What creates waves?
Several factors may create waves but the most important are:
· Human activity, such as shipping
· Special natural phenomena such as earthquakes, land slips and underwater volcanic eruptions create waves known as tsunamis
· Tide; waves with extremely long wavelengths are created.
Waves created by the wind are the most relevant to aquaculture facilities and are further described below. Shipping may also create waves that are unwanted on fish farms. To avoid such waves, sites close to heavily trafficked sea routes should be avoided. Waves created by exceptional natural phenomena (tsunamis) are difficult to avoid even if such phenomena occur more frequently in some areas than others. Tsunamis have a very long wave-length (>100 m) and a long period (around 1000 s). In such waves an enormous amount of energy is stored. They do not represent a great danger if a boat is on the sea, because the wavelength is so long. However, when they reach shallower water, and especially when they reach the shore and start to break, all the energy that is stored in them is released, and the consequences can be fatal. Such waves can be up to 30 m high when the shoreline has forced them to increase in height to dissipate their energy; they cause enormous destruction when they hit the shore. Waves created by the tide normally present no problems for cage farms. The wavelength here is so long that it is not interpreted as a wave, and the wave period is 12.5 h. Such waves can, however, create very strong tidal currents.
Wind created waves: The main ingredient in the formation of waves on the open ocean is wind. Wind created waves are normally also what inhibits site selection for cage farming. When winds blow across water, a drag is applied on the surface and pushes the water up, creating a wave. The height will increase as long as the wind is strong enough to add energy to the wave. After a period of time there will be equilibrium between the energy in the wind and the energy in the waves; the wave height will now be stable. Once a wave is generated, it will travel in the same direction until it meets land or is dampened by an opposing force such as winds blowing against it in the opposite direction, or by friction.
The height of wind created waves depends on the wind velocity (Uv), the duration of the wind (tv), the fetch length (F) and the presence of other waves when the wind begins to blow. The fetch length is the distance where wave development can take place (Fig. 15.7).
The Beaufort wind scale gives the expected wind velocity for the different wind strengths (see below). Some scales also present the normal wave height with different wind strengths; however, this is on open sea with no protection from land or islands.
In protected water the fetch length where the wind can blow will limit the ability of the wind to create waves. The fetch length can be read from a chart and is the length of the free water surface. If the fetch where the wind is blowing is narrow, as in a fjord, the wind effect will be less because of the friction against land on both sides reducing the velocity. To calculate the effective fetch length a compensation factor called the fetch length factor is used (Fig. 15.8). Calculations of effective fetch length in narrow fjords are given in the following two examples.
The length where the wind blows is 10 km and the width of the fjord is 2 km.
Effective fetch length = fetch length × fetch length factor
In shallow water the Sverdrup–Munk–Bretsnei-der (SMB) method may be used to estimate wave height. Formulae and diagrams have been developed to find wave heights based on wind velocity (Uv), wind duration (tv) and effective fetch length (Fe) (Fig. 15.9). It must be remembered that some diagrams use the traditional sea units of foot (ft), knot (kn) and nautical mile (nm) (1 ft = 0.3048 m; 1 kn = 0.5144 m/s; 1 nm = 1852 m).
To use the diagram in Fig. 15.9, knowledge of the three factors, wind velocity (Uv), wind duration (tv) and effective fetch length (Fe) is required. First the wave height is found based on Uv and tv, and after-wards based on Uv and Fe. Of the two different values found, the lower will be the wave height on the site under the specified conditions because either wind duration or fetch length will limit the maximum wave height. For instance, if the wind duration is short a maximum wave height will not be attained; if the fetch length is also short it will also inhibit maximum development of waves, even if the wind duration indicates higher waves.
Use SMB to estimate the wave height and wave period for a site if a fresh breeze of 20 kn blows for 2 h and the fetch length is 10 nm.
First calculate wind velocity and fetch length in SI units:
20 kn= 20 × 0.5144 m/s = 10.29 m/s
10 nm = 10 × 1.852 km = 18.5 km
Using wind velocity and duration criteria and Fig. 15.9
Significant wave height = ca. 0.6 m Significant wave period = ca. 3.3 s
Using wind velocity and fetch length criteria and Fig. 15.9
Significant wave height = ca. 0.8 m
Significant wave period = ca. 3.9 s
This means that wind duration is the limiting factor for development of waves; the wind does not blow long enough to create maximum wave height in pro-portion to the fetch length. Critical values for the site will therefore be:
Significant wave height = 0.6 m
Significant wave period = 3.3 s
The SMB method with the values and diagram given above may be used for depths greater than 15 m, which is normal in sea cage aquaculture. For intermediate depths and shallow water other for-mulae and diagrams apply.11
In open sea conditions with no limitation of the wind duration, the wave height will only depend on the effective fetch length and the wind velocity. The wave height created is the maximum possible with the given fetch length. A simplified method can then be used to calculate wave height in shallow water11 (Table 15.1):
Hs=5.112×10−4× UA F1/2(m)
U = wind velocity (10 min average value 10 mabove sea level) (m/s)
UA=adjusted wind velocity (m/s)
F = fetch length (m)
Hs = significant wave height (m)
ts = significant wave period (s).
The following may be used to estimate the maximum wave height from the significant wave height:
Use the simplified method to calculate the significant wave height and wave period for a near gale with wind velocity of 15 m/s and fetch length of 3 km.
UA=0.71×27.96= 19.9 m/s
Hs=5.112×10−4×19.9×30001/2= 0.56 m
Ts=6.328×10−2×(19.9×3000)1/3= 2.44 s
Swell: Swell comprises wind generated wavescreated far away, which can also be called ocean waves; these may also affect cage farms when they come in from the sea. This is another reason for sitting cage farms in sheltered positions behind holms and breakwaters. Swells are characterized by quite large wave heights and long wavelengths. A swell can be recognized by its higher wave period compared to a local wind generated wave: typical swell periods are in the range 9–20 s, compared with 2–11 s for wind generated waves.
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