Frequency analyses of peak discharge and suspended sediment concentration in the United States

Abstract

Compound extreme events with concurrent peak discharges and suspended sediment concentrations have substantial impacts such as floods, infrastructure failure, pollution, increased water treatment costs, and threats to aquatic life. It is thus essential to understand the spatiotemporal dynamics of water and sediment transport during such extreme flood events. Copulas are used to conduct multivariate flood frequency analyses and provide relationships between design variables and recurrence intervals that are useful in hydrology and water resource management. We used hydrological data from 20 gauging stations on important rivers across the United States with daily records spanning at least the last 30 years. We extracted annual maximum discharges (Qmax) and corresponding suspended sediment concentrations (SSCmax) at these 20 stations. We analyzed the temporal trends of Qmax and SSCmax by using the Mann–Kendall non-parametric statistical test. Then, we investigated the interdependency of Qmax and SSCmax through various correlation coefficients and used the Multivariate Copula Analysis Toolbox (MvCAT) for flood frequency analyses. We found Qmax decreased significantly for one river, whereas SSCmax decreased significantly for 10 of the 20 rivers over the analysis period. Five of rivers selected as statistically suitable for copula analysis were of the smallest size and with the fewest dams. Seven of rivers showed significantly decreasing SSCmax trends were of the largest size and with the most dams. The remaining eight rivers were of moderate size. In larger watersheds, the number of dams more strongly affected sediment transport dynamics. Soil condition and forest cover exerted stronger influences on SSCmax in smaller than in medium-sized watersheds, and soil condition was better correlated with the spatial SSCmax distribution than forest cover. Using MvCAT, we computed the optimal continuous marginal distribution functions for the five suitable rivers, and the optimal copula was different in each case. Our results indicate reduced sediment transport during extreme flood events. Copula functions are very useful in such flood frequency analyses, and measures should be applied according to local conditions to achieve optimal water resource management and planning.