It is a fragmented material, originated either from chemical or physical disintegration of rocks. These materials can vary from a big boulder to a colloidal particle. The fluvial sediments move in the stream as bed load or suspended load through water flow.
In broad sense, the sediment may be defined as any fragmented material, which is transported and deposited by water, air or ice as natural agents. Among all, the water is the most widespread agent of sediment transport. Therefore, sediment yield by water is commonly considered for study purposes.
Sources of Sediments:
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The sources of sediments can be as below:
1. Erosion from agricultural, forest and waste lands.
2. Movement of soil mass due to land-slides, slumps and soil creeps.
3. From gully by concentrated runoff.
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4. Stream bank erosion including cutting of banks and scouring from bed.
5. Erosion caused by floods in the watershed.
6. Mining and dumps left as waste materials over the ground surface.
In sediment load analysis, the estimation of total sediment load carried through any stream has primary importance because based on the total sediment load several measures to check it can be laid. The relative contribution of sediment loads from different sources varies from catchment to catchment. In general, the consideration is given to those sources of sediments which contribution is very effective.
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Types of Sediment Load:
Sediment loads are classified into following types:
1. Contact load
2. Saltation load
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3. Suspended load; and
4. Bed load
1. Contact Load:
Contact load may be defined as the materials rolled or sliding along the stream bed in continuous contact of stream bed.
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2. Saltation Load:
It is the material bouncing along the stream bed or moving directly by the impact of bouncing particles with each other.
3. Suspended Load:
Suspended loads are the materials, either (1) Moving in suspension form with the fluid, or (2) Measured or computed from the samples collected by the sampler. The samplers used for this purpose are known as suspended load samplers.
The suspended loads are further sub-classified into following three grades on the basis of particles diameter:
(i) Coarse Sediment – Particles above 0.20 mm diameter
(ii) Medium Sediment – Particles between 0.20 to 0.075 mm diameter
(iii) Fine Sediment – Particles below 0.075 mm diameter
The sediment diameter is defined as the diameter of a spherical particle which has same specific weight and the same terminal settling velocity in the same sedimentation fluid.
4. Bed Load:
It is the sediment load, which moves on or near the stream bed. Movement of particles can be in the form of rolling, sliding or hopping (i.e. the saltation). The mode of movement depends on the flow velocity. Saltation is changed into suspension form as the flow velocity gets increase.
In other words, due to increase in flow velocity the particles which are responsible for movement under saltation process, are carried away in suspension form by the upward components of turbulent flow. In this way, the transportation of sediments in suspension form is an advance stage of bed load movement, intensified by eddies in vertical direction of stream flow.
The depth-wise variation in sediment movement in the stream is in the following sequence:
i. The large size sediment particles are moving along the stream bed and constitute the bed load.
ii. Immediately, just above the stream bed a thin layer of relatively coarse materials with high concentration is carried away either in suspension, saltation or in both the forms depending on the particles size.
iii. Above it, extending up to the top of water surface, relatively fine grained materials with their decreasing concentration upward in the order of depth are carried away in suspension form.
Sediment Transportation:
The transportation of soil particles (i.e. sediment) from the land surface begins by the action of precipitation. When raindrops are falling on the soil surface from greater heights, they break the larger soil particles into smaller grains, which are thrown upward by the energy of rain drops and are carried away in suspension form after falling over the sheet of water or soil surface by the overland flow.
The water flowing over the land surface also detaches additional particles from the soil surface which are transported towards the streams channels etc. The rain drops have a considerable kinetic energy, while falling on the earth surface. The relationship between kinetic energy and rainfall intensity, developed by Wischmeir (1959) is given as under –
K.E. = 210.3 + 89 log10I … (23.1)
In which, K.E. is the kinetic energy, expressed in metric tonnes per hectare cm, and I is the rainfall intensity in cm/h. The rainfall intensity is closely related to the mass of raindrop and its terminal velocity. The mass of raindrops is directly proportional to their diameter, which is also related to the terminal velocity. The energy of rain drop increases rapidly as its size increases.
According to Gilbert, the transportation of sediment takes place under following three types of movement:
1. Saltation
2. Suspension; and
3. Surface creep
In saltation process, relatively larger particles of sediment are transported in the form of bed load slide, rolling or short skips. The saltation movement changes into suspension, either when the size of sediment becomes small or velocity of flow gets increase, suddenly.
Suspension process is referred to the small size particles that are transported by the fluids in suspension form. The movement of particles takes place due to turbulence in the flowing water.
Similarly, the surface creep type movement is related to the large size particles, which are transported by creeping action generated by the water flow.
Sediment particles are thus carried to downstream side either by traction or in suspension form, being supported by the vertical component of turbulent eddies. In sediment transportation by stream flow the saltation and surface creep actions are sometimes combined together, and is referred as bed load movement.
Sediment Transport Mechanism:
The mechanism of sediment transportation in stream flow is described by Gilbert as – the transportation of some particles takes place by bed load slide, rolling and some are by the short skips or heaps, are included under saltation process. The saltation changes into suspension, when small size particles are moved together in the form of rolling. During course of stream flow the interaction of the particles within a sediment laden flow develops a kind of sediment waves or bars, is known as ripples.
These ripples are extended to some length, with flat upstream slope and steep slope towards downward. The sequence of ripples is likely to change as the mean flow velocity gets increase. Further, if the velocity gets increase more than to move the bed load, then bed load consisting of fine sands (less than 2 mm diameter) form saw tooth type ripples, is shown in Fig. 23.3. Similarly, when velocity further increases, the ripples are superimposed on each other and form equally spaced dunes (Fig. 23.4).
The shape of dune is generally round and size is larger than the ripples. It can get form by any size sediment. The ripples and dunes are short crested waves; the crest does not extend across the stream width. Again, if there is increase in the flow velocity then formed dunes are changed into a flat bed, as shown in Fig. 23.5.
Further at high velocity, there generates sand waves, which are associated to the surface waves; these become too steep when Froude number (V/√g.d) is unit, because the whole sand wave is breaked gradually towards upstream. The formation of sand waves by the interaction of stream bed and the water surface is known as anti-nodes.
Distribution and Transportation of Suspended Load:
The settling of suspended materials from the water is based on the theory of “Stoke’s law”, which states that the falling velocity of any particle through a liquid depends on the particle’s radius. The equation describing the falling velocity of particle is given as under –
in which, V is the falling velocity of the particle; ρs and ρ are the density of sediment particles and liquid, respectively; r is the radius of particle; μ is the absolute viscosity of liquid and g is the acceleration due to gravity.
This equation follows following assumptions:
1. The particle’s size is greater than the molecule of the liquid.
2. The particle is rigid, smooth and spherical.
3. The falling velocity of particle does not impede by the adjacent particles.
The above equation is applicable for the size of sediments ranging from 0.0002 to 0.2 mm diameter.
The settlement rate of particle in still water is the same as in the steady laminar flow condition, because in this flow condition there is no vertical displacement of sediment particles. In either cases, the concentration of suspended materials tend to decrease with time because of deposition over the bottom. The suspension takes place in unstable condition of flow and did not occur equilibrium. In still water the suspended materials are settled down due to gravitational force. The gravitational settling produces a concentration gradient.
In turbulent flow, the transportation of sediment in upward direction by eddy current is more than those moving downward. The transportation of sediment in ‘suspension’ is a continuous process with scouring and settling actions. If scouring and settling processes act at equal rates, then bed configuration remains unchanged with lime, and suspension reaches at equilibrium state.
If the flow velocity and bed roughness are such that the turbulent transfer exceeds the gravitational settling, then stream bed is being scoured or eroded. On the other hand, if gravitational settling exceeds the turbulent transfer then sediment is being deposited over the stream bed.
At equilibrium state the basic theory of turbulent transport is described by the following relationship –
in which, Cs is the average sediment concentration at any depth y above the stream bottom and ԑ is the mixing coefficient also known as “eddy conductivity”, diffusion coefficients, mechanical viscosity or eddy viscosity. The value of coefficient ԑ depends on the distribution and transport of sediment momentum or any other form of turbulent transfer. Integrating equation (23.9),
In which, Csa is indicated for concentration of the sediment at some reference depth, say ‘a’. The right-hand side of equation 23.10 can be evaluated by assuming and substituting the value of ԑ as the function of y.
The equation 23.10 is based on the assumption that the mixing coefficient ԑ for sediment transfer is the same to the momentum. For this condition the value of ԑ can be determined from turbulence theory, i.e. the value of unit shear τ at any point is equal to –
In which, V is the velocity of flow at depth y above the stream bed and ρ is the density of fluid.
The Prandt-Von Karman have developed an equation for vertical velocity distribution of sediments in a wide stream, given as under –
Where, Vm is the mean velocity distribution of sediment in a wide stream, can be determined by using Manning’s formula, and K is the Von Karman’s constant, taken as 0.4. Thus,
At the centre of wide stream the value of unit shear force increases approximately in linear fashion, from zero at the top surface to τo at the stream bed, where τo is equal to τ. D.g.s. In other form the τ can also be expressed as –
Thus, knowing the value of settling velocity ‘Vs‘ at the depth y above stream bed, the sediment distribution pattern can be determined. The above equation is applicable for the small depth as compared to ‘a’ especially where suspension gets mixed with contact load of sediment.
The combine form of equation 23.12 and 23.13 can be used to predict the total sediment transport through the stream, as the sum of all points values of the product of velocity and sediment concentration.
Lane and Kalinske (1941) made further assumption that the value of ԑ is constant with stream flow depth, which is equivalent to the parabolic velocity distribution. Thus, from equation (23.14) the average value of ԑ can be expressed as D√gDs/15. Thus, the equation (23.9) can be rewritten as –
Where, x = Vs/√g.Ds for a wide stream.
Using equation (23.14) the concentration of sediment Cs with settling velocity (Vs) at any point ‘a’ in vertical section of stream flow, can be determined. After determining the value of Cs, a curve is drawn by plotting the values of Cs and Csa on semi-log paper and sketching a straight line through the plotted points.
Thus, the total suspended load Mi carried through the stream flow per unit time and per unit width, is given by –
Where, q is the discharge rate per unit stream width and ξ is the constant, which depends on x and the relative roughness (n/D1/6) of stream bed.
Analysis of Suspended Sediment:
The sediment samples collected from the gauging station of stream/river are analysed by following two methods:
(a) Gravimetric method; and
(b) Hydrometric method
Analysis of sediment samples using hydrometric method requires a trained staff, and is suitable for that location where laboratory facility is available for analysis work. To make analysis of suspended load, it is required to classify the categories of suspended sediments, first. Because analysis is started first for coarser particles and then proceeded to medium and fine sediments.
On the basis of size of sediment particles, the following standard is followed for classifying the coarse, medium and fine suspended sediments:
1. Coarse Sediment – Particles size is more than 0.2 mm in diameter
2. Medium Sediment – Size ranges from 0.075 to 0.20 mm
3. Fine Sediment – Particles size is less than 0.075 mm in diameter
Apart from suspended particles, there are various kinds of salts and organic matters are also dissolved in the stream water. For precise estimation, they are also counted in suspended load analysis. Estimation of suspended sediment particularly for fine sediment, the use of hydrometric method is very difficult, and if used, yields inaccurate estimation. For such cases, the use of gravimetric method is most suitable and obtained result is also being more reliable.
1. Analysis of Coarse Sediment:
For analysis, the sediment samples collected from different sections of stream, are thoroughly mixed and allowed to pass through sieve BSS-100 (IMM-70). The retained coarse particles (0.2 mm dia) on the sieve are then thoroughly washed by clean water jet and dried in ovan or in sun light to remove, the moisture. The weight of coarse sediment obtained so, is for the collected volume of sample. The obtained amount of sediment can be expressed in terms of g/l or hectare-cm per 100 sq km of watershed area.
2. Analysis of Medium Sediment:
The residue of sediment sample, which is obtained after passing the original sample through B.S.S. 100 mesh sieve, is collected in an enamel bucket or in a beaker for analysis of medium size sediments. In case, if the residue sample is not sufficient to make the depth of 10 cm in beaker, then clean water can be added to it for making desired depth of sample.
Now, the sediment sample prepared so is kept for stirring with mechanical stirrer or by hand using glass rod fitted with rubber pad. The stirring is done in clock-wise direction for few seconds, so that a uniform sediment distribution in the sample may take place. It is now allowed to stand for some time according to the temperature of water in beaker, which is given in Table 23.1.
After lapse of requisite time, the supernatant (water above the settled sediment) is poured off for separating the medium size sediments, known as decantation process. This process is repeated till the supernatant water becomes clear.
The medium size sediment which is settled in the beaker, is transferred into a weighed beaker and dried in oven or in sun light to remove the moisture. The dry sediment is then weighed, carefully. The difference of the two weights gives the amount of medium sediments in the collected sample, which can be expressed in g/lit.
Analysis of Fine Sediment:
In decantation process for determining the medium size sediments, the residual sample is collected and filtered with a filter paper of known weight to get the fine sediments. On filtration the fine sediments are retained on filter paper. Now, wet filter paper along with fine sediments is removed from the funnel and is placed in a glass dish for drying over the sand or in sun light.
It should always be kept in mind that the sun light should be free from dust. The dried filter paper along with fine sediments is then weighed. If W2 is the weight of filter paper and fine sediments and W1 is the weight of filter paper, then weight of sediments is equal to W2 – W1. The sediment concentration can be expressed in terms of g/liter.
Selection of Sediment Sampling Point in Vertical Section of Stream:
The concentration of sediments in stream water varies depth-wise, i.e. near the stream bed it is more; in middle relatively less, while near the top of the water surface it is too less. In this condition, it is very difficult to select the sampling point, which can accurately represent the sediment concentration in vertical cross-section of stream flow. However, the followings are the recommended points for sediment sampling in point-integration method.
1. In case of single point sampling, the sample should be collected from the depth 0.6 d from the top water surface (d is the depth of stream flow).
2. In case of two points sampling, one sample should be collected from the point near the top of water surface, i.e. at the depth about 0.2 d, and other near to the stream bed at about 0.8d. The concentration of sediment is weighted equally.
3. For three points sampling, one sample should be taken near the top of water surface; second from mid depth of the stream flow and the third near the stream bed; and their sediment concentration is weighted equally.
4. Similarly, in case of multi-sampling, the samples should be taken from several points of vertical section of the stream flow. This helps to elaborate the vertical sediment distribution in the stream flow.
Frequency of Sediment Sampling:
Frequency of sampling depends on the sediment concentration in the stream flow. As it is common phenomena that the sediment concentration increases rapidly on rising limb of the hydrograph than the falling limb. Therefore, the sediment samples should be collected at greater frequency at the beginning of runoff, and it should be continued up to the peak stage of runoff. The samples should be taken at every 15 minutes interval.
Number of Sediment Monitoring Stations:
In watershed from where total runoff is disposed off through a single point called outlet, the collection of sediment samples should be carried out from the outlet point. The outlet is an ideal location as monitoring station for the entire watershed. Similarly, in drainage system, where flow is drained from more than one points, there should be installed gauging station at each outlet point for sampling.
Location of Sediment Observation Posts:
The silt observation posts are located on the basis of following points:
1. Wherever possible, the stream gauging site should be utilized for installation of silt observation post. The outlet of catchment can also be suitably used for observation post because it represents the point where sediments get concentrate from the entire watershed area.
2. The observation sites should be free from disturbing points, such as change in site configuration, protrusion, confluent point as well as the points with back water effect.
3. The silt observation site should be located at that point of stream, from where a constant soil erosion is taking place due to flow turbulence. The observation post should be installed at the upstream face licensure heading of water for disturbing the soil particles throughout the vertical section of stream.
4. The site should be easily accessible, and preference should also be given to that site, where transport facilities such as road etc. are available.
5. The sediment monitoring station with overhead platform wherever available, should be used. It helps in collection of sediment sample from the entire width of stream flow.
6. In hilly regions, where torrential flow exists, the cable bridge may be used for taking the silt observations.
7. The site should have a straight stream reach of about 150 m, both towards upstream and downstream face.
Stream-Width Wise Sediment Sampling:
The total number of sampling points to be considered for collecting the sediment samples, depending on the width of stream flow, is given in Table 23.2.
Apart from width-wise sediment sampling the depth-wise sampling is also carried out, using the following points:
1. Under single point sampling. The samples are taken at the depth 0.6 d (d is the flow depth). The sediment concentration obtained is multiplied by a suitable factor to get the correct value.
2. Under two point sampling, one sample is taken near the top water surface and other near to the bottom of the stream. The sediment loads obtained at these two depths, are weighted equally to get an average value.
3. In three point sampling, one sample from near the top water surface and others two samples are taken from the mid as well as bottom of the stream flow.
4. To determine the vertical distribution of sediments in the stream flow, there should be collected more number of samples from several points of stream flow depth.
5. To collect accurate samples, the sampler should be kept in vertical position from the stream bed.
6. The mouth of sampler should be opened exactly after reaching at the desired depth of stream flow.
7. If it is expected that the distribution of sediment is uniform in the stream flow, then sampling should be done only at 0.6d of the stream flow.