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Summary: Accurate prediction of a flood inundation area constitutes an essential part of a flood forecasting system. When a river reach receives significant lateral inflows, flood inundation modelling requires the joint application of a hydrological model to calculate lateral inflows and a hydraulic model to calculate water levels along the river reach. In this study, we compared different strategies to couple the GR4J lumped rainfall–runoff model and the linearised diffusive wave propagation model. These strategies introduced variations in the nature of the connections between the two models using combinations of point and uniformly distributed lateral inflows. The coupled model was then applied to the Illinois River case study for which 10 years of hourly data were made available within the Distributed Model Intercomparison Project, now in phase 2. The simulations were assessed at the downstream end of the reach and at two interior points considered to be ungauged during the calibration process. The results first show that including uniformly distributed inflows made the coupled model more robust and stable compared to only using point flow input. A similar level of performance was reached with models using point inflows only, but at the cost of more uncertain parameters and less stable model performance when changing test periods. Second, identifying the optimal number of tributaries to be modelled individually by the hydrological model was easier when a combination of uniformly distributed and point inflows was used. In this case, model performance was less sensitive to the number of tributaries used and the inclusion of two or three tributaries appeared sufficient to obtain satisfactory performance for the simulations on the main channel. Last, the results on the main channel differed from those obtained on tributaries: overall performance was better on the main channel and required a lower degree of lateral inflow resolution, which suggests that upstream flow measurements provide valuable information for flow simulation on ungauged points. [ABSTRACT FROM AUTHOR]

The one-dimensional unsteady-state hydraulic model (UNET) was developed by the U.S. Army Corps of Engineers and has been used for flood protection in some large rivers. In its use for the real-time management of levee and drainage districts for flood protection, such as on the Mississippi River Basin, the required number of simulations for combinations of scenarios would present a great challenge for modelers to provide data for operational-mode decision making. The goal of this paper is to identify the most sensitive parameters for the spillway, simple levee failure, and complicated levee failure options in UNET so that simulations for real-time operation can focus on the most sensitive parameters without sacrificing much modeling accuracy. The sensitivity analysis was performed for the 50-year frequency flood in the lower Illinois River during which the Thompson Lake Levee and Drainage District or the Emiquon Area were used as temporary flood storage by using a state-of-the-art variance-based sensitivity analysis method-extended Fourier amplitude sensitivity test (FAST). The first-order and total-effect sensitivity indexes computed from the FAST method were used to evaluate the affect of the parameters to flood-peak reduction. As a result, the least and most sensitive variables for the spillway, simple levee failure, and complicated levee failure options in UNET were identified, and this knowledge helps significantly reduce the number of simulations for management scenarios by focusing on only the most sensitive parameters. [ABSTRACT FROM AUTHOR]