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LONGSHORE AND SEASONAL VARIATIONS IN BEACH SAND,
HUMBOLDT COUNTY, CALIFORNIA: IMPLICATIONS FOR
BULK LONGSHORE TRANSPORT DIRECTION
by
Paul Bodin
A Thesis
Presented to
The Faculty of Humboldt State University
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
March, 1982
LONGSHORE AND SEASONAL VARIATIONS IN BEACH SANDS
FROM HUMBOLDT COUNTY, CALIFORNIA: IMPLICATIONS FOR
BULK LONGSHORE TRANSPORT DIRECTION
by
Paul Bodin
Approved by the Master's Thesis Committee
Robert W. Thompson, Chairman
Steven Costa
Gary Carver
Natural Resources Graduate Program
Approved by the Dean of Graduate Studies
Alba M. Gillespie
What is that feeling when you're driving away from
people and they recede on the plaint till you see their
specks dispersing?--it's the too-huge world vaulting us, and
it's good-by. But we lean forward to the next crazy venture
beneath the skies.
Jack Kerouac (from On The Road)
ABSTRACT
Subaerial beach sediments were
sampled
at
80
stations along 62 kilometers of the central Humboldt County
shoreline.
Samples were taken during winter and summer to
represent both the fully eroded and fully accreted
conditions of the beach. Beach samples were analyzed for
the grain size distribution parameters of mode, mean,
standard deviation, and skewness. The 1.75 phi to 2.75 phi
size fractions were recombined and analyzed for both heavy
mineral to light mineral ratios and
for heavy mineralogy.
Three samples each from the Mad and Eel Rivers, which are
the chief sediment contributors to the study area, were
subjected to a similar petrographic analysis. Results of
the grain size analyses and of the analyses of the ratio of
heavy minerals to light minerals indicate that the net
annual longshore drift is generally northward, and that the
Eel River is the chief source of sediment in the study area.
The entrance channel to Humboldt Bay appears to be
selectively trapping coarse particles (>500 microns) as
sediments are passed through the channel system by longshore
drift.
Heavy/light ratios along the beach show
strong
seasonal differences probably related to selective sorting
associated with cross-shore sand movements.
Heavy mineral comparisons
i
ii
indicate
that
river
iv
sediments undergo a selective sorting in transport to the
beach which obscures the provenance of beach materials.
ACKNOWLEDGEMENTS
While performing this research,
I was supported
almost entirely by a University of California Sea Grant
College Program traineeship, for which I am very grateful.
Much technical support was given to my research by the
Sea-Grant funded project, "A Study of the Entrance Problems
at Humboldt Bay, California", and I would like to
acknowledge the principal investigators: the late professor
John D.
Isaacs, Dr. Theodore Kerstetter, and Dr.
Robert
W. Thompson. Drs. Steven L. Costa and James T. Stork of
the Sea-Grant project gave much of their time and of
material assistance throughout the period of data collection
and analysis.
Mr.
Phillip
Buttolph
enthusiastically
assisted in sample collection and processing above and
beyond the call of duty.
Mr.
Hugh Whelan, Ms.
Seanna
Willit, and Ms. Edna Rodrigues assisted with the technical
analyses--thank you.
This thesis owes its present form and readability to
Dr. Robert W. Thompson, whose knowledge and experience
were relied upon throughout the entire undertaking.
I am
grateful he was able to stick it out to the end.
The staff of the Fred Telonicher Memorial Marine
Laboratory were specially wonderful to work with. In
particular, I would like to acknowledge Ms. Carol
Mr. James Rusconi, Mr.
Wardrip,
Hal Genger, Mr. John Smith, and
vi
Mr. David Hoskins. Without their cheerful support I
might
still be in prison.
For the ease of preparation of the final
the thesis,
I
drafts of
am grateful to Dr. Richard J. Seymour and
Ms. Martha Rognon of the Nearshore Reasearch group at
Scripps Institution of Oceanography for the use of their
word-processor.
Finally, my morale through the more difficult times
was maintained only by the loving support of Joan and my
parents.
TABLE OF CONTENTS
page
ABSTRACT
iii
ACKNOWLEDGEMENTS
v
TABLE OF CONTENTS
vii
ix
LIST OF TABLES
LIST OF FIGURES
x
INTRODUCTION
1
STUDY AREA DESCRIPTION
4
Physiographic Features
4
Sediment Input to the Beach System
Oceanographic Features
8
12
METHODS OF STUDY
Sample Collection and Processing
12
Grain Size Analysis
15
Mineralogical Analysis
16
Statistical Analysis
17
AERIAL DISTRIBUTION OF GRAIN SIZE PARAMETERS
19
Results
19
Discussion
31
Bulk Northwards Longshore Transport Hypothesis
31
Evidence for Selective Trapping in the Bay Mouth
33
Effects of Mad River Sediment on the Beach
34
Alongshore Wave Energy Gradient
36
vii
viii
TABLE OF CONTENTS (continued)
HEAVY MINERAL-LIGHT MINERAL RATIOS
page
38
Results
38
Discussion
38
HEAVY MINERALOGY
44
Results
44
Discussion
46
CONCLUSIONS
53
Bulk Northward Transport
53
Suggestions for Further Work
55
REFERENCES CITED
57
APPENDICES
A.
B.
C.
Numerical values of
grain-size parameters--winter
60
Numerical values of
grain-size parameters--summer
62
Numerical values of
heavy mineral abundance
64
LIST OF TABLES
Table
page
1
Heavy mineral percentages--river samples...
45
2
Beach heavy mineral percentages--winter
47
3
Beach heavy mineral percentages--summer
48
ix
LIST OF FIGURES
Figure
page
1
Location map of study area
5
2
Beach morphology and names;
River sampling sites
7
3
Wave power roses for the study area
9
4
Beach sampling south of
the harbor entrance
13
Beach sampling north of
the harbor entrance
14
Most common size constituent (mode)
of the beach samples--winter
20
Most common size constituent (mode)
of the beach samples--summer
21
8
Modal difference between seasons
22
9
Mean grain size--winter
23
10
Mean grain size--summer
24
11
Standard deviation of the
grain size distributions--winter
26
Standard deviation of the
grain size distributions--summer
27
Skewness of the grain size
distributions--winter
29
Skewness of the grain size
distributions—summer
30
Heavy mineral abundance
along beach
39
Glaucophane abundance along
the beach--summer
49
5
6
7
12
13
14
15
16
INTRODUCTION
As a coastal area becomes economically developed,
anthropogenic stresses can alter bordering beaches, with
consequences varying from aesthetic irritation to
large-scale economic
loss.
are often the result of
Undesireable changes in beaches
ignorance about
local
shore
dynamics. Due to rapid economic development of coastal
areas, scientific knowledge of shore processes tends to
accumulate after problems arise. The magnitude of
difficulties which may follow coastal development, and the
uniqueness of each beach system, indicate that knowledge of
beach dynamics at unstressed coastal localities is required
to forestall beach degradation related to coastal
development. The beaches of Humboldt County, California,
have not yet been subjected to the same degree of stress as
have beaches on other parts of the California coast.
As
developmental pressures grow, however, these relatively
unscathed beaches will come
under
increasing
stress.
Investigating the dynamics of Humboldt County beaches will
help isolate areas of potential
problems and enhance our
knowledge of unaltered beach systems.
An important problem arising from earlier
investigations of the study area is a controversy over the
dominant direction of
littoral drift.
Previous
investigators have noted that the prevalent longshore
transport direction at any one time in the study area varied
1
2
depending on the season, being northward during the winter
and southward at other times (Noble 1971, Ritter 1972,
DeGraca et al.
1974).
reached about which,
However,no clear consensus was
if any, longshore transport direction
dominated in the annual cycle.
I will refer to average
In order to avoid confusion,
longshore transport caused by a
particular wave train, an instantaneous rate varying
seasonally, as "net longshore transport". The overall
sum
of net longshore transport effects over the entire course of
a year,
which
is the controversial value, will be referred
to as "bulk longshore transport".
Elucidation of the bulk longshore sediment transport
direction is related to the question of which of the waves
arriving obliquely at the study area, when refracted to
shore, concentrates the largest longshore component of wave
power on mobile beach sediments. A mathematical model of
this process was used by DeGraca and Eckar (1974) to
determine the potential
bulk longshore transport direction
north of the Humboldt Bay Jetties.
Their results were
inconclusive and yielded patterns of convergence and
divergence of potential sediment transport for which there
is no physical evidence.
Noble (1971) examined changes
in
beach
shape
resulting from the emplacement of jetties at the entrance to
Humboldt Bay.
He interpreted greater accretion in the lee
of North Jetty as evidence of the dominance of southward
transport.
3
Ritter (1972) performed grain-size analyses
and
examined abundance of the accessory mineral garnet along the
study area.
He concluded that northward sediment flow in
the winter was of sufficient magnitude to make the Eel River
the chief contributor to the beach as far north as the
Humboldt Bay Jetties.
Snow's (1962) survey of
beach
morphology
and
grain-size along the Northern California coast included
several stations in the study area. Snow theorized that the
beach sediment south of the mouth of the Eel River was of
different origin than beach sediments north of the Eel, and
that the Eel's sediment may be the chief source of the
northern beach.
The purpose of my work was to investigate the
longshore variation of beach sediment grain size and
mineralogy, and to apply these to the question of the bulk
longshore transport direction. My investigation was carried
out as part of a Sea-Grant funded investigation of the
shoaling of the entrance channel
system at Humboldt Bay,
California (Kerstetter 1980). My work was to be used in an
effort to determine the source of sediments which comprise
the entrance channel shoals.
Results and conclusions of this and future work
apply directly to several real-life problems in the study
area. These include: harbor shoaling, coastal erosion
problems, disposal
of dredge spoils from Humboldt Bay, and
potential consequences of dams on the Eel and Mad Rivers.
STUDY AREA DESCRIPTION
Physlographic Features
The coast of central Humboldt County between 40°31'N
and 41° 02'N is a 62 kilometer sand beach bounded by rocky
headlands and containing the entrance to Humboldt Bay, a
major shipping and fishing port (Figure 1).
The beach is the seaward extension of two extensive
flood plains composed of recent sedimentary materials
(Evenson, 1959). The southern plain is the Eel River flood
plain, or delta.
The northern plain contains both the
Humboldt Bay system, into which drain several small streams,
and the Mad River flood plain.
The tectonic framework which provides an underlying
"skeleton" for the shoreline of the study area is somewhat
unusual compared to that of many other California beaches.
The majority of the California coastline
strikes
north-westerly and, typically, a rocky headland at the
northern end of a sand beach in California extends farther
west than does the southern boundary.
In the study area,
however, the northern boundary (Trinidad Head) is east of
the southern bounding rocky promontories of False Cape
(Figure 1).
The overall strike of the study area shoreline
is, therefore north-north-easterly.
The beach, and the nature of its landward backing,
4
5
Figure 1.
Location map of study area. Stippled areas
are areas of Holocene sedimentation.
6
change along the study area.
South of the Humboldt Bay
jetties, the beach is generally steep and narrow, and is
backed by either:
(1) cliffs which occasionally are eroded
actively by waves, or (2) a thin band of low dunes (Figure
2).
North of the jetties, the beach tends to be broader and
flatter than
it is to the south (Snow 1962), and is backed
by well developed dune systems.
Where cliffs, eroded by
both wave and river action, occur in the northern part of
the study area, they are separated from the ocean by as much
as 0.75 kilometers of vegetated dune sand (Figure 2).
Sediment input, to the Beach System
There are two major sediment sources to the study
area at the present time, the Mad River in the north and the
Eel River in the south (Figures 1 and 2). These rivers
drain geologically similar terraines, and rocks of the same
formations (Bailey 1966).
The Eel River contributes an
estimated average of 4.5X106 tons of sand per year; the Mad
approximately one tenth of this amount (Ritter 1972).
The Mad and Eel Rivers have the
largest ratios of
sediment yields to drainage areas in .North America, and
carry extremely high sediment loads for their sizes (Judson
et al.
1964, Karlin 1980). Most of this sediment load is
carried at high flows attained in the winter months,
large portion of the overall
and a
total is moved during major
flood events with a long recurrence interval (Brown et al.
7
Wide, flat beach.
Extensive dunes.
"Abandoned" cliffs.
Intermediate-to-wide
beach. Generally very
low steepness. Extensive
high backing dunes. Gravel
on beach in some placesdunes.
Beach is seaward edge
of Humboldt Bay barrier.
Wide beach, low steepness.
High backing dunes.
Beach is seaward edge
of Humboldt Bay
barrier. Steep &
narrow in S. to
broad & flat in
N.
Steep beach,
stable cliffs?
Beach is
seaward edge
of Eel R.
delta. Steep,
narrow beach.
Low backing
dunes..
Verysteep,
narrow
beach.
No dunes
eroding
cliffs.
Figure 2. Beach morphology and names; river sampling sites.
Morphological character of the shoreline is shown
for seven sections of beach. River samples from
the Mad River were given the identification numbers: m-20, m-30, and m-40. Eel River samples
were numbered: e-20, e-30, and e-50.
8
1971, Kelsey 1980).
The large amount of sediment input to
the beach system is another characteristic which
distinguishes the study area from other California coastal
locales.
Oceanographic Features
More wave power arrives at the study area (and to
the north) than at coastal
localities to the south in
California (N.M.C. 1960, D.N.O.D.
The incoming
1977).
wave power is distributed among three recognizable
"families" of waves, each with a particular season-of-effect
and average angle-of-approach (Johnson et al.
1971)(Figure
3):
1) From April through November, the dominant wave
energy component at the study area is the "prevailing
swell". These waves are generated by storms in the
north-central and northeastern Pacific. These waves are low
energy wave forms, generally not higher than three meters
and averaging under one meter high, and dominate the total
wave power as a result of their long season-of-effect. The
tectonic structure, and the high sediment input previously
discussed have allowed the beach to become oriented roughly
perpendicular to the direction
from which the prevailing
swell arrives (another northern California beach exhibiting
these characteristics is discussed
by
I.E.E.,
1968).
Therefore, these waves should not, on the average, cause net
9
Figure 3. Wave power roses for the study area. Values
were obtained by averaging the results of calculations by Johnson (1971) for two deep-water
stations approximately 200 kilometers north and
south of the study area.
10
longshore transport.
2) The winter months, November through March, are
characterized by very high energy "seas" which arrive from
the south (Figure 3) (Johnson et al.
1971).
These
southerly waves average about three meters high and range
over eight meters high.
They are associated with storm
fronts passing through the study area during winter. The
winter storms also contribute the bulk of the annual average
precipitation of about one meter (Rantz 1969).
Therefore,
during this season the river runoff and stream sediment
contribution are high, coinciding
with
net
northward
transport engendered by the high southerly waves of the
season.
3) A third group of waves arriving at the study area
are seas generated by NNW winds which occur most strongly in
the months of May to August. This family of waves has an
average deep-water approach direction of NW to NNW (Figure
3), and an average height of one to two meters with a
maximum height of five meters.
Southward drift during the
late spring and summer months is a result of the arrival
of
this group of waves. The total wave-power of this northerly
group is about equal to the total wave-power of the winter
southerly waves.
This description of wave power in the study area is
based on data hindcast for deep-water stations off the west
coast (N.M.C. 1960, D.N.O.D. 1977). These data can only
be used to generate a rough estimate of the actual wave
11
climate of the study area.
It is hoped that wave data
collections underway at the present time will provide a more
detailed view of this important aspect of the local beach
processes (Seymour et al.
1980).
Tides in the study area are mixed semi-diurnal, with
a mean tide range of about 1.5 meters, and a maximum tide
range of about 3.4 meters (N.O.A.A.
1980).
METHODS OF STUDY
Sample Collection and Processing
River sediment samples were taken at the waterline
on the depositional (inside) bank of meanders. Three
locations from the floodplains of the Mad and Eel Rivers
were sampled (Figure 2).
Samples consisted of about two
kilograms of sediment, scooped by hand to a depth of 10
centimeters.
Beach samples were selected from approximately one
thousand samples of beach sediment taken as part of the
Sea—Grant project on harbor entrance shoaling (Kerstetter
1980). Samples were taken monthly from stations located at
0.8 kilometer intervals along the beach except near the
Humboldt Bay jetties where the sampling interval was 0.2
kilometers (Figures 4 and 5). Mileage along the beach was
determined with the odometer of the sampling vehicle.
Sampling sites were marked with stakes placed on the first
line of permanent dunes, where permitted.
Samples were
identified by numbers corresponding to their distance from
the Eel River mouth in kilometers, negative values being
south of the river mouth and positive values being north of
the river mouth.
Samples consisted of approximately 200 cm.3 of sand.
Each sample represented four sub—samples taken from the
12
13
Figure 4.
Beach sampling south of the harbor entrance.
Next to each sampling location there are symbols
indicating which analyses were performed on that
sample. See legend for meaning of symbols.
14
Figure 5. Beach sampling north of the harbor entrance.
Next to each sampling location there are symbols
indicating which analyses were performed on that
sample. See legend for meaning of symbols.
15
corners of a two meter grid.
Samples were taken from
approximately mid-beach face to a depth of three centimeters
to represent the most recent beach-face transport (Bascom
1951).
Samples from two of the monthly sampling sets were
analyzed for sedimentary properties. The set collected in
late January and early February of 1979 represents the
winter high-energy condition. The set taken in September of
1979 represents the prevailing low-energy summer swell
condition.
River samples were hand sieved to remove particles
larger than -1
phi (2000 microns) and finer than 4 phi (63
microns), then treated the same as beach samples.
Beach
samples were rinsed in fresh water and dried for two days in
an oven.
158 samples (79 each from winter and summer) were
analyzed for grain size parameters (Figures 4 and 5).
Of
the samples analyzed for size, 41 (20 from winter, 21 from
summer) were further analyzed for heavy mineral content.
Samples were selected for mineralogical examination to
obtain equal coverage of the study area at approximately 3.5
kilometer intervals (Figures 4 and 5).
Grain Size Analysis
A thirty to fifty gram subsample was obtained from
each sample for grain-size analysis using a soil splitter.
Samples were sieved to quarter-phi intervals by placing them
16
in a Ro—Tap mechanical sieve shaker for twelve minutes. The
fraction of sample retained on each sieve was weighed to a
precision of 0.005 grams.
The mean, standard deviation,
skewness, and kurtosis values of the grain size
distributions (based on the weight percent of the sample in
each size class) were calculated using central
derived measures (Blatt et al.
1972).
moment
In addition the
mode, or the most prevalent size class of the distribution
by weight, was calculated. Details regarding derivation of
statistical descriptors of the grain size distributions are
given in Blatt et at.
1972.
Mineralogical Analysis
The heavy mineral
fraction of the size interval
between 1.75 phi and 2.75 phi was separated by gravity
settling in tetrabromoethane (specific gravity=2.90).
The
restricted grain size was used for two reasons: 1) to
facilitate
petrographic
grain
identification
by
standardizing the grain thicknesses observed through the
microscope, and 2) to make the results more
indicative of
provenance differences rather than sorting effects. The
1.75 phi to 2.75 phi size fraction was used
because
particles in this size range were found in nearly every
sample. A two to three gram split of the sample was used in
the separation. Heavy minerals were mounted on microscope
slides in Canada Balsam (refractive index=1.56).
17
Prior to examination,
each slide
was
randomly
assigned a letter identification symbol in order to
eliminate bias in counting. Samples were counted using a
petrographic microscope which was arranged to allow
reflected light examination of the grains as well
transmitted light observation.
in two steps:
as
The slide counts were made
1) counting one hundred grains of all
types
in order to determine the percentage of non-translucent
grains, and 2) counting at least one-hundred translucent
grains. Minerals were identified using
standard
petrographic techniques, tables, and references (Krumbein et
al.
1941, Milner 1962). During mineralogical examination,
grain shape and surface texture (roundness) were observed
and compared to standard figures (Powers 1953).
StaisclAnye
I have analyzed the data for grain size trends by
statistically treating the aerial distribution of modal
values. Modal values were used instead of mean values
because they have use in ascribing provenance (Curray 1960),
and because the mean values (since they are based on
logarithmically transformed weight percentage measures) have
been questioned by some authorities as
statistical comparison (Blatt et al.
a
basis
for
1972).
Aerial trend analyses were performed on the modal
grain size values by performing linear regressions of the
18
size values on alongshore location. The coefficient of the
resulting regression' was tested for significance. Unless
otherwise stated, the significance level for all tests of
significance was 0.01. Seasonal differences in modal grain
size were tested for si g nificance by comparing samples taken
in both seasons from identical (staked) sites, and applying
the paired sample t-test.
In order to test for separable
regions in sorting and sqewness values during a single
month, regime boundaries were selected visually, and, in the
cases of both sorting and sqewness in winter, a two-sample
t-test was used. For summer sqewness values, aerial trends
were discerned by testing the significance of the regression
of sqewness on alongshore location.
Mineralogical results were not subjected to
statistical analysis because of low precision in the results
of the examination of about 100 translucent grains per
slide. The mineralogical results, however, can be viewed as
being indicative of major trends which may be deserving of
more thorough statistical treatment in the future.
The error involved in determining ratios of heavy
minerals to
light minerals is unknown, depending to a great
extent upon the skill of the worker, the materials and
methods used
(Twenhoffel et al.
this phase of analysis may
1941). Error involved in
compound
difficulties
in
interpreting results of the final petrographic analysis, but
does so to an unknown extent.
AERIAL DISTRIBUTION OF GRAIN SIZE PARAMETERS
Results
The most common size constituent of the
beach
sediments became significantly finer with distance northward
in the study area during both winter and summer. Modal
values ranged from 0.125 phi (875 microns) in the south to
2.625 phi
(225 microns) in the north (Figures 6 and 7).
During the winter high river-flow season the coarsest beach
sediments were located at, or Just north of, the Eel River
mouth (kilometer 0). Average and modal grain size decreased
in both directions from this point.
In the summer, the
coarsest sediments were 8.1 and 8.9 kilometers south of the
Eel River. Most of the modal values south of the Eel during
the summer clustered around 1.125 phi
(450 microns),
with
the gradual fining trend beginning Just north of the Eel
River.
There was more variance in modal
grain size among
samples south of the jetties than among samples north of
them in both seasons.
difference overall
There was
in modal
not
a
significant
grain size between seasons
(Figure 8).
Mean size values calculated from central moment
statistics showed the same general trend of fining to the
north as in the mode (Figures 9 and 10).
mean values, a region of
However in the
lower phi values (coarser mean
Figure 6.
20
Most common size constituent (mode) of the beach samples--winter. Best-fit
line indicates trend of decreasing modal size toward north. Correlation
is significant at the 0.01 level (correlation coefficient=0.8673, number
of samples=79).
21
Figure 7. Most common size constituent (mode) of the beach samples--summer. Best-fit
line indicates trend of decreasing modal size toward the north. Correlation
is significat at the 0.01 level (correlation coefficient=0.8392, number of
samples=79).
22
Figure 8. Modal difference between seasons. Open circles represent the summer value
minus the winter value for sites marked with stakes. Line through data
represents line of no seasonal difference. Points above the line were sites
finer in summer, those below the line were sites coarser in summer.
Figure 9.
Mean grain size--winter.
23
Figure 10.
Mean grain size--summer.
24
25
sizes) was observed Just south of the Mad River mouth,
between about kilometer 32 and kilometer 42 in the winter,
and between about kilometer 35 and kilometer 41
in the
summer. The excursion to lower mean values implies the
presence of a coarse "tail" or secondary modes on the beach
in this region.
In the area noted by coarser mean values,
the beach was observed to include occasional
patches of
gravel on the surface, or an underlying layer of
coarse-medium sand.
Sorting of beach sediments, as reflected by the
standard deviation of a sample's grain size distribution,
showed distinct seasonal differences in aerial distribution
(Figures 11 and 12).
In the winter significantly lower
values of standard deviation were evident south of the
Jetties compared to north of them (Figure 11). North of the
jetties, along North Spit (from kilometer fifteen to about
kilometer thirty), during the winter there was generally
less variability in sorting than south of the jetties in the
same season, and values were
low, indicating well-sorted
sediment. An exceptional point was found at kilometer 23.3,
which during the winter was much
samples from North Spit.
less sorted than other
This sample was taken from a
disposal site of dredge spoils materials of a generally
coarser nature than the surrounding beach, and is an
artifact of the active erosion of these materials onto the
beach at this point during the winter cut-back of the berm.
Adjacent to and south of the Mad River mouth (kilometer 32
Figure 11.
26
Standard deviation of the grain size distributions--winter. South of
the.Humboldt Bay mouth (kilometer 14.5), the samples are significantly
less sorted than north of the bay mouth (significance level=0.05,
t=2.83, degrees of freedom=77).
Figure 12.
27
Standard deviation of the grain size - distributions—summer. South of the
Humboldt Bay mouth, the samples are significantly less sorted than from
the same area in winter (significance level=0.01, t=4.92, degrees of
freedom=36).
28
to kilometer 41), there were several poorly sorted samples.
North of the Mad River mouth the samples were very well
sorted.
In the summer set, the same general regions of poor
and well sorted sediments were observed.
However,
differences in the boundaries of, and values within, the
regions of elevated values were evident (Figure 12). South
of the Jetties during summer there was more variation among
samples than during winter, with significantly higher values
of standard deviation.
In summer, the zone of poor sorting
south of the Mad River mouth extended southward only to
Kilometer 35.
Skewness values in winter appear to be divided into
two regimes north and south of kilometer 31 (Figure 13).
For samples south of kilometer 31, the average sqewness
value was a near-symmetrical -0.003, while north of this
point the average sqewness value was -0.833.
Significantly
smaller (more negative) sqewness values from north of
kilometer 31 than from south of this point showed either the
presence of a coarse tail
or
loss of fines north of
kilometer 31 compared to samples to the south.
In summer a two-regime pattern was also displayed by
the sqewness values. However in summer the break between
the two regimes was located at the Humboldt Bay jetties
(Figure 14). North of the Jetties there was a significant
correlation of
increasingly negative sqewness with distance
northward. There was a visually observed offset between
Figure 13.
29
Skewness of the grain size distributions--winter. Skewness values south
of kilometer 31 average 0.003, significantly different than values north
of kilometer 31, which average -.0833 (significance level=0.01, t=9.922,
degrees of freedom=77).
Figure 14.
30
Skewness of the grain size distributions--summer. Best-fit lines
through points were tested for significance. Significant regressions
of values from alongshore locations north of the bay mouth (significance
level=0.01, correlation coefficient=0.735, degrees of freedom35) indicates northward trend to smaller skewness values. Offset between lines
at the bay mouth is such that higher values are from just north of the
jetties than just south of them.
31
south and north of the Jetties, with larger skewness values
Just north of the Jetties than just south of them.
A trend to smaller values of skewness may
indicate
the gradual loss of the finest part of the size distribution
(Inman 1949).
An abrupt increase in values may represent a
sudden loss of coarse or gain of fine particles and a sudden
decline in skewness may represent the opposite.
Discussion
Northward Longshore
A hypothesis of northward bulk
longshore transport
is supported by the grain size data. The general fining of
the beach to the north may reflect sorting accomplished
during longshore transport.
travel alongshore
more
Specifically, fine grains may
rapidly
than
coarse
grains,
establishing a larger proportion of fines on the beach with
increasing distance from the source.
The literature is
divided on the issue of which grain size is most readily and
rapidly advected downcurrent.
Inman (1949) argues on both
theoretical and empirical grounds that the 2.6 phi (170
micron) size fraction is most rapidly transported in a
variety of environments. Also, Ingle (1966) has assembled
data which show that the 2.5 to 2.75 phi (180 to 200 micron)
sizes take the
least energy to move alongshore (see also:
32
Raudkivi 1967). However, in measurements obtained on a low
energy beach, Komar (1977)
found that the -0.25 phi (1200
micron) grain size fraction moved alongshore most rapidly,
with finer grains traveling more slowly. Komar's (1977)
results probably indicate the importance of rolling and
saltation grain transport modes in his low energy study
area.
In a high energy beach, such as the present study
area, suspension load becomes more important, and rolling
may be only a minor mode of transport. Finer grains become
suspended in the water column more easily than coarse
grains, and spend more time exposed to longshore currents.
Therefore, fines would probably be transported more rapidly
alongshore in the study area than coarse grains because of
the high wave energy level.
The general improvement in sorting for samples from
North Spit compared to samples from south of the Jetties
indicates that the beach sediments undergo sorting with
northward drift.
The existence of poorly sorted samples
north of kilometer 32 in winter and north of kilometer 35 in
summer may indicate the addition of relatively poorly sorted
Mad River materials.
The rather
31-kilometer mark
abrupt
sqewness
decrease
at
the
in the winter distribution is indicative
of a coarse tail being added to the grain size distribution
north of this point, presumably by Mad River derived
materials. During summer, the regions both south and north
of the Humboldt Bay jetties show trends of northward
33
decrease in sqewness, which is consistent with net northward
transport of material (Inman 1949).
The northward bulk longshore transport hypothesis
implies that the Eel River is the chief sediment supplier to
the entire study area.
Evidence for Selective Trapping in the Bay
Mouth
In addition to selective longshore transport rates,
another mechanism which may produce alongshore trends in
grain size parameters is selective removal of particular
grain sizes by size-selective trapping. Some aspects of the
grain size data
indicate that the entrance to Humboldt Bay
represents a size-selective littoral trap.
Patchiness in
grain size from south of the Humboldt Bay Jetties, as
indicated by variability in modal, mean, and sorting values,
is in sharp contrast to the finer and better
sorted
sediments along North Spit. The entrance channel may act as
a "littoral
sediment filter",
reducing the range of grain
sizes present as longshore movement passes sediment through
the channel
system from south to north. Since the beach is
generally finer north of the harbor mouth, and
since
sqewness values in summer indicate a sudden loss of coarse
particles at the Jetties, it must be coarse particles which
are preferentially removed from littoral transport at the
channel. The sediments north of the Jetties may be viewed
as a fine, relatively homogeneous differentiate of a more
34
heterogeneous and generally coarser "lag" sediment from
south of the harbor mouth.
Effects of Mad. River Sediment on the Beach
The occurrence of coarser, poorly sorted samples
(presumed to be the effect of the input of Mad River
materials) at about kilometer 32 to 35, Just south of the
Mad River mouth appears to contradict the bulk northward
drift hypothesis.
However, these observations
may
be
explained by other processes.
Part of the solution to this problem may involve the
northward migration of the Mad River outlet.
During the
two-year period November 1978 to November 1980 the mouth of
the Mad River migrated 500 meters north, an average rate of
0.25 kilometers per year.
This migration is apparently
cyclic, with the mouth opening as far south as about
kilometer 36, then migrating northward for a period of
years, and finally re-opening at the southernmost point
again. The observed coarse underlying sediment could
represent materials deposited in the Mad River mouth when it
was both farther south and at flood stages with
the
competence to move a coarse sand and gravel load. The
coarse materials deposited under high-flow conditions might
remain on the beach as a sluggish lag deposit at the site
where the Mad River outlet deposited them.
The lag of gravel and very coarse sand is subject to
35
seasonal burial by the onshore movement of finer particles
from the surf-zone during the relatively lower energy
non-winter months. The coarsening of the beach extends
farther southward, and is greater in magnitude, during
winter when the beach is in an eroded condition. The coarse
deposits are buried during beach accretion in the southern
part of the disturbed zone. However, in the northern part
of the disturbed zone, beach accretion is apparently not as
efficient in covering up the lag.
A possible additional process in maintaining coarse
particles south of the Mad River mouth is size selective
trapping of
littoral
materials by the
lower Mad River
channel analagous to that proposed for Humboldt Bay. Such a
littoral sediment filter would explain the nearly uniform
fineness of the beach north of the Mad.
Lower wave energy north of the Mad than south of it
may also be a contributing factor to the maintenance of
coarser samples south of the mouth than north of it.
Refractive and diffractive wave energy losses in the lee of
Trinidad Head, and wave energy
dissipation
over
the
generally shallower offshore waters north of the Mad River
may cause the beaches north of the Mad to have a lower wave
energy level.
This is a complicated region which will
require more detailed examination.
The grain size data generally support the model
of
Eel River dominance of the beach sediment system and of a
bulk northward longshore transport. The Mad River may only
36
become an important contributor during extremely high flows.
Alongshore Wave Energy Gradient
An alternative hypothesis to that of bulk northward
drift as an explanation for the general northward fining
-trend in the grain size data is that a wave energy gradient
exists alongshore in the study area.
DeGraca and Eckar
(1974) suggested that a zone of wave convergence may exist
in the southern half of the study area due to refraction by
the Eel River delta offshore.. Convergence would cause the
southern half of the study area to have a higher wave energy
level than the northern sector.
This suggestion has not
been tested, but could lead to the observed gradation
in
average particle size observed in the study area because,
all else being equal, beaches with low energy waves tend to
be finer-grained than beaches with high energy waves (Bascom
1951).
While the
northward
bulk
longshore
transport
hypothesis suggests that much of the relatively unsorted
material arriving at the shoreline from the rivers remains
in the
littoral
zone and
is subjected to size-sorting by
longshore currents, the wave gradient hypothesis suggests
that the relatively unsorted materials from the river may be
immediately size-sorted by wave action upon arrival at the
shore.
In order to create the observed northward fining by
an alongshore wave gradient mechanism, fines would have to
37
be lost (presumably to offshore) in the southern half of the
study area, while fine and coarse particles would both be
retained in the northern half.
This process may occur to some extent, but it is
difficult to perceive it as a controlling process in
producing the grain size trend in the study area.
Skewness
values do not show truncation of the fine tail of the grain
size distributions in the area of high energy hypothesized
by DeGraca and Eckar (1974). Also, sorting does not become
markedly and gradually poorer toward the north, which would
be expected if a wider range of grain sizes were generally
being maintained on the beach in the north than
in the
south. Instead, poor sorting is restricted to an area
immediately adjacent to the Mad River mouth,
as previously
discussed, which indicates that the input of Mad River
materials. causes a very localized disturbance unrelated to
the major fining trend throughout the study area.
In
addition, the relative smootheness of the northward fining,
showing no area in the south which appears to be the focus
of wave energy by having elevated modal sizes, argues
against the wave energy difference hypothesis.
HEAVY MINERAL-LIGHT MINERAL RATIOS
Results
The ratio of heavy minerals to light minerals in the
1.75 to 2.75 phi size range, expressed as the percentage of
heavy minerals in that size range, varied both seasonally
and spatially in the study area (Figure 15).
The winter
values ranged from 1.53 percent to 5.07 percent, with a mean
of 2.58 percent.
during winter.
pattern emerged.
There were no regular longshore trends
During summer, however, a
three-region
South of the Humboldt Bay jetties, for
fifteen kilometers on either side of the Eel
River mouth
there were somewhat variable values with an average of 1.82
percent, and no value below one percent. From the jetties
north to kilometer 32 the values were all low; none of the
six adjacent samples from North Spit contain even one
percent heavy minerals.
From kilometer 35 to the northern
end of the study area, the values were again elevated to
about two percent, with the exception of a single point at
kilometer 41. The anomalous sample was taken in part from a
beach placer deposit and was influenced by this sampling
error.
Discussion
Alongshore patterns in heavy mineral abundance may
38
39
Figure 15. Heavy mineral abundance along the beach. The curves display the weightpercentage of the 1.75 phi to 2.75 phi size fraction which is contributed
by heavy minerals (specific gravity= 2.9).
40
express the effects not only of sedimentary source
(provenance), but of sorting related to sediment movement
subsequent to arrival at the shoreline (White et al.
Lowright et al.
1967).
(1972) noted that heavy minerals were found
in higher proportion to lights near source areas. DuBois
(1972) showed that cross shore movements may also
be
important in determining the heavy-light ratio expressed on
the beach face.
The relatively high concentration of heavy minerals
on the beach near both the Eel and Mad River mouths during
summer are probably an expression of the overall
longshore
spreading of sand in both directions from both sources.
Within a restricted grain size such as I examined, the
heavies may be less easily entrained than the lights and
thus tend to lag behind them in transport. This would form
a region near the source which is enriched in heavies.
In
addition, heavies have been found to become preferentially
buried in the beach sediments, and may become removed from
transport this way (Clifton 1969), also leading to decreased
heavy-light mineral ratios downcurrent from the source.
The elevated heavy mineral content throughout the
area south of Jetties
implies that the Eel River is the
primary source and that heavy minerals must be transported
northward from the Eel river to at least the Humboldt Bay
Jetties.
If lights outstrip heavies in transport, then Eel
River materials must be transported north of the Jetties and
along North Spit. This evidence supports the bulk northward
41
drift hypothesis proposed to explain the grain size data.
The rather sudden drop off of heavy minerals during
the summer north of the jetties compared to the south may be
explained by the existence of a selective trap in the
entrance channel system. The heavies in the analyzed sizes
would be filtered out along with coarser light grains in
accordance with the principle of hydraulic
(Rubey 1933;
Blatt et al.
equivalence
1972). This selective removal
process would augment the selective
transport
process
described above in creating the observed small heavy-light
mineral ratios from along North Spit.
heavy
The occurrence of elevated quantities of
minerals south of the Mad River mouth (north of kilometer
35) coincides with the region of coarser grain sizes caused
The same
by the introduction of Mad River materials.
processes (chiefly northward river mouth migration)
responsible for the coarser grain sizes may be responsible
for the elevated heavies.
In the heavy mineral
fraction
abundance, however, unlike in the coarse grain sizes, the
elevation of values extends north of the Mad River to the
northern end of the study area. This elevation may
represent northward mixing of Mad River materials with
sorted Eel River sand. Alternatively it may be an
expression of the proclivity of heavies to be concentrated
in the finer sand grain sizes (Pettijohn et al.
Since these
fine
sizes
are
the
major
1972).
sedimentary
constituents north of the Mad, the elevation of heavies may
42
be an artifact of this association. More detailed analyses
are needed to resolve these possibilities.
The larger area of elevation in heavy
mineral
content around the Eel than around the Mad may be evidence
of the dominance of the southern sediment source.
It could,
however, be the expression of more rapid transport in the
southern sector, as proposed by DeGraca and Eckar (1974).
I
believe that it is a combination of more sediment and faster
transport.
The generally higher abundance of heavy minerals
in
the winter samples compared with summer samples reflects the
seasonal effects of beach cutback in winter. Light grains
have been found to move offshore more readily than heavy
grains during episodes of beach cutback (Rao 1957). This
process allows the beach face to become enriched in heavy
mineral particles during winter.
Simultaneously, the winter
surf-zone becomes enriched in light particles which have
been preferentially moved off of the beach face.
During
spring and summer, onshore movements, which build the broad
summer berm, "swamp" the beach face with particles which had
resided in the surf zone during winter.
Therefore the
summer samples are depleted in heavy minerals relative to
the winter samples.
The locations discussed previously which display
either enrichment or depletion of heavy minerals in the
summer samples, then, reflect to some extent either the
enriching or the depleting .effects of sorting associated
43
with winter movement in the surf zone.. The summer samples
should therefore not be interpereted as indicating the
effects of longshore movement taking place during the summer
months.
HEAVY MINERALOGY
Results
The translucent fraction of the heavy mineral
suite
of Eel and Mad River samples was dominated by amphibole
minerals including hornblende, glaucophane, and members of
the tremolite-actinolite series.
In both rivers, pyroxenes
comprised the second most numerous class of minerals.
The
metamorphic mineral epidote was the next most abundant
mineral in both rivers, followed by the accessory minerals
sphene, garnet, and sillimanite (Table 1).
Differences between the two rivers were observed in
the amount of hornblende and glaucophane, which comprised a
greater proportion of the translucent fraction of the Mad
River (54 percent) than of the Eel (40 percent). The
increase of amphiboles in the Mad relative to the Eel
was
apparently at the expense of garnet and sphene--both of
which occured in relatively elevated percentages in the Eel
samples.
Substantial variations in relative percentages of
some minerals occured among samples from the same river.
Examples of this variation could be seen in the amount of
hornblende, garnet, clino-pyroxenes, and opaques from the
Eel River, and in the opaques and garnet content of the Mad
River samples.
The beach samples displayed mineral suites similar
44
45
Table 1. Heavy mineral percentages--river samples.
Measured for one hundred grain count. T-A=
tremolite-actinolite; HO=hornblende; GL=glaucophane;
CPX=clinopyroxenes (augite-diopside); OPX=orthopyroxenes
(enstatite-hypersthene); EP=epidote; GA=garnet;
Sl=sillimanite; SPH=sphene; AP=apatite; OTH=other
translucents; TNT=total non-translucents; LF=lithie
fragments; OPQ=opaques.; Values given are percentage of
translucent fraction, except for 0TH, LF, and OPQ, which values
indicate percentage of total heavy mineral fraction. np=not
present.
46
to both of the rivers sampled (Tables 2 and 3), however the
data showed that important differences existed between river
and beach samples. Beach samples typically contained more
hornblende and
less sphene and opaques than either of the
stream sources. The only alongshore trend apparent in the
mineralogical data was the glaucophane abundance, which
showed an irregular decrease to the north (Figure 16)..
South of the Jetties, summer glaucophane abundance varied
North of the
around an average of about fifteen percent.
Jetties there was an overall decrease except for elevated
values between kilometer 32 and the mouth of the Mad River
at kilometer 41.
Discussion
Similarities between the suite of heavy minerals
found in a typical beach sample during either season and the
suites displayed by 'both the Mad and Eel Rivers strongly
suggest that the active
beach
sediments
are
wholly
contributed by these streams. Only minor contributions can
be expected from sources which
might
have
differing
mineralogical character, such as material contributed by
cliff erosion and by "leakage" around headlands.
Although no unique tracers were detected,
it is
possible that the relative proportions of certain minerals
in the heavy mineral suite may be different for each river.
The present mineralogical data are not adequate for
47
Table 2. Beach heavy mineral percentages--winter.
Measured for one hundred grain count. T-A=
tremolite-actinolite; HO=hornblende; GL=glaucophane;
CPX=clinopyroxenes (augite-diopside); OPX=orthopyroxenes
(enstatite-hypersthene); EP=epidote; GA=garnet;
Sl=sillimanite; SPH=sphene; AP=apatite; OTH=other
translucents; TNT=total non-translucents; LF=lithic
fragments; OPQ=opaques.; Values given are percentage of
translucent fraction, except for 0TH, LF, and OPQ, which values
indicate percentage of total heavy mineral fraction. np=not
present.
48
Table 3. Beach heavy mineral percentages--summer.
Measured for one hundred grain count. T-A=
tremolite-actinolite; HO=hornblende; GL=glaucophane;
CPX=clinopyroxenes (augite-diopside); OPX=orthopyroxenes
(enstatite-hypersthene); EP=epidote; GA=garnet;
Sl=sillimanite; SPH=sphene; AP=apatite; OTH=other
translucents; TNT=total non-translucents; LF=lithic
fragments; OPQ=opaques.; Values given are percentage of
translucent fraction, except for 0TH, LF, and OPQ, which values
indicate percentage of total heavy mineral fraction. np=not
present, *=data unavailable.
Figure 16.
Glaucophane abundance along the beach--summer.
49
50
statistical analysis, but variations between the two rivers
in the content of hornblende, garnet, and sphene may
ultimately prove distinctive for each river. Variations in
hydraulic properties between different locations in a river
leads to variations in relative mineral
abundances at the
different sites, and indicates that the problem of
separating the rivers mineralogically is very complex and
would require extensive investigation.
Differences between either river's heavy mineral
suite and that of any beach sample suggests that sorting
occurs between the river
environment
and
the
beach
environment. The sorting acts to enrich primarily the
hornblende component of the beach sediment in the analyzed
size fraction while depleting the opaques and sphene.
Clearly, this implies that simple mixing of the two sediment
sources is not the only process operating to create the
heavy mineral suite of the beaches. Sorting between river
and beach environments is
intervening step.
an
important
As all mineral
and
unknown
suites from along the
beach were very similar and there were no trends
or
disturbances alongshore, even close to the river outlets,
and as there are no trends leading away from the river
mouths (with the exception of glaucophane), this initial
sorting must occur prior to the arrival of sediments to the
shoreline. Since all river sampling sites were located
upstream of the region of tidal
action in the rivers,
I
propose that the sorting takes place in the lower reaches,
51
or "estuary", of the rivers under the action of oscillating
tidal currents. Despite differences in the relative amounts
of certain minerals between the two rivers, therefore, it is
not possible to ascribe absolute provenance to any beach
sample.
The alongshore density sorting observed in
the
heavy-light ratios were not, apparently, carried out to any
appreciable degree among the heavy minerals themselves.
This implies that the density sorting process associated
with longshore transport is fairly gross and
is
not
sensitive to small density differences between minerals.
Because both rivers contained substantial amounts of
glaucophane, and since the Mad river displayed
if anything
more glaucophane than the Eel River, the apparent general
northward decrease of glaucophane is
surprising.
The
decrease of glaucophane from near riverine abundances in the
south to lower than riverine values in the north implies
that glaucophane is somehow preferentially lost to the beach
transport system with bulk transport northward.
A plausible explanation of the
trend
is
the
restriction of glaucophane to the coarser of the analyzed
sizes. Then glaucophane would- occur in lower abundances
away from the source as it lags behind in transport with the
rest of the coarse grains. If in the source rocks
glaucophane crystals were larger than the other heavies were
in their source rocks, then a size restriction
ultimately result on the beaches (Briggs 1963).
might
52
Restriction of glaucophane to the coarse fractions
would also explain its paucity in the uniformly fine grained
beach north of the Mad River. The peak in values south of
the Mad may be generally associated with
the
coarse
components which occur there.
Another, although less plausible, explanation for
the trend in glaucophane is its possible loss to the
littoral transport system as a result of its
mineral
character. For example, glaucophane could conceivably be
more friable, or less stable, than other minerals and be
gradually removed from transport by eroding away as it
spends time on the beach. The problem with this explanation
is that glaucophane shares many mineralogical
properties
with other minerals in the heavy suite, yet no similar trend
in abundance is observed for any other mineral.
CONCLUSIONS
Bulk Northward Transport
The grain size and heavy/light mineral
ratio data
suggest that bulk longshore transport direction in the study
area is northward.
This is a response to wave energy
arriving from the south during winter, at the same time that
the majority of fresh sediment is mobilized to the beach by
the river outflow. Apparently there is a very rapid sorting
of grains by size during winter by longshore drift
mechanisms in which the finer
grains
advect
rapidly
northward from the Eel River, which is the major source of
beach sand to the study area. Sediments near the mouth of
the Eel River represent a heterogenous lag deposit. The
entrance channel to Humboldt Bay appears to filter the
coarser, heavier grains out of the beach sediments,
resulting in a more uniform beach along North Spit.
The Mad River probably
is not a very important
sediment source except to the fine beach north of the Mad
River mouth. Also, at extremely high flows coarse sand and
gravel are deposited. These coarse deposits remain to some
extent, as markers of previous river mouth locations, while
the active river mouth migrates northward. Mixing of Eel
River and Mad River sediments takes place near the Mad River
outlet. North of the Mad River mouth,
it appears that
refractive, diffractive, and dissipative wave energy
53
54
losses--perhaps in concert with size selective trapping of
beach sand by the lower reaches of the Mad--maintain a
relatively homogeneous fine-grained beach.
The bulk northward transport in the study area,
in
contrast to the southward transport exhibited by most other
California beaches, results from differences in wave climate
and shoreline orientation between the study area and other
locales in California. The coastal tectonic orientation of
the study area, in concert with the extremely high sediment
yields of the provenance areas for the local coastal
sediments, has allowed the shoreline to orient
facing
approximately WNW, the direction from which the greatest
proportion of the wave power comes. A very high southerly
component of wave power arriving during winter, the season
of the largest sediment input, effects an efficient drifting
of the beach sediment northward.
Then, apparently, the
onshore movements. associated with wave power arriving from
directions more normal to the strike
material which was
of the beach, moves
in the surf zone during winter onto the
subaerial beach, with little southward drift.
By contrast, the majority of California beaches to
the south of the study area are forced by their tectonic
skeleton to face WSW.
This orientation causes a major
proportion of the total annual wave power, that from west to
northwest, to impinge onto the surf zone at a northerly
oblique angle to the beach normal,
bulk drift.
engendering southward
Also, on beaches to the south there is a
55
smaller proportion of southerly wave energy in the total
wave climate, and these waves tend to arrive in summer, when
beach sediments are less mobile (Komar 1976).
The likelihood that the majority of the
longshore
drifting of sand takes place during winter and is directed
northward should be taken into account by designers of
coastal engineering projects.
Disposal sites of dredge
spoils, for example, might be located in such a way that
they do not immediately shoal channels again.
Coastal
projects which would interfere with longshore drift should
not be located to the south of regions which have but little
beach sediment to protect the backshore from destructive
wave energy. The possible starvation of already sand-poor
areas should be considered prior to interfering with the
downstream sediment transport of the Eel River. Projects to
improve the entrance conditions at Humboldt Bay should be
aware of the mechanisms which give rise to shoaling.
Suggestions for Further Work
This study should be regarded as preliminary.
Much
more knowledge is needed before a satisfactory level of
assurance about beach processes in the study area can be
reached.
Surely the accumulation of high quality, shallow
water wave data is fundamental to our further knowledge of
the system.
I believe that such information from at
least
56
two widely-spaced stations within the study area is
necessary to
adequately
respond
to
questions
about
alongshore wave energy variation. In addition, the
acquisition of reliable and detailed bathymetry in the
shallow nearshore would allow for the elaboration of a
mathematical transport model including the entire study
area.
Sediments must be sampled from difficult surf zone
locations, because we need a better understanding of the
structure and composition of such a high energy surf zone.
Samples from offshore of the breakers might allow for a
satisfactory assessment of offshore losses. Closely spaced
observations should be made of beach processes at the mouth
of the Mad River, where curious things are apparently
happening. Finally, carefully planned tracer studies may be
used to confirm predictions about beach response in the
study area.
REFERENCES CITED
Bailey, E.H., ed. 1966, "Geology of Northern California",
Cal. Div. Min. Geol. Bull. 190.
Bascom, W.H., 1951, "The relationship between sand size and
beach face slope", Trans. Am. Geophys. Union.,
32:866-874.
Blatt, H., G. Middleton, and R. Murray, 1972, The _Origin
Of Sedimentary Rocks, Prentice Hall, New Jersey, 624
P.P.
Briggs, L.I., 1965,
"Heavy
mineral
correlations
provenances", J. Sed. Pet., 35:935-955.
and
Brown, W.B.III, and J.R. Ritter, 1971, "Sediment transport
and turbidity in the Eel River basin, California", U.S.
Geol. Surv.., Water-Supply Paper 1986.
Clifton, H.E., 1969, "Beach lamination: nature and origin",
Mar.. Geol., 7:553-559.
Curray, J.R., 1960, "Tracing sediment masses by grain size
modes", International _Geological Congress, Report of
the 21st Session Norden, Copenhagen.
DeGraca, H.M., and R.M.
Eckar,1974, "Sediment transport,
coast of Northern California", Am. Soc. Civ, Engin.,
National meeting, Los Angeles, California.
D.N.O.D., 1977, (Department
of
Navigation
and
Ocean
Development), Deep-Water. Wave. Statistics for _the
California Coast-Station 1, D.N.O.D., Sacramento.
DuBois, R.N., 1972, "Inverse relation between foreshore
slope and mean grain size as a. function of heavy
mineral content",
83:871-876.
Evenson, R.E., 1959, "Geology and ground-water features of
the Eureka area, Humboldt County, California", U.S.
Geol. Surv., Water-Supply Paper 1470.
Ingle, J.C., 1966, The Movement of Beach Sand,
Amsterdam, 221 pp.
Elsevier,
Inman, D.L., 1949, "Sorting of sediments in the light of
fluid mechanics", J. Sed. Pet., 19/2:51-70
I.E.E., 1968, (Interstate Electronics and
Engineering),
Drift
Littoral
of
Preliminary Investigation
57
58
Characteristics
Bolinas ,
Lagoon
California,
IEC-Oceanics Report 445-027, prepared for Bolinas
Harbor District, Bolinas, California.
Johnson, J.W., J.T. Moore, and E.B. Orret, 1971, "Summary
of annual wave power for ten deep water stations along
the California, Oregon, and Washington coasts", Univ.
Tech. Report. HEL-24-9.
Judson, S., and Ritter, D.F., 1964, "Rates of regional
denudation in the United States", Jour. Geophys..
Res., 69, pp. 3395-3401.
Karlin, R., 1980, "Sediment sources and
clay
mineral
distributions off the Oregon coast", J. Sed. Pet.
50/2:543-560.
Kelsey, H.M., 1980, "A Sediment budget and analysis of
geomorphic process in the Van Duzen River basin, north
coastal California, 1941-1975: a summary", Geol._ Soc.
Am. Bull. part 1, 91:190-195.
Kerstetter, T.A., 1980, "A study of the entrance problems at
Humboldt Bay", California Sea _Grant College Program,
Final Report, R/CZ-47
Komar, P.D., 1976, Nearshore Processes. and Sedimentation,
Prentice-Hall, New Jersey, 429 pp.
Komar, P.D., 1977, "Selective longshore transport rates of
different grain-size fractions within a beach, J. Sed
Pet„ 47/4:1444-1453.
Pettijohn, 1941,
Krumbein, W.C., and F.J.
Manual of
Appleton-Century Co., New
Sedimentary Petrology, D.
York, 549 pp.
Dachille, 1972, "An
Lowright, R., E.G. Williams, and F.
analysis of some factors controlling deviations in
Sed.
hydraulic equivalence in some modern sands",
Pet., 42/3:635-645
Milner, H.B., 1962, ...Sedimentary Petrography, MacMillan Co.,
II 715 pp.
New York, Vol. I 643 pp., Vol.
Murray, S.P., 1967, "Control of grain dispersal by particle
75:612-634.
size and wave state",
J. Geol.,
N.M.C.
(National Marine Consultants), 1960, Wave Statistics
for Seven Deep Water Stations Along the California
Coast, Santa Barbara, California.
59
N.O.A.A., 1980, United_ States Coast Pilot 7 16th ed.,
National Ocean Survey, Washington, D.C.
Noble, R.M., 1971
"Shoreline
changes,
Humboldt
California", Shore and Beach, 39:11-18.
Bay,
Pettijohn, F.J., P.E. Potter, and R.
Stever, 1972, Sand
and Sandstone, Springer-Verlag, New York, 618 pp.
Powers, M.C., 1953, "A new roundness scale for sedimentary
particles", J. Sed. _Pet., 23:117-119.
Rantz, S.E., 1969, "Mean annual precipitation
in
the
California region", U.S. Geol.
Surv., Open-File Maps.
Rao, C.B., 1957, "Beach erosion and concentration of heavy
mineral sands",
Sed. Pet., 27:143-147.
Raudkivi, A.J., 1967, Loose, Boundary Hydraulics, Pergamon
Press, New York, 331 pp.
Ritter, J.R., 1972, "Sand transport by the Eel and its
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Geol. Surv.,
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Rubey, W.W., 1933, "Settling velocities of gravel, sand, and
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Hyd. _Eng.. Lab., Tech. Report series
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APPENDICES
A. Numerical values of grain-size parameters--winter.
Site
49.1
48.3
47.5
46.7
45.9
45.1
44.3
43.5
42.7
41.9
41.1
40.9
40.7
39.9
39.1
38.3
37.5
36.7
35.9
35.1
34.3
33.5
32.7
31.9
31.1
30.3
29.5
28.7
27.9
27.0
26.2
25.4
24.6
23.8
23.3
23.0
22.2
21.4
20.6
19.8
19.0
18.2
17.4
Mean
2.436
2.427
2.439
2.399
2.415
2.446
2.426
2.462
2.336
2.295
2.258
2.017
2.169
1.809
2.256
2.160
1.776
1.704
2.210
2.288
2.045
2.027
1.965
1.948
2.096
2.061
2.179
2.068
2.149
2.187
2.207
1.982
1.895
1.853
1.475
1.724
2.044
1.775
1.897
1.873
1.845
1.886
1.764
MSt.Devd
2.625
2.625
2.375
2.375
2.375
2.625
2.625
2.625
2.375
2.375
2.375
2.375
2.375
2.375
2.375
2.375
1.875
2.375
2.375
2.375
2.375
2.125
2.125
2.375
2.125
2.125
2.375
2.125
2.125
2.125
2.375
1.875
1.875
1.875
1.625
1.875
1.875
1.875
1.875
1.875
1.875
1.875
1.875
St. Dev.
0.289
0.292
0.240
0.259
0.257
0.249
0.283
0.258
0.299
0.299
0.440
0.506
0.460
1.019
0.304
0.380
0.553
0.767
0.348
0.355
0.659
0.407
0.484
0.637
0.354
0.340
0.299
0.304
0.289
0.287
0.293
0.339
0.386
0.360
0.575
0.344
0.306
0.369
0.305
0.388
0.312
0.291
0.343
60
Skewness
-0.984
-1.055
-0.372
-0.374
-0.494
-0.647
-0.651
-0.591
-0.658
-0.651
-1.234
-0.893
-0.985
-1.271
-0.675
-0.554
-0.376
-0.589
-0.522
-1.269
-2.309
-0.918
-0.820
-1.350
-0.909
-0.514
-0.244
-0.173
-0.078
-0.103
-0.119
-0.137
-0.037
0.174
-0.657
-0.033
0.129
-0.060
0.054
-0.118
0.163
0.080
0.042
Kurtosis
6.549
7.083
3.706
3.154
3.444
4.467
4.811
4.041
4.057
4.765
5.358
3.605
4.994
3.129
4.666
3.495
2.675
1.946
3.985
7.009
9.325
5.267
3.444
4.701
6.026
3.550
3.095
3.101
2.877
2.909
2.929
3.004
2.878
3.385
3.621
3.405
2.976
3.056
3.243
3.167
3.215
3.224
2.909
61
Values of grain-size parameters--winter (continued)
Site.
16.6
16.3
16.2
15.8
15.6
15.5
15.3
15.1
14.5
14.3
14.2
14.0
13.8
13.7
10.5
9.7
8.9
8.1
7.2
6.4
5.6
4.8
4.0
3.2
2.4
1.6
0.8
0.2
-1.6
-4.8
-6.4
-8.1
-9.7
-11.3
-12.9
-13.4
Mean.
1.745
1.996
1.996
1.694
1.754
1.821
1.876
1.949
2.140
2.175
1.975
2.128
2.001
1.754
1.785
2.052
1.697
1.788
1.302
1.466
1.989
1.895
1.559
1.554
1.364
1.350
0.517
0.758
1.100
1.744
1.537
1.371
1.588
1.337
1.050
1.118
_ Mde
1.875
1.875
1.875
1.625
1.875
1.875
1.875
1.875
2.125
2.125
1.875
2.125
1.875
1.875
1.875
1.875
1.875
1.875
1.375
1.375
1.875
1.875
1.625
1.375
1.375
1.375
0.625
0.875
1.125
1.625
1.625
1.625
1.625
1.375
0.875
1.125
St.. Dev.
0.337
0.287
0.287
0.338
0.306
0.305
0.351
0.343
0.331
0.289
0.310
0.272
0.271
0.286
0.347
0.289
0.389
0.370
0.376
0.383
0.300
0.314
0.373
0.349
0.399
0.483
0.491
0.556
0.391
0.344
0.339
0.555
0.302
0.375
0.333
0.282
Skewness
-0.139
0.011
0 011
-0.037
0.102
0.077
-0.367
-0.262
0.042
0.024
0.423
0.502
0.214
0.331
-0.062
0.321
-0.136
-0.204
-0.538
0.346
0.301
0.217
-0.081
0.077
0.077
-0.542
0.506
0.442
-0.032
0.149
-0.225
-1.273
-0.055
-0.061
0.362
0.440
Kurtosis
3.204
3.084
3.084
3.475
3.093
3.307
3.471
3.151
2.903
2.782
3.106
3.773
3.552
3.336
3.533
3.275
2.967
3.518
5.108
3.219
3.555
3.269
2.937
3.484
2.970
3.614
4.180
4.036
5.324
3.432
3.444
5.371
3.644
3.016
2.744
5.016
62
B. Numerical values of grain-size parameters--summer.
Site
48.9
48.3
47.5
46.7
45.9
45.1
44.3
43.5
42.7
41.9
41.1
40.7
40.7
39.1
37.5
35.1
34.3
33.5
32.7
32.3
31.9
31.1
30.3
29.5
27.9
26.2
24.6
23.3
23.0
21.4
19.8
19.0
18.2
16.6
15.8
15.3
15.0
14.5
14.3
14.2
14.0
13.8
Mean
2.551
2.540
2.453
2.470
2.448
2.387
2.457
2.347
2.453
2.427
2.458
2.272
1.991
2.299
0.908
1.235
2.208
2.348
2.239
2.226
2.203
1.988
2.130
1.927
1.840
2.045
1.838
1.693
1.979
1.666
1.720
1.706
1.721
1.464
1.826
1.806
1.698
2.314
2.072
1.979
1.841
2.083
Mode
2.625
2.625
2.625
2.625
2.375
2.375
2.625
2.375
2.375
2.625
2.625
2.375
2.375
2.375
2.375
2.375
2.375
2.375
2.375
2.375
2.125
1.875
2.125
1.875
1.875
1.875
1.875
1.625
1.875
1.625
1.625
1.625
1.625
1.375
1.875
1.875
1.625
2.375
2.125
1.875
1.875
2.125
St.Dev
0.274
0.230
0.288
0.243
0.251
0.263
0.254
0.284
0.251
0.282
0.290
0.321
0.715
0.311
1.091
1.139
0.309
0.269
0.294
0.309
0.300
0.355
0.297
0.298
0.314
0.255
0.311
0.335
0.353
0.327
0.294
0.266
0.258
0.343
0.295
0.343
0.281
0.316
0.338
0.379
0.385
0.298
Skewness.
-0.720
-0.480
-0.533
-0.432
-0.457
-0.362
-0.389
-0.426
-0.418
-0.437
-0.326
-0.349
-1.222
-0.359
0.064
-0.460
-0.171
-0.162
-0.241
-0.207
0.129
-0.101
0.014
0.312
0.032
0.321
0.101
0.288
-0.240
0.127
0.206
0.440
0.201
0.306
0.105
0.185
0.314
0.063
-0.222
-0.561
-0.506
-0.139
Kurtosis
4.371
3.689
3.652
3.245
3.450
2.957
3.193
3.175
3.448
3.106
3.389
2.905
4.408
3.025
1.733
1.820
2.692
3.495
2.860
2.895
2.905
2.984
2.831
3.020
2.982
3.102
3.525
3.353
3.140
3.352
3.456
3.817
4.512
3.239
2.922
2.960
3.697
2.554
3.334
3.729
3.553
3.059
63
Values of grain-size parameters--summer (continued).
Site.
13.7
12.9
12.1
11.3
10.5
9.7
8.9
8.1
7.2
6.4
5.6
4.8
4.0
3.2
2.4
1.6
0.0
-0.8
-1.6
-2.4
-3.2
-4.0
-4.8
-5.6
-6.4.
-7.2
-8.1
-8.9
-9.7
-10.5
-11.3
-12.1
-12.9
-13.7
-14.5
Mean.
1.688
1.751
2.165
1.738
1.689
1.677
1.418
1.422
1.101
1.743
0.780
1.885
1.505
1.557
1.620
1.719
1.652
0.724
1.372
1.307
1.459
1.047
0.893
0.930
0.869
0.924
0.243
0.415
1.511
1.148
0.874
1.383
1.104
1.290
1.454
Mode
1.625
1.875
2.375
1.875
1.875
1.875
1.625
1.625
1.625
1.875
1.375
1.875
1.375
1.625
1.625
1.875
1.625
0.875
1.375
1.375
1.375
1.375
0.875
1.375
0.875
0.085
0.125
0.125
1.625
1.375
0.875
1.375
0.875
1.375
1.625
St. Dev.
0.337
0.356
0.331
0.390
0.376
0.382
0.481
0.538
0.706
8.341
0.774
0.358
0.428
0.431
0.374
0.369
0.393
0.747
0.396
0.386
0.341
0.547
0.588
0.651
0.533
0.561
0.602
0.645
0.355
0.496
0.635
0.371
0.463
0.388
0.358
Skewness
0.470
-0.186
-0.120
-0.510
-0.533
-0.618
-0.991
-0.856
-0.592
-0.192
-0.335
-0.261
0.212
-0.900
-0.328
-0.058
-0.125
-0.153
0.095
-0.347
0.041
-0.678
-0.561
-0.545
-0.446
-0.603
0.189
0.029
-0.310
-0.253
-0.412
-0.028
0.202
0.018
-0.172
Kurtosis
3.307
3.572
2.873
3.552
3.991
3.731
4.813
3.867
2.897
3.345
2.250
3.445
3.090
5.605
3.893
3.150
3.567
2.382
3.366
3.627
3.249
3.706
3.174
2.803
3.050
3.132
2.657
2.382
3.368
2.724
2.602
3.044
2.287
2.725
3.127
64
C. Numerical values of heavy mineral abundances.
Site
47.5
44.3
41.1
40.7
37.5
34.3
31.1
27.9
27.8
24.6
21.4
18.2
13.7
10.5
7.3
4.0
.0.8
-1.6
-4.8
-8.0
-9.7
-11.3
-14.5
Sumer
1.59
2.10
13.4
2.03
2.00
0.96
0.42
0.61
0.74
0.89
1.22
2.79
1.19
2.97
1.36
3.64
1.14
1.01
Winter
1.38
3.16
2.54
0.77
1.72
1.35
1.74
5.07
1.38
1.93
1.06
3.15
3.83
1.75
1.53
2.96
3.88
4.23
4.93