barely covered at low tide. This can cause landing craft
to become hung up or beach prematurely. See figure
4-9.
TIDE CALCULATION
Semidiurnal tides consists of two low tides and two
high tides in a 24-hour period in which each successive
high and low water periods have nearly the same height.
Diurnal (daily) tides consist of one low tide and one
high tide in a 24-hour period. Mixed tides are
semidiurnal tides in which each successive high and
low water periods have different heights.
Tidal information is usually obtained from
Quartermaster personnel, but is available from
publications and prepared tables. A Tidal Prediction
Program (TIDE) is available in TESS and MOSS using
location-specific tide data. The TIDE program
calculates hourly tidal information only for locations
contained in the data base. However, adjustments can
be made for substations using time and height
corrections.
Tidal currents are not predicted by the
TIDE program.
However, tidal currents can be
estimated from tidal current tables produced by the
National Imagery and Mapping Agency.
REVIEW QUESTIONS
Q29.
A flood current is associated with what type of
tide?
Q30.
When are rip currents strongest?
Q31.
Why is the surf zone normally wider at low tide?
Q32.
Where can tidal information be obtained?
SUMMARY
Although you may rarely, if ever, be tasked to
provide SUROBs, you must be familiar with the
observation procedures and the terms used to describe
surf conditions. In this chapter, we have explained the
causes of surf and discussed nearshore hydrography and
several key terms used in surf observations. We
covered the actual surf observation (SUROB) and the
calculation of the Modified Surf Index (MSI). We also
discussed tides and their impact on surf.
Figure 4-9.—Effects of tides on surf zone.
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