On YD 253, five passes (and corresponding measurements) were made along the 

 transect crossing the MSC entrance. Measured discharges ranged fi-om 170,179 to 

 263,370 cfs over the 7.5-hr sampling period. Calculated discharges varied from 36,220 

 to 223,842 cfs over this same period. For Pass Cavallo, where five passes were made, 

 measured discharges ranged from 70,025 to 93,410 cfs, and the calculated discharges 

 varied from -18,957 (i.e., flood flow) to 78,61 1 cfs. 



On YD 274, discharges measmed at the MSC entrance ranged from 179,186 to 

 -210,470 cfs, and the calculated discharges varied from 1 18,286 to -1 14,690 cfs. For 

 Pass Cavallo, measured discharges ranged from 69,639 to -71,250 cfs, and calculated 

 discharges varied from 47,967 to -36,944 cfs. 



Values of the maximum discharge measured at the MSC entrance agree well with 

 those estimated by Harwood (1973) for Pass Cavallo prior to the cutting of the MSC. If 

 peak discharges of 90,000 and 240,000 cfs are taken as representative of Pass Cavallo 

 and the MSC entrance, respectively, in their present (1998) state, then the volume of 

 water now flowing through Pass Cavallo is only 30 percent of that at the MSC Entrance. 



At a given time, discharges for either ebb or flood currents were underpredicted by as 

 much as 48 percent, although there is agreement between trends and magnitudes for the 

 calculations and the measurements. One source of discrepancy is lack of detailed 

 bathymetric data for the ebb and flood shoals at Pass Cavallo. Another source of 

 discrepancy between calculations and measurements concerns driving the open boundary 

 with water-surface elevation collected at the Galveston Pleasure Pier while specifying the 

 wind measured at the East Matagorda station. Ideally, the model should be driven with 

 data collected in closer proximity to the study area. 



Implementation of Alternative 



As discussed in Chapter 1, this study focuses on the feasibility of relocating a section 

 of the GIWW nordiward vidthin Matagorda Bay. Presently, barges and other watercraft 

 traversing the GIWW experience strong currents in the vicinity of the MSC-GIWW 

 intersection during periods of strong winds and peak tidal floods. Furthermore, at its 

 present location, the GIWW near the intersection is subject to extensive shoaling because 

 of the strong crosscurrent. As discussed in Chapter 2, Sundown Island, constructed and 

 maintained through dredging operations, has been identified as a major source of 

 sediment deposited into die GIWW near the intersection. Its position relative to the 

 MSC, on the south side of the GIWW, and its close proximity (1,300 ft) to the waterway, 

 makes the island highly susceptible to erosion induced by the strong currents experienced 

 in this area. 



To assess the consequences of relocating the GIWW and possible reduction of 

 sediment shoaling of the waterway by erosion of an artificially created bird island, the 

 bathymetry grid of the circulation model was modified to represent the proposed 

 aUgmnent. A one-month simulation was then made with the caUbrated model, and time 

 series of water velocity computed at several complementary locations at the existing and 

 proposed GIWW alignments were saved and processed. These time series were analyzed 

 and compared in assessing and qualifying the potential reduction in shoaling at the 

 relocated channel. The complementary stations for the calculated currents allow 

 comparisons for equivalent geometric locations for the existing and relocated waterway 

 and island. 



35 Chapter 3 Circulation Modeling 



