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Thursday, January 26, 2012

RIP Currents

Backwash is the seaward flow of water that had been carried shoreward by breaking waves, but the augmentation (wave set-up) of nearshore sea level by shoreward movement of water due to wave motion must also return seaward, typically as localised rip currents. These occur in distinct (though variable) patterns on many shores (particularly along beaches), related to longshore variations in wave set-up. 

They are usually more or less evenly spaced along the shore at intervals of 50 m up to at least a kilometre): their spacing increases with breaker height (and so surf zone width) and decreasing beach gradient (Short, 1999). Within a rip current zone water flows back through the breaker line in sectors up to 30 m wide, attaining velocities of up to 8 km/hrbeforedispersingseaward.Theyareahazard for swimmers, who can be swept out to sea, and who are advised to swim alongshore until they find water moving back towards the beach. The shoreward movement of breaking waves and the seaward return currents form the main components of the nearshore water circulation.

A light or moderate swell produces numerous small rip currents, and a heavy swell produces a few more widely spaced and concentratedrips,fedbystrongerlateralcurrentsinthe surfzone.Pulsationsinoutflowmayoccurinresponse to variations in the height of waves (or groupsofwaves)breakingontheadjacentshore. Rip currents may cut channels seaward through the nearshore zone (across any nearshore bars), and deposit fans of sediment as velocity falls at their seaward limits; in some places these channels have grown headward to form re-entrants in the beach. At low tide rip current velocity may increase as outflow is concentrated along thesechannels. When waves arrive at an angle to the beach the rip currents head away diagonally through the surf instead of straight out to sea, and cut oblique channels through the nearshore zone. Rising tides and onshore winds raise the water level along the shore, and thus intensify rip currents.

Source : Bird, Eric. 2007. Coastal Geomorphology : an Introduction. West Sussex : John Willey and Sons
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