prevent the failure that can occur if the bottom material is eroded to 

 the extent that the sheet piling may be shoved out of place. 



(4) Water Level in the Backfill . The ground water level in 

 the backfill will rise and fall as the tide rises and falls. This is 

 caused mostly by natural percolation of water through the soil, behind 

 the lower parts of the bulkhead wall, rather than by water passing 

 through cracks in the bulkhead. Because the soil slows down the flow 

 of water somewhat, the ground water level behind the bulkhead is seldom 

 the same as the free water level on the seaward side. As the tide rises, 

 the ground water level rises, but at a slower rate. Similarly, as the 

 tide recedes, the ground water level falls, also at a slower rate. The 

 distance between these two water levels is called the water lag 



(Figure 4). 



When the free water is higher than the ground water there is no 

 particular design problem. The water pressure is resisted by the soil 

 backfill. But when the ground water level is higher than the free water, 

 there is an outward pressure that is resisted only by the wall and its 

 anchorage system. 



The net effect of the ground water pressure is similar to that of 

 the backfill against the bulkhead. The outward pressure of both water 

 and soil must be considered in the design of a bulkhead wall. 



To determine the ground water level, dig a hole behind the sheet piles 

 of any nearby bulkhead. Water will rise and fall to measurable levels. 

 If no bulkheads exist in the vicinity, use a minimum value of 1 foot, 

 or a maximum of one-half the tidal variation, for the water lag. 



As the water level recedes during ebbtide, the water in the ground 

 behind the sheeting will escape through any openings left in the wall, 

 carrying away the finer particles of the soil backfill. During numerous 

 tidal interchanges, large quantities of backfill can escape in this 

 manner, unless tight joints are provided between sheet piles. Joints 

 used to minimize losses are shown in Figure 7. 



(5) Finished Elevations . The criteria used to determine the 

 basic vertical dimensions of a bulkhead are shown in Figure 4. The 

 finished elevation of the bulkhead and backfill was listed by making an 

 allowance for wave action above the high water level. 



Wave height is a function of wind velocity and duration, of expanse 

 of water {fetch) over which the wind blows, and of water depth. Methods 

 for estimating wave height and period under various site conditions are 

 described in the Shore Protection Manual (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1973). 



In protected; areas, where treated timber bulkheads have their best 

 application, the length of fetch and water depth are limited. In most 

 cases, the expected wave heights range from 1 to 2 feet. 



17 



