Floating breakwaters

Floating breakwaters

Floating breakwaters represent an alternative solution to protect an area from wave attack, compared to conventional fixed breakwaters and can be effective in coastal areas with mild wave environment conditions. Therefore, they have been increasingly used aiming at protecting small craft harbours or marinas or, less frequently, the shoreline, aiming at erosion control.

Some of the conditions that favour floating breakwaters are:

1. Poor foundation: Floating breakwaters might be a proper solution where poor foundations possibilities prohibit the application of bottom supported breakwaters.
2. Deep water: In water depths in excess of 6 m, bottom connected breakwaters are often more expensive than floating breakwaters.
3. Water quality: Floating breakwaters present a minimum interference with water circulation and fish migration.
4. Ice problems: Floating breakwaters can be removed and towed to protected areas if ice formation is a problem. They may be suitable for areas where summer anchorage or moorage is required.
5. Visual impact: Floating breakwaters have a low profile and present a minimum intrusion on the horizon, particularly for areas with high tide ranges.
6. Breakwater layout: Floating breakwaters can usually be rearranged into a new layout with minimum effort.

Floating breakwaters are very effective when their width is of order of half the wavelength and/or when their natural period of oscillation is much longer compared to the wave period.

The first requirement is seldom verified, and in this case the performance is uncertain. The performance of a floating breakwater depends on the strongly non-linear interaction of the incident wave (that may partially overtop the module and is in general short-crested and oblique) with the structure dynamics. The interaction becomes complicated by the forces induced by the mooring system and the connections between the modules. Accurate design is necessarily based on the combination of numerical and physical models.

Floating breakwaters are commonly divided into four general categories:

1. box,
2. pontoon,
3. mat,
4. tethered float.

For each category, some types of floating breakwaters are shown in Figures 1 - 4. The first three types have been much widely investigated by means of physical models and prototype experience, than the last one.

Figure 1 Category: Box

Figure 2 Category: Pontoon

Figure 3 Category: Mat

Figure 4 Category: Tethered Float

Box type breakwaters are used most frequently (Fig. 2). Reinforced concrete modules are either empty inside or, more frequently, have a core of light material (e.g. polystyrene). In the former case the risk of sinking of the structure is not negligible. Usually dimensions are limited to a width of a few meters. Connections are either flexible, allowing preferably only the roll along the breakwater axis, or pre or post tensioned, to make them act as a single unit. In the latter case the efficiency is higher, but the forces between modules are also higher. The modular system as applied and the mooring system are primary points of concern for this kind of structures.

Figure 5 Example of floating breakwater (Fezzano,SP-Italy; courtesy of INGEMAR srl)

Large breakwaters are frequently built with used barges, ballasted to the desired draft with sand or rock.

Pontoon type are effective since the overall width can be of the order of half the wavelength. In this case the expected attenuation is significant.

Within the mat category, the most used are made with tires. Although less effective, they have a low cost, they can be removed more easily, they can be constructed with unskilled labour and minimal equipment, they are subjected to lower anchor loads, reflect less and dissipate relatively more wave energy. Tethered float are seldom used.