3 Circulation l\/lodeling 



This chapter presents a numerical modeling study of the circulation patterns in 

 Matagorda Bay, with focus on the current in the vicinity of the MSC-GIWW intersection. 

 The objective of the modeling is to infer sediment paths and their relative magnitudes. 

 The hydrodynamic model and its solution scheme are described, together with the 

 numerical grid representing the study area. Calibration of the model is presented for 

 which calculated water-surface elevations and current velocities are compared to 

 measurements. The model is then modified to represent the relocated GIWW, and 

 comparisons of currents computed with the existing and proposed channel configurations 

 are made. 



Hydrodynamic Model 



The ADvanced CIRC ulation (ADCIRC) numerical model was chosen for simulating 

 the long-wave hydrodynamic processes in the study area. By specifying the 

 tidal-elevation signal at the coastal boundary together with the wind-induced shear 

 stresses over the model domain, the ADCIRC model can simulate tidal- and wind- 

 induced circulation in Matagorda Bay. The ADCIRC model was developed in the 

 USACE Dredging Research Program as a family of two- and three-dimensional finite 

 element-based models (Luettich, Westerink, and Scheffher 1992). Model attributes 

 include the capabihty of the following: 



a. Simulating tidal circulation and storm-surge propagation over large 

 computational domains while simultaneously providing high resolution in areas 

 of complex shoreline and bathymetry. The targeted areas of interest include 

 continental shelves, nearshore areas, and estuaries. 



b. Representing the pertinent physics of the equations of motion. These include 

 tidal potential, CorioHs, and all nonlinear terms of the governing equations. 



c. Calculating reliably and efficiently over time intervals ranging from months to 

 years. 



In two dimensions, the model formulation is based on the depth-averaged shallow- 

 water equations for conservation of mass and momentum. The formulation assumes that 

 water is incompressible, and that the pressure is hydrostatic. Rather than directly solving 

 the Navier-Stokes and continuity equations, ADCIRC employs the Generalized Wave 

 Continuity Equation (GWCE) for computing water-surface elevations and velocities. 

 The GWCE-based solution scheme eUminates several problems associated with those 

 finite-element schemes that solve the primitive forms of the continuity and momentum 



Chapter 3 Circulation Modeling 



21 



