Figure 14 depicts the dredging history for the MSC for the same time interval as in 

 Figure 13 for the GIWW. This MSC segment was dredged the most in the first decade 

 after the MSC entrance was cut. Maintenance dredging dechned somewhat in the 

 subsequent 10 years, and the amount of material dredged increased sUghtly after 1984. 



5 UE+05 



i 



D 1965-74 

 B 1975-84 

 H 1985-94 



Stations 



Figure 14. Volume of material dredged for maintenance from the MSC 



Channel cross section 



Condition surveys were examined for the MSC and GIWW channel segments 

 discussed in the preceding section. It was found that the MSC tended to accumulate 

 sediment in approximately equal amounts on both sides of the channel center Une, with 

 little change in depth in the middle of the channel. It can be reasoned that the motion of 

 the keel and propellers of ships traversing the MSC tend to sweep sediment away from 

 the bottom of the channel. Approximately equal amounts of deposition on either side of 

 the center Une indicate that the crosscurrent is balanced in direction; there was no 

 strongly preferential direction of the crosscurrent. 



The condition surveys of the GIWW showed that the sediment accumulated 

 predominantly on the south side, the side nearest Sundown Island. The pattern of 

 accumulation is consistent from a characteristic or "symptom" identified in the DMS 

 Manual, as depicted in Figure 1 1 . Sediment is shown entering a channel from one side, 

 as transport by the water flow. If the sediments are in the sand size range and do not 

 remain in suspension or are easily suspended, then the bank will fill to reduce channel 

 width and, ultimately, channel depth. 



Sundown Island 



Sundown Island serves as a beneficial uses site for dredged material and forms a 

 portion of the Galveston District's Disposal Area 1 16A. It is an unconfined, 

 partially-emergent disposal area and subject to chronic erosion by waves and 

 currents, especially during strong winds and storms, when the water level in the bay 



Chapter 2 Channel Shoaling 



17 



