correct. However, in observing the underwater groins between Borkum and 
Norderney, a difference in this attitude can be noticed which has not 
created disadvantages of a functional nature. 
Some modern authors consider the line of attack in design of groins; 
some attribute no meaning to the line of attack while other authors do 
not present the subject at all. 
The reason that a pliant line of attack is required in river engin- 
eering is the effort to achieve a regulated flow, particularly for the 
purpose of navigation. On beaches we deal with one shore only. The 
currents change direction and intensity, and even though of low velocity, 
they develop, in combination with the surf, a considerable transport 
Capacity, as the surf action provides the suspended sand. 
The importance of the line of attack is doubtful with regard to short 
groins which do not reach the offshore sand bars where the sand transport 
usually takes place. ; 
h, Construction Material - A variety of construction materials 
used in building seagroins is listed below: 
Wood: piles, spars, timber of available beach types, and brushwork 
(endangered by marine borers). 
Natural stone. 
Steel: gingle or multiple wall groins built of steel sheet piling; 
connecting elements of structural parts (endangered by rusting and 
corrosion). 
Concrete: used experimentally at the beginning of the 20th Century, 
since then systematically. 
Asphalt: for several years used for seagroin construction. 
When one construction material is being replaced progressively by 
another, it is usually because of its economic life. This is influenced 
by changes of wetness, solar radiation, temperatures, surge impact (wave 
action), surge currents, elevation changes of the shore, sand abrasion, 
ice drift and sea bottom composition. 
The economic life of steel and reinforced concrete for seagroins is 
generally only 10 to 20 years. 
Wood can be destroyed in two to three years, when used in regions 
subject to marine borers or other wood-damaging phenomena. Without this 
deteriorating effect, the durability can extend several centuries. 
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