8.4 How do waves change in the surf zone?
most waves generated in sea area by storm winds move across the ocean as swell. these waves then release their energy along the margins of continents in the surf zone
surf zone = zone of breaking waves
breaking waves show power and persistence, sometimes moving objects weighing several tons.
energy from a distant storm can travel thousands of km
Physical Changes as Waves Approach Shore
shoaling = shallow
as deep water waves of swell move toward cont. margins over gradually shoaling water, they eventually encounter water depths that are less than ½ of their wavelength —> become transitional waves
any shallowly submerged obstacle (coral, reef, sunken wreck, sand bar), will cause waves to release some energy
breaking waves indicate dangerously shallow water
as waves approach the shore and encounter water depths of less than ½ wavelength, the waves “feel bottom” —> wave speed decreases, waves stack up against the shore, causing wavelength to decrease
result: increase in wave height —> wave steepness > 1/7 ratio, causing the wave to pitch forward and break in the surf zone
if surf is swell that has traveled from distant storms, breakers will develop relatively near shore, in shallow water. horizontal motion characteristic of shallow-water waves moves water alternately toward and away from shore as an oscillation
if surf consists of waves generated by local winds, waves may not have been sorted into swell, may be mostly unstable, deep-water, high-energy waves w/ steepness already near 1/7 ratio
in this case, waves will break shortly after feeling bottom some distance from shore —> rough, choppy, irregular surf
when water depth is about 1 and 1/3 x wave height, crest of the wave breaks —> produces surf
use this relationship to estimate water depth in the surf zone: depth of the water where waves are breaking is one and 1/3 times the breaker height
when water depth becomes less than 1/20 wavelength, waves in surf zone begin to behave like shallow-water waves, particle motion is greatly impeded, and a significant transport of water toward shoreline occurs
waves break in surf zone bc particle motion near the bottom of the wave is severely restricted, slowing the waveform
at surface, individual orbiting water particles haven’t been slowed bc no contact w/ bottom
wave height increases in shallow water
difference in speed between top and bottom parts of the wave causes the top part to overrun the lower part —> wave breaks
Breakers and Surfing
3 main types of breakers:
(1) spilling breaker: turbulent mass of air and water that runs down the front slope of the wave as it breaks
result from a gently sloped ocean bottom, which gradually extracts energy from the wave over an extended distance and produces breakers w/ low overall energy
longer life span, give surfers a long but less exciting ride
normal for open-ocean waves when gusty wind conditions cause waves to form whitecaps offshore
(2) plunging breaker: curling crest that moves over an air pocket
curling crest occurs bc the particles in the crest literally outrun the wave, then there is nothing beneath them to support their motion
plunging breakers form on moderately steep beach slopes
best waves for surfing
(3) surging breaker: when ocean bottom has an abrupt slope, wave energy is compressed into a shorter distance, and wave will surge forward
build up and break right at shoreline
Wave Refraction
waves seldom approach shore at a perfect right angle —> instead, some segment of the wave will feel bottom first and slow down before the rest of the wave —> wave refraction
wave refraction = bending of each wave crest as waves approach the shore
refraction of waves along an irregular shoreline distributes wave energy unevenly along the shore
orthogonal lines are always oriented perpendicular to wave crests, indicating direction in which they travel
far from shore, orthogonals are evenly spaced, indicating all parts of the wave have the same amount of energy
as waves approach shore and refract, orthogonals converge on headlands that jut into the ocean and diverge in bays
wave energy is focused against the headlands but dispersed in bays
large waves occur at headlands = good surfing
smaller waves occur in bays
**the same waves break against both headlands and in nearby bays, but their energy is diff bc of wave refraction which causes the spacing of orthogonals and wave energy to change. waves approaching shore are also influenced by sea floor features i.e. shallow banks or submarine canyons.
Wave Reflection
not all energy is expended as waves rush onto shore
a vertical barrier i.e. seawall or rock ledge can reflect waves back into the ocean with little loss of energy = wave reflection
similar to how a mirror reflects light
if incoming wave strikes barrier at a right angle, the wave energy is reflected back parallel to the incoming wave, interfering with that wave and creating unusual waveforms.
more commonly, waves approach the shore at an angle, causing wave energy to be reflected at an angle equal to the angle at which the wave approached the barrier
The Wedge: A Case Study of Wave Reflection and Constructive Interference
west of the jetty that protects the harbor entrance at Newport Harbor, California
as incoming waves strike vertical side of the jetty at an angle, they’re reflected at an equivalent angle. bc the original waves and reflected waves have the same wavelength, a constructive interference pattern develops, creating plunging breakers
Standing Waves, Nodes, and Antinodes
standing waves: produced when waves are reflected at right angles to a barrier, sum of two waves w/ same wavelength moving in opposite directions, resulting in no net movement
although water particles continue moving horizontally and vertically, no circular motion
node = point where there is no vertical movement
antinode = crests/troughs, greatest point of vertical movement w/in a standing wave
water is motionless when antinodes reach max displacement
water movement at max when water level is horizontal
movement is vertical beneath the antinodes and max horizontal movement occurs beneath the node
CONCEPT CHECK 8.4
(1) what is the 1:7 ratio? what happens to a wave when the 1:7 ratio is exceeded?
1:7 ratio = wave steepness ratio, wave height / wavelength
if the ratio is exceeded, the wave breaks
(2) Describe the physical changes that occur to a wave’s wave speed, wavelength, height, and wave steepness (h/l) as the wave moves across progressively shallower water to break on the shore.
wave speed (S): decreases
wavelength (L): decreases bc waves stack up against shore
wave height (H): increase
wave steepness (H/L): increases beyond 1:7 ratio, causing wave to break in the surf zone3
(3) Describe the three different types of breakers and indicate the slope of the beach that produces each of the three types. how is the energy of the wave distributed differently within the surf zone by the three types of breakers?
spilling: turbulent mass of air and water that runs down the front slope of a wave as it breaks, gently sloped ocean bottom, low overall energy, longer life span wave
plunging: curling crest that moves over an air pocket, occurring bc the particles in the crest literally outrun the wave, and there’s nothing beneath them to support their motion, best waves to surf, moderately steep beach slopes
surging: abrupt slope, wave energy compressed into a shorter disyance, wave will surge forward, all energy is dumped right where it breaks rather than over time as a plunging breaker or spilling breaker
(4) Using examples, explain how wave refraction is different from wave reflection.
wave refraction = waves pass through the surface or over something weird, then just change direction of energy i.e. over a sand bar that bumps the wave
wave reflection = wave hits something and bounces directly back off of it in the same direction it came from at the same angle i.e. a wave hitting a jetty