Question

You and a friend set up an umbrella and chairs at a beach. Your friend then goes into the surf zone to play Frisbee with another person. Several minutes later your friend looks back toward the beach and is surprised to see that she is no longer near where the umbrella and chairs were set up. Although she is still in the surf zone, she is 30 or 40 yards away from where she started. How would you explain to your friend why she moved along the shore?

Short answer

Longshore drift moves her parallel to the shore, not walking.

Step-by-step solution

Understanding the Scenario

Begin by identifying the key elements of the situation. Your friend starts in the surf zone, near the umbrella and chairs. After a while, she notices she has moved away from her starting point. This movement likely occurred parallel to the shoreline.

Exploring Causes of Movement

Explain that such lateral movement is caused by a process called 'longshore drift' or 'longshore current'. These are ocean currents that move parallel to the shore and are caused by waves hitting the coast at an angle.

Wave Action

Understand that as waves hit the beach at an angle, they transport water and sediment sideways. This process can move objects, including people, stealthily along the shore without them noticing it initially.

Relation to the Scenario

Connect the concept of longshore drift to your friend's situation. Her movement away from the initial point (the umbrella and chairs) is due to the longshore current subtly carrying her down the beach as she plays in the surf.

Recapping Key Points

Summarize by reinforcing that her lateral movement is not due to walking but rather due to the parallel movement of water and sediment caused by angled wave action. It's a common occurrence on many beaches and explains her unexpected shift along the shoreline.

Key concepts

Longshore Current

Imagine standing in the ocean, with waves gently lapping at your feet. As you enjoy the water, you may notice yourself slowly moving sideways along the beach. This subtle movement is driven by a longshore current.
Longshore currents are powerful ocean currents that move parallel to the shore. They arise when waves hit the coastline at an angle, not directly straight on. Because of this angled approach, the energy of the waves pushes water and anything in it, like swimmers or floating objects, down the beach.
The key factors that influence the strength of a longshore current include:

• The direction and angle at which the waves approach the shore.
• The strength and frequency of the incoming waves.

On days with strong angular wave action, the longshore current can be particularly noticeable.

Wave Action

The behavior of waves upon reaching the shore is a fascinating aspect of coastal dynamics. Waves are primarily caused by wind transferring energy to the surface of the ocean. As waves travel, they gain or lose energy based on the wind and other factors such as ocean currents.
When waves reach shallow waters near the coast, they slow down due to friction with the sea floor, and their energy becomes more concentrated. The waves then break onto the shore. It's this breaking action that causes waves to hit the shore at an angle, leading to the movement of water observed on coastlines.
This angular impact is significant because:

• It creates forces that push water along the coastline, contributing to longshore currents.
• It initiates the movement of sediments, an essential process in shaping beaches.

Additionally, the repetition of waves hitting the beach at an angle maintains the continuous flow of water alongshore.

Sediment Transport

Every grain of sand on the beach has a journey, and its movement is largely dependent on sediment transport. Sediment transport is the process by which sand and other materials are moved along the coast by natural forces. Waves, currents, and tides all play a critical role in this movement.
When waves approach the shore at an angle, they not only create longshore currents but also carry sediments along with them. As the waves break, they pick up sand from the seabed and transport it sideways along the coast. This process can lead to various coastal features such as:

• Sand bars, created as sediments accumulate in certain areas.
• Beach erosion in some spots, where sediments are carried away.

The constant movement and re-deposition of sediment are key in forming and reforming the ever-changing landscape of a beach.