Training Dike - 

 BA Y ^^' N ~°^l¥slT h ^el 



OCEAN 



Figure 10. Training dike to control location of 



navigation channel in a weir-jetty system. 



Two factors which influence the required deposition basin capacity are the 

 longshore sand transport rates into the inlet and the estimated frequency at 

 which the basin will be dredged. At sites where physical constraints on depo- 

 sition basin size may exist due to inlet geometry, the longshore transport 

 rate and maximum allowable deposition basin size will establish the frequency 

 of dredging. 



Existing systems are intended to be dredged annually but the deposition 

 basins have been designed to hold a 2-year storage as a safety factor. Thus, 

 the required capacity of the basin will be twice the annual volume transport 

 into it. The amount transported into the basin will be between a minimum 

 equal to the net transport at the inlet and a maximum equal to the total down- 

 coast transport. That is, if the annual downcoast transport is Q R and the 

 upcoast transport is Q L with Q R > Q L , then the amount trapped per year will 

 be between the minimum of Q R - Q^ = Q net and a maximum of Q R , assuming that 

 no sand is lost offshore. If dredging is scheduled for a 2-year cycle, the 

 capacity of the basin should be 2Q R to assure adequate storage volume. If 

 the annual transport is highly variable from year to year, additional depo- 

 sition basin capacity may be necessary to provide sufficient storage for 2 

 consecutive high-transport years. An alternative is to dredge more frequently 

 during such anomalous occurrences. 



In the first few years after jetty construction the amount of sand enter- 

 ing the deposition basin may exceed the normal longshore transport to the 



24 



