Observations of rip spacing, persistence and mobility at a long, straight coastline

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• 作者：Ian L. Turner ; David Whyte ; B.G. Ruessink ; Roshanka Ranasinghe
• 关键词：rip currents ; surfzone morphology ; coastal imaging
• 刊名：Marine Geology
• 出版年：2007
• 期刊代码：57_00253227
• 类别：ear
• 出版时间：1 February 2007
• 卷：236
• 期：3-4
• 页码：209-221
• 文件大小：812 K

摘要

Three years of daily video observations at a long, straight beach were analysed to determine the temporal trends and variability of the location, spacing, persistence and mobility of rips. Rips were identified at this site on 684 days of the total 947 days (72%occurrence) when suitable images were available. A median number of 7 rips were observed within the total 2 km study area, on days when rips were present. No tendency was identified for rips to reoccur in preferred locations alongshore following storm reset events. The average alongshore distance between all rips observed was 209 m, but with a high standard deviation of 98 m or approximately 47%of the mean rip spacing, there was no evidence at this site that rips tended to be regularly spaced alongshore. No clear relationship was identified between the number of rips and the prevailing offshore wave conditions, including significant wave height, peak wave period and incident wave power. The majority of rips persisted for 5 or fewer days, with an average of 8 days and standard deviation of 9 days. Rips were stationary 33%of days, with migration rates of less than 5 m/day observed on 47%of days when rips were present. Maximum migrations rates of up to 50 m/day were observed, and generally though not always occurred in the direction consistent with the prevailing offshore swell direction and resulting alongshore current. The occurrence of inshore sea breezes may have accounted for the less frequent observations of rip migration against the opposing regional swell direction.

The results presented at this site complement a recent four year study at a contrasting short and embayed beach, detailed in Holman et al. [Holman, R.A., Symonds, G., Thornton, E.B. and Ransinghe, R., 2006. Rip spacing on an embayed beach. Journal of Geophysical Research, 111, C01006. doi:10.1029/2005/JC002965 (17p)]. The specific mechanism(s) of rip channel infilling at the long, straight and littoral-drift dominated coastline may have contributed to the observation of shorter rip persistence relative to the embayed site. Mean rip spacing was observed at both sites to be highly variable, irregular, and exhibited no identifiable trend associated with the offshore wave conditions. This latter observation supports the notion that rips may become rapidly topographically controlled soon after a storm reset event, and their location is then primarily determined by the evolving nearshore morphology, rather than hydrodynamic forcing.

This new dataset of daily, multi-year observations at an open coast site cannot be reconciled with the majority of existing template and instability models for rip formation, that predict a relationship between incident wave conditions and regular spacing of rips alongshore. Rather, these observations tend to support emerging theories of rip genesis that predict the irregular and random spacing of rip channels alongshore.