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

Your friend moved because of longshore drift caused by waves hitting the shore at an angle.

Step-by-step solution

Identify the Cause

The movement your friend experiences along the shore is called **longshore drift**. This phenomenon happens because of waves approaching the beach at an angle.

Understand Wave Motion

Waves carry sand and water up and down the shore at an angle. When a wave (swash) comes toward the shore, it moves sand in the direction of the wave's arrival. The backwash then drags the sand directly back perpendicular to the shoreline.

Recognize the Effect

Due to the repeated action of angled incoming waves, objects including swimmers will be slowly moved down the shoreline. This movement happens without the swimmer noticing much because they are focused on playing and the drift is subtle.

Summarize the Drift

Your friend's movement is simply a result of the natural shore dynamics. Over time, playing in the surf leads to temporary displacement along the coast due to the angled wave and sand interaction.

Key concepts

Wave Motion

Wave motion is a fascinating topic that explains how energy travels through water. Waves on the beach result from winds blowing across the surface of the ocean. These waves carry energy from one part of the sea to another. When waves reach shallow waters near the shore, their behaviors change. The base of the wave slows down due to friction with the sea floor, causing the wave to grow taller and eventually break.

It's important to note that the movement of waves is directional. Waves often approach the shore at an angle rather than straight on. This angled approach is what sets the stage for longshore drift, where materials like sand on the seabed are moved in a zigzag pattern along the coast. The energy of the waves, influenced by wind strength and direction, creates the rhythmic dance of water we observe. Understanding this helps in grasping how waves shape the coastline over time.

Swash and Backwash

Have you ever noticed water rushing up the sand after a wave breaks? This is known as the swash. It is the forward movement of water up the beach after a wave has crashed. The swash carries with it sand, shells, and even small pieces of driftwood. The direction of the swash is determined by the angle at which the wave approaches the shore. This is the first part that contributes significantly to longshore drift.

After the swash, gravity pulls water back down the beach, and this is called the backwash. The backwash moves straight down towards the ocean, pulling sand and other materials back with it. The combination of swash moving sand up the beach at an angle and backwash pulling it straight back down to the sea creates a zigzag movement. This ongoing process gradually shifts materials, including swimmers and equipment, along the shore.

Shoreline Dynamics

Shoreline dynamics involve the interaction of various natural forces such as waves, tides, and currents, which all impact how the shoreline changes over time. It's a nonstop process driven largely by wave action, including the longshore drift that moves sand and other materials along the coast.

Understanding shoreline dynamics is crucial for comprehending how and why beaches form, erode, or even disappear. Over time, the buildup or displacement of sand affects the shape of the shoreline. Human activities like building barriers or altering coastlines can also influence these dynamics.

• Longshore drift helps redistribute sand, aiding in the formation of sand bars and spits.
• Natural events, such as storms, can dramatically accelerate erosion and sediment movement.
• Programs aimed at beach nourishment work against natural dynamics to preserve coastal areas from erosion.

By understanding these dynamics, we can better predict and manage changes to coastal environments.