the energy imparted to the waves by U^, with a minimum duration t^ ♦ Z/2
for a minimum fetch Fml + F/2, does not change, U2 will be assumed to
impart energy to waves which already contain energy due to Uj.
Plotted on Figures 3-15 and 3-16 are
are considered lines of constant wave
deepwater wave energy is given by
dotted lines of constant H2T2 which
energy. To a first approximation,
PgH2Lo
E°
8
5.12 pg (HT)2
(3-24)
8
If energy had been imparted to the waves by U2 acting alone, these waves
wotild be of length and height given in Figures 3-15 or 3-16 by the intersection of the U2 ordinate with the constant energy line (plotted or
interpolated) corresponding to energy imparted by Ui with a minimum
duration of t^ + Z/2 or a minimum fetch Fml + F/2. By increasing
the minimum duration at this point by an amount Z/2 or by changing the
minimum fetch by an amount AF/2, wave conditions under U2 at the
time of the second chart may be approximated.
For example, if the wind speed increases so that U2 = 40 knots, and
with Uj = 35 knots, t77îl = 10 hours, Fml = 92 nautical miles, t^ + Z/2 =
13 hours; an interpolated (by eye) dotted line of constant H2T2 would be
followed up to the U2 = 40-knot line where the duration = 6.5 hours. To
this value Z/2 or 3 hours is added and then moving horizontally along
the line U2 = 40 knots to t = 6.5 + 3.0 = 9.5 hours, it is found that
Hp2 = 15.6 feet, T^2 = 8.7 seconds, t^ = 9.5 hours, and
= 95 nautical
miles. If the measured fetch F2 had been less than 95 nautical miles,
this length of fetch would limit the growth of waves. Although the preceding discussion would indicate that AF should be calculated, in practice
this need not be done; the results obtained through calculation of AF
would be found by reading off wave heights at the intersection of U2 and
F2 if F2 is limiting. Therefore, if F2 had equalled 85 nautical miles,
in this case less than 95 miles and therefore limiting, at the intersection
of U2 = 40 knots, then HF2 =15.0 feet, T^2 = 8.5 seconds, Tm2 = 8.8 hours,
and Fm2 = 85 nautical miles. Note this important distinction: tj, Fx and F2
are calculated by use of Figure 3-15. Some of the measured and calculated
values will be the same, but not ail of them.
If the wind velocity U2 is less than Uj, the procedures followed
are nearly the same. From the intersection of Uj and t^j + Z/2 a
constant energy line is followed to its intersection, if there is one,
3-39
for a minimum fetch Fml + F/2, does not change, U2 will be assumed to
impart energy to waves which already contain energy due to Uj.
Plotted on Figures 3-15 and 3-16 are
are considered lines of constant wave
deepwater wave energy is given by
dotted lines of constant H2T2 which
energy. To a first approximation,
PgH2Lo
E°
8
5.12 pg (HT)2
(3-24)
8
If energy had been imparted to the waves by U2 acting alone, these waves
wotild be of length and height given in Figures 3-15 or 3-16 by the intersection of the U2 ordinate with the constant energy line (plotted or
interpolated) corresponding to energy imparted by Ui with a minimum
duration of t^ + Z/2 or a minimum fetch Fml + F/2. By increasing
the minimum duration at this point by an amount Z/2 or by changing the
minimum fetch by an amount AF/2, wave conditions under U2 at the
time of the second chart may be approximated.
For example, if the wind speed increases so that U2 = 40 knots, and
with Uj = 35 knots, t77îl = 10 hours, Fml = 92 nautical miles, t^ + Z/2 =
13 hours; an interpolated (by eye) dotted line of constant H2T2 would be
followed up to the U2 = 40-knot line where the duration = 6.5 hours. To
this value Z/2 or 3 hours is added and then moving horizontally along
the line U2 = 40 knots to t = 6.5 + 3.0 = 9.5 hours, it is found that
Hp2 = 15.6 feet, T^2 = 8.7 seconds, t^ = 9.5 hours, and
= 95 nautical
miles. If the measured fetch F2 had been less than 95 nautical miles,
this length of fetch would limit the growth of waves. Although the preceding discussion would indicate that AF should be calculated, in practice
this need not be done; the results obtained through calculation of AF
would be found by reading off wave heights at the intersection of U2 and
F2 if F2 is limiting. Therefore, if F2 had equalled 85 nautical miles,
in this case less than 95 miles and therefore limiting, at the intersection
of U2 = 40 knots, then HF2 =15.0 feet, T^2 = 8.5 seconds, Tm2 = 8.8 hours,
and Fm2 = 85 nautical miles. Note this important distinction: tj, Fx and F2
are calculated by use of Figure 3-15. Some of the measured and calculated
values will be the same, but not ail of them.
If the wind velocity U2 is less than Uj, the procedures followed
are nearly the same. From the intersection of Uj and t^j + Z/2 a
constant energy line is followed to its intersection, if there is one,
3-39