as a structure because it is built by man. It has long been recognized 
that a beach of proper proportions, including an adequate berm backed by 
dunes, is man’s and nature's best solution to the erosion problem. Where 
possible, an eroded natural beach is rebuilt to the exact characteristics 
of the original beach. In other words, the borrow material is selected to 
conform as nearly as possible with material comprising the natural beach. 
The dimensions of the beach, such as slope, height, and breadth of berm 
are adhered to as closely as possible where similar beach materials are 
available. Where the characteristics of the borrow and native beach ma- 
terials are not the same, the constructed beach is placed to such dimen- 
sions that the material available will be stable under prevailing coastal 
processes as is illustrated by Krumbein's Figures 5 and 7 (foregoing paper). 
The widespread use of constructed beaches has to a large degree been 
brought about by the increasing construction costs of other types of struc- 
tures and the need of beaches for recreational use. The beach at Harrison 
County, Mississippi, illustrated in Figure 6, is a classic example of this 
type of structure. 
BEHAVIOR OF INDIVIDUAL STRUCTURES 
The structures enumerated all have specific functions to perform and 
all react differently in use. The mission of the seawall is to protect the 
upland area and is not expected to protect and preserve the beach (Figure 
7). It is a massive structure designed to take the full impact of the wave 
and is not designed to modify the littoral forcés in the area to preserve 
or enlarge the beach. When a wave breaks on a vertical seawall the energy 
which throws water high into the air also permits it to move downward with 
an equal force, thus eroding the sand at the toe of the wall and carrying 
it seaward. Erosion usually continues until the bottom material ceases to 
be affected by the downward velocities. A great deal of effort has been 
expended in seawall modification to reduce this erosive feature associated 
with it. Modification has helped to some extent, but a seawall should never 
be used alone as protection if the retention of the beach is desirable. 
Bulkheads behave in the same manner as seawalls but are not used 
directly as a bulwark against the sea (Figure 8). They are usually placed 
well back of the waterline to retain low bluffs or dunes and to serve as a 
secondary line of defense during storms - the first line of defense is the 
beach. However, if the beach erodes to a point where the wave strikes the 
structure the bulkhead becomes a seawall and rapid toe erosion normally 
takes place. Since the bulkhead is not designed as a bulwark against the 
sea, the structure generally fails by falling seaward after damaging scour 
or erosion at the base of the structure has occurred. 
A revetment is an alternative to seawalls and bulkheads. Like sea- 
walls and bulkheads, revetments protect the shore area from direct attack 
but they do not modify the littoral regimen of the area to protect or 
preserve the beach (Figure 9). Revetments are generally sloping permeable 
22 
