Table 1. Amplification of factors for long waves. 



Incident 

 wave period 



Amplification 



factor 



Present 





Planned 



(min) 



geometry 





geometry 



68.1 



34 





30 



22.2 



24 





11 



13.3 



14 





7 



9.5 



10 





5 



equation (3), it is predicted that Uj^^^^ in the new channel reduces to about 

 0.7 foot per second. Ippen and Goda's program shows that amplification of any 

 long waves will be reduced. 



(4) External Effects. The hydraulics of Freeport are also influenced 

 by a manmade factor, the Dow Chemical Company's withdrawal of water from the 

 harbor and subsequent discharge into the Brazos River. Figures obtained from 

 Dow (J.M. Kieslich, Galveston District, personal communication, 1977) show 

 that the quantities of water range from about 5.4 x 10 cubic feet per month 

 (winter) to 7.6 x 10 cubic feet per month (summer) or an average flow through 

 the entrance of between 2,000 and 2,900 cubic feet per second. Thus, a land- 

 ward average velocity of 0.14 to 0.10 foot per second would be superimposed on 

 any tidal currents. These velocities will be greater if the instantaneous 

 withdrawal rate is greater than the monthly average. This velocity component 

 may seem small, but consider that sediment transport rate is an exponential 



function of velocity (assume third power). For u 



0.82 foot per second 



(tidal only) and the maximum average artificial current, 0.19 foot per second, 

 "max flood = 0-82 + 0.19 = 1.01 feet per second and u^^ ^^^ = 0.82 - 0.19 = 

 0.63 foot per second. The flood sediment transport capability would then be 

 four times as great as the ebb, a factor which could be contributing to the 

 entrance shoaling problems. 



d. Stability of Freeport Entrance . Freeport Harbor entrance is atypical 

 of other inlets in this study since currents are probably unable to transport 

 sand size material or scour finer sediment. Therefore, the major problem at 

 the entrance is extensive deposition of very fine silts and clays. To 

 quantify the deposition rate of this material, dredging surveys from 1970 to 

 1973 were analyzed, and these rates were computed using equation (4): 



S = 



A + A ^, ^D _^, 

 n n+1 _^ n+1 



".) 



iK^} 



(4) 



where 



An 



^n+1 



- D„ 



shoaling rate (cubic yards per month per foot) width 

 between stations n and n+1 



difference between postdredging and subsequent predredg- 

 ing cross-channel profile (square yards) at station n 



distance between stations n and n+1 



width over which deposition ocurred at station n (feet) 



time between surveys (months) 



27 



