metal) are mounted on a vertical axis, the meter is insensitive to current 

 direction. In operation, the niimber of wheel revolutions per 10-second 

 period is determined visually and converted directly into prototype veloc- 

 ity. A photo cell, fiber optics, an electronic counting circuit, and an 

 alternating current-direct current converter have been added to provide 

 automatic velocity measurements. Light is transmitted to the reflective 

 surface on the cups and back to the photodiode through fiber optics. The 

 pulsations of the photodiode are thiis directly proportional to velocity. 

 The sampling frequency is quite flexible. Miniature electromagnetic cur- 

 rent meters are being developed which should further improve the ease and 

 accuracy of obtaining velocity measurements. Advantages of such sensors 

 include the lack of inertia (which would improve the response to accel- 

 erating and decelerating flow) and automatic determination of velocity 

 vectors. The limitations of the current velocity meters used in estuary 

 models should be considered before making close comparisons between model 

 and prototype velocity data. In models with commonly used vertical scales, 

 the centerline of the meter cup or vane is 3 to 5 feet (prototype) above 

 the bottom, whereas field measurements are usually about 2 feet above the 

 bottom. The model velocities are commonly determined over an interval 

 equivalent to 3 to 16 minutes (depending on the model scales and the type 

 of meter) , whereas prototype velocity observations are often made in less 

 than 1 minute. For common model scales, the horizontal spread of the 

 entire meter cup or vane wheel is equivalent to 50 to 100 feet (depend- 

 ing again on the scales and type of meter). However, prototype velocity 

 meters have a horizontal spread of less than 1 foot. Thus, the distortion 

 of areas (model to prototype) results in comparison of prototype point 

 velocities with model mean velocities for a much larger area. The same 

 is true for the vertical area. The accuracy of the model velocity meters 

 is on the order of ±0.25 foot per second (prototype). 



(18) Current Velocity Photos . In areas where velocities are 

 very small throughout most of the tidal cycle or where water depth is too 

 shallow for a velocity meter, velocity can be determined by timing the 

 travel of a float over a measured distance. Surface current velocities 

 can also be determined from time-exposure photos of confetti floating on 

 the water surface. The camera lens is opened by a solenoid activated by 

 the tide generator and closed by an accurate timing device. A bright 

 light is flashed immediately before the camera lens is closed, resulting 

 in a bright spot at approximately the end of each confetti streak which 

 indicates the direction of flow. Current velocities can be determined 

 from the photos by determining the length scale of the photo, measuring 

 the length of a confetti streak, and knowing the length of the time ex- 

 posure. An example of a current pattern photo is shown in Figure 3-22. 



(19) Salinity Measurement . Salinity concentration is measured 

 either by chemical titration with silver nitrate or conductivity measure- 

 ment. Based on the sample volumes normally used for analysis, salinity 

 concentrations determined by titration are considered accurate to within 

 ±0.02 part per thousand. The conductivity meter assembly consists of a 

 set of three temperature-compensated conductivity cells, a cell switch, 



a conductivity indicator, and a digital readout. The conductivity cells 



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