Rip Currents
Rip currents (often incorrectly referred to as 'rip tides') are strong flowing and relatively narrow currents that originate inside the surf zone and flow seaward (Bowen, 1969). Rip currents often occur near structures such as piers or groins, or due to the geometry of the shoreline such as headlands. In these cases the features of the shoreline deflect the flowing alongshore current offshore creating a rip current. Rip currents can also form along straight coastlines, which is often the greater safety hazard of the two. This is because they can be constantly changing in size and strength both spatially and temporally and perhaps more obviously there is often no shoreline feature identifying a probable rip location. This type of rip is what is being focused on in the Kill Devil Hills study and is discussed in more detail below.
A photo showing a rip current at KDH beach. The rip is in the center of the photograph and can be seen due to the foam traveling offshore and the lack of breaking waves.
Physical Characteristics of a Rip Current System
The characteristics of any rip current system can vary significantly. They can be relatively stable, existing at a certain location for months at a time or be present for only a few minutes before moving alongshore or dissipating. They generally have velocities ranging from 0.1 - 2 m/s (0.2 - 5 mph) and can have either relatively persistent velocities or pulse with varying wave conditions (MacMahan et al., 2006). A singular rip current will generally range from 50-150 meters wide and can extend up to 400 meters seaward.
Why Rips Happen - A brief explanation of rip current dynamics
An example of alongshore variability in wave breaking. Notice how waves are breaking in one spot and not at all directly to the left and right of the location.
A diagram depicting variability in the wave field caused by wave-wave interactions. Here 2 wave trains are intersecting causing increased wave height at some points and decreased at others (also known as constructive and destructive interference).
For rip currents to exist on a straight beach, the most essential requirement from a dynamical perspective is that there must be alongshore variability in the wave field (MacMahan, 2006). In other words, as your position moves along the shoreline there must be differences in the height of the breaking waves, or how far from shore they break. There are a few ways in which this can occur, 2 of which are the most common.
The first is by having wave-wave interactions. Often there are waves originating from different sources approaching the beach at different directions. This causes interference between the different wave trains which can cause higher waves in some locations and smaller in others. This can provide the alongshore variability necessary to generate rip currents. However, rip currents of this variety are usually weaker and short lived due to the constant changing of the wave conditions.
The second method to generate alongshore variability in the wave field is through differences in the alongshore bathymetry or shape of the bottom. As waves enter relatively shallow water they gain height and steepness and eventually begin to break. For a certain wave train this point of breaking will generally occur at a constant depth. Thus, if there is change in depth alongshore, incoming waves will begin breaking at different distances from the shoreline and create the variability needed to generate rip currents. Often changes in depth will occur due to the presence of a sand bar when there are channels or depressions in the bar.
How does this alongshore variability in the wave field generate rip currents? The answer to this question can get quite complex, however in a general sense it is due to differences in the water level alongshore. Simply, shoreward of breaking waves, in locations where there are higher waves there will be a higher water elevation, or set-up, than in locations where there are smaller waves. Since water flows downhill, the current will flow from regions of high waves to regions of low waves. The simplest case to use as an example would be when there is an alongshore sandbar with a channel in it, such that it is bar-channel-bar. Using the figure below as a guide, waves over the sandbar will generally break further from shore and be larger than waves inside the channel. This leads to increased set-up landward of the breaker zone in locations near the bar, and a decreased set-up in locations near the channel. This is the basic mechanism generating rip currents.
A diagram showing a basic rip current system. Notice the higher set-up in the regions of breaking waves and the lower set-up in the region of the channel and rip current.
Identifying Rip Currents and Safety Procedures
Understanding how rip currents function gives clues on how to identify rip currents if you are on the beach or in the water, and how you can safely escape a rip if you are caught in one.
A photo showing a rip current at KDH Beach. The foam and more turbid water heading offshore identifies the location of the rip.
If waves are breaking near the sandbar nearshore, rip currents can be identified by looking for areas where waves are NOT breaking. These are likely locations of deeper water or channels, in which rips are commonly found.
Areas where there is a plume of foam or sediment are often indications of a rip current. The current will carry these and other objects away from shore.
Often while in the water you can see or feel areas of deeper water or channels in the area of the sandbar. These are often locations that will be active with rip currents when wave conditions are sufficient
Finally, if you are caught in a rip current don't panic, and don't try to fight the current. Simply swim along shore until you no longer feel the pull of the current and then swim in or call for help. More information on rip currents can be found on the NOAA rip current website.