EM 1110-2-1100 (Part II)
30 Apr 02
EXAMPLE PROBLEM II-7-3
FIND:
The transmission coefficient for incident waves having periods of 2 and 5 sec.
GIVEN:
The catamaran breakwater whose performance is depicted in Figure II-7-21.
SOLUTION:
For a water depth of 7.6 m, using the linear wave theory, wavelengths would be calculated to be 6.24
m (for T = 2 sec) and 34.4 m (for T = 5 sec). For a width W of 6.4 m, this yields W/L = 1.03 and 0.186,
respectively. From Figure II-7-21, this yields Ct = 0.2 (extrapolating) for the 2-sec wave and Ct = 0.8
for the 5-sec wave. Thus, this catamaran is quite effective for the 2-sec wave but quite ineffective for
the 5-sec wave.
(4) Floating breakwaters.
(a) Moored floating breakwaters have some distinct advantages for harbor installations. They are more
adaptable to the water level changes that occur at harbors that are built on reservoirs and in coastal areas
having a large tidal range. They are usually more economical than fixed breakwaters for deep-water sites,
and they interfere less with water circulation and fish migration. But they also have some significant
limitations. Since they are articulating structures, they are prone to damage at connecting points between
individual breakwater units and between these units and mooring lines. And, their performance is very
dependent on the period of the incident waves. This last factor will establish relatively severe limits on where
floating breakwaters can effectively be deployed.
(b) Several different types of floating breakwaters have been proposed. Hales (1981) presents a survey
of these various types and their performance. Most of the floating breakwaters in use are of three generic
types - prism, catamaran, and scrap tire assembly (see Figure II-7-20). Figure II-7-21 plots the wave
transmission coefficients for a typical representative of each of these three types. The transmission coefficient
is plotted versus the breakwater's characteristic dimension in the direction of wave propagation W divided
by the incident wave length L at the breakwater. The data plotted in Figure II-7-21 are all derived from
laboratory experiments. The prism results are for a concrete box having a 4.88-m width (W), a draft of 1.07
m, and a water depth of 7.6 m (Hales 1981). The catamaran (Hales 1981) has two pontoons that are 1.07 m
wide with a draft of 1.42 m and a total width (W) of 6.4 m. The water depth was 7.6 m. The tire assembly
(Giles and Sorensen 1979) had a width of 12.8 m (W), a nominal draft of one tire diameter, and was tested
in water 3.96 m deep. The three breakwaters were all moored fore and aft; other mooring arrangements
would somewhat alter the transmission coefficient.
(c) Example Problem II-7-3 demonstrates a major limitation of floating breakwaters. For typical
breakwater sizes (i.e., W equals 5 to 10 m) the incident wave period must not exceed 2-3 sec for the
breakwater to be very effective. Thus, for typical design wind speeds, the fetch generating the waves to
which the structure is exposed cannot be very large. Sorensen (1990) conducted an analysis to determine
general wind speed, fetch, and duration guidelines for the three floating breakwaters from Figure II-7-20.
He assumed an allowable transmitted wave height of 2 ft (0.61 m). For example, for a wind speed of 60 mph
(26.8 m/sec) having a duration of 20-30 min, the fetch must not exceed 2-3 miles (3.2-4.8 km) if the
transmitted wave height is to be less than 2 ft.
Harbor Hydrodynamics
II-7-23
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