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Or you can download these pages in several formats that you can include in your course website or local Learning Managment System. Learn more about using, modifying, and sharing InTeGrate teaching materials. Longshore Currents Very rarely do wave trains approach a shoreline aligned perfectly parallel to the trend of the shoreline, much more common is for wave trains to arrive at an angle to the trend of the shoreline. Reuse of InTeGrate Materials We encourage the reuse and dissemination of the material on this site for noncommercial purposes as long as attribution to the original material on the InTeGrate site is retained.
Show terms of use for text on this page » Page Text A standard license applies as described above. Click More Information below. Images image details Provenance Credit: M. We learned in section However, most waves still reach the shore at a small angle, and as each one arrives, it pushes water along the shore, creating what is known as a longshore current within the surf zone the areas where waves are breaking Figure Another important effect of waves reaching the shore at an angle is that when they wash up onto the beach, they do so at an angle, but when that same wave water washes back down the beach, it moves straight down the slope of the beach Figure The upward-moving water, known as the swash , pushes sediment particles along the beach, while the downward-moving water, the backwash , brings them straight back.
With every wave that washes up and then down the beach, particles of sediment are moved along the beach in a zigzag pattern. The combined effects of sediment transport within the surf zone by the longshore current and sediment movement along the beach by swash and backwash is known as longshore transport , or littoral drift. Longshore transport moves a tremendous amount of sediment along coasts both oceans and large lakes around the world, and it is responsible for creating a variety of depositional features that we will discuss in section They are often located in topographic depressions in nearshore bars and thus topographically constricted.
A cell circulation system consists of a slow onshore directed mass transport across bars and two longshore directed feeder currents in the trough that converge on the rip current per se. The rip current is again subdivided into the rip neck, located in the rip channel across the bar, and the rip head seaward of the bar where the rip current expands and slows down.
Rip currents are often rhythmically spaced along the beach, having wavelengths of approximately m and they are forced by longshore setup gradients. Such setup gradients occur in the case of longshore wave height gradients or in the case when the topography is non-uniform alongshore. Longshore gradients in wave dissipation create longshore gradients in setup that force the rip currents.
As rip currents tend to scour out the depressions in the bar, bathymetry and hydrodynamics evolve in a mutually dependent way called 'morphodynamic feedback'. See also: Rhythmic shoreline features. Along a straight shoreline, the above-mentioned shore-parallel and shore-normal current patterns dominate.
The currents discussed here are two-dimensional in the horizontal plane due to complex bathymetries and structures in the nearshore zone. The nature of the obstruction of the shore-parallel currents of course depends on the extension and shape of the coastal structure. If the structure is located within the surf zone , the obstruction leads to offshore-directed jet-like currents, which cause loss of beach material.
If the structure is a port, the current will follow the upstream breakwater and finally reach the entrance area.
The currents in the entrance area will both influence the navigation conditions and cause sedimentation. The design of the entrance is important; it must provide a smooth and predictable current pattern so its impact on navigation is acceptable, sedimentation must be minimised and the bypass of sand must be optimised.
The answer is a smooth layout of the main and secondary breakwaters combined with a narrow entrance pointing towards the prevailing waves.
At the leeward side of coastal structures, current patterns caused by the sheltering effect of the structure in the diffraction area can develop. Sheltered or partly sheltered areas may result in circulation currents along the inner shoreface as well as return currents leading to deep water. The reason for this is that the wave set-up in the sheltered areas is smaller than in the adjacent exposed areas and this generates a gradient in the water-level towards the sheltered areas.
These circulation currents in the sheltered areas can be dangerous for swimmers who are using the sheltered area for swimming during rough weather. Another problem is that the sheltered areas will be exposed to sedimentation and such areas must, therefore, be avoided when planning small ports. If the structure extends beyond the breaker zone , the shore-parallel current will be directed along the structure, where the increasing depth will decrease the speed. The current will deposit the sand in a shoal off the breaker zone upstream of the structure.
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