|Abstract or Summary
- The distribution of surface sediments on the northern Oregon
continental shell is characterized by a nearshore sandy facies and an
outer shelf muddy facies, separated by a mid-shelf zone of mixed
sand and mud. Currents which have been measured at 130 centimeters
above the bottom indicate that the distribution of the surface sediment
is a reflection of the hydraulic regime.
The strongest bottom currents which were measured were in the
nearshore region at a depth of 36 meters. Here currents of over 40
cm/sec generated by surface waves are capable of placing the nearshore
sands in suspension, where they are transported shoreward
by the wave surge. At mid-shelf, in 90 meters of water, the bottom
current veolcity ranges from zero to over 25 cm/sec, although the
mean is normally about 10 cm/sec. The strongest currents at this
depth are capable of eroding some of the fine sediments, but probably
do not rework the older sediments which have been compacted. Currents which are similar in character to those at mid-shelf were observed
at the shelf edge in a depth of 165 meters. A significant
departure, however, is the difference in frequency where the most
energy is found. At the shelf edge the dominant frequency was about
four cpd whereas the dominant frequency at mid-shelf was two cpd or
lower. The dominant frequencies indicate that tides are important
in the generation of continental shelf bottom currents. The twelve -
hour period is that of the semi-diurnal tide; the six-hour period is the
second harmonic of the semi-diurnal component. No indication of
surface wave influence was found at mid-shelf or shelf-edge depths.
Profiles of turbidity made at four east-west transects of the
continental shelf indicate suspended sediment transport occurs principally
at three levels in the water column. An upper layer is at the
level of the seasonal thermocline, a mid-water layer is located at the
level of the permanent pycnocline, and the third layer is at the bottom.
The surface layer is important in transporting suspended sediment
of the Columbia River plume, although there is also a contribution to
the surface layer from the surf zone by the process of diffusion of
The mid-water layer thickens vertically and becomes less
intense seaward, indicating a nearshore source for the suspended
material. This source is diffusion of fine particles from the surf
zone at mid-water depths. The mid-water layer is located at the level
of the permanent pycnocline. The layer is sub-parallel to the bottom
over the shelf but becomes diffuse at the shelf edge. Sediment transport
in the mid-water layer provides a mechanism by which sediment
bypasses the outer shelf and upper slope area.
The bottom layer receives its suspended material from erosion
of the bottom, from the water column above, and from fine material
moving seaward from the surf zone. The amount of eroded material
contributed to the bottom layer depends on the bottom current strength
and on the bottom roughness characteristics. Over a rough bottom
the erosive power of a given bottom current is increased drastically.
For this reason, the presence or absence of rippling is important to
sediment transport on the shelf. The fine material of the bottom
layer may concentrate by settling during quiescent periods, allowing
low-density flows to initiate.
Several time-series observations of turbidity indicate that the
bottom layer thickens and thins in response to increases and decreases
in current velocity. The mid-water layer migrated somewhat in a
vertical direction, but its thickness and intensity remained nearly
the same. The thickness and intensity of the upper layer responded
to changes in the structure of the thermocline, becoming thick and
dispersed when the upper part of the water column is mixed.
A model of sediment transport proposes that mid-water and
bottom currents transport suspended sediments diagonally across
the shelf toward the south-southwest. The sediments of the Columbia
River plume are also transported in a southerly direction in the surface
waters. Relatively little deposition takes place on the shelf and
upper slope, with the bulk of the sediments bypassing the shelf and
depositing on the lower slope and continental rise.