Currents Flashcards
undertoe and rips
Bed Return Flow (aka Undertow)
- offshore directed mean flow near the bed
- Segregated circulation within the water column
- onshore flow in the upper part of the water column
- offshore flow in the lower part of the water column
Onshore flow due to Stokes drift and water thrown landward during breaking
undertoe:
the amount of water that is returned as undertow
=amount of water transferred landward by Stokes Drift and the breaking waves
Undertoe = Q stokes drift + Q breaking waves
Mean velocity
= undertow relative to the water depth:
Velocity = (Q sd + Q bw) / depth
Best developed on steeper beaches, which tend to have larger set-up gradients
Maximum bed return flows:
Planar beach- occur mid surf zone
Barred beach- at or slightly landward of the bar crest
rip dominated vs undertoe dominated
- variations in set-up alongshore
cause horizontally segregated flows which may dominate (Rip Currents)
alongshore variation
Larger waves break in deeper water
(set-up begins further seaward)
§ Set-up for the larger waves achieves higher elevations on the beach
§ Creates a longshore pressure gradient which will drive a longshore current from the large waves to the small waves
Pressure gradient develops between
areas of large set-up and small set-up
cell circulation
Wouldn’t the difference in set-down
between the low and high waves create a current that is opposite to the surf zone because there is a water surface slope from the small waves to the large waves?
§ NO: The larger wave heights will balance the difference in the set-down
§ if they did not remain large waves then there wouldn’t be a difference in the set-down
rip current speed
How does the speed vary with wave height and tide?
Rip current speed is related to the onshore discharge of water, the spacing of the rips and the area of the channel
instability model
positive feedback develops when waves and currents interact
§ Rip currents diminish the power of the incoming waves in that region
§ Negative feedback develops as the larger current starts to diminish too much energy-slows down
§ Distance between rip currents scales to the width of the surf zone
-the amount of kinetic energy at the shore
Rips tend to be evenly spaced
§ Spacing increases with wave height and decreasing beach gradient
§ Rips will attain largest spacing and size under the most dissipative conditions
§ If it grows any further there will be not enough energy to supply it
§ Spacing is probably related to the volume of water
longshore current and transport
wave generated current when waves approach coast at angle
alongshore ≅Qsd + Qb
at max at breakpoint
- dependent on wave height, angle approaching coast, and slope of beach
current stronger where nearshore is steep
(plunging, breaking happens in one spot, concentrated)
-spilling/dissipative conditions spread water over wider area
->moves sediment from source to sink
if alongshore current is starved from shoreline protections in one spot (breakwall, jetty, groin) there will be erosion further down
breakwalls dont stop alongshore current, so just cause erosion vertically rather than horizontally. this means it will likely get undercut and fail
alongshore variation in wave breaking
-> rip current develops
undertoe dominates when waves arrive perpendicular to shoreline
alongshore dominates if wave points arrive at an angle
breaking not uniform due to variation in bar morphology, rips develop at expense of undertoe and alongshore
if breaking uniform alongshore, only undertoe
high set up where breaking is, low set up at rip. water will flow from high to low rather than straight back as undertoe
bathymetrically controlled channel rips
due to variations in nearshore bar morphologies with rips developing in areas where bar crest is deeper
tends to occur when bars move landward after a storm
during a storm, large waves move bar offshore and make them uniform
after storm, small waves move bar back onshore through stokes drift
small variations in crest elevation lead to some areas moving faster than others
small variations get larger over time
wave breaking and set up intensify over shallower bar areas
leads to rip that carves out channel further
stokes gets stronger
rip reinforces rip (positive feedback loop)
rips help move bar landward -> important for storm recovery
as bar wells to beach, rips become less and less pronounced
bathymetrically controlled focus rips
caused by underwater canyons or ridges
deflection and shadow rips
rips develop adjacent to natural and anthropogenic structures
deflection rips -> alongshore current encounters groin or jetty
on other side of the jetty, shadow rips form
rips on both sides of structure
strong if only one long structure
weak if multiple closely spaced or if structure is short
if many structures, rips decreased with decreased spacing between and increased structure length
boundary rips
develop in embayments
waves approaching small embayments lead to rip currents in centre
as embayments increase in size, rips occur on either side
hydrodynamic rip
shear instability rip:
most difficult to see, predict, measure
if alongshore current meanders, rips develop alongshore where current moves offshore
flash rip:
when wave breaking not uniform alongshore
but variation is temporary
rips devolop and disappear in seconds
