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CHAPTER 7

RESERVOIR SEDIMENTATION

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CHAPTER 7

RESERVOIR SEDIMENTATION

7.1 General

Sach Khas H.E. Project envisages construction of a concrete gravity dam with a grossstorageof 27.037 Mcum corresponding to FRL of 2220 m. The reservoir elevation area capacitycurve has been prepared at the project site based on the reservoir contour map.

7.2 Suspended Sediment Observation

The sediment fractions in suspension are classified into three grades on the basis of particlesize:

Coarse: More than and equal to 0.20 mm diameter Medium: 0.075 mm to 0.20 mm diameter Fine: Less than 0.075 mm diameter

7.3 Sediment data availability

Suspended sediment data measurements are carried out at Udaipur G&D Site in Chenab river basin by CWC. The details of the silt at Udaipur are given in Table 7.1.

Table 7.1 Details of Silt data

Sr. No. G&D Site River Catchment area(Sq.km) Period

1 Udaipur Chenab 5922.474 Jan 0 3- May 08 2 Tandi Chenab 1639.662 197 8 - 2 00 4

7.4 Average annual sediment rate

The silt rate estimated at Udaipur site is based on available sediment data and the details isgiven in Table 7.2 . The silt rate estimated at Tandi site is based on available sediment data

and the details is given in Table 7.3.

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Table 7.2 Details of annual silt rate at Udaipur site

YearSed.Conc n.(gm./litre)

Average

Discharge(Cumecs)

Sediment

Yield(tonnes/day)

Sediment

yield(Mm 3/Yr.)

Sediment Rate(Ha-m/Sq.km/Yr.)

2003 0.361 309.921 9660.785 2.351 0.0397

2004 0.272 258.393 6064.352 1.476 0.0249

2005 0.331 226.034 6471.365 1.575 0.0266

2006 0.380 266.343 8743.335 2.128 0.0359

2007 0.190 238.668 3924.544 0.955 0.0161

Average value 0.0286

Table 7.3 Details of annual silt rate at Tandi site

YearSed.Concn.(gm./litre)

AverageDischarge(Cumecs)

Sediment Yield(tonnes/day)

Sediment Yield(Mm 3/Yr.)

Sediment Rate(Ha-m/Sq.km/Yr.)

1978 0.859 163.285 12121.435 2.950 0.1801979 0.515 152.542 6781.227 1.650 0.1011980 0.507 151.237 6623.859 1.612 0.0981981 0.294 142.298 3611.613 0.879 0.0541982 0.295 136.905 3490.777 0.849 0.0521983 0.345 147.692 4398.623 1.070 0.0651984 0.321 149.021 4128.121 1.005 0.0611985 0.264 137.253 3134.923 0.763 0.0471987 0.432 154.359 5756.271 1.401 0.0851988 0.616 169.434 9014.350 2.193 0.1341989 0.602 140.993 7327.483 1.783 0.1091990 0.399 158.573 5465.513 1.330 0.0811995 0.111 164.143 1570.119 0.382 0.0231996 0.210 125.694 2276.966 0.554 0.0341997 0.167 142.644 2054.074 0.500 0.0301998 0.342 214.133 6327.383 1.540 0.094

1999 0.869 213.286 16007.712 3.895 0.2382000 0.298 145.907 3752.495 0.913 0.0562001 0.186 142.535 2290.589 0.557 0.0342002 0.360 181.284 5643.868 1.373 0.0842003 1.006 804.873 69981.454 17.029 1.0392004 0.293 132.157 3349.379 0.815 0.0502005 0.420 198.397 7199.418 1.752 0.1072006 0.460 152.423 6057.913 1.474 0.090

Average value 0.123

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I f 20% of suspended load (as per standard B I S practice) is taken as bed load, then total siltrate becomes 0.0344 Ha-m/sq.km/year and 0.147 Ha-m/sq.km/year at Udaipur and Tandi Siterespectively. As Udaipur site is near to Sach Khas HEP Site, therefore same rate of 0.0344mm /year has been adopted at Sach Khas HEP Site.

7.5 ESTIMATION OF SEDIMENTATION VOLUMEAs per I S: 12182: 1987, it is recommended to adopt full service time & feasible service time for reservoirs of hydro power projects supplying power to a grid as not less than 25 years & 70 yearsrespectively. I n view of above, the New Zero Elevation for Sach Khas Hydroelectric Project isestimated for 25 years and 70 years.

The rate of sedimentation at 0.344mm per year which is equivalent to 0.0344 Ha-m/sq. km/year,the sediment volume expected to be deposited in the reservoir at the end of 25 years and 70 yearshave been computed by considering the variable Capacity I nflow ratio at an interval of 5 yearsand corresponding Trap Efficiency obtained from Brunes Median Curve for normal pondedreservoir. With 6608 km 2 catchment area of Sach Khas at dam site, the annual sediment load

works out to be 0.0344 x 6608 = 227.32 Ha-m/year or 2.27 Mm3/year.

The volume of sediment deposits over a period of 25 years and 70 years works out to be718 Ha-m and 1163 Ha-m respectively as illustrated in table below.

ESTIMATION OF SEDIMENT VOLUMEBasin Area : 6608.00 km 2 Annual Sediment : 2 . 272 Mm 3 /yr.Capacity : 27 .04 Mm 3 Inflow : 8199.40 Mm 3

Time Capacity Inflow(I) C/ITrap

EfficiencyTotal

SedimentTotal in 25/70

Yrs.

(Yrs.) (Mm 3) (Mm 3) (Mm 3) (Mm 3) (Mm 3) (Mm 3)0-5 27 .04 8199.4 0.0033 16.09 1.830

6-10 2 5. 2 1 8199.4 0.0031 14. 2 6 1.6 2 0

11-15 2 3.59 8199.4 0.00 2 9 1 2 .53 1.4 2 0

16- 2 0 22 .1 7 8199.4 0.00 27 10.9 2 1. 2 40

2 1- 2 5 2 0.93 8199.4 0.00 2 6 9.4 2 1.0 7 0 7 .18

2 6-30 19.86 8199.4 0.00 2 4 8.05 0.910

31-35 18.95 8199.4 0.00 2 3 6.83 0. 7 80

36-40 18.1 7 8199.4 0.00 22 5. 7 3 0.650

41-45 1 7 .5 2 8199.4 0.00 2 1 4. 7 8 0.540

46-50 16.98 8199.4 0.00 2 1 3.96 0.450

51-55 16.53 8199.4 0.00 2 0 3. 2 6 0.3 7 0

56-60 16.16 8199.4 0.00 2 0 2 .6 7 0.300

61-65 15.86 8199.4 0.0019 2 .19 0. 2 50

66- 7 0 15.61 8199.4 0.0019 1. 77 0. 2 00 11.63

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7.6 ESTIMATION OF NEW ZERO ELEVATION (NZE)

As the quantity of sediment which will deposit below the normal water surface has been established,the Empirical Area-Reduction Method is used to estimate the distribution. The shape factor wasadopted as the major criteria in deciding the reservoir type. So, a curve is plotted with depth of reservoir at different elevations starting from the FRL as ordinate against the corresponding storagecapacity as abscissa on logarithmic scale. The curve fits into a straight line and the reciprocal of theslope (m) has been worked out to be 1.6 which falls within the range of m = 1.5 to 2.5. Henceaccording to IS: 5477 (Part 2) the reservoir is classified as Type III (H I LL) considering the original areacapacity curve.

Sediment distribution through the depths of reservoir and the NZE have been carried out adapting theEmpirical Area Reduction method using the design curves given in I S 5477 (Part 2). The equation for the design curve is given by empirical relationship.

A p = C x p m (1- p) n where,

A p = a non-dimensional relative area at relative distance p above the stream bed.

C, m, n = non dimensional constants which have been fixed depending on the type of reservoir.

For type III reservoir, the equation is A p = 16.967 x p 1.15 (1 - p) 2.32 .

Thus, the NZE for Sach Khas reservoir after 25 years and 70 years sedimentation works out to be2174.0 m & 2186.80 m respectively. Details are presented in the table below.

The calculation of sediment deposition is attached in Annexure VII . The live storage of the reservoir

will have to be maintained by proper flushing of the reservoir, for this a proper reservoir managementand operation rules have to be framed by model studies.