Joint frequency analysis of river flow rate and suspended sediment load using conditional density of copula functions

Abstract

Abstract In this study, the river flow rate and suspended sediment load at Deh Molla hydrometric station in Zohreh River in the period of 1983–2018 were used for copula-based joint frequency. The Frank copula was selected as the best copula. The joint frequency analysis of the studied variables was performed based on the conditional density of the copula functions, which leads to the conditional occurrence probability of the variables. The results showed that with increasing river flow rate and suspended sediment load, the joint return period increases. Also, the joint return period for the "and" state is longer than the joint return period for the "or" state. When the river flow rate exceeds the threshold of 100 m3/ s and suspended sediment load exceeds the threshold of 2.7 × 104 ton/day, the joint return periods for the "and" state and for the "or" state, are about 11 and 4 years, respectively.