Rip Currents
How to Avoid Getting Caught in a Rip Current
The National
Weather Service (NWS)
o
Watch
Dr. Greg Dusek discuss rip current awareness.
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Check
water conditions before going in by looking at the local beach forecast before
you leave for the beach and talking to the lifeguard at the beach.
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Only
swim at a beach with lifeguards. The chances of drowning at a beach with
lifeguards are 1 in 18 million (U.S. Lifesaving Association).
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Don't
assume! Great weather for the beach does not always mean it's safe to swim or
even play in the shallows. Rip currents often form on calm, sunny days.
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Learn
how to spot a rip current. The Break the Grip of the Rip free online training
will help you learn how to spot a rip current.
o
What
are scientists doing to keep swimmers safer? Find out in this video: Predict
the Rip
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Rip
currents aren't the only deadly beach hazard. Learn more about dangerous waves
and other hazards and why you should never to turn your back on the ocean.
Rip currents are strong, narrow, seaward flows of water
that extend from close to the shoreline to outside of the surf zone.
They are found on almost any beach with breaking waves and
act as “rivers of the sea,” moving sand, marine organisms, and other material
offshore (see pictures below).
Coastal scientists have been studying rip currents since
the 1920s.
While early studies were largely descriptive and
qualitative, recent research has combined quantitative theoretical approaches,
field experiments measuring waves and currents, remote video imaging,
experiments in laboratory wave tanks, and computer model simulations.
This research has focused on a variety of topics including
rip current formation mechanisms, flow characteristics, and the hazard that rip
currents pose to swimmers.
The information here provides a brief description of our
present understanding of the science of rip currents, along with a glossary of
key scientific terms relating to rip currents and beaches. For more detailed
information, a list of key scientific reviews of rip currents is provided
below.
Rip
currents are part of nearshore circulation cells that transport water between
the surf zone and areas offshore.
While
rip current circulation patterns have some common features, the details of the
structure of these currents can vary widely.
Breaking
waves in the surf zone result in onshore movement of water and patterns of wave
setup that drive longshore “feeder currents” flowing along the shoreline,
converging and turning seaward to form a narrow and fast-flowing rip-neck.
The
rip-neck extends through the surf zone and outside of the breaking waves where
it eventually decelerates and dissipates as an expanding rip-head. This water
can then be transported shoreward again by breaking waves, completing the cell.
In
some cases the rip-neck is narrow and flows perpendicular to the beach and
extends well beyond the surf zone (see figure below). In other cases, the
rip may flow at an angle away from the beach, or may even meander.
Other
rip currents are characterized by recirculating flow within the surf zone with
only occasional exits of flow offshore (see figure below).
Both
types of circulation patterns exist and have significant implications for
swimmer escape strategies and the transport of sand, larvae and water borne
material.
Many different types
of rip currents can occur on beaches in the United States and around the world.
Scientists and beachgoers use a range of terms to describe these various rip
currents types, and usage often is inconsistent.
The following list of
rip current types follows a scientific classification from Castelle et al. (2016) that differentiates
rip currents based on their forcing mechanisms: the different causes of the
alongshore variations in breaking waves that are crucial for the formation of
rip currents.
Bathymetrically-controlled rip
currents
The location and
persistence of rip currents in many locations is controlled by the bathymetry,
or shape of the bottom.
This can mean the
bathymetry near the beach, including sandbars that may shift in position and
exhibit different configurations, or the offshore bathymetric features that are
relatively fixed, including submarine canyons, submarine ridges, and reefs.
Bathymetrically-controlled
rip currents may be referred to as either channelized or focused:
Channelized rip currents are the most
documented and well understood rip current type. They occupy deeper channels
that interrupt shallow, mostly shore-parallel sandbars found on many beaches.
They can persist in a
relatively fixed location for days, weeks, or months and are relatively easy to
identify (see figure below).
Channelized rip
currents typically range in width from 5 to 100 yards, have depths on the order
of 3 to 10 feet or more, and may be spaced anywhere from 50 to 500 yards apart,
with greater spacing often occurring for large wave heights.
The dominant visual characteristic of channelized rips is a narrow
path of darker water between areas of whitewater associated with breaking waves
( see figure below).
While the darker water appears seemingly calm, the surface is
often characterized by a choppy, rippled texture.
When channel rip currents remain in the same place for a long
period of time, they can also erode the shoreline creating pronounced scallops,
sometimes referred to as rip embayments or megacusps.
Channelized rips have also been referred to as bar-gap, fixed,
and accretionary rip currents.
Focused rip currents, like channel rips, also occur at relatively fixed locations, but
are controlled by alongshore variations in breaking waves created by offshore
bathymetric features such as submarine canyons, submerged ridges, or offshore
sand bars.
These features result in wave patterns that tend to focus wave
breaking and rip current formation at a particular location.
However, as the direction of wave approach changes, so too can the
location of focused rip currents.
Focused rip currents can occur along flat, featureless beaches
where they typically appear as offshore directed plumes of turbulent water and
sediment (see figure below).
Structurally-Controlled Rip Currents
Rip currents often
occur adjacent to man-made structures such as groins, jetties, and piers as
well as natural geologic features such as headlands and rock/reef outcrops ( see
figure below).
These
structurally-controlled rip currents are persistent in location and can occur
even during small breaking wave conditions.
Structural rips
sometimes are associated with deeper water adjacent to structures, and in these
cases appear as areas of darker water with reduced wave breaking activity.
Structural rips also
have been referred to as boundary, topographic, permanent, and headland rip currents.
They can be grouped
into two types based on how and where they form. In some cases, these types can
exist simultaneously on each side of the structure.
Shadow rip currents flow on the down-wave
side of a rigid structure or boundary (i.e., the sheltered side).
On straight beaches,
the presence of a rigid obstacle creates alongshore variations in breaking
waves due to the wave shadowing effect of the obstacle.
Wave setup tends to
be higher away from the structure due to larger waves, and lower closer to the
structure due to smaller waves, as this region is more protected from wave
energy.
This pressure
gradient forces water “downhill” in the alongshore direction back toward the
obstacle and then offshore as a rip current alongside the obstacle (see figure
below).
Deflection rip currents occur on the up-wave
or exposed side of a rigid structure or boundary.
On straight beaches,
waves approaching and breaking against the shoreline at an angle can create
strong alongshore currents that are physically deflected seaward when they
reach the structure and flow offshore adjacent to it (see figure below).
As
waves approach the shoreline, they usually break at an angle, generating a
longshore current that flows along the shoreline (parallel) to the beach.
When
the longshore current (moving along the shore) encounters coastal structure
(such as a groin, jetty, or pier) it is deflected in an offshore direction.
This offshore-directed flow of water is called a rip current.
Hydrodynamically-Controlled Rip Currents
Unlike
bathymetrically and boundary-controlled rip currents, some rip currents do not
rely on the presence of nearshore or offshore bottom features or the presence
of structures.
These
rip currents are controlled solely by hydrodynamics -- wave and current
interactions -- and can occur on featureless straight beaches.
It
is not possible to predict the exact location and timing of hydrodynamic rip
currents, but it may be possible to estimate the likelihood of their formation
on a given beach.
There
are several mechanisms by which hydrodynamic rips form.
For
example, waves originating from two different sources may approach the beach
from different directions, and interference between the different wave trains
causes higher breaking waves in some locations than in others.
This
variability in wave breaking leads to wave setup patterns that drive rip
currents.
In
addition, waves that break in short sections, or “short-crested waves”, can
drive small swirling flows or eddies that can merge to form “flash” or
“transient” rip currents, lasting for several minutes.
At
other times, “shear instability” rip currents may form when eddies are shed off
of longshore currents flowing along the beach as a result of waves breaking at
an angle to the coast.
Hydrodynamically-controlled
rip currents often present different visual characteristics than channelized
rip currents and typically appear as narrow bands of turbulent whitewater and
sand extending offshore of the surf zone ( see figure below).
Less
is known about their flow speed and duration due to a lack of field
measurements; however, they still represent a risk to swimmers and waders due
to their unpredictability and ability to carry people offshore into deeper
water.
While the best way to
avoid getting caught in a rip current is always to swim near a lifeguard,
however lifeguards are not everywhere at all times.
Therefore a key
aspect of being safe at a beach with breaking waves is to understand what rip
currents are and how to avoid them by learning how to identify them.
The video below shows
a channelized rip current. Note the area in the center of the video where
there is a gap in the line of breaking waves. Also note the slightly discolored
water, sand and foam traveling seaward past the breaking waves. Source: NOAA
The following list
provides some tips on how to best spot rip currents:
§ It is much easier to
spot rip currents from an elevated position overlooking the beach. This might
be from a parking lot, beach access, sand dune, or headland (see figure below).
§ Always watch the
water for several minutes as rip current conditions can change.
§ Channelized rip
currents are the easiest to identify as they typically appear as darker, narrow
gaps of water heading offshore between areas of breaking waves and whitewater.
They can appear as darker paths heading out through the surf so look for gaps
in the lines of breaking waves (see figure below).
§ Look for narrow
regions of choppy, rippled water heading offshore. Waves moving toward shore
may steepen as a result of opposing offshore rip current flows, leading to a
different surface water texture (see figure below).
§ Look for pronounced
scalloped embayments on the beach, which can be caused by channelized rip
currents eroding the shoreline (see figure below).
§ Wear polarized
sunglasses to help identify contrasting colors in the water; deep rip current
channels stand out as darker water.
§ Ask a lifeguard if
there are any rip currents and to point them out for you.
Rip Current Flow, Behavior and Characteristics
Rip currents can
occur along any coastline with breaking waves.
Flow speed and behavior: Rip currents flow at speeds
that can quickly transport unsuspecting swimmers considerable distances
offshore. Channel rip currents typically flow about 1 to 2 feet per second and
may reach speeds up to 8 feet per second!
Even in weaker rip
currents, it can be difficult to stand in place at waist depth against
persistent flows and the combination of breaking waves and currents can be
disorienting.
The speeds of the
strongest rips are on par with the top speeds of some of the fastest Olympic
swimmers in history.
Even for a strong
swimmer, attempting to swim back to shore against the seaward flowing current
can quickly result in exhaustion.
Not all parts of a
rip current flow at the same speed. For example, in a classic rip current
system, flow speeds gradually increase from the alongshore feeders and and peak
about half way offshore in the rip-neck before eventually slowing down and
stopping in the rip head.
Similarly, rip flow
speeds tend to be fastest toward the middle of the rip current and are at their
maximum just below the surface of the water.
Rip pulsing: Rip current flow is
also unsteady, which means it changes over time.
A rip pulse is a
sudden acceleration in flow speed that lasts for a short time period (30
seconds to a minute) and is associated with the breaking of incoming wave
groups (sets of larger waves).
When groups of large
waves break, water depths can rapidly increase due to increased wave setup and
unwary swimmers and waders can suddenly lose their footing, be swept into a rip
current, and carried offshore by the subsequent rip current pulse.
During rip pulses,
the speed of the flow can more than double. It is extremely important to
understand that changes in rip current speed can change rapidly with random
increases in incoming wave heights and water levels.
Rip pulses are often
what lead to people getting in trouble in rip currents.
Tidal modulation: The speed and
presence of rip currents on ocean beaches is related to the stage of the tide
because the tide affects the amount of wave breaking, which is what drives rip
currents.
On beaches with a
tide range of less than 6 feet, rip currents often flow faster about an hour or
two on either side of low tide, when water levels are shallower and breaking
wave activity is at its peak.
Rips may stop flowing
completely around high tide because the greater water depths can dramatically
reduce the breaking wave activity and decrease any alongshore variations in
wave setup or water level (see figure below)
On beaches with large
tidal ranges, the tide may drop low enough that bars become exposed (dry) at
low tide.
In this case, rips
shut off once the bars are exposed and the maximum rip speeds occur at mid
tide, when the water level is low enough that there is strong wave breaking on
the bars but not so low that the bar is dry.
Regardless of the
tidal range, rip currents have not been found to be stronger during a falling
outgoing tide, versus a rising incoming tide; only the water level of the tide
is important to controlling rips, not the direction of the tide.
Rip Current Myths and Misconceptions
Rip currents are
often incorrectly referred to as rip tides. Rip currents are not tides, so this
term can cause confusion.
Tides are a large-scale ocean
process that typically leads to the slow rise and fall of water
level over a period of 6-12 hours. Rip currents are not a rise and fall of
water level, but rather concentrated currents that move water in a particular
direction.
As described in
Section 6, the speed of rip current flow can be modulated by the level of the
tide, which affects wave breaking.
Strong and
concentrated currents often occur in tidal inlets, mouths of estuaries and
harbor openings associated with both the incoming flood tide and outgoing ebb
tide.
While these currents can be very
strong and also represent a hazard, they are best referred to as tidal
currents rather than rip tides.
Rip Currents or Undertow? Rip currents are not
the only way that water brought toward the shoreline by breaking waves is
returned seaward. Offshore movement of water located close to the bottom also
moves water out.
This phenomenon is
known as undertow or a near bed return flow. The term
undertow has sometimes been used mistakenly to refer to rip currents and has
contributed to the myth that rip currents can pull a person under the water.
Undertow and rip
currents are separate phenomena and neither will pull a person under the water!
Undertow is usually slower than rip currents and typically does not present a
hazard to swimmers.
For some swimmers in
distress, the false sensation of being pulled under by a rip current results
from a combination of panic, exhaustion, and being hit by breaking waves.
Undertow also is
often confused with the backwash of broken waves at the shoreline.
After a wave breaks
and runs up the beach face, the returning water is called backwash and can be quite strong, particularly on
steep beaches and in the embayments of beach cusps.
While this backwash
will not carry water very far offshore, it can be strong enough to knock people
over, particularly small children and the elderly, and carry them into deeper
water.
Collapsing Sandbars? Some mass
rescue events and fatal drownings have been incorrectly attributed to the
collapse of a sandbar, which suddenly left waders in deeper water.
Sandbars do not
collapse. Sandbars are large masses of sand and while they do shift in location,
this is a slow and gradual process taking days to months under normal
conditions, or as short as several hours during a large storm.
In many cases, the
real cause of swimmer distress in these situations is a group of larger waves,
which causes a small rise in water level (wave setup), causing waders to lose
footing.
This is often
associated with a rip current pulse or increase in flow speed as described in
Section 6, which can present a further hazard.
§ Backwash: The seaward return of
water following the uprush of waves on the shoreline. Also called backrush or
run down.
§ Beach cusp: One of a series
of crescentic undulations along the shoreline spaced at more or less regular
intervals. Cusps are characterized by ridge-like horns separated by shallower
embayments.
§ Bed Return Flow: A term used to
describe the seaward movement of water near the bottom. Often referred to as
undertow.
§ Breaker: A wave that has
become so steep that the crest of the wave topples forward, moving faster than
the main body of the wave. A breaking wave.
§ Breaker zone: The zone within
which waves approaching the coastline commence breaking, typically in water
depths around 10 to 20 ft. Denotes the seaward extent of the surf zone. Can often be confined to crests of sandbars.
§ Embaymenrt: A large
indentation in a shoreline forming an open bay bound by headlands.
§ Estuary: A semi-enclosed
coastal body of water that has a free connection with the open sea and where
freshwater and saltwater mix. The seawater is usually brackish or measurably
diluted with freshwater.
§ Feeder current: The currents
that flow along the shoreline, converge and form the offshore-directed neck of
a rip current.
§ Groin: A shore-protection
structure (often built to trap littoral drift or slow erosion of the shore). It
is narrow in width (measured parallel to the shore) and its length may vary
from tens to hundreds of yards (extending from a point landward of the
shoreline out into the water). Groins may be classified as permeable (with openings through them) or impermeable (a solid or nearly solid structure).
§ Jetty: On open
seacoasts, a structure extending into a body of water to direct and confine the
stream or tidal flow to a selected channel, or to prevent the navigation
channel from infilling with sediment. Jetties are built at the mouth of a river
or entrance to a bay to help deepen and stabilize a channel and facilitate
navigation.
§ Littoral currents: A current
running along the shoreline (parallel) to the beach and generally caused by
waves striking the shore at an angle.
§ Littoral drift: The sedimentary
material moved along the shoreline (parallel) in the nearshore zone by waves
and currents.
§ Longshore current: A current
located in the surf zone, moving generally along the shoreline (parallel),
generated by waves breaking at an angle with the shoreline, also called
an alongshore or littoral current.
§ Mega-rip currents: A type of
extremely large rip current resulting from the circulation cell of an entire
embayment or pocket beach.
§ Nearshore: The region extending
seaward from the shoreline, beyond the surf zone to where waves first start to
interact with the bottom.
§ Rip Head: That part of a rip
current circulation typically located beyond the breakers, marked by a
spreading out or fanning of the circulation. It is here where the velocity and
strength of the rip current circulation begins to weaken considerably.
§ Rip Neck: That part of a rip current
circulation located in the surf zone, marked by a narrow band of swiftly
moving, seaward flowing water. It is here where velocity of the circulation is
at a maximum.
§ Rip Channel: A deeper
channel cut by the seaward flow of a rip current, usually between sandbars.
§ Rip Current: A strong,
narrow and concentrated seaward flow of water that extends from close to the
shoreline, through the surf zone and variable distances beyond.
§ Rip Tide: This term has
inconsistent usage, and is not used widely in the scientific community. Rip currents are not rip tides. A tidal current is a distinct and separate type of
current, and especially strong and variable tidal currents are sometimes called
a "rip tide" or "tidal rip.: These strong tidal currents occur
in areas where tidal flow is constricted like inlets, mouths of estuaries,
embayments and harbors. These currents may cause drowning deaths, but these
tidal currents are a separate and distinct phenomenon from rip currents.
§ Runup: The rush of water up
a beach due to the breaking of a wave. The amount of runup is the vertical height above stillwater
level that the rush of water reaches. Also referred to as uprush.
§ Sandbar: An offshore
ridge or mound of sand which is submerged (at least at high tide) and either
separated from the beach by a short distance, or connected to the beach. They
are often shore-parallel, but exhibit various shapes and forms and are also
common at the mouths of rivers or estuaries.
§ Shoreline: The
intersection of the ocean water surface with the shore or beach. On tidal
coasts, the shoreline position changes with the rise and fall of the tide.
§ Significant Wave Height: The average
wave height of the one-third highest waves over a period of time..
§ Surf Zone: Area of water
between the high tide level on the beach and the seaward extent of breaking
waves.
§ The combined uprush
and backwash of waves after they break on a shoreline. The swash may be
deflected by beach cusps, concentrating the downrush in cusp embayments.
§ Swash Zone:The region of the
beach that is alternately wet and dry as waves run up and down.
§ Swell: Wind-generated waves
that are no longer directly forced by local winds. Often they have
traveled out of their source region, usually over a considerable distance.
Swell waves exhibit a more regular and longer period (typically between 8 and
20 seconds) with flatter crests than choppy, locally generated wind waves.
§ Tide: The periodic rising
and falling of the water resulting from the gravitational attraction of the
Moon and Sun acting upon the world’s oceans as the Earth rotates.
§ Tide Range: The vertical
elevation difference between water level between high and low tide.
§ Undertow: A scientific term
used to describe the seaward return of water close to the bottom surface that
is driven by breaking waves. Often referred to as bed return flow. Undertow is often mistakenly used to
describe rip currents. It is also often associated with the strong backwash after breaking waves.
§ Uprush: The landward rush of
water up the beach after a wave as broken.
§ Wave Height: The vertical
distance between the crest and the preceding trough of a wave.
§ Wave Period: The time taken for
two successive wave crests to pass a fixed point.
§ Wind Waves: Waves generated by, and directly attributable to local winds, as opposed to swell waves. They typically have wave periods of less than 8 seconds.
Accurate
weather forecasts do not always result in a good outcome. The National Weather Service (NWS) learned this difficult lesson in late
April, 2011, when a tornado outbreak over two days
in Mississippi, Alabama, and neighboring states resulted in over 300 lives
lost. The average tornado lead time, the amount of time between an NWS
warning and the arrival of a tornado in a given location, was over 20
minutes - well over the national average. Yet, property damage was
measured in the billions of dollars, and what could not be measured
was the heartache of loved ones lost and lives changed forever.
Traditionally, forecast accuracy is the measure of success for the NWS,
but April 2011 was a game changer for the NWS and its parent agency, the
National Oceanic and Atmospheric Administration (NOAA). Success cannot
just be measured by the accuracy of forecasts, but by the societal response to
those forecasts and ultimately, the societal outcome. Though this was not
necessarily a new way of thinking, the April 28, 2011 storm
drove it home for us all that more had to be done.
https://www.weather.gov/safety/ripcurrent
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