12.11: Rip Currents and Rip Tides
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Rip Currents and Rip Tides
A rip current (or just “rip”) is a current that flows away from the coast. Rip currents form when wave break strongly in one direction, but weakly in another. In the surf zone, breaking waves produce currents that flow both along the shore and out to sea. Rip currents typical form on beaches with a sand bar and channel system in the nearshore area. A rip current forms as a narrow fast-moving current of water moving in an offshore direction. Obstructions in the water can also deflect current offshore. Rip current vary in size and speed (up to 6 miles per hour [10 km/hr], or faster than an Olympic swimmer). Rip currents move offshore and dissipate beyond the breaker zone. If caught in a rip current, swim parallel to shore to leave the current before heading for shore.
A rip current is different than a rip tide, which is current associated with the swift movement of tidal water through inlets and the mouths of estuaries, embayments, and harbors caused by the rise and fall of tides.
|Figure 12.38. Rip currents are wave-generated currents that move in an offshore direction.||Figure 12.39. Rip current can vary with size and intensity depending of waves and shore geometry.|
Tidal currents (including rip tides) are strong erosional forces where they are restricted at the mouths of inlets and straights between bodies of water. One example include the narrow straights of the Verrazano Narrows (between Staten Island and Brooklyn on Long Island, NY) (Figures 12-40 and 12-41). Another example is the Golden Gate Narrows between San Francisco and Marin County in northern California (Figure 12.42). In both cases, the seabed has been scoured deeply by the daily tidal flows. Tidal flows redistribute sediments building submerged tidal deltas at opposite ends of the channel that need to be dredged frequently to mitigate hazards to shipping.
|Figure 12.40. Dominant winds, waves, and currents of the New York City region.||Figure 12.41. Bathymetry shows tidal current scour in the Verrazano Narrows.||Figure 12.42. Bathymetry shows tidal current scour in near the Golden Gate Bridge.|