Effect of Model Setup Complexity on Flood Modeling in Low‐Gradient Basins

Abstract

The flat topography, large natural storage features, backwater effects, and boundary conditions all play different roles in the flood response of rivers in low‐gradient environments. The combined effects of these factors result in frequent episodes of reverse flows, slow recession of flood waters, and complex flow interactions. This study investigates the value of varying degrees of model complexity and setup features on the model ability to reproduce some of the unique flooding characteristics in low‐gradient basins. The study focuses on (1) effect of streamflow routing techniques; (2) effect of incorporating large natural storage areas; (3) effect of model dimensionality; and (4) effect of downstream boundary conditions. The study assessed six different model setups for the Vermilion River in south Louisiana, during a series of flood‐inducing storm events in May–June 2014. A successful simulation of the repetitive reverse flows in the river was only possible after incorporating the large swamp areas within the basin. The slow recession of the flood peaks was accurately reproduced with the use of a two‐dimensional representation in characterizing the swamp areas. The results of this study have implications for understanding flood dynamics in low‐gradient basins, and for guiding the development of reliable flood models that take advantage of available technologies and information without adding unwarranted complexities that require extensive, yet typically unavailable calibration data.

Assessment of Riverine Dredging Impact on Flooding in Low-Gradient Coastal Rivers Using a Hybrid 1D/2D Hydrodynamic Model

“…The results clearly suggest that the dredging scenarios have an effect on flow exchanges between the river and the swamp system that provide a valuable flood mitigation service for the entire watershed (Saad et al, 2020). During the peak of the storm, the river flows north into the swamp providing relief for the downstream areas of the watershed.…”

“…In this study, an unsteady hybrid 1D/2D hydrodynamic model for the Vermilion River and its main tributaries was used to simulate the existing conditions and the proposed dredging scenarios. The model is an expansion of an earlier version (Saad et al, 2020) that was developed using the Hydrologic Engineering Center's River Analysis System (HEC-RAS) version 5.0.7. The HEC-RAS system allows for 1D and 2D unsteady flow simulations using either dynamic or diffusive wave approximations of the shallow water equations.…”

“…The adjustments were key in improving the model's ability to simulate the prolonged recession of the receding flood waves, a typical behavior of the river during extreme events that is attributed to swamp-to-river flow exchanges. More details about the model performance and the calibration results are available in Saad et al (2020).…”

“…While dredging can increase the hydraulic efficiency of river channels and potentially reduce overbank flooding, the impact on flow hydrodynamics and the overall flooding regime needs to be better understood, especially in watersheds that are located within inland-coastal transition zones. In such regions, river systems are typically characterized with complex flow dynamics due to factors, such as flow reversals and bi-directional flows (e.g., Burton and Demas, 2016;Watson et al, 2017), dynamic connectivity and flow exchanges with large natural storage areas, such as swamps and wetlands (Saad et al, 2020); tidal influences, and the large volumes of synchronized tributary flows from urbanized sub-watersheds (e.g., Pattison et al, 2014;Wang et al, 2019). The complexity of flow regimes in these regions has implications for flood mitigation; therefore, it is imperative to develop a comprehensive understanding of the impacts of riverine dredging that emerge as a possible mitigation strategy in response to decreased channel capacities and increased flood risk.…”

“…Besides the main river, it is also of importance to assess the propagation of any potential flood mitigation benefits into the tributaries that are connected to the river. Unlike most previous studies that depended on 1-dimensional (1D) hydraulic modeling in simulating pre-and post-dredging conditions, the current study utilizes a hybrid 1D/2D approach that allows for more accurate representation of the flow dynamics in low-gradient complex river systems (Saad et al, 2020).…”