2 Nearshore Terminology and Circulation
2.1 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 effect 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.
2.2 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. The average of these currents onshore causes a rise in the mean water level above the still water level known as wave set-up. The other parts of the nearshore circulation system stem from this onshore flow of water.
Click here to see an onshore flow video.
2.3 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 as it moves farther offshore.
Larger waves create faster longshore currents. The angle of wave approach at breaking also affects 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 in the nearshore can also be generated from persistent synoptic-scale winds as opposed to locally-breaking waves.
 | 2.3 Which of the following will always be present with rip currents as part of the nearshore circulation system? - Shoaling zone
- Submarine canyons
- Longshore currents
Click here for the answer key. |
2.4 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 offshore 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.
| 2.4 Rip currents are _____. - a part of the daily nearshore circulation that occurs naturally at surf beaches
- an unnatural part of the daily nearshore circulation system due only to man-made structures
- a natural part of the longshore current
Click here for the answer key. |
2.5 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
- Multispectrum waves
- Wave height
- Slope of the beach
2.6 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 recur every year cause seasonal fluctuations in the beach width and the bathymetry.
Rip current potential is affected 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.
2.7 Surf Zone - Multispectrum Waves
Multispectrum waves are present in the surf zone, and the forecaster needs to understand the numerous and complex motions occuring within the wave spectra.
It should be emphasized that most offshore 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 affecting 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.
| 2.7 Why is it necessary to analyze wave spectral data? - Offshore wave observation systems report this data.
- A wave group contributing to rip current development can be "masked" by a more energetic wave group.
- Rip currents can only occur when multispectrum waves are present.
Click here for the answer key. |
2.8 Surf Zone - Wave Height
Surf zone wave height and period correlate reasonably well with offshore 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. More information on this topic may be presented in a later module.
2.9 Surf Zone - Slope of the Beach
| 2.9 Which type of beach do you think will have a lower probability of rip current formation? - A beach with a shallow slope
- A beach with a steep slope
Click here for the answer key. |
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 effect 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).
3 Nearshore Waves
3.1 Wave Energy Transformation - Shoaling
| 3.1 What changes to wave height and wave speed would you expect as waves enter the shoaling zone? - Both wave height and wave speed increase.
- Both wave height and wave speed decrease.
- Wave height increases while wave speed decreases.
- Wave height decreases while wave speed increases.
Click here for the answer key. |
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 affects the wave form by slowing its base while having less effect 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.
3.2 Wave Energy Transformation - Refraction Due to Depth
While change in water depth affects wave height, it can also affect 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.
| 3.2 Based on the illustration below, do you think submarine ridges or submarine canyons will have an increased probability of rip current formation? - Submarine ridges
- Submarine canyons
Click here for the answer key. |
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 a submarine ridges, which tend to focus the waves and cause a divergence of the longshore current.
3.3 Wave Energy Loss and Dissipation - Diffraction
Wave diffraction occurs when wave crests bend around barriers. Wave energy can be diffracted into the shadow of an object similar to light bending around corners. Waves will diffract on the lee side of small islands and breakwaters.
3.4 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 here to see a wave reflection video.
3.5 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.
4 Rip Currents
4.1 Characteristics of Rip Currents - Rip Current 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 offshore, extending up to a maximum of 2.5 times the surf zone width.
Click here to see a rip current process animation.
| 4.1 If the surf zone on a beach is approximately 100 yards, what is the farthest you would expect a rip current to extend? - 100 yards
- 250 yards
- 500 yards
- 1 mile
Click here for the answer key. |
4.2 Characteristics of Rip Currents - Visual Clues
Clues for identifying a rip current include the following:
- Channel of churning, choppy water
- Difference 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.
4.3 Characteristics of Rip Currents - 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.
Click here to see how wave angle variations affect rip current formation.
4.4 Characteristics of Rip Currents - 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.
| 4.4 Order the beaches listed on the image below from lowest to highest rip current probability. 
- A, B, C
- B, C, A
- C, A, B
- A, C, B
Click here for the answer key. |
4.5 Characteristics of Rip Currents - 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.
Multicell rip currents can form in many different 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 multicell 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.
4.6 Characteristics of Rip Currents - 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 affected by a series of rip currents for more than 10-20 minutes as there could be multicell development that occurs over time along a stretch of beach.
Click here to see a Rip Current Pulsation animation.
4.7 Characteristics of Rip Currents - 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 off 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 offshore velocity. Multiple rip currents in the same area of given wave height tend to reduce the velocity. In general, the larger the spacing between rip currents on a single beach, the larger 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.
Click here to see how rip current spacing relates to rip current velocity.
5 Rip Current Forcing Mechanisms
5.1 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 offshore.
| 5.1 Convergence of longshore currents at _______ causes rip currents to form. - high wave set-up points
- low wave set-up points
Click here for the answer key. |
5.2 Longshore Variations in Incoming Waves - Wave-Wave Interactions
The waves coming into the surf zone are generated by multiple sources and can arrive from different 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 here to see how wave interactions affect rip current formation.
5.3 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.
5.4 Wave-Boundary Interaction - Surf Zone Bathymetry
Changes in surf zone bathymetry can cause incoming wave variations along the shore. We have already talked about the fact that submarine canyons and ridges can refract wave energy, changing the direction of wave propagation. Therefore, 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.
5.5 Wave-Boundary Interaction - Coastal Structures
Natural and man-made obstructions frequently occur in the shoaling and surf zones. Waves that intersect shore-connected piers, offshore groins and jetties, and points of land are bent (partially diffracted) 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 diffracted wave energy interacts with other significant waves in the surf zone.
5.6 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 here to see how rip currents form near sandbars.
| 5.6 Where would changes in rip current frequency and intensity be least likely? - A beach experiencing a large amount of erosion
- A beach that has just been hit by a large storm
- A beach located in a bay
- A beach that has recently undergone major sand replacement
Click here for the answer key. |
5.7 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 effect 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 due to a greater depth between the surface of the water and the top of the sandbar.
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.
5.8 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
- Multispectrum waves
- Wave height
- Slope of the beach
Nearshore waves undergo wave energy transformation through shoaling and refraction due to water depth and diffraction. 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.
Clues for identifying a rip current include the following:
- Channel of churning, choppy water
- Difference 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 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 affect the formation of rip currents. Tides 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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