1、Coastal Hydrodynamics,Chapter 4 NEARSHORE CURRENTS,Stating types of currents,Stating concept of radiation stress(輻射應(yīng)力),Stating phenomena of wave set-up (波浪增水) and wave set-down (波浪減水),Stating characteristics of longshore currents (沿岸流),Ocean currents,Chapter 4,4.1 Outline of Currents,2. Tidal curren

2、ts,3. Nearshore currents,Chapter 4,In the ocean, there exit currents of almost constant direction and speed through the year. Ocean currents are mainly generated by wind and are classified as drift currents and gradient currents, or density currents and compensation currents according to generation

3、mechanism. They are also classified as warm or cold currents depending on whether their temperatures are higher or lower than those of the neighboring ocean region.,1. Ocean currents,Chapter 4,Tidal currents are induced by the horizontal movement of the sea water together with tidal motion. The moti

4、on of tidal currents has periods which correspond to their tidal constituents. Tidal currents occasionally play an important role in the nearshore region. They are easily influenced by the sea bottom and coastal topography, thus they have strongly localized characteristics. Hence it is rather diffic

5、ult to establish a clear relationship between tides and tidal currents in general.,2. Tidal currents,Chapter 4,The nearshore current consists of the mass transport induced by wave action, the longshore current and rip currents.There are two wave- induced current systems in the nearshore zone. These

6、are: a cell circulation system of rip currents and associated longshore currents, longshore currents produced by an oblique wave approach to the shoreline.,3. Nearshore currents,Chapter 4,The slow mass transport, the feeding longshore currents, and the rip currents taken together form a cell circula

7、tion system in the nearshore zone.,Chapter 4,It is well known that when waves approach a straight coastline at an oblique angle, a longshore current is established flowing parallel to the coastline in the nearshore zone.,Chapter 4,It is seen that, under an oblique wave approach with both systems pre

8、sent, the current pattern becomes asymmetrical, with the zero velocity node just updrift from the rip current and a velocity increase extending from there to the next rip current.,Chapter 4,The cell circulation is important in renewing the water in the nearshore zone and removing pollutions such as

9、mud and sewage that may be dumped there. The longshore current is particularly significant in that it is responsible for the net transport of sand or other beach material along the shore.,Chapter 4,Fishermen and others living near the coast have been aware of these currents for several years through

10、 their experiences in nearshore water. Johnson considered the longshore current and undertow as wave-induced currents in his classic treatise. He attempted to treat coastal sediment movement and coastal processes by regarding these currents as the media of sediment transport.,Chapter 4,In 1941, Shep

11、ard et al. published a paper entitled “Rip Currents”, in which they defined rip currents as currents in the offshoreward direction which return the sea water transported shoreward by wave action. The function of rip currents in carrying away fine suspended sediment particles from the surf zone to de

12、eper water region seems to be important in the overall pattern of sediment movement in the nearshore area.,Chapter 4,The rip current is the dark patch. There is intense wave breaking on both sides of the rip currents with little breaking within the deeper rip channel, where bubbles are advected seaw

13、ard.,Chapter 4,Chapter 4,In 1949 Putnam, Munk and Traylor proposed formulae to evaluate the mean velocity of longshore currents. Their results encouraged researchers to undertake further studies to clarify the longshore sediment transport mechanism and to estimate the longshore transport rate.,Chapt

14、er 4,In 1961, Longuet-Higgins and Stewart presented the concept of radiation stress and applied it to analyze various water wave problems such as wave-current and wave-wave interactions. Since the end of the 1960s, the application of the radiation stress concept has been extended to such as wave set

15、-down, wave set-up, lonshore currents, rip currents, and the nearshore current system.,Definition,Chapter 4,4.2 Radiation Stress,2. Expressions,3. Applications,Chapter 4,There is a stress field in water waves, which is similar to the Reynolds stress in turbulence. This force is usually called moment

16、um flux, since it is resulted from the exchange of momentum among the water particles.,1. Definition,Chapter 4,Radiation stress is defined as the excess momentum flux induced by the existence of wave motion. It equals the difference between the total momentum fluxes and the hydrostatic pressure in t

17、he absence of waves.,Chapter 4,The radiation stress across the plane x=constant in the direction of wave advance is,The radiation stress of y-momentum across the plane y=constant,The transverse radiation stress in the direction of waves is,Chapter 4,If the x-axis is placed in the direction of wave a

18、dvance and the y-axis parallel to the wave crests, then there are two nonzero components to the radiation stress: the x- and y- fluxes of x-momentum and y-momentum.,2. Expressions,Chapter 4,In the more general case, when a linear wave is propagating at some angle to the direction of wave propagation, we have,Chapter 4,radiation stresses on unit water column,Chapter 4,Radiation stress has been proved to be a very powerful tool in the study of a variety of oceanographic phenomena. In the context of littoral processes, it has been used to predict ch