Chapter 2
Ocean Surface Waves
Ocean surface waves cause periodic loads on all sorts of man-made structures in the sea.
It does not matter whether these structures are …xed, ‡oating or sailing and on the surface
or deeper in the sea. To understand these loads, a good understanding of the physics of
water waves is necessary.
Looking over the sea, one gets the impression that there is an endlessly moving succession
of irregular humps and hollows reaching from horizon to horizon. If the winds are light, the
irregularities are small. If the winds are heavy, you may be awed by the resulting gigantic
stormy seas. Since water moves ”easily” and ‡at calms seldom occur, an undisturbed
water surface is rarely found at sea. Even when drifting in glassy calm, one will usually
…nd the ocean heaving itself in a long smooth swell whose source is a storm which may
have occurred days before and hundreds of miles away.
Ocean surface waves are generally distinguished in two states: sea or wind waves, when
the waves are being worked on by the wind that raised them and swell, when they have
escaped the in‡uence of the generating wind. Sea is usually of shorter period (higher
frequency) than swell. As a rule of thumb, a period of about 10 seconds may be taken as
separating sea from swell; although one must allow for considerable overlap. Sea is shorter
in length, steeper, more rugged and more confused than swell. Since wind-generated waves
have their origin in the wind - which is proverbially changeable - they too are changeable,
varying both seasonally and regionally.
Wind waves, especially, are short crested and very irregular. Even so, they can be seen
as a superposition of many simple, regular harmonic wave components, each with its own
amplitude, length (or period or frequency) and direction of propagation. Such a concept
can be very handy in many applications; it allows one to predict very complex irregular
behavior in terms of the much simpler theory of regular waves.
B. Kinsman wrote Wind Waves - Their Generation and Propagation on the Ocean Surface
while he was Professor of Oceanography at The Johns Hopkins University in Baltimore,
Maryland. The book, published in 1963 was complete for its time; the wit scattered
throughout its contents makes is more readable than one might think at …rst glance.
First, an introduction to the most relevant phenomena is given here for the case of regular
deep water waves; the (general) shallow water case will be treated in a next lecture.