Transcript L18_CoastalForcing

Coastal Forcing
Coasts are the ribbons that form the interface between regions dominated by
terrestrial and marine surface processes, respectively, and are the most dynamic
(rapidly changing) of the Earth's landscapes
Shaped greatly by storm wind energy transferred to waves
Wave energy is dissipated and converted into work that involves the movement of
sediment and/or the erosion of rock.
The coast is the bottom boundary condition for the terrestrial landscape, which
influences river gradients and cliff heights.
Oceans are the "final" sink for terrestrial sediment and water.
Relative Sea Level Rise
Fundamental variable that
determines whether a coast is
sandy or rocky.
RSL=SL-R
Both eustatic sea level (SL) and
vertical crust motion (R) can be
positive or negative, and if their
particular combination makes
RSL<0, crustal rock is exposed and
can be attacked by waves in the
nearshore.
If RSL>0, a veneer (often times
many meters thick) of sediment
blankets the bedrock and allows
sedimentary landforms to be
shaped by wave-generated
nearshore currents.
Sea Level during
the Pleistocene
Over the past few million
years, the major cause of
eustatic sea level change
is the waxing and waning
of global ice volume.
The shape of the ocean
basins makes it possible
to use the following
relationship between sea
level and ocean volume:
Oxygen isotope data from benthic forams in marine
sediment cores provide the bulk of the Pleistocene
record, and this has been calibrated with oxygen
isotopes of air bubbles trapped in ice cores, whose ice
has been deposited over the Last Glacial Maximum
(LGM).
Sea Level during the
Holocene
Fast growing coral that live
within 5 m of sea level on stable
shelves provide a record of sea
level history since the LGM.
They can be dated with U/Th
methods to provide the time
information.
Sea level rise accelerated from
the LGM to about 14ka, when a
dramatic melting event (MWP
1a) occurred. During that time,
SL rose 25 meters in 500 years.
Calculation of MWP 1a
discharge = approx. 6 Amazon
Rivers being turned on for that
interval.
Sea Level Rise During the Last Century
A: Effect from glacial melt:
Changes in global ice volume, i.e. glacier sizes, are extremely difficult to quantify.
Interestingly, it appears that small Alaskan glaciers constitute the bulk of global
small glacier contribution to sea level rise.
B: Effect from thermal expansion of water (steric effect):
some caveats here:
1. The coefficient of thermal expansion, a, is temperature dependent.
2. The thickness of the column of warmed water, H, is only the well-mixed layer,
whose thickness varies with latitude.
3. The temperature of the well-mixed layer, upon which the coefficient of thermal
expansion depends, also has a latitudinal variability.
Markers of Rock Uplift
Rocky coast markers Marine Terraces - very useful,
because they are known to
originally be horizontal. Can
be dated to reveal long term
uplift rates.
Markers of Rock Uplift
Wave Anatomy
Wave Energy Density Across Frequency
Wave Refraction
Longshore Current
Storm Surge