1. 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

4. Shallow Water Waves :
• Waves change as they move into shallow water
• motion of water molecules changes
http://www.oceanpix.co.uk/wave-simulator.htm

5. 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?

7. 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

12. 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

13. 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

15. • 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

16. Longshore currents :
• waves may approach the coast at angle
• usually <100 due to refraction
• swash approaches at an angle
• backwash returns with gravity
Dr Chris Spencer : Lecture 3

17. 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

19. 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

24. Profile 1 Profile 2
9
9
8 8
Height (m)
7 7
Pre-storm
Height (m)
6
Post-storm 6
5
5
4
0 20 40 60 80 4
0 20 40 60 80 100
Distance along transect (m)
Distance along transect (m)
Profile 3
Profile 4
9
8
9
7
8
7 6
Height (m)
Height (m)
Pre-storm
6 5
Post-storm
5 4
4
3
0 20 40 60 80 100 120 140
0 20 40 60 80 100 120 140
Distance along transect (m) Distance along transect (m)