southeast). At this time, the model underpredicted the measured elevations by 0.17 m. 

 Furthermore, the model tends to underpredict water level whenever winds are blowing to 

 the northwest, such as the period YD 257 through 263. For those periods when the winds 

 are blowing from a different direction, such as the north (e.g., YD 267 through 272), the 

 model reproduces the water-surface elevation well. 



Model predictions were also examined by driving it with the hourly winds measured 

 at the Lavaca Bay station (as opposed to the winds measured at the East Matagorda 

 station). Calculations with the model forced by the local winds are presented in 

 Figure 29. The comparison between calculated and measured water levels improved 

 substantially for the period YD 255 through 269. Furthermore, the model accurately 

 reproduced the water-surface elevations throughout the strong wind event. Discrepancies 

 were less than 0.06 m during this event, demonstrating that the bay is highly responsive 

 to the local wind speed and direction. The conclusion is that for the frequently wind- 

 dominated circulation of the Texas shallow bays and estuaries, local wind measurements 

 at high frequency (6- or 10-min averages) are needed to simulate the velocity and water- 

 surface elevation with reUabihty (Kraus and MiUteUo 1995, 1999). 



To measure the ebb and flood flows exiting and entering Matagorda Bay, discharge 

 measurements were taken with the ADCP across the inland sides of the MSC entrance 

 and Pass Cavallo. Ebb flow measurements were collected on YD 253 (10 September), 

 and flood flows were collected YD 274 (1 October). Comparisons between calculated 

 and measured discharges are presented in Figures 30 through 33. (Positive discharges 

 represent ebb flows, and negative discharges represent flood flows in these figures.) 



1.0 



-0.5 



Port Lavaca 



252 254 256 258 



260 262 264 

 Year Day 1 997 



266 268 270 272 



Figure 29. Comparison of calculated and measured water-surface elevations at Port 

 Lavaca station (Lavaca Bay winds specified in model) 



Chapter 3 Circulation Modeling 



33 



