The de-silting of reservoirs has been proposed – and also acted upon sometimes – ostensibly to serve one or more of three main objectives. The first objective is the local de-silting that is required near the entrances to sluices and other types of openings. The second objective would be to increase the volume of water stored in the reservoir. The third objective would be to take the silt out, wash it clear of the finer particles and thereby recover a supply of clean sand for building construction, and treat the increase in reservoir storage as a bonus.
Local de-silting of the beds of reservoirs is essential whenever accumulated silt tends to block the entrance to a tunnel or other outlet structure thereby compromising the proper functioning of the said entrance. Generally, all that is required to be done is to have the control gates of the outlet opened fully for a short while. As the exodus of water always reaches fairly high velocities close to such openings, unwanted soil deposits in the vicinity will be swept out effectively. Silt that is further upstream of such openings will not be removed because the velocity of the water at such distances would be too low. The volume of silt that is extracted near the control structures would not be large and will be carried away by the flowing water and get deposited elsewhere downstream without creating any significant problems. Hence, one may accept the position that this type of small-scale de-silting is technically necessary in specific situations and that it can be economically effected in an environmentally sustainable manner.
As for increasing reservoir capacities by mechanical de-silting, the first point to be noted is that there is no advantage to be gained by removing any silt below the lowest level to which the water can be drawn down by opening the available outlets. All the water below that level is referred to as “dead storage” and any increase in dead storage does not yield a corresponding benefit for irrigation or power production.
The next consideration is that, when a reservoir is constructed, the water level rises to higher elevations than before. The pressure of the resulting greater depth of water causes increased seepage into the earth not only at the lower levels which may have been always under water but also at the higher elevations which have just got inundated. This is because the soils that are higher up are generally much more porous than those at the lower levels. Fortunately, as fresh silt gets deposited on the entirety of the bed of the reservoir, at all levels, over a period of time, the outward seepage of water tends to drag the finer particles of the silt into the spaces between the existing soil particles. This process begins to reduce the porosity of the soil. The incoming silt then continues to form a progressively thicker water-penetration-resistant membrane, the effectiveness of which increases slowly with time, thereby helping to conserve water. Removing this silt with a view to increasing the capacity of the reservoir would destroy this very valuable low permeability membrane and more water may be lost over several years by this destruction than by the increase in the water storage volume that is sought to be achieved.
Before going further, it may be mentioned that 1 acre-foot of water is the volume of water required to cover an area of 1 acre to a depth of 1 foot. This is equal to 1,234 cubic metres. Paddy cultivation requires about 6 ac-ft of water per acre per annum, though frequently a lot more is wasted.
In the case of large reservoirs, the volumes of soil to be moved during de-silting would be quite enormous and present various logistical, environmental and cost complications of a rather forbidding nature. Indeed, a very recent newspaper report states that de-silting is being done at the 100,000 acre-ft Kantale Reservoir. Twenty-five excavators and 175 trucks are being employed at a cost of Rs375,000,000 to remove just 750 acre-ft of soil at Rs500,000 per ac-ft. The 750 ac-ft to be removed represents only 0.75 per cent of the original storage volume. The farmers themselves are said to have questioned the wisdom of trying to de-silt large reservoirs of this type because of the small return for the moneys spent. They have emphasized the greater importance of preventive measures to control soil erosion to check the flow of silt into reservoirs, eg. by growing plants with extensive root systems that would help to hold the soil together within the watershed area. They are also said to have commented that the amount of money that has to be expended on de-silting a large reservoir would be more usefully diverted to the de-silting of a number of small reservoirs.
No mention is made in the newspaper report of where the excavated soil from Kantale is being dumped and what damage is being done to the roads over which this large number of heavy trucks is running. Where is the land where the dumping is being done? What was it used for originally? Was it marsh, pasture or forest? Was an environmental impact assessment carried out before this land was chosen as the fill area? Will one have to spend millions more to deal with the consequential environmental damage causing by the filling operation – presumably with more work for contractors and more “pocket money” for the politicians who back them?
If we now picture a small “tank” covering 10 hectares (about 25 acres) with an average depth of 1.5 metres (5 feet) over this whole area, and assume that it has sluice gates right at the deepest point of its bund, the total live storage would be in the region of 150,000 cubic metres (125 acre-feet). If we wish to increase the capacity of this reservoir by 10% – and anything less would hardly be worth doing – we would need to remove 15,000 cu.m (12.5 ac-ft or 5,200 cubes or about 2,500 dumper-truck loads) of wet soil. The total cost of this would be of the order of Rs6,250,000.
If any of the silt, however excavated, is to be washed and the sand extracted, from where will one get the water to do so? And whither will the slurry of the unwanted clay and decayed organic matter in the silt be directed? Would not this slurry end up in the existing drains and not only block them up over a period of time but also contaminate the water in the area to the extent that the local population will not be able to use it for drinking, washing or bathing their cattle?
If sufficient meteorological and hydrological data are available to carry out a computer model of the relative timings of heavy rain and the drawing down of water for irrigation, and if suitable arrangements can be made for the safe disposal of the silt that is removed, the case for de-silting of certain small reservoirs may be justified. But without such studies, one is led to acknowledge the inescapable fact that the biggest promoters of de-silting are greedy or stupid politicians and earthmoving contractors, aided by spineless administrators and technical personnel. They are not worried about the negative aspects of blind de-silting, some of which are highlighted above. False hopes are built up in the minds of the farmers who are promised much that cannot be delivered. The vast sums of money spent on this kind of exercise go only to enrich the said promoters and those who cooperate with them.