Figure 1-11. Undermining of structures by storm waves, Potham Beach, Maine. 



For ocean-exposed locations vertical bulkheads alone do not provide a long- 

 term solution because of foreshore erosion and flanking. Unless combined with 

 other types of protection, the bulkhead must be enlarged into a massive 

 seawall capable of withstanding the direct onslaught of the waves. Seawalls 

 may have vertical, curved, stepped, or sloping faces. Although seawalls 

 protect the upland, they often create a local problem. Downward forces of 

 water, produced by waves striking the wall, can rapidly remove sand from in 

 front of the wall. A stone apron is often necessary to prevent excessive 

 scouring and undermining. 



A revetment armors the existing slope face of a dune or embankment. It 

 is usually composed of one or more layers of quarrystone or precast concrete 

 armor units, with a filter layer overlaying a graded in situ soil slope. 

 Revetments are of little benefit if placed at the toe of a marginally stable 

 slope since they are usually only a protective armor and not a retaining 

 structure. Because the sloping face of the quarrystone revetment is a good 

 energy dissipater, revetments have a less adverse effect on the beach in front 

 of them than a smooth-faced vertical bulkhead. 



1-21 



