368 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
marine scientists are closely linked, and it has been said that the 
domain of hydraulics extends seaward as far as low-water mark, 
whereas that of oceanography extends landward as far as high-water 
mark. The bottom current is often different from that at the surface, 
so that silt and salt creep up the river estuary in spite of the greater . 
outward flow of water at the surface. Better understanding of the 
mechanism by which both currents change speed and direction might 
well save some of the large sums spent on dredging. 
Experience and sound engineering are essential to the design of 
coastal defenses, but they will not be able to be used to the best ad- 
vantage until our basic knowledge of tides, waves, and beach currents 
reaches a more advanced stage. This lack of knowledge is usually 
offset by allowing a large margin of safety in engineering construc- 
tion; but this often puts up costs so high as to make them prohibitive. 
Wave recorders, which allow us to gain a better understanding of 
various types of beach movement, are now available and are being 
installed in many places where constructional work is planned. It 
has been found that short, steep waves are most effective in eroding 
beaches and depositing the eroded material at greater depths to sea- 
ward of the beach; low, long swell returns the material to the beach, 
but if it comes at an angle to the beach it is very effective in moving 
material along the coast. Similar factors influence the growth and 
movements of bars, which can be a nuisance in the approaches to a 
harbor. 
Tt is not likely that a simple and elegant solution can be found to 
all these problems. But most of the effort in coastal engineering aims 
at preventing beach material from being moved from some parts of 
the coast where it is needed to protect the land, to other parts where 
it is a hindrance to navigation; and it is only reasonable to assume 
that this work may be done with greater efliciency and economy when 
we have gained a better understanding of the physical processes in- 
volved. It may be added that a better knowledge of beach currents, 
especially the dangerous rip currents, would reduce the number of 
deaths from drowning. 
Coasts and harbors are affected by surges and long waves which 
are of a type intermediate between ordinary waves and tides. One of 
the most common types is the surf beat—an oscillation associated with 
the more or less regular occurrance of groups of high and low swell 
approaching an ocean coast. The water level rises and falls several 
inches—and in exceptional conditions nearly a foot—over a period 
of 2to 3 minutes. Many harbors have this surf beat, and the backward 
and forward oscillations can be sufficient to damage ships moored to 
a quay and tosnap the mooring ropes if these are not kept continuously 
tight. The surf can also set up unpredictable movements near harbor 
