3
Wind Speed over Water
There are two common procedures for determining
the design wind speed. They are:
1. A constant overwater wind speed of 100 mph
(Reclamation, 1987)
2. Site-specific wind speed and duration curves
The 100 mph wind speed recommended by
Reclamation is a simple but conservative approach.
The more detailed site-specific approach is presented
in the following paragraphs.
According to the guidelines titled Reclamation ACER
TM-No. 2 and TR-69, the design wind speed and
duration can be selected by using the observed
maximum wind speed and the effective fetch.
Commonly, the observed fastest mile wind speed is
considered as the maximum overland wind speed, UL,
and can be obtained from the National Oceanic and
The NOAA wind data, including wind speed, duration,
and direction, indicates the overland wind
characteristics at 25 feet above ground.
The duration of a given wind speed that needs to be
maintained to fully develop the maximum waves is a
function of the effective fetch. The longer the effective
fetch, the longer the duration for the sustained wind
speed. Figure 2 graphically shows the selection of
design wind speed based on the relationship between
the maximum wind speed and the effective fetch
response to wind speed. The intersection of the red
curve and blue curve identifies the “design wind
speed.”
The red line on Figure 2 can be developed using the
observed fastest mile wind speed and the information
contained in Figure 5 of TR-69. Alternatively, Table 1 is
provided as a simplification of the information shown
in Figure 5 of TR-69.

Figure 2: Plot of wind speed vs. duration
Table 1: Maximum wind speed relationship
Fastest Mile Wind
Speed, mph
Ratio of Land Wind Speed to the Fastest
Mile Wind for the Durations
1 min* 30 min 60 min 100 min
100 100% 52% 46% 41%
80 100% 57% 51% 47%
60 100% 65% 59% 55%
* Duration of fastest mile wind speed is one minute.
The blue curve in Figure 2 needs to be generated using
Figure 2 in TR-69 or the empirical relationship
(Equation 2) of overland wind speed and duration for
the site specific effective fetch.

(
)
Eq.2
g = Gravitational Acceleration, 32.2 ft/sec2
T = Wave Duration in seconds. Wave duration is
equal to the minimum wind duration required
for generation of wave heights for a specific
effective fetch and wind speed.
UL = Overland Wind Speed in ft/sec
Fe = Site Specific Effective Fetch in ft
Because of smoother and more uniform surface
conditions, overwater wind speeds, Uw, are higher
than overland wind speeds, UL. To consider this speed
enhancement, the overwater wind speed can be
computed using the following equation.
Eq.3
β = Wind Speed Adjustment Factor or Ratio, Uw/UL,
Shown on Figure 3.
Careful!! The units for effective fetch and
wind speed vary for the various equations in this
article. Make note of units required for each eqn.