distance away from the jet to areas of weaker current, the shoaling rate in the waterway 

 decreases. 



Channel cross-section surveys of the MSC and GIWW showed that the MSC tended 

 to fiU from both sides, indicating isotropic sediment transport. Cross sections of the 

 GIWW indicated predominant infilling or transport from the south side, as brought by the 

 flood current. 



Circulation modeling 



A numerical model of the hydrodynamics in Matagorda Bay was established and 

 cahbrated by comparison of predictions to recent (September 1997) measiu-ements of the 

 water level and current. The horizontal current pattern and magnitude of the current were 

 then compared at the present location of the GIWW and at the proposed location. 



The modeling confirmed inferences as given in the previous section based on general 

 characteristics of flood shoals and flood jets. It was found that the flood current flowed 

 past Sundown Island toward the GIWW with sufficient strength to transport sediment 

 into the waterway. 



Comparison of the magnitudes of the calculated current at the present location of the 

 GIWW and at the proposed location showed that the peak current speed at the proposed 

 location was on the order of one-half that at the existing location. Therefore, because 

 sediment transport follows a nonlinear function of current magnitude, typically on the 

 order of magnitude cube, a decrease in transport would be expected, hence infilling at the 

 relocated channel on the order of one-eighth that occurring at the existing channel. This 

 conclusion holds in the dynamic equilibrium condition, a few years after initial dredging 

 for the relocated channel. If Uttle dredging is necessary, then initial transients as 

 slumping of channel walls will make only a minor contribution to the shoaling. 



Recommendations 



The following are recommendations from appUcation of the DMS methodology: 



a. If the GIWW is not relocated northward, shoaUng in the waterway in the vicinity 

 of Sundown Island can be reduced by armoring the island to prevent material eroded 

 from it from being transported into the waterway by the flood current. The large shallow 

 shoals on the northeast side of Sundown Island might also be dredged to eliminate this 

 material as a source of shoaling. 



b. If the GIWW is relocated and Sundown Island is to be maintained or protected, 

 then armoring should be placed around the island to reduce erosion by waves and 

 currents (both tidal and wind-generated currents). 



c. Relocation of the GIWW northward as under consideration by the Galveston 

 District would be beneficial for three reasons: 



(1) ShoaUng of the waterway would be greatly reduced because it would be 

 out of the range of influence of a strong flood current. 



(2) Sundown Island would not provide the large input of sediment that it 

 presently provides. 



(3) Navigation safety would be improved at the new MSC-GIWW intersection 

 because the flood current would be greatly reduced as compared to its 



50 Chapter 4 Summary and Recommendations 



