An Investigation of magnitude-frequency of effective discharge for suspended sediment transport in sub-catchments of Gorgan Drainage Basin

Suspended sediment transport by rivers is important in water quality of cities and villages fed by certain drainage basins. Therefore determination of effective discharge and its characteristics, which carries the highest amount of sediment in long term, is very important. To determine characteristics of effective discharge, probability density function for water discharge is calculated based on daily discharge data and by multiplying coefficients of sediment rating curves by probability density function, sediment density function is drawn. Then, its return period is calculated by statistical analysis of maximum instantaneous discharge. In this research, the above-mentioned method was employed for six headwater streams of Gorgan Drainage basin, using statistical data of suspended sediment records in the past 13 to 16 years in regions with areas between 13 and 165.5 km2. The results have shown that suspended sediment of effective discharge of the stations is between 0.11 to 75.4 (m3s-1), their average frequencies between 0.07 to 16.5 days per year and their return period ranges from 0.96 to 2.07 with of average 1.45 years. The amount of sediment transported by effective discharge is higher than the amount of sediment transported by other discharges, and varies by 3.7 to 15 percent as compared to yearly-suspended sediment. Furthermore, comparing different stations in terms of specific suspended sediment of effective discharge, it has been shown that Nomal and Pasposhte hydrometric stations have the lowest and largest amounts, respectively. Generally, effective discharge for suspended sediment occurs every year or every two years and carries the highest amount of sediment in sub-catchments as compared to other discharges.