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 furtherLecture 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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