waves Flashcards
how do waves work
they propagate energy thru a medium (ocean water)
how do water particles move during a wave cycle
forward-circular motions, yet they remain in place
generating forces of waves
- wind
- geological events
- gravitational pull of moon
restoring forces of waves
- surface tension (L < 1.7cm)
- gravity (L > 1.7cm)
what generates seiches?
- changes in atmospheric pressure
- storm surges
- tsunami
- meteor impacts
- seismic forces
- L = 1km
what generates tsunamis?
- volcanic eruptions
- landslides
- vertical plate motion at a subduction
zone - L = 200km
what generates tides?
the rotation of the earth, moons gravitational pull
L = 20,000km
deep water waves
d > 1/2L
c = 1.25sqrt(L)
c = 1.56T
shallow water waves
d < 1/20 L
c = 3.1sqrt(d)
waves break when…
in open ocean: H/L > 1/7
in shallow water: when H/d = 3/4 or H/L > 1/7, whichever happens first
factors needed to produce big waves
- fast wind
- sustained wind
- long fetch
maximum sea state
fully developed sea
rogue waves
- 3-4 times higher than surrounding waves
- generated by constructive interference
- appear and disappear quickly
seiche definition
standing wave in a partially or fully enclosed basin
longer basin length ->
longer standing wave period
natural resonant period
determines period of seiche based on size, depth and contour of lake
T(s) = 0.64L/sqrt(d)
tsunamis occur from…
large input of energy affecting water mass
most occur in ring of fire
tsunami characteristics
L ~ 200 - 400km
T ~ 10min - 1hr
c ~ 200m/s or 720km/hr
c = 3.1sqrt(d)
H ~ 0.5 - 1m
steepness H/L ~ 0.00002 -> almost flat
tsunamis are always shallow water waves
why are tsunamis so destructive?
bc speed decreases and height increases as waves approach shore
wind waves vs tsunami
short L & T vs long L & T
water particles rotate in circles vs water flows straight forward
when do the effects of a tsunami worse in an enclosed bay?
if wave period is a multiple of natural resonant period
types of breakers
spilling: energy released slowly over a wide area
plunging: sudden release of energy, large & violent, forms pipeline
surging: v large & powerful, fast release over small area, waves don’t break but instead surge onto shore
longshore drift
waves bend as they approach shore
gradually smooth out coastline
mitigation strategies for longshore drift
groins -> trap sediments
jetties -> preserve inlets
breakwaters -> protects shore from erosion
sea walls -> reduce effects of strong waves, wall eventually collapses
