for each of the gulf boundary nodes were assigned according to the following. For those 

 nodes along the lateral boundaries, the nodes located at the land-water interface (or shore) 

 as well the next node removed from the shore, the water-surface elevations were not 

 adjusted. Elevations assigned to the third node removed from the shore were multipUed 

 by 0.98. Similarly, elevations assigned to the fourth, fifth, and sixth nodes removed from 

 shore were multipUed by 0.96, 0.94, and 0.92, respectively. Elevations assigned to aU 

 remaining gulf boundary nodes were multipUed by 0.90. 



The strong sea breeze and fronts experienced along the Texas coast dominate the 

 astronomical tidal forcing and significantly alter the circulation and water level in the 

 shaUow bays and estuaries. Harwood (1973) noted that typical strong winds can 

 contribute as much as 1 ft of change in water level, as compared to a typical tidal range of 

 0.8 ft in the bay. Kraus and MUiteUo (1996, 1999) measured a 2-ft, wind-induced water- 

 elevation change in East Matagorda Bay, and 1-ft changes occurred weekly. The time 

 series of wdnd speed and direction recorded at the East Matagorda Bay station were input 

 to the model. (Wind direction is measured relative to magnetic North.) This station was 

 chosen instead of the Lavaca Bay station because data for the caUbration period in 1997 

 were available in part at 6-min intervals, as opposed to hourly data for the Lavaca Bay 

 station. Data at 6-min intervals were available for the period Year Day (YD) 258 

 (15 September) through YD 294 (21 October), and hourly data were available dining the 

 remaining study period. For input to the model, hourly values were interpolated linearly 

 to achieve a time series with uniform 6-min intervals between data points. 



Field measurements were made to obtain cturents for caUbrating and evaluating the 

 accuracy of the hydrodynamic model. This program consisted of sustained and synoptic 

 monitoring, which for the sustained portion of the program, two current meters were 

 deployed in the vicinity of the MSC-GIWW intersection (Figure 21). One meter was 

 placed adjacent to the MSC, and the other was situated adjacent to the GIWW. 

 Instruments were affixed to existing pilings located along the channels. Measurements 

 were coUected at 15-min intervals over the period 9 September through 2 October. 



For the synoptic portion, Acoustic-Doppler Current Profilers (ADCP) were mounted 

 on two boats and measured cmrents on transects at certain times during two 2-day 

 periods. The first period was 10 and 1 1 September, and the second period was 

 30 September and 1 October. Measurements of the current through the water column 

 were made along 18 transects. Two of the transects crossed Pass CavaUo, and another 

 two crossed the inland side of the MSC entrance. In the caUbration procedure, calculated 

 discharges wiU be compared to the measured discharges at one transect at Pass CavaUo 

 and at one transect at the entrance. 



Five passes were conducted along the MSC inlet and Pass CavaUo transects on 

 10 September. Time intervals between each pass ranged between 1 and 2 hr. An 

 additional seven passes were made on 1 October, at roughly 1-hr intervals. 



Two rivers, the Colorado and Lavaca Rivers, empty into Matagorda Bay. The 

 Colorado River is the larger of the two rivers and discharges into Matagorda Bay through 

 the Colorado River Diversion Channel, located at the eastern end of the bay. Since about 

 1935, the Colorado River had emptied directly into the Gulf of Mexico. Beginning in 

 1992, the bulk of the river flow was diverted through die diversion channel. A minor 

 amount of water volume can pass through navigation locks on the GIWW. This quantity 

 of water is Umited to the amount of water used in operating the locks themselves. For the 

 period of 1960 through 1982, the Colorado River had a mean annual discharge volume of 

 1,720,600 acre-ft (MueUer and Matthews 1987), as measured at the USGS-operated 



Chapter 3 Circulation Modeling 25 



