Longshore currents and rips

CURRENTS AND RIPS NEAR THE SHORE Robert Perry Malibu High School

World surface currents are sometimes not the same as the currents that run along the very edge of the shore.

Longshore currents are currents of water flowing near the shore and parallel to the shore. Longshore currents are formed by water being tranported to the beach by waves striking the shore at an angle.

LONGSHORE CURRENT a water current that runs along the shore LONGSHORE DRIFT the drifting of substances (such as sand) which are transported by the longshore current

Longshore currents Longshore currents flow in one direction along the shore between the breakers and the shore.

The longshore current is strongest when the waves are high and approaching at a greater angle to the shore.

If you are standing on the beach, facing the water, and the waves are approaching at an angle to the shore from the right, the longshore current is probably moving in one direction towards your left.

Longshore currents can be escaped by swimming towards the shore and/or wading to shore.

Q. When waves create a longshore current, where does all that water end up going?

A. It eventually finds (or creates) a low spot, jetty or groin and rushes back out to sea. This is called a “Rip Current.�

Longshore current

Rip Currents Rip currents are channeled currents of water flowing away from shore.

They typically extend from the shoreline, through the surf zone, and past the line of breaking waves.

Rip currents can occur at any beach with breaking waves.

Rip currents most typically form at low spots or breaks in sandbars, and also near structures such as groins, jetties and piers. Rip currents are usually very narrow but can extend in width.

The seaward flow of rip currents varies: sometimes the rip current ends just beyond the line of breaking waves, but more powerful rip currents continue to push hundreds of yards offshore.

Note the effect of the longshore current on these 3 rips.

How do you spot a rip current?

The key signs to look for are: Deeper darker water Fewer breaking waves Sometimes sandy colored water extending beyond the surf zone Debris or seaweed Sometimes it’s easier to look for where the waves are breaking consistently, and then look to each side where they don't break consistently.

That’s the rip current!

LONGSHORE DRIFT ‌. is the movement of sand along the coastline. This movement of sand is influenced by the surf zone currents created by waves and the predominant wave direction.

Under these conditions, waves break at an angle to the shoreline (forming oblique waves) and the sand is moved along the beach in the surf zone.

The direction the sand moves depends on the dominant wave direction.

LONGSHORE DRIFT

GROINS Groins are man-made structures designed to trap sand as it is moved down the beach by the longshore drift. As the longshore drift current approaches the groin, it is forced to slow down and change direction. This chance in velocity causes sand suspended in the current to be deposited on the up-drift side of the groin. As the current then continues around the groin, it becomes turbulent and actually contributes to erosion on the downdrift side of the groin.

GROINS AND LONGSHORE DRIFT

Santa Barbara Harbor - Sand Deposition