Permeable groins are usually built high, the top serving as a recrea- 
tion facility such as a fishing platform or sun deck. This kind of groin 
has openings through it from top to bottom over its entire length to permit 
beach material to pass through. The effectiveness of this structure com- 
pared to the low impermeable groin has not been fully demonstrated to the 
satisfaction of most coastal engineers, and it is interesting to note that 
the Beach Erosion Board has never recommended construction of permeable 
groins in its 33-year history. 
Breakwaters both shore-connected and detached (offshore) are used to 
provide shelter for anchorages and harbor entrances. 
The inner end of the shore-connected breakwater acts in the same 
manner as a jetty (Figure 12). It impounds the littoral material on the 
updrift side and thus excludes these materials from the harbor area. A 
structure of this nature, high and sand-tight, is a total littoral barrier, 
and its construction will result in accretion updrift and erosion downdrift. 
Eventually the impounded material will move along the seaward face of the 
structure and accumulate in the harbor area. Through harbor maintenance 
by pipeline dredge the material can be spoiled downdrift to nourish the 
eroding shore, effecting a sand-bypassing system. The seaward portion of 
the structure or that part roughly paralleling the shore acts essentially 
as an offshore breakwater absorbing the wave energy to create a calm anchor- 
age or harbor area in its lee. 
Offshore breakwaters have been designed more recently for the dual 
purposes of providing shelter to a harbor entrance and to provide a pro- 
tected littoral reservoir from which sand-bypassing operations can be 
conducted (Figure 13). In this application the offshore breakwater is 
installed seaward and updrift of the harbor entrance to intercept wave 
action which would normally strike the shore in the area. This arrange- 
ment provides a relatively calm approach for ships entering the harbor. 
By intercepting the wave action, it also results in arresting the flow of 
littoral material which impounds in the form of a bulge on the shore op- 
posite the center of the structure. Thus it can be seen that this struc- 
tural arrangement improves conditions by stilling the sea and preventing 
littoral material from moving into the navigation channel. 
It has long been recognized that littoral barriers are detrimental to 
the downdrift shores and that where they exist some sort of sand-bypassing 
system should be used. The floating hydraulic pipeline dredge is ideal 
equipment for moving sand from one location to another, but it cannot be 
safely used near shore in an exposed location. In other words it would be 
impracticable to dredge material impounded by a jetty. Dredging behind an 
offshore breakwater is another matter. The shelter afforded by such a 
structure is conducive to bypassing by pipeline or hopper dredge. It is 
considered that this type of bypassing system is superior to others now in 
service. 
28 
