Waves and Tides

wavelength

crest to crest distance (units of length)

wave speed

distance a wave crest travels per unit time

how are constructive waves formed

when two crests are added together, you will accentuate the trough (constructive)

how are destructive waves formed

when troughs get added to the crests

what do waves propagate

energy → this causes everything to come back to the same spot of the wave orbital

what do waves not propagate

mass

what is wave speed a function of

• wavelength

• bottom depth

do long or short wavelengths travel faster

long

do waves travel faster in deeper or shallower water

deeper

in exceptional circumstances, how can bottom depth / wavelength affect wave speed

• in very deep water, small variation in bottom depth has little impact on overall wave speed and wavelength alone determines wave speed

• in very shallow water, small variation in bottom depth has a big impact on wave speed and variations in bottom depth alone determines wave speed

deep-water wave

• bottom depth is deeper than ½ the wavelength

• speed is a function of wavelength only → longer wavelength waves move faster than shorter wavelength waves

shallow-water waves

• bottom depth is shallower than 1/20 of the wavelength

• speed is a function of depth only

• waves in shallower water move slower than they would

what is wave dispersion

self-sorting of deep-water waves leaving a storm region based on wave length

what is the consequence of wave dispersion

we get a nice clean set of waves (only if there is distance to sort out the waves)

what is wave refraction

focusing and defocusing of wave energy on headlands and bays, respectively

wave refraction where there is a headland

the center is deeper so waves travels faster. wave rays will converge to the center, wave crest’s energy per unit increases, height of wave increases

wave refraction at a bay

the center is shallower so waves travel slower

wave rays diverge over larger volume of water, decreasing the energy per unit of wave crest as the wave height decreases

when does longshore sediment transport occur

waves strike the beach at an angle

what happens when waves break

• waves slow down as water gets shallower

• wave energy density increases (with crowding waves and shallower water depths) push waves upward

• the top of the wave moves faster than the bottom of the wave and the wave eventually spills over and breaks

summer beach conditions

gentle waves push offshore sandbar shoreward to create wide and gently sloping sandy summer beaches due to the presence of constructive waves

winter beach conditions

• destructive storm waves drag sand off the beach to be stored in offshore sandbar and frequently resulting in a rocky winter beach

what are rip currents

• initiated when large waves push water onto an elevated beach face

• the beached water is funneled back off the beach through narrow breaks in underwater sand bars through narrow breaks in underwater sand bars to form accelerated jets of water (rip currents)

• these jet-like rip currents can pull a person hundred of meters offshore

what to do if caught in a rip current

swim along the shore for 10/20 meters

swim at an angle back to the beach

and call bruce to let him know he saved your life

what determines the height of a wave

• wind speed

  • sets the upper possible limit on wave height for a fully developed sea

• duration of the wind event

  • modulates the upper possible limit on wave height

• fetch (the distance over which wind can blow without obstruction)

  • modulates the upper possible limit

what is fetch

• duration that the wind can blow without obstruction

  • it is limited by the size of the storm system in the open ocean

• once a wave is generated under the center of a storm region, it begins to propagate outward and away from the region

• if the fetch is large, it takes a long time before the wave finally reaches the outer edge of the storm region where the winds subside

• fetch is connected to the duration over which the wave experiences maximal winds

what does a tsunami require

movement of the seafloor through sediment slumping or tectonic plate

how fast is the propagation of a tsunami

500 miles per hour

what is an example of a tsunami wave generated at a convergent plate boundary

indonesian (sumatra-andaman) earthquake in december 2004

what do you do when you see a tsunami or feel the ground shaking

go to high ground

are all tsunamis generated from an earthquake

no

what are the type of daily wave patterns

• diurnal

• semidiurnal

• mixed semidiurnal

what are the type of monthly wave patterns

• spring pattern

• neap pattern

diurnal pattern

one high tide and low tide per day

semidiurnal pattern

two equal high tides and two equal low tides

mixed semidiurnal

two unequal high tides and two unequal low tides per day

why are there diurnal wave patterns

• pull of gravity by the moon and the centrifugal force creates 2 “tidal bulges” on the opposing sides of the earth, creating a regular rise and fall of the tides over an approximately 24h period as earth rotates on its axis

where is the center of mass of the earth

it is off-centered because we need to consider the mass of the moon as well. this barycenter is where the moon rotates around

how do we get diurnal and mixed semidiurnal tides

the moon’s orbit is inclined 28 degrees relative to the earth’s equator and this impacts the daily pattern of tides

how does the moon’s latitude change with each week

moon orbits 28 degrees above the equator, next week will be at the equator, the week after will be 28 degrees below the equator

in what position of the moon do diurnal tides form

when the moon is 28 degrees above the equator

in what position of the moon do mixed tides form

when the moon is in the middle of 28 degrees and equator

in what position of the moon do semidiurnal tides form

when the moon is at the equator

what causes the monthly pattern of tidal amplitude

combined effects of gravity from the moon and the sun

when do spring tides form

during a new and full moon

why do spring tides form

because the moon pulls along the same line as the sun

how are the spring tides like

tides are more exaggerated

when do neap tides form

during the first and third quarter moon

why do neap tides form

when the moon pulls at 90 degrees to the sun

how are the neap tides like

tides are small

why do we get ocean tides but no tides in small lakes

• in small lakes, centrifugal force is exactly inline with the moon’s gravity and the two forces cancel each other out

• in large ocean/lakes, the centrifugal force at locations not directly under the moon does not align equal and opposite the gravity force of the moon, so the moon’s gravity is not counterbalance by the centrifugal force at these locations

• oceans are pinched upward from the cumulative lateral pull by the moon on the ocean

• lakes are not large enough to experience these broad lateral pinch by the moon

how does coriolis force work

it always acts exactly to the right of the direction of motion in the northern hemisphere and always acts directly to the left of the direction of motion in the southern hemisphere

what is the dynamic theory of waves

• tide wave treated as a forced shallow-water wave that is not in equilibrium with lunar/solar forcing

• affected by coriolis force

• continents interfere with the propagation of the tide wave

what is the amphidromic point

it is the center of the rotary tide where there are no tides (like hawaii)

which areas exhibit the largest tidal ranges

coastal regions (e.g. bay of fundy, canada)

why does wave dispersion occur

longer wavelength waves travel faster than shorter wavelength waves (for deep water)

what does larger fetch give the storm

• the storm has more time to pump up the size of a given wave before the wave propagates out from under the storm center

why does rotary motion of a tide wave in a closed ocean basin occur

coriolis force and blocking by continents

what does dynamic theory of tides explain

• why tidal bulge (high tide) occurs after the moon passes overhead

• the rotary motion of tides