CHAPTER 2 



PHYSICAL DESCRIPTION OF NEARSHORE CURRENTS 



I. FIELD OBSERVATIONS 



One of the earliest known scientific descriptions of coastal currents 

 was made by Johnson (1919). The longshore current was Induced by an oblique 

 angle of wave Incidence. Undertows or rip currents were said to occur when 

 waves struck the shore at right angles. The hydraulic currents were thought 

 to be created by translation and asjmmetric, shallow wave motion piling water 

 up against the shore. This volume increase concept persisted for more than 

 50 years and even became the basis for some theories before 1967. 



1. Measurements of Mean Values . 



a. Longshore Currents . The wave- induced current that flows alongshore 

 and is generally confined between the first breaker and the shoreline is 

 termed the tongshove oiccvent. It was first measured by Putnam, Munk, and 

 Tray lor (1949), using traveltimes and distances of floats and dye in the surf 

 on the California coast. Currents up to 1.7 meters per second were recorded. 

 Although, on the average, these currents are usually much smaller, these re- 

 searchers reported 18 measurements that gave a mean of 0.9 meter per second. 

 Shepard (1950) reported the results of more than 2,000 measurements of long- 

 shore currents over the entire southern California coast. Using floats, the 

 maximum value given was only 0.5 meter per second. More detailed measurements 

 taken at Torrey Pines Beach, California, were reported in Inman and Quinn 

 (1955) . Measurements by floats positioned near midsurf gave a maximum value 

 of 0.4 meter per second with a mean of about 0.1 meter per second. Moore and 

 Scholl (1961) also reported a maximxam value of 0.4 meter per second using dye 

 on an Alaskan beach. Galvin and Savage (1966) reported five measurements 

 using floats at Nags Head, North Carolina. They obtained a maximum current of 

 1.3 meters per second. 



Except for Shepard (1950) , all these experiments included enough field 

 information to be useful to Calvin's (1967) assessment, and they are still 

 being used today. Since Calvin's study, more data sets have been reported 

 but some failed to record the important independent variables. Many are ex- 

 tremely detailed with the introduction of an array of fixed current meters of 

 the propeller or electromagnetic (EM) t3rpe to give continuous velocity read- 

 ings. Appendix A provides a chronological tabulation of 38 experiments re- 

 ported in the literature review. Also given are the range of wave data pre- 

 sent, beach slope, observation method, and location of experiments as reported, 

 if available in the reference. Measurements have been reported from all over 

 the world (South Africa, Mexico, Japan, Australia, the North Sea, England, 

 Egypt, India, Canada, and Poland). Surprisingly, no measurements could be 

 found for the Gulf of Mexico coast of the United States. All other U.S. coasts 

 have reported experiments, including some Lake Michigan measurements. The 

 California coast is the most frequently observed. 



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