diffraction wave

$: Diffraction wave$
COASTAL ENGINEERING

WATER RESOURCE ENGINEERING

FACULTY OF ENGINEERING

UNIVERSITY OF BRAWIJAYA

WAVE DIFFRACTION

GROUP 1

• ALIF RAMADHANI MEDISIA PG (135060400111033)

• MUNFARID(135060400111035)

• GANISA ELSINA SALAMENA (135060401111068)

• YUDHISTIRA A Z R (145060400111005)

• FAIZAH FITRIAH (145060400111006)

• TAMI PRATIWI (145060400111007)

• NUR FADHILILLAH A (145060400111008)

INTRODUCING

Coastal area is one of natural resources which has huge potential especially in

tourism sector. In our country coastal area not only popular in tourism sector,

but also in water transportation. Speaking of coast area there must be a

discussion about waves. There are many type of waves. If the wave moving

towards coast area, it may causes a deformation. This phenomenon caused by

a refraction process, diffraction, reflection, and breaking wave. So that coast

area moves in because of abration and sedimentation in this area moves out.

From this problem, a construction is needed to protect coastal area. This

construction has a specific function according to the condition and problem in

the field.

1.1 General Description

WATER RESOURCE ENGINEERINGSunday, April 10, 2016

INTRODUCING

1.2 Purpose and Goal

Purpose and goal from this assignment is to analyze the deformations of wave which occurs in coastal area

especially diffractions wave with formulas that will be used to solve the wave problems.


THEORY

2.1 The General Deformation of Waves

Deformation of the wave is a changing of wave characteristic that occurred during the moved of waves

propagating toward the shore. Wave deformation process will determine the wave height and wave crest line

pattern or direction of the wave at somewhere in the coast area. When a row of waves moving towards to the

shore, the wave will changes in shape caused by the refraction and the erosion of the waves, diffraction,

reflection, and wave breaking.


THEORY

2.2 Diffraction of Wave

Diffraction happen when the wave height on a point on the crest of a wave larger than a point nearby, which led

to the displacement of energy along the crest of the wave height towards smaller waves. Diffraction happen

when a row of waves resist by restriction such as breaking waves (breakwater) or an island. Wave height and

direction of the waves on the coast area is important, for example, we can use to determining the flow and

sediment transport in coastal areas.

There are parameters that influence the process of diffraction is a wavelength comparison with the end of

distance point of the restriction (r / L), the angle between the barrier and the line joining the end of the

restriction, and the angle between the directions of wave propagation with restriction. The ratio between wave

heights at point being reviewed by the coming wave height is called coefficient of diffraction.


THEORY

Figure 2.1 This phenomenon is known as a wave’s

diffraction. In diffraction of wave, there is a transfer

of energy in a vertical direction to the wave sheltered

areas. As seen in Figure 2.1, if there is no diffraction of

waves, the area behind the barrier will stay still. But,

when diffraction process occurs make an area

affected by a wave of energy into the sheltered area.

Transfer of energy to that place causes the formation

of shelter wave.


The Shadow area with

diffraction area occurs

The area is not

disturb

The area sharp corners, where

waves come and reflection formed

Figure 2.1 Wave Diffraction behind Obstacles

Sunday, April 10, 2016

THEORY

2.3 Calculation of Diffraction Wave

• If Hi is the height of the waves coming at the end of breakwater and Hd is the height of a wave diffracted at the

reference point in a sheltered area, it can be defined diffraction coefficient :

Kd = Hd / Hi

• Value of Kd depends on the location behind backwater which is defined by r and β, and the direction of the

incoming wave is defined by θ. Or a dimensionless shape :

Kd = f (r/L,,),

• with L is the wavelength in the protected area of backwater. Because wavelengths is a function of the wave

period and water depth, the diffraction coefficient for each component spectrum will depend on the direction of

the wave coming and the period.


THEORY

Diffraction Theory for two or more breakwaters with a gap (aperture)

• If there are two or more breakwaters with a gap (aperture) of them, to determine the coefficient used

diffraction chart was developed by Johnson (1952, 1953; in Wiegel, 1964).


Figure 2.2 Forming an angle of diffraction waves behind obstacles gaps


THEORY


Figure 2.3.a Diffraction graph through the Gap (B/L=2)


THEORY

Example :

The waves come with a period of 6 seconds to the breakwater with the angle of incidence at the end of the

breakwater at θ = 600. The water depth behind or at breakwater protected area is 10 m and assumed to be

constant. Find the wave height at an angle β = 300 from point A breakwater and 99.6m distance from the tip of

the breakwater if the wave coming with a height is 2.2 m.


A

THEORY

Answer :

• Lo = 1.56T2 = 1.56 x (6)2 = 56.16 m d/Lo = 6/56.16 = 0.10684

From Figure 2.6 continued Table A-1 can be obtained:

• d/L = 0.1469024 L = 10/0.1469024 = 68.072 m

Distance point A to the end of the hurdles:

• r = 99.6 m r/L = 99.6/68.072 = 1.463 m ≈ 1.5

From the table figure 2.5 Kd table for r/L = 1.5, θ = 600 and β = 300 thus Kd = 0.32

Then the wave height at point A is:

• HA = Kd x Hi = 0.32 x 2.2 = 0.704 m.


Figure 2.5 and 2.6


THEORY

VIDEO OF WAVE DIFFRACTION


CLOSING


3.1 Conclusion

• Deformations wave is one of the change in the nature of the waves that happened at a wave of moving vines

towards the shore. If a row of a wave of moving to the shore, the wave will undergo changes in shape caused

by the process of refraction and shallowed wave, difraction, reflection and brokens wave.

• Difraction happen if high waves one point in the wave crests larger on point in the near, that causing a move

energy on the all the wave crests wave toward the smaller of highers wave. Difraction occurs when a row of

wave of blocked by the obstacles as breakwater or is an island.

• On the breakwater gap or double, diffraction hapens when the wavelength (L) is comparable or larger than

the aperture between the breakwater (B).


THANK YOU

diffraction wave

Engineering