1. The construction industry is at its lowest ebb, due to scarcity of fine aggregate or sand in the industry parlance. As a responsive government, the problem needs to be resolved to help the construction sector which is an area warranting great stimulus to get out of the recession due to global meltdown in Kerala.
So a wonderful idea has come up from the Aruvikkara Panchayath administration to remove the sediment in the reservoir, wash it clean of mud (particle <62 .5=".5" style="mso-spacerun: yes;">
construction are saved in one shot. Obviously a very glamorous proposal
Unfortunately, the lay mind has seen only one side of the issue. At times of contingencies this is the problem with the human mind. But for a trained mind like me, I see a variety of human issues arising out of the desilting of the reservoir at Auvikkara, which is the only drinking water source for the greater Trivandrum
2. The questions begging answers are many. Say the question of disposal of contaminated water used for washing and removal of mud (silt+clay), which constitutes the lions share of (at least 75% or more) of the reservoir sediment. How do we keep this water away from the water source we rely on for daily needs? Where would the remainder of sediment i.e., mud will go? Now, what about the re-released chemical ions (some of which are deleterious to human health and organic life in the waters)? Will they join the drinking water source in the reservoir or will be transported away by some mechanical means.
Though the boring was carried down to 5 or 6 meters in the sand bars in the channel, would anybody think of going down to such depths below the reservoir/water levels at any point in time using conventional methods and not suction dredging. Why in the world, after all, humans are employed to remove the sediment? The longer it takes for removal of sediment, the greater are the chances and risks of contamination of the drinking water of the reservoir.
3. The 80,000 m3 of sediment estimated tentatively cover all the size fractions or only the sand in the sediment? If it is the total accumulated stuff then on 20-30% will be the sand component. Or sand fraction will be like 16,000 m3 (2666 tipper truck loads) to 24,000 m3 (4000 tipper truck loads) only.
Any process of desiltation should be using suction dredges which will cause only the least or minimum disturbance. Finally, is the sand reserve the only consideration among all others, when the waters are used for drinking water supply? Unless all these questions are answered scientifically, the Panchayat should be banned and prohibited from the desilting of the reservoir.
Finally has anybody looked into the worth gem-chrosoberyl (or Vaiduriyam) accumulated along with the sediment? Who will have the ultimate right on the Vaiduriyam.
Bit of History
The Wellingdon Water Works, the fore runner of the Trivandrum Water Supply System was operational like in the 1930”s. At the present Aruvikkara dam site, it was only a weir that had pooled the water for pumping to the processing plants and water supply in the city. Then came second five year plan, when a larger diameter cast iron pipe line was laid to augment the water supply to the town. Then, height of the weir was raised and finally shutters were built to regulate the through flow of the river to ensure drinking water for the city dwellers.
Sedimentation in the reservoir
Sediment accumulation in the reservoir is as much a reality as the reservoir and dams. The difference is only in the degree. All along sediment has been acceleratedly accumulating in the reservoir, since the days of Wellindon Water Works. Enhance sedimentation was primarily due to the location of the reservoir in the habited or populated area, with a cultivated catchment in the midland of Kerala, where primary pursuit of the people (landed and landless) were to cultivate Tapiocca and other seasonal or annual crops.- the right ingredients for sediment loss from the prepared land and deposition in the reservoir pool.
Farming trends in the catchment.
Over the decades, the catchment of the Aruvikkara reservoir in the Karamana R. basin went through intense cultivation of rubber and tea and now with pineapple, seasonal vegetables etc. Undoubtedly, all these farms do make use of the biocides (some quite harmful) as well as chemical fertilizers. And the geochemistry has it that a large portion of it will join the waters as dissolved ions in the waters and finally to the reservoir, and in the reservoir sediment. Another portion however will escape the trap in the reservoir to reach the sea.
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Dissolved ions in water and the problems
To the lay citizens what accumulates in the reservoir is only sediment which is more silt and clay (<62 .5=".5">62.5 micron) and perhaps only under 20-30% of all the sediment in the reservoir. In contrast, along with sediment, also transferred to the reservoir are several naturally supplied dissolved ions and human introduced chemical ions which are trapped and bound along with the fine sediment.
Yet another un-addressed aspect is the amplification of concentrations of the chemical ions in water, in the pore-water (i.e., water held in the inter-granular spaces of sediment particles) and finally in the finer sediment particles like silt and clay. The Unforeseen consequence is that when the sediment is disturbed, removed, washed and cleaned for meeting the need for sand or fine aggregate, the finer particle bound chemical ions are released and will join the waters, that we ultimately drink as well as it might mix with the through flow harming the waters down stream.
Summary
1. Thus the hidden trap in the proposition of the Aruvikkara Panchayat, is that it will release harming chemical ions to the drinking waters supplied in the city of Trivandrum, as well as in the waters that flow downstream of the dam. .
2. Scientific estimates of the potential sand reserve has not been made yet. Only the size of the reservoir fill is known and stands at 80,000 m3.
3. Generally only 20 to 30% of the fill is fine aggregate grade sediment. So until such estimates are made hold the back the implementation. My own back of the envelope estimates show only 16,000-24,000 m3 of sand.
4. Unless plans for disposal of washed waste are designed wisely do not jump into the process with 200 men and women with shovels and baskets- a method never to be implemented in a drinking water reservoir.
5. We need to worry about the type and nature of chemical uions trapped and bound on the fime sediment particles and the degree of harm they can do to people and the society. So make sure of the inert/safe nature of these ions before jumping into manual removal of reservoir sediment.
6. Firstly identify the locations for storage of the mud-separate coming off the washing process of reservoir sediment to obtain the salable sand. In other words 70 to 80 percent of 80,000 m3 needs to be disposed off with out harming the area and people and farmland for several decades to come.
7. Has the period for the manual removal of reservoir sediment been estimated and if so what is the duration? If it is carried out in summer where would one find water in large quantities for washing the sediment.
8. Lastly, has any body thought of the potential of Vaiduriyam in the sediment and its worth?
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1 comment:
i would like to know more about this project. the project have been started today so whether any study has been done for this. you said the harmful substances in the mud seposited under the water will contaminate it. But i heard that from 1973 onwards the river is given to some private owners for sand mining. They took the sand and washed it in the river itself. Then no contamination was happend. So how it will occur now. I not at all very clear about this subject please send me some details and more data about this .
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