Undertow (water waves)

Buhr Hansen and Svendsen ICCE 1984 Fig 1

Okayasu Shibayama Mimura ICCE 1986 Fig 8

Undertow is a term often used to describe a specific type of water current that can occur near the shores of oceans, seas, and large lakes. This phenomenon is particularly relevant in the context of swimming safety and coastal environment management. Understanding the dynamics of undertow is crucial for both swimmers and those involved in lifesaving and beach management practices.

Definition

An undertow occurs when water pushed ashore by waves flows back into the ocean or lake under incoming waves. Contrary to popular belief, an undertow does not pull swimmers under the water; instead, it pulls them out to sea along the bottom. This can make it difficult for swimmers to remain standing or to move towards the shore.

Causes

The primary cause of undertow is the action of breaking waves on the shoreline. When a wave breaks, water is pushed up the slope of the shore. Gravity then pulls this water back towards the sea, creating a seaward flow under the incoming waves. The strength and danger of an undertow can be influenced by several factors, including the wave height, wave frequency, slope of the beach, and the underlying seabed contours.

Differences from Rip Currents

It is important to distinguish between undertow and rip currents, as the two are often confused. While undertow is a return flow of water from the shore towards the sea, rip currents are strong channels of water flowing away from the shore, cutting through the lines of breaking waves. Rip currents can pull swimmers out to deeper water, but unlike undertow, they can occur at various depths and are more noticeable at the surface.

Safety Measures

To ensure safety near water bodies exhibiting undertow, swimmers are advised to:

• Swim at beaches with lifeguard supervision.
• Avoid swimming alone.
• Learn to recognize the signs of undertow and rip currents.
• Not fight against the current if caught in an undertow; instead, swim parallel to the shore until out of the current, then swim back to shore.

Environmental Impact

Undertow can also have significant environmental impacts, influencing beach erosion, sediment transport, and the distribution of nutrients and pollutants. These effects can alter coastal habitats, affecting both terrestrial and marine ecosystems.

Conclusion

Understanding the dynamics of undertow is essential for beach safety and environmental management. By distinguishing undertow from similar phenomena like rip currents, individuals can take appropriate safety measures to protect themselves and others while enjoying coastal areas.