Waves in the Ocean PDF

OCEAN UNIVERSITY OF SRI LANKA Aspects of Coastal Engineering CRM 4022 Waves in the Ocean Eng. Nilupul Senarathne M.Eng (Moratuwa), M.A (Financial...

OCEAN UNIVERSITY OF SRI LANKA Aspects of Coastal Engineering CRM 4022 Waves in the Ocean Eng. Nilupul Senarathne M.Eng (Moratuwa), M.A (Financial Econ).(Colombo), Bsc Eng. (Hons) (Moratuwa), AMIE(SL)  Outline 1.Wave Parameters 2.Wave Types 3.Wave Formation 4.Wave Behavior  1.Wave Parameters Wave Height Vertical distance from crest to trough Wave Period Time required for 2 successive crests or troughs to pass a given point Amplitude Vertical distance from crest to level surface Also = ½ height Wave Parameters Cont.…… Frequency = # of wavelengths which pass a fixed point at a given time period. Frequency= 1/T Steepness = Ratio of height of wave to its wavelength. Steepness = Height (H)/wave length (L) Wave Types & Wave Formation Wave Types Cont.…………..  Wave Formation.…… The variety and size of wind generated waves are controlled by four principal factors (1) Wind velocity (2) Wind duration (3) Fetch – (area of water over which the wind blows) (4) Original sea state  Wave Formation Cont.…… Waves will continue to grow in size until they reach a maximum size that is determined by the wind speed and fetch. At this stage of wave growth, called a fully developed sea  Significant Wave Height Significant Wave Height is the average of the highest one third (1/3) of all the waves present in an area of the sea surface. The significant wave height will always be more than the average wave height Hs≈Hm0=4[E/(gρ)]1/2, where Hm0 is the spectral significant wave height Mean wave period The mean wave period, tm, is the mean of all wave periods in A time-series representing A certain sea state. Peak wave period The peak wave period, tp, is the wave period with the highest energy. The analysis of the distribution of the wave energy as a function of wave frequency F=1/T Mean wave direction The mean wave direction, Dir(m), is defined as the mean of all the individual wave directions in a time- series representing a certain sea state.  Wave Motions Depending on the wave motion wave are divided to two main types Progressive Waves Standing Waves Wind-generated waves are progressive waves Do not move horizontally, but remain stationary, They travel (progress) across the sea surface These waves oscillate back and forth about a fixed point called a node What actually is moving? Progressive Waves Note that the water molecules in the crest of the wave move in the same direction as the wave, but molecules in the trough move in the opposite direction. Summary of Progressive Waves As waves pass, wave form and wave energy move rapidly forward, not the water. Water molecules move in an orbital motion as the wave passes. Diameter of orbit increases with increasing wave size and decreases with decreasing water depth. Wave base is the depth to which a wave can move water. If the water is deeper than wave base, orbits are circular and there is no interaction between the No forward motion of mass, no matter how many bottom and the wave, but if the water is shallower waves pass through the area. than wave base, orbits are elliptical and become Thus, wave energy, not water particles, travels across increasingly flattened towards the bottom. the sea surface. Wave classification – Based on depth Deep-water wave: Travels through water that is deeper than its wave base, which is one-half its wavelength Intermediate-water wave : travels through water depths that are between one-half and one-twentieth the wavelength, Shallow-water wave : travels through water depths less than one-twentieth the wavelength  Wave Parameters Cont.…… Celerity (Wave Speed) Celerity is the velocity or speed of the wave form, not the water The formula for speed applies to the individual waves traveling through the group A wave group itself, and therefore wave energy, travel across an undisturbed sea surface at a speed that is less than the speed of the individual waves. CHAOTIC WAVES Analysis of chaotic seas  Wave Interference These confused seas are really a composite of numerous waves having different lengths, heights, and periods that are continually merging and separating as they move in different directions and at different speeds through the fetch area. This interaction of several waves is called wave interference  Wave Transformation Regular ocean swell develops as waves sort themselves according to speed by the process of dispersion Wave Transformation Cont.…. 1. Transformation of the wave’s properties 2. Wave refraction 3. Wave breaking Wave Refraction  Wave Refraction Cont.…. 1. Refraction is, the bending of the wave crest in response to changes in wave speed. 2. The Crestline of the wave lies at some angle to the shore, so the water depth beneath the crest varies 3. Since the speed of shallow-water waves depends directly on depth, different parts of the same crest are traveling shoreward at variable speeds 4. This differential speed along the crest causes the wave to reorient itself by refracting, or “bending,  Wave Breaking simple ratio that divides the wave height by the wave length (H/L) gives the wave steepness As waves enter shallow water, their height (H) increases and their wavelength (L) decreases. A critical wave steepness occurs when the wave height is about equal to one-seventh the wavelength (H/L =1/7, which means that for a 1-meter-high wave, H=1 meter, and L = 7 meters).  Spilling breaker In a spilling breaker, the upper part of the crest becomes over-steepened and “spills” down the front side of the advancing wave, continually breaking, and slowly losing its energy across the surf zone. (Gentle slope) Forms wide surf zone  Plunging breaker In a plunging breaker, the entire wave front steepens, curls, and collapses, (tunnel collapse) or “plunges” forward, releasing much of its energy instantaneously Forms narrow surf zone  Surging breaker In a surging breaker, the flat, low waves do not become over-steepened or actually break; instead they move smoothly up and then down the face of the beach, reflecting much of their energy seaward. Crest does not attain a critical wave steepness, fails to break, and is reflected off the beach and back into the sea  Storm Surges Sudden changes of coastal water levels produced by storms (due to strong wind conditions and atmospheric pressure changes associated with a storm) are called storm surges Storm surges are particularly destructive when they happen to coincide with unusually high tides.  Other Types of Progressive Waves-- Tsunamis Tsunamis consist of a series of long-period waves characterized by very long wave length (up to 200 km), high speed (up to 760 km/hr) in the deep ocean with an average period of 15 mins. Because of their large wave length, tsunamis are shallow-water to intermediate-water waves as they travel across the ocean basin. They only become a danger when reaching coastal areas where wave height can reach about 10 m. – Tsunamis originate from earthquakes, volcanic explosions, or submarine landslides, Asteroid collision  Caused major displacement of the ocean by: – Earthquakes Tsunami – Volcanic Eruptions – Submarine landslides – Asteroid collision  Common in tectonically active basin Historic Tsunami – Volcanoes Tsunami Krakatoa in 1812, killed 35,000 people – Earthquakes Hilo, Hawaii in 1946 as a result of an Alaskan earthquake Nicaragua 9/18/92 – Landslides The French Riviera (10/16/79)  Internal waves form within the water column on the pycnocline. Internal Because of the small density difference between the water masses above and below the pycnocline. Waves Internal waves display all the properties of surface progressive waves including reflection, refraction, interference, breaking, etc. Any disturbance to the pycnocline can generate internal waves. Internal Waves Generated where have density contrasts Standing waves Standing waves do not move horizontally, but remain stationary These waves oscillate back and forth about a fixed point called a node  A node is an imaginary line across the surface which experiences no change in elevation as the standing wave oscillates. It is the line about which the surface oscillates. Antinodes are where there is the maximum Standing displacement of the surface as it oscillates and are usually located at the edge of the basin. Waves Geometry of the basin controls the period of the standing wave. A basin can be closed or open. Standing waves can be generated by storm surges. Thank you ! Email : [email protected] Wave heights Descending order Average of highest 1/3 1.5 2.6 1.7 2.5 2.4 2.4 2.5 2.5 2.6 1.8 1.8 2.6 1.7 1.2 1.5 Average - 2.5 2.4 1.4 0.8 1.2 1.4 0.8