b. Interruption of Material in Transport . This factor is probably the 

 most important cause of man-induced erosion. Improvement of inlets by both 

 channel dredging and channel control and by harbor structures impounds lit- 

 toral material (see Fig. 1-9). Often, the material is permanently lost from 

 the downcoast beach regime either by the deposition of dredged material 

 outside of the active littoral zone or the building of bars, shoals, and 

 wider updrift beaches. This can be mitigated by sand-bypassing systems. 

 Construction of protective works at the source of littoral material, such as 

 an eroding cliff or bluff, can also result in disruption of supply. Realine- 

 ment of the shoreline by the use of such structures as groins also interrupts 

 the transport of littoral material. These structures may not only reduce the 

 rate of a longshore transport but also may reduce littoral material reaching 

 downcoast beaches by entrapment. 



c. Reduction of Sediment Supply to the Littoral Zone . In some areas the 

 transport of sediment to the coast by rivers form the major source of material 

 to the littoral zone. Dams constructed on these rivers not only form sediment 

 traps but also reduce peak floodflows, thereby reducing the sediment supply to 

 the coast which results in coastal erosion. 



d. Concentration of Wave Energy on Beaches . The building of coastal 

 structures (such as a vertical wall) either in the active beach zone or on 

 the backshore can increase the amount of wave energy being dissipated by the 

 beach material fronting the structure, resulting in an increase in the rate of 

 erosion. 



e. Increase Water Level Variation . The deepening and widening of navi- 

 gation inlets may adversely affect the tidal range within a harbor or bay, 

 and may permit larger waves to enter the harbor area and adjacent beaches. An 

 increase in tidal range will expose more of the harbor or bay beach face 

 to the erosive effects of waves and cause a change in the beach profile. 



f. Change Natural Coastal Protection . The dredging of nearshore bars and 

 shoals can change the pattern of energy dissipation on a beach face. If the 

 change increases the wave energy acting on a given section of beach, erosion 

 will likely result at that section. Onshore, the leveling of dunes, 

 destruction of beach vegetation, paving of large backshore areas, and con- 

 struction of boat channels on the backside of a narrow barrier island can 

 further increase the overwash erosion and island breaching potential. 



g. Removal of Material from the Beach . Excavation of beach material is 

 undertaken in many parts of the world. This material is sometimes mined for 

 the minerals it contains; in other places it is used for construction purposes 

 (landfills, construction aggregate). For whatever purpose, it is a direct 

 loss of available supply of material for littoral transport. 



VI. COASTAL PROTECTION METHODS AND NAVIGATION WORKS 



The sloping beach and beach berm are the outer line of defense in absorbing 

 most wave energy; dunes are the last zone of defense in absorbing the energy 

 of storm waves that overtop the berm. Although dunes erode during severe 

 storms, they are often substantial enough to afford complete protection to the 



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