Prediction of sedimentation in Kaptai reservoir

Abstract

The study has been conducted to predict the sediment deposition and annual sedimentation rate for the Kaptai reservoir at Rangamati which lies in the south-eastern portion of Bangladesh and its catchment area is 11003 sq.km. Sediment distribution pattern at different depths and the expected life of the reservoir has also been estimated. Prediction model has been used to investigate the sediment deposition. and sedimentation rate using available reservoir inflow data collected from the Karnafulli Hydropower Station. The important variables that influence the siltation in the study area are also carefully investigated and analysed. A Prediction model developed by Swamee has been used to determine the sediment deposition and silting rate of Kaptai reservoir. The rates obtained by field measurement and Prediction model are 3.33 and 4.09 ha-m/1 00 sq.kmIyear respectively. Validity and applicability of the model was then tested by comparing with field data. In the light of comparison made between the sedimentation rate obtained by both the method, it seen that the Prediction model can be applicable to the Karnafulli reservoir with reasonable accuracy .. Sedimentation rate was also estimated by using Joglekar's, Khosla's, Garde and Swamee's equation and these results were compared with.the results of Prediction model and actual measurement. It is found that the maximum amount of sedimentation rate by Joglekar's equation is 6.4 ha-m/lOO sq.kmIyear and the minimum amount of this rate by Garde and Swamee's equation is 2.25 ha-m/100 sq.km/year of watershed area. Percentage discrepancy ratio among this empirical equations with Prediction model varies from -56.48% to 41.56% and this ratio between three empirical equations and Prediction model with field measurement varies from 22.82% to 32.43 %. The sediment distribution pattern at different water level has also been estimated by using empirical area-reduction and area-increment methods. It is also found that the percentage of discrepancy ratio of sediment volume at different elevations by both the methods varies from 0.05 % to 25.19 %. Such range of variation is not significant compared to the size of reservoir. So, anyone of this two methods may be suggested to determine the distribution pattern of the reservoir. Dead storage space of the reservoir was determined and it is found 47.35% of the total reservoir space. Finally, expected life of the reservoir has been predicted by the method provided by Khagawa University, Garde and Swamee's equation and Step method which is related to the Brune's trap efficiency curves. It is observed that the expected life of the reservoir by using this methods are 312, 4527 and 1110 years respectively. Method provided by Khawa University obtained by 312 years has been chosen for estimating the life of the Kaptai reservoir due to safety factor. Finally it may be observed that the Kaptai reservoir will be completely filled up by sediment within 312 years .