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Accretional (Depositional) Coastal Features Netarts Spit, Oregon Spits Tied to the coast at one end...
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Transcript of Accretional (Depositional) Coastal Features Netarts Spit, Oregon Spits Tied to the coast at one end...
![Page 1: Accretional (Depositional) Coastal Features Netarts Spit, Oregon Spits Tied to the coast at one end Fundamental in smoothing an irregular coastline Often.](https://reader036.fdocuments.us/reader036/viewer/2022070414/5697c0011a28abf838cc22a0/html5/thumbnails/1.jpg)
Accretional (Depositional) Coastal Features
Netarts Spit, Oregon
Spits
Tied to the coast at one end
Fundamental in smoothing an irregular coastline
Often exhibit a “recurved” hook at the downdrift end
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Examples of Spit Growth
Older hooks reflect paleo- spit ends
Geomorphic history of Fire Island, NY - from aerial photography - growth in direction of L.S.T.
Spit growth can deflect the course of a river
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Google Earth Exercise - Go Spit Hunting
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Tombolos Occur where spit growth extends to an offshore island
Island often influences wave field - refraction
Italian term, where they are numerous and well-developed
see Figure 2.16 in Komar
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Cuspate Forelands / Capes
World class example: Outer Banks, NC
Mechanism of formation is debated:
1. Form at points of LST convergence from 2 directions?
2. May have a geologic control?
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Barrier Islands
Occur along 13 % of the world’s shorlines
Prevalent in areas of small tidal range = 1-2 meters
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Blackboard Example: barrier island migration
set up slope of substrate, rate of sea level rise, retreat rate of barrier island
washover mechanics
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Washover of Barrier Islands
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Regressive vs. Transgressive Sequences
Rooted in Walther’s Law
Regressive Sequence - Landward sediments are deposited on top of seaward seds, reflect a seaward shift of shoreline.
Transgressive Sequence - Seaward sediments are deposited on top of landward seds, reflect a landward shift of shoreline.
Not necessarily diagnostic of SL rise or fall - think about sediment supply.
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Theories of Barrier Island Formation
Hoyt (1967) - sand dunes drowned in place by relative SL rise
de Beaumont (1845) - cross-shore sed. transport, from an excess, “traps” a lagoon.
Gilbert (1885) - Longshore sediment transport downdrift of a headland, or inlet, forms spits, that are occasionally breached.
(aside on inlets, tidal prisms, and ebb tidal deltas)
3 “models” not mutually exclusive - D.J.P. Swift (1968)
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Inlet Dynamics: Spit Growth to Equilibrium Length
Thought to be controlled, in part, by tidal prism:
1.Within each tidal cycle, a “prism” of water must be moved into, and out of, the bay.
1.Spit extends freely until tidal currents, through narrowing inlet, disallow longshore transport - tidal currents generate shear stress sufficient to block longshore sediment transport.
2.Sediment “sinks” are the flood tidal and ebb tidal shoals (deltas)
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Inlet SpacingRelated to sediment supply / distance from source
Near the sediment source (eroding headland, river mouth, etc.), inlet spacing is wide.
Flood and ebb-tidal shoals are a net sink for sediment, LST rate decreases downdrift.
At the distal end of the barrier island system (relative to source), inlet spacing becomes close, due to decreased LST rate
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Beach Ridge Sets
Ubiquitous Landforms, without a clear explanation of their formation, preservation, and evolution.
One clear requirement – sediment supply.
Rink & Forrest (2005) combined OSL dating with Brooks (1972) RC dates to establish land accretion rate from beach ridges to be ~135 m/century
Examine numerous beach ridge sets through google earth.
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Beach Ridge Generation by Changes in Wave Climate & Sediment Supply
St. Vincent Island, Apalachicola Delta, FL