Sediment Characteristics of the Nan River Above the Sirikit Dam

Reservoir sediment can result in loss of reservoir storage capacity and shortening reservoir service life. It is of crucial necessary to know how much sediment entering a reservoir in the time period of concern. Unfortunately, observed sediment loads, especially bed loads, for rivers in Thailand are limited or unavailable to be used for reliably estimating the amount of river sediment transport rates. This study was aimed at investigating sediment characteristics of the Nan River above the Sirikit Dam based on observed river survey data. The survey data included river cross sections, flow velocities, water discharges, suspended sediment and bed loads, and river bed material which were observed and collected during dry and wet season of the year 2018. From the study, it has been shown that the river discharges varied between 7.4 m³/s to 873.4 m³/s. The observed total sediment loads ranged from 6 tons/day to 75,000 tons/day and ratio of bed-to suspended load narrow ranged about 0 to 0.003. From the analysis of sediment grain size indicated very fine sand size of bed load materials with the median size (D₅₀) ranged from 0.23 to 0.30 mm. Meanwhile, bed materials were medium to coarse sand with D₅₀ varied between 0.88 to 4.78 mm. In addition, it has been found that the median size of river bed material decreased at upstream reaches of the Sirikit Dam.