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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 Atmospheric Administration (NOAA) National Climatic Data Center websites shown at the end of this article. 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.