40 



Spatially integrating methods 



To date, experiments in spatially averaging velocity by observing induced 

 electrical fields have been conducted by towing electrodes from ships or by 

 sending voltages in abandoned underwater telephone cables. Some of these 

 experiments have been for the purpose of measuring barotropic flow in the 

 North Pacific (Chave, Luther, and Filloux 1990; Spain 1990 - these two pa- 

 pers provide a substantial summary of the mathematics and methods). This 

 author (Morang) is unaware of whether these techniques have been tested in 

 shallow water or in restricted waterways such as channels. At this time, 

 therefore, spatially integrating methods appear to have no immediate applica- 

 tion to coastal engineering studies. 



Point source (Eulerian) and related technology 



In channels, bays, and offshore, direct measurements of the velocity and 

 direction of current flow can be made by instruments deployed on the bottom 

 or at various levels in the water column. Two general classes of current 

 meters are available: mechanical (impeller-type) and electronic. Several types 

 of electronic current meters are in common use, including electromagnetic, 

 inclinometer, and acoustic travel-time (Fredette et al. 1990, McCullough 

 1980, Pinkel 1980). 



Impeller current meters measure currents by means of a propeller device 

 that is rotated by the current flow. They serve as approximate velocity 

 component sensors because they are primarily sensitive to the flow component 

 in a direction parallel to their axle. Various types of propeller designs have 

 been used to measure currents, but experience and theoretical studies have 

 shown that ducted propellers are more satisfactory in measuring upper ocean 

 currents than rotor/vane meters (Davis and Weller 1980). Impeller/propeller 

 meters are considered to be the most reliable in the surf zone (Teleki, 

 Musialowski, and Prins 1976), as well as the least expensive. One model, the 

 Endeco 174, has been widely used by CERC for many years throughout the 

 country. Impeller gages are subject to snarling, biofouling, and bearing 

 failures, but are more easily repaired in the field and are more easily cali- 

 brated than other types (Fredette et al. 1990). 



Electronic current meters have many features in common, although they 

 operate on different principles. Their greatest common advantages are rapid 

 response and self-contained design with no external moving parts. They can 

 be used in real-time systems and can be used to measure at least two velocity 

 components. The degree of experience of the persons working with the 

 instruments probably has more influence on the quality of data acquired than 

 does the type of meter used (Fredette et al. 1990). The InterOcean Systems 

 S4 electromagnetic meter has been successfully used recently by CERC at 

 field experiments. 



Chapter 3 Field Data Collection and Observation 



