Saville (1954) proposed a method to détermine the effect of fetch
width on wave génération. Figure 3-13, based on this method^ indicates
the effective fetch for a relatively uniform fetch width. The following
démonstrates the use of Figure 3-13.
problem
************** EXAMPLE PROBLEM **************
GIVEN: Consider a channel with a fetch length F - 20 miles, a width
W = 5 miles, an average depth d = 35 feet, and a windspeed U = 50 mph
along the long axis.
FIND: Estimate the significant wave height Hg, and the significant
wave period Tg.
SOLUTION: Compute W/F = 5/20 =0.25
From Figure 3-13 for W/F = 0.25, Fg'/F = 0.45
Compute VE = 0.45 x 20 = 9 miles or 47,500 feet.
Using the forecasting relations given in Section 3.6, Wave Forecasting
for Shallow Water, for a fetch of 47,500 feet and a wind speed of
50 mph and an average uniform depth of 35 feet, the significant wave
height may be determined from Figure 3-27 to be Hg = 5.3 feet, say
5 feet and the significant wave period will be Tg = 3.4 seconds, say
3 seconds.
*************************************
The preceding example présents a simplified method of determining the
effective fetch. Shorelines are usually irregular, and the uniform-width
method indicated in Figure 3-13 is not applicable. A more general method
must be applied. This method is based on the concept that the width of a
fetch in réservoirs normally places a very definite restriction on the
length of the effective fetch; the less the width-length ratio, the shorter
the effective fetch. A procedure for determining the effective fetch
distance is illustrated in Figure 3-14. It consists of constructing 15
radiais from the wave station at intervals of 6° (limited by an angle of
45° on either side of the wind direction) and extending these radiais
until they first intersect the shoreline. The component of length of
each radial in a direction parallel to the wind direction is measured and
multiplied by the cosine of the angle between the radial and the wind
direction. The resulting values for each radial are summed and divided
by the sum of the cosines of ail the individual angles. This method is
based on the following assumptions:
(a) Wind moving over a water surface transfers energy to the
water surface in the direction of the wind and in ail directions within
45° on either side of the wind direction.
(b) The wind transfers a unit amount of energy to the water along
the central radial in the direction of the wind and along any other radial
an amount modified by the cosine of the angle between the radial and the
wind direction.
3-30
width on wave génération. Figure 3-13, based on this method^ indicates
the effective fetch for a relatively uniform fetch width. The following
démonstrates the use of Figure 3-13.
problem
************** EXAMPLE PROBLEM **************
GIVEN: Consider a channel with a fetch length F - 20 miles, a width
W = 5 miles, an average depth d = 35 feet, and a windspeed U = 50 mph
along the long axis.
FIND: Estimate the significant wave height Hg, and the significant
wave period Tg.
SOLUTION: Compute W/F = 5/20 =0.25
From Figure 3-13 for W/F = 0.25, Fg'/F = 0.45
Compute VE = 0.45 x 20 = 9 miles or 47,500 feet.
Using the forecasting relations given in Section 3.6, Wave Forecasting
for Shallow Water, for a fetch of 47,500 feet and a wind speed of
50 mph and an average uniform depth of 35 feet, the significant wave
height may be determined from Figure 3-27 to be Hg = 5.3 feet, say
5 feet and the significant wave period will be Tg = 3.4 seconds, say
3 seconds.
*************************************
The preceding example présents a simplified method of determining the
effective fetch. Shorelines are usually irregular, and the uniform-width
method indicated in Figure 3-13 is not applicable. A more general method
must be applied. This method is based on the concept that the width of a
fetch in réservoirs normally places a very definite restriction on the
length of the effective fetch; the less the width-length ratio, the shorter
the effective fetch. A procedure for determining the effective fetch
distance is illustrated in Figure 3-14. It consists of constructing 15
radiais from the wave station at intervals of 6° (limited by an angle of
45° on either side of the wind direction) and extending these radiais
until they first intersect the shoreline. The component of length of
each radial in a direction parallel to the wind direction is measured and
multiplied by the cosine of the angle between the radial and the wind
direction. The resulting values for each radial are summed and divided
by the sum of the cosines of ail the individual angles. This method is
based on the following assumptions:
(a) Wind moving over a water surface transfers energy to the
water surface in the direction of the wind and in ail directions within
45° on either side of the wind direction.
(b) The wind transfers a unit amount of energy to the water along
the central radial in the direction of the wind and along any other radial
an amount modified by the cosine of the angle between the radial and the
wind direction.
3-30