11 Sedimentacion en Embalses (Ingles)

8/9/2019 11 Sedimentacion en Embalses (Ingles)

1/8

Chapter11

As the silt originates from the water shed, the characteristics of the catchment such its

areal extent, soil types, land slopes, vegetal cover and climatic conditions like temperature,

nature and intensity of rainfall, have a great significance in the sediment production in the

form of sheet erosion, gully erosion and stream, channel erosion. In regions of moderate rain-

fall, sheet erosionis the dominant source of total sediment load while in arid and semi-arid

regions, gullying and stream-channel erosion furnish the greater part of the load.

Experiments have shown that the erosive power of water, flowing with a velocity V,

varies as V2 while the transporting ability of water varies as V6. Sediment moves in the

stream as suspended load (fine particles) in the flowing water, and as bed load (large parti-

cles), which slides or rolls along the channel bottom. Sometimes, the particles (small particles

of sand and gravel) move by bouncing along the bed, which is termed as saltation, which is a

transitional stage between bed and suspended load. The material, which moves as bed load atone section may be in suspension at another section.

The suspended sediment load of streams is measured by sampling the water, filtering to

remove the sediment, drying and weighing the filtered material.

Sediment load, (ppm) =weight of sediment in the sample

weight of sediment laden water sample 106

The samplers may be of depth-integrating type or point samplers. Point samplers are

used only where it is not possible to use the depth integrating type because of great depth of

high velocity, or for studies of sediment distribution in streams. The sample is usually col-

lected in pint bottle held in a sample of stream-lined body so as not to disturb the flow while

collecting a representative sample.

The relation between the suspended-sediment transport Qsand stream flow Qis givenby

Qs=KQn ...(11.1)

log Qs = logK+ nlog Q ...(11.1 a)

and is often represented by a logarithmic plot ofQsvs. Q(Fig. 11.1); Qs=Kwhen Q= 1, and n

is the slope of the straight line plot and 2 to 3.

The sediment rating curve from a continuous record of stream flow provides a rough

estimate of sediment inflow to reservoirs and the total sediment transport may be estimated

by adding 10-20% to the suspended sediment transport to allow for the bed load contribution.

RESERVOIRSEDIMENTATION


2/8

When the sediment-laden water reaches a reservoir, the velocity and turbulence are

greatly reduced. The dense fluid-solid mixture along the bottom of the reservoir moves slowly

in the form of a density current or stratified flows, i.e.,a diffused colloidal suspension having

a density slightly different from that of the main body of reservoir water, due to dissolved

minerals and temperature, and hence does not mix readily with the reservoir water (Fig. 11.2).Smaller particles may be deposited near the base of the dam. Some of the density currents and

settled sediments near the base of the dam can possibly be flushed out by operating the sluice

gates. The modern multipurpose reservoirs are operated at various water levels, which are

significant in the deposition and movement of silt in the reservoir.

The total amount of sediment that passes any section of a stream is referred to as the

sediment yieldor sediment production. The mean annual sediment production rates generally

range from 250-2000 tons/km2or 2.5-18 ha-m/100 km2and the Indian reservoirs are losing a

storage capacity of 0.5-1% annually.


3/8

The following figures give a general idea of the silt carried by some of the big rivers inthe world during floods:

River Silt content (% by weight)

Colorado (USA) 1.00

Mississippi-Missouri (USA) 0.20

Yangatze (China) 0.04

Yellow river (China) 4.00

Indus (Pakistan) 0.42

Sutlej (India) 1.67

Nile (Egypt) 0.15

Krishna (India) 1.00Cauvery (India) 0.14

Tungabhadra (India) 0.67

Sone (India) 0.56

The sedimentation rate worked out by experts for different projects in India are given

below:

Project Annual sedimentation rate

(ham/100 km2)

Bhakra 6.00

Hirakund 3.89

Gandhisagar 10.05Nizamsagar 6.57

Panchet 9.92

Maithon 13.02

Ramganga 17.30

Tungabhadra 6.00

Mayurakshi 20.09

Tawa 8.10

Dantiwada 6.32

Manchkund 2.33

The useful life of a reservoir gets reduced due to sediment deposition causing a decrease in its

storage capacity.The factors affecting the pattern of sediment deposition in reservoirs are:

(i) sediment load (i.e.,sediment inflow rate)

(ii) sediment size (i.e.,gradation of silt)

(iii) compaction of sediment (iv) river inflow pattern

(v) river valley slope (vi) shape of reservoir

(vii) capacity of reservoir (its size and storage period)


4/8

(viii) vegetal growth at the head

(ix) outlets in the dam (their types, location and size)

(x) reservoir operation

(xi) upstream reservoirs, if any.

It has been found by experience that a low sediment inflow rate, large fraction of fine

particles, steep slope, no vegetation at head of reservoir, low flow detention time in the reservoir

(by operation of outlets of suitable size at different levels), possibly series of upper tanks or

reservoir upstream (where deposition occurs) do not favour sediment deposition and compaction.

The silt carried in the rainy season may be excluded from the reservoir by means of scouring

sluices slightly above the deep river-bed, which discharge the heavily silt-laden water at high

velocity. The percent of the inflowing sediment, which is retained in a reservoir is called the

trap efficiencyand it is a function of the ratio of reservoir capacity to total annual sedimentinflow, since a small reservoir on a large stream passes most of its inflow quickly (giving no

time for the silt to settle) while a large reservoir allows more detention time for the suspended

silt to settle. The relation between trap efficiency of reservoir vs. capacity-inflow ratio is shown

in Fig. 11.3 (Brune, 1953), on the basis of data from surveys of existing reservoirs. The rate at

which the capacity of a reservoir is reduced by sediment deposition depends on

(i) the rate of sediment inflow, i.e.,sediment load.

(ii) the percentage of the sediment inflow trapped in the reservoir, i.e.,trap efficiency.

(iii) the density of the deposited sediment.

In estimating the useful life of a reservoir, the correct prediction of the density of the

deposited sediment is an important factor. Lane and Koelzer (1943) gave the equation for the

dry specific weight tafter time tyears as

t= i + K log10 t ...(11.2)

where i= initial specific weight

K= a constant for the rate of compaction


5/8

If the deposited sediment consists of a mixture of materials like sand, silt and clay, aweighted average specific weight is calculated as (Lane and Koelzer).

t= 1x1+ ( 2+K2log t)x2+ ( 3+K3log t)x3 ...(11.2 a)

where t= average specific weight of reservoir sediment after tyears

1, 2, 3= specific weight of sand, silt and clay after tyears

K1, 2, 3= constant for the rate of compaction of sand, silt and clay, respectively (K1 0, for

sand)

x1, 2, 3

= fractional part of total sediment, of sand, silt and clay, respectively

t= time in years ( 1 yr)

The useful capacity of reservoir lost each year by sediment deposition is

Vs= Qs trap ...(11.3)

where Vs= volume of useful capacity of reservoir lost each year

Qs= annual sediment inflow into the reservoir

trap= trap efficiency of the reservoir

while allocating space for the dead storage in the reservoirs (i.e.,to provide space for sediment

deposition during the life of the project) the trap efficiency is taken as at least 95% and rarely

below 90%. Sediment deposits in the upper end of the reservoirs generally become covered by

vegetation resulting in heavy evapotranspiration loss of the available water, which is more

critical in arid regions.

Example 11.1A proposed reservoir has a capacity of 400 ha-m. The catchment area is 130 km2

and the annual stream flow averages 12.31 cm of runoff. If the annual sediment production is

0.03 ha-m/km2, what is the probable life of the reservoir before its capacity is reduced to 20% of

its initial capacity by sediment deposition. The relation between trap efficiency and capacity-inflow ratio is given below.

Capacity-inflow Trap Capacity-inflow Trap

ratio,C

Iefficiency, ratio,

C

Iefficiency,

trap(%) trap(%)

0.1 87 0.002 2

0.2 93 0.003 13

0.3 95 0.004 20

0.4 95.5 0.005 27

0.5 96 0.006 31

0.6 96.5 0.007 360.7 97 0.008 38

1.0 97.5 0.01 43

0.015 52

0.02 60

0.03 68

0.04 74

0.05 77

0.06 80

0.07 82


6/8

SolutionThe useful life may be computed by determining the number of years required foreach incremental loss of reservoir capacity (i.e.,for the decreasing values of capacity-inflow

ratios) upto the critical storage volume of 400 0.20 = 80 ha-m as tabulated below:

Trap efficiency Loss of No. of years

Capacity Capacity* trap(%) Annual** reservoir for the

C inflow sediment capacity capacity

(ha-m) ratio for the Ave. for trapped C loss

C

I

C

Iratio *increment V s= Qs trap (ha-m) C V s

400 0.25 94

320 0.20 93 93.5 3.64 80 22.0

240 0.15 90 91.5 3.57 80 22.4

160 0.10 87 88.5 3.45 80 23.2

80 0.05 77 82.0 3.20 80 25.0

Total = 92.6

say, 93 yr

*Average annual inflow, I=1231

100

130 10

10

6

4

.

= 1600 ha-m

For reservoir capacity C= 400 ha-m,C

I=

400

1600= 0.25

**Annual sediment inflow into the reservoir

Qs= 0.03 130 = 3.9 ha-m

Note:If the average annual sediment inflow Qs is given in tons, say Qs= 43600 tons and for

trap= 93.5% (for the first incremental loss), assuming a specific gravity of 1.12 for the sediment depos-

its, annual sediment trapped Ws= 43600 0.935 = 40750 tons.

Vs=

Ws

s

=40750 1000 kg

1.12 1000 kg/m3

=40750

1.12m3= 3.64 ha-m.

Usually the specific gravity of sediments deposits ranges from 1 to 1.4.

Sediment deposition in reservoirs can not be actually prevented but it can be retarded by

adopting some of the following measures:(i) Reservoir sites, which are prolific sources of sediment should be avoided.

(ii) By adopting soil-conservation measures in the catchment area, as the silt originates

in the watershed. See art 8.7 in Chapter 8.

(iii) Agronomic soil conservation practices like cover cropping, strip cropping, contour

farming, suitable crop rotations, application of green manure (mulching), proper control over

graze lands, terracing and benching on steep hill slopes, etc. retard overland flow, increase

infiltration and reduce erosion.


7/8

(iv) Contour trenching and afforestation on hill slopes, contour bunding gully pluggingby check dams, and stream bank stabilisation by the use of spurs, rivetments, vegetation, etc.

are some of the engineering measures of soil conservation.

(v) Vegetal cover on the land reduces the impact force of rain drops and minimises

erosion.

(vi) Sluice gates provided in the dam at various levels and reservoir operation, permit

the discharge of fine sediments without giving them time to settle to the bottom.

(vii) Sediment deposits in tanks and small reservoirs may be removed by excavation,

dredging, draining and flushing either by mechanical or hydraulic methods and sometimes

may have some sales value.

IMatch the items in A with the items in B

A B

(i) Gullying and stream (a) V6

Channel erosion

(ii) Measurement of suspended sediment (b) V2

(iii) Erosive power of water (c) Capacity-inflow ratio

(iv) Transporting capacity of water (d) Soil conservation

(v) Saltation (e) Bouncing along bed

(vi) Trap efficiency (f) Semi-arid regions

(vii) Sedimentation control (g) Point samplers

IISay true or false; if false, give the correct statement:

(i) While sheet erosion is dominant in regions of moderate rainfall, gullying and stream channel

erosion are characteristic of arid and semi-arid regions.

(ii) Density current is a stratified flow along the bottom of the reservoir and mixes readily with

the reservoir water.

(iii) The per cent of the inflowing sediment in a stream retained by a reservoir is called the trap

efficiencyof the reservoir.

(iv) The trap efficiency of a reservoir is a function of the ratio of the reservoir capacity to the

average annual sediment inflow, and as this ratio decreases the trap efficiency increases.

(v) Silting of reservoirs can be controlled by

(a) proper agronomic practices.

(b) adopting soil-conservation measures.

(c) by providing sluice gates at various levels in the dam and proper reservoir operation.

(false: ii, iv)

IIIChoose the correct statement/s:

Siltation of reservoir can be reduced by

(i) proper reservoir operation.

(ii) providing sluice gates at different levels.

(iii) land management.


8/8

(iv) gully plugging, check dams and contour bunds in the catchment area.(v) strip cropping, contour farming and afforestation of hill slopes.

(vi) providing dead storage.

(vii) all the above steps. (except vi)

1 (a) What are the factors which contribute for silt in a natural stream?

(b) How will you determine the quantity of silt deposited in a reservoir?

(c) Recommend the measures for controlling the silt entry into reservoirs.

2 (a) How would you determine the sediment load carried by a stream?

(b) What do you understand by density current?

(c) Explain the terms:

(i) Saltation (ii) Suspended load

(iii) Bed load (iv) Contact load

3 (a) Briefly give the theory of distribution and transporation of suspended material and derive

the formula connecting the concentration of sediment and settling velocity under equilib-

rium conditions.

(b) Explain how sedimentation in a reservoir can be controlled.

(Hint:for Q 3 (a) supplementary reading)

4 (a) Describe a method by which silt accumulation in a reservoir can be computed and its useful

life determined.

(b) An impounding reservoir had an original storage capacity of 740 ha-m. The catchment area

of the reservoir is 100 km2, from which the annual sediment discharge into the reservoir is

at the rate 0.1 ha-m/km2. Assuming a trap efficiency of 80%, find the annual capacity loss of

the reservoir in percent per year. (1.3%/year)

5 A proposed reservoir has a capacity of 600 ha-m. The catchment area is 147 km2and the annual

streamflow averages 17 cm of runoff. If the annual sediment production is 0.035 ha-m/km2what

is the probable life of the reservoir before its capacity is reduced to 20% of its initial capacity by

sediment deposition. The relation between trap efficiency and capacity-inflow ratio is given

below:

Capacity

Inflowratio Trap

Capacity

Inflowratio Trap

efficiency efficiency

(%) (%)

0.1 87 0.01 43

0.2 93 0.02 60

0.3 95 0.03 68

0.4 95.5 0.05 77

0.5 96.0 0.07 82

0.6 96.5

0.7 97

1.0 97.5

(97 yr)

Documents

11 Sedimentacion en Embalses (Ingles)