MATHEMATICAL MODELING OF RESERVOIR SEDIMENTATION

Abstract

Sediment is one of the significant problems in the reservoir nowadays because deposition affects the dam’s life. For controlling the deposited sediment in the reservoir. Sediments is divided into two types, suspended sediment and bed sediment. More than 80% of the river sediments are suspended sediment, but this amount changes in every river. It depends on the river bed and the river bed material. Mathematical modeling of reservoir sedimentation as a case study has been carryout in this research for analysis of sediment deposition profile for the long-term and flushing of the sediment from the reservoir. HEC-RAS 6.0 has been used for Simulation of the sediment profile/flushing of sediment as 1D&2D modeling. The Simulation of sediment profile for the long-term, 1D sediment model is used to analyze the sediment deposition and flushing profile for 25 years. The 2D sediment model is used to get accurate and detailed results of the sediment deposition profile for the representative year. The study area is Bichom River, the main tributary of Kameng river, near the township of Nafra in the West Kameng district of Arunachal Pradesh. A run-of-river hydropower project of capacity 260 MW is planned. The project site is about 3 km from Nafra town and about 220 km from the nearest railhead / nearest airport, Tejpur. Nafra dam site has a 776 km2 catchment area of the Bichom river. Bichom HE project site available flow and sediment data downstream of Nafra HE project are transferred to the Nafra site in the proportion of catchment areas at these two sites. A smooth average curve was fitted through the plotted from average daily flow data derived from the Bichom HEP site and observed at Nafra dam site for a representative one-year period (June 1 to May 31). In this study, the smooth average curve is considered as representative one-year flow data for the mathematical model. The sediment carrying capacity of the virgin river is adequate to carry its sediment load without any deposition on the bed. The simulated water level at FRL 981 m is found to extend up to about 2.4 km upstream of the dam and the reservoir capacity has been estimated as 6.93 Mm3 at FRL level. The velocity in the river at chainage 2599 is 1.75 m/s, it decreases in the reservoir upstream to 1.04 m/s, and the iii iv velocity downstream of the reservoir is 1.38 m/s. The decrease in velocity indicates the deposition of the sediments. HEC-RAS is using five sediment transport functions for 1D; all are tested in this research. The Engelund-Hansen formula shows a good agreement for analyzing sediment deposition profile and flushing of sediment from reservoir. The model is run for the representative year inflow, at the beginning of the monsoon, it is assumed that the reservoir is free from sediment deposition. The sediment deposition extends up to 1200 km upstream. The total cumulative volume is 384690.4 m3 in the first year from (June 1 2010, to May 31 30, 2011), which is 5.55% of the reservoir volume (6.93 Mm3). The flushing has been carried out for 6hr, 8hr, 12hr, 24hr, and 36hr, as for the result of 8hr and 12hr as well 24hr is not that much different, 8hr of flushing time was considered the most efficient. The amount of sediment flushed from the reservoir in the peak flow flushing time of July 31 in 25 years is 95.2%, and the sediment which remains in the reservoir is 4.72%. This amount of flushing on August 20 is 99.2% sediment flushing from the reservoir, 0.74% is the reaming sediment in the reservoir. According to this study, the peak flow flushing is not more efficient. In comparing the 1D&2D sediment deposition profile in the representative year, no particular differences can be observed. For example, the bed elevation changes can be seen up to 1.25 km upstream of the dam site. The bed elevation changes in 1D can be seen up to 1.18 km upstream of the dam sit. Also, the maximum deposited sediment level in 1D can be seen up to 1.4 km upstream, which is 978.1 m, and such a level in 2D is up to 1.55 km upstream, 981.6 m. This observation means that the 1D&2D modeling matches in many cases.