exceeds other volume rates in the sediment budget, but if the beach is 

 approximately in equilibrium, this may not be easily noticed. 



The erosion of beaches and cliffs and river contributions are the 

 principal known natural sources of beach sand in most localities. Inlets, 

 lagoons, and deep water in the longshore direction comprise the principal 

 known natural sinks for beach sand. Of potential, but usually unknown, 

 importance as either a source or a sink is the offshore zone seaward of the 

 beach. 



The works of man in beach replenishment and in mining or dredging may 

 provide major sources or sinks in local areas. In a few U.S. localities, 

 submarine canyons or wind may provide major sinks, and calcium carbonate 

 production by organisms may be a major source. 



*************** EXAMPLE PROBLEM 5************** 

 GIVEN ; 



(a) An eroding beach 7.1 kilometers (4.4 miles) long at root of spit that 

 is 16.1 kilometers (10 miles) long. Beaches on the remainder of the 

 spit are stable, and the tip of the spit is accreting (see Fig. 4-50a.) 



(b) A uniform recession rate of 0.9 meter (3 feet) per year along the 

 eroding 7.1 kilometers. 



(c) Depth of lovEst shore-parallel contour is -9.1 meters (-30 feet) MSL, 

 and average dune base elevation is 4.6 meters (15 feet) MSL. 



(d) Sand is accumulating at the tip of the spit at an average rate of 

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



(e) No sand accumulates to the right of the erosion area; no sand is lost 

 to the offshore. 



(f) A medium-width jettied inlet is proposed which will breach the spit as 

 shown in Figure 4-50. 



(g) The proposed inlet is assumed to trap about 15 percent of the gross 

 transport Q^ . 



(h) The 2.1-kilometer- (1.3-mile-) long beach to the right of the jettied 

 inlet will stabilize (no erosion) and realign with y at the inlet 

 assumed to be 3.5 (see eq. 4-34). 



(i) The accumulation at the end of the spit will continue to grow at an 

 average annual rate of 305,000 cubic meters (400,000 cubic yards) per 

 year after the proposed inlet is constructed. 



FIND: 



(a) Annual littoral drift trapped by inlet. 



(b) After-inlet erosion rate of the beach to the left of the inlet. 



4-129 



