Inlet Operation
The bay is separated from the ocean by two large sand spits with an inlet to
the ocean protected by two massive jetties. The jetties, as shown in Figure 2, are
approximately 6,000 ft long and 2,200 ft apart. Dredged navigation channels
have been constructed across the outer bar, through the Entrance Channel, and
into the Fields Landing Channel to the south and Samoa Channel and Eureka
Channel to the north. The jetties have fixed the Entrance Channel orientation and
stabilized the channel. A large shoal occurs on the north side of the Entrance
Channel near the north jetty. The navigation channel is located to the south side,
near the south jetty. This configuration of channel and shoal must be related to
the horizontal circulation patterns associated with the ebb and flood currents and
their redirection to and from the basins to the north and south of Entrance Bay.
Estimates of inlet stability indicate that the throat of the inlet is probably
deeper than its equilibrium depth, particularly following the recent (1999)
deepening project to 45 ft. Currents in the entrance are strong, with average
maximum flood of 1.6 knots and average maximum ebb at 2.0 knots. Maximum
flood and ebb currents can reach 2.8 and 3.5 knots, respectively, on extreme tidal
ranges. The strong ebb currents often steepen the waves so that vessels cannot
safely traverse the harbor Entrance Channel during ebb tide.
Adjacent Beaches
The Humboldt Bay entrance is located at the approximate midpoint of a
40-mile-long littoral cell, an anomalously long cell for the northern California
coastline (Figure 3). The cell stretches from the south at False Cape, directly
north of Cape Mendocino, north to Trinidad Head. The two major rivers within
the cell are the Mad River, located about 14 miles north of the entrance to
Humboldt Bay, and the Eel River, located about 7 miles to the south. The Eel
River is the major source of sand to the cell. The large sand dunes at the north
end of the bay, Lamphere Dunes, appear to be the major sink.
Several previous estimates have described the net alongshore transport in the
Humboldt Bay littoral cell as being from north to south. This is consistent with
the observation of a seaward shoreline offset at the north jetty compared to the
landward offset at the south jetty. Although the apparent offset across the
stabilized entrance (jetties) is consistent with a north to south transport path, the
source of available sand material does not readily support this conclusion. The
primary source of sand material in the cell is from the Eel River, located near the
south end of the cell. A distinct seasonal variation in the alongshore transport
reconciles the apparent inconsistencies in observations. This seasonal shift in the
nature of the transport is consistent with the wave climate previously described.
The net summer transport is to the south, and the surf zone is not sufficiently
wide to move sand past the end of the north jetty; thus, an accumulation and the
observed offset is formed. During winter storms, however, the surf zone can
extend well beyond the jetties and provides a wide northward transport path that
can bypass sand readily from south to north. Winter storms involve intense wave
action, with nearshore profiles changing as much as 20 ft or more vertically
overnight. Therefore, there is little opportunity for accumulation adjacent to the
south jetty in winter storms, and the observed offset persists throughout the year.
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Chapter 2 Description of Study Area
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