A bathymetric survey conducted by the Galveston District and the DMS Work Unit 

 in September 1997 was the third source of data and provided the bathymetry for those 

 nodes composing Pass Cavallo and the MSC entrance. Both of these areas are 

 geomorphicaUy active. As discussed in Chapter 1, Pass CavaUo is tending to close 

 because of the presence of the MSC entrance, whereas the entrance is deepening because 

 of the strong current flowing through it. Thus, the bathymetry data were collected to 

 represent present-day width and depth of Pass Cavallo, as compared to anachronistic 

 information on the NOS charts, and to better represent the MSC entrance. Depths at Pass 

 CavaUo and the MSC entrance must represent present conditions to allow water to flow 

 through the inlets to accurately model the circulation and water level in the bay. 



Forcing mechanisms specified in the model were tide, wind, and the CorioUs force. 

 A time series of water-surface elevations was assigned to each node composing the open 

 gulf boundary. Serving as boundary conditions, these water levels induce tidal 

 circulation in the model. Time series of wind speed and direction are processed in the 

 model by converting the wind speed into shear stress, and shear stress components are 

 then computed with the wind direction. 



Measurements 



Time series of measured water-surface levels and wind-speed and direction data were 

 obtained from a worldwide Web site maintained by the Texas A&M University-Corpus 

 Christi, Conrad Blucher Institute, which archives data collected through the Texas 

 Coastal Ocean Observation Network (TCOON). The Texas General Land Office, the 

 Texas Water Development Board, and the Galveston District sponsor this network. 

 Stations at which time series of measured water-surface levels are available include East 

 Matagorda Bay, Lavaca Bay, Port Lavaca, Galveston Pleasure Pier, Bob Hall Pier 

 (Corpus Christi), and RawUngs Bait Camp (located in the Colorado River Navigation 

 Chaimel between East Matagorda Bay and Matagorda Bay). (The Galveston and Bob 

 Hall stations are operated by the NOS.) Data on water level are provided at 6-min 

 intervals and are measured with respect to each station's staff zero. All data were 

 subsequently converted to local msl for defining depth in the model. 



The time series of water-surface elevation collected at the Galveston Pleasure Pier 

 served to drive the open gulf boundary in the model. The model requires that each node 

 composing an elevation-specified boundary have assigned to it a continuous series of 

 elevations input at equal time intervals and at identical times. The Galveston gauge time 

 series contained four gaps in the data string, with the longest gap being 18 min. To form 

 a complete or continuous time series, missing data entries were estimated via linear 

 interpolation, where each missing entry was computed from measurements recorded 

 immediately prior to and after that particular gap. 



The measured elevations imphcitly contain tidal wave shoaling produced as waves 

 enter shallower water. Shoaling increases the amplitude of the tidal signal measured at 

 the nearshore station as compared to that expected at the offshore boundary of the grid 

 located in deeper water. Wave shoaling contributions to the signal must be removed 

 ft-om the time series of the forcing data because the model computes shoaling. Sensitivity 

 simulations were conducted, by which the elevations were multiplied by a correction 

 factor (which is less than unity), and comparisons were made between the measured and 

 computed water-surface elevations at the Port Lavaca station. Applying an iterative 

 approach to determine the optimal corrections, the time series of water-surface elevation 



24 Chapter 3 Circulation Modeling 



