September 2019 (ROTM#129) Tijuana, Mexico

The rip current this month is a great example of a 'boundary' rip current, which occur against physical structures, such as headlands, rock platforms, groynes, jetties and piers. These types of rip currents are often channelised and therefore appear as narrow dark gaps along the structure where wave breaking is reduced. Their flow is quite strong and can extend significant distances beyond the structure itself.

 Boundary rips are sometimes called headland rips, topographic rips, or permanent rips. The reason for the latter name is that as long as wave breaking is taking place near physical structures, there will almost always be a rip current running offshore along the structure - sometimes on both sides.

 If waves approach the beach and break, a longshore current will be generated that will be physically deflected offshore by the structure. That's fairly easy to understand. But water will always flow from areas where there is lots of wave breaking activity to areas where there is less breaking wave activity. This means that on the other side of the structure (the downwave and 'protected' side) the water will flow back towards the structure and back offshore. So even the seemingly calm waters on the protected side of a structure can mask strong offshore rip current flow.

 The main message here is: never swim next to physical structures regardless of whether they are natural or anthropogenic.

 The fascinating aspect of this picture is that the metal wall is separating the border of Mexico (to the left) and the United States (to the right). It therefore acts as a physical boundary, almost like a groyne and rip current flow will be present on either side of the wall. Riding the rip may actually take you into the United States from Tijuana although I wouldn't recommend it.

 Thanks to Jonathan Webber, a big name in the drowning prevention world, and manager of Aquasafe New Zealand Limited, for the picture.

Ride the rip from Mexico (left) to the US (right)