Neither the total time occupied by
the flood and ebb tides nor the rate of the rise and fall are
equal, except in the open sea, where there are fewer disturbing
conditions. In restricted areas of water the ebb lasts longer
than the flood.
Although the published tide-tables give much detailed
information, it only applies to certain representative ports,
and even then it is only correct in calm weather and with a
very steady wind, so that in the majority of cases the engineer
must take his own observations to obtain the necessary local
information to guide him in the design of the works. It is
impracticable for these observations to be continued over the
lengthy period necessary to obtain the fullest and most
accurate results, but, premising a general knowledge of the
natural phenomena which affect the tides, as briefly described
herein, he will be able to gauge the effect of the various
disturbing causes, and interpret the records he obtains so as
to arrive at a tolerably accurate estimate of what may be
expected under any particular circumstances. Generally about 25
per cent. of the tides in a year are directly affected by the
wind, etc., the majority varying from 6 in to 12 in in height
and from five to fifteen minutes in time. The effect of a
moderately stiff gale is approximately to raise a tide as many
inches as it might be expected to rise in feet under normal
conditions.
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