P1254156765bfekT - PowerPoint PPT Presentation

Title: P1254156765bfekT


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Tidal Inlet Formation Barrier Island Breaching
Shinnecock Inlet, 1938
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Tidal Inlet Formation Barrier Island Breaching
Pikes Inlet, 1992
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Beach Morphology and Sediments
Beach Morphology reflects the composition of its
sediments and physical process of waves,
currents, and sediment transport The Beach is
usually defined as an accumulation of
unconsolidated sediment (sand, gravel, cobbles,
and boulders) extending from the mean low-tide
line to some physiographic change such as a sea
cliff or dune field or the point where permanent
vegetation is established. Shoreline the line
of demarcation between the water and the exposed
beach.
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Terminology used for Waves and Currents
Nearshore Zone region extending seaward from the
shoreline to just beyond the point where waves
break. Offshore relatively flat region beyond
the breaker zone extending out to the edge of the
continental shelf. Breaker Zone the nearshore
region in which waves arriving (propagating) from
the offshore become unstable and break. Depending
on incident wave conditions and bottom slope this
region could appear more like a line. .
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Terminology used for Waves and Currents
Surf Zone the nearshore region in which
bore-like waves occur following wave breaking.
This region extends from the breaker zone to the
swash zone. Swash Zone the portion of the shore
or beach face alternately covered by the run-up
of the wave swash and exposed by the backwash.
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Terminology used for Nearshore Morphology
Littoral Zone term used to denote the zone
extending across the exposed beach into the water
to a depth at which the sediment is less actively
transported by surface waves, (depth of closure,
10-20m). Backshore The zone of the beach profile
extending landward from the sloping foreshore to
the point of development of vegetation or change
in physiography (dune field, cliff etc.). Berm
the nearly horizontal portion of the beach or
backshore formed by the deposition of sediments
by waves. Some beaches have more than one berm at
slightly different levels separated by a scarp.
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Terminology used for Nearshore Morphology
Scarp A nearly vertical escarpment cut into the
beach profile by wave erosion. Generally scarps
are less than a meter. Beach Face The sloping
nearly planar section of the beach profile below
the berm, which is normally exposed to the swash
of waves. Foreshore the sloping portion of the
beach profile lying between a berm crest and the
demarcation of the low-water backwash, extends
slightly seaward of the beach face region.
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Terminology used for Nearshore Morphology
Inshore The zone of the beach profile extending
seaward from the foreshore to just beyond the
breaker zone. Longshore Bar An underwater ridge
of sand running roughly parallel to the shore,
sometimes continuous over large distances, at
other time having roughly even breaks along its
length. It may become exposed during low tide,
often several longshore bars are present in
different water depths. During large storms waves
will break on the outer bars which are not
visible during normal conditions. Longshore
Trough elongated depression extending parallel
to the shoreline and any longshore bars that are
present.
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Dominant Factors Controlling Mean Grain Size on
Beaches
Sediment Source for New York, bluffs (glacial
till), shelf sands. Wave Energy wave energy
variations from Montauk to NY Harbor. General
Offshore Slope steep slopes (coarse material),
shallow slopes (fine material).
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General Relationships Between Settling Velocity
Sediment Removal
Bagnold 1963 Autosuspension grains of a certain
size will remain permanently in suspension for a
given condition and be transported away from the
beach (crosshore or longshore). Ws settling
or fall velocity u maximum horizontal orbital
velocity due to waves (related to H and
T) S beach slope (3o is a 1 on 20 slope or 0.05)
.
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General Relationships Between Settling Velocity
Sediment Removal
Dean 1973/1983 Relates transport to height of
suspension and wave period. h local water
depth percentage of local water depth Maximum
suspension is occurring in the most turbulent
regions of the beach breaker zone and surfzone.
Using relationships between the water depth and
wave height the equation can be modified to
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Example S 3o, 0.05 u 25 cm/sec Ws lt 1.25
cm/sec /h 10 cm T 10 sec Ws lt 1 cm/sec
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Grain Sorting on Beaches
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