************** example problem **************
GIVEN: A wind speed U = 35 knots (40 mph) and a duration t = 10 hours.
FIND: The significant wave height Hp, and the significant period Tp,
at" the fetch front for:
(a) A fetch length, F = 200 nautical miles and
(b) a fetch length, F = 80 nautical miles.
SOLUTION:
(a) Enter Figure 3-15 from the left side at U = 35 knots and move
horizontally across the figure from the left toward the right, until
intersecting the dashed line for a duration of 10 hours that cornes
before the line indicating a fetch length of 200 nautical miles. At
the 10-hour duration line F = 92 nautical miles; this is the minimum
fetch Fn for this case.
With U = 35 knots, t = 10 hours, and F = 200 nautical miles then
Hp = 13.1 feet, Tp = 8.0 seconds, t^ equals 10 hours, and
= 92
nautical miles.
(b) Entering Figure 3-15 as above, when F = 80 nautical miles
and t = 10 hours, then the heights, periods, minimum duration and
fetch would be Hp = 12.6 feet, Tp = 7.8 seconds, and tm = 9.0 hours.
The minimum duration t^ is 9 hours, corresponding to the miles
which limit génération, and cornes before a duration of 10 hours.
In this example problem, the wave pattern in (a) is limited by the
duration; the wave pattern in (b) is limited by the fetch.
**************************************
When a sériés of surface synoptic weather charts (Fig. 3-9) are used
to détermine wave patterns, the values of U, F and t can be tabulated
for the first chart. For the same fetch on a later chart drawn for a time
Z, after the first chart, U, F, and t are again tabulated. Using
the subscript 2 to refer to those of the second chart and subscript 1
to refer to those of the first chart, if U2 = Uj, the above procedures
should be followed using either t2 = tmï + Z or F2. If, however, U2 / Up
certain additional assumptions must be made before using the forecasting
curves.
A change in wind speed from
to U2 in a time Z between charts may
be assumed to take place ins tant aneously at a time Z/2. Waves due to U;
ma> then be calculated by assuming that the first chart minimum duration
time has been lengthened by an amount Z/2 or that its minimum fetch has
been changed by AF/2, where AF represents the change in fetch length
between weather charts. Since at the assumed abrupt change in wind speed,
3-38
GIVEN: A wind speed U = 35 knots (40 mph) and a duration t = 10 hours.
FIND: The significant wave height Hp, and the significant period Tp,
at" the fetch front for:
(a) A fetch length, F = 200 nautical miles and
(b) a fetch length, F = 80 nautical miles.
SOLUTION:
(a) Enter Figure 3-15 from the left side at U = 35 knots and move
horizontally across the figure from the left toward the right, until
intersecting the dashed line for a duration of 10 hours that cornes
before the line indicating a fetch length of 200 nautical miles. At
the 10-hour duration line F = 92 nautical miles; this is the minimum
fetch Fn for this case.
With U = 35 knots, t = 10 hours, and F = 200 nautical miles then
Hp = 13.1 feet, Tp = 8.0 seconds, t^ equals 10 hours, and
= 92
nautical miles.
(b) Entering Figure 3-15 as above, when F = 80 nautical miles
and t = 10 hours, then the heights, periods, minimum duration and
fetch would be Hp = 12.6 feet, Tp = 7.8 seconds, and tm = 9.0 hours.
The minimum duration t^ is 9 hours, corresponding to the miles
which limit génération, and cornes before a duration of 10 hours.
In this example problem, the wave pattern in (a) is limited by the
duration; the wave pattern in (b) is limited by the fetch.
**************************************
When a sériés of surface synoptic weather charts (Fig. 3-9) are used
to détermine wave patterns, the values of U, F and t can be tabulated
for the first chart. For the same fetch on a later chart drawn for a time
Z, after the first chart, U, F, and t are again tabulated. Using
the subscript 2 to refer to those of the second chart and subscript 1
to refer to those of the first chart, if U2 = Uj, the above procedures
should be followed using either t2 = tmï + Z or F2. If, however, U2 / Up
certain additional assumptions must be made before using the forecasting
curves.
A change in wind speed from
to U2 in a time Z between charts may
be assumed to take place ins tant aneously at a time Z/2. Waves due to U;
ma> then be calculated by assuming that the first chart minimum duration
time has been lengthened by an amount Z/2 or that its minimum fetch has
been changed by AF/2, where AF represents the change in fetch length
between weather charts. Since at the assumed abrupt change in wind speed,
3-38