IV. REVETMENT TOE 



The revetment toe often requires special treatment because the toe is 

 subjected to both hydraulic forces and the changing profiles of the beach 

 fronting a revetment. Toe stability is an essential consideration since 

 toe failure usually triggers failure of the upper revetment. Toe protec- 

 tion is generally difficult to construct since it is usually underwater 

 and must be placed without visual guidance. 



The mechanisms which cause toe scour are wave breaking, if near the 

 toe, and the runup and runback involved in reflection of the incident 

 wave. Incident wave runup and runback are dependent on the revetment 

 roughness and slope, water depth, and wave steepness. Therefore, wave 

 reflection is also dependent on these parameters. An estimate of wave 

 reflection for each revetment type is presented in the Appendix. These 

 estimates compare revetment porosity and roughness for revetments of the 

 same slope. The present knowledge of the causes and effects of toe scour 

 is too limited to estimate reflection coefficients for both revetment 

 slope and roughness. Generally, the flatter the revetment, the lower the 

 wave reflection, and the smaller the toe scour due to runup and runback. 

 Also, the rougher and more permeable the revetment surface, the lower the 

 wave reflection, and smaller the toe scour. A flat slope, rough, perme- 

 able revetment should have the least toe scour. 



Another factor that affects toe scour-hole size is the beach sand 

 size that fronts the revetment. Fine-grain sand beaches are more erod- 

 able than coarse- grain beaches. 



Toe-scour problems can be avoided by adequate toe protection in the 

 revetment design. Some common types of toe protection are shown in 

 Figure 4. 



V. REVETMENT DESIGN CONSIDERATIONS 



If a revetement is selected as the most suitable means of protecting 

 an eroding shore front, the next step is to determine the best revetment 

 type. This requires an analysis of how alternative revetments will inter- 

 act with site conditions and the resulting cost of these revetments. 

 Considerations for an adequate revetment design are: 



(a) Design Life of Project . Design life is short for tem- 

 porary structures like embankment protection for a coffer dam, 

 but long for nuclear powerplant embankment revetments. 



(b) Design Wave . Select a wave height and wave period con- 

 sistent with the project design life. The wave characteristics 

 are based on analysis of wave gage, observed wave data, or wave 

 hindcasts as described in the SPM, chapter 3. This report 

 assumes the zero-damage wave height (see App.) is measured at the 

 revetment toe. The design wave is needed to determine the size 

 of the revetment armor units, estimate wave runup, and predict 

 potential toe scour. 



10 



