Why do piers have rips?

Recently, I was surfing at Scripps Pier when I experienced the peculiar power of pier rips firsthand. Starting on the north side, I paddled about 20 feet before the current abruptly forced me through the pier to the south side. Another 20 feet later, the rip ordered me north again, threatening me with barnacles if I didn't comply. I was a bit spooked by how forcefully the water moved me around those pilings, but this pokey meander got me thinking about the currents under the pier.

If there's a pier in sight, I'll usually paddle out alongside it. Similar to jetties, piers reliably generate rip currents that can provide an express route to the lineup. But unlike the straightforward rips along jetties, pier rips create more complex flow patterns that deserve closer examination.

The fundamental mechanism begins with breaking waves. When waves break, they push significantly more water toward shore than unbroken waves do. This water accumulation can only run up the beach so far before gravity pulls it back. With nowhere else to go, it flows sideways as alongshore currents.

Two different types of rips in panels (a) and (c). These are for a jetty, but the same idea applies for piers, with some modification since flow comes from both sides. Credit: B. Castelle et al. 2016, “Rip current types, circulation and hazard” on Elsevier.

On beaches without structures, these currents flow freely until they converge between breaking peaks, forming transient rips that carry water back offshore. Since wave breaking patterns constantly change, these rips migrate along the beach over time.

Piers disrupt this natural rhythm, at least partially. Unlike solid jetties that completely block alongshore flow, a pier's pilings create drag that slows water moving parallel to shore. This slowdown causes water to "pile up" near the pier, creating convergence zones where accumulated water can only push so much volume up the beach, forcing the rest to flow offshore as a rip current. The more pilings, the closer the flow is to that of a solid jetty.

The interactions with existing currents and waves beneath the pier further complicate these dynamics. When waves break on both sides, water accumulates underneath and heads offshore. As this cross-shore current interacts with the pilings, it experiences drag that generates eddies and turbulent features. The influx of water from the sides meets these swirling currents under the pier, creating those complex, sometimes unpredictable flow patterns that sent me pinballing between pilings.

A relatively low Reynolds number flow past a cylinder. While the behavior is different for ocean conditions and pier pilings, this demonstrates some of the eddy formation from flow past a cylinder. Credit: Cesareo de La Rosa Siqueira.

This interaction between alongshore currents, cross-shore flows, and structural elements explains why pier rips often don't provide the straight offshore shot that jetty rips do. Instead, they create a dynamic environment where currents can shift direction and intensity as you move through them.

While pier rips may send you closer to the pilings than might be comfortable, they remain valuable tools on those short-period days that would otherwise keep you diving through endless sets. The tradeoff between slightly sketchy pier navigation and an easier paddle out is an ever-changing preference for me.

Further Reading:

Types of Rips

Eddies

Reynolds Number