Modeling the effects of sediment concentration on the propagation of flash floods in an Andean watershed

Abstract

Rain-induced flash floods are common events in regions near mountain ranges. In peri-urban areas near the Andes the combined effects of the changing climate and El Niño-Southern Oscillation (ENSO) have resulted in an alarming proximity of populated areas to flood-prone streams, increasing the risk for cities and infrastructure. Simulations of rapid floods in these watersheds are particularly challenging, due to the complex morphology, the insufficient hydrometeorological data, and the uncertainty posed by the variability of sediment concentration. High concentrations produced by hillslope erosion and rilling by the overland flow in areas with steep slopes and low vegetational covering can significantly change the dynamics of the flow as the flood propagates in the channel. In this investigation, we develop a two-dimensional finite-volume numerical model of the nonlinear shallow water equations coupled with the mass conservation of sediment to study the effects of different densities, which include a modified version of the quadratic stress model to quantify the changes in the flow rheology. We carry out simulations to evaluate the effects of the sediment concentration on the floods in the Quebrada de Ramón watershed, a peri-urban Andean basin in central Chile. We simulate a confluence and a total length of the channel of 10.4 km, with the same water hydrographs and different combinations of sediment concentrations in the tributaries. Our results show that the sediment concentration has strong impacts on flow velocities and water depths. Compared to clear-water flow, the wave-front velocity slows down more than span classCombining double low line"inline-formula"70/span % for floods with a volumetric concentration of span classCombining double low line"inline-formula"60/span % and the total flooded area is span classCombining double low line"inline-formula"36/span % larger when the sediment concentration is equal to span classCombining double low line"inline-formula"20/span %. The maximum flow momentum at cross sections in the urban area increases span classCombining double low line"inline-formula"14.5/span % on average when the mean concentration along the main channel changes from span classCombining double low line"inline-formula"30/span % to span classCombining double low line"inline-formula"44/span %. Simulations also show that other variables such as the arrival time of the peak flow and the shape of the hydrograph at different locations along the channel are not significantly affected by the sediment concentration and depend mostly on the steep channel morphology. Through this work we provide a framework for future studies aimed at improving hazard assessment, urban planning, and early warning systems in urban areas near mountain streams with limited data and affected by rapid flood events.