SPEED, HEIGHT, AND 

 PROPAGATION DIRECTION 



The speed, height, and direction 

 of propagation of internal waves at the 

 tower are determined by three NEL- 

 developed isotherm follower units. As 

 the depth of an isotherm changes, sig- 

 nals from a sensor suspended in the 

 thermocline create an imbalance in the 

 bridge circuit. The circuit in turn causes 

 a winch to raise or lower the sensor. 

 Thus the vertical oscillation is followed 

 as the isotherm passes the tower. 



Approximately 90 percent of the 

 internal waves measured propagated in a 

 westerly and southwesterly direction. 

 The speed of the progressive internal 

 waves, c, varies directly with the thick- 

 ness of the two layers and their density. 

 For internal waves that are long com- 

 pared with the water depth 



ghh' 

 h+h' 



P-p ' 

 P 



Where h' is the thickness of the upper 

 layer, h is the thickness of the lower 

 water layer, and p' and p are the respec- 

 tive water densities. In summer, the 

 speed is about 0. 3 knot. 



Thus as internal waves approach 

 the shore, they decelerate, become more 

 closely spaced, refract, develop long 

 crests, and finally move onshore. 



Simultaneous depth-time recordings 

 of the same isotherm are made by 

 isotherm followers suspended on 

 40-to-50-foot booms out from three 

 corners of the tower. The stream- 

 lined sensing units of the followers 

 move vertically with the depth of 

 any selected isotherm. 



INTERNAL 



The short-period waves described by an 

 isotherm in the middle of a summer thermo- 

 cline show a median height of 5.4 feet, a 

 period of 7.3 minutes, and a speed of about 

 0.3 knot. 



36 



