A vertical or nearly vertical face structure lends itself to use as a 

 quay wall, docking or mooring place. Where a light structure is required, 

 a vertical face (of sheet piling, for example) may often be constructed 

 more quickly and more cheaply than other types. This ease or speed of 

 construction is important where emergency protection is needed. A verti- 

 cal face is less effective against wave attack, and specifically against 

 overtopping, than the concave curved and reentrant face. The use of 

 vertical or nearly vertical face walls can result in severe scouring when 

 the toe or base of the wall is in shallow water. Waves breaking against 

 a wall deflect energy both upward and downward. The downward component 

 causes scouring erosion of the material at the base of the wall. To pre- 

 vent scouring, toe protection should be provided in the form of a toe or 

 armor stone of adequate size to prevent displacement, and of such grada- 

 tion as to prevent the loss of the foundation material through the voids 

 of the stone and consequent settlement of the stone. 



Convex curved face and smooth slopes are least effective in reducing 

 wave runup and overtopping. The rubble sloping seawall and revetment is 

 effective in dissipating and absorbing wave energy, and reduces wave run- 

 up and overtopping. Sloping face structures, generally reduce scouring, 

 and may have an advantage over vertical face structures. 



Concave curved or reentrant faced structures are the most effective 

 for reducing wave overtopping when onshore winds are light. Where the 

 structure crest is to be used for a road, promenade, or other purpose, 

 this design may be the best shape for protecting the crest and reducing 

 spray. This is especially true if the fronting beach is narrow or non- 

 existant, or if the water level is above the structure base. If onshore 

 winds occur at the same time as high waves, a rubble slope should also 

 be considered. 



A stepped-face wall provides the easiest access to beach areas from 

 protected areas, and reduces the scouring of wave backwash. 



5.25 LOCATION OF STRUCTURE WITH RESPECT TO SHORELI NE 



A seawall, bulkhead, or revetment is usually constructed along that 

 line landward of which further recession of the shoreline must be stopped. 

 Where an area is to be reclaimed, a wall may be constructed along the sea- 

 ward edge of the reclaimed area. 



5.26 LENGTH OF STRUCTURE 



A seawall, bulkhead, or revetment protects only the land and improve- 

 ments immediately behind it. These structures provide no protection to 

 either up- or downcoast areas as do beach fills. Usually, where erosion 

 may be expected at both ends of a structure, wing walls or tie-ins to 

 adjacent land features must be provided to prevent flanking and possible 

 progressive failure of the structure at the ends. Short-term beach changes 

 due to storms, as well as seasonal and annual changes, are design con- 

 siderations. Erosion updrift from such a structure will continue unabated 

 after the wall is built, and downdrift erosion will probably be intensified. 



5-4 



