Song
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Song as PDF for free.
More details
- Words: 2,631
- Pages: 9
gls214_waves
1 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Definition: Ocean waves are undulations of the water's surface resulting from the transfer of energy. The disturbance is propagated by the interactions of disturbing (e.g. wind) and restoring (e.g. gravity) forces. The energy in most ocean waves originates from the wind blowing across the water's surface. Large tsunami or seismic sea waves are generated by earthquakes, volcanic eruptions or large marine landslides. On the other hand, Tides, largest of all ocean waves result from the combined gravitational force exerted on the oceans by the sun and the moon.
Wave spectrum and the classification of waves Waves can be classified according to whether the waves pass through the body of a material or move along an interface. Body waves (Examples: secondary and primary seismic waves) Surface waves (Examples: Rayleigh waves and Love waves) Water waves are Rayleigh-type surface waves characterized by an orbital motion. Components of a surface wave
Length (L): Distance from equivalent points on a wave (e.g. crest to crest) Height (H): Distance from the crest to the trough. Period (T): The time it takes a complete wave to pass by a fixed point Parts of a wave by Edward A. Zobel
Internal waves: Waves formed at the interface between two layers that differ in density. The spectrum of ocean surface wave shown below categorizes waves according to wave period, or frequency. The period is the length of time it takes for an entire wave to pass a point. Frequency is the inverse of the period (1/T). Ocean waves with the longest periods are tidal wave produced by the gravitational forces exerted on the Earth by the Moon and Sun. Tides move sediment perpendicular to the shore and controls the daily movement of the surf up and down the foreshore. Wind generates the ocean waves we sea breaking in the surf zone. Breaking waves produce the longshore currents that transport sediment parallel to the shore.
4/9/2008 8:22 PM
gls214_waves
2 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Approximate distribution of ocean surface wave energy (after Kinsman 1965)
Water waves are classified by the disturbing and restoring forces involved Wind waves (wind is the disturbing force and gravity is the restoring force) (Estimating wind speed - Beaufort scale and more form Almanac online) Tides (transtidal waves-gravitational pull of the moon and sun is the disturbing force) Capillary waves (small waves where wind is often the disturbing force and surface tension of the water is the restoring force)
Other terms used to describe waves Throughout the literature the following terms often appear. Any other terms can typically be found in the USAC dictionary Periodic: Essentially all waves are periodic, which means that the motion (e.g. crest-trought-crest) is repetitive over a time. The period is the time it takes for one cycle, or repetition to occur. Progressive: Any wave that propagates through or across the surface of a material. Translatory: A wave in which both the wave form and water move forward. Breakers are translatory. The water particles are significantly transported forward with the wave. Standing wave: The wave form appears to oscillate in one place; standing waves are the product of two progressive waves moving in opposite directions. Standing wave formed by reflection (applet) A kelvin wave is a rotating standing wave. Oscillatory: wave form travels forward but water remains stationary; the wave orbitals close as one complete wave passes. Most waves are not purely oscillatory. A small forward movement of the water does take place and is referred to as wave drift or mass transport. Forced wave: a wave that exists as long as the disturbing force is acting on it (e.g. tides) Irrotational: The individual particles of water do not spin when a wave moves. Classification based on water depth relative to wavelength. The orbital motion of water decreases exponentially with depth. At depths greater than .5L the orbital motion is minimal and the wave no longer feels bottom. Waves that don't feel the bottom are deepwater waves.
4/9/2008 8:22 PM
gls214_waves
3 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
The orbital motion of water decreases exponentially with depth. At depths greater than one half the wave length the orbital motion is minimal and the wave no longer feels bottom. Deep water waves are those where the depth is greater than .5 the wave length.(1 d/L >.5 or d>.5L)
Intermediate (.5>d or L<.05) and shallow water waves (d/L <.05) deform in response to interactions with the sea floor. Shoaling waves are waved that are deforming in response to decreasing depth.
Wave theories: Different wave theories are used to predict and describe wave shape and wave behavior: Airy Wave theory: sinusoidal waves (Linear wave applet by Dalrymple) most accurate for low amplitude waves in deep water less accurate for predicting wave behavior in shallow water most commonly used wave theory because it is the least mathematically complex does not take into account the effects of wave height in determining wave velocity Stokes Wave theory: trochoidal waves can be used for deep-, intermediate- and shallow-water waves mathematically complex Takes into account the effects of wave height on velocity more accurately describes orbital velocity asymmetries Solitary wave theory: (Solitary wave calculator applet by Dalrymple) an isolated crest moving in shallow water none oscillatory progressive waves (translatory) use only to describe shallow-water waves (breakers) Most equations used here are based on the Airy Wave theory
Wave Parameters Wave Length
4/9/2008 8:22 PM
gls214_waves
4 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
the horizontal distance between successive wave crests generally difficult to measure. But can be measured from air photos. most commonly calculated from wave period (T)
r = kd k (wave number) = (2∏)/L d =depth Note: when d/L>.5, as for deepwater waves, tanhr approaches kd, and when d/L<.5, as for shallow water waves, tanhr approaches 1.
Wave Height (H) the vertical distance from crest to trough measurements are made with a staff in the surf zone or from a pier Wave height is equivalent to the diameter of deep-water wave orbitals The energy of an individual wave is proportional to the square of the wave height.
Significant Wave Height (H3): The average of the highest 1/3 of the waves from a wave spectrum. An observer standing on the shore is incapable of measuring all waves that approach the shore and typically records those that are larger. According to Komar (1998, p. 143) such visual measurements of wave height roughly corresponds to the significant wave height. Period (T) the time (t) it takes for an entire wave (L) to pass a given point. T=L/t field measurement: time the passage of 11 wave crests and divide by 10 The wave period for the significant wave (highest 1/3) can be measured in the surf Period does not change from deep to shallow water Frequency (F) - cycles or waves per unit time Number of waves to pass a point per unit of time (F=1/T) Wave steepness (H/L) If a wave steepness exceeds 1/7 then the wave breaks and reforms Relative depth (d/Lo ) Used to distinguish between deep-water, intermediate, and shallow-water waves. Celerity (C=L/T): Phase velocity = speed of an individual wave. The second equation below is known as the dispersion equation and shows that waves of different periods travel at different velocities, which is why waves become sorted as they move beyond the influence of the generating wind.
Group Speed (Cg): the speed of the wave train, not the individual wave Cgo = .5 Co for deep water Cg = C for shallow water (phase velocity decreases in shallow water) Amplitude
4/9/2008 8:22 PM
gls214_waves
5 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Vertical distance between the crest or trough and the still-water level. (1/2H)
Generation of waves (Read Davis and Dolan, 1993) Important factors governing the formation of wind waves storm duration wind speed Fetch: restricts the time during which individual waves can be influenced by the wind. Fetch is controlled by basin size storm diameter The amount of energy obtained from wind and stored in waves controls the dimensions of the various wave parameters. Forecasting wind-generated waves (see Gardiner and Dackombe, 1983) Fetch- and and depth-limited Wave Calculation applet (Chris Sherman) Mechanisms of wave formation by the wind Energy transfer by tangential shear Differential pressure forces turbulent velocity eddies sheltering effect Storm surges High water levels that accompany storms that are the result of Low pressure system produces a bulge in the water's surface Wind blowing on shore: Surface currents and increased mass transport the results in water to piling up on shore
Deep-water waves: Sea and swell (Lo, Do, To etc: d is greater than .5L, waves are not affected by the bottom) Sea: Waves found within the area of generation. Wave are chaotic and the spectrum is broad. Characteristics: waves are typically steep and chaotic; Ho, Lo, and To are variable breaking waves reform into broader waves with longer periods and A fully developed sea contains the largest waves capable of being formed under the prevailing set of conditions (fetch and wind speed) Swell are broad crested, sinuous waves that have traveled out of the area of generation. Swell waves are uniform in length and period and have a narrow spectrum. This transformation is the result of dispersion, the sorting waves according to period (C=1.56T).
Changes which occur when waves reach shallow water Shoaling (changes in size, shape and speed of waves) resulting from interactions of the wave with the bottom Wave orbitals become elliptical H increases L and C decreases T remains constant H/L increases to 1/7 and the wave breaks Breakers waves change from oscillatory waves to translator waves (breakers) and a shoreward mass transport of water occurs. Breaking waves are responsible for most of the suspension and transportation of sediment.
4/9/2008 8:22 PM
gls214_waves
6 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Factors causing waves to break: Wave will break when the water velocity (U) in the crest exceeds the wave celerity, or velocity at the trough. According to the solitary wave theory wave velocity is dependent on the water depth plus the instantaneous wave height These conditions occur when the angle of the wave crest exceeds 120°, this occurs when Ho/Lo > 1/7 or Hb=.78d
Energy is expended when waves break. Much of the energy is used in the suspension of sediment.
Breaker types: breaker type
slope
d/H
Phase difference
Dispersal of Energy
spilling
<3°
1 (high)
>1
Energy is dissipated over a broad distance
plunging
3-11°
.9-1 1-.5
collapsing 11-15° .8-.9 surging
>15°
<.8
<.5
Energy is concentrated where waves break Energy is released along the beach face Large amount of energy is reflected
phase difference = Tswash/Twave Table 1. Breaker type, conditions and dispersal of energy. phase difference = Tswash/Twave
<.5
low phase
.5-1
medium
>1
high
swash/backwash cycle completed before next bore swash cycle is interrupted causing turbulence (hydraulic jump visible) no backwash; overlapping swash
Table 2. Phase difference between swash and backwash. Significance of breaker type: As shown in table 1, breakers distribute wave energy differently depending on type. Plunging breakers concentrate energy in one location, usually at the step or where the water gets suddenly shallower. Spilling waves distribute energy over a broad region of shoaling. How breaker-type and height (Hb) influence suspension and transportation of sediment
4/9/2008 8:22 PM
gls214_waves
7 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Longshore current velocity increases with Hb The height to which sediment is suspended increases with Hb Height of swash increases with Hb
Wave phenomena Wave interference An increase or decrease of wave energy resulting from the superposition of wave forms. (See Dalrymples wave superposition applet) Constructive interference: increase in wave height results from the superposition of waves in phase Destructive interference: interactions of waves that are out of phase cancel each other out Partial reinforcement or cancellation can occur if waves are partially out of or in phase When you're lying on a beach with your eyes closed you can hear a rhythmic pounding or beat produce by the interference of waves having different frequencies or periods. Explore surf beat with this applet by B. Surendranath Reddy. Another beat applet by Walter Fendt Wave refraction bending of wave crests resulting from differential reduction in wave speed (C) as portions of the wave reach shallow water at different times Wave refraction diagrams: Illustrate the shoreward transmission and distribution of wave energy Wave refraction is caused by the interaction of waves with: the sea floor shoals and islands currents (e.g. tidal current from an inlet) Terms: orthogonal (wave ray): line showing the direction of wave propagation. Drawn perpendicular to wave crests. Procedure for drawing a wave-refraction diagram from airphotos Draw a series of evenly spaced orthogonals along the crests of a set of deepwater waves Extend the lines shoreward into shallow water. The orthogonal must always be perpendicular to wave crests assumptions: the Energy contained along a wave crests between two wave rays is equal Interpretation: Where orthogonals converge E is concentrated Where they diverge E is dissipated Energy is concentrated on headlands and spread out along embayments Local erosion may be governed by variations in offshore topography that my not be reflected by the shape of the coast Scripps Canyon wave refraction diagram Wave diffraction: The lateral transmission of energy along the crest of a wave (how does this phenomena conflict with the assumptions given above?) Importance: Energy is transmitted into the shadow zones behind islands and breakwaters diffracted waves typically experience refraction as energy varies along the crest Wave reflection
4/9/2008 8:22 PM
gls214_waves
8 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
waves are commonly reflected from seawalls, steep beach faces, breakwaters, etc. with little loss in energy results in the formation of standing waves which form offshore bars and some beach cusps reflection of waves from the foreshore may result in the formation of edge waves (standing waves in which the node and antinodal line are oriented perpendicular to the shoreline)
Local photos of wave phenomena
Sites to Explore Java Applets Demonstration Sediment-Transport Applets, Chris Sherwood, USGS Woodshole Linear wave calculations (wavenumber and orbital velocity) Dalrymples Coastal Engineering Java Page: Prof. Robert A. Dalrymple, Center for Applied Coastal Research,University of Delaware Wave Calculator: Using incident deep water wave data, it calculates the local wave variables in shallower water. It includes solving the dispersion relationship and Snell's Law. Linear Wave Kinematics: Given wave properties, the orbital motions of the water particles are shown graphically. Wind-induced Surge in a Basin: The effect of a steady wind stress on a constant depth body of water. Superposition of Waves: Up to four waves can be superimposed to show wave groups and standing waves. Edge Waves on a Planar Beach: The first three edge wave modes on a planar beach of given slope. Depth of Closure: Determine the depth of closure for a sandy beach, given wave height and period. Sand Transport Calculator: Calculates sand transport and diffusitivity parameter, given shallow water values. Academic Sites Water Waves: M.S. Krammer Waves: Bob Sica University of Ontario Ocean of waves in the world of Physiscs Coastal Carolina University Wave Refraction at Jaws, Maui : Katie M. Fearing and Robert A. Dalrymple Center for Applied Research Coastal University of Delaware Shoaling, refraction and defraction of waves Animation Numerical Models For 1960 Chilean Tsunami Velocities under water waves, Robert A.Dalrymple Wave animation, Science Websites: Virtual Labs and Simulation Java Applets on Physics by Walter Fendt Other Radar imaging of waves: Miguel A Tenorio-González, South Hampton University, UK Southern California Swell Model: USAC Coastal Data Information Program National Data Buoy Center Northeast Noaa Wave Watch Center How are spectral wave data derived from buoy motion measurements? Oceanweather NOAA/NCEP WAVEWATCH III Plots 1
* some authors (e.g. Komar, 1998 and Pethick, 1984) use d>.25L to define deep water waves
4/9/2008 8:22 PM
gls214_waves
9 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
[GeoHotsitesHome][GeoIndex][QkRef][GLS214]
Lindley Hanson/email /Gls214 Department of Geological Sciences, Salem State College, Salem, MA last updated 7/19/03
4/9/2008 8:22 PM
1 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Definition: Ocean waves are undulations of the water's surface resulting from the transfer of energy. The disturbance is propagated by the interactions of disturbing (e.g. wind) and restoring (e.g. gravity) forces. The energy in most ocean waves originates from the wind blowing across the water's surface. Large tsunami or seismic sea waves are generated by earthquakes, volcanic eruptions or large marine landslides. On the other hand, Tides, largest of all ocean waves result from the combined gravitational force exerted on the oceans by the sun and the moon.
Wave spectrum and the classification of waves Waves can be classified according to whether the waves pass through the body of a material or move along an interface. Body waves (Examples: secondary and primary seismic waves) Surface waves (Examples: Rayleigh waves and Love waves) Water waves are Rayleigh-type surface waves characterized by an orbital motion. Components of a surface wave
Length (L): Distance from equivalent points on a wave (e.g. crest to crest) Height (H): Distance from the crest to the trough. Period (T): The time it takes a complete wave to pass by a fixed point Parts of a wave by Edward A. Zobel
Internal waves: Waves formed at the interface between two layers that differ in density. The spectrum of ocean surface wave shown below categorizes waves according to wave period, or frequency. The period is the length of time it takes for an entire wave to pass a point. Frequency is the inverse of the period (1/T). Ocean waves with the longest periods are tidal wave produced by the gravitational forces exerted on the Earth by the Moon and Sun. Tides move sediment perpendicular to the shore and controls the daily movement of the surf up and down the foreshore. Wind generates the ocean waves we sea breaking in the surf zone. Breaking waves produce the longshore currents that transport sediment parallel to the shore.
4/9/2008 8:22 PM
gls214_waves
2 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Approximate distribution of ocean surface wave energy (after Kinsman 1965)
Water waves are classified by the disturbing and restoring forces involved Wind waves (wind is the disturbing force and gravity is the restoring force) (Estimating wind speed - Beaufort scale and more form Almanac online) Tides (transtidal waves-gravitational pull of the moon and sun is the disturbing force) Capillary waves (small waves where wind is often the disturbing force and surface tension of the water is the restoring force)
Other terms used to describe waves Throughout the literature the following terms often appear. Any other terms can typically be found in the USAC dictionary Periodic: Essentially all waves are periodic, which means that the motion (e.g. crest-trought-crest) is repetitive over a time. The period is the time it takes for one cycle, or repetition to occur. Progressive: Any wave that propagates through or across the surface of a material. Translatory: A wave in which both the wave form and water move forward. Breakers are translatory. The water particles are significantly transported forward with the wave. Standing wave: The wave form appears to oscillate in one place; standing waves are the product of two progressive waves moving in opposite directions. Standing wave formed by reflection (applet) A kelvin wave is a rotating standing wave. Oscillatory: wave form travels forward but water remains stationary; the wave orbitals close as one complete wave passes. Most waves are not purely oscillatory. A small forward movement of the water does take place and is referred to as wave drift or mass transport. Forced wave: a wave that exists as long as the disturbing force is acting on it (e.g. tides) Irrotational: The individual particles of water do not spin when a wave moves. Classification based on water depth relative to wavelength. The orbital motion of water decreases exponentially with depth. At depths greater than .5L the orbital motion is minimal and the wave no longer feels bottom. Waves that don't feel the bottom are deepwater waves.
4/9/2008 8:22 PM
gls214_waves
3 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
The orbital motion of water decreases exponentially with depth. At depths greater than one half the wave length the orbital motion is minimal and the wave no longer feels bottom. Deep water waves are those where the depth is greater than .5 the wave length.(1 d/L >.5 or d>.5L)
Intermediate (.5>d or L<.05) and shallow water waves (d/L <.05) deform in response to interactions with the sea floor. Shoaling waves are waved that are deforming in response to decreasing depth.
Wave theories: Different wave theories are used to predict and describe wave shape and wave behavior: Airy Wave theory: sinusoidal waves (Linear wave applet by Dalrymple) most accurate for low amplitude waves in deep water less accurate for predicting wave behavior in shallow water most commonly used wave theory because it is the least mathematically complex does not take into account the effects of wave height in determining wave velocity Stokes Wave theory: trochoidal waves can be used for deep-, intermediate- and shallow-water waves mathematically complex Takes into account the effects of wave height on velocity more accurately describes orbital velocity asymmetries Solitary wave theory: (Solitary wave calculator applet by Dalrymple) an isolated crest moving in shallow water none oscillatory progressive waves (translatory) use only to describe shallow-water waves (breakers) Most equations used here are based on the Airy Wave theory
Wave Parameters Wave Length
4/9/2008 8:22 PM
gls214_waves
4 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
the horizontal distance between successive wave crests generally difficult to measure. But can be measured from air photos. most commonly calculated from wave period (T)
r = kd k (wave number) = (2∏)/L d =depth Note: when d/L>.5, as for deepwater waves, tanhr approaches kd, and when d/L<.5, as for shallow water waves, tanhr approaches 1.
Wave Height (H) the vertical distance from crest to trough measurements are made with a staff in the surf zone or from a pier Wave height is equivalent to the diameter of deep-water wave orbitals The energy of an individual wave is proportional to the square of the wave height.
Significant Wave Height (H3): The average of the highest 1/3 of the waves from a wave spectrum. An observer standing on the shore is incapable of measuring all waves that approach the shore and typically records those that are larger. According to Komar (1998, p. 143) such visual measurements of wave height roughly corresponds to the significant wave height. Period (T) the time (t) it takes for an entire wave (L) to pass a given point. T=L/t field measurement: time the passage of 11 wave crests and divide by 10 The wave period for the significant wave (highest 1/3) can be measured in the surf Period does not change from deep to shallow water Frequency (F) - cycles or waves per unit time Number of waves to pass a point per unit of time (F=1/T) Wave steepness (H/L) If a wave steepness exceeds 1/7 then the wave breaks and reforms Relative depth (d/Lo ) Used to distinguish between deep-water, intermediate, and shallow-water waves. Celerity (C=L/T): Phase velocity = speed of an individual wave. The second equation below is known as the dispersion equation and shows that waves of different periods travel at different velocities, which is why waves become sorted as they move beyond the influence of the generating wind.
Group Speed (Cg): the speed of the wave train, not the individual wave Cgo = .5 Co for deep water Cg = C for shallow water (phase velocity decreases in shallow water) Amplitude
4/9/2008 8:22 PM
gls214_waves
5 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Vertical distance between the crest or trough and the still-water level. (1/2H)
Generation of waves (Read Davis and Dolan, 1993) Important factors governing the formation of wind waves storm duration wind speed Fetch: restricts the time during which individual waves can be influenced by the wind. Fetch is controlled by basin size storm diameter The amount of energy obtained from wind and stored in waves controls the dimensions of the various wave parameters. Forecasting wind-generated waves (see Gardiner and Dackombe, 1983) Fetch- and and depth-limited Wave Calculation applet (Chris Sherman) Mechanisms of wave formation by the wind Energy transfer by tangential shear Differential pressure forces turbulent velocity eddies sheltering effect Storm surges High water levels that accompany storms that are the result of Low pressure system produces a bulge in the water's surface Wind blowing on shore: Surface currents and increased mass transport the results in water to piling up on shore
Deep-water waves: Sea and swell (Lo, Do, To etc: d is greater than .5L, waves are not affected by the bottom) Sea: Waves found within the area of generation. Wave are chaotic and the spectrum is broad. Characteristics: waves are typically steep and chaotic; Ho, Lo, and To are variable breaking waves reform into broader waves with longer periods and A fully developed sea contains the largest waves capable of being formed under the prevailing set of conditions (fetch and wind speed) Swell are broad crested, sinuous waves that have traveled out of the area of generation. Swell waves are uniform in length and period and have a narrow spectrum. This transformation is the result of dispersion, the sorting waves according to period (C=1.56T).
Changes which occur when waves reach shallow water Shoaling (changes in size, shape and speed of waves) resulting from interactions of the wave with the bottom Wave orbitals become elliptical H increases L and C decreases T remains constant H/L increases to 1/7 and the wave breaks Breakers waves change from oscillatory waves to translator waves (breakers) and a shoreward mass transport of water occurs. Breaking waves are responsible for most of the suspension and transportation of sediment.
4/9/2008 8:22 PM
gls214_waves
6 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Factors causing waves to break: Wave will break when the water velocity (U) in the crest exceeds the wave celerity, or velocity at the trough. According to the solitary wave theory wave velocity is dependent on the water depth plus the instantaneous wave height These conditions occur when the angle of the wave crest exceeds 120°, this occurs when Ho/Lo > 1/7 or Hb=.78d
Energy is expended when waves break. Much of the energy is used in the suspension of sediment.
Breaker types: breaker type
slope
d/H
Phase difference
Dispersal of Energy
spilling
<3°
1 (high)
>1
Energy is dissipated over a broad distance
plunging
3-11°
.9-1 1-.5
collapsing 11-15° .8-.9 surging
>15°
<.8
<.5
Energy is concentrated where waves break Energy is released along the beach face Large amount of energy is reflected
phase difference = Tswash/Twave Table 1. Breaker type, conditions and dispersal of energy. phase difference = Tswash/Twave
<.5
low phase
.5-1
medium
>1
high
swash/backwash cycle completed before next bore swash cycle is interrupted causing turbulence (hydraulic jump visible) no backwash; overlapping swash
Table 2. Phase difference between swash and backwash. Significance of breaker type: As shown in table 1, breakers distribute wave energy differently depending on type. Plunging breakers concentrate energy in one location, usually at the step or where the water gets suddenly shallower. Spilling waves distribute energy over a broad region of shoaling. How breaker-type and height (Hb) influence suspension and transportation of sediment
4/9/2008 8:22 PM
gls214_waves
7 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
Longshore current velocity increases with Hb The height to which sediment is suspended increases with Hb Height of swash increases with Hb
Wave phenomena Wave interference An increase or decrease of wave energy resulting from the superposition of wave forms. (See Dalrymples wave superposition applet) Constructive interference: increase in wave height results from the superposition of waves in phase Destructive interference: interactions of waves that are out of phase cancel each other out Partial reinforcement or cancellation can occur if waves are partially out of or in phase When you're lying on a beach with your eyes closed you can hear a rhythmic pounding or beat produce by the interference of waves having different frequencies or periods. Explore surf beat with this applet by B. Surendranath Reddy. Another beat applet by Walter Fendt Wave refraction bending of wave crests resulting from differential reduction in wave speed (C) as portions of the wave reach shallow water at different times Wave refraction diagrams: Illustrate the shoreward transmission and distribution of wave energy Wave refraction is caused by the interaction of waves with: the sea floor shoals and islands currents (e.g. tidal current from an inlet) Terms: orthogonal (wave ray): line showing the direction of wave propagation. Drawn perpendicular to wave crests. Procedure for drawing a wave-refraction diagram from airphotos Draw a series of evenly spaced orthogonals along the crests of a set of deepwater waves Extend the lines shoreward into shallow water. The orthogonal must always be perpendicular to wave crests assumptions: the Energy contained along a wave crests between two wave rays is equal Interpretation: Where orthogonals converge E is concentrated Where they diverge E is dissipated Energy is concentrated on headlands and spread out along embayments Local erosion may be governed by variations in offshore topography that my not be reflected by the shape of the coast Scripps Canyon wave refraction diagram Wave diffraction: The lateral transmission of energy along the crest of a wave (how does this phenomena conflict with the assumptions given above?) Importance: Energy is transmitted into the shadow zones behind islands and breakwaters diffracted waves typically experience refraction as energy varies along the crest Wave reflection
4/9/2008 8:22 PM
gls214_waves
8 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
waves are commonly reflected from seawalls, steep beach faces, breakwaters, etc. with little loss in energy results in the formation of standing waves which form offshore bars and some beach cusps reflection of waves from the foreshore may result in the formation of edge waves (standing waves in which the node and antinodal line are oriented perpendicular to the shoreline)
Local photos of wave phenomena
Sites to Explore Java Applets Demonstration Sediment-Transport Applets, Chris Sherwood, USGS Woodshole Linear wave calculations (wavenumber and orbital velocity) Dalrymples Coastal Engineering Java Page: Prof. Robert A. Dalrymple, Center for Applied Coastal Research,University of Delaware Wave Calculator: Using incident deep water wave data, it calculates the local wave variables in shallower water. It includes solving the dispersion relationship and Snell's Law. Linear Wave Kinematics: Given wave properties, the orbital motions of the water particles are shown graphically. Wind-induced Surge in a Basin: The effect of a steady wind stress on a constant depth body of water. Superposition of Waves: Up to four waves can be superimposed to show wave groups and standing waves. Edge Waves on a Planar Beach: The first three edge wave modes on a planar beach of given slope. Depth of Closure: Determine the depth of closure for a sandy beach, given wave height and period. Sand Transport Calculator: Calculates sand transport and diffusitivity parameter, given shallow water values. Academic Sites Water Waves: M.S. Krammer Waves: Bob Sica University of Ontario Ocean of waves in the world of Physiscs Coastal Carolina University Wave Refraction at Jaws, Maui : Katie M. Fearing and Robert A. Dalrymple Center for Applied Research Coastal University of Delaware Shoaling, refraction and defraction of waves Animation Numerical Models For 1960 Chilean Tsunami Velocities under water waves, Robert A.Dalrymple Wave animation, Science Websites: Virtual Labs and Simulation Java Applets on Physics by Walter Fendt Other Radar imaging of waves: Miguel A Tenorio-González, South Hampton University, UK Southern California Swell Model: USAC Coastal Data Information Program National Data Buoy Center Northeast Noaa Wave Watch Center How are spectral wave data derived from buoy motion measurements? Oceanweather NOAA/NCEP WAVEWATCH III Plots 1
* some authors (e.g. Komar, 1998 and Pethick, 1984) use d>.25L to define deep water waves
4/9/2008 8:22 PM
gls214_waves
9 of 9
http://w3.salemstate.edu/~lhanson/gls214/gls214_waves.html
[GeoHotsitesHome][GeoIndex][QkRef][GLS214]
Lindley Hanson/email /Gls214 Department of Geological Sciences, Salem State College, Salem, MA last updated 7/19/03
4/9/2008 8:22 PM
