across the boundary between Reaches 3 and 4 is 



*+ 

 ^n(3,4) ~ \(2,3) "^ ^3(3) 



(3) ("^3(3)) 



^n(3.4) = \(2.3) + ^ 



Q ._ ,. = 243,400 cubic yards per year + (5 kilometers) (1000 meters 

 ' per kilometer) (12.33 cubic meters per year per meter) 



Q .„ ,, = 305,000 cubic meters per year (400,000 cubic yards per year) 



This Q„C3 4) moves left across Reach 4 with no additions or subtractions, 

 and since the accretion rate at the end of the spit is 305,000 cubic meters 

 per year, the budget balances. The bef ore-inlet sand budget is shown in 

 Figure 4-50b. Now the aftev-inlet condition can be analyzed. 



Q ,, ,N = 217,500 cubic meters per year (same a "before-inlet") 



Q -.- o\ = Q /] n) ~ 217,500 cubic meters per year (284,500 cubic 

 ' * yards per year) (Reach 2 is stable) 



The gross transport rate across the inlet with the new y = 3.5 , using 

 equation (4-34), is: 



Q =Q,. ^^^ 



V ^n (1 - Y) 



V2,3) (1 - Y) 



Qn(2,3)^^^^^ 

 (1 - Y) 



Q .„ „, = (217,500 cubic meters per year ( „ ' 1 



Q (2 2) " 319,500cubic meters per year (512,000 cubic 

 ^ ' yards per year 



The inlet sink Q„ = 15 percent of Q ,^ -.^ 

 2 *^ V(2,3) 



Q = 391,500 cubic meters per year x 0.15 



Q„ = 58,700 cubic meters per year (76,800 cubic 



yards per year) 



The erosion value from Reach 3 now becomes 



Reach 3 erosion = spit end accretion 

 + inlet sink 

 - net littoral drift right of inlet 



4-132 



