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Assessing environmental flow needs in streams, rivers, bays, and estuaries often requires the use of numerical models to simulate water movement, and nutrient and sediment transport. For example, numerical models may be used to determine the relationship between flow and available habitat in streams, which may then be used in developing instream flow requirements. Flow requirements for estuarine health are often related to salinity levels, distributions, and fluxes for which general circulation models of estuarine currents and freshwater inflows are required.
Numerical hydrodynamic models differ in the number of dimensions (1-D, 2-D, or 3-D) in which water movement is simulated, and in how each model represents the physical environment. For typical environmental flow studies, environmental systems are studied holistically using lower-resolution models, with representative portions of the system studied in detail with higher resolution models. For example, two-dimensional models of portions of the Brazos River (TX) were developed to determine mesohabitat suitability for numerous species including the red shiner (Cyprinella lutrensis) and western mosquito fish (Gambusia affinis) under a variety of flow conditions. The results from these models were then extrapolated upward to the entire river system, for which the flow regimes were modeled and characterized with the 1-Dimensional model HEC-RAS. Results from the 1-D and 2-D models of the river were then used as input to a 3-D simulation of the Brazos River estuary, in order to assess the relationship between freshwater inflows, estuarine salinity distributions, and shellfish habitat suitability. Additional 1-dimensional modeling efforts were performed to determine the frequency and duration of water exchanges between the mainstem Brazos River and adjacent oxbow lakes, linking such exchanges to breeding habitat preferences for Brazos River fauna.
This poster presents a case study of the Brazos River-Estuary system, as an example of how INTERA employs numerical modeling in environmental flow assessment projects. INTERA staff have undertaken such efforts in many basins across the US and abroad, and have been deeply involved in the development of the methodology for determining environmental flows in Texas.
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