When the wind has imparted its maximum energy to the waves, the sea is said to be fully developed. The maximum frequency range for that wind will have been produced by the fetch, and this maximum frequency range will be present at the leeward edge of the fetch. Once the sea is fully developed, no frequency is produced with a value lower than that of the minimum frequency value for the wind speed in question, no matter how long the wind blows. In brief, the waves cannot grow any higher than the maximum value for that wind speed.

When the sea is fully developed, the area near the windward edge is said to be in a steady state, because the frequency range does not increase any more. If the wind continues to blow at the same speed and from the same direction for a considerable period of time, the major portion of the fetch reaches the steady state.

When the wind is unable to impart its maximum energy to the waves, the sea is said to be nonfully developed. This can happen under two circumstances. First, when the distance over which the wind is blowing is limited or when the fetch is limited. Second, when the wind has not been in contact with the sea for a sufficient length of time, or when the duration time is limited. Now lets look at each situation.

FETCH LIMITED SEA. When the fetch length is too short, the wind is not in contact with the waves over a distance sufficient to impart the maximum energy to the waves. The ranges of frequencies and wave heights are therefore limited, and the wave heights are less than those of a fully developed sea. The process of wave generation is cut off before the maximum energy has been imparted to the waves and the fetch is in a steady state. This leads to the conclusion that for every wind speed, a minimum fetch distance is required for the waves to become fully developed, and that if this minimum fetch requirement is not met, the sea is fetch limited.

DURATION TIME LIMITED SEA. When the wind has not been in contact with the waves long enough, it has had insufficient time to impart the maximum energy to the waves, and the growth of the frequency range and wave heights ceases before the fully developed state of the sea has commenced. Such a situation is known as a duration time limited sea. This leads to the conclusion that for every wind speed, a minimum duration time is required for the waves to become fully developed; and that if this minimum duration time requirement is not met, the sea is duration time limited. The state of the sea, then, is one of three conditions: fully developed, fetch limited, or duration time limited.

Table 6-1 shows the various wind speeds, fetch lengths and minimum wind duration times needed to generate a fully developed state of the sea. When conditions do not meet these minimum requirements, the properties of the waves must be determined by means of graphs and formulas.

As we have discussed, wind is the cause of waves. It therefore stands to reason that in order to accurately predict sea conditions, it is necessary to determine wind properties as accurately as possible. Miscalculation of fetch, wind speed, or duration will lead to inaccuracies in predicted wave conditions.

In this section, we present methods of determining the wind properties as accurately as possible with the available data.

Table 6-1.-Minimum Wind Speed (V), Minimum Fetch Length (F), and Minimum Duration Time (t) Needed to Generate a Fully Developed Sea

Figure 6-4.-Typical fetch areas.

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