3.43 FORECASTS FOR LAKES, BAYS, AND ESTUARIES 



3.431 Wind Data . The techniques referred to for determination of wind 

 speeds and directions from isoharic patterns apply generally to ocean 

 areas. The friction that causes winds to spiral when crossing isobars 

 and to have a velocity lower than geostrophic or gradient winds is more 

 variable over land areas. When a fetch is close to land, this variability 

 will alter anticipated wind directions and velocities. In enclosed or 

 semienclosed bodies of water, such as lakes and bays, wind speeds and 

 directions should be taken from actual weather station reports whenever 

 possible. 



In enclosed bodies of water, or in other areas where the wind blows 

 off the land, differing frictional effects of land and water should be 

 considered, and indicated wind speeds should be adjusted for these effects. 

 Studies by Myers (1954) and Graham and Nunn (1959) indicate recommended 

 adjustments in wind speeds. (See Table 3-2.) The adjustment factor may 

 vary considerably depending on the shoreline frictional characteristics. 

 This adjustment is used only for short fetches such as those in reservoirs 

 and small lakes. 



Often, over small or well-defined fetch areas, it is not convenient 

 or even possible to utilize surface charts to determine wind characteris- 

 tics. Where wind records exist for locations in or near a fetch area, 

 these may be utilized. The accuracy of the forecast will depend on the 

 completeness of the records, the extent of fetch, and the wave prediction 

 technique employed. Where wind duration records are not available, local 

 wind speed reports may still be utilized to forecast waves assuming un- 

 limited durations, that is, wave growth is limited by the available fetch. 

 Wave characteristics deduced in this way are only qualitative. 



(Graham and Nunn, 1959) 

 *Ratio of wind speed at location to overwater wind speed 

 (both at 30- ft. level) . 



3.432 Effective Fetch . The effect of fetch width or limiting ocean wave 

 growth in a generating area may usually be neglected since nearly all ocean 

 fetches have widths about as large as their lengths. In inland waters 

 (bays, rivers, lakes, and reservoirs), fetches are limited by land forms 

 surrounding the body of water. Fetches that are long in comparison to 

 width are frequently found, and the fetch width may become quite important, 

 resulting in wave generation significantly lower than that expected from 

 the same generating conditions over more open waters. 



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