SEDIMENT INFLOW FORECASTING FOR ALMATTI RESERVOIR

Abstract

The Almatti dam has been constructed on river Krishna. First filling of reservoir was carried out in the year 2001. The sediment entering in to the reservoir was estimated earlier manually by conventional methods to asses the dead storage capacity required for design purpose. A detailed study is attempted to asses the sediment flowing in to the reservoir and asses the time likely to take for filling the dead storage capacity, in a scientific way by simulating the water and sediment flow using modern tools and long term data available. The Gross Storage capacity of Almatti reservoir is 3485.698 MCM (123.08 TMC) and :,e storage is 2985.440 MCM (105.43 TMC) Total sediment transport capacity is required for assessment of sediment. deposit in the channel and sediment flowing in to the reservoirs. Total sediment load includes bed load, suspended load and wash load. This study envisages the computation of sediment transport capacity, amount of sediment deposit in the Almatti reservoir over a period of time. The River Krishna, situated covering large areas in the States of Maharashtra, Karnataka and Andhra Pradesh, drains an area of 2,58,948 sq. km has about 13 major tributaries along its 1400 km. course. Central Water Commission has been maintaining total 55 Nos of Hydrological observation stations (as on the year 2005-06) in the Krishna Basin in addition to stations maintained by respective state governments. Daily/ hourly water level of the river is measured as its elevation above MSL datum. Discharge observations are conducted once or alternative day. Measurements of suspended sediment concentration are being conducted at 23 important stations in the Krishna basin by taking river water samples at selected sites while discharge measurement. bata'of Gauging sites Galgali & Kurundwad on Krishna, Bagalkot & Mudhole on tributary Ghataprabha were used for simulation purpose The physical laws which govern the flow of water in a channel are the principle of conservation of mass (continuity) and the principle of conservation of momentum. The widely accepted procedure for solving the one dimensional unsteady flow equation is the four point implicit scheme. Unconditionally stable condition is considered for stability of the module. Water surface profiles are computed from one cross section to the next by solving the energy equation with an iterative procedure called standard step method. •The total river course distance considered for analysis is 230 km. There are 36 surveyed cross sections in this reach and interpolation of cross section between two available cross sections was made. As the monthly average rate of flow varies from 13.31 to 3532.58 m3/ sec, the option of having the computation increment based on the f low rate was fixed from 0.5 to 48 hrs so that the hydraulic parameter during the peak flow are not missed without computation. Water temperature, the monthly average of temperature data collected at site Kurundwad has been used. .9 set of average flow rate and corresponding sediment load was used. Sieve analysis and siltometer data available were used for assessing the grain size distribution for each rate of sediment M. Tech course in WRD (Civil), 2006-08 xiv Dissertation Report on Almatti reservoir sedimentation load. 4 set of bed material data were used. Specific gravity, shape factor of sediment, density of sand/ gravel, silt and clay were suitably considered Toffaleti transport function and Ruby fall velocity, sorting method of Exner 5 was found more suitable and same were chosen in simulation. Trap efficiency computed various from 78% to 98% As the simulation was carried out for the main river and the river Ghataprabha separately the sediment contribution also computed separately. As no data is available for the sediment contribution from the intermittent catchment between the Kurundwad station and dam excluding from river Ghataprabha, the assessment has been made based on the drainage area proportionate to Kurundwad site and Almatti reservoir. but excluding the Ghataprabha. Similarly the Ghataprabha contribution from simulated result using data of the site Mudhole/ Bagalkot and intermittent contribution between Mudhole/ Bagalkot and dam has been proportionally assessed separately. The study was conducted based on the data obtained from the CWC field organizations. The accuracy of the study is depending on the quality of gauge, discharge, sediment, river cross section, Manning's n value and other data used in the study. From the study it is found that the average sediment entry in to the reservoir is 11.2166994 M. m3/year. Average water _inflow into the reservoir is 6955.8393 M. m3/Year. Dead storage of Almatti will be filled by sediment in about 37 years and 80% of gross storage is likely to be filled in about 237 years. Hence, the usefulness of this reservoir would start reducing after 37 years, and after 237 years it would be nothing but a collection of sand and sediment with no water in it, provided the- silt and water entry rate remains constant.