Lecture Waves

8/11/2019 Lecture Waves

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WIND WAVES

Waves on the surface of the sea are caused principally

by wind.

Other factors:

-submarine earthquakes,

-volcanic eruptions

-the tide.


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Waves elementsCreasta-linia cea mainalta valului n raport cu suprafaaapei linitite

The Crest - the highest part of the wave.. The wave height is the overall verticalchange in height between the crest and the trough and distance between twosuccessive crests (or troughs) is the length of the wave or wavelength.

Talpa -adncimea (golul) valului, partea cea mai de jos a valului n raport cusuprafaaapei linitite

The Trough- the lowest part of a wave

L Lungimea de und (sau lungimea valului) este distana orizontal dintre 2creste msuratparalel cu direciade deplasare a valului.

The length of the wave (wavelength)is the distance between 2 successive crests

(or troughs)

T Perioada valului este intervalul de timp n secunde n care cele 2 crestesuccesive de val trec prin dreptul unui punct fix: T = L/C

The wave Periodis the time period when 2 successive pass through a fix point

f Frecvenavaluriloreste numrulde valuri care trec printr-un punct ntr-o unitatede timp. Este invers proporionalcu perioada valului: f = 1/TWaves frequency is the number of waves that pass through a point/passage

duration

H nlimea valuluipe vertical ntre creasta i depresiunea valului: H = 2a. Dinformaii asupra gradului de agitaie al mrii

The wave heightis the overall vertical change in height between the crest and the

trough


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Waves elements

a amplitudinea valuluieste distana vertical dintre creasta valului sisuprafaa apei linitite

wave amplitudeis the vertical distance between the crest and the calmsea level

h adncimea apeidistanta dintre suprafata apei linistite si fundul apei

water depththe distance between the calm sea level and the bottom

C viteza valului(sau distana parcurs de creasta valului)

wave speed= length/period C=L/T

Generally, the longer the length of the wave, the faster it moves through the water.


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Wind waves in deep water (RO)

Mrimea valului se reduce cu adncimeapnla dispariie

La suprafa particulele de ap au otraiectorie aproape circular,micarease transmite spre straturile inferioareunde particulele au traiectoriielipsoidale cu raze din ce n ce maimici spre fundul mrii unde micareaeste doar o oscilaie liniar (care

dispare la o adancime egala cu L).Diametrul cercului (orbitei) scad cuadncimea; De ex. la h = L/4, diametrulorbitei se va reduce cu 1/5 din formaoriginalideplasarea apei se va face

nainte i napoi i mai puin n formcircular.

Viteza particulelor scade cu adncimea: pt.perioada Pw = 10 sec deplasareadevine neglijabil la adncimi mai maride 100 metri.


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Wind waves in deep water (EN)

The passage of each wave = a circular motion.The motion is not exactly circular but is trochoidal (line form traced by a

point on a rolling wheel).While the motion in a wave over deep water move is an almost closed

circular path there is a tiny forward motion with the passage of each wave,particularly in large waves.

Also, in deep water, the motion changes as the depth increases fairlyrapidly. The trochoidal shape at the surface flattens with increasing depth as well

as a decrease in the total motion.This flattening of motion/decreasing size continues with increasing depth, it

transforms into an elipsiodal motion until all that remains is a small back andforth movementand even that will cease to be noticed which occurs at one-halfof the waves's total length(1/2 L).

For shallow water waves, the same flattening in the motion occurs but thereis no decrease in the forward/backward motion.


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Waves heightHow big wind waves get depends on 3 things: Wind strength. The wind must be moving faster than the wave crests for

energy to be transferred. Wind duration. Strong wind that does not blow for a long period will not

generate large waves. Fetch. This is the uninterrupted distance over which the wind blows without

significant change in direction.H max = 0,029914V2m/sObs: The height and length of the wave increase to a maximum generated by the

increasing speed and duration of the wind.

Relation between the wave maximum height and the wind speed


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High waves

Storms can generate larger waves in the open Pacific Ocean as compared to theother oceans due to the long open distance of water.If the wind continues or strengthens :

-the waves get higher from trough to crest, and length + period become longer.-the water first forms whitecaps and eventually the waves start to break = a fullydeveloped sea. (see sea statepictures).Rogue waves= unusually large waves appearing in a set of smaller waves.There are many sailor tales of "rogue waves", "freak waves", "three sisters" andother "killer waves = "extreme storm waves" . These tales were ridiculed and

mariners were accused of using them as an excuse to cover their own mistakes inwrecks. Characteristics of rogue wavesare:their height is greater than twice the size of surrounding waves,they often come unexpectedly from directions other than prevailing wind andwaves, andthey are unpredictable.Rogue waves formation: they form because of swells, while traveling across theocean. As these swells pass through one another their crests, troughs, and lengthshappen to coincide and reinforce each other, combining to form unusually largewaves that tower then disappear. If the swells are traveling in the same direction,these mountainous waves may last for several minutes before subsiding.Normally sailors do not even see huge waves over 20 m because ships nowadayswill try to avoid such conditions by altering course before the storm hits.


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Wind waves classification

1.Wind waves

2.Swell 3.Breakers

1.Wind waves (the sea)valurile de vnt:

-sunt neregulate, cu perioade inlimidiferite, direciide deplasare diferite.

-atingnlimimari -sunt alimentate permanent de vnt, de aceea panta valului este mai abrupt

n direciapropagrii (n fa)imai lin n dosul valului (valurile de vntsunt considerate valuri asimetrice).

-cnd prsescaria unde au fost generate iunde erau sub influenadirecta vntului, valurile cu perioade i viteze mari depesc valurile mai mici,

astfel c valuri cu dimensiuni similare vor avea tendina de a se deplasaparalel (valurile de hul).

-irregular, with different periods, heights and directions.

-can reach important heights

-the impulse force is the wind, which explains their asymmetric profile.


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Wind waves classification

2.Swell - Hula

Nu se produc neapratsub influenavntului. Apar dupcdereavntului ca fenomende amortizare sau se pot propaga dintr-o perturbaie baric aflat la distan. Elecresc ca lungime iperioad,dar scad nnlime.

When the wind stops or changes direction, waves that continue on without relation tolocal winds are called swell.

OBS: Valurile de vnt ihula determinstarea de agitaiea mrii.

n funcie de nlimea valuri lo r de vnt s-au stabilit 9 trepte pentru starea mrii (ncorelaie cu scara Beaufort a forei vntului). Pentru gradul 9, amplitudinea valuluidepete14 metri, excepional16 metri.

n funciede hul, s-au stabilit 3 trepte(slab,moderat,puternic)dar cu lungimi deundmai mari dect pentru valurile de vnt.

Obs. Wind waves (the sea) and the swell determine the sea state.

The sea scale contains 9 degrees (the scale is correlated to the wind force scale=theBeaufort scale. The 9thdegree = the phenomenal sea corresponds to waves height16 metres.

The swell scale contains 3 degrees (low, moderate, heavy); the swell wavelength is

bigger than the wind waves wavelength.


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3. Breakers form near the shore, due to decreasing depths. They are dangerousfor small boats.

RO: Brizanii (valurile de resac)

As the wavelength decreases, the height generally becomes greater. The lowerpart of a wave, being nearest the bottom, is slowed more than the top. at thepoint where the depth of the water is one-half of the wave's length (h=L/2), itbegins to "feel" the bottom.

The wavepeaks reach a height where they become unstable and, moving fasterthan the water below, they break forward.

Such a wave is called a breaker, and a series of breakers is surf.

Wavesclassification


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Sea state

The Douglas Sea Scalewas created by H. P. Douglas in 1917 while he was thehead of the British Meteorological Navy Service.

Its purpose is to estimate the roughness of the sea for navigation.The Scale has 2 codes:

- one code is for estimating the state of the sea;

- the other code is for describing the swell of the sea.


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Sea state

WMO (Douglas)

Sea State CodeWave Height (meters) Characterist ic s

0 0 Calm (glassy)

1 0 to 0.1 Calm (rippled)

2 0.1 to 0.5 Smooth (wavelets)

3 0.5 to 1.25 Slight

4 1.25 to 2.5 Moderate

5 2.5 to 4 Rough

6 4 to 6 Very rough7 6 to 9 High

8 9 to 14 Very high

9 Over 14 Phenomenal


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Character of the sea swel l

0. None

Low 1. Short or average

2. LongModerate 3. Short

4. Average

5. Long

Heavy 6. Short

7. Average

8. Long

9. Confused

Direction from which swell is coming should be recorded.

Confused swell should be recorded as "confused northeast," if coming from the

direction of northeast.

Th B f t Wi d S l


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The Beaufort Wind Scale

BeaufortForce

Wind SpeedKnots

Description Sea Condition

0 0 Calm Sea like a mirror

1 1 - 3 Light air Ripples but without foam crests

2 4 - 6Lightbreeze

Small wavelets. Crests do not break

3 7 - 10 Gentlebreeze Large wavelets. Perhaps scattered white horses

4 11 - 16Moderatebreeze

Small waves. Fairly frequent white horses

5 17 - 21

Fresh

breeze Moderate waves. Many white horses

6 22 - 27Strongbreeze

Large waves begin to form; white foam crests.Probably spray

7 28 - 33 Near gale

Sea heaps up and white foam blown in streaks

along the direction of the wind


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8 34 - 40 GaleModerately high waves. Crests begin to break intospindrift. The foam is blown in well marked streaks alongthe direction of the wind

9 41 - 47 Severe galeHigh waves. Dense foam along the direction of the wind.Crests of waves begin to roll over. Spray may affectvisibility

10 48 - 55 Storm

Very high waves with long overhanging crests. The surface

of the sea takes a white appearance. The tumbling of thesea becomes heavy and shock-like. Visibility affected

11 56 - 63 Violent stormExceptionally high waves. The sea is completely coveredwith long white patches of foam lying in the direction of the

wind. Visibility affected

12 64+ HurricaneThe air is filled with foam and spray. Sea completely whitewith driving spray. Visibility very seriously affected


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Other types of waves

Seiches-A seiche is a stationary wave that oscillates (changes aspect) without

progressing.-Also called a "standing wave-Seiches are caused by strong winds and/or changes in barometric pressure.-Found in enclosed / semi-enclosed areas.-Seiche period is determined by the length and depth of the water.-Currents associated with seiches are at maximum speed near the axis (node)

and minimum speed (or not at all) at either end (loops).


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Other types of waves

Rip currentsare powerful, channeled currents of water flowing away from shore.-They typically extend from the shoreline, through the surf zone, and past the line of

breaking waves.-Rip currents most typically form at low spots or breaks in sandbars, and also near

structures such as groins, jetties and piers.-Rip currents can be very narrow or extend in widths to hundreds of yards.-The seaward pull of rip currents varies: sometimes the rip current ends just beyond

the line of breaking waves, but sometimes rip currents continue to push hundreds of yardsoffshore.

-Rip currents form as incoming waves create an underwater sandbar close to shoreand the waves push more and more water in between the sandbar and the shore until asection of this sandbar collapses and the water rushes back toward the sea through anarrow gap. Once the flowing water passes through the narrow gap it begins to spread out.It is here where the velocity and strength of the rip current circulation begins to weakenconsiderably.

Rip currents can be killers as they are the leading surf hazard for all beachgoers.

They are particularly dangerous for weak or non-swimmers.A rip current is a horizontal motion not a vertical motion. Rip currents do not pull

people under the water; they pull people away from shore. Signs that a rip current ispresent are very subtle and difficult for the average beachgoer to identify. Look fordifferences in the water color, water motion, incoming wave shape or breaking pointcompared to adjacent conditions.


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Waves impact

In RO:

Efectele valurilor:

-asupra navelor: efectele se datoreaz energiei ridicate a valurilor i se manifest prinsleming, respectiv cdereanavei n golul dintre valuri sau ridicarea elicei deasupraapei imersul ei n gol etc. Pentru navele cu lungime mic, tangajul este puternic.Zona de pericol apare daclungimea valului este egalcu lungimea navei astfel nctnava rmnepe 2 creste.

-asupra zonei de rm: prin aciunea de eroziune (abraziune) modic configuraiarmului. n timpul furtunilor, cnd valurile deferleaz aducnd n zona de rmcantiti mari de ap, eroziunea este mult mai rapid. Jetul de resac arunc() cuviolen pietri i bolovni asupra rmului, provocnd erodarea lui. Prin frecareacontinu a acestora se produc particule mai fine; cele care rmn n apropierearmului formeazplaje ibare. Datoritcontracurenilorde splaredup deferlare,se produce deriva plajei. Plaja se poate lrgi(progradare) sau micora(retrogradare).

Curentul litoral i fenomenul de abraziune produc deriva de coast i eroziuneagradata plajei.


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Waves observations and measurements

1. Analitical2. Measurements

Analitical: based on formula, calculation tables.Hw (waves height) determination is based on wind speed, wind duration (hours) waterdepth, and the fetch se determina functie de viteza vantului, durata vantului in ore,adnacimea marii si fetch (the uninterrupted distance over which the wind blowswithout significant change in direction).

Tables contain values determined by using empirical formula (formula take into accountthe relations between the wind and the waves elements).

Wind Fetch Waves elements

WindforceBeaufort

knots Time (windduration - hours)

Distance(km)

Height(m)

Length(m)

Period(sec)

5 19 9,2 65 1,32,5 21 5,4

7 30 24,0 290 4,16,0 80 8,7

10 48 73,0 1550 14,0 225 14,5


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MeasurementsIn RO:

a) Instrumente-valograf (masuratorile se bazeaza pe diferenta de presiune hidrostatica;traductorul de presiune hidrostatica este atasat unei geamanduri careoscileaza pe val, atasata este o contragreutate; se transmite electric princablu printr-un potentiometru la nava.

-perspectometru se foloseste la coasta, este un aparat optic care vizeaza direct ogeamandura ce oscileaza liber pe val. In ocularul lunetei este o retea

reticulara care permite determinarea amplitudinii, lungimii si directiei depropagare a valurilor.b) Observati i vizuale val semn if icat iv = expresia matematica a Hw observata

de o persoana cu exper ienta (trainedobserver".)VAL SEMINIFICATIV = media a 1/3 din tre cele mai inalte valu ri obs ervate

Este folosita frecvent ca Hw la ocean.

Visual observations:the significant wave = the average height of the highest 1/3 of the waves

(represented on maps received onboard ship)


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Waves maps received on board ship:

For ex. The maps issued by the The Fleet Numerical Meteorology andOceanography Center. The Center produces synoptic analyses andpredictions of ocean wave heights using a spectral numerical model.The wave information consists of heights and directions for differentperiods and wavelengths.

Verification of projected data has proven the model to be very good.Information from the model is provided to the U.S. Navy on a routine basisand is a vital input to the Optimum Track Ship Routing program.


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Waves maps


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Documents

Lecture Waves