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<br /> <br /> <br /> <br />3 <br />Wind Speed over Water <br />There are two common procedures for determining <br />the design wind speed. They are: <br />1. A constant overwater wind speed of 100 mph <br />(Reclamation, 1987) <br />2. Site-specific wind speed and duration curves <br />The 100 mph wind speed recommended by <br />Reclamation is a simple but conservative approach. <br />The more detailed site-specific approach is presented <br />in the following paragraphs. <br />According to the guidelines titled Reclamation ACER <br />TM-No. 2 and TR-69, the design wind speed and <br />duration can be selected by using the observed <br />maximum wind speed and the effective fetch. <br />Commonly, the observed fastest mile wind speed is <br />considered as the maximum overland wind speed, UL, <br />and can be obtained from the National Oceanic and <br />Atmospheric Administration (NOAA) National Climatic <br />Data Center websites shown at the end of this article. <br />The NOAA wind data, including wind speed, duration, <br />and direction, indicates the overland wind <br />characteristics at 25 feet above ground. <br />The duration of a given wind speed that needs to be <br />maintained to fully develop the maximum waves is a <br />function of the effective fetch. The longer the effective <br />fetch, the longer the duration for the sustained wind <br />speed. Figure 2 graphically shows the selection of <br />design wind speed based on the relationship between <br />the maximum wind speed and the effective fetch <br />response to wind speed. The intersection of the red <br />curve and blue curve identifies the “design wind <br />speed.” <br />The red line on Figure 2 can be developed using the <br />observed fastest mile wind speed and the information <br />contained in Figure 5 of TR-69. Alternatively, Table 1 is <br />provided as a simplification of the information shown <br />in Figure 5 of TR-69. <br /> <br />Figure 2: Plot of wind speed vs. duration <br />Table 1: Maximum wind speed relationship <br />Fastest Mile Wind <br />Speed, mph <br />Ratio of Land Wind Speed to the Fastest <br />Mile Wind for the Durations <br />1 min* 30 min 60 min 100 min <br />100 100% 52% 46% 41% <br />80 100% 57% 51% 47% <br />60 100% 65% 59% 55% <br />* Duration of fastest mile wind speed is one minute. <br />The blue curve in Figure 2 needs to be generated using <br />Figure 2 in TR-69 or the empirical relationship <br />(Equation 2) of overland wind speed and duration for <br />the site specific effective fetch. <br /> <br /> <br /> ( <br /> ) <br /> Eq.2 <br />g = Gravitational Acceleration, 32.2 ft/sec2 <br />T = Wave Duration in seconds. Wave duration is <br />equal to the minimum wind duration required <br />for generation of wave heights for a specific <br />effective fetch and wind speed. <br />UL = Overland Wind Speed in ft/sec <br /> Fe = Site Specific Effective Fetch in ft <br />Because of smoother and more uniform surface <br />conditions, overwater wind speeds, Uw, are higher <br />than overland wind speeds, UL. To consider this speed <br />enhancement, the overwater wind speed can be <br />computed using the following equation. <br /> Eq.3 <br />β = Wind Speed Adjustment Factor or Ratio, Uw/UL, <br />Shown on Figure 3. <br /> Careful!! The units for effective fetch and <br />wind speed vary for the various equations in this <br />article. Make note of units required for each eqn.