6 Evaluation of Project Design 



General Discussion 



Most of the data presented in this report were collected within the period 

 1990 to 1992. This time frame is after completion of the jetties and 

 impoundment basin, but just prior to the diversion of the Colorado River flow 

 into west Matagorda Bay. This diversion may have a significant impact on the 

 hydrodynamics and sediment dynamics at the mouth of the river. Thus, some 

 of the conclusions drawn from this data set may not be useful for long-term 

 forecasting of conditions at the mouth of the Colorado River. 



The principal flaw in the Colorado River weir jetty system design was the 

 substantial underestimation of the longshore transport rate. This led to a 

 clearly inadequate design for the impoundment basin, and to a lesser extent, to 

 a general underdesign of several parts of the system. To operate effectively, 

 the impoundment basin should be dredged more frequently than called for in 

 the design (once every 2 years) or the size of the impoundment basin should be 

 substantially enlarged. Unfortunately, not much room is available to enlarge 

 the impoundment basin. 



If the primary purpose of the project were to have minimal impact on the 

 adjacent beaches, then the project could have been judged a success. However, 

 as stated on page 1 of the General Design Manual: 



The primary purpose of the authorized project is to provide safe and dependable 

 navigation between the Gulf of Mexico and the Gulf Intracoastal Waterway. 

 (U.S. Army Engineer District, Galveston 1977). 



Shoals form in the entrance channel between dredging events, making 

 navigation hazardous. Jetties that were wider apart would have allowed for a 

 larger impoundment basin, and thus, less shoaling. Longer jetties would also 

 have decreased the shoaling problem. However, these changes would have 

 caused greater impacts on the adjacent beaches. 



In spite of these flaws, some aspects of the design have worked well, as 

 discussed below. 



Chapter 6 Evaluation of Project Design 53 



