4 Summary and 

 Recommendations 



This chapter summarizes study findings on the cause of excessive shoaling in the 

 GIWW near the MSC-GIWW intersection. Based on the findings, recommendations are 

 given for moving the open-bay portion of the GIWW to the north. The recommendations 

 are developed from a synthesis of observations of the morphology at Pass CavaUo, 

 hydrodynamic processes at the MSC entrance, dredging records, and circulation 

 modeUng, by applying the DMS methodology and rapid-assessment techniques. 



Summary 



Results are summarized according to the category of process. 



Morphology and circulation 



The present location of the GIWW is within about 13,000 ft of the entrance of the 

 MSC. This distance is within the range occupied by the historical flood shoal of Pass 

 Cavallo. Similarly, Sundown Island is located within this range of extent of the historic 

 flood shoal. A flood shoal occupies the region of effective sediment-transporting 

 capacity of the inlet and its flood current, as schematically depicted in Figure 47, which is 

 adapted from the DMS Manual (Kraus et al., in preparation). A flood current deposits 

 sediment at a distance from the inlet where the current sufficiently weakens. The flow 

 weakens because of horizontal expansion of the flood jet in the calm bay water with 

 distance from the entrance into the bay. A substantial flood shoal wUl not form at the 

 MSC entrance because the jetties prevent Uttoral sediment from entering the chaimel, and 

 the strong ebb current tends to sweep out Uttoral sediment that is deposited in it. 



Because the MSC entrance is capturing the tidal prism from Pass Cavallo and 

 because it is a jettied and deep channel, it is much more efficient to water flow than the 

 wide and shallow historic pass. Therefore, the effective range of sediment-transporting 

 capacity for the flood current of the MSC entrance is greater than that of Pass Cavallo. 

 Any sediment mobiUzed by waves within this range will be transported bayward by this 

 current. The GIWW segment in this range will tend to shoal, and material eroded from 

 Sundown Island wiU tend to be transported into the GIWW by the flood current. 



Figure 48, also taken from the DMS Manual (Kraus et al., in preparation) is a 

 schematic of channel infilling by a current with a predominant direction. Inspection of 

 condition surveys at the GIWW high-shoaling rate section shows a pattern similar to that 

 of the schematic. 



48 Chapter 4 Summary and Recommendations 



