Rip Currents: Nearshore Fundamentals

Ocean Lecture & Educator’s Night May 16, 2012

Below is an overview of the activities Rip Currents: Nearshore Fundamentals (University Corporation for Atmospheric Research, COMET modules) to incorporate information learned from Dr. Herrington’s presentation and subsequent discussion.

Lesson Overview Students learn how nearshore circulation and wave dynamics are involved in the formation of rip currents.

Lesson Rationale Students will participate in an online exploration of the influence of nearshore circulation and wave dynamics on the formation of rip currents. They are exposed to the science behind nearshore waves, from the transfer of energy to the loss and dissipation of energy in waves. After gaining insight into how waves form and act in the nearshore environment, students will further explore the science behind rip currents. First, students will gain a deeper understanding of the different characteristics of rip currents. Second, students will learn what a forcing mechanism is and how longshore variations in waves, wave-beach interactions, and tides can all influence the formation and strength of rip currents. Students understanding will be assessed throughout with small quizzes to check for comprehension and application of the knowledge they are gaining.

Key Concept Students are exposed to the science behind how rip currents form.

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Rip Currents: Nearshore

Fundamentals2012 Update

Produced by The COMET® Program Print Version

Table of Contents

Introduction Nearshore Terminology and Circulation Zones Circulation Onshore Flow Longshore Current Rip Currents

The Surf Zone Characteristics Variable and Fixed Bathymetry Multi-spectrum Waves Wave Height Slope of the Beach

Nearshore Waves Wave Energy Transformation Shoaling Refraction Due to Depth Difraction

Wave Energy Loss and Dissipation Reﬂection Breaking

Rip Currents Characteristics Circulation Visual Clues Wave Angle Location Spacing Duration Velocity

Rip Current Forcing Mechanisms Longshore Variations in Incoming Waves Wave Set-up Wave-Wave Interactions Other Temporal Modulations

Wave Boundary Interaction Surf Zone Bathymetry Coastal Structures Longshore Sandbar

Tidal Forcing Modulation by Tides

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Summary

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Introduction

This module provides insight into how nearshore circulation and wave dynamics are involved in rip current formation. (Wave characteristics, types, and life cycles are discussed in greater detail in other COMET modules.)

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Nearshore Terminology

Nearshore Terminology Zones

Waves absorb energy from the wind. That energy is transmitted across the water surface by waves. At the shoreline, waves break and unleash that energy on the beach.

There are four deﬁned zones in the nearshore environment. When a wave passes from deep to shallow water, it enters the shoaling zone. In this zone the depth of the water is less than half the wavelength. The vertical orientation of the wave changes as it begins to feel the efect of the lake or ocean bottom. As the wave nears the shore, its steepness increases and the wave eventually breaks.

Where the wave begins to break is known as the breaker zone. Not all waves begin to break in the same location due to wave height variation. The breaker zone is where the majority of waves reach their steepness limit for a given wave spectrum. After the wave begins to break, it enters the surf zone where water is transported toward the beach in the form of smaller, broken waves known as bores. These bores can be thought of as continually breaking waves. As bores reach the beach, water particles are pushed onshore and then retreat seaward. This area of run-up and backwash of water is known as the swash zone.

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Question

In which zone is the depth of water less than half of the wave length? (Choose the best answer.)

a) Shoaling zone

b) Breaker zone

c) Surf zone

d) Swash zone

Done

The correct answer is a.

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Nearshore Terminology Circulation

Nearshore Terminology » Circulation Onshore Flow

In the deeper water beyond the shoaling zone, water particles of non-breaking waves have a decreasing orbital motion to a depth equal to half the wavelength, but little to no net ﬂow in the wave direction. As waves move into shallower water, the circular orbits become progressively more distorted and upon breaking are highly disrupted. Following the wave breaking, water particles still oscillate moving landward with the wave crest and seaward with the trough. A rise in the mean water level above the still water level, known as wave set-up, is determined by the average of the onshore currents. The other parts of the nearshore circulation system stem from this onshore ﬂow of water.

Click to see an onshore flow animation. (media/flash/onshore_flow2.swf)

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Nearshore Terminology » Circulation Longshore Current

When waves break on the shoreline, they create currents parallel to the shoreline called longshore currents. Longshore currents occur most often when waves approach the shoreline at an angle. The angle of the incoming wave causes a progressively breaking wave that moves along the shoreline and a longshore current that moves in the same direction as the breaking wave. The longshore current spans the entire width of the surf zone. It reaches maximum strength in the middle of the surf zone and diminishes in strength farther ofshore.

Larger waves create faster longshore currents. The angle of wave approach at breaking also afects the speed of the current. Peak currents occur when the wave approaches from 45 degrees. Higher or lower angles produce slower currents. Waves breaking parallel to the shoreline will have no longshore current generated by the wave angle.

Like rip currents, longshore currents are subtle but can be seen or felt while standing in the surf zone. Longshore currents will always be present with rip currents as part of the nearshore circulation system.

Large-scale currents moving at slower speeds nearshore can also be generated from persistent synoptic-scale winds as opposed to locally-breaking waves.

Question

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Which of the following will always be present with rip currents as part of the nearshore circulation system? (Choose the best answer.)

a) Shoaling zone

b) Submarine canyons

c) Longshore currents

Done

The correct answer is c.

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Nearshore Terminology » Circulation Rip Currents

Rip currents are jet-like currents of water that typically extend from near the shoreline out past the line of breaking waves.

Rip currents can be caused by several wave phenomena that will be presented later in this module. These include ofshore ﬂow through channels in sandbars, natural variability of breaking wave heights, and longshore current interaction with man-made structures. Rip currents are a natural part of the dynamic nearshore circulation system. A portion of the longshore current enters into "feeder currents," which are the segments on the shore-side of a rip current. A rip current also has a neck and a head, as illustrated in the drawing.

Question

Rip currents are _____. (Choose the answer that best completes the sentence.)

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a) a part of the daily nearshore circulation that occurs naturally at surf beaches

b) an unnatural part of the daily nearshore circulation system due only to man-made structures

c) a natural part of the longshore current

Done

The correct answer is a.

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Nearshore Terminology The Surf Zone

Nearshore Terminology » The Surf Zone Characteristics

Of the four zones deﬁned in the nearshore environment, the surf zone is the most important for the marine forecaster’s analysis of rip current potential. There are several aspects of the surf zone that must be assessed, including:

Variable and ﬁxed bathymetry Multi-spectrum waves Wave height Slope of the beach

Question

In which zone do rip currents originate? (Choose the best answer.)

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a) Shoaling zone

b) Breaker zone

c) Surf zone

d) Swash zone

Done

The correct answer is c.

Nearshore Terminology » The Surf Zone Variable and Fixed Bathymetry

Beaches exist because of the erosion and recovery of sand as shorelines adjust to the forces that shape them. Varying weather and storm patterns that occur every year cause seasonal ﬂuctuations in the beach width and the bathymetry (data derived from measurements of water depth are compiled in a topographic map).

Rip current potential is afected by the interaction between waves and the bathymetry of the surf zone. Beaches can contain man-made bathymetric structures such as groins, jetties, and piers and natural bathymetric features such as canyons, ridges, and sandbars.

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Nearshore Terminology » The Surf Zone Multi-spectrum Waves

Multi-spectrum waves are present in the surf zone, and the forecaster needs to understand the numerous and complex motions occurring within the wave spectra.

It should be emphasized that most ofshore wave observation data systems report only the signiﬁcant wave height, thus making it necessary for forecasters to look at the full spectral wave analysis to obtain a comprehensive depiction of the sea state environment afecting the surf zone.

Notice the wave spectra density plot shows two wave groups of twelve- and ﬁve-second periods, but only one will be in the observation record. However, depending on the direction of the wave groups, either may be contributing to rip current formation.

Question

Why is it necessary to analyze wave spectral data? (Choose the best answer.)

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a) Ofshore wave observation systems report this data.

b) A wave group contributing to rip current development can be "masked" by a more energetic wave group.

c) Rip currents can only occur when multi-spectrum waves are present.

Done

The correct answer is b.

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Nearshore Terminology » The Surf Zone Wave Height

Surf zone wave height and period correlate reasonably well with ofshore buoy measurements of directional wave spectral data. Given the lack of any real-time shoreline data in many locations, distant buoy data can be used to forecast waves and, hence, the rip current potential in the surf zone. The process of determining this correlation involves the use of large amounts of data to ﬁnd a regression equation.

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Nearshore Terminology » The Surf Zone Slope of the Beach

Question

Which type of beach do you think will have a lower probability of rip current formation? (Choose the best answer.)

a) A beach with a shallow slope

b) A beach with a steep slope

Done

The correct answer is b.

Another important aspect of the surf zone is its slope.

In general, ﬁne sands result in ﬂatter beaches. Coarse sand results in a steeper beach face. Variations in sand size occur due to the source of the sand, most often resulting from the underlying or adjacent geology that is being reworked by the waves.

The slope of the beach has a direct efect on the size of the surf zone. For a given wave height, steeper beaches will have a narrower surf zone and, hence, reduce the ﬂux of water in the longshore current. This decreased ﬂow tends to cause weaker rip currents on steep beaches (Murray et al. 2003).

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Nearshore Waves

Nearshore Terminology Wave Energy Transformation

Nearshore Waves » Wave Energy Transformation Shoaling

Question

What changes to wave height and wave speed would you expect as waves enter the shoaling zone? (Choose the best answer.)

a) Both wave height and wave speed increase.

b) Both wave height and wave speed decrease.

c) Wave height increases while wave speed decreases.

d) Wave height decreases while wave speed increases.

Done

The correct answer is c.

Wave shoaling occurs as waves travel toward shore in shallow water. Shoaling is the changes in wave characteristics that occur when a wave reaches shallow water. The decreasing depth causes:

An increase in wave height. The conservation of energy results in more energy forced into a smaller area. Since wave energy is proportional to wave height squared, this increases wave height as it propagates toward shore even though some of the energy is dissipated by bottom friction. A decrease in wave speed. Remember that waves in shallow water have speeds that are dependent on the square root of water depth. As the depth decreases, so too will the wave speed.

As the wave moves into shallower water, shoaling afects the wave form by slowing its base while having less efect on the crest. At some point, the crest of the wave is moving too fast for the bottom of the wave form to keep up. The wave then becomes unstable and breaks.

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Nearshore Waves » Wave Energy Transformation Refraction Due to Depth

While change in water depth afects wave height, it can also afect the direction of wave propagation. If a wave is moving in a direction that is not perpendicular to bathymetry contours, then the wave speed will vary along the length of the wave crest. The section of the wave over deeper water will move faster than other parts of the wave. This causes a turning or “refraction” of the wave direction. This refraction occurs before the surf zone and within it. Refraction of waves in the surf zone implies a lower wave incidence angle from shore, which is not conducive to rip current formation.

Question

Based on the illustration below, do you think submarine ridges or submarine canyons will have an increased probability of rip current formation? (Choose the best answer.)

a) Submarine ridges

b) Submarine canyons

Done

The correct answer is b.

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Submarine canyons will cause a convergence of longshore currents. This area is favorable for rip current formation due to the localized decrease in wave heights near shore. The opposite is true for submarine ridges, which tend to focus the waves and cause a divergence of the longshore current.

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Nearshore Waves » Wave Energy Transformation Diffraction

Wave difraction occurs when wave crests bend around barriers. Wave energy can be difracted into the shadow of an object similar to light bending around corners. Waves will difract on the lee side of small islands and breakwaters.

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Nearshore Terminology Wave Energy Loss and Dissipation

Nearshore Waves » Wave Energy Loss and Dissipation Reflection

Reﬂection is dependent on the breaking state of the wave as well as the slope and hardness of the surface. It can vary from 0% for breaking waves on low-sloped, porous beaches to nearly 100% for non-breaking waves in deep water on a hard, vertical surface such as a sea wall.

Click to see a wave reﬂection video. (media/ﬂash/WLMCINTR.swf)

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Nearshore Waves » Wave Energy Loss and Dissipation Breaking

As described in the marine module Wave Types and Characteristics, increasing wave steepness is a cause of wave breaking. Wave breaking will occur at or prior to a wave steepness of 1/7 (wave height/wavelength). Remember that wave breaking is also dependent on water depth. Waves are forced to break nearshore when the wave height is approximately 78% of the water depth.

Breaking waves are the key to rip current production because they help produce the wave set-up which piles water onshore. The non-uniform height of wave set-up within the surf zone helps generate longshore currents which, in turn, promote the formation of rip currents.

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Rip Currents

Rip Currents Characteristics

Rip Currents » Characteristics Circulation

Rip currents are jet-like ﬂows of water moving away from the beach with a root at the shore, a neck across the breaker zone, and a mushrooming head. A rip current neck can be very narrow or more than 50 yards in width. A rip current head is typically seen just beyond the breaker zone.

The seaward extent of rip currents can vary from just beyond the line of breaking waves to hundreds of yards ofshore, extending up to a maximum of 2.5 times the surf zone width.

Click to see a rip current process animation. (media/ﬂash/ripcurrent_3.swf)

Question

If the surf zone on a beach is approximately 100 yards, what is the farthest you would expect a rip current to extend? (Choose the best answer.)

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a) 100 yards.

b) 250 yards

c) 500 yards

d) 1 mile

Done

The correct answer is b.

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Rip Currents » Characteristics Visual Clues

Clues for identifying a rip current include the following:

Channel of churning, choppy water Diference in water color (Suspended sediments may be transported back to sea in the rip current.) Line of foam, seaweed, or debris moving out to sea Break in the incoming wave pattern

The rip current head is often brown and foamy from sediment that has been disturbed. When viewed from above, this head provides a clear indication that a rip current is present. However, rip currents may not be as evident from the perspective of a swimmer standing on a beach. This risk can be minimized by swimming in beach areas monitored by lifeguards.

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Rip Currents » Characteristics Wave Angle

Rip currents are frequently generated when the incoming wave direction is nearly perpendicular to the shoreline. The orientation of the shoreline is therefore a key feature to note when assessing the potential for rip current formation at a particular beach. Some studies suggest the ideal wave angle for rip current formation is within 20 degrees of normal. However, in the vicinity of shoreline structures waves approaching at angles of greater than 20 degrees create faster longshore currents and are therefore more likely to cause rip currents.

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Click to see animated version of how wave angle variations afect rip current formation. (media/ﬂash /waveangle_2.swf)

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Rip Currents » Characteristics Location

Rip currents are commonly observed on beaches with a relatively gentle slope. This occurs because there is generally a wider surf zone on gently sloping beaches. A gently sloping beach has a typical slope of around 1/40 or 0.025. The wider surf zone allows waves to break for a longer period of time and, hence, transport more water toward the beach. Steep beaches are less likely to have rip currents. Rip currents that do occur on steep beaches will be weaker due to waves breaking close to shore and the lack of a wide surf zone. Note that this relationship with beach slope puts the public at greater risk since more people tend to swim at gently sloping beaches.

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Question

Order the beaches listed on the image below from lowest to highest rip current probability. (Choose the best answer.)

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a) A, B, C

b) B, C, A

c) C, A, B

d) A, C, B

Done

The correct answer is c.

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Rip Currents » Characteristics Spacing

A single-cell rip current can occur solely in the vicinity of jetties or other man-made structures as we will discuss in the next section.

Multi-cell rip currents can form in many diferent conditions. Here are a few common examples of when multiple rip currents occur:

One current is being created as another dissipates near the same location. Multiple gaps in the sandbar exist. Wave angles are close to normal on a cuspate beach.

Due to their nature, the spacing between multi-cell rip currents is generally observed to be less than 500 m and will vary based upon beach slope, shoreline orientation, wave height, and wave period.

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Rip Currents » Characteristics Duration

Rip currents are transitory and temporal. The pulsation or intermittency of an individual rip current is approximately 10 to 20 minutes. It is rare to see a rip current sustained for an hour or more. This is not to say that an area of beach will not be afected by a series of rip currents for more than 10-20 minutes as there could be multi-cell development that occurs over time along a stretch of beach.

Rip currents caused by man-made structures are the exception because they can result in much longer duration rip currents that can even be semi-permanent.

Click to see a Rip Current Pulsation animation. (media/ﬂash/rip_pulsation.swf)

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Rip Currents » Characteristics Velocity

Rip current velocity is intermittent and may rapidly increase within minutes due to larger incoming wave groups or nearshore circulation instabilities. It is important to understand that changes in rip current velocity occur in response to changes in incoming wave height and period as well as changes in water level. The rapid increase in velocity can catch unwary beach goers and swimmers of guard. While rip current velocities average 1-2 ft/s, moderate to strong rip currents can have speeds at or over 8 ft/s.

Without the use of velocity measurements, one can attempt to estimate the potential strength of rip currents based on their spacing along a single beach. A single rip current in a given area of wave height usually indicates high ofshore velocity. Multiple rip currents in the same area of given wave height tend to reduce the velocity. In general, increasing spacing between rip currents on a single beach increases the potential velocity in the current. However, under the right wave and water level conditions, high velocities should be a concern on any beach, no matter what the spacing of the rip currents.

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Click to see how rip current spacing relates to rip current velocity. (media/ﬂash/rips_spacing.swf)

Rip Current Forcing Mechanisms

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Rip Current Forcing Mechanisms Longshore Variations in Incoming Waves

Rip Current Forcing Mechanisms » Longshore Variations in Incoming Waves Wave Set-up

When waves break on a beach, they produce a set-up, or rise in the mean water level above the still water level. Wave height and period variations among incoming waves cause the water level in the surf zone to have areas of non-uniformity. This varying of high and low set-up causes a horizontal pressure gradient that induces the current ﬂow from the high to low set-up areas. Convergence of the longshore current at a low set-up point causes a rip current to form and transport the displaced mass of water back ofshore.

Click to see a wave set-up animation. (media/ﬂash/wave_setup_ani.swf)

Question

Convergence of longshore currents at ______causes rip currents to form. (Choose the best answer.)

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a) high wave set-up points

b) low wave set-up points

Done

The correct answer is b.

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Rip Current Forcing Mechanisms » Longshore Variations in Incoming Waves Wave-Wave Interactions

The waves coming into the surf zone are generated by multiple sources and can arrive from diferent directions. For example, the southeastern U.S. coast can experience long-period swells from a distant tropical cyclone traveling toward shore from the southeast while wind waves are impacting the same shore from the northeast. These bidirectional incoming wave components can produce variations in the water transported through the surf zone, leading to rip current formation. Rip currents will develop in the areas between wave intersections due to the low point in water level at these locations. The rip currents will migrate along the beach relative to these wave intersections unless the wave groups have periods and wavelengths that develop a constant intersection point.

Click to see how wave interactions afect rip current formation. (media/ﬂash /rips_wave_intersections.swf)

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Rip Current Forcing Mechanisms » Longshore Variations in Incoming Waves Other Temporal Modulations

In addition to large-scale wave interactions in the surf zone, the following factors may also inﬂuence the type, duration, and strength of rip currents:

Sea breezes Infra-gravity waves Current-refracted waves Energy from wave groups

These motions are beyond the scope of this module, but are mentioned to illustrate the complexity of the surf zone environment.

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Rip Current Forcing Mechanisms Wave Boundary Interaction

Rip Current Forcing Mechanisms » Wave Boundary Interaction Surf Zone Bathymetry

Changes in surf zone bathymetry can cause incoming wave variations along the shore. Coastal zones with complex or signiﬁcant bathymetry will have areas where wave energy converges and diverges for a given wave path. These areas, if predetermined, can be used as a guide for forecasting areas of potential rip current formation.

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Rip Current Forcing Mechanisms » Wave Boundary Interaction Coastal Structures

Natural and man-made obstructions frequently occur in the shoaling and surf zones. Waves that intersect shore-connected piers, ofshore groins and jetties, and points of land are bent (partially difracted) around these objects. Some of the energy from the wave is bent into the “shadow region” of the object. This results in wave height variations as the difracted wave energy interacts with other signiﬁcant waves in the surf zone.

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Rip Current Forcing Mechanisms » Wave Boundary Interaction Longshore Sandbar

Often, rip currents form where a cut in a longshore sandbar is already present. Typically, the incoming waves will break on the sandbar. The sandbar acts as a dam that holds water deposited by the breaking waves. As wave set-up occurs and the longshore current develops, the low spots in the sandbar become the path of least resistance for the return ﬂow of water. The speed of the rip current is strong enough to move bottom sand and carve out a rip channel across the sandbar's low point. For these reasons, natural changes (sediment transport and major storms) as well as manmade changes (major sand replenishment projects) can signiﬁcantly impact the surf zone environment, leading to changes in rip current frequency and intensity.

Click to see how rip currents form near sandbars. (media/ﬂash/rip_current.swf)

Question

Where would changes in rip current frequency and intensity be least likely? (Choose the best answer.)

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a) A beach experiencing a large amount of erosion

b) A beach that has just been hit by a large storm

c) A beach located in a bay

d) A beach that has recently undergone major sand replacement

Done

The correct answer is c.

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Rip Current Forcing Mechanisms Tidal Forcing

Rip Current Forcing Mechanisms » Tidal Forcing Modulation by Tides

Rip currents are modulated by tides due to changes in the position of the breaker zone and the size of the surf zone. Higher water levels during high tide cause the breaker zone to move closer to shore, while the opposite efect occurs at low tide. Lower water levels during low tide produce a wider surf zone and, hence, larger water mass transport. If a sandbar is present in the surf zone, high tide allows easier transport of water away from the beach than at low tide. This is due to a greater depth between the surface of the water and the top of the sandbar. On the Great Lakes, lake levels and seiches have the same efect on increasing the strengths of rip currents that tides do on the oceans.

There is some evidence suggesting rip currents are more prevalent in the hours immediately before and after the time of low tide. However, rip currents can occur at high tide depending on various factors, and the forecaster should not assume that the danger of rip currents will not be present.

Question

In general, at which of the points in the tidal cycle below would a rip current be more likely

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to occur? (Choose the best answer.)

a) A

b) B

c) C

Done

The correct answer is a.

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Rip Current Forcing Mechanisms Summary

There are four deﬁned zones in the nearshore environment:

Shoaling zone Breaker zone Surf zone Swash zone

The nearshore circulation system consists of onshore ﬂow, longshore currents, and rip currents. Onshore ﬂow brings water into the surf zone. Longshore currents move the water through the surf zone, parallel to the beach face. Rip currents take water beyond the breaker zone.

Of the four nearshore environment zones, the surf zone is the most important for the marine forecaster’s analysis of rip current potential. There are several aspects of the surf zone that must be assessed, including:

Variable and ﬁxed bathymetry Multi-spectrum waves Wave height Slope of the beach

Nearshore waves undergo wave energy transformation through shoaling and refraction due to water depth and difraction. Wave energy can be lost due to reﬂection and breaking.

Rip currents are jet-like ﬂows of water moving away from the beach with a root at the shore, a neck across the breaker zone, and a mushrooming head. Rip currents can be identiﬁed by:

The orientation of the coastline and the angle of incoming waves are key features to note when assessing the potential for rip current formation at a particular beach. Waves moving directly onshore (i.e., near normal angle of incidence) are more conducive to rip current formation while waves at smaller incident angles are less likely to cause rip currents.

Longshore variations in incoming waves and wave-wave interactions are the primary forcing mechanisms that cause variations in wave set-up and lead to rip current formation. Wave-boundary interactions such as surf zone bathymetry, coastal structures, and longshore sandbars also afect the formation of rip currents. Tides on the oceans, lake levels, and seiches on the Great Lakes inﬂuence rip current formation and strength by varying the width of the surf zone as well as the depth of water over the sandbar.

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