Conventional revetments typically 

 provide protection from well above the 

 mean high water line to well below the 

 mean low water line. Conventional re- 

 vetments thus extend from the terres- 

 trial zone to the subtidal zone. Upper 

 beach revetments extend from above the 

 mean high water line to an area between 

 the mean high water line and the mean 

 low water line. This type of revetment 

 generally lies within the region extend- 

 ing from middle intertidal zone to the 

 terrestrial zone. Revetments can also 

 be used entirely above the mean high 

 water line for protection against storm 

 generated tides. Revetments are usual- 

 ly constructed parallel to the natural 

 shoreline. 



Placement Constraints 



Engineering . Several factors should 

 be considered when evaluating the de- 

 sign of a revetment. Design considera- 

 tions include design life of structure, 

 design wave, seasonal changes in beach 

 profile, water level range (e. g. .changes 

 due to tides, storms, and for the Great 

 Lakes seasonal lake level), beach compo- 

 sition, and beach use (McCartney 1976). 

 Once these site conditions are known, 

 alternate types of revetments may be 

 evaluated. Armor facing requirements, 

 wave runup heights, toe scour depth, 

 toe protection needs, revetment slope, 

 revetment length, and filter require- 

 ments vary with different types of re- 

 vetments. 



Revetment slope length and place- 

 ment on the shoreline should be such 

 that waves do not overtop the structure 

 and erode away the supporting beach or 

 saturate the soil and cause structural 

 failure due to the hydraulic processes. 

 Wave runup, an important factor in the 

 determination of revetment slope length, 

 depends upon water depth at the toe of 

 the structure, slope of the beach in 

 front of the structure, and the slope, 

 shape, roughness, and porosity of the 

 revetment (U.S. Army Corps of Engi- 

 neers 1973b, McCartney 1975). Other 

 factors which determine revetment slope 

 length include water level range, beach 

 slope, toe scour depth, and minimum 

 water depth allowed at the toe of the 

 structure (McCartney 1976). 



Toe protection is necessary to pre- 

 vent scouring at the base and to protect 

 the structure against changing beach 

 profiles. Revetments possess very little 

 internal stability, relying on the un- 

 derlying beach which they protect (Fig- 

 ure 32). Undermining of the structure 

 at its toe can lead to failure of the 

 entire structure. Wave energy is de- 

 flected both landward and seaward as 

 waves break against revetments. Wave 

 energy which is deflected seaward can 

 cause scouring of material at the foot 

 of revetments (U.S. Army Corps of Engi- 

 neers 1973b). Factors affecting the 

 amount of toe scour include slope, per- 

 meability and roughness of the revet- 

 ment, water depth, hypothetical surface 

 of wave reflection, wave height and 

 steepness, and beach sand size (McCartney 

 1976, Sato et al. 1968). 



In general, rougher, flatter, and 

 the more permeable revetment surfaces 

 cause less toe scour and require less 

 toe protection. Structural failure due 

 to scour may be avoided by incorporating 

 adequate toe protection into the design 

 of revetments. Common toe protection 

 methods are addressed in the construc- 

 tion materials section. 



The supporting materials under 

 structures may also be washed away if an 

 adequate filter is not used. A filter 

 prevents undermining of the revetment, 

 distributes armor unit weight, and pro- 

 vides for relief of hydrostatic pres- 

 sures (Collier 1975, McCartney 1976). 

 Ideally, a filter layer prevents scour- 

 ing of supporting shore material and al- 

 lows water drainage. The amount and type 

 of filter material needed is determined 

 by beach composition, water depth, type 

 of armor units, and current velocities. 

 In areas of heavy wave action, armor 

 units are often placed on a scour pad of 

 plastic filter material (filter cloth) 

 and stone. Special care must be taken 

 in design and construction of imperme- 

 able revetments to prevent excessive 

 landward hydrostatic pressure. Design- 

 ing the structure with gravel or with 

 rock weep holes are ways to help prevent 

 this potential problem (McCartney 1976). 



Materials used for armor facings 

 should be designed to remain intact 



62 



