Tuesday, August 23, 2005

Rip Currents

I have posted about rip currents and ocean wave theory before and recently I came across some interesting material describing some of the key causes of dangerous rip currents as well as well as the theory behind how rip currents form and how strong they become. The National Weather Service has some great resources on rip currents for anyone who does not know what a rip current is or how to identify one on their website.

The first paper, which is on the NWS’s rip current page, describes a study to improve forecasting of dangerous rip currents in Volusia County Florida. The study concluded that four factors contributed to the formation of dangerous rip currents – those rip currents that tended to lead to large numbers of rescues by local lifeguards. These four factors are: wave height, wave direction, wave period, and a tidal factor. The first two factors seemed pretty obvious to me because wave height and wave direction clearly play an important factor in the strength and formation of rip currents. The later two, wave period and the tidal factor, were interesting to me. I had not thought much about wave periods influencing rip current strength, but it certainly makes sense that:
Long period waves will be effected by the bottom at deeper depths and will break further from shore, resulting in a larger wave set up. Long period waves may also have significance due to their ‘groupiness’ creating pulses of the rip currents (Shepard and Inman 1950), catching bathers by surprise.

When I think back to days when I have made the most rescues, the majority of rescues tend to happen after a larger set of waves breaks over the sandbar. I would hypothesize that long period waves cause increased rescues due to 1. the increased strength of the rip current themselves and 2. the temporarily higher set up height which causes swimmers in relatively deep water to suddenly find themselves in water over their heads and hence become venerable to the currents.

The tidal factor undoubtedly plays a major role in the formation and strength of rip currents. This study found that 62% of rescues occurred during the 42% of the time when the tide was between 0.45 m and 0.75 m below the mean with 0.6 m below the mean being ideal. It is interesting that evidence from Volusia County shows this empirically. I would expect that the tidal component might be somewhat more complex than this because of differences in the structure of the ocean floor at various different beaches. The empirical study shows that when the tide falls below 0.75 m of the mean the number of rescues decrease, perhaps because of sandbar exposure. The tidal level where sandbar exposure begins likely varies from beach to beach and also over time at a single beach as the height of the sandbar changes.

I will not comment as much on the second article that I found here, which describes some ongoing research to determine the causes of rip currents and lists some hypothesis about why the form in the first place. Many people who swim in the Ocean have long been able to describe what a rip current is, how to identify it, and how to avoid and escape from one. It looks like science is still working to identify the underlying physics of why they from and how to predict where and when they will form.

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