dr. chris spencer university of the west of england, bristol
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Understanding coastal processes and problems to produce successful management solutions. A case study of Dawlish Warren. . Dr. Chris Spencer University of the West of England, Bristol. Shallow Water Waves : Waves change as they move into shallow water motion of water molecules changes. - PowerPoint PPT PresentationTRANSCRIPT
Understanding coastal processes and problems to produce successful
management solutions.A case study of Dawlish Warren.
Dr. Chris Spencer
University of the West of England, Bristol
Shallow Water Waves :• Waves change as they move into shallow water• motion of water molecules changes
http://www.oceanpix.co.uk/wave-simulator.htm
Moving into shallow water the sea floor starts to interfere
As water shallows wavelength and wave velocity decrease• wave height increases• wave steepens• crest becomes narrower and steeper, can’t continue• the wave breaks
The point at which this happens varies
How does this vary on steep / shallow beaches?
The wave breaks :• water runs up the beach until stopped by gravity – the swash• water then flows back down the beach due to gravity – the backwash
Beaches : Shore Normal Morphology :• beach – loose pile of sand and gravel• survives in high energy environment…….walls get destroyed??
How do beaches survive?• beaches are mobile• adapt shape to conditions
• walls are fixed
Low energy conditions (summer)– beach relatively steep - reflective
– waves break onto the beach, relatively well spaced out– backwash returns before the next swash– swash not slowed
Lecture 3 : Dr Chris Spencer
How will this effect sediment movement?
Beach Shape?
– these are constructive waves– give us summer profiles– berms at the back of the beach
Under high energy conditions (winter)– beach slope is more gentle – absorb / dissipate– spilling breakers, breaking over beach
– waves arrive in rapid succession– backwash interferes with the next swash– reduced ability to move seidment
Lecture 3 : Dr Chris Spencer
• net seaward movement of sediment– destructive waves– winter profile– wider flatter beach
Beach shape is also influenced by particle size• this is due to the variable permeability
• coarse beaches are more permeable
• swash dominates, backwash returns through the beach
• net sediment movement up the beach– increases gradient
Lecture 3 : Dr Chris Spencer
Dr Chris Spencer : Lecture 3
Longshore currents :• waves may approach the coast at angle• usually <100 due to refraction• swash approaches at an angle• backwash returns with gravity
Longshore Drift :• wave process leads to characteristics landforms
• landforms become detached from the coast
Spits– narrow, elongate beaches– detached from the coast
• longshore drift transports sediment along the coast
• where the coast is ‘indented’ some of this sediment is deposited
• the longshore drift can now extend further
Lecture 3 : Dr Chris Spencer
The spit then extends across the indentation in the coast• can extend right across bay bar
– if an estuary the high discharge will keep this open
• spits have a characteristic hook shape at the end, a recurve
• often a whole series of recurves visible
Example : Dawlish Warren
2 theories– waves approach and slow in the shallows– curve around the spit– angle of the wave varies– builds up a recurve
Lecture 3 : Dr Chris Spencer
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