the shore is affected by diffraction caused by naturally occurring changes
in hydrography. An aerial photograph illustrating the diffraction of
waves by a breakwater is shown in Figure 2-27.
Putnam and Arthur (1948) presented experimental data verifying a
method of solution proposed by Penny and Price (1944) for wave behavior
after passing a single breakwater. Wiegel (1962J used a theoretical
appxoach ta study wave diffraction around. a single breakwater.
B lue and
Johnson (1949) dealt with the problem, of the behavior of waves after
passing through a gap, as between two breakwater arms.
The assumptions usually made in the development of diffraction
théories are:
(1} Water is an idéal fluid, i.e., imviscid and incompressible.
(2) Waves are of small-amplitude and can be described by linear
wave theory.
(3) Flow is irrotational and conforms to a potential function which
satisfies the Laplæce équation.
(4) Depth shareward of the breakwater is constant.
2.42
DIFFRACTI0K CALCULATIONS
2.421 Waves Passing a Single Breakwater. From a présentation by Wiegel
(1962), diffraction diagrams hâve been prepared which., for a uni form depth
adjacent to an impervious structure, show limes of equal wave height réduction. These diagrams are shown in Figures 2-28 through 2-39; the graph
coordinates are in unit s of wave length. Wave height rédaction is given in
ternis of a diffraction coefficient K7 which is defined as the ratio of a
wave height H, in the area affected by diffraction to the incident wave
height
in the area unaffected by diffraction. Thus, H and
are
determined by H = KZH£.
The diffraction diagrams shown in Figures 2-28 through 2-39 are constructed in polar coordinate form with arcs and rays centered at the structure'1 s tip. The arcs are spaced one radius^Dao&l^ng'th. unit apart and rays
15 apart . In application', a given diagram must be scaled up or down so
that the particular wavelength corresponds to the scale of the hydrographie
chart being used. Rays and arcs on the refraction diagrams provide a
coordinate System that makes it relatively easy to transfer Unes of
constant Kz on the scaled diagrams.
When applying the diffraction diagrams to actual problems, the wavelength must first be determined based on the water depth at the tip of the
structure. The wavelength L, in water depth d^, may be found by computing dg/Lp = ds/5.12T2 and using Appendix C, Table C-l to find the
corresponding value of d^/L. Dividing dg by ds/L will give the shallow
water wave length L.
It is then useful to construct a scaled diffraction
2-8>
in hydrography. An aerial photograph illustrating the diffraction of
waves by a breakwater is shown in Figure 2-27.
Putnam and Arthur (1948) presented experimental data verifying a
method of solution proposed by Penny and Price (1944) for wave behavior
after passing a single breakwater. Wiegel (1962J used a theoretical
appxoach ta study wave diffraction around. a single breakwater.
B lue and
Johnson (1949) dealt with the problem, of the behavior of waves after
passing through a gap, as between two breakwater arms.
The assumptions usually made in the development of diffraction
théories are:
(1} Water is an idéal fluid, i.e., imviscid and incompressible.
(2) Waves are of small-amplitude and can be described by linear
wave theory.
(3) Flow is irrotational and conforms to a potential function which
satisfies the Laplæce équation.
(4) Depth shareward of the breakwater is constant.
2.42
DIFFRACTI0K CALCULATIONS
2.421 Waves Passing a Single Breakwater. From a présentation by Wiegel
(1962), diffraction diagrams hâve been prepared which., for a uni form depth
adjacent to an impervious structure, show limes of equal wave height réduction. These diagrams are shown in Figures 2-28 through 2-39; the graph
coordinates are in unit s of wave length. Wave height rédaction is given in
ternis of a diffraction coefficient K7 which is defined as the ratio of a
wave height H, in the area affected by diffraction to the incident wave
height
in the area unaffected by diffraction. Thus, H and
are
determined by H = KZH£.
The diffraction diagrams shown in Figures 2-28 through 2-39 are constructed in polar coordinate form with arcs and rays centered at the structure'1 s tip. The arcs are spaced one radius^Dao&l^ng'th. unit apart and rays
15 apart . In application', a given diagram must be scaled up or down so
that the particular wavelength corresponds to the scale of the hydrographie
chart being used. Rays and arcs on the refraction diagrams provide a
coordinate System that makes it relatively easy to transfer Unes of
constant Kz on the scaled diagrams.
When applying the diffraction diagrams to actual problems, the wavelength must first be determined based on the water depth at the tip of the
structure. The wavelength L, in water depth d^, may be found by computing dg/Lp = ds/5.12T2 and using Appendix C, Table C-l to find the
corresponding value of d^/L. Dividing dg by ds/L will give the shallow
water wave length L.
It is then useful to construct a scaled diffraction
2-8>