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. locali- 

 ties, submarine canyons or wind may provide major sinks, and calcium car- 

 bonate production by organisms may be a major source. 



************** EXAMPLE PROBLEM *************** 



GIVEN: 



(a) An eroding beach 4.4 miles long at root of spit that is 10 miles 

 long. Beaches on the remainder of the spit are stable. (See 

 Figure 4-50a.) 



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

 4o4 miles. 



(c) Depth of lowest shore parallel contour is -30 feet MSL, and 

 average dune crest elevation is 15 feet MSL. 



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

 400,000 cubic yards per year. 



(e) The variation of y along the beaches of the spit is shown in 

 Figure 4-51. (y = Q.it/^rt'' Equation 4-23.) 



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

 lost to the offshore. 



(g) A medium-width jettied inlet is proposed which will breach the 

 spit as shown in Figure 4-50a. 



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

 gross transport, Q^. 



(i) The 1.3-mile long beach to the right of the jettied inlet will 

 stabilize (no erosion) and realign with y changing to 3.5. 



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

 at an average annual rate of 400,000 cubic yards per year after 

 the proposed inlet is constructed. 



4-134 



