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Abrasi Pantai Balongan, Indramayu
Abrasi Pantai Malibur (P. Padang)
GELOMBANG
Ocean Waves Historical background: Aristotle (384-322 BC): relationship between wind and waves 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 angin 1. 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, C
wave number,
L T
2 k L
Angular frequency,
How could C be expressed in term of k and
2
L k C T 2 k
?
2 T
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
gL 2 d C tanh 2 L g= the acceleration due to gravity L=wavelength d=water depth When d>0.5L deep water
gL C 2
From the equation,
L C T We obtain
L,
and
gL C 2 L gL T 2 L2 gL 2 T 2 2 T gL 2 L 2 gT 2 L 2
Shallow-water Waves
When d<0.05L shallow water
C gd
o 0.5 0.5 o 3
o 3.5 3 o 3.5
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
