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.
The current study investigates the effect of large-scale channel modifications via riverine dredging on flood dynamics in low-gradient river systems located in inland-coastal flood transition zones. The study site is the Vermilion River in south Louisiana, US, which is characterized by complex flow regimes, reversal and bi-directional flows, presence of large swamps with significant river-swamp interactions, and large volumes of runoff contributions from lateral tributaries. The study aims to understand the interplay of these factors and how they modulate and get affected by different dredging approaches that vary in spatial extent and the modifications introduced to the channel. The study deploys a hybrid, one-/two-dimensional (1D/2D), hydrodynamic model that simulates flow and stage dynamics in the main river and its major tributaries, as well as the flow exchanges with the interconnected swamp system. Overall, the results show that the dredging activities can significantly alter the flow regime in the watershed and affect flow exchanges between the river and the swamp system. In terms of flooding impact, only dredging approaches that are extensive in spatial extent and modifications to channel longitudinal slope can result in sizeable reductions in flood stages. However, these benefits come at the expense of significant increases in the amplitude and inland propagation of the Gulf tidal wave. On the other hand, less-extensive dredging can still provide moderate and spatially limited flood mitigation; however, they further expose downstream communities to increased levels of flooding, especially during more frequent events. The results reveal that while dredging can increase the hydraulic conveyance of the river system, the large runoff volumes delivered by the urbanized tributaries seem to outweigh the added improvement in the in-channel storage, thus reducing the anticipated flood relief. The results suggest that a watershed-centered approach, instead of a riverine-centered approach is needed for flood management in these systems so that the relative benefits and tradeoffs of different mitigation alternatives can be examined.
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