5.27 HEIGHT OF STRUCTURE 



Seawalls, bulkheads, and revetments can be built so high that no 

 water would overtop the crest of the structure, regardless of severity of 

 wave attack and storm-surge levels, though it is sometimes not economically 

 feasible to do so. Wave runup and overtopping criteria on which the height 

 of a structure should be based can be estimated from data presented in 

 Section 7o2 "WAVE RUNUP, OVERTOPPING, AND TRANSMISSION." Specific model 

 tests for the design case can be carried out if greater detail or accuracy 

 is warranted. 



5.28 DETERfnNATION OF GROUND ELEVATION IN FRONT OF A STRUCTURE 



Seawalls and revetments are usually built to protect a shore from 

 the effects of continuing erosion and to protect shore improvements from 

 damage by wave attack. The exact effect of such a structure on erosion 

 processes cannot be fully determined, but can be estimated by the method 

 given in this sectiono For safety, even though erosion processes seem 

 to have been halted or reversed, the designer must assume that they will 

 continue. A determination of the beach profile that will exist after 

 construction of the structure can be estimated through experience, obser- 

 vations, and general guides. 



Scour may be anticipated at the toe of the structure as an initial 

 short-term effect. Scour will form a trough with dimensions governed by 

 the type of structure face, the nature of wave attack, and the resistance 

 of the bed material. At a rubble-mound seawall, scour may undermine the 

 toe stone, causing it to sink to an ultimately lower stable position. The 

 resultant settl jment of stone on the seaward face may be offset by over- 

 building the cross-section to allow for settlement. Another method is to 

 provide excess stone at the toe to fill the anticipated scour trough. The 

 face of a vertical structure may be protected similarly against scour by 

 the use of stone, A gravity wall must be protected from undermining by 

 scour by providing impermeable cutoff walls at the base. As a general 

 guide y maximum depth of a sooup trough below the natural bed is about 

 equal to the height of the maximum unbroken wave that can be supported 

 by the original depth of water at the toe of the structure. For example, 

 if the controlling depth of water seaward of the face of the structure is 

 10 feet, the offshore bottom slope is 1 on 30, and a design wave period 

 of 8 seconds is assumed, the maximum unbroken wave height that can be 

 supported is 10.4 feet. (See Section 7.1.) Tlierefore, the maximum depth 

 of scour at the toe of the structure would be 10.4 feet below the original 

 bottom, or a total of 20.4 feet below the design water level. The place- 

 ment of a rock blanket with an adequate bedding material seaward from the 

 toe of the structure will prevent erosion at the toe, and will result in 

 a more stable structure. (See Section 7.3 for design methods.) 



For long-term effects, it is preferable to assiome that the structure 

 would have no effect on reducing the erosion of the beach seaward of the 

 wall. Such erosion would continue as if the wall were not there. Since 

 the determination of scour can only be approximate, general guides are 

 usually adopted. 



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