Desilting is becoming a major component of any Lake Conservation Project now a days. The basic reasons given for necessity of desilting are - increasing the storage capacity and checking eutrophic conditions.
As it is not practicable to reverse the slow process of silting in the bed of lakes through which only silt can be removed, there is no word like 'desilting' in any standard English dictionary. What we are doing in the name of 'desilting' is practically 'digging' or 'excavation' of lake bed. By so called desilting, the original lake bed is disturbed which has far reaching adverse effects on the performance of the lake. Most visible effect is the increase in percolation rate resulting in heavy seepage losses through the lake bed as observed after massive desilting at Pushkar lake in 2009.
Increasing storage capacity by digging lake bed is the most costly proposal when compared to other alternatives and therefore needs study of alternatives as well as detailed hydrological investigations considering available yield, existing storage capacity, down-stream needs, etc. As overflowing of a lake is necessary to keep the river alive in the downstream and to flush out the static water of the lake, if a lake does not overflow at least once in four years on an average, there is no need to increase the storage capacity. Instead of desilting, the long term solution is to treat the catchment area so that silt load in the incoming flow is permanently reduced. This can be done by contour bunding, check dams, massive plantation, etc. which will be less costly and will have far reaching positive effects of permanent nature.
Desilting, if not done in a planned way, creates isolated pits of considerable size in the submergence area which may have lower bottom levels than the main storage. Because of this isolation, water collected in these pits never reach to the main storage, it only seeps or evaporates. Thus, desilting said to be carried out for increasing storage capacity, practically reduces the actual utilizable storage in most of the cases.
Removing surface soil to check eutrophication, is like giving treatment for symptoms instead of the disease. Unless we check the inflow of untreated sewage in lakes, it is not possible to reduce the nitrate or phosphate contents. Therefore, it is better to invest funds on checking of sewage inflow than on removing surface soil for the purpose so as to have permanent solution of the problem.
When used in farms and gardens, silt acts as a natural manure and increases water holding capacity of the soil. It is also an essential ingredient of the soil mix used for making bricks, tiles, etc. Because of this, silt is regularly taken away by the beneficiaries at their own cost and the off take is sometimes even more than the deposition. Thus, instead of spending money on desilting, the government should charge royalty for the silt used by beneficiaries considering it as a mineral. This will generate funds for regular maintenance of lakes.
Gyan Prakash Soni holds an ME in Water Use Management from IIT Roorkee. This article was presented at the National Seminar on Lake Conservation organized by College of Technology and Agriculture Engineering at Udaipur in February 2010.