entrance. The inlet gorge is self-scouring and, since construction, no dredging has been 

 required at or seaward of the MSC entrance. 



Gulf Intracoastal Waterway 



The Gulf Intercoastal Waterway (GIWW) is a major water artery that runs along the 

 inland coastal waters of Texas for 435 miles. The first navigable segment of the GIWW 

 in Texas was completed in 1853 (Texas Department of Transportation 1994) and 

 connected West Galveston Bay and the Brazos River, which he to the north of 

 Matagorda. Thereafter, the GIWW in Texas was constructed in a series of segments, and 

 the earUer waterways were typically 9 ft deep by 100 ft wide. 



The present design dimensions of the GIWW are 500-ft width at the top, 125-ft width 

 at the bottom, and 12-ft depth (to mlt for the case of Texas). Legislation to maintain the 

 entire waterway to its present dimensions was passed by Congress in 1942. In Texas, the 

 GIWW is dredged to 14-ft depth, which includes 1 ft of advance dredging and 1 ft of 

 over-dredging (allowance for operational practicahty and inaccuracy in dredging). 

 Atturio, Basco, and James (1976) studied the shoaling characteristics of the GIWW from 

 Port Arthur to Brownsville (380 miles) by evaluating dredging records in 5,000-ft 

 segments along U.S. Army Corps of Engineers (USAGE) waterway stations. Among 

 their conclusions are as follows: 



a. The average shoaling rate over the entire reach was 10.5 in. per year. 



b. Shoaling rates in open-bay areas were on average 3 in. per year greater than in 

 land-cut areas. 



c. The combination of dredged-material mounds, or fetch greater than 5 miles, with 

 water depths less than 6 ft (surrounding bay depth) increased average shoaling 

 rates to 5 in. per year. 



d. The placement of dredged material in mounds on the windward side of the 

 waterway increased the average shoaling rate of open-bay areas by 7 in. per year. 



e. In bay areas with long fetches and depths less than 3 ft, it was found that 

 windward placement of dredged material reduced the shoaling rate as compared 

 to neighboring sections. 



/. Hurricanes did not appear to have catastrophic consequences for the inland 

 waterway. 



In a study on the cause of chronic shoaling in the Lower Laguna Madre (see 

 Figure 1), Mihtello and Kraus (1994, 1995) found that redirection of the tidal and wind- 

 generated flow in the large lagoon moved opposite to the circulation inferred from the 

 predominant wind direction. At this high-shoahng area, they recommended placing the 

 dredged-material islands upwind rather than downwind. Their smdy showed that the 

 cause of increased sediment shoaling in a waterway may not be intuitively obvious and, 

 together with modest field measurements, an apphcation of a numerical model of the 

 hydrodynamics can be helpful for inferring sediment paths and the driving mechanisms 

 controlling the shoahng. 



Chapter 1 Background and Problem Statement 



