As for bulkheads, the critical design condition is the tendency of 

 the wall to move outward under the driving force of active earth pres- 

 sure and unbalanced water pressure. The combined effects of these 

 forces on seawalls are determined in the same way as for bulkheads. 

 The forces due to oncoming wave action do not establish a critical 

 design condition because any tendency for the wall to move shoreward is 

 resisted by the force of passive earth pressure behind the sheet piles. 



In locations subject to "rare" occasions of severe storms that raise 

 the general water level by several feet (storm surge) , it is not econom- 

 ical or practical to build timber seawalls sufficiently high to prevent 

 wave overtopping. Consequently, it may be necessary to replace eroded 

 backfill after the storm surge recedes. 



In establishing the allowance for unbalanced water pressure, due 

 consideration should be given to the past history of unusual water 

 levels in a particular locality. For such abnormal conditions, a reduc- 

 tion in the usual safety factors can be tolerated. 



2. Treated Timber in Concrete Seawalls . 



Figure 9 is a seawall for intermediate exposure to wave action. 

 This is a rigid-type cross section of reinforced concrete with stepped 

 face. The structure is supported on round, treated timber piles and has 

 a treated timber sheet-pile cutoff wall at the toe. This type of sea- 

 wall has been used for exposure to 6- foot waves in storms along the gulf 

 coast. The parapet wall may be added to prevent overtopping if scouring 

 of the backfill material occurs as the result of unusually high wave 

 action during storm surge. Adequate drainage is required behind the 

 seawall for removal of surface water from rain and overtopping waves. 



IV. DESIGN OF GROINS 



Groins are fingerlike barrier structures built perpendicular to the 

 shoreline for extending and maintaining a protective beach. 



At locations where the supply of sediments is not sufficiently large 

 to fill a groin system without causing erosion of downdrift shores, 

 artificial filling for the groin system is required so that the natural 

 supply of sediments in transport may pass without reduction in volume. 

 Lawsuits by owners of eroded property may be expected. 



1. Types . 



Groins are classified as impermeable, or permeable corresponding to 

 their tightness as barriers to sediment movement. The current flow is 

 completely obstructed by impermeable groins. Openings through permeable 

 groins permit partial current flow and passage of a part of the sus- 

 pended sediments past the barrier, often resulting in deposits on both 

 sides of the groin. The sizes and spacings of the openings are some- 

 times varied to increase the flow progressively from shore to the seaward 



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