impoundment basin was designed to hold a 2-year supply of sediment, 

 460,000 cubic meters (U.S. Army Engineer District, Galveston 1977). After 

 construction, it was found that the impoundment basin filled and the navigation 

 entrance charmel shoaled substantially more rapidly than expected. Analysis of 

 data collected for this monitoring program indicates that the beaches around the 

 mouth of the Colorado River have a gross littoral drift transport rate of the 

 order of 670,000 cubic meters/year, and a net littoral drift transport rate of the 

 order of 510,000 cubic meters/year to the southwest. In addition, river-borne 

 sediments are also deposited in the impoundment basin. Thus, the 

 impoundment basin is less than half as large as needed for a 2-year dredging 

 cycle to be adequate. 



e. Hypothesis No. 5: The impoundment basin trapping efficiency is high. 

 Analysis of elevation profiles taken across the impoundment basin indicates that 

 its trapping efficiency does not approach 100 percent and that the efficiency 

 continuously decreases as the basin fills. The major accumulation of sediment 

 in the entrance channel, estimated at more than 200,000 cubic meters/year, also 

 indicates that substantial quantities of sediment are not trapped in the 

 impoundment basin. Thus, the impoundment basin trapping efficiency is not 

 high. 



f Hypothesis No. 6. The northeast jetty length should extend 460 m past 

 the end of the weir section (to the -3. 7-m contour). The original experiment 

 plan called for making sediment transport measurements at the tip of the 

 northeast jetty. However, this was not possible due to hazardous waves and 

 currents at this location. However, the major shoaling problems appear to be 

 in the entrance channel between the jetties, rather than on a bar off the tip of 

 the jetties. This would indicate that the major sediment pathway was through 

 the weir and impoundment basin and then around the southwest jetty to the 

 downstream beach, rather than around the tips of both jetties. Thus, from that 

 standpoint, the northeast jetty length is satisfactory. However, a longer 

 northeast jetty would help decrease the shoaling in the entrance channel by 

 forcing the shoals to form in deeper water. 



Chapter 6 Evaluation of Project Design 55 



