equations, including spurious modes of oscillation and artificial damping of the tidal 

 signal. Forcing functions include time-varying water-surface elevation, wind shear 

 stress, and atmospheric pressure gradient. The Coriolis force is included in the GWCE. 

 Also, the study area can be described in ADCIRC through either a Cartesian (flat earth) 

 or spherical coordinate system. For representing Matagorda Bay, the grid was defined in 

 the spherical coordinate system. 



The ADCIRC model is based on a finite-element algorithm for solving the defined 

 governing equations over compUcated bathymetry encompassed by irregular sea and 

 coastal boundaries. This algorithm allows for flexible spatial discretization over the 

 computational domain wlule retaining high stability. The advantage of this flexibihty in 

 developing a computational grid is that larger elements can be specified in open-ocean 

 regions where less resolution is needed, and smaller elements can be specified in the 

 nearshore and estuary areas where finer resolution is required to resolve hydrodynamic 

 details in channels and around islands. 



Numerical Grid 



A study area is defined by means of an unstructured grid composed of triangular 

 elements. Water depths are specified at the vertices, referred to as nodes, of each element 

 composing the grid. The water-surface elevations and the horizontal velocities are 

 computed at the nodes. Figure 19 shows the numerical grid, with the existing GIWW 

 position, developed for this study. Figure 20 displays the grid in the vicinity of the MSC 

 entrance. 



The grid encloses Matagorda Bay entirely and incorporates Lavaca, Carancahua, Tres 

 Palacios, Chocolate, Keller, and Espirita Santo Bays. The alongshore open-ocean 

 boundary is situated approximately 65.2 km in the seaward direction from Matagorda 

 Peninsula. Lateral open-ocean boimdaries are positioned 91.7 and 69.5 km to the 

 northeast and southwest, respectively, from the MSC. 



The grid consists of 20,320 elements and 1 1,575 nodes, and elemental areas ranged 

 from 1.021 10^ to 1.31 10^ sq m. The largest element lies on the Gulf boundary, whereas 

 the smallest resides in the GIWW at the channel's northeast entrance. Generally, smaller 

 elements are specified for resolving the various dredged channels in the study area, with 

 larger elements being placed in areas with relatively modest depth gradients and in the 

 open bay and Gulf. 



Bathymetry data specified in the grid were obtained from three sources. The general 

 sovu-ce was the NOS bathymetric database, which was accessed to define water depths for 

 those nodes residing in the open-ocean portion of the grid. Depths contained in this 

 database are defined relative to the Gulf Low Water Damm and were subsequently 

 converted to msl (by adding 1.05 ft). Depths assigned to grid nodes were found by 

 interpolating the three nodes contained in the database that encloses a given grid node. 

 Nodal depths are interpolated with an algorithm that weights each sounding or data point 

 inversely proportional to its distance from that node. 



NOAA Chart 11319 served as the second source and provided the depths for nodes 

 located in Matagorda Bay and in adjacent, smaller bays such as Lavaca Bay. Depths 

 were digitally extracted from this chart and processed in the same manner as those 

 extracted from the NOS database. 



22 Chapter 3 Circulation Modeling 



