r&b: beach processes and coastal environment

beach

land that extends from mean lower low water to dunes/bluffs/cliffs

shoreline

where the land and ocean meet

spit

linear extension of land; forms from an accumulation of sediment and is maintained by tides

barrier

a spit or island seaward from land; usually is parallel to land

bars/troughs

seabed features in surf zone

berm

relatively flat region of beach behind shoreline

foreshore

seaward sloping; area seaward of berm

backshore

berm and dunes

flood-tide delta/washover delta

features formed when beach sediment is transported landward due to tides/storms respectively

longshore drift/transport

movement of water and sediment parallel to beach

MSL

mean sea level

winter berms

higher up a beach relative to summer berms due to more active tides and storms

summer berms

lower down a beach relative to winter berms due to calmer tides and less storms

swash

upward movement of waves when they break

why is swash more capable of moving sediment than backwash?

some of the water goes into pore spaces rather than returning to ocean

backwash

downward movement of water after waves have broken

what is the single most important physical process on beaches?

surface waves

what snuffs out surface waves?

gravity

what propagates surface waves?

wind stress

wavelength

the distance from the crest of one wave to another (or other analogous point)

wave height

distance between the trough and crest of a wave

still water level

mean between trough and crest heights; approximately where water would sit if it wasn't disturbed

crest

the highest point of a wave

trough

the lowest point of a wave

how does water move in waves?

in an orbital fashion

wave period

amount of time it takes for two adjacent wave crests to cross the same point; proportional to wave length

in deep water, wave speed increases with...

wavelength and period

as waves move away from the source, they...

sort themselves out; larger waves move ahead of smaller waves

swell

waves with large wavelengths that reach beaches first

the energy of a wave is proportional to

the wave height

as you move down to wave base, sediment transport capacity...

lowers

as waves move across sea, they become...

muted due to gravity

once a wave forms, it will keep going until...

it breaks on a beach

fetch

distance a wind blows

bigger waves are the result of

strong winds, longer duration of winds, bigger fetches

sea condition waves

very jumbled structure (not organized with swell ahead of smaller waves); confused waves

post-generation wave conditions

waves are now organized; single wave shape characteristic

surf

where waves break on shore; pointed wave crest characteristic

in deeper water, how do things move in waves?

in a more circular orbit; influence decreases with depth

wave base

1/2 wavelength

in shallow water, how do things move in waves?

in a more elliptical orbit; as you go deeper, orbit gets thinner until it's just moving back and forth in a line

transition zone

occurs when waves are 10-50 m of shore (wherever water depth is less than or equal to 1/2 of wave base)

what happens to the seabed at the transition zone?

gets eroded; mud in suspension, sand as bedload

as depth decreases, what happens to wave speed and wavelength?

decreases as well (due to friction of bed); leads to wave refraction (change in wave direction)

as depth decreases, what happens to wave height?

increases; crest increases until height-length ratio is greater than 1/7, in which case wave breaks

what determines break shape of a wave?

gradient of seabed and grain size

spilling breakers

occur at nearly horizontal beaches with fine sediment

plunging breakers

occur at steeper beaches with sandier sediment

surging breakers

occur at steepest beaches with biggest grain sizes

water infiltration

when water seeps into permeable beach sediment; again, this reduces sediment transport capacity of backwash

how does water infiltration affect beach gradient?

coarser sediments have more pore spaces between them allowing more water to seep in; thus further reduces backwash's transport capacity; thus bigger ratio between landward and seaward transportation (landward is much bigger); thus beaches that have coarse grain sizes are steeper; this also contributes to formation of berms

what controls berm elevation?

wave energy and tidal range

rip currents (riptides)

method for water to return to sea after breaking on beach; moves in parallel direction to longshore transport until washing out to rip head (location of this depends on gradient and wave morphology)

undertow

a weak, non-dangerous riptide

wave refraction

bending of the wave crest

swash can move up at an angle due to refraction. what about backwash?

no, it stays perpendicular because it is driven by gravity

wave ray

imaginary line that moves perpendicular to wave crest to allow us to see movement of energy

when waves approach a straight shoreline, what happens?

wave refraction due to lowering of speed

when waves approach an irregular shoreline, what happens?

shallow water of headlands gets hit with most energy, backlands not hit with as much energy; leads to erosion of headlands and sediment deposition at backlands

what causes longshore transport?

wave refraction of swash (angled) and perpendicular direction of backwash

summer beach characteristics

tall dunes and wide berm

winter beach characteristics

dune and berm erosion, washover, and offshore bar/higher berm formation

to put it simply, the beach grows seaward when?

in the summer

to put it simply, the beach recedes landward when?

in the winter

storm surge

occurs in winter; when strong wind and waves push against shore with high energy, eroding it

what happens to the sediment eroded during storm surge?

builds offshore bars/winter berms; washes through dunes during washover and forms washover deposits

what happens to the sediment stored offshore during the summer?

gets pushed back back up to shore; forms summer berm

longshore bar

deposit of sediment just off the shore that has been eroded during storm surge

why are spits and barrier islands common on trailing edge margins?

gentle gradients and little tectonic uplift/subsidence

can spits/barrier islands appear on collision margins?

yes, only if the sediment load is abundant

where are these coastal barriers migrating today?

landward due to sea level rise (however, some places see seaward migration due to being right next to river mouths)

how do spits form?

longshore transport; inlets maintained by tides

barrier island characteristics

tidal inlets are both ends; elevation depends on wind/sand supply

what determines length of barrier islands?

tidal prism (vertical amount of water flowing in and out during tides)

large prism

short barrier island

small prism

long barrier island

can barrier islands be covered by washover?

yes, if they are low enough

two kinds of coastal barriers

spits and barrier islands

what are the three ways a barrier island forms?

spit elongation, bar submergence, and bar emergence

how are spits built?

built from tip out seaward

spit elongation

when a spit becomes too long for water to efficiently transport across it, an new inlet will form, making the spit a barrier island

bar submergence

when an old dune or other topographic high is surrounded by water as sea-level rises

bar emergence

when a longshore bar is formed and then rises out of sea when storm surge subsides

what is the time interval for tide fluctuations?

24 hours, 50 minutes

diurnal tide

one high and one low each day

semidiurnal tide

two nearly-equal highs and two nearly-equal lows each day (most common type of tide)

mixed semidiurnal tide

two unequal highs and two unequal lows each day

when are currents strongest in regards to tides?

during period of greatest water level change

slack water

period of least movement of water; occurs during max high and max low

what are the two causes of tides?

gravitational attraction of the sun and moon and centrifugal force around earth

tidal range

the vertical difference between max high and max low during SRPING TIDE; can be cm or m

ebb tide

when water level falls

flood tide

when water level rises

how often do the sun and moon cycle to affect the tides?

every 28 days or so

spring tides

larger tidal range

neap tides

smaller tidal range

every 28 days, how many neap and spring tides are there?

2 neap tides and 2 spring tides

when do neap tides occur?

during first quarter and third quarter moons (moon and sun are working against each other)

when do spring tides occur?

during new and full moons (moon and sun work together)

can local geometry differences influence tidal range?

yes; can increase or decrease depending on how much flow the seabed allows