Sriramsagar siltation level under control Report

from The Hindu Dt. 12th August, 1999

Engineers here are happy that Sriramsagar Project is back in cheers again. Sriramsagar Project has won the first round of its battle against siltation and promises to live for a century more after its 100 year lifespan certified by its designers.

The catchment and consequently, the Godavari were virile and merciful, notwithstanding two major projects upstream in Maharashtra - Jaykwadi and Vishnupuri, and the reservoir was full to the brim far in advance of the season, showing up 70 tmc, and not 62 tmc as would have been if siltation had maintained the rate estimated by the planners.

On 9th August the tamed river restlessly swirled at 1085.7 ft level flapping the rare of the gargantuan dam, eight - storey high and nearly a km long in contrast to the 1995 situation when it sank to the rock - bottom, forcing the Government to make a plea for release of atleast 5 tmc from Maharashtra projects to save the standing crops. The plea, however, was not responded to. To the naked eye, the water spread was endless. In the monsoon mist and vertical streaks of rain, it appeared as though touching the skies in the distant horizon. Visitors who were flowing in the hundreds, standing on top of the dam, could fulfill a fantasy finding the riverbed downstream ironically empty and dry, revealing the bottom rocks up to the confluence of the Swarna, three kms away - a typical character of the Godavari synonymous with its highly 'erratic behaviour'.

The maintenance engineers who scurried from their offices to the site by jeeps keeping an eye on the swelling reservoir for opening the flood-gates at an appropriate time are doubly happy because the slit joining the project from upstream has plummeted to a tolerable level. According to Mr. S. Maruti, Chief Engineer, the structure will last for two generations and the reservoir will have a cut of about 60 tmc in its gross storage capacity (GSC) instead of being filled with mud-silt completely. The encouraging development has been attributed to "a small beginning" of vegetation in the catchment area and on the ageing bunds under sustained efforts independently made by both Andhra Pradesh and Maharashtra.

A cumulative analysis of extensive surveys made at different points of time by various expert groups, covering a 24-year period from 1972 (when water was first released) to 1994 during which the reservoir was found shrunk to 334.12 sq km from 434.8 sq km, has indicated a "small let-up" by silt, and pointed out that the silt is reaching the place at "only half the rate orginally estimated".

Considering the notoriety of the Godavari as a silt-gathering river in view of the loose black-cotton soils through which it passes, designers of Sriramsagar had allowed a provision for 1.02 acre ft of silt a year for a square mile of the catchment and this works out to a loss of 6 tmc ft out of the GSC. But what was observed during the studies for which even satellite maps had been used was that the siltation was at the rate of 0.5 acre ft a year working out to 0.3 tmc loss. Among the surveyors, were the Andhra pradesh Engineering Research laboratory (APERL) and the AP Remote Sensing Application Centre, both in Hyderabad.

The total loss for the dam's assumed 100-year lifespan, which would have been 60 tmc at the estimated rate, has thus come down to 30 tmc, and at this rate, the reservoir will lose 60 tmc after two centuries. This implies that the reservoir survives even after 200 years but with the GSC, which was 98.9 tmc when the dam was constructed and which today remains at 90 tmc after a loss of 8 tmc, being reduced to 38.9 tmc. If the estimated silt-rate were to be allowed to have its go, the loss would have been 120 tmc for the two centuries and in that case, the reservoir will cease to exist even earlier.

There has been confusion among the present generation of engineers about true figure relating to GSC. Citing APERL "conclusive studies", they argue that the GSC was erroneously calculated at 112 tmc by the original designers while it should have been only 98.9 tmc. With the loss on account of siltation being 8 tmc so far, what today remains as GST is only 90 tmc, they say, even while there has been a hot debate among the engineering circles about this 'dispute" the project engineers are counting 90 tmc as the present capacity for all practical purposes.

Slicing off water though the Maharashtra projects upstream Jayakward and Visnupuri being the main ones - has arrested the silt to a great-extent. Further fall in silt-flow has become possible due to the afforestation of the catchment area which was deprived of the existing vegetation by ambitious farmers and wood smugglers, and a series of minor irrigation tanks and check-dams constructed at the mouth of the scores of tributaries, rivulets and hill-streams which join the Godavari. A total of 152 minor irrigation (MI) tanks and 26 check- dams are under execution by the AP Government with Rs. 18.72 crores, say N.Narayana Reddy, and Mr. P. Sivakantantham, Superintending and Deputy Engineers at the site. The share of the Madhole and Bainsa areas in Adilabad district, which form the core of Sriramsagar catchment, alone is 40 MIs. Lands were acquired up to the contour of 1093 ft but the dams FRL was later fixed at two contours is, thus, lying vacant and proposals are afoot to fill this with a green belt. As the first step, the extent running into thousands of acres is being handed over to Forest Department.