Abstract
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 70 % for floods with a volumetric concentration of 60 % and the total flooded area is 36 % larger when the sediment concentration is equal to 20 %. The maximum flow momentum at cross sections in the urban area increases 14.5 % on average when the mean concentration along the main channel changes from 30 % to 44 %. 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.
Highlights
Flash floods with high sediment concentrations are common natural events in mountain rivers, which generate hazards in cities and other smaller human communities located near river channels (EEA, 2005; Wilby et al, 2008)
In this case we compare the maximum force produced by the flow in the urban area of the watershed, considering flows with different sediment concentrations coming from the Quebrada de Ramón and the Quillayes streams
The results evidence the competition between two main factors that control the dynamics of the flow in mountain rivers at different spatial and temporal scales: (1) the geomorphological features of the river represented by the bathymetry, the slope, and the channel width and (2) the flow resistance due to the internal sediment dynamics that changes the rheology of the mixture
Summary
Flash floods with high sediment concentrations are common natural events in mountain rivers, which generate hazards in cities and other smaller human communities located near river channels (EEA, 2005; Wilby et al, 2008). The development of efficient and accurate numerical models to simulate flash floods, is far from trivial, since multiple factors control the dynamics of the flow This is true in mountainous regions, where rivers are characterized by three important features that complicate their representation: (1) complex bathymetries and steep slopes produce rapid changes on velocities and water depths, formation of bores, and wet–dry interfaces; (2) large sediment concentrations directly affect the flow hydrodynamics by introducing additional stresses that alter the momentum balance of the instantaneous flow; and (3) there is a lack of accurate field data, used for validation, due to the difficulties on measuring hydrometeorological variables in high-altitude environments, with difficult access, and during episodes of severe weather.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
