estimated from site hydrography, measured current velocities, and model 

 studies (Hudson et al., 1979). Special attention must be given to sections of 

 the structure where scour is intensified; i.e. to the head, areas of a section 

 change in alinement, bar crossings, the channel sides of jetties, and the 

 downdrift sides of groins. Where waves and currents occur together, Eckert 

 (1983) recommends increasing the cover size by a factor of 1.5. The stone 

 size required for a combination of wave and current scour can be used out to 

 the width calculated for wave scour protection; smaller stone can be used 

 beyond that point for current scour protection. Note that the conservatism of 

 the apron width estimates depends on the accuracy of the methods used to 

 predict the maximum depth of scour. 



(7) Revetments . Revetments commonly are typically the smallest and 

 most lightly armored of coastal protective structures, yet their failure leads 

 directly to loss of property and can put protected structures in jeopardy. 

 They commonly are constructed above the design water level or in very shallow 

 water where their toes are likely to be exposed to intense wave and current 

 forces during storms. For these reasons, their toes warrant special pro- 

 tection. 



Based on guidance in EM 1110-2-1614 (U.S. Army Corps of Engineers, 1984), 

 the cover for the toe apron of a revetment exposed to waves in shallow water 

 should be an extension of the lowest cover layer on the revetment slope. Only 

 the cover thickness is varied to increase stability. The toe apron should be 

 buried wherever possible, with the revetment cover layer extended into the 

 bottom for at least the distance of 1 meter or the maximum expected unbroken 

 wave height, whichever is greater. If scour activity is light, the thickness 

 of the cover on the buried toe can be a minimum of two armor stones or 50 

 percent size stones in a riprap gradation, the same as on the slope. For more 

 intense scour, the cover thickness should be doubled and the extension depth 

 increased by a factor of up to 1.5. For the most severe scour, the buried toe 

 should be extended horizontally an additional distance equal to twice the 

 toe's depth, that is, 2 to 3 times the design wave height (see Fig. 7-121). 



If the apron is a berm placed on the existing bottom and the cover is 

 quarrystone armor, the cover thickness may be as little as one stone and the 

 apron width may be three to four stones. A thickness of two stones and a 

 width equal to that of a buried toe is more conservative and recommended for a 

 berm covered by riprap. For the most severe wave scour the thickness should 

 be doubled and a width equal to 3 to 4.5 design wave heights used, as 

 illustrated in Figure 7-121. According to EM 1110-2-1601 (U.S. Army Corps of 

 Engineers, 1970), the width of a toe apron exposed to severe current scour 

 should be five times the thickness of the revetment cover layer, whether the 

 toe is buried or a berm. 



If a geotextile filter is used beneath the toe apron of a revetment or a 

 structure that passes through the surf zone, such as a groin, the geotextile 

 should not be extended to the outer edge of the apron. It should stop about a 

 meter from the edge to protect it from being undermined. As an alternative, 

 the geotextile may be extended beyond the edge of the apron, folded back over 

 the bedding layer and some of the cover stone, and then buried in cover stone 

 and sand to form a Dutch toe. This additionally stable form of toe is 

 illustrated as an option in Figure 7-121. 



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