diagram shoreward to the breakwater; (b) at this point, constructing a
diffraction diagram carrying successive crests three or four wavelengths
shoreward, if possible; and (c) with the wave crest and wave direction
indicated by the last shoreward wave crest determined from the diffraction
diagram, constructing a new refraction diagram to the breaker line. The
work of Mobarek (1962) on the effect of bottom slope on wave diffraction
indicates that the method presented here is suitable for medium-period
waves. For long-period waves the effect of shoaling (Section 2.32) should
be considered. For the condition when the bottom contours are parallel to
the wave crests, the sloping bottom probably has little effect on diffraction. A typical refraction-diffraction diagram and the method for determining combined refraction-diffraction coefficients are shown in Figure
2-59. When a wave crest is not of uniform height, as when a wave is undergoing refraction, a latéral flow of energy - wave diffraction - will occur
along the wave crest. Therefore diffraction can occur without the wave
moving past a structure although the diffraction effects are Visually more
dramatic at the structure.
2.5
WAVE REFLECTION
2.51 GENERAL
Water waves may be either partially or totally reflected from both
natural and manmade barriers. (See Figure 2-60.) Wave reflection may
often be as important a considération as refraction and diffraction in the
design of Coastal structures, particularly for structures associated with
development of harbors. Reflection of waves implies a reflection of wave
energy as opposed to energy dissipation. Consequently, multiple reflections
and absence of sufficient energy dissipation within a harbor complex can
resuit in a buildup of energy which appears as wave agitation and surging
in the harbor. These surface fluctuations may cause excessive motion of
moored ships and other floating facilities, and resuit in the development
of great strains on mooring lines. Therefore seawalls, bulkheads and
revetments inside of harbors should dissipate rather than reflect incident
wave energy whenever possible. Natural beaches in a harbor are excellent
wave energy dissipaters and proposed harbor modifications which would
dccrease beach areas should be carefully evaluated prior to construction.
Hydraulic model studies are often necessary to evaluate such proposed
changes. The importance of wave reflection and its effect on harbor
development are discussed by Bretschneider (1966), Lee (1964), and
LeMehaute (1965); harbor résonance is discussed by Raichlen (1965).
A measure of how much a barrier reflects waves is given by the ratio
of the reflected wave height H^, to the incident wave height FLwhich
is termed the reflection coefficient X; hence x =
The magnitude
o
x varies from 1.0 for total reflection to 0 for no reflection; howeyer, a small value of x does not necessarily imply that wave energy is
dissipated by a structure since energy may be transmitted through such
AffïciPntSmaS TT?16: rubble’mound breakwaters. A transmission coefficient may be defined as the ratio of transmitted wave height Hy., to
inci ent wave height H^. In general, both the reflection coefficient
2-110
diffraction diagram carrying successive crests three or four wavelengths
shoreward, if possible; and (c) with the wave crest and wave direction
indicated by the last shoreward wave crest determined from the diffraction
diagram, constructing a new refraction diagram to the breaker line. The
work of Mobarek (1962) on the effect of bottom slope on wave diffraction
indicates that the method presented here is suitable for medium-period
waves. For long-period waves the effect of shoaling (Section 2.32) should
be considered. For the condition when the bottom contours are parallel to
the wave crests, the sloping bottom probably has little effect on diffraction. A typical refraction-diffraction diagram and the method for determining combined refraction-diffraction coefficients are shown in Figure
2-59. When a wave crest is not of uniform height, as when a wave is undergoing refraction, a latéral flow of energy - wave diffraction - will occur
along the wave crest. Therefore diffraction can occur without the wave
moving past a structure although the diffraction effects are Visually more
dramatic at the structure.
2.5
WAVE REFLECTION
2.51 GENERAL
Water waves may be either partially or totally reflected from both
natural and manmade barriers. (See Figure 2-60.) Wave reflection may
often be as important a considération as refraction and diffraction in the
design of Coastal structures, particularly for structures associated with
development of harbors. Reflection of waves implies a reflection of wave
energy as opposed to energy dissipation. Consequently, multiple reflections
and absence of sufficient energy dissipation within a harbor complex can
resuit in a buildup of energy which appears as wave agitation and surging
in the harbor. These surface fluctuations may cause excessive motion of
moored ships and other floating facilities, and resuit in the development
of great strains on mooring lines. Therefore seawalls, bulkheads and
revetments inside of harbors should dissipate rather than reflect incident
wave energy whenever possible. Natural beaches in a harbor are excellent
wave energy dissipaters and proposed harbor modifications which would
dccrease beach areas should be carefully evaluated prior to construction.
Hydraulic model studies are often necessary to evaluate such proposed
changes. The importance of wave reflection and its effect on harbor
development are discussed by Bretschneider (1966), Lee (1964), and
LeMehaute (1965); harbor résonance is discussed by Raichlen (1965).
A measure of how much a barrier reflects waves is given by the ratio
of the reflected wave height H^, to the incident wave height FLwhich
is termed the reflection coefficient X; hence x =
The magnitude
o
x varies from 1.0 for total reflection to 0 for no reflection; howeyer, a small value of x does not necessarily imply that wave energy is
dissipated by a structure since energy may be transmitted through such
AffïciPntSmaS TT?16: rubble’mound breakwaters. A transmission coefficient may be defined as the ratio of transmitted wave height Hy., to
inci ent wave height H^. In general, both the reflection coefficient
2-110