EM 1110-2-1100 (Part II)
(Change 1) 31 July 2003
Figure II-2-7. Ratio RL of windspeed over water UW to windspeed over land UL as a function of
windspeed over land UL (after Resio and Vincent (1977))
marine boundary layer and winds from other directions fall within a land boundary layer. In areas such as
this, a land-to-sea transform (similar to that shown in Figures II-2-7 and II-2-8) can be used for all angles
coming from the land. Depending on the distance to the water and the elevation of the measurement site,
winds coming from the direction of open water may or may not still be representative of a marine boundary
layer. Guidance for determining the effects of fetch on wind speed modifications can be found in Resio and
Vincent (1977) and Smith (1983). These studies indicate that fetch effects wind speeds significantly only at
locations within about 16 km (10 miles) of shore.
(e) Wind speed transition from land to water. The net effect of wind speed variation with fetch is to
provide a smooth transition from the (generally lower) wind speed over land to the (generally higher) wind
speed over water. Thus, wind speeds tend to increase with fetch over the first 10 miles or so after a transition
from a land surface. The exact magnitude and characteristics of this transition depend on the roughness
characteristics of the terrain and vegetation and on the stability of the air flow. A very simplistic
approximation to this wind speed variation for the Resio and Vincent curves used here could be obtained by
fitting a logarithmic curve to the asymptotic overland and overwater wind speed values. However, for most
design and engineering purposes, it is probably adequate to simply use the long-fetch values with the
recognition that they are somewhat conservative. The one situation that should cause some concern would
be if overwater wind speed measurements are taken near the upwind end of a fetch. These winds could be
considerably lower than wind speeds at the end of the fetch and underconservative values for wave conditions
could result from the use of such (uncorrected) winds in a predictive scheme.
II-2-14
Meteorology and Wave Climate
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