Director, Centre for Environmental Studies,
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Godavari is the largest river in South India, it starts at Nasik in the western ghats and runs South East area is about 1.2 lakh sq.miles, greater than the area of Britan. Its annual average flow is 3600 Thousand Million Cubic ft. (TMC ) (83 MA ft.). But its highest yield is 5,860 TMC and the lowest in 65 years is 960 TMC. Its peak flood flow is more than 20 lakh cusecs. Godavari water is used by construction of an anicut near Rajahmundry during 1850-60 by Sir Arthur Cotton. The anicut is 11,850 long excluding 3 Islands which it connects and irrigates about 10 lakh acres in the delta. The second crop depends upon low summer flows and hence higher food production required new irrigation projects and hence Rampada Sagar was conceived during the British rule to augment irrigation land. This project consists of a 428 ft. height dam , 20 miles upstream of Rajahmundry and 1.5 miles above Polavaram with two major canals, one extending on the left upto Visakhapatnam and the other on the right extending upto Gundlakamma river with a hydro power of 150 MW on the right bank. The canals proposed to irrigate 27 lakh acres in addition to stabilizing existing irrigation in about 20 lakh acres under Krishna and Godavari deltas. The dam is 6600ft. long and 428 ft. high from deepest foundation. FRL of the Reservoir is +198ft, with a water spread of 527 sq.miles with a gross storage of 690 TMC. Crest level of drum is +180ft. road level 237.82, Tail water is at +43ft. Foundation bed level rock is between bed width at foundation is 303 ft. under spillway section. To dispose of the maximum floods the spill way is 4,200 ft. long with 16 drum gates of 135ft. x 18ft. There will be 100 sluices of 10ft. x 20ft with sill at +83 ft. to dispose of sill laden floods. The river flow from the middle of June to middle of September will be diverted into the canals and the sand sluices will dispose of the floods at the diversion level of +145. The annual silt deposition is estimated at 2 TMC in the initial stages and 0.33 TMC during the later periods and hence the silt capacity is provided for 168 TMC for a life of 400years for the reservoir.
The Mahanandeeswara hill in the middle of the river will help in the construction of a coffer dam and diversion of river water. After consulting foreign experts including Dr.J.L.Savage, Dr.Karl Terzaghi, Sir Murdoch Macdonald and S.O.Harper. The technical experts the construction of the dam was found feasible. The investigation started in 1943 was almost completed. The excavation of the bed upto rock level by nearly 230ft. is big task. The let canal upto Visakhapatnam provides water to irrigate 4.66 lakhs of Ist crop and 3 lakh acres of II crop. The canal will be 130 miles long with a bed width of 350ft. and depth of 10ft. with a flow of 9139 cusecs. The right canal will be through a twin tunnel of 32ft diameter each. The canal will be 110 miles long upto Krishna river and irrigates 3 lakh acres under the I crop and 2 lakh acres under II crop. It will also provide water for he II crop under Krishna delta. An aqueduct will be constructed across Krishna to extend the canal for 105 miles from Krishna to Gundlakamma river to irrigate 4.2 lakh hectares. The canal will be 470ft. wide with water depth of 15ft. to carry a discharge of 22,000cusecs. It will produce about 10 lakh tonnes of rice and 150 MW of power.
To sum up the salient features of the dam and canals are tabulated as reported in the Indian Concrete Journal, December 15th 1950 are as follows.
Catchment area of the river Godavari -- 1,21,500 sq.miles
Average Total Annual flow of the Godavari -- 3,600,000 million c. ft.
Maximum food discharge -- 2.1 million c .ft per second
Full Reservoir Level -- +198
Level of roadway over the dam -- +237.82
Level of deepest foundation -- 190
Height of dam from bottom-most point -- 428 ft.
Water spread of reservoir total -- 527 sq.miles
Water spread of portion lying in Madras -- 316 sq.miles
Water spread of portion lying in Hyderabad -- 181 sq.miles
Water spread of portion lying in Madhya Pradesh -- 16 sq.miles
Water spread of portion lying in Orissa -- 14 sq.miles
Length of Left canal --130 miles
Length of right canal upto crossing of Krishna -- 124 miles
Length of right canal beyond Krishna -- 89 miles
Area of Irrigation New I crop in uplands -- 11.86 lakhs acres
Area of Irrigation New II crop in uplands -- 5.10 lakh acres
Additional New II crop in Godavari delta -- 3.50 lakh acres
Additional New II crop in Krishna delta and single crop in collair lake area -- 7.00 lakh acres
Annual food production -- 1.1 million tonnes of rice
Firm electric power developed -- 1,50,000 KW
Approximate cost of Project -- 129 crores
The claim of some experts that the rockfill Polavaram dam is designed with a reduced height by 50ft. to the original Polavaram dam known as Rampadsagar dam is not correct because Rampadsagar dam was a concrete dam with a higher storage capacity of 690TMC with an FRL at +198ft. to transfer more water of Godavari for the benefit of the coastal districts from Visakhapatnam to Prakasam district. The present Polavaram dam is not made with concrete but with earth and rockfill materials which make the embankment dam highly susceptable to collapse for several reasons and hence this dam possess very serious risks to people living downstreamof the dam in Godavari delta resulting in a potential economic loss of more than Rs.1 lakh crores due to a dam burst either due to human errors or unanticipated extreme floods due to global warming or intense cyclones of longer duration which pose a threat to storage dams in the upstream areas when sudden flash floods are relased from the upper dams to safeguard their own interests. The Concrete dam at Srisailm was able to withstand the extreme floods in October 2009 when the floods touched over 902ft and the floods flowed over the top of the dam for about 10ft depth without resulting in collapse of the dam.