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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 64 (1980)

Issue: 9. (September)

First Page: 1565

Last Page: 1565

Title: Nearshore Bars Along United States Gulf Coast: ABSTRACT

Author(s): Dag Nummedal

Article Type: Meeting abstract


The microtidal, low-wave energy, United States coast of the Gulf of Mexico is characterized by an abundance of shoreface bars. Four distinct component bars can be identified, though interference patterns exist. Vertical air photos since 1945 and numerous overflights and field measurements during the last two years have shown that the equilibrium bar configuration at any given site is time-invariant. During storms, however, the bars go through cycles of change in a manner similar to those identified by Short.

Based on plan geometry the following four bar types are identified.

Multiple (10 or more) longshore bars:
Multiple longshore bars, without interference from other bar types, occur on the low-tide platform in front of a steep beach face, along shoreline segments of convex plan form and low wave energy. The bars have a sinusoidal cross-profile, constant height, and constant spacing suggesting that they form in response to a standing wave pattern established through interference between incident wind waves and waves reflected from the steep beach face.

Transverse bars:
Bars oriented at steep angles relative to the shoreline dominate in low energy environments (Mississippi Sound and other sheltered embayments) along the central Gulf coast. Intersecting transverse bars, forming an overall rhombic pattern, dominate the lagoonal platform behind the Mississippi-Alabama barrier chain. The mainland shoreface of Mississippi Sound, however, is dominated by intersecting transverse and multiple longshore bars.

Generally, transverse bar spacing appears to be proportional to bar amplitude. The transverse bar morphology reflects a complex interaction between incident, high-frequency waves and the resultant longshore currents.

Crescentic bars:
The Gulf beaches of barriers and spits of the Florida panhandle, Alabama, and Mississippi are characterized by crescentic bars or single longshore bars. The crescentic form is best developed adjacent to, or between, shore-normal structures which might be jetties, groins, or natural headlands. This suggests that the crescentic bar morphology forms in response to a wave motion transverse to that of the incident waves, i.e., edge waves which are expectedly best developed trapped between reflecting shore-normal structures.

Longshore bars:
These bars characterize the entire Texas coast and many Louisiana barriers. This coastline has the highest wave energy in the study area. The constant crest-to-trough water depth ratio, the asymmetric cross-profile, and the landward decrease in size and spacing suggest that these are breakpoint bars.

These four component bars, all resulting from different mechanisms of generation, occur either individually or in different patterns of superposition creating six commonly observable bar morphologies along the Gulf Coast barriers and mainland shores. Specific coastal subenvironments are characterized by time-invariant equilibrium bar configurations.

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