REMOTE SENSING BASED SEDIMENTATION STUDY FOR SHARAVATHY RESERVOIR, KARNATAKA

Abstract

As a result of runoff from rainfall or snowmelt, soil particles on the surface of a watershed can be eroded and transported through the processes of sheet, rill, and gully erosion. Once eroded, sediment particles are transported through a river system and are eventually deposited in reservoirs, in lakes, or at sea. The estimation of the deposited sediment volume is one of the most important tasks in river engineering. Sediment particles originating from erosion processes in the catchments are propagated along with the river flow. When the flow of a river is obstructed by a dam the water is stored in a reservoir, the sediment settles down and reduces its capacity. Reduction in the storage capacity of a reservoir beyond a limit hampers the purpose for which it was designed. Thus assessment of sediment deposition becomes very important for the management and operation of reservoirs. Some conventional methods, such as Hydrographic survey and Inflow-Outflow approaches, are used for estimation of sediment deposition in a reservoir, but these methods are cumbersome, time consuming and expensive. There is a need for developing simple methods, which require less time and are cost effective. In this study, a Satellite Remote Sensing (SRS) approach has been attempted for assessment of sedimentation in Sharavathy Reservoir, located on the Sharavathy River in the Shimoga district of Karnataka. Multi date remote sensing data (IRS P6, Li S S III) provided the information on the water-spread areas of the reservoir at different dates, which were used for computing the sedimentation volume and in turn to assess the revised capacity. The revised capacity of the reservoir was then compared with the original capacity of the year 2002 so as to give the loss in capacity from 2002 to 2010 i.e. in 9 years. It was found that the capacity was reduced to 53.682 Mm3 from 58.689 Mm3 showing 8.53 % of loss in capacity. The rate of sedimentation was estimated as 0.000259 Mm3/km2/yr which is equivalent to 311 T/km2/yr considering total catchments of 2148 km2. Considering free catchments of 151.50 km2 the rate of sedimentation comes to 0.00367 Mm3/km2/yr or 4406 T/km2/yr. Considering the same percent reduction as above, the dead and live storages of Sharavathy reservoir will have a loss in storage to the tune of 6.158 Mm3 and 11.165 Mm3. On the basis of analysis of impact of whole catchments and free catchments,. contributing sediment to Sharavathy reservoir the rate of sedimentation seems to be on higher side with respect to standard sedimentation rates in zone-7 of India.