Effect of model setup complexity on hydraulic simulations in low-gradient watersheds: Application in the Vermilion River Basin, south Louisiana.

Haitham Saad, University of Louisiana at Lafayette, Department of Civil Engineering, Lafayette, LA, United States, Emad H Habib, University of Louisiana at Lafayette, Civil Engineering, Lafayette, LA, United States and Robert Miller, University of Louisiana at Lafayette, Civil Engineering, Lafayette, United States
Extreme flood events are increasing on global and local scales. A wide variety of modeling approaches, in terms of scale and representation of physical details, have been proposed and implemented to provide quantitative information about floods and their impacts on communities. However, the selection of the proper level of detail in the model setup process remains a fundamental challenge, especially in low gradient basins that are controlled by both inland and coastal processes. In such regions, the concept of upstream and downstream direction is ill-defined, and the flow direction becomes a time-dependent variable controlled by the hydraulic gradient. Backwater effects are significant and wind and tidal effects may also play a significant role in the total flood response. The relative significance of the physical drivers of flood dynamics in such systems is challenging to define. This presentation addresses the question of model setup complexity in the challenging context of low-gradient basins. A series of numerical tests are developed using a range of model setups featuring a HEC-RAS hydraulic model of varying geometric complexity forced by hydrologic inputs furnished by the National Water Model (NWM). An evaluation of the role of hydraulic model setup complexity is made by testing a range of spatial geometry combinations (1-D, 1-D/2-D, 2-D), tailwater boundary conditions, inflow hydrographs, representation of wetlands and natural storage areas). The test site is the Vermilion River watershed located in South Louisiana. The Vermilion features a broad range of interacting effects including rapid urbanization, tidal influence, flow regulation, and significant storage affects related to hydraulic interaction with the former Mississippi River alluvial floodplain. The model results are compared against available measurements for a moderate flood event that occurred in the late spring of 2014. The outcomes of the various test provide insight on the potential advantages and disadvantages of the range of model setups considered with particular relevance to low-gradient hydraulic simulations.