Nearshore waves, currents and rips

Question 1

what is wave refraction?

Answer 1

the bending of waves as they move from deep to shallow water .

Question 2

why do waves refract?

Answer 2

shallower water slows part of the wave while the deeper part keeps moving causing the wave to bend

Question 3

crests in bays vs headlandsn(energy)

Answer 3

crest - stretched (energy more dispersed), headlands - compressed (energy focused)

Question 4

refraction over uneven topography (headlands?)

Answer 4

causes H to vary longshore, so coastal wave energy varies longshore and currents are created..

Question 5

wave diffraction

Answer 5

process of wave energy along the wave crest rather than direction of propagation irrespective of water depth

Question 6

what does diffraction do to wave energy

Answer 6

reduces and spreads out wave energy into sheltered zones (around obstacles/ barrier)

Question 7

diffraction vs refraction

Answer 7

R - bends waves due to depth changes, D - bends/spreads waves due to obstacles/openings. different processes but can operate alongside.

Question 8

reflective beach

Answer 8

narrow surf zone, steep beach slope, plunging to surging breakers waves, one line of breakers, no energy dissipated due to steep foreshore

Question 9

dissipative beach

Answer 9

wide surf zone, flat beach slope, breaking points (troughs) in beach profile causing waves to break before reach shore - wave energy dissipated, spilling breakers to swash bore.

Question 10

spilling breaker wave

Answer 10

gentle beach gradients, steep waves, gradual peaking of crest till instability and crest spills forward. mostly likely to accrete beach.

Question 11

surging breaker type

Answer 11

steep beach gradient, low steepness wave, front face and crest remain smooth and intact, wave doesn’t break just slides directly up beach. longterm erosion (doesnt touch seabed to transport sediment on - only erodes)

Question 12

plunging

Answer 12

steeper beach gradient, intermediate wave steepness, shoreward face of wave becomes vertical, curls over and plunges forward as intact mass of water. suspends and scarps the most sediment on beach.

Question 13

breaker types

Answer 13

spilling, plunging, surging

Question 14

wave reflection

Answer 14

instead of breaking, waves reflected at shore - energy back out to ocean. (e.g surging wave on steep beaches with sea wall/cliff)

Question 15

infra-gravity waves (storm conditions)

Answer 15

long period wave ride on top of normal wind or swell waves. (common in storms), cause elevated water levels, strong swash, increase erosion. dont break in surf zone

Question 16

backwash

Answer 16

offshore accelerating flow of water driven by gravity

Question 16

swash

Answer 16

onshore declerating flow of water driven by wave energy

Question 17

run up

Answer 17

overall vertical movement of water line on beach influenced by initial sea level and dynamic forces of waves (increases significantly in storms - high energy conditions)

Question 18

what drives coastal landform change in nearshore zone?

Answer 18

sediment transport, driven by nearshore currents.

Question 19

wave-generated nearshore current types

Answer 19

longshore, undertow, rip

Question 20

effects of nearshore currents

Answer 20

transport sediment, transport water, forms topographic features like spits, bars, channels.

Question 21

longshore currents

Answer 21

transfer sed and energy, max strength ~1m/s. stronger in shallow water by beach

Question 22

undertow / bed return flow

Answer 22

movement of sed offshore. max ~0.1 to 0.3m/s. mid surf zone. breaking waves drive water onshore but underneath some already hit shore bed so moving backwards - returning water offshore

Question 23

how rips formed?

Answer 23

form from wave interaction with surf zone bathymetry and barriers. variation in breaker height from differences in wave set up, (caused by refraction around bath/barriers) or infragravity waves. excess water flows back offshore through low spots forming rip.

Question 24

what is a rip current?

Answer 24

narrow, fast-moving current flows from shore out to sea through surf zone. moves sed, people and water offshore through deep channels.

Question 25

where do rips form?

Answer 25

in low spots in sandbars or channels where the excess water escapes back to sea.

Question 26

features associated with rip currents

Answer 26

troughs, bars, rip channels, and beach cusps - formed through wave action and rip currents. (rip removes sediment, longshore deposits sed on cusp)

Question 27

rips more common on what type of beach? and how to spot

Answer 27

dissipative beach (flat), spot in narrow gaps in breaking water, calmer looking water or discoloured water with moving sediment.