ERDC/CHL CHETN-IV-33
June 2001
jetties, and slope of the nearshore shelf in great part determine the offshore extent of the ebb
shoal. Deposition of sediment carried by the ebb current into the ebb shoal is enhanced through
refraction of the waves around the ebb shoal complex, tending to generate a longshore current
directed toward the inlet on both sides. The ebb shoal shelters the area behind it from waves,
creating a zone of low wave energy where sediment can deposit (Dean and Walton 1973), so that
formation of an ebb shoal creates a self-preserving mechanism. Hubbard, Oertel, and Nummedal
(1979) found that an ebb shoal developed at wave-dominated inlets lies closer to the inlet
opening than the ebb shoal at a tide-dominated inlet. Ebb shoals formed on low-wave energy or
tide-dominated coasts are longer and narrower with a more defined ebb channel and terminal
lobe (Hayter et al. 1988).
The distance from the shoreline to the furthest seaward extent of the ebb shoal was determined
through examination of the breaking wave pattern in aerial photographs. On nautical charts, the
furthest seaward point of the ebb shoal was identified through bathymetric contours. The
distance from the shoreline to the furthest seaward extent of the ebb shoal increases with
increasing tidal prism (Figure 7). Although a clear visual trend exists, inlets with ebb-shoal
offshore distances less than 2 km are found under a wide range of tidal prisms. Table 3 lists the
coefficients of the trend lines in Figure 7 as given by Equation 2. In Equation 2, WA2 was
replaced with the variable L defined pictorially in Figure 2.
Static factors controlling the asymmetry of the ebb shoal include the length and condition of the
jetties. At a mature inlet with large longshore sediment transport, it is expected that the greater
the distance the jetties extend offshore, the greater distance to the offshore terminus of the ebb
shoal. Consequently, the more seaward ebb shoal will produce a greater distance to the
downdrift and updrift attachment bars. However, it is feasible that in some situations the jetties
are sufficiently long and, possible, relatively closely spaced such that the resultant ebb shoal can
never form bypassing bars; material comprising the shoal is jetted so far seaward that wave
action cannot return it under typical wave conditions. This is the situation at Grays Harbor, WA.
Seaward migration of the ebb shoal alters the amount and location of sediment bypassing.
The condition of the jetties plays a role in determining asymmetry inlet morphology. If the
jetties are permeable or low, sediment can enter the entrance channel. At impermeable jetties,
the sediment accumulates on the updrift side of the structure until it can move around the tip of
the jetty (jetty becomes fully impounded). An impermeable jetty will be a more effective sand
by-passer to the downdrift shoreline because more of the sediment will be transferred from the
updrift shoreline through the ebb shoal and ultimately deposit on the downdrift shoreline.
The relationships developed in this Technical Note for estimation of asymmetry indicators may
assist in preliminary study for the design and maintenance of inlet navigational channels.
Equation 2 may be employed, as part of an evaluation plan, if the addition or modification of
jetties is being considered. In addition, these observations may be beneficial in estimating
required bypassing and optimal location for placement of dredged material on the downdrift
beach.
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