instance corresponding to each x^(t). After the concentration contours 

 are derived, the evaluation of the eddy diffusivity coefficients, e^, 

 is simplified. 



b. Selection and Use of Dye Tracers . Water-tracing dyes must be 

 chosen according to whether their colorimetric or fluorometric properties 

 are to be measured or imaged. Generally, the emissivity of fluorescent 

 substances (measured in milliwatt per square centimeter steradians) is 

 much higher than nonfluorescing chemicals and the bandwidth of transmit- 

 tance is usually narrower, all of which aid the spectral separation of 



one tracer from another. In tests at Oceanside and Point Mugu, California, 

 and at Pentwater, Michigan, three dyestuffs in green, yellow, and red 

 bands of the visible spectrum were commonly used (Table 4). These sub- 

 stances have been used in pollution research and rescue operations; none 

 is toxic to living organisms. 



c. Dye Injection Procedures . Dyes are normally injected by placing 

 100 grams of the material in a water-soluble film or bag, which in turn is 

 placed in a porous sand-sample bag. The inner container retards solution 

 of the dye until the film dissolves; the porous sand-sample bag allows the 

 solution to pass through and keep the dye fixed in position. Six bags are 

 usually attached to an anchored buoy line and positioned 0.3048 meter 



(1 foot) below the water's surface. This method does not satisfy any of 

 the injection schemes discussed previously since the dye release is 

 neither instantaneous nor at a constant discharge; however, this type of 

 injection is useful in aerial photography of nearshore circulation. Where 

 the spatial integration method is used, the dye packets are tied to a 

 free-floating buoy which marks the advective path, and the surrounding 

 developing dye patch will indicate the rate of diffusion outward from the 

 source. In steady dilution, a pump must be used to inject the tracer at 

 a known discharge rate. 



A typical experiment involves designing a control grid to locate the 

 centroid or leading edge of the dye; e.g., where a combination of six 

 buoys and six shore targets were used to provide sufficient control to 

 (a) determine the position of the cloud, and (b) rectify oblique images 

 to vertical equivalents (Fig. 35) . 



Although drogues and floats are used in nearshore experiments for the 

 charting of currents, their physical size is not representative of a 

 volume of water small enough to have the scales of molecular diffusion. 

 Wind stress on the surface also tends to modify their path to the extent 

 of overriding the advective properties of the fluid. Where offshore ba- 

 thymetry is undulating (bars, shoals), these objects often ground for 

 short periods. Therefore, correctly traced dye clouds will better depict 

 the true course of nearshore currents. 



4. Aerial Photography . 



a. Survey Flight Lines and Frequency of Coverage . To piece together 

 the distribution of currents both in the lateral (y-coordinate) and the 



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