5.5 SAND BYPASSING 



5,51 GENERAL 



An inlet is a short narrow waterway connecting the sea or major 

 lake with interior waters. Inlets, either natural or improved to meet 

 navigation requirements, interrupt sediment transport along the shore. 

 Natural inlets have a well-defined bar formation on the seaward side of 

 the inlet. A part of the sand transpbrted alongshore ordinarily moves 

 across the inlet by way of this outer bar - natural sand bypassing. How- 

 ever, the supply reaching the downdrift shore is usually intermittent 

 rather than regular, and the downdrift shore is usually unstable for a 

 considerable distance. If the tidal flow through the inlet into the in- 

 terior body of water is strong, part of the material moving alongshore is 

 carried into and permanently stored in the interior body of water as a 

 middleground shoal, reducing the supply available to nourish downdrift 

 shores. The outer bar normally migrates with a migrating inlet, but the 

 middleground shoal does not. Thus the middleground 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, additional storage capacity is created to 

 trap available 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 down- 

 drift shore may be nearly eliminated. The resulting erosion is propor- 

 tional to the reduction in rate of supply. 



An often-used 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 chan- 

 nel, to serve as training walls for inlet tidal currents, to stabilize the 

 position of the navigation channel, to increase the velocity of tidal cur- 

 rents and flush sediments 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 avail- 

 able 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. 



Stability of the shore downdrift from 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 mechani- 

 cally by using the available littoral drift from updrift sources, the pro- 

 cess is called sand bypassing. 



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

 generally employed in connection with inlet and harbor improvement are 



5-24 



