Bars and bedforms Flashcards
What is the surf zone?
From the beach to where the waves first start breaking as they come ashore.
What does wave breaking do?
Stir up sediment
Bigger waves break…
What does this mean?
Farther offshore.
The wider the surf zone.
Means more of the nearshore bed is in motion.
Where is suspended sediment most concentrated?
Near the bed.
What creates net offshore transport?
Undertow but gradient in sediment transport creates a bar.
What is undertow?
Is a return flow compensating for the onshore waves approaching a shore
What happens if you move sediment to edge of the surf zone which is wide during a storm?
Sand piled up offshore where it builds an alongshore bar.
This is sand removed from the SZ/beach beach erosion extends as far as the waves can reach (eg, toe of dunes)
BUT this erosion is temporary: the sand comes back!
What happens as a longshore bar transitions into a crescentic bar and then to a beach?
Longshore bars are created as straight bars following storms.
As they gradually get more altered and reworked by different wave conditions (because different waves can reach the bed to different extents).
Eventually, as it works its way towards shore bar welds into a beach (welded bar).
Cycle restarts with next large wave event (high set-up resets bar offshore…)
Beach comes back…but how fast, and why?
Post-storm recovery
Waves, by themselves, tend to sweep sand back onshore because of velocity asymmetry
More velocity at crest than trough = more transport under crest (toward shore) than under trough.
and…
swells (fair weather waves) are better sweepers -> shoal more, more velocity asymmetry, reach bed in deeper water
vel asymmetry affects coarser sed more
coarser sed spends more time on bed, affected by asymm
limit of fine sed ~always suspended, moves with water
What are ripples?
Bump in the bed (because nature is never mathematically perfect)
The small cluster of grains grows to form the crest of a ripple and separation occurs near this point.
Sand grains saltate up to the crest on the upstream stoss side of the ripple. Flow is squeezed over crest of bump (+ feedback)
Avalanching of grains occurs down the downstream or lee side of the ripple as accumulated grains become unstable at the crest (Gravity causes collapse (- feedback))
Slight lad between crest and deposition will tend to move bump forward
Symmetry in ripples shape depends on
Flow direction
If you have equal equal and approx opposite flow directions what kind of ripple do you get?
Peaky
If you have one dominant flow direction what kind of ripple do you get?
Tilted (and/or flat)
What happens if if ripple crests move at different speeds?
You get “defects”
- Bifurcations
- Termination
- Loops
How is a bar created?
When there is a gap in the coastland with water in it. This could be a bay or a natural hollow in the coastland. The process of longshore drift occurs and this carries material across the front of the bay.
Material is pushed up onto beaches at an 45 degree angle when the swash brings it onto the coastline. The backwash takes it back out towards the sea at a right angle to the coast. Through this process material is constantly moved along the coastline.
The deposited material eventually joins up with the other side of the bay and a strip of deposited material blocks off the water in the bay. The area behind the newly formed bar is known as a lagoon.
