Water level changes and its effect on nearshore wave forecasting

Thu 24 November 2022

2 minutes read

Photo: Adobe Stock / Remi Piotrowski

As you travel along any coastline, the water level may strongly vary. This variation in water level results in a larger difference in wave heights than you might expect. In this article we will explain four influences on how the depth influences wave height.

Water motion in a wave

To thoroughly understand what happens in the water when a wave passes, we not only have to consider the water at the surface, but also the sub-surface. A ‘water particle’ or droplet in the sub-surface only slightly moves along with the wave, it remains more or less in place as a wave passes. The droplet actually makes a small circular motion as the wave passes. It goes up on arrival of the wave, and goes down after the wave has passed. The further we go down in the water, the smaller this motion gets.

Effect of depth on waves

A wave is affected by the seabed when the movement of these water particles are limited by the seabed. In case there is friction of the wave motion at the bottom of the ocean, the part of the wave near the surface travels faster than the wave underwater. If the wave motion is limited long enough, eventually the top of the wave will topple over: we talk about wave breaking here.

Tidal influences of sun and moon

The depth of the water can be largely influenced by tidal effects. The gravity of both sun and moon affect movement of water. When the sun and moon is nearest, we have a spring tide (or: king tide), the sun and moon are both in line with each other. As a result a bulge of water, attracted both by the sun and the moon makes water rise significantly. With neap tide, the sun and moon are not aligned and the bulges of water cancel each other out. Depending on the time and location, this tidal effect may result op to an increase of water depth over 16 meters (in the Bay of Fundy, Canada). Also in Europe, the water level may vary significantly, (in the Bristol Channel, United Kingdom) it can cause up to a 15 meter difference. And as we know now, the deeper the water is, the higher the waves may get. So, with spring tide we can expect way higher waves than with neap tide.

Water offset by wind

The offset of water is not only affected by sun and moon, also the wind carries along bodies of water. This is most notably seen in the Antarctic Ocean (South Pole). Here, storms can cause a significant offset since the ocean is not limited by a land mass. Not only in large bodies of water, this can be achieved, also in inner seas like the Black Sea or the Caspian Sea, the water offset by wind can cause a significant change in water level and therefore can significantly affect the expected wave height.

Wave course

At Infoplaza, we strongly believe that incorporation of the water level is important to get a reliable weather forecast. Especially in coastal areas, tidal dominated areas or in shallow inner seas the water level effect has to be incorporated. We made an online wave course. In six video’s we explain principals about waves and wave forecasting.

Watch wave course

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