generalized schematic of the stages of lagoonal filling behind a barrier 

 island by tidal inlet flood deposits is presented in Figure 4-86). Thus, 

 while the ebb-tidal delta or outer bar, being subject to the action of the 

 currents and waves of the sea, generally does not increase beyond a definite 

 stage; the flood-tidal delta, being subject to much milder forces in the bay, 

 often continues to increase, even to the complete filling of the interior 

 waters by the formation of marshes and the closure of the inlet. While this 

 progressive deterioriation of interior bays and sounds on sandy shores may not 

 be a rapid process as measured by the span of human life, it is a very rapid 

 process geologically as many of these interior waters show a marked deteriora- 

 tion within the century after their creation. 



Drum Inlet, North Carolina, is an example of an inlet which deteriorated 

 very rapidly, became hydraulically unstable, and eventually closed. Drum 

 Inlet shoaled closed within 5 years after excavation. The volume of sand 

 stored in the flood shoals as measured from a survey made 5 years after the 

 inlet was opened was calculated to be 1,600,000 cubic meters (2,100,000 cubic 

 yards) (Foreman and Machemael, 1972). Foreman and Machemael (1972) noted that 

 the material in the flood shoal was similar in quality to, although somewhat 

 finer than, that on the beaches adjacent to the inlet and that the median 

 material size and standard deviation of size decreased with distance from the 

 inlet throat. 



*************** EXAMPLE PROBLEM 7*************** 

 GIVEN : Plans have been made to construct a new unstructured inlet on the east 



coast of the U.S., through a barrier island into the back lagoon. The 

 overall bay surface area which the inlet will serve is 3 x 10 square meters 

 (9.8 X 10 square feet). The ocean tide range is 1.3 meters (4.26 feet). 



FIND : A rough approximation of the volume of sand which will eventually be 

 "captured" from adjacent beaches by the ebb-tidal delta of the inlet system. 



SOLUTION : Use equation (4-72) for the Atlantic coast to calculate ¥ . 

 A (conservative) approximation for tidal prism P is 



P = (Ocean tide range) x (Bay area) 



P « 1.3 meters x (3 x 10 square meters) = 3.9 x 10 square meters 



From equation (4-72a) 



¥ = 2.384 X 10"'^ (3.9 x lO^)^*^^ = 518,000 cubic meter (677,600 



cubic yards) 

 of beach material. 



*************************************** 

 6. Channel Shoaling and Dredging Effects . 



When a channel is dredged through a tidal inlet, increased shoaling is 

 expected to occur in the channel over and above that which would occur in the 

 natural channel. Little research exists on this subject, although one 

 untested methodology for predicting channel shoaling has been presented by the 

 U.S. Army Engineer District, Wilmington (1980). It should be noted that the 



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