Cross-shore transport - bedforms - Lecture 5

Question 1

What is the surf zone?

Answer 1

Bigger (frothy white) waves breaking offshore - meaning more of the nearshore bed is in motion

Question 2

How are bars formed?

Answer 2

Less force moving sediment around further backwards you go - more coming in than going out = bar
Waves breaking stir up sediment and suspended sediment is more concentrated near bed, undertow creates net offshore transport, but gradient pushes sediment backwards creates a bar in the surf zone

Question 3

What are the surf zone interactions during storms?

Answer 3

SZ is wide during a storm, sediment is moved to the edge of the surf zone and piled up offshore, building alongshore bar (sediment has been removed from SZ/beach). Deeper the bar goes out, harder to get back
(Wave set up increases during a storm, washes further up beach, bar moves further offshore, SZ widens)

Question 4

What are the processes involved in bar formations?

Answer 4

Bars straighten out in the storm and get altered (crescentic) as bar is reworked by wave conditions, bar welds to beach and cycle restarts with next large wave event

Question 5

What are the seasonal differences to bars?

Answer 5

Wave height increases summer -> winter
Welded bar (summer - wide beach), tranverse bar, crescentic attached bar, crescentic bar, longshore bar (winter - narrow beach)

Question 6

Beaches come back after storms - how fast and why?

Answer 6

Waves become smaller and brake up shape of bar

Swells and shoaling move sediment onto beach

Question 7

What is the formation of ripples?

Answer 7

Bump in the bed, flow squeezed and accelerates over crest of bump, lag between crest and deposition moves bump forward
Gravity/disturbance at water surface limits growth

Question 8

What is the formation of cusps?

Answer 8

Initial bump in beach face, swash slows at deposits at high (horn), accelerates and erodes at low (bay).
Will compete until no gradient in velocity (sediment flux)

Question 9

How are patches formed?

Answer 9

Sharply defined regions of sand and gravel that can occur at large scales - NOT formed by cross-shore processes

Question 10

What is imbrication?

Answer 10

On cusps, they will follow flow patterns - used in sedimentology as paleo-flow (direction) indicators