becomes elevated. The emergent area of Sundown Island has been variously estimated as 

 between 15 and 50 acres, depending on the state of erosion and subsequent fill. The 

 island was formed of material removed in cutting of the MSC during 1962 to 1963, and it 

 may have been placed in its location as suggested by Simmons and Rhodes (1966) to 

 reduce the crosscurrent along the MSC that was observed in the physical model. Material 

 dredged from the GIWW in the vicinity of the island is placed on the island during 

 permitted environmental windows. From 1983 to 1998, Galveston District records show 

 that approximately 2.6 miUion cu yd have been placed on the island, aU originating from 

 the GIWW segment between sta 600+000 to 616+000. 



A high-altitude photograph of Sundown Island and the MSC taken in 1991 is given in 

 Figure 15. The two jetties of Port O'Conner, through which the GIWW passes, are seen 

 on the left side of the photograph. This photograph indicates a process of erosion of 

 Sundown Island consistent with the dredging records and observations of the morphology 

 of the island itself. Sediment is removed from the island by waves, especially during 

 times of high water and fronts out of the northwest, which would create waves moving 

 toward the northeast comer of the island. The flood current entering the bay then 

 transports northward the sediment mobiUzed from the platform and foreshore of the 

 island. The sediment is then deposited into the GIWW. In the DMS Manual, this 

 transport and deposition process is termed "horizontal expansion," as depicted in 

 Figure 16, and is discussed further in Chapter 3. 



Boat reconnaissance of Sundown Island made in May and September 1997, indicated 

 that it is situated on a broad shallow platform of sediments consisting primarily of sand. 

 The platform tends to be skewed to the northeast, approaching the GIWW at an angle. 

 Older and more stable portions of the island are located in its center. It has been reported 

 that vertical cuts appear in the island as it erodes, and these cuts tend to be on the 

 northeast side, which is the side facing the longest fetch in the bay with respect to the 

 location of the island. 



Sundown Island is a significant natural resource for bird nesting, because its location 

 far from shore in the bay hinders mainland predators from reaching it. Bird species that 

 nest on the island include brown peUcans (endangered species), herons, egrets, ibises, 

 roseate spoonbills, gulls, terns, and black skinners (Texas Department of Transportation 

 1994). Figures 17 and 18 show the perimeter of Sundown Island, as seen from the MSC 

 entrance upon entering the bay. Much of the perimeter of the island is of relatively low 

 rehef and susceptible to erosion by waves. The tidal and wind-generated current can then 

 easily move the mobilized sediment. 



If the erosion and sediment-transport processes are as described in the preceding 

 paragraphs, then a numerical model of circulation in the bay will reveal a strong flood 

 current aroimd the perimeter of Sundown Island. The model and calculation results are 

 described in the next chapter. 



1 8 Chapter 2 Channel Shoaling 



