100 



80 



60 



O 40 



20 k- 



sta1 



Measured 

 Modeled 



252 254 256 258 260 262 264 266 268 270 272 

 Year Day 1997 



Figure 26. Comparison of calculated and measured velocities at sta 1 



At sta 2, shown in Figure 27, calculated velocity matched the measured current in 

 phase, but overestimated the peak magnitude by as much as 50 percent during some 

 periods (e.g., YD 253 at 1800). During other periods, such as YD 255 at 0600, the 

 calculated peak velocity matched the measured velocities to within 4 cm/sec. Inspecting 

 currents at sta 2 together with both the wind directions recorded at the East Matagorda 

 station and the water-surface elevations recorded at the Galveston Pleasure Pier, it 

 appears that the model overestimates water velocity if the wind is blowing in the same 

 direction as the current. For example, the model over-predicts water velocity during ebb 

 flow when the wind is from the north. During periods when the wind direction differs by 

 more than 50 deg from the current direction, the calculated velocities nearly match the 

 measurements. 



The matching of calculated and measured velocity for these transient wind events is 

 difficult because the current meter measures at a point in the water column, whereas the 

 model calculates an average over a relatively large grid area (approximately 1,000 sq m). 

 A second cause for these discrepancies at sta 2 can be attributed to driving the model with 

 water-surface elevation and wind measured at sites outside the study area. Galveston 

 Pleasure Pier and East Matagorda Bay stations are located about 70 and 20 miles, 

 respectively, to the northeast from the MSC entrance. Given these relatively large 

 distances, the wind-induced setup and setdown contained in the time series of water- 

 surface elevation measured at the Galveston station may not be synchronous with the 

 wind measured at East Matagorda. Thus, the forcing imposed on the model is not 

 synchronous with the current measured at sta 2. 



Calculated water-surface elevation was compared to measurements at the Port 

 Lavaca station, shown in Figure 28. The model rephcates the phase of the tidal signal, 

 and the calculated elevations are generally within 0.05 m of the measurements. The 

 maximum discrepancy between the calculated and measured water-surface elevations 

 occurs at YD 266 at 1200, or when the wind is blowing at 135 deg (i.e., from the 



Chapter 3 Circulation Modeling 



31 



