periods. The actual formula for the speed of the wavetrain isC=1.515Twhere C is the speed of the wave train and T is thewave period in the wave train.All of the different wave trains (series of waves allhaving the same period and direction of movement) inthe fetch can be compared to a group of long distancerunners at a track and field meet. At first all of therunners start out at the starting line at the same time. Asthey continue on, however, the faster runners moveahead and the slower runners begin to fall behind. Thusthe field of runners begins to string out along thedirection of travel. The wave trains leaving a fetch dothe same thing. The stringing out of the various groupsof waves is called dispersion.In a swell forecast problem it is necessary todetermine what wave trains have already passed theforecast point and which have not yet arrived. After thishas been determined, the wave trains that are left are theones that are at the forecast point at the time ofobservation.Angular SpreadingAs the wave trains leave the fetch, they may leaveat an angle to the main direction of the wind in the fetch.Thus, swell waves may arrive at a forecast point thoughit may lie to one side of the mainline of direction of thewind. This process of angular spreading is depicted infigure 6-6.The problem in swell forecasting is to determinehow much of the swell will reach the forecast point afterthe waves have spread out at angles. This isaccomplished by measuring the angles from the leewardedge of the fetch to the forecast point. These anglesmust be measured as accurately as possible, figure 6-7,and are determined by the following five rules:1. Draw the rectangular fetch.Figure 6-6.-Angu1ar spreading.6-10
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