Assessment of the Role of Tributaries on Flood Dynamics in Low-Gradient Coastal Rivers

Abstract:

Climate change and rapid urbanization have significant ramifications on the flood risk in low-gradient coastal riverine environments. The Vermilion River located in south Louisiana, USA, is an example for rivers that experience such conditions. The Vermilion River has a long history of severe flooding events that have been increasing over recent years. Different physical drivers stand behind the flood dynamics in the Vermilion River. Streamflow hydrographs in the main river results from the convolution of several large tributaries that have witnessed increasing activities of land-use changes and channel modifications. We hypothesize that the peak tributary relative timing, a function of both the geometry of the watershed network and the dynamics of individual flood events, can exert a significant impact upon flood magnitudes in the main river. The study takes advantage of the availability of the NOAA National Water Model Reanalysis dataset that provides hydrologic outputs from multi-decade retrospective simulations (V2.0 1993-2018), in addition to high-resolution rainfall radar data to enforce a hydrodynamic model developed for the Vermilion River basin. The model is used to simulate historical events that occurred during a 12-year period (2007-2018) and analyze the relative timing between the peak flows from the main tributaries draining into the Vermilion River and the peak flow occurring inside the river itself. We undertake a sensitivity analysis of the effects of changing the tributaries’ peak flow timing on the flooding regime in the main river. The analysis shows a significant impact on the flooding regime in the Vermilion River by manipulating the timings and magnitudes of peak flows of the downstream tributaries asynchronously from the upstream tributaries. The analysis also examines the impact on the complexity of flow regime in the river (e.g., presence of reverse flows during extreme events) and its interaction with large swamps that act as natural storage areas in low-gradient coastal systems.