INTRODUCTION 



Flowmeters are not normally designed for the purpose of measuring shallow- 

 water, steady-state currents in the presence of orbital velocities. A need therefore 

 exists for information on the value of existing meters used for this purpose. 



This report presents a feasibility study of the relative merits of various 

 instruments and techniques. The dynamic response, filtering, linearity, and direc- 

 tionality of various devices were investigated to identify the most accurate flow- 

 meter to employ at depths where the magnitude of orbital velocities caused by 

 surface waves can be several times larger than steady-state flows. 



PROCEDURES 



Selection 



Typical magnitudes of orbital velocities are from 5 to 50 centimeters per 

 second (cm/ sec) with periods from 10 to 15 seconds. Typical steady-state signals 

 are actually very-low-frequency velocity fluctuations. Typical magnitudes are 

 from 2 tolO cm/ sec with periods of interest at 5 to 10 minutes and at semidiurnal 

 and diurnal regions. Since the frequency of these oscillations is so low compared 

 with the predominant orbital or swell frequencies, they may be considered dc or 

 steady-state signals for most analytical work. 



The primary concern will be the selection of a transducer with inherently 

 good features for this particular type of measurement. Of secondary concern will 

 be items such as the amount of electronic equipment required or the necessary 

 power and maintenance. The accessibility of flowmeters installed in shallow 

 water (compared, for example, with deep-water buoy installations) allows more 

 flexibility in the specifications. If sizable amounts of electronic equipment need- 

 ing ac power are required, this will not be considered an inherent bad feature of 

 the system. However, if the electronic package has to be so close to the sensor 

 that it interferes hydrodyn ami c ally with the flow, then it would become an inherent 

 fault. 



Operation 



Two basic procedures, continuous and discrete, can be used operationally 

 for the measurement of the velocities of currents. The continuous method can use 

 a transducer: 



