Shorelines Flashcards
what are shorelines
dynamic environments
Erosional + depositional processes combine along coasts to create landscapes that undergo rapid change
Ancient shoreline deposits occur as rock bodies that are important oil + gas reservoirs
Wind-generated waves provide most of the energy that shapes + modifies shorelines
what is the processes of waves?
Energy in normal water waves (I.e. except tsunamis), is derived from the wind
Tops of waves (crests) are separated by wave troughs
Wavelength: horizontal distance btwn wave crests
Wave height: vertical distance btwn trough + crest
Wave period: the time it takes for 2 successive crests to pass a given point
how do waves move in deep water
In deep, open waters, water molecules move in a circular path during the passage of a wave
The circular motion rapidly diminishes w/ increasing water depth
At a depth equal to abt half the wavelength, the movement of water particles becomes negligible (this depth is called wave base)
how do waves move in shallow water
As waves approach a shoreline, their characteristics will change
Waves will “feel bottom” at a water depth equal to abt half their wavelength (wave base)
Friction w/ the seafloor slows down advancing waves at their bases (at the sea surface, crests are still moving quickly)
Wavelength decreases, wave heigh increases
Eventually, the waves collapse (“break”) in the surf zone
what happens after a wave breaks
After a wave breaks, water moves up the slope of a beach as a sheet (this is called “swash”)
The sheet of water slows, stops, then returns to the sea as “backwash”)
Backwash > swash = erosion (destructive)
Swash > backwash = deposition (constructive)
waves are powerful agents of what?
erosion
most sediment deposited along most shorelines are deliver to the sea by what?
streams
sediment can travel alongshore by ____
beach drift
Along most shorelines, waves approach the shore at an oblique angle
Swash carries suspended particles up the beach at an oblique angle
Water always refunds to the sea under the influence of gravity (as backwash) in a direction perpendicular to the shoreline
As a result, each sand particle on the beach moves in a zig-zag pattern down the length of the beach
what is a offshore drift
Seaward of a beach, oblique waves can also produce turbulent longshore currents
Longshore currents easily move fine suspended sand + roll larger + gravel along the bottom
If it were not for beach drift + longshore currents, redistributing sediment from river mouths, many beaches would be nearly sandless
what is a tide
Repetitive daily changes in elevation of ocean surface due to gravitational attraction btwn Earth + the Moon (to lesser extent, the Sun)
The Moon’s gravitational force produces a bulge in the ocean. Due to inertia, a matching bulge also develops on the opposite side
what are tidal cycles
Bc position of Moon changes very little in a single day, we can imagine the tidal bulges remaining in place while Earth rotates through them
The pattern is little more complicated bc of Earth’s tilt (tides tend to be unequal due to this)
Bulges migrate very slightly as the Moon orbits around the Earth
as tide rises, landward- moving water generates what?
flood currents
as tide falls, seaward-moving water generates waht?
ebb currents
where do wave-cut cliffs originate from
originate by the undercutting action of the waves against the base of coastal land
what are shorelines where deposition dominates over erosion
“Fair-weather wave base” (FWWB) is the maximum water depth affected by normal, day-to-day
“Storm wave base” (SWB) is the water depth affected by storm waves (deeper than FWWB)
what are the main “zones” from sea to land?
Offshore -> Shoreface -> Foreshore (beach) -> Backshore
where is the offshore zone
Below FWWB
Due to lack of wave agitation, mud tends to be deposited here
During storms, sand may be deposited (if above storm wave base)
where is the shoreface
Above “fair-weather wave base” but below low tide level
Concave-upward shape
Mostly sand (mud tends not to settle here as it is swept away by fairweather waves)
Sand bars form in upper shoreface due to breaking waves
Loss of wave energy -> local deposition of sand from suspension
where is the foreshore
Lower, “wet” part of “beach”
Btwn low + high tide levels (aka “intertidal zone”)
Exposed to air during low tide
Affected by swash + backwash (sand may or may not accumulate depending on which is dominant)
where is the backshore
Upper part of “beach”
Above high-tide level
Most sediment transported + deposited by wind (and is mobile due to lack of anchoring plants; commonly form aeolian dunes)
A storm beach face (marking the position of the beach during storms (affected by large waves)) on the seaward edge of the backshore
where beach drift + longshore currents are active, what depositional features may develop?
spit
baymouth bar
what is a spit
an elongated ridge of sand that projects from the land into the mouth of an adjacent bay
what is a baymouth bar
is applied to a sand bar (sometimes originating as a spit) that completely crosses a bay, sealing it off from the open ocean
what is barrier islands
Narrow buildups of sediment that are separated from the mainland by a lagoon
Can form during marine transgression
what is a tidal delta
Gaps in barrier island system are produced by tidal channels
Tidal deltas are lobes of sediment deposited by tidal currents as they move shoreward + seaward w/ the changing tides
Tidal deltas can also develop in baymouth bars
how to protect shorelines from erosional + depositional processes?
Construction of seawalls (protecting shoreline from breaking waves)
Breakwaters (built away from shoreline to reduce wave energy before waves reach the shore)
what are groins
A barrier built at a right angle to the beach to trap sand that is moving parallel to the shore (by longshore current)
Advantage: sand is caught on the upcurrent side
Disadvantage: Sand is eroded on the downcurrent side (+ an angry neighbour)
