Field experiments in the past 10 years have generally recognized the 

 variability of longshore currents across the surf zone and down the coast. 

 Mean values distort the true physical process. To date, the maximum long- 

 shore current value reported (see App. A) is less than 2 meters per second. 

 Hom-ma, Horikawa, and Sonu (1962)^ reported measurements (methods not stated) 

 of surf zone swash reaching 3 meters per second during a typhoon in Japan 

 (from Gourlay, 1978) . 



b. Nearshore Circulation and Rip Currents . Rip currents are narrow, 

 strong return flows directed through the surf to sea. Together with longshore 

 currents they can create a two-dimensional coastal current system within and 

 beyond the surf zone termed nearshore ciroulablon. The first detailed and 

 well-doctnnented measurements of such a system of currents were reported in 

 Shepard and Inman (1950) for La Jolla, California (Fig. 1). Surf floats, 

 drogues, and dye were used to measure the magnitude and directions. Mean 

 values reported ranged from less than 0.1 meter per second outside the surf 

 areas, 0.5 to 1.0 meter per second alongshore in the surf zone, and 0.25 to 

 1.0 meter per second entering into the rip currents. Detailed current speeds 

 along the rips were not reported. 



As early as 1936, Shepard reported on the qualitative aspects of rip cur- 

 rents (Shepard, 1936). Shepard, Emergy, and LaFond (1941) cited maximum rip 

 speeds greater than 1 meter per second and occasionally extending more than 

 500 meters from the shoreline. These observations in California were confirmed 

 by McKenzie (1958) in Australia and by Dobson and Draper (1965) on the Atlantic 

 coast of England. The latter researchers used a theodolite to observe the 

 speed of floats on the foam line. A maximum rip current speed of 2.5 meters 

 per second in the surf zone was reported with a mean value of about 1.4 meters 

 per second in the surf and 0.7 meter per second beyond the breakers. Rips 

 were observed more than 300 meters from shore. Horikawa (1978) sximmarized 

 Japanese field observations and concluded that speeds near the root are about 

 1 meter per second and can reach 2 meters per second under rough sea condi- 

 tions. Surprisingly, there have been few actual rip current velocity measure- 

 ments reported. Dalrymple (1976) presents a bibliography on rip currents that 

 includes only 14 references to field observations. Many of these concentrated 

 on nearshore circulation patterns as discussed below. 



A series of rip currents is usually found along the coast with longshore 

 currents feeding the rips and forming independent circulation cells. Spacings 

 on the California coast have been reported as 30 to 100 meters (Inman and 

 Bagnold, 1963)^ and 400 meters on the average (Shepard and Inman, 1951). 

 Horikawa (1978) observed some Japanese coasts with 200 to 300 meters in rip 

 spacings and others with only 80- to 150-meter intervals. Little is knovm 

 about the actual width of rip currents. Widths of 10 meters or less near 

 their root and 30 meters at the head (beyond the breakers) are crude estimates. 



^HOM-MA, M., HORIKAWA, K., and SONU, C, "Field Investigation at Tokai, Japan 

 Conducted by Combined Procedure of Macroscopic and Microscopic Approaches," 

 Coastal Engineering in Japan, Vol. 5, 1962, pp. 93-110 (not in bibliography), 



^INMAN, D.L., and BAGNOLD, R.A., "Littoral Processes," The Sea, M.N. Hill, 

 ed., Interscience, Vol. 3, New York, 1963, pp. 529-553 (not in bibliography). 



24 



