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Rip-current type, spacing and persistence, Narrabeen Beach, Australia
In: Marine Geology. Elsevier: Amsterdam. ISSN 0025-3227; e-ISSN 1872-6151, meer
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| Abstract |
Long-term daily rip observations on Narrabeen Beach, Australia, were made together with daily measurements of wave height, period, direction, surf-zone width and beach state. A total of 3513 rips were observed on 270 days over a 19-month period. Results show that rips accompany moderate to high waves and are associated with intermediate beach types. Rip spacing follows wave conditions increasing in spacing, size and intensity as waves rise, and conversely as they fall. Rips are a function of both the prevailing and antecedent wave conditions and the rate and direction of change in wave conditions. Rips are classified into three types: erosion, mega and accretion. Erosion rips are initiated by rising seas in the transverse bar and rip and higher beach states. They accompany general beach erosion. They are widely spaced (Narrabeen ys = 300–500+ m, s = 100–200 m) increasing in size and intensity with the waves until the fully dissipative (ripless) state is reached. They are both temporally and spatially highly variable persisting in one location for only hours to a day or so, except where topographically controlled. They are a major mechanism for the seaward transport of sediment and under extreme conditions may extend over a kilometer seaward of the breaker zone. Mega rips are large-scale (>1 km) topographically controlled erosion rips that persist when nearshore and/or embayment topography prevents the development of the fully dissipative state by inducing wave refraction and persist longshore gradients in surf-zone dynamics that in turn drive the rip circulation. Accretion rips prevail during stable or falling wave conditions, usually following erosion rip formation. They are associated with general beach accretion, are more closely spaced (Narrabeen ys = 170–250 m, s = 100 m), less intense and become increasingly topographically arrested by crescentic bars and rhythmic bar-beach morphology of the longshore bar-trough, rhythmic bar and beach and transverse bar and rip beach states. They are relatively stable both spatially and temporally and may persist in one location for days to weeks if wave conditions remain favorable. They are fair weather rips which may disappear by infilling (in the low-tide terrace beach state) or be destroyed by rising seas. Rips generally are absent from the reflective, low-tide terrace and fully dissipative beach states. |
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