Figure 27 indicates that the maximum infilling rate of the basin is around 

 400,000 cubic meters/year. However, even initially, the basin is not 1 00 percent 

 efficient at trapping the sediment that enters it. If it were, the dashed line in 

 Figure 27 would be expected to be shaped like a (backwards) lazy "S." It would 

 start out as horizontal at the value of the transport rate. As the basin shoaled, at 

 some point, the line would slope down to the right, and then as the basin 

 completely filled, the infilling rate would approach zero and the line would again 

 become horizontal. Thus, it is clear that substantially more than 400,000 cubic 

 meters of material enters the basin yearly. 



However, the sediments deposited in the impoundment basin are a mix of 

 river and beach sediments. The river does carry a substantial sediment load. 

 In the past, an average of 371,000 cubic meters of material has been removed 

 yearly from a settling basin upstream of the Gulf Intracoastal Waterway 

 (U.S. Army Engineer District, Galveston 1977). Even though the river has 

 been diverted into Matagorda Bay upstream of its mouth, grain size analysis 

 indicates that significant volumes of riverine material continue to reach the 

 coast. 



The volumes of material dredged from the impoundment basin and entrance 

 channel are shown in Table 8. In the 5-year interval following the first dredging 

 event (March 1990 to February 1995) more than 2,000,000 cubic meters of 

 material were removed in three major dredging operations, with about equal 

 parts coming fi-om the entrance channel and impoundment basin. This averages 

 to 430,000 cubic meters/year. Part of this material may have come from the 

 offshore bar which was at the mouth prior to jetty construction and/or prior to 

 river diversion. If so, infilling rates in the impoundment basin would be 

 expected to diminish in the future. 



However, it appears likely that the large majority of this material is carried to 

 the area by longshore transport down the beach or by riverine transport. Thus, it 

 is prudent to consider that 430,000 cubic meters is approximately the amount 

 that will need removal on a yearly basis. Taking averages of the sediment 

 samples, 75 percent of the accumulation in the entrance channel and 55 percent 

 of the accumulation in the impoundment basin can be assumed to be derived 

 from longshore drift. This would imply that of the 430,000 cubic meters/year 

 average accumulation, about 280,000 cubic meters/year is from the littoral drift 

 and the remainder, 150,000 cubic meters/year, is supplied by the river. Thus, 

 from this conservative estimate, the long-shore transport rate can be assumed to 

 be in excess of 280,000 cubic meters/year. 



Summary 



In Table 1 1 , the longshore transport estimates based upon some change in 

 bathymetry are in general substantially less than the transport estimates based 

 upon wave data. Both techniques have flaws. Unless some shoreline feature 

 acts as a total block to the longshore transport, a bathymetry-based technique 



Chapter 5 Prediction of Sediment Transport Rates 5 1 



