waves + beach morphology

• how do waves form

  produced by friction between wind + sea surface
• define a wave

  a transfer of energy from one water particle to a neighbor particle with individual water particles moving in circular orbit
• what is wave height

  vertical distance from peak to trough
• how is wave height determined

  by energy transferre from the wind and the water depth
• how is wave energy determined

  wind strength

  fetch

  wind duration - longer time = more energy

  water depth - depth decreases, wave height increases
• whats fetch

  uninterrupted distance of open water over which wind blows

  -Large fetch allows wind to push against water for longer time, transferring more energy.
• 2 types of waves

  sea waves

  swell waves
• swell waves

  wave energy which is continually being transferred creating waves without wind

  - as they travel away from the origin they may absorb small waves and gain energy height

  -can travel long distances before loosing energy and dissapating

  -produce waves at the coast even when no local wind

  -can form periodically larger waves among smaller local waves
• sea waves

  caused by wind blowing

  - they vary in height and direction
• what is wavelength

  horizontal distance from crest to crest
• how do waves reach the offshore zone

  When a wave approaching the shore reaches a water depth of ½ its wavelength, friction between the sea bed begins to distort the wave particle orbit from circular to elliptical

  = wave enters the offshore zone
• how do waves reach the nearshore zone

  water depth decreases further in offshore zone,

  - wave velocity slows,

  - wavelength shortens & wave height increases.

  Ultimately the wave breaks (NEARSHORE ZONE) & water flows up the beach as SWASH which gravity can then pull down as backwash
• what is wave frequency

  number of waves passing a certain point over a given period of time
• define a constructive wave

  -low energy wave

  -low wave height <1m

  -long wavelength up to 100m

  -low frequeny, 6-9pm
• where are constructive waves 'encouraged'

  in the nearshore zone as the long and shallow water causes friction to slow the wave and reduce its energy
• what is a spilling constructive wave

  spilling constructive:

  -have stronger swash than backwash (due to a low angle of wave impact)

  - their low frequency means that previous backwash does not impede current swash

  - a long and shallow nearshore allows the backwash water to percolate into the beach sediment which reduces the backwash's energy further
• define a destructive wave

  -high energy

  -large wave height >1m

  - short wavelength about 20m

  -high frequency, 13-15pm
• where are destructive waves 'encouraged'

  at a short and steep nearshore zone which quickly drops away into deeper water

  - this means there is little energy loss through friction
• what is a plunging destructive wave

  -have a strong backwash and weak swash due to a steep angle of impact

  - steep impact directs most of the energy downwards and backwards

  - this creates a strong backwash

  - strong backwash erodes material from the top of the beach and carry sit down the beach to the offshore zone
• define beach morphology

  the shape of the beach
• define a beach sediment profile

  a pattern of distribution of different size or shaped deposited material
• how do constructive waves alter beach morphology

  by causing net movement of sediment up the beach which steepens the profile

  - produces a berm where the swash reaches at high tide
• define a berm

  a ridge of material across a beach

  - produced by constructive waves
• how do constructive waves alter beach sediment profile

  -strong swash can carry all sediment sizes up the beach but the weaker backwash can only bring back down the smaller sediment

  - this leads to a sorting of material in the foreshore zone with bigger+ heavier at the back and sand/lighter closer to the sea

  
  

  - further sorting occurs with coarser sands deposited in the middle of the beach and finer ones to the sea
• how do destructive waves alter beach morphology

  -the weak swash and powerful backwash produces a net transportation of sediment down the beach which reduces the beach gradient

  
  

  however: some sediment is thrown foreward in the spray of a high impact breaking wave

  -this accumulates above the high tide mark as a storm ridge
• how do destructive waves alter beach sediment profile

  -powerful backwash drags down larger sediment to form a wide ridge of material below the low tide mark at the start of the offshore zone

  
  

  -if the friction is enough, it may cause the backwash to drop some sediment in the middle or lower beach
• define temporal variation

  change over time
• seasonal variation of waves

  -destructive high energy waves dominate in winter

  
  

  -in summer, constructive low energy waves dominate
• monthly variation of waves

  -tide height varies over the course of a lunar month: highest high tide occuring twice a month at spring tides and two very low high tides at neap tides

  
  

  -in the summer, a berm is made

  -as the months progress from spring to neap tides a series of berms may be built consecutively lower down the beach

  -when neap passes and spring tides come back the next year, the berms will be destroyed by the strong swash pushing material back up the beach
• daily variation of waves

  -storm events in summer will produce destructive waves that reshape the beach profile in a few hours

  -calm anticyclonic conditions in winter can produce constructive waves that begin to rebuild the beach, steepening the profile for a few days before the next storm
• decadal variation of waves

  -climate change is expected to produce more extreme weather events

  -'winter' profiles may be present for longer times of the year

  -more frequent + powerful destructive waves may reduce beach size: high tides reach more inland which increases the rate of coastal erosion in a previous backshore zone