Rip Currents: A Process of Geological Importance

Author(s): F. P. Shepard, K. O. Emery and E. C. La Fond

Source: The Journal of Geology , May - Jun., 1941, Vol. 49, No. 4 (May - Jun., 1941),
pp. 337-369
Published by: The University of Chicago Press

Stable URL: https://www.jstor.org/stable/30063339

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VOLUME XLIX NUMBER 4

THE

JOURNAL OF GEOLOGY
May-June 1941

RIP CURRENTS: A PROCESS OF GEOLOGICAL

IMPORTANCE'

F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

University of Illinois and Scripps Institution

ABSTRACT

Rip currents are defined as seaward-moving streaks of water which return the wa
carried landward by waves. These currents are believed to be an almost universal ac
companiment of large waves breaking on an exposed coast.
Unlike the largely hypothetical undertow, rip currents flow principally at and near
the surface. They attain velocities up to at least 2 miles an hour and extend on occa-
sions for 1,000 feet or more from the shore. Geologically the currents are of importance,
since they carry a suspended load of fine sediment out from the shore. Small channels
in the sand are produced by the flow in the near-shore portions of the rip currents. The
development and changes of these channels have been investigated. The strong outflow
of the current and the presence of the channels along the bottom constitute a serious
danger to inexperienced swimmers.

INTRODUCTION

The observation that a stick thrown into the breakers from a

beach will usually return to the shore demonstrates that surface
water is carried landward in the breaker zone in the absence of an
offshore wind. Floating objects seaward of the breaker zone are
also observed to move toward land but at a slower rate. Thus
objects thrown into the sea at the end of the I,ooo-foot Scripps pier
(Fig. i) almost invariably move shoreward, although their move-
'Contribution from the Scripps Institution of Oceanography. New Series No. 126.
Assistance in the preparation of these materials was furnished by the personnel of the
Works Progress Administration, Official Projects No. 665-07-3-141 and No. 65-1-07-
2317.

337

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338 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

ment usually has a longshore component. A return flow required

by this landward movement has led to the idea that the water re-
turns beneath the surface. The popular idea of violent undertow
which is said to pull bathers beneath the surface was first challenged
by W. M. Davis," and a considerable discussion of the subject en-
sued. Conflicting evi-
dence was presented,
centered chiefly on
whether bathers were
dragged out from the
beaches or whether the
waves had as much for-
ward as backward
transport.3
After discussing with
lifeguards the causes for
surf rescues and making

some preliminary ob-

servations along the

beaches, Shepard4 called

attention to definite evi-
FIG. i.-Showing the area in which most of the
dence of outward drag of
investigations were undertaken. Nature of sea floor
shown by contours, swimmers in relatively
narrow seaward-moving

belts of water. These lanes of agitated water extending out at right

angles to the beach are well known to lifeguards and to experienced
swimmers but seem to have largely escaped the notice of scientists.
They are known as "rip tides" or "sea pusses," but the name "rip
current" was proposed as being more appropriate, and the term "rip"
might also be used as an abbreviation, which removes the unfortu-
nate tidal connotation of the popular term "rip tide."
2 "The 'Undertow,' " Science, Vol. LXII (new ser., 1925), pp. 206-8.
3 Walter C. Jones, "The Undertow Myth," Science, Vol. LXI (new ser., 1925),
p. 444; Wallace Craig, "The 'Undertow,' " Science, Vol. LXII (new ser., 1925), p. 30;
I. Brant, "The 'Undertow,' " Science, Vol. LXII (new ser., 1925), pp. 30-31; Davis,
"The Undertow Myth," Science, Vol. LXII (new ser., 1925), p. 33.
4 "Undertow, Rip Tides or Rip Currents," Science, Vol. LXXXIV (new ser., 1936),
pp. 181-82.

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 RIP CURRENTS 339

These rip currents were described as moving outward both at the

surface and underneath the surface, so that they are something
quite distinct from the supposed undertow. Subsequently, O. F.
Evans,5 on the basis of observations with colored water in lakes,
showed that with an onshore breeze water may move shoreward at
the surface and return underneath, thus suggesting the reality of
undertow under conditions of an onshore wind. Evans' suggestion
that such subsurface-return currents, while inconsequential in his
experiments, might attain dangerous proportions, led to observa-
tions by Shepard and La Fond6 which showed that at the Scripps
Institution pier during strong onshore winds no appreciable under-
tow could be discerned but that the water moved principally
alongshore both at the surface and near the bottom. Observations
made by the writers subsequently have failed to produce any con-
firmation of the idea of dangerous undertow. While slow outward-
moving subsurface currents may exist, the chief seaward return of
water moved in by the waves seems to be in the form of rip currents.7
Recent investigations of the rips have shown that they are, in all
probability, a phenomenon associated with all coasts where surf

comes in more or less directly onto the shore. Surveys initiated

recently by Emery have shown the erosional effects of these currents
in shallow water. All the authors have combined in surveys to
determine the nature of water movement in the rip currents. Also
records compiled over several years by La Fond, Shepard, and
others have been analyzed to find the relation of rip currents to
other phenomena. The present report is an attempt to summarize
the information obtained by these studies.

s "Undertow," Science, Vol. LXXXVIII (new ser., 1938), pp. 279-81.

6 "Undertow," Science, Vol. LXXXIX (new ser., 1939), pp. 78-79.

7 In an article to appear shortly in Science, O. F. Evans refers to undertow as a sub-
surface drift in various directions both landward and away from the land but in a direc-
tion opposite to the drift at the surface and existing at some distance from the shore.
The existence of an undertow of this nature is not denied by the present writers. On
the contrary, evidence could be supplied to show such counterposed drifts in the La Jolla
district, although for the most part in directions alongshore. The writers have used
undertow in the popular sense of a dangerous, subsurface current moving out from the
shore under the oncoming waves.

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340 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

GENERAL DESCRIPTION OF RIP CURRENTS

SURFACE MANIFESTATIONS

Rip currents, like double rainbows or sun dogs, are phenomena

which are not seen by casual observers until they have been pointed
out by someone familiar with them. However, after their charac-
teristics are known, they can be easily recognized along shorelines
where large breakers form. Rips are most readily identified by an

FIG. 2.-Showing a rip current extending out from the beach at Coronado, Cali-
fornia. Note portions of other less pronounced rips on either side. Official United States
Navy photograph.

observer looking down from an elevation, but careful scrutiny from

the beach will also serve to show their presence. The following are
their chief characteristics:
I. Sediment-laden water.-Rip currents located off a shore con-
taining relatively fine sand or silt can be identified by a brown streak
of water which protrudes beyond the breaker zone. This brown
color is due to the sediment suspended in the water by the breaking
waves and dragged seaward in the rip current. These brown streaks
can be seen to move seaward spreading out as they go. New belts
join them from time to time (Fig. 2).

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 RIP CURRENTS 341

2. Green water.-Shoreward of the main breaker belt one can

often see a zone where the water has a distinctly greenish appear-
ance. This color is due to the greater depth generally found along
the course of a rip current. As will be shown later, the trend of these
inner channels is likely to be diagonal to the shoreline.
3. Foam belts.-Where the bottom is rocky, so that the rip cur-
rents do not contain brown water, their presence may be observed
by bands of foam which project seaward from the breaker belt
(Fig. 3). Foam is also associated with the brown water and is often
confined to the outer rim of an advancing rip.

FIG. 3.-Showing a rip current indicated by foam extending out from a rocky point
without any beach.

4. Agitated water.-In a rip current the surface of the water is

usually very much agitated, resembling in appearance a tidal cur-
rent moving against oncoming waves. TIhis agitation cannot only
be seen but can be heard and felt both by a swimmer and by an oars-
man rowing through the outer tongue of a rip current. Beyond
the breakers the irregular waves in the rip sometimes have the
appearance of whitecaps. According to reports of lifeguards, the
water in a rip may be so choppy as to make breathing very difficult.
5. Gap in advancing breakers.-While waves do break in the rip-
current zones, they often break much nearer shore than on either
side, so that it is a common thing to see an advancing breaker with
a gap at the point where the rip current is moving seaward (Fig. 4).
However, a gap in the breakers does not necessarily show the pres-
ence of a rip current, since it may be due to a greater depth of the
water along a belt where no rip current is operating or to inter-
ference of waves.

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342 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

6. Seaward movement of floating objects.-When one is in doubt

concerning the reality of a supposed rip current, drifting objects
will often serve to test the case. For example, the existence of a rip
was established where a stick floated out from the base of a 20-foot
rock cliff directly against a storm wind and against large waves.
Possibly such occurrences are common but have not been observed

FIG. 4.-Showing the gap in an advancing wave crest where a rip current is moving
seaward.

because no one would suspect the presence of an outward-moving

surface current at such a place. The drift of objects in typical rip
currents will be discussed below.

WIDTH AND OUTWARD EXTENT

The maximum width and outward extent of rip currents cannot

be determined until more data are available. Estimates made at
Venice, California, by Mr. C. P. L. Nicholls8 limited the outward
projections to about mile. At La Jolla a convenient index is pro-
vided by the I,ooo-foot Scripps pier. It is not unusual for a rip

8 Supervisor of aquatics of the Los Angeles department of playground and recreation.

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 RIP CURRENTS 343

to extend beyond the end of this pier, and on some occasions they
appeared to extend more than twice that distance from shore. From
the cliffs along the San Simeon-Carmel coast highway, observations
of rip currents suggested an outward extent of about a half-mile,
under what were probably typical wave conditions. Since average
wave conditions at La Jolla are accompanied by relatively short rip
currents and large waves by greatly extended rips, it might be sup-
posed that considerably greater lengths would accompany excep-
tionally large waves in this exposed San Simeon-Carmel area.
Rip currents are narrow near shore, commonly not over 5-100oo
feet across, but they widen farther from shore. Off La Jolla, beyond
the breakers, they have been measured as being from ioo to 440
feet across. In the widest of these rips the current was especially
concentrated in a belt i80 feet across. The rip currents observed
from the San Simeon-Carmel highway appeared to have greater
width compared to length than those seen at La Jolla or off other
sandy beaches. The former were probably over 1,000 feet across.
It was noted that all the currents are sufficiently narrow near shore
so that swimming out of them should not be difficult, but a longer
swim would be required beyond the breaker belt. However, currents
in the outer zones are not so strong.

RELATION TO COASTAL AND SHORELINE CONFIGURATION

The statements from lifeguards regarding rip currents refer

exclusively to areas where there are long, relatively straight beaches
such as the one south of Scripps Institution (Fig. i). However,
investigation has shown that the currents are by no means confined
to shores of this type. As far as could be determined, rip currents are
found off all types of coasts where waves pound in more or less
directly against the shore.
Relation to indentations.-Rip currents are found in many cases
off the center of small beach indentations. In rare cases these in-
dentations are reported to be the result of drains entering the sea
at that point, but in general they are due to natural causes and
processes which allow them to shift in position. Rips are also ob-
served in larger coastal indentations. In the cases where there is no
prevailing longshore current, they generally move out at the center

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344 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

of the indentation, or they form on the downcurrent side of the bay

where strong drifts exist.
At La Jolla Cove (Fig. i) large waves do not ordinarily occur, but
under some conditions resonance or some type of interference causes
the building-up of a series of large waves, and these waves sweep
into the cove with great force. After a series of waves of this type,
a current is observed moving out from the rocky point which lies
east of the cove.
Relation to rocky coasts.-An example has been referred to pre-
viously (Fig. 3) where a rip developed off a rocky coast where there
was no beach. Other cases were observed along the mountainous
coast of the San Simeon-Carmel highway. There are also a few rips
off the rocky cliffs of Point Loma near San Diego. However, it is
likely that rips are less common on rocky coasts than off sand
beaches. Examination of the two types on the same day revealed
fewer rips off the rocks. On the other hand, it should be borne in
mind that it is not easy to recognize the rips where there is no bot-
tom sediment to be thrown into suspension to indicate the outward
movement beyond the breakers.
Relation to artificial structures.-Rip currents are frequently found
running seaward along piers and groins. The slight deflecting in-
fluence of a pier evidently plays an important part in localizing
these currents. At many beaches guards have posted warnings for
bathers to avoid the vicinity of these structures largely because of
the dangerous rips.

NATURE OF CURRENTS

Surface movement.-Investigations at La Jolla have given some

information concerning the movement of water particles in what
appear to be typical rip currents. Conversations with lifeguards
have confirmed these observations. Figure 5 gives what is believed
to represent typical movement of water in one of these rip currents.
It can be seen to consist of three main parts, which have been called
the "feeders," the "neck," and the "head." Just outside the beach
there is a flow of water nearly parallel to the shore. An object drift-
ing in this feeder current is carried in a path diagonal to the shore
into the main outward-flowing current, or the neck, which moves
essentially at right angles to the general coastal trend. Ordinarily

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 RIP CURRENTS 345

the water coming into the neck enters from a feeder on either
although one current may flow more swiftly than the other. In
these feeders, currents have been observed to move at about 2 miles
an hour, although higher velocities may exist during storm waves.
Water enters the neck
not only from the feed-
ers but also to some ex-
tent from the sides far-
ther out.

In the inner portion

of the neck, shoreward
of the point where the
large waves break, cur-
rent velocities are high,
but the current becomes
more intermittent in
character because of the
effects of the breaking
waves. With each ad-
vancing wave the out-
moving current is
checked or may be
given a strong side mo-

tion. After the crest has

passed, the outward
FIG. 5.-Diagrammatic sketch of a rip current
flow is resumed. In the
showing its component parts and the common as-
outer portion of the sociation of currents with it.
neck the counteracting
effect of large breakers is particularly noticeable. Frequently the
forward surge of the waves completely interferes with the seaward
passage of a floating object which simply moves back and forth
until it is thrown out of the rip by a lateral movement of the
current. As will be shown subsequently, this is the most dangerous
position for a swimmer, particularly if he is trying to get out of
the neck on the side from which the main current is moving (Fig. 5).
According to Mr. T. V. Warren,9 the currents may enter the neck
9 A lieutenant in the Los Angeles County Lifeguards, who has had twenty years of
experience with rip currents.

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346 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

from both sides so that a floating object may be held within the
neck until a pause in the onslaught of the breakers allows it to move
outward. Also, on two occasions it was observed that, with small,

FIG. 6.-Showing the direction and speed of floating objects observed in rip cur-
rents south of the Scripps Institution pier. Shows also the contours of the bottom
determined subsequently from the same area and during times of strong rip currents.
The near-shore channel was surveyed on the day subsequent to the survey of the outer
portion. Dotted line shows motion of a float submerged 2 feet below the surface.
Time refers to clock time for the surface objects and to minutes from start in the case
of the float. Contour interval i foot. Soundings are reduced to mean lower low water.

short-period waves accompanying an onshore wind, floating objects

moved through the breakers without much delay.

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 RIP CURRENTS 347

A floating object which is thrown out of the rip at the neck moves
shoreward and then is caught by one of the feeders and carried sea-
ward once more. In moving seaward the path weaves back and
forth under the influence of the oncoming breakers. A piece of kelp
may describe several circles before being carried up onto the beach
or carried seaward through the neck. Lieutenant Warren informed
the writers of cases where lifeguards in making a rescue had also
been forced into the same circular path, so that they went through
the rip several times before making their escape.
In the outer portion or head of the rip current the water spreads
out, and the rip becomes much wider (Fig. 5). Observations of the
surface currents in the head were made from rowboats in the vicinity
of the Scripps Institution pier. Paths of floating objects, both on
the sides of rip currents and within them were plotted (Fig. 6).
It will be noted that the general direction of movement was out-
ward, but ample evidence of longshore movement away from the
rip was seen on either side of the main current. At a distance of
about 900 feet from the shore an outward movement of mile an
hour was observed. Other objects moved outward respectively at
and 5 of a mile an hour.
To find the lateral extent of the rip, three objects were set adrift
over a width of 350 feet along a line about 900 feet from the shore.
Only one of these was thought to be definitely in a rip current, but all
of them drifted outward, showing that the effect of the rip was much
more widespread than was supposed. Another object set out on the
south side of the head of a rip current was observed to move south-
ward at first. Returning to the locality after observing other floating
objects, we were unable to find the south-drifting float. Later it was
picked up on the beach about 400 feet south of where it had been
set out. Presumably it had described an arc and turned toward
shore.

Along the side of an advancing head, eddies are often observable

(Fig. 7). These whirls turn to the right on the right side of the rip
and to the left on the left side. A turning movement is also ex-
perienced in a rowboat allowed to drift in the advancing end of a
rip current.

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348 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

Subsurface movement.-The movement of currents beneath the

surface has not yet been completely explored either in rip currents
or in breaker zones on either side. According to Lieutenant Warren,
the flow may be stronger either at the surface or underneath. It is

FIG. 7.-Showing the curling edges of an advancing rip current. Note that a new
addition to the current is moving out from the shore. Official United States Navy
photograph.

his observation that where the neck is very narrow the flow under-
neath is stronger than at the surface and where the rip is spread
over considerable area the flow decreases with depth. The presence
of gullies in the sand (to be described later) underneath the feeders
and the neck testify to swift movement where the rip is confined.
Also observations from the Scripps piero showed that rip currents,

o0 Shepard and La Fond, "Undertow," Science, Vol. LXXXIX (new ser., 1939), PP.
78-79.

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 RIP CURRENTS 349

evident at the surface, were also operating along the bottom in a

depth of 5 feet of water. In another rip the outward movement
along the bottom was found to be limited to a depth of 6 feet, al-
though the outward movement at the surface continued more than
twice as far from the shore to a point where the water was 16 feet
deep. At another time current measurements were being made at
the end of the Scripps pier in water about 18 feet deep, when a rip
current was seen to move out over the spot where the measurements
were in progress. Since the current meter near the bottom recorded
continuous movement parallel to the coast while the current at the
surface was moving seaward, it was evident that the rip had passed
over the bottom water. Further indication that the currents do not
reach great depth is derived from the fact that soundings show the
absence of important channels along the bottom out beyond the
neck of the rip current (Fig. 6).
The observation that floating objects are ordinarily carried out
the neck of a rip current only to the point where the larger waves
break, while the rip itself is seen to extend well beyond the breakers,
indicates that there must be a greater transfer of water outward
beneath the surface than at the surface. To test the nature of this
subsurface current, large crossed vanes were suspended from a float
to a depth of about 2 feet from the surface. A flag was placed on
the float so that its motion would be observed. Since the movement
would be more dependent on the pressure against the vanes than
against the small float, the motion would be indicative of the sub-
surface current. The apparatus was put into a feeder channel of a
rip current. After some lateral movements caused by oncoming
breakers, it moved with a velocity of nearly I mile an hour out
through the breaker zone and continued seaward beyond (Fig. 6).
This movement was in contrast to that of kelp, which is often held
up in the breakers as explained above.
On the other hand, it should not be supposed that this greater
velocity just below the surface produces any appreciable tendency
to draw a swimmer beneath the surface. Observations by the
writers are supported by information from lifeguards showing that,
while a swimmer's body may be tilted by the subsurface flow, there
is little if any tendency to be pulled under the surface.

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350 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

CHANNELS IN UNDERLYING SAND

The existence of channels in the sand along the path of a rip cur-
rent has long been observed by lifeguards. Their presence was re-
ported by all guards with whom the writers have talked. Surveys
of these channels were initiated after a chance discovery of one
several feet deep off the Scripps Institution. Several surveys of that
channel were made and subsequently of other channels along a

stretch of beach extending for of a mile south of the Scripps

Institution.

Methods of survey.-Since the inner portion of a rip current is

within the breaker belt, surveying of the bottom is very difficult.
Some attempts were made to take soundings from a rowboat within
the breaker belt, but this proved to be both difficult and dangerous,
and the results were somewhat uncertain, particularly since it was
found to be hard to concentrate on the work sufficiently to judge
average wave levels.
A more successful method for the breaker zone consisted of
wading and swimming out along prepared ranges on shore to cross
ranges along the Scripps pier. The depth was determined by ob-
serving the water level on the surveyor's body. Help given the
project by Dr. Roger Revelle was particularly valuable in this con-
nection, since Dr. Revelle has a height of 6 feet, 4 inches.
Seaward of the principal breakers and to some extent within the
belt, wire soundings were made from rowboats and depths deter-
mined with the usual sounding sheave. In the wading-swimming
surveys, periods of low tide were chosen, while periods of high tide
were picked for the boat surveys so as to have the least interference
from breakers. The tide corrections were made after the surveys.
Probably errors of as much as foot were due to the uncertainty
as to mean level between the trough and crest of waves. Also, in
studying the charts illustrating these gullies, it should be borne in
mind that the outer portions were ordinarily not surveyed on the
same day as the inner channels. Possible changes of depth may
have intervened.
Relation to breaker zone.-The soundings and contours in Figures
6, 8, 9, and o10 will show the nature of the channels which are related
to rip currents. Figure 8 indicates the possibility that the channel
may continue out beyond the breakers. However, the sounding

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 RIP CURRENTS 351

data were scarce outside, and there is no definite proof of the exist-
ence of the outer channel. Furthermore, in a more extensive survey
shown in Figure 6, the outer portion, which was in an intensive rip

DISTANCE SOUTH OF SCRIPPS PIER IN FEET

FIG. 8.-A survey of one of the channels associated with a rip current. Note the
curving feeder channel extending into the neck. The inset shows a resurvey of a por-
tion of the same area made after an interval of i2 days. Note that in the inset the
feeder channel enters from the north rather than from the south. In the main diagram
the two sets of soundings along the range 600 feet from the pier were made on successive
days, with those on the left made first. The symbol T indicates the presence of terrace
material on the channel floor. The symbol C indicates the presence of loose cobbles.
Soundings are reduced to mean lower low water.

head, had no clear indication of a channel. Therefore it seems likely

that the channels are features largely confined to the breaker zone.

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352 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

Character of the channels.-Some of the channels are developed

only in the neck of the rip current, but most of them have one arm
which is located on the side of the feeder current having the greater
velocity. These feeder channels are found so near the shoreline at
low tide that a child may step out from the beach and fall directly
into water beyond its depth. In Figure 8 it will be seen that one of
these channels ran parallel to the shore for 200 feet. Seaward of the
channel the water was very shallow above a bar. The term "ball and
low"" might be applied to the bar and adjacent channel. During
the survey a very conspicuous current was moving along this chan-
nel toward the outer trough. Also there was a weaker current mov-
ing south along the shore on the other side, but no feeder channel
appeared on that side.
In Figure 8 it will be seen that the steeper side of the inner channel
is toward the breakers. It happens that this is the inside of the
curve which is contrary to the usual steep side in a land gully. How-
ever, in another case (Fig. 9), the shoreward side of the channel
proved to be the steeper, so that no rule can be established regarding
this steepening.

The channel in Figure 9 is cut to a depth of about 3 feet below

its surroundings with the deepest entrenchment near the point
where the channel assumes an outward direction. It will be seen
that beyond this point there is a gradual decrease of the depth of
the channel below its surroundings.
The floor of most of the channels was found to be decidedly ir-
regular, so that it was difficult to walk along them for this reason as
well as because of the strong current. In some of the channels there
was a series of disconnected basins (Fig. 9) having a depth of half a
foot or more below their sills.
Hard bottom.-The bottom of the inner channels off the Scripps
Institution was found to consist in part of the same Quaternary
terrace alluvium that is found in the low cliffs inside the area in-
vestigated. Since the channel shown in Figure 8 headed off a gully
in the land cliffs, it was thought possible that it might be following
an old submerged stream course that had been re-excavated by the
" D. W. Johnson, Shore Processes and Shoreline Development (New York: John
Wiley & Sons, 1919), p. 486.

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 VERTICALx2

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354 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

rip current. However, probing of the sand showed that the rip had
not excavated below the level of the wave-cut terrace (Fig. 10) and
that the relation of the channel to the land gully was only coinci-
dental. The presence of fresh animal boringsI2 in the terrace mater-
ial of the channel bottom confirmed the absence of any excavation
by the rip current into the underlying material.

FIG. II.-Showing the channels appearing at a very low tide on Mission Beach,
where rip currents had been operating. Photo by Captain C. W. Hardy of the San Diego
lifeguards.

Shifting of the channels.-There are some places where permanent

rip currents are said to exist, and, therefore, more or less permanent
channels can be expected. These rips are controlled by some arti-
ficial structure such as a pier, a groin, or in some cases the entry of a
drain. At Scripps Institution the rip currents were found to vary
in position considerably. Also successive surveys showed relatively
rapid changes in the position of the accompanying channels.
The changes in such a channel are illustrated best in the cross
sections of Figure o10. Another illustration of such a change is seen
12 The freshness of the borings is indicated by the presence of live Exosphaeroma
amplicauda within the holes.

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 RIP CURRENTS 355

in Figure 8 in the inset showing a resurvey of a portion of the area

of Figure 8. It will be seen that here the feeder channel is entering
from the north, while in the main portion of Figure 8, based on a
survey made i2 days earlier, it was entering from the south. It is
notable that the main feeder current had also changed direction,
showing the clear relationship between the currents and the chan-
nels. Three days after the survey of Figure 8 inset, the inner channel
had completely disappeared and no appreciable rip could be ob-
served in the locality.
By repeating the surveys some idea of the history of development
and of the changes in the channels was obtained. It seems likely
that in the earlier stages of channel development the bottom is
relatively smooth and slopes outward more or less continuously as
in Figure 8. Later, when the currents are less pronounced, bars
develop along the course of the channels, and the outward con-
tinuity is lost. This stage is illustrated in Figure q. Small, some-
what circular holes in the beach are the only remnants of the channel
at a later stage. These are sometimes exposed at low tide (Fig. i1).

RELATIONSHIP TO OTHER PHENOMENA

GENERAL

Daily records have been kept for several years at the Scripps
Institution showing the character of the waves and the nature of
the currents. During one year the height of the sand under the pier
was measured daily at forty-five stations, and during five years it
has been measured at periods varying from semi-weekly to weekly.'3
Also for many years the pier has been a tidal station of the Coast
and Geodetic Survey, and other records, such as water temperature,
wind velocity, and rainfall, have been recorded. In addition, since
November, 1938, the height of the sand at various points south of
the Scripps Institution pier has been measured at intervals varying
from daily to weekly. During more than one year, rip-current in-
tensity was evaluated daily along the beach south of the Institution.
While the evaluation is probably none too exact, it seems likely that
important relationships can be drawn between the values given to
'3 Shepard and La Fond, "Sand Movements along the Scripps Institution Pier,"
Amer. Jour. Sci., Vol. CCXXXVIII (1940), pp. 272-85.

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356 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

rip-current intensities and other conditions for which a numerical

value can be given.

RELATION TO WAVES

Records at La Jolla.-Undoubted relation between int

rip currents and height of waves has been found from t
maintained at the Scripps Institution (Figs. i2 and 13). It
shown by daily nota-
tions made for approxi-
mately a year in con-
nection with the sand

survey of the Scripps

pier. These records,
however, were made by
various workers and are

subject to a personal
equation that makes
them uncertain in
value. A more system-
atic record was started

by the writers in Febru-

ary, 1940, and is still
in progress. The daily
measurements of wave

heights obtained on a
wave machine4 during
FIG. 12.-Showing the relation of rip-current in- this period are com-

tensity to wave height, longshore currents, pared with the rip-cur-

wind, and
sand height based on observations at the Scripps
rentIn-
evaluation in Fig-
stitution during 1938. The data were obtained daily
ure I It Will be noted
but are plotted as weekly averages.
that the rip currents in-
crease with every period of large waves, so that the largest values
for rips correspond with the largest waves.
Besides the height of waves, the period between crests and the
smoothness of the wave records were examined to see if these charac-

teristics had any bearing on rip-current development. It was found

14 Ibid., P1. I.

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 RIP CURRENTS 357

that in general small waves were m

form and had slightly longer peri
large waves. As a result, the sm
would be accompanied by larger r
to determine whether the smooth
were important in development o
currents did not follow wave heig

FEB. MARCH APRIL MAY JUNE

WAVES & TIDE RIPCURRENTS

FIG. I3.-Showing the relation of rip-current intensity to wave height and tidal
ranges at the Scripps Institution in 1940. These observations were made daily and are
plotted as such. The high points on the tidal curve indicate spring tides and the low
points neap tides.

of this comparison showed that there is no pronounced relation

between these wave characteristics and the rip evaluation.
Wave and rip variations along different coasts.-Along the coast
south of the Scripps Institution (Fig. i) there is a distinct variation
in the size of the waves. In the vicinity of the Institution the waves
are relatively large, but to the south inside the head of the La Jolla
submarine canyon the waves are almost invariably smaller. Still
farther south, despite the protection of La Jolla Point, the wave
size increases. Rips are largest somewhat to the south of the Scripps
Institution but are negligible inshore from the submarine canyon.

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358 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

South of the canyon rip currents of large size have been observed
during periods of particularly large waves. The same relationship
between the smaller waves and absence of rips landward of a sub-
marine canyon is shown in air photos of the shore at the head of
Monterey Canyon.
Observations along the outer exposed coast south of Carmel,
California, were made on the same day as observations along the
somewhat protected but otherwise similar coast of Santa Monica
Mountains. Along the former there were large breakers, but along
the latter only small waves were rolling in. Off the mountainous
coast with the large waves there were impressive rip currents, where-
as no appreciable rips could be distinguished along the mountainous
coast with small waves.

Similarly, along the east coast of the United States t

phenomena appear to be uncommon in the calm embayments of the
Maine coast.'s However, there are reports of these features (called
"sea pusses") along the unprotected shorelines of New Jersey.
Relation to wave fronts.-The typical form of a wave front coming
in to the shore south of Scripps Institution is shown in Figure i. It
will be noted that the wave front is relatively straight and the
coast is broadly indented. It has been found that the rip currents
are ordinarily greatest at the middle of the indentation. The waves
come in most directly onto the coast in this section, while they are
somewhat diagonal to the shoreline on either side. A similar relation
to indentations of smaller scale has been referred to above. Possibly
also the usual absence of rip currents along the Santa Monica
Mountain coast and their presence at Venice and Playa del Rey
farther south may be due to the fact that the waves come in more
diagonally along the Santa Monica Mountains than they do farther
south.

RELATION TO CURRENTS

The relation of the rips to the currents measured at the end of the
pier is shown in Figure 12. The weekly averages which are shown in
the 1938 series of measurements decidedly indicate that the rip
is On the other hand, tide rips, which are actually due to the tide, are very pro-
nounced along many of the narrow passages of the Maine coast. Rip currents also
exist on the .open coast. One was observed along a rocky promontory at Deer Isle,
Maine, during a storm.

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 RIP CURRENTS 359

currents vary with the direction and intensity of the currents a

pier end. The periods of currents flowing to the south are in ge
those of strong rips, whereas the periods of northerly currents cor-
respond with weak rips. This relationship is less impressive where
the more exact rip-current data of 1940 are compared with the daily
record of the currents at the pier end. However, even in this record,
the change from north-flowing to south-flowing current shows in
general an increase in the rip currents. In a number of instances the
rips show a growth when a north current slackened, in spite of the
fact that the waves had subsided from their peak. Examples of this
relationship are to be seen on March o10 and May 7.
On the other hand, the relation of the current to the rips in both
records is decidedly less constant than is the relation of the wave
height to the rips. Thus we find large waves and large rip currents on
April 26, despite a decided northward current. On May 12 the re-
lationship of rips to current direction is also reversed from the
normal, but in this case the relatively large waves had no rips
associated with them, so that neither influence appears to have
played a part.

An unfortunate feature of the current record preve

pletely satisfactory correlation with the rip-tide evaluations. The
current meter at the end of the pier has to be taken out of the water
during excessively large waves in order to preserve it from damage.
Therefore, the nature of the currents on days when the rips are
especially large is not well known. Future direct observations may
remedy this condition.

RELATION TO WINDS

Indirectly the rip currents are related to winds, since winds

produce most of the waves. However, along the coast of southern
California, the large waves appear to come largely from storm
centers at a considerable distance from the coast. Frequently large
waves roll in accompanied by no appreciable wind. Also, the period
of relatively strong winds at La Jolla extends somewhat longer into
the late spring than does the period of large waves. The rip currents
subside with the large waves, and strong rips do not occur on the
windy days of late spring and early summer when the waves are
small.

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360 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

On the other hand, the rip currents may be distinctly related

the direction of the wind. Rip currents appear to be small when
the wind is blowing along the coast. A good example of this was
observed near San Simeon, California. To the north there was little
wind blowing, and large rips were in operation; but to the south,
where the land juts out at Point Piedras Blancas, the wind was
blowing violently along the coast, and rip currents were incon-
spicuous. However, on one occasion, rip currents were observed
at La Jolla when there was a strong longshore wind, but in this
case heavy runoff due to a violent rain was piling up water along
the shore and supplementing the effects of the breaking waves.

RELATION TO TIDES

Since the tides produce currents, it might be supposed that the

rips would be stronger at spring tides, when these tidal currents are at
their maximums. However, a plotting of the daily tidal range in the
same curve with the rip-current evaluation shows that the rips have
no evident relation to the spring and neap tides (Fig. I3). The cases
where the wave-height and rip-current curves are at slight variation
are not explicable on the basis of tides.
In one way, however, the tides may have some influence on rip
currents. During low tide the water is sufficiently shallow to con-
centrate the flow of the current within the rip channels, whereas the
deeper water at high tide is less confined by the channels. Observa-
tions at the Scripps Institution give the impression that rip currents
are more pronounced at low tide. On the other hand, in a paper by
C. P. L. Nicholls,6 the statement is made that "it is usual for the
rip tides to disappear at low tide, and it is likely that most rip tides
will occur at high tide." Whether this inconsistency is due to a
difference of locality of observation is uncertain.

RELATION TO CUT AND FILL OF BEACHES

The numerous measurements of sand height along the Scripps

Institution pier"7 and along the beach to the south of the Institution
i6 "Rip Tides and How To Avoid Their Perils," Calif. Beaches Assoc., Vol. I, No. 9

(1936), p. 12.
'7 Shepard and La Fond, "Sand Movements along the Scripps
loc. cit.

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 RIP CURRENTS 361

afford another check on the various influences on rip currents

times of year when the sand is cut low along the inner portion
Scripps pier are also times when the rip currents are strong, while
the times of high sand along the inner pier correspond with times of
weak rips (Fig. 12). However, this correspondence may be ex-
plicable purely on the basis of wave size, since the large waves of
winter produce cutting of the sand and the small waves of summer
produce fills. Furthermore, large waves are accompanied by large
rips, whether or not the beach is cut away. On the other hand, there
is reason to believe that the locations of the principal rip currents
near the Scripps Institution are associated with beach indentations
and thus with sand level at these places. For several years it has
been found that the beach is cut to its lowest level for the year off
the Institution several months later than in a zone about 1,000 feet
to the south. The rip currents also appear to be somewhat larger off
the southern zone at an earlier date and off the Scripps Institution
later.

ORIGIN OF RIP CURRENTS

PREVIOUS SUGGESTIONS

In describing a phenomenon which he called "undertow" but

which was clearly a rip current, M. P. Hite'8 referred to the piling-
up of water along the shore behind a submerged bar and to the re-
turn of this water along channels. Nicholls9 attributed rip tides to
any features which "break up this littoral current resulting in vari-
ance in the ocean bottom." He also states: "Rip tides usually are
caused .... by converging currents." These opinions are upheld
by the present investigation.

INADEQUACY OF UNDERTOW

The common landward movement of floating objects20 and the

rapid movement in that direction in the breakers show that the
surface water over most of the area is moving landward. This mo-
i8 "The 'Undertow,' " Science, Vol. LXII (new ser., 1925), pp. 31-33.

9 Op. cit., p. 12.

20 T. Levi-Civita, "ber die Transportgeschwindigkeit einer stationaren Wel-

lenbewegung," Vortrdge aus dem Gebiete der Hydro- und Aerodynamik (Berlin, 1
pp. 85-96.

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362 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

tion may be compensated either by a return seaward in the fo

undertow along the bottom or as a concentrated outward flow of s
face and near-surface water as a rip current. The prevalence of rip
currents shows that the return is largely by the second method. The
reason for the dominance of the rip currents is that undertow is im-
peded by two factors. In the first place, there is more friction along
the bottom so that outward flow is easier at the surface. Second, sea
water is stratified to a certain extent with the heavier water under-
neath. As a result the light surface water cannot displace the water
in this heavier zone. The effect of density stratification, however,
cannot be important in the breaker zone where the water must be
quite thoroughly mixed, but in the head of the rip beyond the
breakers this factor may be very important. The frictional effect is
evidently of considerable importance in the breaker zone, since
near-shore observations along the Scripps pier have shown that
objects in suspension near the bottom of a rip do not move outward
nearly so fast as objects near the surface.

CHANNELS AS A CAUSE OF THE RIP CURRENTS

While there can be little doubt that groins and piers are an im-
portant cause of rip currents, as stated by Nicholls,2" the relation of
channels in the sand as a cause of these currents is not so clear. Rips
have been observed in the absence of channels. Also the longshore
channels or lows are found in the absence of rips. On the other hand,
the channels associated with necks of the rip currents have not been
observed as yet without currents.

Investigations of ball and low along the Scripps Institution pier

have shown that these longshore channels and bars are developed
principally as an accompaniment of longshore currents and may
form under conditions of either small or large waves. During the
development of the phenomenon the chief process appears to be
the excavation of the low rather than the building-up of the ball.
According to Lieutenant Warren,22 such channels grow in length
in the direction of the prevailing longshore currents until they
make contact with a minor gap in the ball. This gap serves as a
means for the water moving alongshore in the low to escape sea-
21 Loc. cit. "" Personal communication.

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 RIP CURRENTS 363

ward, forming a rip current. Thus the low becomes also a feeder
channel of a rip current. This rip current presumably deepens the
initial gap and forms the outward-trending channel beneath the neck
of the rip current. The more pronounced the channel, the more the
rip tends to remain in this place.
It seems likely that the channels are caused primarily by the rip
current, although an initial shallow indentation may localize the rip,
as is shown by the channel on the right of Figure 9, which was not
present a few days earlier in the survey shown on the left of Figure
8. The rapid filling of channels when conditions change, as above,
shows the partial independence of rip currents from channel loca-
tions.
EFFECT OF CURRENTS AND WAVES

It has been shown that, in general, abundance of rip currents

appears increased by southward-flowing currents as measured from
the pier and decreased by northward-flowing currents. The cause
of this relationship can perhaps be determined from an examination
of the coastal configuration (Fig. i). Relatively straight waves
piling onto the broadly curved beach south of the pier should cause
a northward transport at the south end of the beach and a south-
ward transport at the north end. Such opposite movements tend
to pile up water in the middle of the beach, producing rips. Thus
times of southward currents at the pier should tend to correspond
with periods of stronger rips in the middle of the beach.
Since the principal rips are found south of the pier, currents flow-
ing to the south would feed these rip currents, whereas currents
flowing to the north would tend to remove water from the rip area.
Furthermore, a general longshore current flowing to the north does
not meet with any obstruction which might lead toward the piling-
up of water. The relative straightness of the coast continues for
almost ioo miles in that direction. On the other hand, Ir miles
to the south the coast bends outward to form La Jolla Point, which
may cause a piling-up of the water during times of south-flowing
currents. This excess water may escape partly in the form of rip
currents.

When once an outward-flowing current is initiated, it tends to

weaken the breakers locally. The smaller the breakers along the

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364 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

path of the rip current, the faster it can flow. Thus the rip current
accelerates until the head of water onshore, which was due to the
piling-up effect of the two opposing currents, has been sufficiently
lowered so that the rip dies down awaiting the buildup of another
head. This sequence accounts at least in part for the spasmodic
flow of the rips (Fig. 7).
While the local longshore currents play an important part in
the development of rip currents, the close relationship shown be-
tween the rip evaluations and the size of waves shows that the
waves must be the dominant factor. The larger the waves, the more
water is piled onto the shore, and, therefore, the more water has to
return seaward. However, the height and trend of the waves are
largely responsible for the development of the local longshore cur-
rents, so that these two factors are interrelated.

SIGNIFICANCE OF RIP CURRENTS

EFFECT ON BEACHES

Rip currents have both a direct and an indirect effect on beaches.

Their direct effect is the production of channels and holes in the
portion of the beach exposed at lowest tides. The indirect effect
comes from the presence of indentations inshore from the channels
which allow the waves to wash higher on the shore so that they may
have more erosive action inside the rip current.

OUTWARD TRANSPORTATION OF SEDIMENT

Sorting of beach sands.-It has been generally assumed that beach

sands become sorted by the breaking waves stirring the sand and
the backwash carrying the fine material into the undertow current,
which in turn carries the load seaward.23 If undertow is eliminated
as an important process, rip currents are still available as a means
of carrying the fine sediments well beyond the beaches. In this
process the wave-stirred sediment is carried along the feeders to the
neck and thence beyond the effect of breaking waves.
As an example of the operation of this process observations made
near the Scripps Institution may be cited. After the great southern

23 See, e.g., C. R. Longwell, A. Knopf, and R. F. Flint, Textbook of Geology (2d ed.;
New York: John Wiley & Sons, 1939), p. 200.

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 RIP CURRENTS 365

California flood of March, 1938, the beach beneath low cliffs

of the Institution was covered with a thin mantle of mud. After one
or two high tides this material was largely removed from the beach.
Much of it could be seen moving seaward in the large rip currents
which were in operation. Samples taken along the Scripps pier
showed that the mud had not lodged within a thousand feet of the
shore. Similarly, after winter floods had carried large masses of
muddy sediments down the slopes of the Santa Lucia Mountains
onto the beaches, large rip currents were observed carrying the fine
sediment seaward.

A source of offshore sediment.-The evidence given above indi-

cates that rip currents carry fine material seaward, but their limited
outward extent might be supposed to restrict them to near-shore
zones as a transporting agent of sediment. Off southern California
the chief sites of sedimentation are thought to be the offshore basins
and troughs.24 However, since the chief source of sediments for
these basins must be the land, any process which can carry fine
sediment out from the shore must be important. Furthermore, it
should be remembered that there are other types of currents out
beyond the zone where rip currents predominate, and the sus-
pended sediment carried out by the rips must be transferred to a
considerable extent into the offshore currents in which it may be
carried to the various basins before settling-out of suspension.
An example of this ferrying of sediment from shore out into the
main circulation was observed from the "E. W. Scripps" while
anchored off the mouth of the San Dieguito River following the
March, 1938, flood. Muddy-water zones were seen to move out
from the shore in a series of pulsations with marked color boundaries
at the front of each advancing mass. These outward-moving masses
of muddy water closely simulated typical rip currents except that
they had much more suspended sediment. However, the outward
movement was presumably due to the piling-up of water by the
entering river rather than by the waves; but, whatever the cause,
these outward-moving masses were supplying the offshore currents
with large quantities of suspended mud.

24 Roger Revelle and F. P. Shepard, "Sediments off the California Coast," Recent
Marine Sediments (Tulsa: Amer. Assoc. Pet. Geol., i939), pp. 245-82.

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 366 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

POSSIBLE RELATION TO SUBMARINE CANYONS

As yet no one has attempted to show that rip currents are

cause of submarine canyons. However, this oversight may be due
to lack of information concerning these currents. The concentrated
outward flow of the rips on open coasts gives them a distinct ad-
vantage as a canyon cause over other known types of oceanic cur-
rents. Also, their associated gullies may serve to stimulate interest
concerning possible relation between the rips and submarine can-
yons. Furthermore, the absence of large breakers at the heads of
canyons and their presence on either side suggest the possibility
that water piled up by the breakers might return seaward along
the axes of the canyon. There is some evidence that currents do
move toward the canyon heads.25 Also the evidence accumulated
in work on the "E. W. Scripps" has shown that where canyon heads
approach the California coast little sedimentation has taken place
on the canyon floors.
On the other hand, the importance of rip currents either as a
cause of canyons or even as a means of keeping them clear of sedi-
ments appears unlikely for the following reasons:
I. The observations given above suggest that the currents do not extend more
than a few feet below the surface.
2. The density stratification of ocean water must act as a serious impediment
to deep circulation of such a current.
3. The rip currents are not known to extend outward more than about mile,
which is in great contrast to the submarine canyons which extend out for as
much as 100 miles.

4. After the occurrence of large waves, with their accompanying large rips,
examination of canyon heads has failed to show evidence of deepening.
5. Evidence given above shows that rip currents in the vicinity of La Jolla are
much less pronounced around the head of La Jolla Canyon than they are on
either side. Airplane photographs of the coast at the head of Monterey
Canyon also show that there are rips on either side and an absence of rips
at the canyon head.
6. The scoured condition of the canyon heads can be explained by the mud
flows and other types of submarine slipping which are known to occur at these
places.

25 F. P. Shepard and K. O. Emery, "Submarine Topography of

Canyons and Tectonic Interpretations," Geol. Soc. Amer., Sp
p. 103.

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 RIP CURRENTS 367

Accordingly, the present state of our information gives little

ground for connecting the submarine canyons with rip-current
movements. Whether these currents may play some slight part in
canyon development or in clearing out the canyon heads during
unusually large wave periods remains to be seen.

A MENACE TO SWIMMERS

Lifeguards who have been consulted by the writers are of the

opinion that rip currents are the dominant cause of trouble en-
countered by bathers. There appear to be two principal locations
where swimmers get into difficulties. The first and most common
location of rescues is in the rip-current neck (Fig. 5), at the point
where the large waves are breaking. A bather may find himself
in this position either by slipping into a feeder channel, which may
be very near the shore, and being swept out into the neck or by
jumping through breakers in the zone next to the rip-current neck
and being pulled gradually toward the neck until, having reached
the main channel, he finds that he is in water beyond his depth.
Ordinarily a person getting into the neck will not be carried beyond
the breakers, but the outward-moving current may prevent all but
a very good swimmer from progressing landward. As a result, the
victim finds he is being buffeted by each oncoming wave but is
making no progress toward the shore. Furthermore, if he has
slipped into the neck from shallow water along the side of the
channel, his attempts to swim parallel to the coast back to shoal
water may not succeed because the water may be moving toward
the neck with considerable speed from that side (Fig. 5). The solu-
tion for a swimmer caught in this situation is to try to swim out of
the rip to the side opposite that from which he entered the neck
(Fig. 5). If he does not get out on one side, he should try the other.
Probably there are times when this method would not succeed,
since water may come in forcibly from both sides. In this case a
good swimmer can go out through the breakers and escape as ex-
plained below.
The second situation of danger is beyond the breakers. A swim-
mer may have gone through the breakers in a rip current or may
have swum into a rip while swimming along the coast outside of the

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368 F. P. SHEPARD, K. O. EMERY, AND E. C. LA FOND

breakers. Turning toward shore, he may find that he can make n

progress. The current in the head is not ordinarily very strong,
most swimmers are not very fast. In some cases the rip is of shor
duration so that only a little patience is necessary; but at other
times the rip, even if somewhat pulsatory, continues to move out-
ward over a long period of time. The solution for a bather caught
in this situation can be seen from Figure 5. He should swim parallel
to the shore and, after a relatively short distance, will be helped
by the laterallymoving current of the rip. Then, after getting out
of the turbulent rip, in coming landward he will be further assisted
by the shoreward-moving surface water next to the rip. Two pos-
sible difficulties may be encountered in attempting this procedure.
Sometimes new rips develop adjacent to old ones, so that the swim-
mer might have to go a considerable distance parallel to shore before
escaping. Another trouble may be that the swimmer having reached
shallow water successfully may stumble into the feeder channel
and be swept out into the neck of the rip in a fatigued condition.
This last difficulty can be avoided by approaching the shore with
caution and going around any feeder channel.
The dangers of rip currents are such that swimmers should exer-
cise considerable precaution in their surf swimming. Careful scru-
tiny of the breakers will usually show whether there are dangerous
rips present. Also the swimmer will do well to go out from broad
projecting points rather than from beach re-entries where rips are
most common. Particularly should he avoid piers and groins or
other engineering structures which might deflect outward a long-
shore current. Since the danger exists only when the waves are
large, bathing in summer is ordinarily free from such difficulties
even on open coasts.

SUMMARY AND CONCLUSIONS

Along coasts where large breakers move landward over a

water platform the cumulative landward movement of the surface
water is compensated by seaward-moving lanes of water which
extend out normal to the beach. These water masses, moving con-
trary in direction to the wave approach, are given the name "rip
currents." According to available evidence they appear to take the

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 RIP CURRENTS 369

place erroneously assigned to undertow, in that they return the

water which tends to be piled onto the beach by the waves and in
doing this carry seaward fine sediment derived from the land.
Rip currents are ordinarily fed by water moving along the shore
from either side. The two currents join and extend out in what is
known as the neck, where the water rushes through the breakers in
a narrow lane. Beyond the breakers the current spreads out in what
is called the "head" and dissipates. The outward-moving columns
can be recognized by their brownish color if they contain abundant
sediment, by their agitated water, or by an extension of foam belts
well outside the line of breakers.
These currents are ordinarily long and narrow, but in their outer
zone they widen considerably. They extend out from a few hundred
up to about 2,500 feet from the shore and vary from narrow belts
50 or 1oo feet across in the feeders and neck to as much as 500 feet
or more in the heads. Their velocity is known to be as great as 2
miles an hour in some instances. This rate of flow is very inconstant,
being greatly checked or even stopped by advancing wave fronts.
The flow is capable of erosion near the shore where it produces
channels a few feet deep. These channels may change in location
and in form in periods of a day or even during the change of a tide.
Rip currents are important as a means of transporting fine ma-
terial from the shallow water along the shore into the main-current
circulation outside a coast. They are also a considerable source of
danger to bathers, since their velocity is such that a swimmer may
not be able to make progress against them. Most rescues from the
surf along the coast of southern California are made in these rip
currents.

ACKNOWLEDGMENTS.-The writers wish to express their appreciation for the

suggestions and interest taken in the work by Dr. H. U. Sverdrup and for
the help provided on two occasions by Dr. R. R. Revelle in mapping the bottom
beneath the currents. Also the help of various W.P.A. assistants in making
daily records of rip currents is gratefully acknowledged.

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