Beaches are Rivers of Sand: The Key to Making Groins and Breakwaters Work

Have you ever gone to the beach, walked down to the shoreline, and gone for a dip in the ocean… only to find out minutes later that you’ve ended up down the beach from where you got in?

Here’s why that happens and why it’s relevant to understanding how groins and breakwaters work.

Incoming waves rarely strike parallel to the shoreline and instead break at an angle to the shoreline. Those incoming waves move sand up the beach at an angle -and in the same direction of the incoming waves, but the rush of water back to the ocean carries the same sand particles straight down the beach, perpendicular to the shoreline and not at an angle. Over time, this “longshore current” and zig zag pattern move sand parallel to the beach and shoreline by a process called “longshore transport.”

Estimates of how much sand moves in the longshore currents may surprise you. Along the ocean side of Virginia’s coasts, anywhere between about 115,000 cubic meters per year (about 150,000 cubic yards per year) to about 460,000 cubic meters per year (about 601,000 cubic yards per year). The lower number is enough to cover a football field from goalpost to goalpost 7 stories high and the higher number would cover a football field 28 stories high!

That’s the sand that groins and breakwaters attempt to capture being delivered to them by the longshore currents.

However, Chesapeake Bay beaches have much less sand transported along its beaches than the ocean side beaches. That’s because the waves don’t get nearly as large in the Bay as they do on the oceanside and there isn’t always as much sand available. While there aren’t many estimates of longshore transport rates in the Chesapeake Bay, an older study suggested that the football field would only fill up with about ¼ of an inch of sand.

In summary, longshore currents produce “rivers of sand” that move along the coast. They are a natural part of the coastal system and play a big role in maintaining wide beaches. This is the sand that groins and breakwaters aim to capture in a local area.

One last important point: You can’t rob Peter to pay Paul as they say… in other words, if you trap sand at one location, it gets taken away from another location downdrift and causes erosion at that location.