海岸动力学

Under mild comoderate waves (less than two to three feet high), this return flow is small.
When the waves are high and approaching nearly perpendicular to shore, the undertow is strongest.
Undertow回卷底流
Undertow is the flow which is dispersed along the shore and moves offshore in the lower half of the water column beneath the waves with the highest velocities close to the bed between the breakers and the shore.
Longshore Currents沿岸流
Obliquely incident water wave climate and nearshore bathymetry are responsible for the wave-induced longshore currents. When waves break at an angle to the beach, the momentum of the breaking wave generates onshore currents that flow in the direction of propagation of the breaking wave.
Forces
Nearshore currents can be forced by a combination of wave breaking, winds, pressure gradients, and topographic effects. As waves approach shore, they drag some of the water with them, assisted by the wind when it is blowing onshore. This excess water piles up between the breakers and the beach.
Influence
Nearshore current is also important to recreation. How strong are nearshore currents? Longshore currents may be moving at two to four miles per hour. Such high velocities can sweep a person along the shore towards a rip current. Rip currents can sometimes have relatively high velocities of four to nine miles per hour and they can carry people offshore into deep water. Undertow can also be quite strong. So nearshore current is critical to beach tourism.
In addition, the salinity content of coastal water is usually lower than that in the deep ocean due to the discharge of fresh river water.
For these reasons, the characteristic nature of neashore water cannot be assumed to be homogeneous.
Goal
The goal of this chapter is to understand basically the forms and dynamics of the flows near shore.
coastal currents induced directly by waves
The longshore current沿岸流, rip current裂流 and undertow 回卷底流 are wave-induced currents.
Influence
Nearshore current may affect the coast processes. For example the longshore current can be responsible for moving materials from one part of the beach to another. When a structure is built out into the sea, the longshore current results in sand building up on one side of the structure.
4 Near shore Currents 4-1 Introduction
4-1-1 Ocean Currents
In the ocean there exist currents of almost constant direction and speed throughout the year. They are mainly generated by wind and are classified as drift currents 漂流 and gradient 梯度currents, or density currents and compensation currents according to their generation mechanisms. They are also classified as warm or cold currents depending upon whether their temperatures are higher or lower than that of the neighboring ocean region.
The water that comes ashore must return offshore.
Influence
Nearshore currents may have significant influence on the sediment transport. When the waves approach the beach at an angle, they generate a longshore current which moves parallel to the beach. While this current is not in itself strong enough to pick up sediment particles from the sea bottom, it can move material that has already been stirred up by the waves. So while waves are the most important process moving sediment particles on a beach, currents may also have a role to play.
The current records obtained at a given point of an ocean region indicate a complicated pattern of velocity vectors矢量 as shown in Fig. 4.1.2.
The velocity components for east-west and north-south are treated by harmonic 调和analysis, and analyzed results are plotted as shown in Fig.4.1.3, in which a constant velocity vector and the tidal ellipses of semidiurnal半日的and diurnal 全日的tides are drawn.
The density differences found there generate complicated flow patterns.
4-1-3 Nearshore Currents
Direction
If we look at the coast, we may find nearshore currents. Floating debris, foam patches or foam lines are interesting indicators. If they are drifting parallel to shore, there is a longshore current. If the debris or foams are drifting offshore, there is probably a current offshore. Also you may observe a backwash of breaking waves and this is caused by undertow.
The predictions of the nearshore currents play a crucial role in management and protection of coastlines, designs of coastal structures, and environmental assessments in coastal zones.
Importance
So the estimation of wave-driven nearshore currents is important to understand
the current itself, the subsequent sediment movement
and the evolution of morphology in coastal zones.
全日的
半日的
density current
Due to the shallow water depth of the neashore area, the amount of sunshine, wind, precipitation, and other meterological factors have rather stronger influences on the environmental conditions there than they do in deep ocean areas. Thus the seasonal variation of water temperatures and salinity in shallow regions is comparatively great.
4-1-2 Tidal Currents
Tidal currents play an important role in the nearshore region. These currents are induced by the horizontal movement of the sea water together with tidal motion. For this reason, the motion of these currents has periods which correspond to their tidal constituents.