Both the updrift and downdrift beaches gain sand so that the updrift beach 

 (Bogue Banks) will advance at a rate of 1.5 feet per year and the downdrift 

 beach (Shackleford Banks) will advance at 1.8 feet per year. The problem is 

 the same with this strategy as with the preceding one — the beach updrift of 

 the weir is stabilized by construction of the weir and any extra sand is 

 carried into the deposition basin. It will not be possible to limit accum- 

 ulation in the deposition basin to 231,000 cubic yards per year. 



d. Strategy 4. A fourth strategy (probably the most realistic one for 

 operating a weir-jetty system) is to keep losses from the updrift beach at 

 zero to stabilize the updrift beach and generally ensure quasi-steady state 

 operation of the weir (see Fig. 28). A disadvantage of this strategy is that 

 it might require a greater volume of sand to be transferred than other strate- 

 gies. Under strategy 4 the volume change on Bogue Banks, A^g> is taken as 

 zero and the Bogue Banks equation becomes 



415.8 (gain from west) - 98.3 (loss to west) 



- 32 (lost offshore - B £ = 



and the Shackleford Banks equation is 



B E - 223.0 (loss to east) + 79.4 (gain from east) 



- 33 (lost offshore) = A¥ g 



Solution of the two equations gives 



B E = 285.5 and A¥ g = 108.9 



The rate of accumulation on the downdrift shoreline (Shackleford Banks) is 

 thus 108,900 cubic yards per year and the shoreline moves seaward at 3.6 feet 

 per year if the accumulation is uniformly distributed along the shoreline. 

 The amount of sand to be bypassed is 285,500 cubic yards per year, the volume 

 that would probably accumulate in the deposition basin if the updrift beach is 

 stabilized by construction of a weir jetty with a sandtight landward section 

 and a weir section positioned to establish a dynamically stable beach 

 planform. 



BOGUE BANKS 



ATLANTIC 



B E = 285.5 

 AV S = 108.9 



Figure 28. Weir-jetty construction sediment budget for strategy 4. 



47 



