2 Channel Shoaling 



In this chapter, the shoaling rates are analyzed for the high-frequency dredging 

 section adjacent to the MSC-GIWW intersection and to Sundown Island. The analysis is 

 based on Galveston District dredging records. Inferences about sediment paths are made 

 by inspection of aerial photographs and reconnaissance surveys of Sundown Island. 



Review of Dredging Records 



The Galveston District provided maintenance dredging records dated from 1965 to 

 1995 that include Ustings of dredging frequency and amount of material dredged for 

 maintenance by location according to District stationing. The area experiencing greatest 

 navigation discrepancies and dredging is located at and around the MSC-GIWW 

 intersection, and the studied reaches were selected according to their proximity to this 

 intersection. The stretch covered by the records for the MSC extends from the 

 Matagorda Peninsula south of the channel to Point Comfort to the north. The stretch for 

 the GIWW extends across Matagorda Bay. 



The analyzed records comprise 3.4 miles of the GTWW and 1 .9 miles of the MSC. 

 Records of the GIWW were analyzed from sta 600+000 to 618+000 (east to west). The 

 MSC stretch from sta 11+000 to 21+000 was studied (south to north). Both reaches 

 include the MSC-GIWW intersection. The studied stretch of the GIWW was divided into 

 2,000-ft-long segments, and the MSC stretch was divided into 1,000-ft-long segments for 

 analysis. This division into equal lengths allows more reliable comparison of volume 

 distributions. 



Dredged volume 



Figure 8 plots the volume of material dredged from the GIWW stretch under study. 

 The MSC-GIWW intersection is located between sta 616+000 and 617+000. The 

 quantity of material dredged increases significantly as distance to the intersection 

 decreases. The amounts of dredged material and distance to the intersection have a high 

 correlation coefficient of 0.91, statistically significant at 99 percent confidence level. 



Volumes of most of the material dredged from the MSC section under study are 

 displayed in Figure 9. The intersection of the GIWW with the MSC is located at 

 sta 616+000. Again, the figure shows a substantial increase in volume of dredged 

 material at and northward of the intersection with the GIWW. 



Chapter 2 Channel Shoaling 1 3 



