SILTATION OF TUNGABHADRA RESERVOIR- A STATUS NOTE


 
 
Introduction
Design Parameters Periodic Hydrographic Surveys
Hydrographic Surveys of 1993 Trends of Siltation
Feasibility of desiltation Soil Conservation Works

Introduction

Water Management in a reservoir, among others, calls for a fairly accurate assessment of the capacity of the reservoir to store water at various levels. Tungabhadra Reservoir with a drainage area of 28,180 sq.km was designed for storing 133 TMCft. of water and was impounded for the first time in the year 1953. The full reservoir level (FRL) was fixed at RL 1633 ft. Capacity of the Reservoir at various levels was worked out based on the regular contour survey conducted in the river basin upstream of the Dam prior to the impounding of water in the reservoir. The survey was conducted with a contour interval of 10 ft and the capacity of the reservoir computed for every one-foot interval. According to this the Gross capacity of Tungabhadra Reservoir was taken to be 133.0 TMCft., (3766.1 M.cum) at an FRL of 1633 ft. (497.738M).


Design Parameters.

At the planning stage it was envisaged that the annual silt deposition in the reservoir would be between 420 to 430 MCft., with about 25% of the silt flowing down the reservoir. The yield from the catchment of 28,180 Sq.Km so adopted by the planners works out to 4.29 ha.m per100 sq.km per year (90 acres ft per 100 sq.miles) which tallies fairly well with the recommendations made by Dr. A.N.Khosla.


Periodic Hydrographic Surveys.

In order to assess the rate of siltation in the reservoir, Mysore Engineering Reserach Station (MERS), K.R.Sagara was entrusted with hydrographic surveys of the reservoir in 1963 for the first time under CBIP programme for scientific hydrographic surveys of eleven important reservoirs in the country. In order to carry sedimentation surveys regularly, 133 permanent range pillars were established on the periphery of the reservoir for positioning the sounding points. The location of these permanent range pillars was fixed by MERS by triangulation surveys using the straight portion of the main dam measuring 1689.735 m, as base line.

Necessary benchmarks along the periphery of the reservoir at suitable intervals were also established. Subsequently, hydrographic surveys were conducted during the years 1972,1978, 1981, 1985 and 1993 using these range pillars. Capacity of the reservoir worked out on the basis of hydrographic surveys in the past are as follows.

Sl. 
No
Year of Survey Capacity at FRL(1633 ft.) 
M.Cum(TMCft)
1 1953 (Topographical survey) 3751.17      (132.473)
2 1963 3246.79      (114.660) 
3 1972 3428.60      (121.080) 
4 1978 3332.75      (117.696) 
5 1981 3275.68      (115.681) 
6 1985 3166.74      (111.832)
7 1993 3157.53      (111.508) 


Hydrographic Surveys of 1993.

Under UNDP sponsored programme, Tungabhadra Reservoir was also included, along with  Bhakra and Hirakud reservoirs, to carry hydrographic surveys using HYDAC 2000  system. This system comprises of Digital Distance Measuring Unit (DDMU) along with transponders for positioning the boat; DESO echo-sounder for measuring depths and one HP computer for plotting the grid of reservoir bed and carrying areas and capacities calculations at different intervals. These components were housed in a high speed Jet Boat purchased for the purpose.

As per the directions of the Board given in 143rd meeting, the positioning equipment and echo-sounder were used in a joint survey conducted by Karnataka Engineering Research Station (KERS) and Andhra Pradesh Engineering Research Laboratories (APERL). The joint survey was completed by the two organisations in association with the Board Engineers in 1992-93. The report submitted by KERS and APERL was approved by the Board in its 152nd meeting held on 23-11-94 for adoption of capacity elevation tables obtained from 1993 sedimentation surveys with effect from the water year 1995-96, superseding the capacity elevation tables of 1981 surveys. As per 1993 surveys, the capacity of Tungabhadra Reservoir at FRL, 1633 ft. is assessed as 111.508 TMCft, which is presently being adopted.

On review of various hydrographic surveys conducted from 1963 to 1993, it can be observed that the rate of siltation of Tungabhadra Reservoir was of the order of 1.78 TMCft per year during the first 10 years. However, during the last 12 years, i.e., from 1981 to 1993, the rate of siltation has come down to the order of 0.41 TMCft per year. For the last 40 years,the siltation rate works out to an average of 0.52 TMCft per year. A comparison of rates of siltation in other major reservoirs in India (Annex-6.1.D) may indicate that the situation in Tungabhadra is not alarming. However, the ground fact is that due to siltation, Tungabhadra Reservoir has lost 21 TMCft of its useful storage. This has contributed to reduction in the irrigation utilisation from the reservoir to about 187 TMCft in recent years (Annex-6.1.E) against 212 TMCft., of water allocated to the project by Krishna Water Dispute Tribunal excluding evaporation losses.


Feasibility of desiltation.

An attempt was made by the Board to study the possibility of desiltation of reservoir economically by dredging in 1989. As per the preliminary feasibility report of the Dredging Corporation of India, Visakhapatnam "on" The desiltation of Tungabhadra Reservoir by way of  Dredging" the cost of dredging worked out to Rs.100 crores per 1 TMCft, according to 1989 rates. The cost of removal of 21.5 TMcft of silt involves a huge expenditure of Rs. 2,150 crores excluding other charges of transportation of silt to safer zones and setting up of required infrastructural facilities. In addition to the economical aspects of desiltation, the problem of removed silt without affecting down stream projects in the river basin needs to be addressed. Keeping in view all the aspects it was considered that desilting is not techno-economicaly feasible and is environmentally detrimental.
To combat the menace of soil erosion, Government of India, Ministry of Agriculture, Department of Agriculture & Co-operation has initiated soil conservation programmes in the catchments of River Valley Projects (RVPS) and Flood Prone Rivers (FPRS). From beginning of 3rd Five Year Plan, a total of 27 catchments were identified assisted by Ministry of Agriculture, Government of  India (Ministry of Agriculture and Cooperation, 1992).

The All India Soil & Land Use Survey (AIS & LUS) organisation has been entrusted with the task of conducting rapid reconnaissance surveys in the catchments of RVPs and FRPS for prioritizing sub watersheds based on adjusted sediment yield/runoff potential for planning programmes. Under this programme, the Government of Karnataka has selected 93 sub watersheds in Tungabhadra catchments.