middle-ground shoal does not. Thus the middle-ground shoal increases in 

 length as the inlet migrates, and the volume of material stored in the inlet 

 increases. 



When an inlet is deepened by dredging through the outer or inner bars or 

 through the channel, an additional storage capacity is created to trap avail- 

 able littoral drift and the quantity which would naturally pass the inlet is 

 reduced. If the dredged material is deposited in deep water or beyond the 

 limits of littoral currents, the supply to the downdrift shore may be nearly 

 eliminated. The resulting erosion is proportional to the reduction in rate of 

 supply. 



A common method of inlet improvement has been to flank the inlet channel 

 with jetties or breakwaters. These structures form a barrier to longshore 

 transport of littoral drift. Jetties have one or more of the following 

 functions: to block the entry of littoral drift into the channel, to serve as 

 training walls for inlet tidal currents, to stabilize the position of the 

 navigation channel, to increase the velocity of tidal currents and flush sedi- 

 ments from the channel, and to serve as breakwaters to reduce wave action in 

 the channel. Where there is no predominant direction of longshore transport, 

 jetties may stabilize nearby shores, but only to the extent that sand is 

 impounded at the jetties. The amount of sand available to downdrift shores 

 is reduced, at least until a new equilibrium shore is formed at the jetties. 

 Usually, where longshore transport predominates in one direction, jetties 

 cause accretion of the updrift shore and erosion of the downdrift shore. 



The stability of the shore downdrift of inlets, with or without jetties, 

 may be improved by artificial nourishment to make up the deficiency in supply 

 due to storage in the inlet. When such nourishment is done mechanically, 

 using the available littoral drift from updrift sources, the process is called 



sand bypassing. 



Types of littoral barriers (jetties and breakwaters) which have been 

 generally employed in connection with inlet and harbor improvement are shown 

 in Figure 5-8. If littoral transport predominates in one direction, any of 

 these types can cause accretion to the updrift shore and erosion of the 

 downdrift shore, unless a provision is made for sand bypassing. 



At a jettied inlet (Fig. 5-8, type I), bypassing can normally be performed 

 best by a land-based dredging plant or land vehicles. A floating plant can 

 be used only where the impounding zone is subject to periods of light wave 

 action, or by breaking into the landward part of the impoundment and dredging 

 behind the beach berm thus leaving a protective barrier for the dredge. Such 

 an operation was performed at Port Hueneme , California, in 1953 (see Ch. 6, 

 Sec. V, 2, a). In any type of operation at such a jettied inlet, it is 

 unlikely that bypassing of all the littoral drift can be attained; some 

 material will pass around the updrift jetty into the channel, especially after 

 the impounding capacity of the jetty has been reached. 



Dredging of a sand trap in the protected waters of an inlet or harbor 

 entrance (Fig. 5-8, type II) provides a practical sand-bypassing technique, 

 particularly when the inlet tidal currents are strong. These currents move 

 the sediment into the inlet where it is deposited into the sand trap. Peri- 

 odic dredging of the trap and depositing of the dredged material on the 



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