The threat of ‘siltation’ to lifelines

Sri Rama Sagar Project (SRSP) Reservoir is built on River Godavari near Pochampad village of Nizamabad district. It is a multipurpose irrigation project. Foundation stone for the project was laid in 1963 by the then Prime Minister Jawaharlal Nehru. The project consists of SRSP Reservoir, Lower Manair Dam reservoir, Kakatiya Canal of 284 kms length, Saraswathi Canal of 77 kms and Lakshmi Canal of 3.5 kms. Its original capacity to hold water was 112 TMC, catchment area 91,751 sq km and designed flood discharge 16 lakh cusecs.

With this the total irrigation potential was 9.69 acres. While Kakatiya canal benefited Nizamabad, karimnagar and Warangal districts, Saraswati and Lakshmi canals irrigated Adilabad and Nizamabad districts respectively. Regarding siltation the engineers were quite happy with the position at one time. Normally speaking any project on River Godavari is expected to face high siltation problem. It is much higher than the ones on Krishna River. This is because Godavari region has black-cotton soils, less rocky lands and forests have been getting denuded at a faster pace. However, the engineers noticed about a decade ago that rate of sedimentation was much less than the original calculations.

The project’s original life span was to be100 years. But, if the rate of siltation was at such a low level the project may continue to benefit farmers for even another 100 years, they thought. For instance in 1992, the storage capacity reached a high of 70 TMC instead of 62 TMC, which should have been the level if siltation continued at the originally estimated rate. The engineers thought the hopeful situation was due to beginning of the growth of vegetation in the catchment area and on the old bunds under sustained efforts made by the governments of Andhra Pradesh and Maharashtra. Thus the siltation rate was thought to have got reduced by half.

To put it in other terms, the satellite pictures had established that the loss was only 0.3 TMC per year, instead of national average of 0.5 TMC and 0.5 acre ft instead of an allowed provision for 1.02 acre ft. Andhra Pradesh Engineering Research laboratory (APERL) and the AP Remote Sensing Application Centre carried on this survey.

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 Gross Storage capacity, which was 98.9 TMC when the dam was constructed and which in 1999 when the survey was conducted remained at 90 TMC after a loss of eight 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.

However these situations appear to have altered in recent years. Some engineers who visited the area of ancient Brahmeswar temple in Adilabad district in early 2013 found the silt deposits in the river to have accumulated to a high level thickness of over 30 ft. That was said to be the position in the entire 45 km backwater stretch of the river up to Basar town.

From the original 112 TMC it has come down to 80 TMC as per a survey done by the A P Remote Sensing Agency in 2006, though the government claims the present (2006) capacity to be at over 90 TMC.

Last year an interesting experiment was undertaken by the district authorities for desiltation of the project. Under the NREGA scheme the Collector planned to remove the alluvial soil from the project bed by engaging 10,000 labourers and transporting the same to agricultural lands owned by SCs and STs within a radius of five km. However, it is not known how the work was executed with what results for the solution of siltation problem.

The multipurpose Nagarjunasagar Project on the River Krishna, near the then Nandikonda village of Nalgonda District,is the largest and highest masonry dam in the world. It was inaugurated in 1955 by the then Prime Minister Jawaharlal Nehru. Water was released into the Jawahar Canal (right one) and Lalbahadur Canal (left one) by late Prime Minister Indira Gandhi in 1967.

The total catchment area of the Nagarjunasagar Project is 2.15 lakh sq km with maximum observed flood of 10.61 lakh cusecs of water. Gross capacity of the reservoir was 408 TMC at the time of construction. Contemplated ayacut potential was 10 to 11 lakh acres under right main canal and 8.5 to 9.5 lakh acres under left main canal. The project was to irrigate lands mostly in the districts of Nalgonda, Khammam, Krishna, Guntur and Prakasam ( in lakhs of acres) and a few thousand acres in West Godavari.

As far as power is concerned, the project was to generate 110 MW in conventional unit and 100 MW in reversible method. Besides this there are three units of 30 MW each on right side and two units of 30 MW each on left side. This makes it a total of 360 MW. However, the Nagarjunasagar project has been siltation-prone, belying the expectation that irrigation schemes on River Krishna face least of that threat. To make the point, the Major irrigation Minister P Madhusudhan Reddy told the State Assembly in 2012 that the project had lost almost “20 per cent of its storage capacity due to extensive siltation”. That is 408 TMC to around 320 TMC: Loss of about 80 TMC in 45 years. This is a little less than two times the loss in Srisailam. The Minister also stated that the survey was conducted in 2009-10 by Andhra Pradesh Engineering research laboratory (APERL). After that in the last three years the project must have lost capacity of five more TMC.

As per one recent estimate by the Central water Commission (CWC) after it conducted a bathymetric survey the loss in capacity is actually 120 TMC and the capacity has come down to 288 TMC. If that is true one can only imagine the threat posed to the ayacut under Nagarjunasagar project. The sedimentation levels are also surprising because it was expected that the upstream reservoirs such as Almatti,Narayanpur, Jurala and Srisailam would hold back much of the silt. Some experts say the situation would not have been this bad had the dam engineers utilized the diversion channel once a year. The channel pushes out sediments from reservoir.

In any case, the reduction in capacities of the Sagar canals is causing severe irrigation problems to the farmers in Krishna, Guntur and other districts, like under any other project. However, Nagarajunasagar is particularly important since the State acquired its status as ‘Annapurna’ mostly because it emerged as a mega granary under Nagarjunasagar. If it loses its storage capacity at this rate it may not take more than one generation for farmers of these five major districts to face deprivations. Actually silting of reservoirs on Krishna river was extensively studied by the government at the time of preparation of Pulichintala project and later at the time of Krishna-Pennar project and lastly for Nagarjunasagar. All these studies have shown extended life for reservoirs. Col Elis calculated the useful life of the Nagarjunasagar reservoir as 370 years for a dead storage capacity of 164 TMC. At Srisailam the storage capacity up to MDDL (Minimum Draw down Level) is 158 TMC.

And therefore the useful life of the reservoir based on the study is predicted to be more than 300 years by irrigation authorities. The life may perhaps be even longer due to the fact that a large number of reservoirs have been constructed or under construction in the upper reaches which will trap most of the silt. Still one cannot possibly claim that the problem along with the silt has not been piling up gradually for the irrigation projects in the State.

Experts say the immediate steps required are construction of silt-arresting tanks on the slopes of river banks, afforestation in the catchment area and growing of grass on river margins to prevent erosion and cutting.

Neelam Sanjeeva Reddy Sagar Srisailam Project, popularly referred to as Srisailam Project, was constructed in deep gorge across River Krishna at about three km from the famous Srisailam temple. In Kurnool district. Although originally envisaged as a purely power project, irrigation and water supply were also included subsequently. It was completed in 1984.

Out of the 800 TMC of Krishna water allocated to Andhra Pradesh in 1973, the Srisailam Right Bank Canal Scheme was cleared by the Planning Commission in 1981. With 19 TMC of water it was formulated to irrigate 1.90 lakh acres in three Talukas of Kurnool and one Taluka of Kadapa districts. In addition to this, Right Bank Canal was contemplated to provide irrigation to drought prone areas of Nalgonda and drinking water to fluoride affected villages en route. This Lefty bank canal was renamed Alimineti Madhava Reddy Project later on. This one is a lift-irrigation project with two canals.

Srisailam Project has a catchment area of 2.06 lakh sq km. Its Gross storage at Full Reservoir Level, as per the 1976 survey was 308.00 TMC. But when a survey was conducted in 1997, twenty years since the project was built, the capacity had gone down to 263.63 TMC, evidently due to siltation problem. That was about 45 TMC in just 20 years. Scientifically the loss estimates are considered to be 0.82 % a year. And any loss beyond 0.5 % annually is considered to be “serious”.

And by now in 2013 it must have lost the capacity by another six TMCs taking it to a total of 51 TMC, pulling down the project capacity from original 308 TMC to an alarming 257 TMC in 26 years flat. The surveys were conducted by the central water Commission (CWC) and the Andhra Pradesh Engineering Research Laboratories (APERL) using hydrographic, remote-sensing and other techniques to study the problem. Siltation in Srisailam is attributed to soil erosion due to uncontrolled deforestation, forest fires, overgrazing and “unwise” farm practices.

The general belief till then had been that projects on River Krishna had less siltation problem compared to those on River Godavari. Because Krishna flows through rocky terrain and the catchment areas have dense vegetation both of which hold back sedimentary soils to considerable extent. But the survey results shocked the experts. The sedimentation problem cuts into both dead and live storages, placing a huge question mark on the survival of the existing projects as also the feasibility of new ones.

With this water “head” (pressure) has also fallen at the hydel stations, affecting the output from installed capacity of 1,740 MW. When Srisailam along with Nagarjunasagar power units was shut down some years ago due to poor monsoon, Genco officials said that the situation could have been avoided had the government de-silted the reservoirs to increase water storage capacities.

However, it cant be ignored that siltation is a world -wide problem and de-siltation techniques have not been developed anywhere. Some technologies and methods were applied from time to time but nothing succeeded except very marginally. Thus it remains as good as an impossible task.