Rip Current Science
Where and Why Rip
Currents Form
Coastal scientists have been
investigating rip currents
for more than 75 years. This
research has been
conducted through field
observations and measurements,
laboratory measurements and
wave tank experiments, and
computer and numerical modeling.
The mechanics of rip current
development are complex and
involve interactions between
waves and currents, waves and
water levels, waves and the
shape of the nearshore bottom
(bathymetry), as well as wave-wave
interaction.
Photo
courtesy University
of Delaware Sea
Grant College Program
|
Rip currents can occur along
any coastline that features
breaking waves. Scientific
investigations of wave and
current interactions along
the coast have shown
that rip currents are likely
present on most beaches every
day as a component of the complex
pattern of nearshore circulation.
As waves travel from deep
to shallow water, they eventually
break near the shoreline.
As waves break,
they generate currents that
flow in both the offshore
(away from the coast) and the
alongshore directions. Currents
flowing away from the coast
are called rip currents. Rip
currents are a result of
complex interactions between
waves, currents, water levels
and nearshore bathymetry.
These current systems
form an integral part of
nearshore circulation patterns
such as alongshore and
cross-shore (onshore/offshore)
water motion. Along all coastlines,
nearshore circulation cells
may develop when waves break
strongly in some locations
and weakly in others. These
weaker and
stronger wave breaking patterns
are most often seen
on beaches with a sand bar
and channel system in the
nearshore zone. A rip current
forms as the narrow, fast-moving
section of water travels
in an offshore direction.
Rip currents can also result
from a wave's natural variability
or when a current traveling
along the shoreline encounters
a structure such as a groin
or jetty and is forced offshore.
Rip current strength and speed
varies. This variability makes
rip currents
especially dangerous to uninformed
beachgoers. Rapid fluctuations
or pulses in wave groups can
quickly generate rip currents
with velocities measured
up to 8 feet per second.
Waves, Currents
and Water Level Variations
As waves break along a shoreline
or over a sandbar, an increase
in water level occurs. This
increase in water level is
known as set-up. The increase
in water level is especially
evident as waves break over
a sandbar, resulting in an
increased set-up of water on
the landward side of the bar.
Thus, waves breaking over a
sandbar can result in mass
transport of water between
the bar and the shoreline.
There is usually a difference
in set-up (water heights) between
the bar, where the waves are
breaking strongly, and the
channel between bars, where
little or no wave breaking
occurs.
One of the ways this water
returns seaward is through
rip currents, which flow seaward
against the incoming waves.
This seaward flow of water
typically occurs through a
break in the sandbar, where
water is channelized into a
narrow current known as a rip
current.
Diagram
courtesy NOAA's National
Weather Service |
Brian Sapp, a graduate student
at the Georgia-Tech Savannah,
has provided the following
explanation of how a rip current
develops over a sand bar. The
accompanying graphic is provided
courtesy of NOAA-National Weather
Service:
Basic Rip Current Mechanics
- Waves break on the sand
bars before they break in
the channel area.
- Wave breaking causes an
increase in water level over
the bars relative to the
channel level.
- A pressure gradient is
created due to the higher
water level over the bars.
- This pressure gradient
drives a current alongshore
(the feeder current).
- The longshore currents
converge and turn seaward,
flowing through the low area
or channel between the sand
bars.
Nearshore
Bottom: Bathymetry
The shape of the shoreline
and nearshore bottom
(bathymetry) may influence
rip current development.
In regions where the
coastline is characterized
by cuspate features (i.e.
the shoreline is “scalloped”),
rip current may be found
between the cusps.
Examples of this type of
shoreline can be found in
some portions of the Florida
coast (e.g. Perdido
Key), and in some locations
along the California
coast (Monterrey Bay).
View from the ground and taken at low tide; this channel has formed at
a break in the sand bar. Photo
courtesy University of Delaware Sea Grant College Program |
Viewed from the air, rip currents can be
seen flowing past the line of breaking
waves. Rip current spacing along an open
coast may be dependent on many factors,
including the shape of the nearshore bottom. Photo
courtesy University of Delaware Sea Grant
College Program . |
The presence of longshore
bars can also have an impact
on rip current development
and location. In some geographic
locations, the nearshore bottom
may be characterized by fixed
structures such as reefs. In
other areas, nearshore bathymetry
is more ephemeral, with offshore
sand bars constantly changing
their shape and location. Along
shorelines where sand is deposited
in an offshore bar, the rip
current often flows through
a low spot or channel cut through
the sandbar.
Man-Made Structures: Groins, Jetties and Piers
Rip currents may occur at fixed locations such as groins, jetties, piers, or other man-made
structures where water can be funneled out to sea in a narrow channel. In coastal
areas with structures, rip current may result when currents running parallel to the shore
are deflected offshore by the structure.
Brownish-colored water can be seen flowing seaward, deflected offshore by the timber groin. Photo
courtesy University of Delaware Sea Grant College Program |
Rip current formation adjacent to coastal structure. Currents that flow in the longshore direction are deflected
offshore by structures. Photo courtesy University of Delaware Sea
Grant College Program. |
As waves approach the shoreline, they usually break at an angle, generating a longshore current
that flows parallel (along) 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.
Wave-Wave Interaction
and Wave-Current Interaction:
Generation of Circulation
Patterns
Rip currents can develop due
to interactions between waves
and currents, even in the absence
of nearshore sand bars and
without shoreline or bathymetric
influence on their formation.
Breaking waves force narrow
regions of offshore-directed
flows, known as rip currents.
As waves break near the shore,
complex wave interactions may
generate circulation patterns
that result in the formation
of rip currents, sending water
back out to sea. Along open
coasts (coasts without sand
bars), rip currents are caused
by variations in breaking waves,
with waves breaking strongly
in some locations and less
strongly in others. These variations
generate circulation cells
causing water to be deflected
offshore as a rip current.
Periodic rips: Beach face currents
along the coast of Germany. Photo
courtesy of Dr. Robert A. Dalrymple. |
Rip Current Size and
Speed
Rip currents can occur along
any coastline with breaking
waves. Although rip currents
are often present daily on
many beaches, the velocities
may be too slow to be a threat
to experienced swimmers. However,
their inherent variability
makes them especially dangerous
to unwary or uninformed beachgoers.
Changes in the size of the
incoming waves can cause pulses
in the strength of a rip current,
which can be dangerous to all
swimmers and anyone entering
the surf.
Wave Groups and Rip
Current Pulsations
Rip currents are very unsteady
and may increase in strength
within a short time frame (a
few minutes) because of larger
incoming wave groups or current
instabilities. It is extremely
important to understand that
changes in rip current velocity
can occur very rapidly with
random increases in incoming
wave heights and water levels.
Water depths can rapidly increase
in rip current channels, catching
unwary beachgoers and swimmers
off-guard. If the higher waves
and water levels sweep bathers
off their feet, they may be
transported offshore by the
rip current.
Rip Current Characteristics
and Velocities
Although rip currents are
not caused by tides, the water
level (tide elevation) at the
coast may have an impact on
rip current speed and strength.
Generally, rip current velocities
increase as water levels (tide
elevation) decrease.
Rip current velocities also
typically increase as wave
heights increase. An increase
in the height of incoming waves
can result in sudden increases
in water depth and rip current
velocities. These sudden changes
or pulses in water depth and
current speed can catch bathers
off-guard. Rip current pulsations
are extremely dangerous to
all swimmers!
While average rip current
velocities of 1 to 2 feet per
second do not pose serious
hazards to strong swimmers,
rip currents may rapidly reach
or exceed velocities of 3 feet
per second. Also, rapid fluctuations
or pulses in wave groups can
quickly generate rip currents
with extreme velocities that
have been measured up to 8
feet per second – this is faster
than an Olympic swimmer can
sprint! If a swimmer is caught
in a rip current, attempting
to swim directly back to shore
against the seaward flowing
current can result in exhaustion
and possible drowning.
Rip currents are usually narrow
(~ 20 to100 feet in the alongshore
direction), may extend hundreds
of feet offshore, and generally
span the entire water column.
However, offshore, or outside
the surf zone, they tend to
be confined near the surface.
Rip currents do not pull people
under water – they pull people
away from shore. Drowning deaths
usually occur when people are
unable to keep themselves afloat
and swim back to shore. This
may be due to fear, panic,
exhaustion, a lack of swimming
skills, or any combination
of these factors.
Rip Current Duration
Some shorelines are characterized
by permanent rip currents which
may be found in a fixed location
such as a break in a reef or
other hard structure. Some
rip currents are persistent,
lasting for many days or months
in one location. Rip currents
may also migrate along a stretch
of coastline. Rip currents
may also be ephemeral, forming
quickly and lingering for a
few hours or days before dissipating
and disappearing.
Miscellaneous/General
Information
Rip Currents vs. Rip
Tides
Warning sign posted
adjacent to tidal inlet
in South Carolina. Photo
courtesy University of
Delaware Sea Grant College
Program |
Rip currents
are not rip tides. A specific
type of current associated
with tides may include both
the ebb and flood tidal currents
that are caused by egress and
ingress of the tide through
inlets and the mouths of estuaries,
embayments, and harbors. These
currents may cause drowning
deaths, but these tidal currents
or tidal jets are separate
and distinct phenomena from
rip currents. Recommended terms
for these phenomena include ebb
jet , flood jet ,
or tidal jet .
What is Undertow?
Undertow, an often misunderstood
term, refers to the backwash
of a wave along the sandy bottom.
After a wave breaks and runs
up the beach face, some of
the water percolates into the
sand, but much of it flows
back down the beach face creating
a thin layer of offshore-moving
water with a relatively high
velocity. This backwash can
trip small children and carry
them seaward. However, the
next incoming wave causes higher
landward velocities, pushing
them back up on the beach.
Undertow does not pull you
under water or out to sea.
For additional science
and information on rip currents,
see our links and Frequently
Asked Questions pages.
This page was developed courtesy
of Wendy Carey, Delaware Sea
Grant. Assistance and input
provided by Dr. Andrew Kennedy,
University of Florida, Gainesville;
Dr. Jamie MacMahan, University
of Delaware; Brian Sapp, Georgia
Tech, Savannah; Spencer Rogers,
North Carolina Sea Grant; and
the many rip current research
scientists who participated
in the Rip Current Technical
Workshop held in April, 2004.
|