The general design objectives of any inlet stabilization or harbor project 

 are to provide a safe navigation channel with adequate dimensions, to minimize 

 the need for channel maintenance dredging, and to preclude or minimize any 

 adverse effects of the project such as downdrift beach erosion. The mitiga- 

 tion of downdrift erosion is a purpose of bypassing sand as is keeping the 

 navigation channel free of sand. A weir-jetty system will meet these objec- 

 tives if (a) the navigation channel is kept in a fixed location and relatively 

 free of sediments, (b) the weir section and outer jetty section of the updrift 

 jetty provide wave protection to a dredge in the deposition basin, and (c) the 

 overall jetty complex provides wave protection to vessels using the channel. 



Another hydraulic function of a weir-jetty system is to allow flood 

 currents to enter the inlet over the weir during floodflow with subsequent 

 channeling of ebb flows out of the inlet between the jetties. During ebb flow 

 most of the tidal prism should exit the inlet through the navigation channel 

 (between the jetties) with a minimum of flow exiting across the weir. This 

 ebb-flow dominance results in higher ebb tidal currents in the navigation 

 channel and tends to flush sediments from the channel. These sediments may be 

 deposited in an outer bar, indicating a need for longer jetties. In addition, 

 the weir and jetty provide wave protection to vessels navigating the inlet and 

 provide protection to a dredge bypassing sand from the deposition basin to the 

 downdrift beach. 



The weir jetty also serves as a structure for controlling sediment trans- 

 port into the inlet by providing a low sill over which sand is transported by 

 waves and thus determining the location within the inlet where deposition 

 occurs. By limiting deposition to a predetermined area, sand is kept out of 

 the navigation channel and deposited where a dredge can safely operate. 



The ideal weir-jetty system will minimize the amount of sand which needs 

 to be bypassed. Optimally, this minimum is the net sand transport into the 

 inlet. If q(t) is the rate of longshore sand transport at a point on the 

 beach at a time, t, then the transport of sand as a function of time at that 

 point can conceptually be described as shown in Figure 6. The sign convention 

 of the figure assumes that q is positive when transport is toward the right 

 of an observer looking seaward from the beach; when q is negative, transport 

 is toward the left. The net transport rate is simply the average value of 

 q(t) given by 



«net ="t / t0+T *'<*> dt CD 



where x is usually chosen to be 1 year. The total net transport during the 

 time interval, t, is 



Qnet = <Wt T - t/° +T *<t> dt (2) 



The total transport to the right during x is 



Qr " t z' 1 qCt) dt + t J* 3 qCt) dt + ... + t j T ~ tn q (t) dt o) 



2 n 



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