Impoundment Basin and Dredging Measurements 



The initial dredging of the impoundment basin was completed in March 

 1990. Bathymetric surveys show that by the time of the second dredging in the 

 fall of 1992, the impoundment basin had almost completely filled and the 

 channel was badly shoaling (see Figure 20). The survey data shown in Table 9 

 were used to establish the volumes of material within the impoundment basin at 

 different times. Pairs of volumes and times were used to determine shoaling 

 rates for the basin. The results are shown graphically in Figure 27; the infilling 

 rate over various time intervals between the two dredging events. While every 

 survey was done on several (or all) range lines, a few pairs of surveys had only 

 a single overlapping range line. Since the basin did not fill completely 

 uniformly, these survey pairs were not considered as reliable and were not 

 plotted in Figure 27. 



Figure 27 shows a series of horizontal lines that represent the average 

 infilling rate over the time span indicated, as determined by the surveys at the 

 endpoint dates. The jagged (dashed) line that slopes down to the right connects 

 the midpoints of the horizontal lines. This diagonal line shows that just after 

 dredging in early 1990 the infilling rate was just under 400,000 cubic 

 meters/year, but that by the end of the year the rate had dropped to about half 

 that amount, and a year later, to essentially zero. Thus, as the basin filled 

 (became shallower), it became less efficient at retaining the sediment that 

 entered it because the bottom was subjected to increasing wave and current 

 forces. 



400,000 

 350,000 



|g 300,000 

 O 



n 250,000 



O 200,000 



"cS 



CC 



D) 150,000 



c 



^ 100,000 - 

 50,000 

















1 



■ 



\ 



*. 



^'l i \ 



















" 











































~ 







— •= — 





-*: 





















- 













--, 









^^ 





1 1 ''^\! 



- 



1 1 





1 1 







1 1 i 1 



; ~N^ 



1 \ '■ 



9003 9005 9008 



9104 



Survey Dates (Months) 



9204 



9209 



Figure 27. Plot of impoundment basin infilling rates 



50 



Chapter 5 Prediction of Sediment Transport Rates 



