predominates and seas from the southwest are the most severe. Incident wave
energy on the north coast is an order of magnitude more intense than found along
the Atlantic coast of the United States. This wave climate exerts a significant
control on the dynamics and behavior of the Humboldt Bay entrance.
Physical Oceanography
In the absence of significant freshwater inflow, the predominant driving
forces in Humboldt Bay are the tides. Waves are a significant driving force as
well, particularly near the entrance. Wind can be a significant secondary forcing,
particularly in the large shallow portions of the bay at high tide. Approximately
70 percent of the bay is tidal mud flat exposed at low-water elevations. The
water-surface area of the bay is about 25 square miles at high tide and 8 square
miles at low tide. There is little natural salt marsh remaining around the bay,
following a long history of diking, dredging, and filling.
Humboldt Bay tides have mixed semidiurnal variations, which typically
show some amplification and phase lag with distance from the entrance. The
mean range at the entrance is approximately 4.97 ft and the diurnal range is about
6.93 ft. Amplification is most pronounced to the north with an increase in range
of up to 1 ft. The tidal prism of the bay has been estimated by a number of
investigators, and appears to be about 3.4 to 3.5 109 cu ft on a spring tidal range
and about 70 percent of that value on a mean tidal range. Approximately
50 percent of the prism is contributed by North Bay (Arcata Bay) and nearly
30 percent by South Bay.
There is a distinct seasonal pattern in the winds. From March through
October, the winds are from the north to northwest sector almost exclusively.
Mean wind speeds during this period range from 5.5 to 8 mph. During the winter
rainy season, November through February, the winds are primarily from the
southwest through the southeast, owing to storms that move from west to east.
These storms overlap with the northwest wind patterns and are common from
October through March (October and March are transitional). The southern wind
patterns result in higher maximum wind speeds associated with distinct storms,
whereas the summer winds tend to be more constant, with a typical sea breeze
pattern (calm at night and increasing in the afternoon).
The orientation of Humboldt Bay and the prevailing wind direction result in
locally generated wind waves throughout the interior of the bay, resulting in
localized erosion in the interior portions. Most of the interior shoreline, although
previously showing signs of erosion, appears to be fairly stable today. The
incident ocean waves are often aligned (wave crest perpendicular) with the
entrance, and the alignment of the jetties and the offshore bar (convex outward)
appear to focus the waves into Entrance Bay. Extreme erosion has occurred in
Entrance Bay, and much of the shoreline is now heavily armored. This apparent
focusing of wave energy has resulted in large changes in the channelization
(filling) and smoothing of bathymetry of Entrance Bay. It is likely that the wave
action also influences mixing, flushing, and circulation within Entrance Bay.
7
Chapter 2 Description of Study Area
Previous Page
