the channel or within ebb tidal jet flow, together with bypassing bars on each side
with volumes dependant on the balance of left-directed and right-directed
longshore transport (Kraus 2000). At wave-dominated inlets, the longshore
transport of sediment occurs by exchange of sediment among these morphologic
components of the ebb shoal system. Inspection of the morphology at Goldsmith
Inlet indicates that sediment bypasses the mouth under wave action, but without
apparent ebb shoal or bypass bar to the east.
A third type of bypassing, episodic bypassing, occurs when large portions of
sediment detach from the ebb shoal complex and migrate to shore (FitzGerald
1988; Gaudiano and Kana 2000). Episodic bypassing is not applicable to
Mattituck Inlet and Goldsmith Inlet, because these inlets do not have ebb-tidal
shoals or significant bypassing bars.
Bruun and Gerritsen (1959, 1960) first identified the mechanisms of
bypassing by littoral transport across the outer bar (ebb shoal complex) and tidal
bypassing. They defined a ratio r as:
P
r=
(6-1)
Qg
where P.= tidal prism, and Qg = gross or total longshore sediment transport rate
in 1 year (giving a volume). The tidal prism in this relation corresponds to spring
tide, when the strongest current scours the inlet channel. The parameter r
expresses the relative strength of the tidal flow that acts to sweep the inlet clear
of sediment brought into its entrance by wave-generated longshore sediment
transport. Based on the observations of Bruun and Gerritsen (1959, 1960), the r
ratio allows for classification of inlet channel stability and mode of sand
bypassing. Inlet characteristics associated with different r ratio ranges are
summarized in Table 6-1. Inlets with low r-values have channels that are
unstable experience bar bypassing (wave-dominated inlets), whereas inlets with
large r-values tend to have stable channels and experience tidal bypassing (tide-
dominated inlets).
Table 6-1
Inlet Bypassing and Channel Cross-Sectional Stability
Classification of Inlets (Modified from Bruun and Gerritsen (1959,
1960) and Including Additional Information)
r-value
Channel Stability
Dominant Bypassing Mode
Bar bypassing
r < 20
Unstable. Inlet may be closed by
deposition of sediment during a
storm. Typically no navigable
channel.
Bar bypassing; may have several bars
r = 20-50
Highly variable in location and area,
with multiple channels possible.
Dredging and jetties typically required
to maintain navigable depths.
r = 50-150
Clear main channel and well-
Bar bypassing and tidal bypassing
developed ebb shoal.
r > 150
Reasonably stable channel.
Episodic bypassing, tidal bypassing
267
Chapter 6 Inlet Morphology and Stability
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