Experimental study of longshore currents on a plane beach

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Experimental study of longshore currents on a plane beach

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Galvin, Cyril Jerome Jr. / Eagleson, Peter S.

Measurements were made of the characteristics of breaking waves and the resulting longshore currents for 34 combinations of wave height (up to 0.22 foot), period (0.90 to 1.50 seconds), and breaker angle (up to 32°), along a 20-foot test section of a 30-foot plane, smooth concrete beach with a slope of 0.104. Observation and measurement show that most of the fluid in the surf zone stays there, and that longshore current velocity initially increases downstream from an obstacle. Velocity increases along the beach because the fluid forming the breaking wave has been withdrawn from the surf zone, and thus already has a longshore component of motion, to which is added the longshore component of motion of the breaking wave. A differential equation for this non-uniform flow agrees qualitatively with the measured variation of velocity with breaker angle and with distance from an obstacle. The non-uniformity of the flow was also indicated by the mean water level, which increased, and the breaker position and runup limit, which moved shoreward, downstream from the obstacle, but there is a possibility that these measurements were affected by the experimental apparatus. The energy used to maintain the flow of longshore currents is a small fraction (less than 1/10) of the energy brought to the surf zone by shoaling waves