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NPS ARCHIVE
1964

BRENNAN, J.

OBSERVATION OF THE NEARSHORE WATER
CIRCULATION OFF A SAND BEACH

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JOHN F, BRENNAN

RICHARD ft MEAUX

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US NAVAL POSTGRADUATE SC
MONTEREY, CALIFORNIA

OBSERVATIONS OF THE NEARSHORE
WATER CIRCULATION
OFF A SAND BEACH

* * * *

John F. Brennan

and

Richard P. Meaux

OBSERVATIONS OF THE NEARSHORE
WATER CIRCULATION
OFF A SAND BEACH

John F. Brennan
Lieutenant, United States Navy
and
Richard P. Meaux
Lieutenant, United States Navy

Submitted in partial fulfillment of the
requirements for the degree of

MASTER OF SCIENCE

United States Naval Postgraduate School
Monterey, California

1964

-^Thisdocument is subject to speoial export
controlsand-^aehtransi»ittal'to foreign govern-
ment or foreign natro^a^s- Ifiav be
prior approval of the U.S.
School (Code 035) .

only with
raduate

OBRARY

U.S. NAVAL POSTGRADUATE SCHOOl

MONTEREY, CALIFORNIA

OBSERVATIONS OF THE NEARSHORE
WATER CIRCULATION
OFF A SAND BEACH

By
John F. Brennan
and
Richard P. Meaux
This work is accepted as fulfilling
the thesis requirements for the degree of
MASTER OF SCIENCE
from the
United States Naval Postgraduate School

ABSTRACT

The nearshore circulation off a long sand beach at the
southern end of Monterey Bay, California, was studied during
February and March 1964. In preparation for the study, various
types of floats were tested, and a resume of the advantages and
disadvantages of each type is included as an appendix. Gathering
the field data entailed the use of aircraft for aerial photo-
graphy and an amphibious vehicle for launching and recovering
the floats. The wind, wave, and tide conditions prevailing
during all of the surveys were nearly the same. The circulation
patterns found are presented in the form of schematic charts for
each of the five surveys made. The dominant drift was observed
to be directly onshore in the area seaward of the surf zone, but
inside the surf zone the flow was to the north. Weak circulation
cells were found to exist in the surf zone at varying locations
along the beach. Current speeds are presented for the onshore
drift, the dominant longshore current, the opposing feeder
currents to rips, and the rip currents. The speed of the onshore
drift was found to be greater than that of the opposing rip
currents.

TABLE OF CONTENTS

Section Title Page

1. Introduction and Acknowledgements 1

2. Previous Field Observations 4

3. Beach and Wave Conditions in the Survey Area 6

4. Survey Procedure \\

5. Observed Circulation Patterns 20

6. Analysis of the Observed Currents 33

7. Conclusions 48
References 50
Appendix 51

LIST OF ILLUSTRATIONS

Figure Page

1. Chart of Monterey Bay 2

2. The Survey Area 7

3. Examples of Wave Refraction 8

4. Double Innertube Floats 12

5. Army BARC 13

6. BARC Underway Just Seaward of a Breaker 13

7. A Portion of the Work Chart for Survey Four 17

8. Chart of Survey One 22

9. Chart of Survey Two 24

10. Chart of Survey Three 27

11. Chart of Survey Four 29

12. Chart of Survey Five 31

13. Current Components Inside Surf Zone, Composite 38

14. Current Components Inside Surf Zone, Survey Two 39

15. Current Components Inside Surf Zone, Survey Three 40

16. Current Components Inside Surf Zone, Survey Four 41

17. Current Components Inside Surf Zone, Survey Five 42

18. Current Components Outside Surf Zone, Composite 43

19. Current Components Outside Surf Zone, Survey Two 44

20. Current Components Outside Surf Zone, Survey Three 45

21. Current Components Outside Surf Zone, Survey Four 46

22. Current Components Outside Surf Zone, Survey Five 47

23. Launching of a Weather Balloon Float 54

24. Fresh-water-filled Weather Balloon Afloat 56

LIST OF TABLES

Table Page

I. Summary of Weather and Sea Conditions Prevailing 32
During Each Survey

II. Mean Current Speed Components and Frequency of 35
Observations

III. Maximum Current Speeds 37

1. Introduction and Acknowledgements.

The research described herein was undertaken to determine
the nature of the nearshore circulation patterns off the long
crescent-shaped sand beach that marks the inner shoreline of
Monterey Bay, California (Fig. 1). The beach area studied is
located in the extreme southern end of the bay and includes the
beach property owned by the United States Naval Postgraduate
School (USNPGS) , which was previously known as Del Monte Beach.
Five field surveys were made to observe the wind, wave, and tide
conditions prevailing during each survey period.

The observational procedure followed in each survey was to
place from 14 to 30 free-drifting floats in the water a short
distance seaward of the surf zone and to take successive aerial
photographs of the floats at known time intervals as they moved
in and through the surf zone. The float positions were then
plotted and their trajectories thus obtained. From these plots
and from visual examination of the photographs, the general
circulation patterns and the speeds of the currents near and in
the surf zone were established. The causes of the circulation
patterns were then examined with respect to the wind, wave, and
tide conditions prevailing. Because floats were used in measur-
ing currents, only the surface circulation was examined in this
s tudy .

Figure 1. Chart of Monterey Bay.

This thesis problem was suggested to us by Professor W. C.
Thompson of the Department of Meteorology and Oceanography, who
provided much assistance and encouragement in all stages. In addi'
tion, we would like to express our thanks to all those who gave
us help and without whose support this project could not have
been accomplished. They include Captain W. H. Craven, Commanding
Officer, U. S. Naval Air Facility, Monterey, California, who made
helicopters available to us, and Lieutenant R. A. Rucks, USN,
who piloted them; Commander R. W. Haupt, USN, and the men of the
Departmental Library who assisted us in the early stages while
testing the floats; Lieutenant J. A. Gould, USA, Commanding
Officer of the 14th Transportation BARC Platoon, Fort Ord,
California, who placed his vehicles at our service, and all his
men who assisted us so ably and willingly throughout; Photo-
graphers Mate First V. 0. McColly, USN, for his excellent photo-
graphic support; Mr. H. C. Green and the City of Monterey Engi-
neer's Office for supplying advice and charts; and Mr. R. D.
Loftus, Physical Science Aid at the USNPGS, for the many varied
tasks which he performed.

2. Previous Field Observations.

Comparatively few field observations have been made of near-
shore circulations off sand beaches and most of these were made
in Southern California, primarily in and around Scripps Institu-
tion of Oceanography at La Jolla, California 1,3,4 and 5
These have shown that the circulation in and near the surf zone
appears cell-like and consists essentially of the mass transport
of water toward the beach by wave currents, the resulting long-
shore currents which flow parallel to the shore in the breaker
zone, and rip currents which return the excess water to sea at
intervals along the beach.

The net movement of water particles in the. direction of the
shoreward -advancing wave crests causes an inflow of water into
the surf zone in the form of a diffuse wave current. Areas of
wave convergence and divergence along the beach result in large
and small transport into the. surf zone, thus producing differential
elevations which lead to longshore currents.

The direction and speed of longshore currents off a rela-
tively uniform sand beach are known to be determined by two basic
controlling factors, the direction and angle of wave approach,
and the occurrence of convergence and divergence zones along the
beach. Considering the first factor alone, when waves approach
the shoreline at an angle, the resulting longshore current direc-
tion is determined by the component of the wave direction parallel

to the shore. The current speed varies directly with the angle
of incidence, but is also affected by the period and height of the
waves and the foreshore slope of the beach. An increase in wave
height and beach slope generally leads to an increase of the
current, whereas an increase in wave period leads to a decrease
of the current.

The second factor results in the differential mass transport
of water into the surf zone by the wave current along the beach
due to wave refraction on the shelf offshore. In convergence
areas, for example, the resulting higher waves will raise the
water level locally so that a current will flow along the beach
in both directions away from the convergence center. Where long-
shore currents are caused by both the angle of incidence and
differential refraction along shore, the phenomenon that is the
stronger can be expected to determine the direction, although the
strength of the current may be reduced by the opposing effect.

At intervals along a sand beach, the longshore current
turns abruptly seaward and flows through the surf zone as a rip
current. These currents then diffuse in all directions where
some of the water is brought back into shore by the wave current.
In some instances feeder currents, which flow opposite to the
predominant longshore drift, contribute water to the rip currents.
Convergence of both the longshore and feeder currents at the
base of a rip current produces a cellular type of circulation.

3. Beach and Wave Conditions in the Survey Area.

The surveys were carried out along a portion of a beach a
little over one mile in length, running from the southern boundary
of the USNPGS property at Sloat Avenue to the Seaside City Limit
at the Laguna Grande outflow (Fig. 2). The selected beach is a
segment of the long continuous sand beach in southern Monterey
Bay that extends uninterrupted, except when interrupted season-
ally at the Salinas River mouth, from Moss Landing to Monterey
Harbor, where it ends against the rocky shoreline of the Monterey
Peninsula. The sea floor off the beach studied slopes uniformly
seaward and the bottom contours closely parallel the beach trend.

Southern Monterey Bay is distinctive from the standpoint
of prevailing wave conditions. The extreme southern end is so
deeply indented that the beach is sheltered from wind waves most
of the time and swell predominates. In addition, refraction of
waves arriving from all directions in deep water is so extreme
that the crests approach the beach with breaker angles that are
very small or negligible most of the time (Fig. 3). Also,
increasing divergence of wave energy occurs toward the southern
end of the bay due to refraction of nearly all swell arriving
from the open ocean, and results in successively smaller breaker
heights toward the south end of the beach.

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Figure 3« Examples of Wave Refraction.
Orthogonals are shown for two sets of 12-second
waves arriving from two directions in deep water.

As a result of these predominant wave conditions, a net
longshore current associated with the breaker-height gradient
along the beach should be directed toward the south, whereas a
net longshore current related to the breaker angle should be
directed toward the north. In addition, the characteristic average
speed of the longshore current should be small due to the dominance
of swell of small size. When considered all together, these
factors suggest that the net longshore current is either very
weak (and in a yet undetermined direction) or that it is absent.
This deduction is supported by the observation that there has
been no significant accretion or erosion of sand on the local
beach as a result of construction of a bulkhead in 1962 along-
side Monterey Municipal Wharf No. 2, located two-thirds of a
mile to the south of the beach area studied. Furthermore, the
beach has displayed no apparent change in position from examina-
tion of Coast and Geodetic Survey Charts dating back to the
earliest survey of 1851 Q 2 J .

Although the net longshore current is negligible or absent,
mass transport of water into the surf zone occurs with the swell.
The frequent occurrence of rip currents observed on this beach
indicates that they are a principal means of return of this
water to the offshore area, and further indicates that localized
longshore currents between the rip currents are common on this
beach0

The dominance of swell and the absence of any appreciable
net longshore current characterizes this beach as a natural labora-
tory in which circulation on the beach may be unique and may
differ from the nearshore circulation patterns that have been
observed on other beaches. Accordingly, the nearshore circulation
along this beach merits study.

4. Survey Procedure,
a. Field Work.

It was decided to trace the nearshore circulation by the
use of floats photographed from the air at known time inter-
vals. Before the surveys began, the writers experimented
extensively with a variety of types of floats in order to
find a design that would best satisfy the requirements of
this survey procedure. The principal requirements were that
the floats accurately depict the surface currents and that
they be readily visible in aerial photographs both seaward
of and within the surf zone. The types of floats tested
and their advantages and disadvantages are described in the
Appendix.

The float that was selected for the surveys was devised
by the writers and is illustrated in Fig. 4. It consisted
of two automobile innertubes lashed together, the lower one
being water-filled and the upper one air-filled and painted
to enable good visibility from the air. Dye-marker packets
were attached to each float for added visibility. A 40-foot
motor launch owned by the USNPGS was used to test the various
floats, but it was not used in the surveys that followed
because of the hazard of its getting caught in a breaker
while launching or retrieving floats a short distance sea-
ward of the surf zone.

Figure 4. Double Innertube Floats

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Figure 6. BARC Underway Just Seaward of a Breaker,

In the actual surveys, an Army BARC was used to launch
and retrieve the floats. The BARC, shown in Figures 5 and 6,
is a large amphibious vehicle, 65-feet long and weighing
100 tons. Its ability to retrieve floats in the surf zone
after it had launched them and to travel onto and along the
beach made it an invaluable craft for the task.

Each survey was carried out as follows: The floats
were placed overboard from the BARC one after another in a
line just seaward of the outermost breakers. It was anti-
cipated that most or all of them would be carried into the
surf zone, which proved to be the case. (A similar float
deployment was used by Shepard and Inman 5 1 in an earlier
field study in Southern California). Fourteen and 16
floats spaced approximately 400-feet apart were used in the
first two surveys, and 25 to 30 floats spaced at about
200-foot intervals were used in the last three. Supplementary
redwood boards and single dye -packet floats were used in
the first two surveys only and were discontinued after they
proved difficult to locate. In each survey, two different
colors for the floats were used so that adjacent floats
were different in color. This facilitated the identifica-
tion of individual floats in the aerial photographs.

A Navy helicopter made successive passes over the beach
at an altitude of 300 feet and at a speed of 60 knots at
measured intervals of one to six minutes, from which oblique
photographs were taken using 35mm color film. Experimenta-
tion showed it was best to take the photographs over the
water looking toward the beach, as topographic and man-made
features on the beach, which served to orient each photo,
were more readily apparent. The overlap in the photographs
helped identify individual floats and to fix their positions
in relation to one another and to known objects on the beach.
Eight to 12 photographs were taken on each pass along the
beach by the helicopter, and 5 to 18 passes were made
during each survey. The time required to conduct each survey
was between 20 and 40 minutes, whereas the preparation time
for each survey was about four hours and involved consider-
able coordination between the helicopter, BARC, and personnel
on the beach.

b. Plotting Procedure.

In order to assimilate the data from a given survey into
a usable form, the 35mm slides were projected onto a screen,
the floats were located and identified, and their positions
were transferred to a work chart of scale of 1:1200. The
locations of all circulation features noted in the photo-
graphs were also plotted on the work charts, such as rip
currents and dye trails. A portion of the work chart from
Survey No. 4 is shown in Fig. 7. In the figure, the dashed
lines extending from individual floats represent elongate
trails of dye emanating from the float.

The positions of the floats and circulation features
could not be plotted on the work charts exactly because of
errors inherent in the transfer of data from the oblique
photographs to the chart due to parallax in the photos.
However, because the photographs were taken looking shoreward
it was possible to fix positions reasonably accurately from
the many land features evident in the slides, such as poles,
streets, fencelines, pipelines, tanks, and painted beach
markers. These features were so numerous that it is esti-
mated the lateral or along-shore position of each float
was determined to within five feet of its actual position.
In determining the distance of the floats from shore, the
effects of parallax were much harder to overcome due to the

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lack of adequate references in and near the water. By using
the known width of the beach (at known tide stages), the
estimated surf-zone width, the dimensions of the BARC (when
present in the pictures), and a few other features of known
size, it is estimated the offshore-onshore position of each
float was established to within 20 feet of its true position,
In addition, three or four floats were ordinarily present in
each slide so that their relative positions also aided in
fixing their true positions. After the successive positions
of each float were plotted, their speeds were found by
measuring the distance between the successive plots and
dividing by the time interval. The times assigned to each
plotted position were accurate to within 15 seconds.

In examining the slides, long, narrow dye trails were
observed to extend away from many of the floats as far as
100 feet in several cases. If the floats and the water
moved exactly together, a roughly circular dye patch would
be expected around each float due to normal eddy diffusion.
The existence of elongate dye trails indicates shear between
the float and the water, for which two explanations may be
offered. One possibility was that the upper innertube
offered a sail area to the wind so that some wind effect
was probably experienced by the float. Another possible
explanation is that vertical shear in the upper few inches

of the water under the stress of the wind may also have
carried away a thin surface layer of dye from the more deeply
embedded float. No estimate was made of the relative impor-
tance of the wind stress exerted on the float to the water
resistance; however, the fact that in spite of the direction
of the wind, the floats travelled in a variety of directions
in the surf zone leads to the conclusion that wind effect
was not very important, and that the floats accordingly re-
flected the water circulation in the surf zone quite closely.
The work charts for each survey are very long and narrow
so that they could not easily be included in this thesis;
therefore schematic charts in which the width of the surf
zone has been expanded were prepared for presentation. These
charts are shown in Figures 8 through 12, and represent the
results of this study.

5. Observed Circulation Patterns.

The near shore surface water circulations prevailing during
each of the five field surveys are shown in Figures 8 through 12.
In each schematic diagram the beach is marked at 400-foot intervals
for the purpose of describing the locations of floats, rip currents
and other circulation features. The tracks followed by the indi-
vidual floats are indicated by irregular trajectories. Current
speeds along each trajectory are variable and are not shown.

The positions of rip currents are shown by arrows directed
seaward. The rip currents were lettered alphabetically from the
south toward the north independently in each survey. If a rip
current was definitely apparent in successive slides or indicated
by the plotted float trajectories, it is shown by a solid shaft,
but if the rip current only occurred briefly, a dashed shaft is
shown to indicate its temporary existence. Though the beach has
a NE-SW orientation, all directions are referred to as simply
north for northeast and south for southwest. All floats in all
surveys were ultimately recovered on the beach.

In the pages to follow each survey is discussed individually
and the circulation pattern is described with comments on the
float trajectories and observed current speeds. A summary of
the weather and sea conditions prevailing during each survey is
presented in Table I following the survey descriptions.

A. Survey One (Fig. 8); 16 floats.

On the southern part of the beach (Stations 0-11),
convergence and divergence of the float trajectories indi-
cated a fairly good cellular circulation in the surf zone,
although all floats came ashore more or less directly. Two
floats were carried slightly seaward in rip currents before
beaching. On the northern end of the beach the flow was
predominately to the north, with the trajectories not
indicating the presence of the two well-defined rip currents
in that vicinity.

Eleven floats followed trajectories toward the north
and five toward the south in traversing the surf zone.
The five southward -moving floats were directly associated
with rip currents that were evident in the photographs.

A total of seven distinct rip currents were observed.
The three northernmost rips (E, F, and G) were separated
by 1000 feet, whereas the five rips to the south were about
700 feet apart. No current speeds are available for this
survey because the fly-over intervals of the helicopter
were not recorded.

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B. Survey Two (Fig. 9); 14 floats.

The general circulation tended to be cellular, with
the overall flow being toward the north. Four rip currents
were observed, being almost equally spaced at 1100 feet
apart.

Nine floats moved northward and five went to the south.
The southbound floats were all associated with rip currents.
In two cases float trajectories passed shoreward of the
rip currents and gave no indication of their presence; however
at the time of passage these rips were not observed in the
photographs.

Between stations 0 and 10, the speeds of the floats
through the surf zone as they approached the beach ranged
from 50-130 ft/min, and decreased to 10-30 ft/min as they
moved parallel to the beach after touching the beach face.
Between stations 11 and 13 the opposite occurred, the
floats travelling from 13-30 ft/min through the surf zone
and 50-90 ft/min as they moved along the beach.

There was no southward flow between stations 11 and
13, and one float drifted persistently northward even after
passing shoreward of Rip D.

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C. Survey Three (Fig. 10); 30 floats.

Strong northward flow dominated along the entire beach
as indicated by the extremely long float trajectories
roughly paralleling the beach.

Four persistent rip currents were observed throughout
this survey, with two additional rip currents appearing
briefly during part of the time. Rip A was not observed in
the photographs but was indicated only by the float trajec-
tories. Rips A, B, and C, were 1200-1600 feet apart, while
Rips C, D, E, and F were 500-800 feet apart. All rip
currents, except Rip A, had considerable effect on most
floats passing through them, with most of these being carried
out from the beach but not far beyond the outer line of
breakers. These floats moved northward, in some cases at
relatively high speeds, and all ultimately beached. At the
northern end of the beach the rip currents were stronger
but no southward -moving feeder currents to the rip currents
were detected.

Twenty-six floats moved to the north and four went
south. The northerly trajectories were generally flat and
long, extending up to 1500 feet along the beach, whereas
the southerly tracks were much shorter and were associated
only with well-defined rip currents. Two floats had tra-
jectories involving considerable north and south movement.

One of these passed through Rip A with no apparent effects,
but the other was caught in Rip B and moved offshore to
seaward of the surf zone. The southernmost float travelled
northward, passing to seaward and to landward of other
floats that eventually beached before it did.

The speeds of individual floats varied markedly
throughout the survey.

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D. Survey Four (Fig. 11); 25 floats.

The flow was dominantly to the north along the entire
beach. The floats generally came into shore directly and
did not travel far parallel to the beach. Only two rip
currents were observed, near the center of the survey area
(stations 10 and 13), and these affected the floats locally.
Two floats were caught in Rip A and moved seaward but not
beyond the outer breakers. No trajectories were especially
unusual.

Float speeds were fairly uniform along the entire
beach, both seaward of and within the surf zone, and ranged
from 25-40 ft/min. Speeds in the rip currents, obtained
from floats moving away from the beach, were about 10 ft/min,
Floats grounding on the sand traveled at 15 ft/min along
the beach.

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E. Survey Five (Fig. 12); 25 floats.

This and the preceding survey were conducted on the
same day about one hour apart. Survey Five covered an addi-
tional 1500 feet of beach at the northern end. This extension
disclosed two distinct rip currents in that area. This
survey also showed a weak rip current off station four that
did not occur in the previous survey. The twc rip currents
appearing in Survey Four were also present in Survey Five.
Thus, Survey Five had four strong rip currents , and one that
was apparent only in the trajectory plots and not in the
photographs.

Northward flow predominated as in the earlier survey;
however, the circulation was more cellular in nature all
along the beach. Two floats were carried seaward but they
were not carried far beyond the outer line of breakers and
eventually beached of their own accord. As in Survey Four,,
the floats generally did not parallel the beach for any
distance but tended to beach rapidly.

All floats moved northward except three which had
distinct southward tracks. The latter were all directly
associated with one of the rips. Speeds of the rip currents
were about 10 ft/min, whereas the floats moving into the
beach drifted at 10-30 ft/min.

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6. Analysis of the Observed Currents.

The distances travelled by each float in known time intervals
were available from the work sheets for each survey (see example
in Figure 7). This yielded a considerable number of current
measurements. These individual current increments were analyzed
in terms of their onshore -offshore components and their north-
south components parallel to the beach.

The onshore component can be considered to give a measure of
the speed of the incoming wave current at the surface, and the
offshore component to represent rip currents. Since in all surveys
the dominant flow was to the north, the northward component gives
a measure of the dominant longshore current that prevailed, and
the southward component represents the local feeder currents to
the rip currents.

In order to obtain a comparison between the current velocities
outside the breakers with those in the surf zone, it was decided
to group the various tracks on the basis of whether the unit tra-
jectories lay outside the surf zone, astraddle the outer breaker
line (transition zone), or within the surf zone. Histograms were
then prepared of the current speeds in the areas outside and in-
side the surf zone, and these are presented in Figures 13 through
22 for Surveys Two through Five (no current speeds are available
for Survey One). As the wind and wave conditions were similar
during all of the surveys, composite histograms were prepared by
combining all the data, and these composites are shown in Figures
13 and 18. From the data presented in the histograms, frequency -

weighted mean values of the current components were computed, and
are tabulated in Table II for the offshore zone, the transition
zone, and the surf zone. Also tabulated in the table (the figures
in parentheses) are the number of observations on which the mean
current speeds are based; these give a measure of the frequency
of the observations in the four directions. The maximum current
speeds in each category are tabulated in Table III.

From examination of the histograms and tables, it may be
seen that outside the surf zone an onshore drift was dominant,
but some weak offshore flow also occurred. In Survey Three , however ,
the rip currents were stronger than in the other surveys and ap-
proximated the onshore currents, although their frequency was
definitely lower, There was no significant north-south drift sea-
ward of the surf zone in any survey except Survey Three. The wind
was somewhat different in direction (from the west) during that
survey and it accordingly induced a flow to the north.

Inside the surf zone, northerly flow dominated in all surveys
although there were some weaker southerly feeder currents asso-
ciated with many of the rip currents. The speed of the offshore
or rip-current flow was less than the onshore flow in each case,
except for Survey Three where the rip-current speed was a little
greater. Also in that survey, the southerly feeder currents
associated with the rips were stronger.

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Table III. Maximum Current Speeds (ft/min).

Survey
Number

Comp

»onent

rections

Outside Surf
Zone

Offshore
0

Onshore
44

South
20

North
168

120

100

360

Composite

120

360

Inside Surf

100

180

Zone

100

Composite

100

180

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I E

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35
30
25

20
15

""2

I £5

<4
u

-20

- I'l 10

_ —

— - --o

80 60

COMPOSITE
SOUTH NORTH

INSIDE S'JrtF ZOKB

Ift

LE^

Values off graph:
180 N
180 N
360 N
100 S
96 S

x] a.

80 6o 40 20

) 20 1)0 60 8$ 100 . 120 HO
SPEED (ft/if in) \ !

•OFFSHOSE ONSHORE

IFL,

;

Figure 13. Current Components Inside Surf Zone, Composite*

VKY TWO

INSIDE SUBF TON"

SOUTH NOJ?TH

•

Valuer of;' graph

180 N
100 S

i n r — i

I! rJ i nn

H n

20 0 20

':0

60 £0 ICO- !20 140

. SPEED < ft/at n)

0FFS1 ORE , QHSHORE

\ •

- -

• 30-

R
E

0.
U

. -

....

.10.

Figure 14* Current Components Inside Surf Zone, Survey Two.

35
30
25

20
t5

10
5

35
30

R
E
Q
-X.
E
H
C
v

ao.j

15-

o.L

SUHVEY THREE

SOUTH NORTH

INSIDE SUSF ZONE

Values off graph:
96 S

rH 11 n

80 60 i+0 20

0 20 ifO 60 80 100- 120 140

SPEED (ft/min)

' 0FFS30KE " ONSHORE

In.

ie So

fed 'j &•? ioo uo ).((•

Figure 15. Current Components Inside Surf Zone, Survey Three.

! F
R

! S

; K

i C

35
30
25
20
15
10

SURVEY FOUR
SOUTH NORTH

INSIDE 5VRP '/.ONE

. Pi n IV ~

20 0 20 40 SO
EPKED (ft/adn)

OFFSHOKE ONSHORE

8o ioo lao ii+o

0. 20 40 60 80
Si-EFD (ft/iain)

100 1.10 140

Figure 16* Current Components Inside Surf Zone, Survey Four,

35
30
25
20

15
to

SURVEY FIVE
SOUTH NORTH

80 60 kO

INSIDE SURF Z0;?2

Values off nraph:

360 N

180 N

20 . 0 20 40 60
SPEED (ft/rain)

80 100 }20 140

15
to

So Co

OFFSHORE i ONSHORE

R .
E

„-U

l c

lb-XL

20 0 £0 WJ bO

SPEED (ft/toitO

,o . i... kv< — rr. ■

Figure 17, Current Components Inside Surf Zone, Survey Five.

I F
R

Q
u

i E

N
C

' y

COMP
SOUTH

03ITE

OUTSIDE SU3? ZONE
NORTH

■ -

Values off graph

120 S

n m

1 ;h n ( — 1 nl h n n

80 60 401 20

0 ao i)D 60 8o 100 120 140

SPEED (ft/toin)

OFPSKORE I ONSHORE

^ jn

80 60 40 20 0 20 Z»p 60 SO 100 120. llfO

SPEED (ft/idn)

Figure 18 • Current Components Outside Surf Zone, Composite,

F
R

i I

; Q

E
i N
I C
: Y

35
30
25

20
15
10

'80 60

- :— tO

SURVEY TWO
SOUTH NORTH

OUTSIDE SUHf ZONE

n .n 1 — 1

kO 20 0 20 40 60
SPEED (ft/ain)

i OFFSHORE i ONSHORE

^.jl_h

Id.

£0

•„

SPEEDS (ft/ntia)

TOO 120 HO

Figure 19* Current Components Outside Surf Zone, Survey Two,

I F
i R
I E,
I Q
'.. u
s

C
Y

•

survey t::pee
south north

OUTSIDE 8URF ZONE

Values off graph:
120 S

□J

rtn^r

11 n n

0 60

1»0 20-0 20 IfO 60

80 100 120 1^0

•

SPEED (ft/mln)

OFFSHORE ONSHORE

■- .

JU L

£} H.

5 30 t\0 6<J>

ft/kin)

Jej ' too • iao !■'!)

Figure 20. Current Components Outside Surf Zone, Survey Three,

' F
1 R
> E
i Q
U
E
. N
C
I

' SURVEY FOUR
SOUTH NORTH

OUT.

SIDE SURF ZONE

•

... .

- -

8b 60 ifO 20

0 20 ^0 60 80
SPEED (ft/riin) I

100 120 11*0

OFFSHORE ONSHORE i

n- rinn

60 ;b 2ft a 20 4<f 60 50 100 !Z0 lifO

SPEED (ft/iiin)

Figure 21* Current Components Outside Surf Zone, Survey Four,

1
1

; f

! 8
! E

j.Q

t E

i N

' C

; t
t

i
i

■

SURVEY
SOUTH

Frvz

NORTH

OUTSIDE SURF .ZONE

•

,. ■

„n

1 i

8o 60 kO 20 6 20 4t> 60 80 100 120 HO

SPEED (ft/atn)

20
15

5
0

OFFSHORE

ONSHORE

Iki

hxl

80 60 k'9

20 0 20 /fD 60

SPEED (ft/nln)

8K) 1C0 120 1i»«>

t„

Figure 22; Current Components Outside Surf Zone, Survey Five.

7. Conclusions.

Wind and wave conditions were nearly the same during the five
surveys, so that the observed nearshore circulations can be
considered to be comparable to each other. The wind was directly
onshore, except in Survey Three when the wind had a component along
the beach toward the north. In all surveys, the flow was dominant-
ly to the north along the beach, with some cellular circulation
always present.

The onshore current approached normal to the beach and its
shoreward drift velocity was greater than the speed of the oppos-
ing rip currents both outside and inside the surf zone. Survey
Three was an exception, because, although onshore flow dominated,
the wave current and the rip currents were nearly equal in
strength. The onshore drift speed was the same (about 15.0
ft/min) outside and inside the surf zone, but was a little greater
(23.7 ft/min) in the transition zone from sea to surf.

The longshore current inside the surf zone was northerly
in every survey, but seaward of the surf zone there was no
definite longshore flow except in Survey Three when it was to the
north. Southerly feeder currents were present with many of the
rips, and in many cases caused a convergence of floats on the
beach at the base of the rip.

Rip currents occurred all along the beach. Their positions
usually varied from one survey to the next, but one rip (near
station 13) was observed to occupy the same location during all
of the surveys. Rip-current speeds were moderate (10.2 ft/min)
within the surf zone, greatest (21.3 ft/min) where the rips
passed through the breaker line, and weak (4.8 ft/min) seaward of
the surf zone where they became diffuse.

Because the wind and wave conditions were nearly the same for
all surveys, no conclusions can be drawn from the data about the
nearshore circulation under other weather and sea conditions.
Different conditions of wind, waves, and tides would also have been
desirable to obtain a broader picture of the circulation patterns
present on the selected beach, but the complexity of coordinating
the surveys required that the survey dates be fixed well in advance,
thus preventing the gathering of data when the environmental condi-
tions were most favorable. The similarity of conditions during
the surveys was entirely coincidental and not pre-planned.

Numerous equations for the prediction of longshore currents
on a sand beach have been presented in the literature I 3 J .
Examination of the survey results contained herein indicates
that caution should be used in developing or applying such equa-
tions to describe the velocity of longshore currents because
they probably are neither uniform nor simple on any s*id beach
along the coasts of the oceans.

REFERENCES

1. Beach Erosion Board, Corps of Engineers. Longshore current
observations in Southern California, Technical Memorandum
No. 13. 1950.

2. Coast and Geodetic Survey, Smooth Sheet No. H-296. 1851.

3. King, C. A. M. Beaches and Coasts. Edward Arnold, Ltd.
1959.

4. Shepard, F. P., K. 0. Emery, and E. C. LaFond. Rip currents
a process of geological importance. The Journal of Geology,
v. 49, no. 4, 1941: 337-369.

5. Shepard, F. P., and D. L. Inman. Nearshore water circula-
tion related to bottom topography and wave refraction.
Transactions, American Geophysical Union, v. 31, no. 2, 1950:
196-212.

APPENDIX

EXPERIMENTATION WITH VARIOUS TYPES OF FLOATS

Before the circulation studies began, the writers experi-
mented extensively with a variety of types of floats in order
to find a design that would best satisfy the requirements of
the aerial survey procedure that was selected. Some of the basic
requirements were that the floats had to reflect the surface
water motion as closely as possible, be visible from an aircraft,
and show up well in the color photographs against both a dark-
blue background outside the surf zone and a white background
in the surf zone. As a result, the floats had to be large and
brightly colored. The various kinds of floats that were tested,
along with their advantages and disadvantages, are outlined
below:

I. Wooden boards:

A. Sheets of wood: Sheets of wood veneer (one-
eighth inch) were cut into two -foot squares and
painted red, orange, and silver.

1. Advantages: Worked well outside the surf
zone and were easily visible from an aircraft
at 1000 feet elevation.

2. Disadvantages: Upon entering the surf zone the
boards either planed or turned end-over -end; they
did not withstand the beating of a strong surf as
they were broken apart.
B. Redwood planks: Inch-thick redwood planks were
cut into pieces one foot by three feet and painted red.

1. Advantages: Worked well outside the surf
zone, did not break up in the surf, and were
recoverable.

2. Disadvantages: Planed over the surf or
turned end -over -end, were hard to see from
the air, and were not easy to detect in the
photographs .

II. Cardboard sheets: Sheets of heavy cardboard were cut into
two-foot squares and painted red, orange, and silver. It was
hoped that the paint would not only improve their visibility,
but also improve their water resistance.

A. Advantages: Worked well outside the surf zone
and were easily visible from the air at 1000 feet.

B. Disadvantages: Rolled up or were torn apart in
the surf zone; paint did not improve their durability
as much as was desired.

III. Weather balloons: A weather balloon filled with water
was found effectively to form a unit of water which responds
exactly to surface water motions while offering no sail area
to the wind. Fluorescent dye was placed inside each balloon
prior to filling to indicate leaks or position if the balloon
was destroyed. The balloons proved very difficult to handle
when full. The best system devised for filling and launch-
ing the balloons was to fill each in a 35-gallon trash can
lined with kraft paper to avoid puncturing on the rough
surfaces of the can, and to throw the can and balloon over
the side (Fig. 23). The bottom of the can took the brunt
of the impact and was retrieved by an attached line after
the balloon floated free. When brightly painted the
balloons were readily visible from the air. Prior to fill-
ing with water they were inflated with air and spray-
painted various colors, although the results were incon-
clusive as to which color was best. The colors used were
red, orange, yellow, metallic copper, metallic brass,
metallic silver, metallic gold, and fluorescent red.
Standard 300 -gram weather balloons were used in these
experiments and were filled with both fresh water and salt
water.

Figure 23. Launching of a Weather Balloon Float .

A. Fresh-water filled: Balloons were filled completely
with fresh water.

1. Filled until distended:

a. Advantages: Accurately represented the surface
current and offered no wind resistance.

b. Disadvantages: Due to the fact that the balloon
was filled until distended, it assumed a spherical
shape so that only a small area, about one foot in
diameter, was visible from the air. The balloon
broke easily upon contact with a rough object and

so required very careful handling. It also broke
easily in the breakers.

2. Filled but not distended:

a. Advantages: Buoyancy of the fresh water
flattened the balloon out so that an area about
two or three feet in diameter was exposed at the
water surface, making the float clearly visible
from the air at 1000 feet (Fig. 24). The balloon
floated awash, unaffected by the wind, and did not
break on contact with a rough surface.

b. Disadvantages: The balloons ruptured in a
ten-foot surf although they worked well in a
three-foot surf.

Figure 24. Fresh-water-filled Weather Balloon Afloat

B. Salt-water filled: A small air space was left in these
balloons to provide buoyancy.

1. Advantages: Since they were not filled to the
stretching point these balloons did not puncture easily
when they came in contact with rough objects.

2. Disadvantages: Since there was no density difference
inside or outside the balloon it assumed different
shapes depending upon the direction of forces acting

on it. The only visible area exposed, about one foot
square, was the air pocket which acted as a low sail
and was thus subjected to slight wind stress,

IV. Innertubes: Water-filled automobile innertubes were tried.

The tubes were painted international yellow, international red,

and silver, all of which were satisfactory.

A. Single tubes: Innertubes were painted and filled with
fresh water.

1. Advantages: The rubber was strong enough to permit
rough handling of the floats in launching and recovery,
and withstood rough treatment in the surf zone. The
tubes floated flush with the water surface so that there
was no wind effect, and apparently conformed well with
the currents. They did not plane in the surf.

2. Disadvantages: The single tube was difficult
to see in aerial photographs due to its floating flush
with the water surface.
B. Double tubes: Two innertubes were lashed together,
one on top of the other, with the top tube filled with air
and painted for visibility and the bottom one filled with
fresh water (Fig. 4). Dye-marker packets were attached to
each double inner tube float. The floats were observed to
leave a well-defined trail of dye which not only gave an
excellent means of locating the float in an aerial photo-
graph, but also gave an indication of the track of the float
with respect to the surface water motion.

1. Advantages: Although heavy and bulky, these
floats could be handled without special launching
schemes and were recoverable. They were always visible
due to the air-filled tube on top. They were durable
and withstood surf action very well with no planing
effect.

2. Disadvantages: Exposure of the upper float to
the air introduced some wind effect, but presumably
inertia of the float largely negated this.

V. Dye packets: Single dye packets tied to small pieces of wood
for buoyancy were also tested as individual floats.

A. Advantages: Dye was easy to see in the aerial
photographs seaward of the surf zone and the packets
were easy to handle.

B. Disadvantages: Dye was difficult to locate in
the aerial photographs once the markers reached the
surf zone due to the intense turbulent mixing.

Double inner tube floats were selected for use in this study
because of their durability and their visibility from the air,
both in the surf zone and to seaward of it. Although balloons
were not used, they are believed to have good potential as a
float for tracing surface currents in open water seaward of the
surf zone and further experimentation is recommended.

lhesB8034

Observation of the nearshore water circu

3 2768 002 07245 6

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