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

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

Google Earth Exercise - Go Spit Hunting

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

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?

Barrier Islands

Occur along 13 % of the world’s shorlines

Prevalent in areas of small tidal range = 1-2 meters

Blackboard Example: barrier island migration

set up slope of substrate, rate of sea level rise, retreat rate of barrier island

washover mechanics

Washover of Barrier Islands

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.

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)

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)

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

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.

Beach Ridge Generation by Changes in Wave Climate & Sediment Supply

St. Vincent Island, Apalachicola Delta, FL