ERDC/CHL CHETN-IV-33
June 2001
confounding processes are changes in shoreline orientation, which change the direction of
transport locally; impoundment in the downdrift shadow region of a jetty, which may make the
downdrift side appear as an updrift side; seasonal sediment drift reversals (Oertel 1975); and
changes in the back bay that might realign the channel. Stauble and Morang (1992) give
additional information on determining net drift in complex systems.
SAND BYPASSING PROCESS: Sediment bypassing paths control much of the geomorphic
asymmetry at inlets. The natural mechanism of sediment bypassing from the updrift shoreline to
the downdrift shoreline through the ebb shoal complex is significant because it mitigates possible
erosion downdrift of the inlet. Bruun and Gerritsen (1959, 1960) described sand bypassing at
inlets and classified the ease of navigability through the prominence of an entrance bar (ebb
shoal complex) in the channel. The parameter introduced for this classification r is defined as
r = P / M tot
(1)
where P is the tidal prism (amount of water passing through the inlet during half a tidal cycle,
typically during spring tide), and Mtot is the average annual longshore transport at the inlet.
Therefore, Mtot is equivalent to the gross longshore transport rate at the inlet multiplied by
1 year. Inlets with a value of r > 150 (approximate) tend to have stable, deep channels and are
poor "bar bypassers" from updrift to downdrift, whereas inlets with r < 50 (approximate) tend
toward closure and are good bar bypassers.
Kraus (2000) quantified the growth of the ebb shoal and sediment bypassing rates from the
updrift shoreline to the downdrift shoreline, assuming continuous transport. The model also
predicts the delay in sediment transfer from the updrift shoreline to the downdrift attachment bar
and is compatible with the concepts of Bruun and Gerritsen (1959, 1960). Guidance on the
symmetry of the inlet ebb shoal complex involved in the bypassing is provided in the following
sections.
The processes envisioned in the preceding formulation are considered as being primarily
continuous. Gaudiano and Kana (2001) found that local sediment accretion on South Carolina
shorelines is also associated with event-based bypassing in which a portion of the ebb shoal
breaks off and moves on to the shore. Although episodic in nature, the bypassing occurred over
a sufficiently long time interval, i.e., over a long averaging interval, that this bypassing might be
modeled as a semicontinuous process. Temporal changes in inlet asymmetry are discussed in the
following paragraphs.
IDENTIFICATION OF ASYMMETRIES: The outlines of ebb shoals in Figure 3 were
digitized from the wave-breaking patterns observed in aerial photographs and plotted to
determine representative shapes these shoals may take. The shoreline served as the baseline for
the x-axis, and the inlet center line (Figure 1) served as the y-axis with positive values occurring
downdrift of the origin and offshore. In the figure, positive x values (to the right of the origin)
indicate normalized distance downdrift, and negative x values (to the left of the origin) indicate
normalized distance updrift. Alignment of the digitized ebb shoal outlines to a common origin
allows comparison of the offshore and alongshore extents of the shoal. The measured offshore
and alongshore distances were normalized by their respective minimum or critical width of the
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