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Near shore currents

Name: Waleed Waheed Mohamed

Sec:7

B.N:42

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Current in the Nearshore Zone

Nearshore mean currents which occur within the surf zone are principally driven

by the breaking waves. For purposes of simplification, nearshore mean currents are

usually separated into their cross-shore and longshore components: Undertows and

rip currents have their principal axes oriented perpendicular to the beach (offshore)

while longshore currents act parallel to the beach. These currents are all driven by

cross- and/or longshore components of  radiation stress gradients (in practive wave

energy gradients) that arise through wave breaking.

Shore-parallel currents

The longshore current is the dominant current in the nearshore zone. The

longshore current is generated by the shore-parallel component of the stresses

associated with the breaking process for obliquely incoming waves, the so-called

radiation stresses, and by the surplus water which is carried across the surf zone

towards the coastline. This current has its maximum within the breaker zone. 

During storms the longshore current can reach speeds exceeding 2.5 m/s. Thelongshore current carries sediment along the shoreline, the so-called littoral drift;

this mechanism will be discussed further in Coastal Hydrodynamics And Transport

Processes. 

The longshore current is generally parallel to the coastline and it varies in strength

approximately proportional to the square root of the wave height and with sin2αb,

where αb is the wave incidence angle at breaking. As the position of the breaking

line constantly shifts due to the irregularity of natural wave fields and since the

distance to the breaker line varies with the wave height, the distribution of thelongshore current in the coastal profile will vary accordingly.

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Shore-normal currents

Undertow

Fig. 2. Undertow velocities measured on a Danish beach during high and moderate

wave conditions. Velocities are seaward direcetd and hence negative in the

figure.</ 

[1]

The undertow is defined as a longshore homogeneous current flowing offshorenear the seabed and it is driven by the cross-shore setup gradient, i.e. the radiation

stress decay. The offshore discharge of water is compensated by the onshore

directed mass transport and roller transport in the upper layers of the water column.

Fig. 2. below shows typically occurring undertow velocities on a beach during

moderate and high wave conditions. During moderate conditions, only few waves

break on the outer bar, and undertow velocities are small. In conditions with large

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waves, undertow velocities may be up to about 50 cm/s.

Rip currents

Fig. 3. Distribution in longshore current in a coastal profile and rip current pattern.

At certain intervals along the coastline, the longshore current will form a rip

current. It is a local current directed away from the shore, bringing the surplus

water carried over the bars in the breaking process back into deep water. The ripopening in the bars will often form the lowest section of the coastal profile; a local

setback in the shoreline is often seen opposite the rip opening. The rip openingtravels slowly downstream.

[1]Rip currents are narrow, jet-like currents which are directed seaward across the

surf zone. They are often located in topographic depressions in nearshore bars and

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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 100-1000 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. Such non-uniform alongshore

topography can consist of alternating shoals/bar horns where wave dissipation is

strong, and depressions in the bar where dissipation is weaker. 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, a positive

morphodynamic feedback can exist between bathymetry and hydrodynamics. 

Cross-currents along the shore-normal coastal profile

Cross-currents occur especially in the surf-zone. Three contributions balance each

other:

  Mass transport, or wave drift, is a phenomenon occurring during wave

motion over both sloping and horizontal beds. Water particles near the

surface will be transported in the direction of wave propagation when waves

travel over an area. This phenomenon is called the mass transport. In the

surf-zone the mass transport is directed towards the coast.

  Surface roller drift. When the waves break, water is transported in the

surface rollers towards the coast. This is the so-called surface roller drift.

  Undertow. In the surf-zone, the above two contributions are concentrated

near the surface. As the net flow is zero, they are compensated for by a

return flow in the offshore direction, which is concentrated near the bed.

This is the so-called undertow. The undertow is important in the formationof bars.

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Two-dimensional currents in the nearshore zone

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.

Two-dimensional current patterns occur, especially in the following situations:

1.  When the bathymetry is irregular and very different from the smooth shore-

parallel pattern of depth contours characteristic of sandy shorelines, and also

when the coastline is very irregular. This can, for example, be at partially

rocky coastlines or along coastlines where coral reefs or other hard reefs are

present. Irregular depth contours give rise to irregular wave patterns, which

again can cause special current phenomena important to the understanding

of the coastal morphology. Irregular bathymetry combined with an irregular

coastline adds further to the complexity of the wave and current pattern.

Reefs provide partial protection against wave action. However, they also

generate overtopping of water and compensation currents behind the reef. Atlow sections of the reef or in gaps in the reef, the surplus water returns to the

sea in rip-like jets. This is the pattern for both submerged reefs and emerged

reefs with overtopping during storms. Such current systems are of great

importance to the morphology behind the reef. Changes in reef structure,

natural or man-made, can cause great changes in the morphology.

2.  In the vicinity of coastal structures, such as groynes, coastal breakwaters and

port structures. Such structures influence the current pattern in two

principally different ways: by obstructing the shore-parallel current and bysetting up secondary circulation currents.

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Obstructions

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Fig. 4. Lee circulation patterns for a coastal breakwater and a small port. The

optimal shape of a small port, avoiding the lee area.

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, consequently 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.

Leeward side

At the leeward side of coastal structures, special 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.

Beyond breaker zone

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. In the case of a major port, the longshore current will not reach theentrance area. In the lee area of a major coastal structure, the effect of return

currents towards the sheltered area will also be pronounced, but the current

circulation pattern will be smoother and less dangerous for swimmers. Thesheltered areas will act as a sedimentation area adding severely to effects of the lee

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side erosion outside the sheltered area of such structures. Once again, shelteredareas should be avoided.

Special morphological features

Fig. 5. Ebb and flood shoals at tidal channel, Cay Calker, Belize. This area is

mainly exposed to the tidal currents, whereas the wave climate is very mild.

Adjacent to special morphological features such as sand spits, river mouths and

tidal inlets. The current patterns and the associated sediment transport at such

locations can be very complicated. Only a few general comments will be given inthis overview of currents and their impacts.

In tidal inlets and river mouths there are often concentrated currents in the gorge

section of the mouth, but seawards of this area the current pattern expands and the

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current speed decreases. This is also the case landwards of the gorge section in

tidal inlets. The gorge section is often deep and narrow, whereas the expanding

currents on either side tend to form the ebb and flood shoals respectively. The ebb

shoal tends to form a dome-shaped bar on littoral transport shorelines, on which

the littoral transport bypasses the mouth/inlet.

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References:

http://www.marbef.org