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).
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.
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.
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.
at FRL(1633 ft.)
Surveys of 1993.
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.
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
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.
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.
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).
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