become focused and cut gaps or channels in the
offshore bar. This return current is called a rip
current. There may be several sandbars present
in a surf zone.
The following factors normally will change in a
short period of time(within 24 hours) and will affect the
surf conditions you observe from the beach:
Deep-water sea-wave height. direction, and
period.
Deep-water swell-wave height, direction.
period, and the pattern of the waves within a
swell-wave group.
Presence of secondary wave systems that may
create a more confused surf zone. These are
discussed later in this chapter.
Stage of the tides; that is, the height and pattern
of the normal rise and fall in the water level.
Set (direction) and drift (speed) of rising and
falling tidal currents.
Set and drift of semipermanent coastal water
currents (usually obtained from hydrographic
charts).
Surf beat (the rise and fall of the entire water
level within the surf zone). The surf beat is
significant to landing craft approaching
submerged obstacles, such as sandbars.
Wind speed and direction.
Although all these factors must be evaluated when a
forecast of surf conditions is calculated, you, as the
observer. need to be aware of the factors that change
with the weather and tides. Normally, your report of surf
conditions. as affected by these changing factors, is
evaluated by the forecaster. The forecaster then will
interpret how forecasted deep-water waves will react
when approaching the beach under various conditions.
WAVE REFRACTION
Refraction is the bending of waves toward areas of
slower wave speed. As a wave travels over shallower
water and begins to interact with the bottom, the wave
speed decreases. If the wave is traveling at an angle to
the beach. the portion of the wave in shallow water
slows down while the portion of the wave in deep water
continues at the same speed, causing the wave to
become more parallel to the beach. Wave refraction
may also occur due to irregularities of the coastline and
bottom contours.
Waves will bend toward points
protruding from shore due to the shallower water
surrounding them.
This process concentrates much
more wave energy onto the protrusions than the
embayments between them, as illustrated in figure 4-2.
To predict variation in surf along a coastline, forecsater
personnel create refraction diagrams. Several different
wave direction, height. and wave period scenarios may
be used.
In the following text, we will discuss how to report
observed surf conditions.
Figure 4-2.—Wave refraction due to coastal irregularities.
4-3
