Adaptation of Calibrated l\/lodel 



The grid constructed for the existing Matagorda Bay condition was adapted to 

 represent the proposed GIWW alignment. The existing-condition grid was modified by 

 increasing nodal resolution in the vicinity of the proposed channel and by positioning 

 additional nodes to depict this channel. The modified grid thereby contains an accurate 

 channel in both aUgnment and width. Chaimel depths assigned to these nodes are 

 consistent to those composing the existing GIWW (with respect to displacement away 

 from the MSC) for areas that are shallower than the channel. For areas where the 

 proposed channel depth is equal to or less than the ambient conditions, the ambient 

 depths were used. 



Because the existing GIWW can be expected to eventually fill with sediment, this 

 channel was removed from the grid, and nodal positions were adjusted in this region to 

 allow elemental sizes to approximate neighboring elements. Depths assigned to these 

 new nodes correspond to the ambient bath3mietry in the surrounding area. For similar 

 reasons, the existing Sundown Island was removed from the new grid to represent a long- 

 term perspective. 



The existing and proposed GIWW locations are presented in Figure 34. The MSC- 

 GIWW intersection for the existing channel is located at approximately 96 deg 21.9 min 

 W long, 28 deg 27.3 min N lat. The proposed intersection is located at 96 deg 22.9 min 

 W long, 28 deg 28.0 min N lat. Figure 35 presents the grid in the vicinity of the 

 proposed MSC-GIWW intersection. This grid consists of 21,940 elements and 12,382 

 nodes (as opposed to 20,320 elements and 11,575 nodes for the existing-configuration 

 grid). 



Each hindcast simulation began on YD 243 (31 August 1997) at 0000 GMT and 

 ended on YD 273 (1 October) at 0000 GMT. The model was run witii a 4-s time-step. 

 The wind input over the entire model domain and the tidal-elevation forcing imposed at 

 the gulf open boundary were identical to those of the caUbration for simulating the 

 existing and proposed GIWW configurations. 



Based on dredging records, extent of the flood shoal at Pass Cavallo, and quahtative 

 observations on site and of aerial photography, it was hypothesized in Chapter 2 that 

 Sundown Island is a significant source of sediment being deposited in the GIWW. 

 Sediment is being eroded and entrained by waves striking the island from the northeast; 

 therefore, flood currents entering through the MSC enttance can readily transport this 

 sediment towards the GIWW, where it is subsequently deposited. Calculated tidal-flood 

 currents (with the existing-configuration grid) in the vicinity of die project area are 

 displayed in Figure 36, which plots velocities at 1 September at 0600 GMT. Arrows 

 indicating current magnitude and direction are displayed at each grid node, and the MSC 

 and GIWW can be observed in the figure by the close proximity of those nodes and their 

 ahgnment composing the channels. 



Chapter 3 Circulation Modeling 



37 



