• Point source and related technology. 



• Acoustic Doppler Current Profilers (ADCP) and related technology. 



The large number of instruments and methods used to measure currents under- 

 scores that detection and analysis of fluid motion in the oceans is an exceed- 

 ingly complex process. The difficulty arises from the large continuous scales 

 of motion in the water. As stated by McCullough (1980), "There is no single 

 velocity in the water, but many, which are characterized by their temporal and 

 spatial spectra. Implicit then in the concept of a fluid 'velocity' is knowledge 

 of the temporal and spatial averaging processes used in measuring it. 

 Imprecise, or worse, inappropriate modes of averaging in time and/or space 

 now represent the most prominent source of error in near-surface flow 

 measurements." McCullough's comments were addressed to the measurement 

 of currents in the ocean. In shallow water, particularly in the surf zone, 

 additional difficulties are created by turbulence and air entrainment caused by 

 breaking waves, by suspension of large concentrations of sediment, and by the 

 physical violence of the environment. Trustworthy current measurement 

 under these conditions becomes a daunting task. 



Lagrangian 



Dye, drogues, ship drift, bottles, temperature structures, oil slicks, radio- 

 active materials, paper, wood chips, ice, trees, flora, and fauna have all been 

 used to study the surface motion of the oceans (McCullough 1980). Some of 

 these techniques, along with the use of mid-depth drogues and seabed drifters, 

 have been widely used in coastal studies. A disadvantage of all drifters is that 

 they are only quasi-Lagrangian sensors because, regardless of their design or 

 mass, they cannot exactly follow the movement of the water (Vachon 1980). 

 Nevertheless, they are particularly effective at revealing surface flow patterns 

 if they are photographed or video recorded on a time-lapse basis. Simple 

 drifter experiments can also be helpful in developing a sampling strategy for 

 more sophisticated subsequent field investigations. Floats, bottom drifters, 

 drogues, and dye are used especially in the littoral zone where fixed current 

 meters are adversely affected by turbulence. 



High-frequency (HF) radar surface-current mapping systems have been 

 tested since the 1970's. The advantage of using the upper high radar frequen- 

 cies is that these frequencies accurately assess horizontal currents in a mean 

 water depth of only 1 m (total layer thickness about 2 m). Hence, HF radar 

 accurately senses horizontal currents in the uppermost layers of the oceans, 

 where other instruments such as moored current meters and ADCP's become 

 inoperable (Barrick, Lipa, and Lilleboe 1990). Nevertheless, HF radar has 

 had limited success in the oceanography community because of the difficulty 

 in proving measurement accuracy and because of relatively high system costs 

 (Appell and Curtin 1990). 



Large-scale coastal circulation can be observed in satellite images, as seen 

 in Figures 13 and 14. 



Chapter 3 Field Data Collection and Observation 



39 



