Coastal Hydrodynamics_复习海岸动力学课件

1、“Bilingual Course”ZHENG Jinhai April 2009Coastal HydrodynamicsHOHAI UNIVERSITY CONTENTSChapter1 IntroductionChapter2 Wave TheoryChapter3 Wave TransformationsChapter4 Nearshore CurrentsChapter5 Coastal SedimentChapter6 Coastal Processes1/130Chapter 1 INTRODUCTIONIntroducing characteristic features of

2、 coast of China Introducing development of coastal zone resources in China Introducing background information of coastal engineering Introducing methods to study coastal hydrodynamics 2/130Chapter 2 WAVE THEORYStating classifications of waves Stating the basic equations of wave motion Stating the sm

3、all amplitude wave theory Stating the finite amplitude wave theory Stating wave theory limits of applicability 3/1301. Classification of waves 2.1 Description of Wave Motion2. Methods of describing fluid motion 3. Theories commonly used to describe wave motion 4. Basic parameters of regular waves 4/

4、130 analytical wave theories: linear wave theory (线性波理论) Trochoidal wave theory (余摆线波理论) Stokes wave theory (STOKES波理论) cnoidal wave theory (椭圆余弦波理论) solitary wave theory (孤立波理论) numerical wave theories: stream function wave theory, etc. (流函数波理论)6/130Wave height (波高) is the vertical distance between

5、 a crest and a trough of the wave train. Wave length (波长) is the horizontal distance between two successive wave crests.Water depth (水深) has notable influence on the wave characteristics, especially when waves are in shallow water. Wave period (波周期), T, is the time required for two successive crests

6、 or troughs to pass aparticular point. 1. Assumptions 2.2 Basic Equations of Wave Motion 2. Governing equation 3. Boundary conditions 4. Discussion 8/130B.B.C. on z= -h L.B.C. K.F.S.B.C. on z= D.F.S.B.C. on z= G.D.E. L.B.C. G.D.E. B.B.C. K.F.S.B.C. D.F.S.B.C. L.B.CG.D.EB.B.CD.F.S.B.CVelocity Potenti

7、alAssuming that the wave slope is small (H/L1) and that the water depth is much greater than the wave height (h/L1), the solution of velocity potential is: While the elevation of the water surface is Substituting the velocity potential and the surface elevation into the K.F.S.B.C yields the dispersi

8、on relationship. A deep water wave is a wave whose wavelength is very small compared with the water depth. The dispersion relationship is: ororThe water particle path of linear wave isThe pressure field associated with a progressive wave is determined from the unsteady Bernoulli equation. The pressu

9、re equation contains two terms: the hydrostatic pressure (静水压强) & the dynamic pressure (动水压强) The maximum value or the minimum one appears when wave crest or trough reaches a given point respectively. Hydrostatic &dynamic pressure at various phaseChapter 3 WAVE TRANSFORMATIONS Stating ocean wave cha

10、racteristics Stating transformations of waves entering shallow water 43/130Statistical characteristics of ocean waves 3.1 Ocean Wave Characteristics2. Wave height distribution and wave period distribution 3. Ocean wave energy spectra 4. Deep-water wave propagation 44/1301. Statistical characteristic

11、s Zero-up crossing method Statistically representative waves The significant wave (有效波高) corresponds to the average of the heights of the one-third highest waves.3. Wave spectrum Wave frequency spectrum Directional wave spectrum 4. Wave propagation in the ocean If R is the distance from the leading

12、edge of the storm fetch to point A on the coast, then time tob of first observation of arrival of the waves is Conservation of wave equations 3.2 Wave transformations in shallow water 2. Wave transformation in shoaling water3. Wave refraction 4. Wave reflection 5. Wave diffraction 6. Wave breaking50

13、/130The conservation of wave equation can be expressed as 1. Conservation of wave equationsThis equation states that any temporal variation of the wave number vector must be balanced by spatial changes of the wave angular frequency. 2. Wave transformation in shoaling waterAssuming that the energy fl

14、ux is conserved in the process of wave propagation, the wave height at a given water depth can be determined by: 3. Wave refraction For straight coasts with parallel contours, Wave convergence or divergence The reflection coefficient is defined as which varies with the angle of the slope, the incide

15、nt wave steepness and the characteristics of the slope. 4. Wave reflection 5. Wave diffraction The diffraction coefficient （绕射系数） is defined as the ratio between the diffracted and incident wave heights.6. Wave BreakingThe limiting steepness （极限波陡） isA widely accepted breaker criterion isThere are t

16、hree types of breakers : spilling（崩破波）, plunging（卷破波）, surging （激破波）. Chapter 4 NEARSHORE CURRENTS Stating types of currentsStating concept of radiation stress Stating phenomena of wave set-up and wave set-down Stating characteristics of longshore currents58/130Ocean currents 4.1 Outline of Currents

17、2. Tidal currents3. Nearshore currents59/130The 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 are: a cell circulation system of rip currents and associated longsh

18、ore currents, longshore currents produced by an oblique wave approach to the shoreline. 3. Nearshore currents Definition 4.2 Radiation Stress2. Expressions 3. Applications 63/130Radiation stress （辐射应力） is defined as the excess momentum flux induced by the existence of wave motion. It equals the diff

19、erence between the total momentum fluxes and the hydrostatic pressure in the absence of waves. 1. DefinitionRadiation 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 changes in t

20、he mean water level (set-up and set-down) in the nearshore region and to analyze the generation of longshore currents. Other applications have been to the generation of surf beat, the interaction of waves with steady currents. 3. Applications Wave set-down 4.3 Wave Set-down and Wave Set-up2. Wave se

21、t-up 69/130Consider a train of waves encountering the coast with normal incidence. For a short distance dx, a force balance can be developed. The pressure gradient of the sloping water surface balances the change of the incoming momentum. There is therefore a change in mean water surface slope whene

22、ver there is a change in the radiation stress. Wave set-down is a phenomenon that the mean sea level falls below the still water level from offshore to the breaking point.1. Wave set-downWave set-up is a phenomenon that the mean sea level rises above the still water level inside the surf zone. 2. Wa

23、ve set-upMean velocity 4.4 Longshore Currents2. Horizontal velocity distribution 75/1301. Mean velocityFollowing Bowens suggestion and development, Longuet-Higgins (1970) derived the following relationship The sand transport studies of Komar and Inman (1970) had earlier suggested that In order to es

24、tablish a relationship between the longshore current and coastal sediment phenomena, it is necessary to know the horizontal and vertical longshore current velocity distributions in the nearshore area. Extensive analyses have been carried out on the horizontal distribution. However, the vertical dist

25、ribution is still a problem to be solved in the future. 2. Velocity distributionUsing the radiation stress approach leads to the solution The inclusion of horizontal mixing effectively couples together the adjacent elemental water columns, resulting in a diffusion in a direction perpendicular to sho

26、re. The lateral diffusion of momentum also enables the momentum flux inside the surf zone to drive longshore currents outside the surf zone. Chapter 5 COASTAL SEDIMENTStating characteristics of coastal sedimentStating bed load transport under wave action Stating suspended sediment transport under wa

27、ve action Stating longshore sediment transport 79/130Physical properties of coastal sediment5.1 Characteristics of Coastal Sediment2. Modes of coastal sediment transport 3. Threshold of coastal sediment motion 80/130Coastal sediment particles are transported by the influence of waves and nearshore c

28、urrents in the onshore or offshore directions, or parallel to the shoreline. There are two modes of sediment transport: suspended sediment movement and bed load movement. 2. Modes of movement The threshold of sediment motion under wave action will govern whether shingle is being transported on a bea

29、ch under certain wave conditions. It also determines to what water depths sand is in motion and could therefore be carried onshore to add to the beach volume, or carried alongshore to contribute basically to a sediment drift parallel to the shoreline. 3. Threshold of motion The review by Komar and M

30、iller in 1973 found (medium sands and finer), the threshold is best related by For grain diameters greater than 0.5mm (coarse sands and coarser), the threshold is best predicted with For a given grain density and diameter, the threshold under waves can therefore be established by a certain wave peri

31、od and orbital velocity or semi-diameter. Only two of these three parameters need to be established in defining the threshold since For engineering applications, knowledge of the water depth where sediment particles move significant distances due to wave action is important for determining the initi

32、ation point for the beach profile change of the offshore region. Numerous research efforts have been conducted during the last five decades concerning the critical water depth for the inception of sediment movement. Sato and Tanaka proposed two expressions based upon their laboratory and filed data

33、obtained using radioactive glass sand as a tracer. The first one gives the critical water depth for surface layer movement, which is defined as the state where almost all sand particles in the first layer move due to wave action. The second gives the critical water depth for completely active moveme

34、nt or significant movement of sediment particles judging from the movement of radioactive glass sand particles: The surface layer movement defined here corresponds to the state where the first layer of particles of the sea bed move collectively in the direction of wave propagation.Completely active

35、movement corresponds to such great movement of bed material as to produce a water depth variation. 5.3 Suspended Sediment Transport1. Sediment suspension in the vicinity of ripples 2. Suspended sediment transport rate 98/1301. Sediment suspension Chapter 6 COASTAL PROCESSESStating beach nomenclature

36、 Stating beach profile Stating coastal change 109/130In the treatment of coastal sediment transport, it is quite common to consider separately sediment movement perpendicular to the shoreline and that parallel to it. The sediment movement perpendicular to the shoreline is considered to be the more s

37、ignificant one for the short-term variation of coastal processes, while that parallel to the shoreline is the more significant one for the long-term variation of the coast. 6.1 General Beach Nomenclaturenearshore zone offshore zone inshore foreshore backshore coastline longshore bar longshore trough

38、 beach face berm crest berms beach scarp cliffEquilibrium beach profile 6.2 Beach Profile2. Beach profile changes 113/130During the progress of the experiment in which a constant wave input is maintained, the beach profile will reach a steady state condition, that is, the beach profile in the flume

39、will approach a particular one, which is named the equilibrium beach profile （平衡海滩剖面）. 1. Equilibrium beach profile Typical types of beach profile Experimental study2. Beach profile changes115/130The storm beach profile with bars versus the swell profile with a pronounced berm that occurs under swel

40、l waves conditions bar trough sea cliff storm profile swell profile bermWith storm waves the sand seaward of the breaker zone moves shoreward, while sand in the surf zone is transported in an offshore direction. This convergence of the sand transport directions must result in an accumulation of sand

41、 at the breaker position, forming a bar. With flatter swell waves, the sand is moved landward at all depths, within the surf zone as well as beyond the breaker zone, so that it accumulates on the berm. According to their laboratory investigations,Sunamura and Horikawa proposed the following semi-empirical criterion on whether a beach will erode or accrete. Shoreline