c * •
c;™™ ?
with cot B = 5, and using the curve for H^/Lq =
0n017 I value of' x2 = 0.41 is found. Assuming that since the beach
is imperméable, x} = 0.8 and
X = Xi X2 = 0.8(0.41) = 0.33.
The steepest incident wave which will be nearly perfectly reflected from
the given revetment is, from Figure 2-62,
/H\
—
= 0.005.
X^o/max
It is interesting to note the effectiveness of fiat beaches in dissipating wave energy by considering the above wave on a beach having a slope
of 0.02 (1:50). From Equation 2-87 (noting that B « sin B ~ tan B = 0.02)
/H \
—
= 0.000014.
\^ol max
Hence
X = Xt X2 = 0.8(0.0014) = 0.0011,
or the height of the reflected wave is about 0.1 percent of the incident
wave height.
As indicated by the dependence of reflection coefficient on incident
wave steepness, a beach will selectively dissipate wave energy, dissipât -
ing the energy of relatively short steep waves and reflecting the energy
of the longer, flatter waves.
*********************************A***
2.6
BREAKING WAVES
2.61 DEEP WATER
The maximum height of a wave travelling in deep water is limited by
a maximum wave steepness for which the wave form can remain stable. Waves
reaching the limiting steepness will begin to break and in so doing, will
dissipate a part of their energy. Based on theoretical considérations,
Michel1 (1893) found the limiting steepness to be given by,
Ho
1
— = 0.142
.
(2-88)
Lo .
7
which occurs when the crest angle as shown in Figure 2-64 is 120°. This
limiting steepness occurs when the water particle velocity at the wave
cres just equals the wave celerity; a further increase in steepness would
2-120