The Forces of Waves

Clarification Statement: Examples could include that an unbalanced force on one side of a ball can make it start moving and that balanced forces pushing on a box from both sides will not produce any
motion at all.

Assessment Boundary: Assessment is limited to one variable at a time: number, size, or direction of forces. Assessment does not include quantitative force size, only qualitative and relative. Assessment is limited to gravity being addressed as a force that pulls objects down.

Table of Contents
Representative Image

This activity builds on the content below.


Balanced and Unbalanced Forces

Forces acts on one particular object with both a strength and a direction. An object at rest typically has multiple forces acting on it, but they add to give zero net force on the object. Forces that do not sum to zero can cause changes in the object’s speed or direction of motion. For example, when the ocean is still in a particular area, the forces are balanced and the sand will remain on the beach. If the energy stored in waves is active and the waves are strong, the beach may experience erosion as the sand particles are pulled off shore. Depending on the strength of the waves, the erosion on a beach may be more or less severe. 

The Force of Waves


Image caption

Fig. 1. The force of a large wave in Waimea Bay, Hawaiʻi, carries a surfer.

Image copyright and source

Image by Byron Inouye.

Most ocean waves are caused by wind (see The Forces of Wind topic). The type of waves tend to change with seasonal changes in wind. For example, during the winter, waves on the northern shores of the Hawaiian islands can be very large. The force of these large winter waves pry sand and rocks from the shore and carry them into deeper waters. In contrast, small summer waves on the northern Hawaiian shores are gentle, but the force of them helps to return sand and rocks back to shore. The opposite is true of southern shores in Hawaiʻi, which tend to have higher swells in the summer than in the winter.

Waves are strong. The force of waves can carry people, plants, animals, and trash (Fig. 1). The force of waves can also cause dramatic change to the beach and shore. Large, storm waves can pull the sand from a beach! Small waves can slowly build up and washing away of sand on a beach.



Wave-Coast Interactions

The study of wave and beach interactions contributes to our understanding of the processes of erosion (loss) and accretion (buildup). Sand erosion is often a problem for property owners because it removes valuable property (Fig. 2). Sand accretion can also be a problem; for example, a sandbar can block boat channels, and sand deposits can fill harbors.


Image caption

Fig. 2. Intense waves and high sea level events contribute to coastal erosion, which threatened this house on the North Shore of O‘ahu.

Image copyright and source

Image courtesy of Dr. Bradley Romine.


In an undisturbed coastal area, sand stored on the low-lying coastal plain is released to the beach as sea level rises. This allows the beach to maintain a wide sandy shoreline even as a sinking island or rising sea level causes the beach to migrates landward. However, the hardening of the shoreline, which is the construction of vertical seawalls and revetments to protect coastal lands from marine erosion, protects the immediate area for a limited time but ultimately causes erosion by preventing waves from accessing sandy reservoirs (Fig. 3).


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Fig. 3. Coastal erosion and “hard armoring” using sea walls has resulted in stretches of shoreline that have no beach, such as this location at Lanikai, O‘ahu.

Image copyright and source

Image courtesy of Dr. Bradely Romine.

Studies conducted at the University of Hawai‘i show that hardening the shoreline of Oʻahu has caused 10.7 miles of beach to narrow and 6.4 miles to be lost. This is ~24% of the 71.6 miles of originally sandy shoreline on Oʻahu. The loss of beaches not only negatively impacts human activities and property but also affects the environment by smothering local marine life with eroded sediment and causing sewage spills into nearshore waters. In order to save beaches, scientists recommend replenishing sand, keeping coastal areas free of hardened structures and requiring large setbacks of new development. See the example photo taken at central Kailua beach on Oʻahu, showing adjacent property lots with sufficient and insufficient setback (Fig. 4). 

The main causes of coastal erosion are:

  1. High waves and currents
  2. Human impacts
  3. Sea level rise


Image caption

Fig. 4. Kailua beach on Oʻahu, showing adjacent property lots with sufficient and insufficient setback.

Image copyright and source

Image Courtesy of Dr. Bradley Romine.

Wave Erosion Vocabulary:

  • Accretion: the process of growth or increase, typically by the gradual accumulation of additional layers or matter
  • Erosion: the process of eroding or being eroded by wind, water, or other natural agents
  • Revetment: a retaining wall or facing of masonry or other material, supporting or protecting a rampart, wall, etc.
  • Seawall: a wall or embankment erected to prevent the sea from encroaching on or eroding an area of land
  • Swell: a slow, regular movement of the sea in rolling waves that do not break
  • Waves: a long body of water curling into an arched form and breaking on the shore


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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawai?i, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.