ERDC/CHL CHETN-IV-33
June 2001
Unrectified aerial photos of different scales were analyzed, and individual photograph scales
were determined through comparison of distance between two stationary objects, such as jetties,
to that same distance found on nautical charts. Uncertainties introduced for the distance
measurements are estimated to be 25 to 150 m for the inlets examined, depending on the scale
and on the distortion and parallax on the aerial photograph.
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Representative Ebb Shoal Outlines
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8
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4
2
0
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-12
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Normalized Distance Alongshore
Figure 3. Digitized planform ebb shoal outlines (distances
normalized by channel critical width)
ASYMMETRIES IN EBB-TIDAL SHOALS: Data from the ebb shoals are plotted in Figure 3
to fall into groups according to distance offshore, distance alongshore, and symmetry or
asymmetry. The shoal outlines lying close to the origin exhibit symmetry about the y-axis and
are similar in alongshore extent. A band of medium outlines is located between the smallest and
largest ebb shoal outlines. These show larger normalized distance offshore and alongshore than
the smaller outlines as well as exhibiting a greater variation of symmetry. The largest outlines
have the greatest asymmetry. These inlet outlines show that the greater the distance offshore the
greater the distance to the attachment bars.
Asymmetry in ebb tidal shoals occurs at inlets with a clear direction of net sediment transport.
The ebb shoal profile at Old Topsail Inlet, NC, can be identified in Figure 3 as the blue profile
that extends the furthest offshore. The outline indicates a great ebb shoal asymmetry in
comparison to the other outlines in Figure 3. At Old Topsail Inlet, net longshore sediment
transport is dominant to the south, which has given rise to the asymmetry of the shoal and a spit
that has constricted the tidal channel, leading to a further asymmetrical tidal channel. Spit
growth from the updrift side of the inlet, submerged at high tide, has constricted the channel,
causing a stronger ebb jet to transport sediment further offshore. This spit growth pattern has
contributed to the asymmetrical development of the ebb shoal complex. FitzGerald, Kraus, and
Hands (2001) describe the migration process for this type of channel and asymmetrical shoal
development in conceptual models of inlet migration and spit breaching.
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