The currents lose strength quickly just outside of the surf zone, but they can be dangerous to swimmers who get caught in them and are pulled away from shore. Swimmers caught in a rip current should not try to swim directly back to shore, as it is difficult to fight the current and the swimmer can quickly tire. Instead, swim parallel to the beach for a short distance until you are outside of the rip current, and then you can easily swim to shore.
Rip currents are visible in Figure As is evident from the photo, the rips correspond with embayments in the beach profile. Longshore transport refers to the cumulative movement of beach and nearshore sand parallel to the shore by the combined action of tides, wind, and waves and the shore-parallel currents produced by them.
These forces usually result in an almost continuous movement of sand either in suspension or in bedload flows see entry on Cross-Shore Sediment Transport. This occurs in a complex, three-dimensional pattern, varying rapidly with time. At any moment, some sand in the area of interest may have an upcoast component while other sand is moving generally downcoast.
The separation of the total transport into components parallel and perpendicular to the shore is artificial and is done as a convenience leading to a simpler understanding of a very complex environment. To be meaningful, the rate of longshore transport must be averaged over intervals of at least many wave periods and is typically predicted or measured over much longer times, ranging up to a year see entries on Gross Transport Skip to main content Skip to table of contents.
This service is more advanced with JavaScript available. Encyclopedia of Coastal Science Edition. Barrier Island Migration. Coastal Pollution. Cape Fear River Pollution. Effect of Development on Coastal Ecosystems. Human Impact on Freeman Park.
Invasive Species. Overfishing and Use of Fish Hatcheries. Sea Level Rise. Water Quality. Brackish Marsh. Limestone Sink. Pine Savanna. Longshore transport is the process of beach material being gradually shifted laterally due to waves meeting the shore at an oblique angle. This is essentially the driving force behind the movement and ever-changing face of our beaches.
This page will discuss the process of longshore transport , and how it relates to shoreline erosion , a process that affects all development located on the coastline.
North Carolina is currently looking to reverse a law banning hard structures along the coastline, which has caused enormous controversy throughout the state between economists and environmentalists. This page examines the effects of hard structures, some examples of hard structures already in place in NC, as well as some of the effects seen in other states along the coast. The very factors that draw people to the beach, such as the wind and waves, are what contribute to a constantly changing coastline that washes away in one area as it builds up in another.
These natural processes made the North Carolina coast the way it is, and continue to shape it now. Instead, it breaks diagonally, at an angle onto the beach. The wind and current patterns that influence the diagonal movement of the waves result in non-stop sand movement along the coast.
Physical Factors Before a wave breaks, it picks up sand and other sediments. When the wave breaks on the shore, the sediments are washed up onto the beach diagonally, at the angle that the wave was moving. As the wave washes back out to the ocean, gravity draws it straight down the beach perpendicular to the shoreline, carrying the sediment with it. The result is that the ocean water, and the sediment it carries, moves down the beach in a zig-zag pattern.
Over and over, the sand and sediment gets picked up in one spot and deposited downstream onto another spot. It is through this process that sand is gradually eroded away in one area, and accumulated further down the beach in another area. On the North Carolina coast this pattern of longshore transport happens primarily in a north-to-south direction, although it can change direction during certain times of the year.
In addition to the north-south movement of sand, barrier islands also continually move west, toward the mainland. Resulting from wind, rising sea levels, and strong storms, such as hurricanes, these islands are gradually pushed closer to the landward side. In the latter case, suspended sediment is confined to a relatively thin layer near the bed.
In both cases, the vertical distribution of sediment in the water column can be approximated by:. Littoral transport is the term used for the transport of non-cohesive sediments, i. The littoral transport is also called the longshore transport or the littoral drift.
Littoral transport is often described under the assumption that the shoreline is nearly straight with nearly parallel depth contours. This assumption is very often valid, especially if the sections of the shore are not too long and if a gradual transition between such sections is assumed. Under these circumstances, the littoral transport can briefly be described as follows. When waves approach the shoreline obliquely, refraction tends to turn the wave fronts so that they are almost parallel to the shoreline.
At the same time, when approaching the breaker zone , they undergo shoaling, which means that they become steeper and higher. Finally, the waves break. During the breaking process, the associated turbulence causes some of the seabed sediments to be brought into suspension. These suspended sediments, plus some of the sediments on the seabed, are then carried along the shoreline by the longshore current, which has its maximum near the breaker line.
The two transport modes are referred to as suspended transport and bed load, respectively. The sum of these is the littoral drift. It can be seen that the littoral drift varies strongly with several parameters.
It is therefore crucial to have exact data when making littoral drift calculations. It is an important point that the littoral drift over the coastal profile depends not only on the hydrodynamics but also very much on the variation of the sediment characteristics over the profile. Hence, the sediment distribution along the coastal profile should be taken into account whenever possible. A littoral drift budget for a coastal profile is the sum of littoral transport contributions caused by all the possible combinations of wave heights and directions, as well as tide and storm surge.
Consider, for example, a coastline oriented north-south with the sea to the west. All wave components from south to west will yield northward littoral drift contributions, and all wave components from west to north will yield southward littoral drift contributions.
The sum of the northward drift components is called the northward littoral drift , and similarly is the sum of the southward drift components referred to as the southward littoral drift. The difference between the northward and the southward littoral drifts is called the net littoral drift , which is associated with a net littoral drift direction. The sum of the northward and the southward drift rates is called the gross littoral drift , which has no direction.
Littoral drift budgets can be made for any period relevant for the site under study as long as there are sufficient data. An overview of the magnitude of littoral drift is provided in the following table as a function of the following parameters:. An important parameter in relation to the littoral drift conditions is the variation of the net transport with varying orientation of the coastline. This means that the sand will accrete upstream of the groyne forming a coastline with the orientation, which gives zero transport.
The efficiency of the groyne depends very much on the angle between the present orientation of the coastline and the orientation of net zero transport.
If this angle is small, the groyne will be efficient, as it will be able to hold a long sand filet. If the angle is large, which is the case with a very oblique wave exposure, the groyne will only be able to hold a very short sand filet, which means that a groyne will not be an applicable type of coast protection in this case.
When discussing the littoral transport along a coastline in general, it is always the net littoral drift that is referred to unless otherwise specified. Gradients in the net littoral drift along a section of coast lead to coastline erosion or accretion. The littoral drift also depends on the sea current, although to a much smaller extent than it depends on the longshore current.
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