Qfi~Qnet ' n Storage for 

 Appropriate Time Period 



Figure 7. Sand transport and storage at an ideal weir- jetty system. 



The longshore transport environment does not remain the same from year to 

 year. In some years the net transport may be in one direction; in other years 

 it may be in the opposite direction. If the period of record used to deter- 

 mine transport quantities and directions is atypical, serious errors in pre- 

 dicting the performance of a weir system can result. Another factor which 

 complicates predicting bypassing quantities is that transport conditions at a 

 given time may differ from one side of the inlet to the other because of wave 

 refraction caused by complex bathymetry near inlets and differences in shore- 

 line alinement. Although the optimum bypassing situation which requires 

 minimum material handling may not be achieved, it is a goal toward which the 

 design should be aimed. In addition to controlling the amount of sand enter- 

 ing the deposition basin, the sandtight landward leg of the weir jetty and the 

 overall jetty layout act to control the planform of both the updrift and down- 

 drift beaches. The location of the landward end of the weir determines how 

 wide the updrift beach will be, how much sand stored in the updrift beach is 

 available for transport back up the beach during reversals in wave direction, 

 and how much sand will find its way into the deposition basin. 



The various functions of the weir dictate different system characteristics 

 which sometimes conflict with each other. For example, to maximize wave pro- 

 tection for a dredge operating in the deposition basin, the weir-crest eleva- 

 tion should be as high as practical; however, to achieve the desired control 

 of sedimentation, a lower weir crest is needed. A lower weir crest is also 

 desirable for maximizing the amount of flow entering the inlet during flood- 

 tide. A higher weir crest would be desirable during ebbtide to contain the 

 flow between the jetties. These conflicting functions require trade-offs to 

 achieve an optimum overall system. They suggest that flexibility be engi- 

 neered into any weir-jetty design so that adjustments can be made after con- 

 struction when project performance has been observed. 



At the present state-of-the-art and with the limited information usually 

 available on longshore transport at a site, an optimum weir- jetty design may 

 not be attainable. Additionally, the transition from the equilibrium that 

 exists before a weir-jetty system is constructed to a new postproject equi- 

 librium requires some as yet undetermined time to be attained. During this 

 transition, performance of the system may not truly reflect its capability, 

 and adjustments based on observations during this time may ultimately prove 



16 



