66 



whether or not the proposed outfall should be constructed must 

 be based on the current patterns, empirical methods for the 

 measurement of direction and velocity were employed. Neither 

 current meters nor drogues give more than an instantaneous 

 trace of the water motion in a restricted locality, and drift 

 cards represent merely the upper mixed layer and its net 

 motion. Furthermore, the distribution of surface and sub- 

 surface water units and their impression on the water structure 

 of the bay as a whole can is only a casual way be interpreted 

 by these methods of measurement. 



On the other hand, there is no theoretical or mathematical 

 method by which density patterns can be used to compute current 

 flow in shallow water overlying continental shelves. Deter- 

 minations of ocean currents from the slopes of isobaric sur- 

 faces assume level surfaces at some depth along which no hori- 

 zontal component of gravity is active. Such level surfaces 

 are usually taken at depths of 500 meters or more. Thus, the 

 preparation of dynamic height anomalies and the subsequent 

 construction of the geopotential topography of isobaric sur- 

 faces by conventional methods is not possible in shallow 

 water . 



However, it is believed that density currents can exist 

 in shelf water and the basic problem becomes one of determining, 

 at least in a qualitative way, their directions if not their 

 velocities. It is only logical to assume that the larger scale 

 differences in temperature in Santa Monica Bay are related to 

 the character of circulation. A temperature profile, such as 

 that obtained on August 18, 1955 (Fig. 22), is an example of 



