Changeability of the RelationshipBetween the Flow Competence and the Suspended Sediment Particle Size Distribution in the Kojour River

Awareness of the existence of a complex behavior، and the variability of the suspended sediment particle size are the most important factors in modeling of sediment transport. Therefore، the present study had aimed to investigate the influence ofdischarge competence on the suspended sediment particle size distribution changes in the Kojour River draining from a 50،000 ha of Educational and Research Forest Watershed of the TarbiatModares University. The study was conducted on a daily and storm bases. The flow velocity was measured by using a current meter and floaters in the normal and flood durations، respectively. The suspended sediment was determined through sampling using the integration method، and the particle size distribution of the suspended sediment was ultimately analyzed based on the Stokes’ law using the modified pipette method. The relationship between the discharge and particle size distribution of the suspended load in the study period was assessed by applying the bivariate regression method supported by different statistical criteria. The results revealed that the best correlation was established between the discharge and silt in the normal situations، and the discharge and sand، silt and clay under storm conditions، respectively، with corresponding correlation coefficient، relative estimation errors، verification errors، and coefficient of efficiency of 0. 46، 0. 63; 129. 36، 32. 66; 46. 00، 49. 61; and 0. 95، 0. 93، respectively.