should be scheduled for times during the year when there is a high probability 

 that bypassed sand will be carried away from the inlet to downdrift beaches. 

 Consideration of all these factors requires knowledge of the transport 

 environment, particularly seasonal variations in transport direction and 

 magnitude. 



Planning should take into account acquisition of easements along areas of 

 both the updrift and downdrift beaches for placement of bypassed sand and for 

 stockpiling sand. The areas, which serve as feeder beaches to provide sand to 

 adjacent areas, must be located far enough from the inlet to avoid the wave 

 shadow of the jetty structures and to preclude large amounts of sand from 

 returning to the inlet during short-term reversals in transport. 



The same methods for predicting updrift beach response to the construction 

 of jetties can be applied to the downdrift beach (discussed in more detail in 

 Section X). 



IV. WEIR HYDRAULICS 



Weir jetties serve a hydraulic function. During floodtide when water 

 levels exceed the weir-crest elevation a part of the inlet's tidal prism flows 

 across the weir into the inlet. During ebb flow, much of the water that has 

 entered the inlet over the weir flows out through the navigation channel. 

 The weir thus causes a greater ebb flow out between the jetties than enters 

 between the jetties during floodflow. 



Greater ebb flow causes ebb current velocities in the navigation channel 

 to exceed flood current velocities and results in natural flushing of sedi- 

 ments from the channel. The amount of ebb dominance resulting from the weir 

 depends on several factors. The phase lag of the tide level on the weir's 

 channel side behind the tide level on the oceanside causes a head difference 

 and drives a current across the weir. In addition, the relative amplitude of 

 the tide on each side of the weir influences the current. Typical tidal 

 curves measured across a laboratory weir jetty are shown in Figure 11. Phase 

 lag and tidal amplitude difference across the weir depend on the inlet 

 hydraulics as characterized by Keulegan's K (see Sorensen, 1977) and by the 

 proposed jetty system geometry. 



A major factor influencing velocity assymetry is weir-crest elevation. 

 Lower weirs allow more flow to enter the inlet during floodtide, but also 

 allow more ebb flow to escape across the weir. Waves also transport water 

 over the weir and contribute to ebb-flow dominance. Since higher waves gen- 

 erally act only on the weir's oceanside, wave transport of water across the 

 weir is into the inlet. There is little or no corresponding seaward wave 

 transport out of the inlet. Wave setup on the oceanside of the weir also 

 contributes to the head difference across the weir and causes flow into the 

 inlet. Each of these factors contributes toward keeping ebb current veloci- 

 ties greater than flood current velocities through the navigation channel and 

 thus assists in preventing channel shoaling. 



The amount of water carried over the weir because of tidal phase and 

 amplitude differences between the ocean and channel sides of the weir can be 

 estimated using an appropriate weir flow formula. If the weir section has a 

 well-defined crest elevation such as would exist for a sheet-pile weir, the 

 discharge per unit length of weir crest can be calculated from (see Fig. 12) 



26 



