Definitions regarding waves
Let’s start this lesson with the following definitions:
Sea: created by the wind at that moment.
Swell: created by the wind that was blowing earlier.
Waves = sea + swell.
Below is a table with the translation in English, French and German.
| NL | Zeegang | Deining |
| EN | Sea | Swell |
| DU | Windsee | Dunung |
| FR | Mer | Houle |
Douglas sea scale
| Height (m) | Description | |
|---|---|---|
| 0 | smooth sea | Calm (Glassy) |
| 1 | 0-0.10 | Calm (rippled) |
| 2 | 0.10-0.50 | Smooth |
| 3 | 0.50-1.25 | Slight |
| 4 | 1.25-2.50 | Moderate |
| 5 | 2.50-4.00 | Rough |
| 6 | 4.00-6.00 | Very rough |
| 7 | 6.00-9.00 | High |
| 8 | 9.00-14.00 | Very high |
| 9 | 14.00+ | Phenomenal |
Waves height predictions
At windy.com, for example, you can check wave forecasts.
Waves have the following characteristics:
– Wavelength
– Period
– Wave velocity
– Steadiness
– Wave height
Significant wave height is average of the highest 1/3rd.
Maximum wave height = H max = 1.8 x H signi.
Breakers, ground sea, surf
Long waves are not necessarily dangerous if they are long. Breakers, on the other hand, are more dangerous because they can capsize the ship. The occurrence of breakers can be explained by the following formula: Wavelength = speed x period. A shallow depth e.g. beach, sandbank inhibits waves and the wave speed thus decreases. The wave period will remain the same, so wavelength becomes shorter. Mass remains the same so wave gets higher and steeper, breakers occur. This happens at when the water becomes shallow (ground sea) and in front of the beach (surf). When there are ground seas in front of the harbour, the waves also contain a lot of sand. The waves are then also sandy.
Current against wind
Tidal current against or in a river greatly affects waves. Current against wind shortens wavelength and increases wave height. If current and wind come from the same direction, we have longer wavelength and lower wave height.
Cross Seas
When a depression passes over quickly, we get different wind directions in quick succession and therefore different wave patterns. Those wave patterns then get mixed up and cross seas form. Sailing is very difficult and unpleasant. A cross sea can also be caused by waves rebounding against a steep deep shore.
Calculate wave height
Wave height is determined by the following factors:
– Duration certain wind
– Wind speed
– Depth of water
– Wind path/fetch
The height depends on wind speed, the duration in which the wind blows, the wind path (fetch). From the graph below, you can get the wave height. The y-axis shows the wind speed in knots and the x-axis the wind path (Fetch). The curved lines show the wave height in feet.
Source: NOAA
More comprehensive graphs also include wave period and duration.
The chart above shows:
- On the left axis, the wave height in metres.
- On the right axis, the wind speed in metres per second.
- On the horizontal axis the duration of the wind in hours.
- The transverse lines show the wind path in kilometres.
