Modeling Hydraulic Control Structures in Estuarine Environments with EFDC

Numerical modeling provides an efficient tool for simulating hydrodynamics in estuarine environments. It is particularly useful when extensive field data collection is impractical, or when impacts of proposed restoration or engineering alternatives must be evaluated. Often surface water flow within an estuarine system is controlled by hydraulic structures such as culverts, flap gates, weirs, and/or sluice gates. These types of structures typically require special treatment within hydrodynamic model codes due to spatial scale limitations and/or physical assumptions (e.g., free surface flow). The Environmental Fluid Dynamics Code (EFDC) provides a means to model hydraulic structures using withdrawal-return pairs of model grid cells. However, the application of withdrawal-return cells in an EFDC model requires a priori knowledge of the relationship between water level and flow rate for the particular structure (e.g. a head-discharge relationship, rating curve, look-up table). In many cases it is difficult or impractical to obtain this information. The flow regime (e.g., outlet control, inlet control, pressure flow) may change as well. To remedy this, additional subroutines have been implemented within the EFDC code to compute discharge through various types of flow control structures (e.g., pipe culverts, box culverts, sluice gates, flap gates). Flow rate is determined at each model time step based on the computed water surface elevation using standard engineering equations for the particular structure. The modeler is required to input the geometry of the structure (e.g., pipe length and diameter) and discharge coefficients or friction factors. There is no need to determine a head-discharge relationship for the structure a priori. Flux between the assigned withdrawal-return cells is accounted for using the original code, which maintains the conservation of mass and other scalar variables. Application of the additional subroutines is demonstrated using EFDC models of actual estuarine systems and validated using field observations.