2B.4A: Waves and Beach Morphology

Different wave types (constructive/destructive) influence beach morphology and sediment profiles, which vary at a variety of temporal scales from short term (daily) through to longer periods. 

Waves in general

• A wave is created through friction between the wind and water surface, transferring energy from the wind into the water. This generates ripples, which grow into waves when the wind is sustained. 
• A wave is the transfer of energy from one water particle to its neighbour with individual water particles moving in a circular orbit
  • The size of the wave particle orbit decreases with depth
• Wave height is the vertical distance from peak to trough
  • it's determined by the energy transferred from the wind, and the water depth
• Wave length is the horizontal distance from crest to crest (or trough to trough)
• Wave frequency is the number of waves passing a particular point over a given period of time

​Waves in open sea

• Waves are simply energy moving through water
• The water itself only moves up and down, not horizontally
• There is some orbital water particle motion within the wave, but no net forward water particle motion. 

Wave size depends upon:

• the strength of the wind
• the duration for which the wind blows
• water depth
• wave fetch
  • this is the uninterrupted distance across water over which the wind blows, and therefore the distance waves have to grow in size. 

How a wave breaks:

• ​When waves reaching the shore reach a wave depth of 1/2 their wavelength, the internal orbital motion of water within the wave touches the sea bed.
• Friction between the sea bed begins to distort the wave particle orbit from circular to elliptical, and slows down the wave.
  • The wave has entered the offshore zone 
• The wave depth decreases further, and the wave velocity slows, wavelength shortens, and wave height increases. Waves 'bunch' together.
• The wave crest begins to move forwards much faster than the wave trough
• Eventually the wave crest outruns the trough and the wave topples forwards - breaking. 
• The wave breaks in the nearshore zone, and water flows up the beach as swash
• The wave then losses energy and gravity pulls the water back down the beach as backwash

​Constructive Waves

• Low energy waves
• Low, flat wave height (<1m)
• Long wavelength (up to 100 m)
• Low wave frequency (about 6-9 per minute)
  • This means their swash is unimpeded by previous backwash
• A strong swash that pushes sediment up the beach, but a weaker backwash is unable to transport all particles back down, so they are deposited it as a ridge of sediment (berm) at the top of the beach
• A backwash that percolates into the beach material
  • encouraged by a long, shallow nearshore, so friction slows down the wave and releases energy
  • Constructive (spilling or surging) waves have a stronger swash than backwash due to a low angle of wave impact. 

Destructive (plunging) Waves

• High energy waves
• Large wave height (>1 m)
• Short wavelength (about 20 m)
• High wave frequency (13-15 per minute)
• They're encouraged by a short, steep nearshore zone, quickly dropping away into deeper water, so that there is little energy loss through friction
• They have strong backwash and weak swash due to the steep angle of impact
  • this directs most energy downwards and backwards, so the particle orbit is more circular than constructive breakers(?)
• Strong backwash erodes material from the top of the beach, carrying down the beach to the offshore zone
  • it's often deposited as a offshore ridge or berm

Beach Morphology and Sediment Profiles

Beach morphology is the shape of the beach. 
A beach sediment profile is the pattern of distribution of different sized or shaped deposited material. 

Constructive waves alter beach morphology by causing net movement of sediment up the beach, steeping the beach profile. 
They produce berms at the point where the swash reaches the high tide line. (A berm is a ridge of material across the beach)
Swash carries sediment of all sizes up the beach, but weaker backwash can only transport smaller particles down the beach. 
This leads to a sorting of material in the foreshore zone, with larger, heavier shingle (pebble-sized sediment) at the back of the beach, and sand drawn back closer to the sea. 
Since the backwash flows down the beach and loses energy through friction and depletion of water through percolation, sediment is further sorted as coarser sands are deposited in the middle of the beach and only fine sands are carried to the area of beach closest to the sea.  

Destructive Waves

• Weak swash and powerful backwash produces a net transport of sediment down the beach, reducing beach gradient.
• Some sediment thrown forwards in detached spray of high impact breaking wave. Accumulates above high tide mark as storm ridge.​
• Large, pebble-sized sediment dragged down beach by backwash to form wide ridge of material below low tide mark at start of offshore zone.
• Friction may be sufficient to cause backwash to down some sediment on middle or lower beach, with deposited sediment size decreasing towards sea.

Decadal Variation​

• Climate change is expected to produce more extreme weather events in the UK.
• Winter profiles may be present for longer time over course of year
• More frequent and more powerful destructive waves may reduce beach size, allowing high tides to reach further inland and increasing rate of coastal erosion in what was backshore zone.

Seasonal Variation in the UK

• Destructive, high-energy waves dominate in the winter, lowering angle of beach profile and spreading shingle over the whole beach. Offshore ridges/bars formed by destructive wave erosion and subsequent deposition of sand and shingle offshore. 
• In summer, constructive, low-energy waves dominate, steepening beach angle and sorting particles by size, with larger shingle particles towards back of beach. ​​In summer, constructive waves build berm ridges, typically of gravel/shingle at high tide mark
• Low channels and runnels between berms

Monthly Variation

• ​Tide height varies over course of lunar month, with highest high tide occurring twice a month at spring tide and two very low high tides (neap tides)​
  
• As month progresses from spring down to neap tide, successively lower high tides may produce a series of berms at lower and lower points down the beach.
  
• Once neap tide passes and move towards next spring tide, berms successively destroyed as material pushed further up beach by rising swash reach.  
  

Daily Variation

• Storm events during summer will produce destructive waves that reshape beach profile in a few hours.
• Calm anticyclonic conditions in winter can produce constructive waves that begin to rebuild beach, steepening profile for few days before storm. ​
• Destructive waves change to constructive ones as the wind drops. 
• Storm beaches, high at the back of the beach, result from high energy deposition of very coarse sediment during the most severe storms
  

​Other info:

The energy transferred by the wind depends upon:

1. ​wind strength
2. wind fetch 
   1. (the uninterrupted distance of open water over which the wind blows)
   2. a larger fetch allows the wind to push against the water for a longer time, transferring more energy
3. wind duration
   1. the longer the wind blows, the more energy it transfers​

Sea waves and swell waves

• Sea waves are produced by winds currently blowing in the local area, and vary in height and direction
• When the wind drops, wave energy continues to be transferred across the ocean in the form of swell waves
• As swell waves travel away from their origin they may absorb smaller sea waves and gain energy and height
• They can travel long distances before they lose energy and dissipate
• They produce waves at the coast even when there is no wind
• Swell waves can form periodically larger waves amongst smaller, local sea waves at the coast