Anatomy of Water Waves
A Creation Shaped From Wind, Current, and Water Depth
Understanding the structure, development and behavior of water waves is an important seamanship skill that boaters should understand.
Water waves shape the very basis of boating. The character of the wave climate influences marine safety, mooring choices, waterfront development, and vessel design. Regardless of their origin, all waves possess the same basic structure. The top of a wave is its crest, the gully following it is the trough, and they are experienced as a series or train of waves. A discussion of waves seldom focuses on an individual event, but rather the greater climate of wave conditions.
Wave Anatomy - Wave height is the most significant aspect of a wave's anatomy to a boater. Wave height is the total distance from the bottom of the wave trough to the peak of its crest. Another height term used is the crest height, which is the distance from the imaginary still water level to the top of the wave. Most marinas are designed to reduce wave heights to less than thirty centimeters (one foot) to ensure a safe mooring environment. Waves in the open ocean can easily exceed fifteen meters (fifty feet) in height, far exceeding the capability of most recreational boats.
Waves are a cyclical series of crests and troughs. An important measurement of this behavior is the length from peak to peak and the time it takes for a number of these crests to pass an observer. This measurement is the wave period. A wave with a length of fifteen meters (fifty feet) will be traveling at a speed of just under ten knots. The third factor of wave formation involves water depth. Since waves are part of a cycle of movement, the wave motion extends down into the water a distance equal to about half of the wavelength. The impact of this effect is seen in buildup of a wave approaching a beach that creates surf conditions and exaggerated wave heights.
Wave Development - Waves are produced by the wind traveling over the water surface. Friction arising from contact between the two fluids moves the water. Stronger winds moving over vast open stretches of water will produce the greatest waves. Mariners describe the open water a wind can pass over as fetch. Many natural water bodies like fiords present a long fetch under specific weather conditions. Wave energy will build as the wave train moves downwind. Natural topography and local currents greatly affect the size and shape of waves. Waves will refract off of rocks or manmade structures like breakwaters. Wind traveling along the water surface will also create an underlying current that boaters need to consider.
Wave Development Factors:
- Wind speed and direction
- Overall period of wind exposure
- Distance wind travels over open water (Fetch)
Wave Behavior - Waves break when the width of their base can no longer support the wave's height. Water crashing down from a breaking wave can produce five to ten tons of force per square meter. High winds can shear the top of wave crests. The Beaufort Wind Force Scale uses this phenomena to describe a gale force wind. Boaters can avoid the effects of larger waves by sailing on the lee side of landmasses, traveling upwind in heavy weather and avoiding shallow water. Monitoring marine weather forecasts can help a sailor predict the behavior of future wave conditions.
