Multivariate Frequency Analysis of Peak Discharge and Suspended and Bed Sediment Load in Karaj Basin

Estimation of suspended and bed sediment load transferred by streamflow is important for planning and storing water in dam reservoirs, watershed management, coastal and environment protection. In this study, multivariate frequency analysis was performed between the maximum annual values of flood discharge, suspended sediment load and bed sediment load in Sierra Karaj hydrometric station with different copula functions. The common time period between the variables of suspended sediment load and bed sediment load was determined from the water year of 2009-2010 to 2019-2020. The results showed that the best copula functions in the analysis of dependency between the variables of flood discharge-suspended sediment load, flood discharge-bed sediment load and suspended sediment load-bed sediment load are Tawn, Shih-Louis and Gaussian, respectively. The results showed that for the joint return period equal to 10 years for the “OR” scenario, the values of flood discharge, suspended sediment load and bed sediment load are respectively equal to 125 cubic meters per second, 100 thousand tons per day and 2500 tons per day. Maximum probability of occurrence of suspended sediment load and bed sediment load for joint return period equal to 10 years for “AND” scenario are equal to 45000 tons per day and 1500 tons per day, respectively. According to the "AND" scenario for the joint return period, the multivariate design quantiles of the suspended and bed sediment loads are smaller than the univariate quantiles. Therefore, ignoring the correlation between suspended and bed sediment load and flood discharge may significantly overestimate the actual sediment value and, consequently, overestimate the corresponding occurrence probability.