For this purpose, small oscillating room fans are placed throughout the 

 model in the pattern and number determined during the model verification 

 process. The fans are directed down on the water surface rather than 

 across the surface. In estuaries with low tidal range and low freshwater 

 inflow, mixing in the navigation channels is often primarily due to the 

 passage of deep-draft ships. Simulation of this type mixing can be 

 achieved by releasing air bubbles at a low rate at intervals along the 

 bottoir, of the channel. 



(15) Ship Simulators for Sediment Resuspension . In estuaries 

 where current velocities are weak, the movement and deposition of sedi- 

 ments in the navigation channel may be strongly influenced by the passage 

 of deep-draft ships. Deposited sediments are resuspended and subsequently 

 moved by the weak tidal currents. Since the model tidal currents are also 

 too weak to move the deposited sediment along the bed of the model, a ship 

 simulator is used to resuspend the sediment. The ship simulator consists 

 of a small propel lor which can be rotated at various speeds in either 

 direction. It is towed back and forth along the navigation channel 

 during shoaling tests. 



(16) Water Level Measurement . Water level is usually measured 

 with manually operated point gages, recording float gages, electronic 

 water level followers, air-capacitance gages or air-bubbling systems. 

 Point gages can be mounted permanently or on portable racks. If it is 

 necessary to obtain a continuous record of the water (as during a hurri- 

 cane surge), one of the other types of measuring devices must be used. 

 The recording float gage consists of a float-supported pen which inks a 

 continuous record of water level on a roll of recording paper on a drum, 

 which in turn is mounted on a tripod permanently located at gaging sta- 

 tions. The electronic water level follower detects conductivity through 

 probes continuously moved up and down by a servo drive, thus continuously 

 making and breaking contact with the water. The air- capacitance gage is 

 also mounted on a servomechanism, but this gage monitors the position of 

 the water surface by maintaining a constant electrical capacitance in the 

 air gap between the gage and the water surface. Vertical movement of the 

 gage probes is recorded on a strip chart, or the voltage outputs can be 

 converted to digital signals and recorded on magnetic tape. The bubble 

 tube positioner follows the water level by moving the bubble tube up or 

 down as required to maintain a constant pressure at the open end (bottom) 

 of the tube. Water level changes are detected by a potentiometer on the 

 servo drive. Bubbler systems are also available which determine water 

 level directly from the pressure in a fixed bubble tube. Water level can 

 be determined to within ±0.001 foot with any of these instruments, and is 

 generally the accuracy of duplicating identical conditions on the model. 



(17) Current Velocity Meters . Current velocity is normally 

 measured by miniature Price-type current meters (Fig. 3-21) capable of 

 measuring velocities as low as 0.02 foot per second (~0.01 knot). Their 

 calibration is quite stable unless the meters are damaged or the jeweled 

 bearings become fouled. Because five cups (constructed of plastic or 



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