Giant Kelp

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Abrasi Pantai Balongan, Indramayu

Abrasi Pantai Malibur (P. Padang)

GELOMBANG

Ocean Waves

Aristotle (384-322 BC): relationship between wind and waves

Historical background:

Until at the present day, understanding of the mechanism of

wave formation and the way that waves travel across the ocean

is not complete yet.

Partly because observations of wave characteristics at the sea

are difficult,

Partly because mathematical models of wave behavior are

based on the dynamics of idealized fluids, and ocean waters

do not conform precisely with ideal fluid

Very complex and random

Nevertheless, some facts about waves are well established.

Wave height (H): the overall vertical change in height between the wave crest (peak)

and the wave trough

H=2a; a is wave amplitude

Wave length (L): the distance between two successive peaks (or two successive

trough)

Wave steepness: wave height divided by wave length (H/L)

Wave period (T): time interval between two successive peaks (or two successive

troughs) passing a fixed point

Wave frequency (f): the number of peaks (or number of troughs) which pass a fixed

point per second

Susunan gelombang

• Crest = titik tertinggi (puncak) gelombang

• Trough = titik terendah(lembah) gel.

• Wave height = jarak vertikal antara crest dan trought

• Panjang gel = jarak berturut-turut antara dua buah crest atau dua buah trough

• Periode gel = waktu yg dibutuhkan crest untuk kembali pd titik semula scr berturut-turut.

• Kemiringan gel = perbandingan antara panjang gel. Dengan tinggi gel.

Types of Waves

Progressive waves: energy is moving through, or across the surface of material

Standing waves: sum of two progressive waves of equal dimensions, but traveling

in opposite directions.

Surface waves: the most familiar surface waves occur at the interface between

atmosphere and ocean, but surface waves can occur at the

interface between any two bodies of fluid oceanographers

usually refer as internal waves

ERS-2 SAR image of the interanal waves in the Sulu Sea (left).

SPOT 2 image of the internal waves in the Sulu Sea taken on 15 Sep 1999, at 0242 UTC

(right). Viewing angle 0.5,

sun elevation 69.7, sun

azimuth 103.2.

© ESA

Type of surface waves, showing the relationship between wave height,

wave period and generating force of each type of wave

Wind-generated Waves on the Ocean

In 1774, Benjamin Franklin said “Air in motion, which is wind,

in passing over the smooth surface of the water, may rub, as

it were, on that surface and raise it into wrinkles, which, if the

wind continues, are the elements of future waves”

In other words, if two fluid layers having differing speeds

are in contact, and there is frictional stress between them,

there is a transfer of energy.

At the sea-surface, most of the transferred energy results in

waves, although a small portion is manifest as wind-driven

currents

Gelombang yang

dibangkitkan oleh angin1. SEA :

Gelombang yang terjadi yang diakibatkan oleh pengaruh angin secara langsung pada permukaan laut, dicirikan dengan bentuk deolbang yang tdk teratur.

2. SWELL :

Gelombang yang terjadi yang diakibatkan oleh angin secara tdk langsung dan telah jauh meninggalkan tempat asal pembentukannya, dicirikan dengan bentuk gelombang yang teratur.

The fully developed sea

Size of Waves in deep water is governed by:

(1) Wind speed

(2) Wind duration (the length time the wind has been blowing at

certain speed)

(3) Fetch

Surface Wave Theory

Wave speed,L

CT

wave number, 2

kL

Angular frequency,

2

T

How could C be expressed in term of k and ?

2

2

L kCT k

Contoh : suatu gel.mempunyai panjang

gel.35 m dengan perioda 4 s dan tinggi 2

m, berapakah kecepatan, amplitudo dan

jumlah gelombang tsb ?

Deep-waters Waves

2tanh

2

gL dC

L

g= the acceleration due to gravity

L=wavelength

d=water depth

When d>0.5L deep water

2

gLC

From the equation,

LC

T and

We obtain L,

2

gLC

2

2

22

2

2

2

2

2

L gL

T

L gL

T

T gLL

gTL

Shallow-water Waves

When d<0.05L shallow water

C gd

0.5o

0.5 3o

3.5o

3 3.5o Collapsing

Wave crest

Longshore Current

Terima kasih

bye......

Provided the fetch is extensive enough, and the wind blow at constant

Speed for long enough, an equilibrium is eventually reached, in which

Energy is being dissipated by the waves at the same rate as the waves

Receive energy from the wind fully developed sea