Effects of extreme rainfall events on sediment and contaminants transport

Abstract

Frequency and intensity of extreme rainfall events is expected to increase. Extreme rainstorms cause redistribution of contaminated sediments which is a significant problem due to the presence of many contaminated sites in heavily populated urban coastal areas. This study evaluates contaminated sediment transport and redistribution in rivers due to extreme rainfall. To do so, a conceptual model that couples hydraulic, sediment and contaminant transport is formulated to predict erosion and deposition of sediments and the transport of adsorbed heavy metals in a river after extreme rainfall.One dimensional energy equation and Manning's equation were used to predict water surface elevations, the sediment continuity equation is used for sediment transport predictions and calculate sediment erosion and deposition, hydrodynamics and sediment transport predictions were linked to predict contaminated sediment transport assuming equilibrium and linear soil-contaminant partitioning. The hydraulic model was calibrated using water surface elevations from USGS. The sediment transport model was calibrated using USGS sediment loads data. Metal concentrations reported by USGS were also used to calibrate contaminant transport model and simulate contaminant-sediment sorption. Lead is used as a model contaminant due to its high sorption on sediment particles. A sensitivity analysis was first carried out to assess the impacts of key parameters on contaminated sediment transport. The approach was then implemented to evaluate the effect of Hurricane Harvey, which hit Texas and Louisiana in 2017, on contaminated sediment transport at the Neches River watershed, one of the most contaminated waterways in Texas, as a case study. The study shows that extreme rainfall events cause substantial transport and redistribution of contaminated sediments in rivers. Redistribution of heavy metals contamination is controlled by transport of fine particles due to their high sorption capacity. This causes significant spreading of contamination after such event as fine particles constitute the majority of transported sediments. Further, the increase in Lead concentrations in surface water and sediments from upstream to downstream the river lasts for long periods after the rain event.