2. Postproject Sediment Budget. 



After construction of a sand-bypassing system, the amount of sand trans- 

 ferred from the updrift to downdrift beach can be controlled by the amount of 

 dredging performed; consequently, the amount of bypassing is no longer an 

 unknown in the sediment budget but depends on the operation plan for the 

 project. Likewise, the amount of sand entering and leaving the system at the 

 extreme ends of the adjacent beaches can, as a first approximation, be assumed 

 to be unaffected by inlet modifications. The Beaufort Inlet example is used 

 to illustrate four project operation strategies (see Fig. 24). It is assumed 

 that the net amount of sand accumulating in the inlet is zero and that bypass- 

 ing is from west to east (i.e., from Bogue Banks to Shackleford Banks). The 

 optimum system is one that will keep sediment from the navigation channel, 

 minimize any adverse effects of the navigation structures on adjacent beaches 

 (to preclude updrift or downdrift beach erosion), and minimize the amount of 

 dredging required to bypass or backpass (pump sand from the inlet to updrift 

 beaches) sand. 



No Loss to Inlet 

 A 



BOGUE BANKS 



1.1 Q E 

 0.26 Q E 



/ 



No Beoch 

 Nourishment 



Figure 24. 



No Hopper Dredging 

 (All Bypassing from West 

 to East - B w = 0; Weir on 

 Bogue Banks Side ) 



Typical sediment budget conditions 

 after weir-jetty construction. 



a. Strategy 1 . A first bypassing strategy at Beaufort Inlet might be to 

 limit updrift beach losses to those that result from sea level rise (i.e., 

 losses that occur naturally along the beach away from the influence of the 

 inlet) which specifies that the amount of sand lost from Bogue Banks is 32,000 

 cubic yards per year. The volume change occurring on Shackleford Banks, A¥ , 



and the amount of sand to be bypassed, 



E> 



become the unknowns in the 



resulting system of two simultaneous equations (see Fig. 

 for Bogue Banks is 



25) . The equation 



415.8 (gain from west) - 98.3 (loss to west) - 32 (lost offshore) 

 - Bg = -32 (net volume lost) 

 and for Shackleford Banks is 



B E - 33 (lost offshore) - 223.0 (lost to east) 

 + 79.4 (gain to east) = AV g 



44 



