Entering the open bay through the MSC entrance, tidal-flood currents exceed 

 2.5 m/sec. These currents progressively weaken as they propagate to the northwest, 

 along the MSC, to about 0.3 m/sec approximately 2 miles inland from the entrance. This 

 weakening is in great part a consequence of the tidal jet spreading laterally over an 

 increasingly widening area as the jet moves into the bay. The lateral extent of this jet is 

 greater for the eastern side of the MSC than for the western side. The shallower water on 

 the western side (compared with the eastern side) impedes intrusion of the jet into the 

 western side. Also, the flood current at Pass Cavallo enters the bay at a northeasterly 

 heading and tends to deflect or displace the MSC current to a northwesterly heading. 



The eastern lateral extent of the tidal jet is deflected further eastward by Sundown 

 Island, subjecting the island to strong currents. The jet is divided into two portions, with 

 one portion flowing south of the island and towards the east-northeast and the other 

 flowing between the island and the MSC and towards the northeast. Consequently, the 

 western and southern faces of the island are exposed to the strongest currents and can be 

 expected to experience the greatest sediment transport potential. Weaker currents, with 

 headings towards the north-northeast, are shown along the northern side of the island; 

 because this area is sheltered from the tidal jet, sediment deposition is likely to occur in 

 this area. 



Figure 37 presents a vector map of the current for the relocated GIWW alternative 

 route. (To facUitate comparison of current patterns between existing- and proposed- 

 chaimel configurations, this figure covers the identical spatial area, and with the same 

 vector scaling, as Figxure 36.) The current is reduced on the western side of the bay; 

 without Sundown Island obstructing the jet, less water is diverted to the west of the 

 location of the island. Without Sundown Island, the heading of the jet changes to the 

 northeast of the MSC. Under the existing-configuration conditions, the jet is divided into 

 northwestern and northeastern currents; if Sundown Island is allowed to erode, the jet 

 will have a northwestern heading, reducing the current moving towards the northeast. In 

 addition, the current in the vicinity of the proposed Sundovm Island is weaker than that in 

 the vicinity of the existing island. 



To quantify and compare the current in the vicinity of the relocated GIWW and 

 proposed Sundown Island, time series of the current were saved at 12 locations in 

 Matagorda Bay (Figure 21). Locations were chosen so that computed velocities at six 

 sites under the proposed GIWW configuration can be compared with their 

 complementary or equivalent site with respect to the existing GIWW location. 



One complementary pair of time series was computed at the existing and proposed 

 MSC-GIWW intersection. A second pair was chosen to depict equivalent currents with 

 respect to sta 1, the location where currents were measured during the calibration period 

 using a current meter placed in the GIWW. For the existing configuration, the current 

 was computed at the actual meter position; an equivalent position, relative to the distance 

 from the station to the MSC-GIWW intersection, was selected for the proposed 

 configuration at the relocated waterway. 



The remaining four complementary sites surround the existing and the proposed 

 Sundown Islands. One pair of sites is located off the western extent of the islands, and a 

 second pair was specified at the eastern extent of each island. The fifth pair was placed 

 midway between the island and the channel, and the sixth pair lies on the opposite side of 

 the island from the channel. 



40 Chapter 3 Circulation Modeling 



