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1、Coastal Hydrodynamics,Chapter 5 COASTAL SEDIMENT,Stating characteristics of coastal sediment,Stating bed load transport under wave action,Stating suspended sediment transport under wave action,Stating longshore sediment transport,1/48,Physical properties of coastal sediment,Chapter 5,5.1 Characteris
2、tics of Coastal Sediment,2.Modes of coastal sediment transport,3.Threshold of coastal sediment motion,2/48,Chapter 5,The review by Komar and Miller in 1973 found that for grain diameters less than 0.5mm(medium sands and finer),the threshold is best related by,3/48,Chapter 5,For grain diameters great
3、er than 0.5mm(coarse sands and coarser),the threshold is best predicted with,4/48,Chapter 5,The above two relationships are empirical equations.The former can be used to evaluate the threshold for grain sizes at least as fine as the lower silt range,where cohesive effects can be expected to cause de
4、partures from the established relationships.The latter gives good results for grain sizes larger than 0.5mm and as coarse as 5cm.,5/48,Chapter 5,The threshold evaluated with these two equations have been used to calculate water depths to which a range of sediment grain sizes could be set in motion b
5、y waves.It is seen that waves of this period would be capable of moving sediments to depths of 100m and more.This conforms with the observations of oscillatory ripple marks on continental shelves to these depths.,8/48,Chapter 5,For engineering applications,knowledge of the water depth where sediment
6、 particles move significant distances due to wave action is important for determining the initiation 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
7、movement.,9/48,Chapter 5,Various formulae for determining the critical water depth of sand inception can be expressedin the following common form:,10/48,Chapter 5,The reasons why different values of coefficient are taken in each formulae are that different criteria were used for the inception of mot
8、ion,and the relationship were established using data obtained under certain limited conditions.,11/48,Chapter 5,Sato and Tanaka proposed two expressions based upon their laboratory and filed data obtained using radioactive glass sand as a tracer.The first one gives the critical water depth for surfa
9、ce layer movement,which is defined as the state where almost all sand particles in the first layer move due to wave action.,12/48,Chapter 5,The second gives the critical water depth for completely active movement or significant movement of sediment particles judging from the movement of radioactive
10、glass sand particles:,13/48,Chapter 5,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 movement corresponds to such great movement of bed material as to produce
11、a water depth variation.,14/48,Quasi-steady approach 准稳定流方法,Chapter 5,5.2 Bed-load Transport,2.Energetics model 能量模型,15/48,Chapter 5,Stokes waves in shallow water give a forward orbital motion under the wave crests that is short in duration but high in velocity,while the return flow under the trough
12、s is slower but of longer duration.,1.Quasi-steady approach,16/48,Chapter 5,The asymmetry of the wave orbital motions with a strong onshore velocity versus a weaker offshore velocity but a longer duration,causing a net shoreward transport of the coarser sediment.,17/48,Chapter 5,Madsen and Grant(197
13、6)introduced a formula applicable to oscillatory flow,by adapting from Browns formula for steady flow.The bed load transport rate averaged over one-half wave period,is given in dimensionless by,18/48,Chapter 5,In 1956 Bagnold introduced the idea of the bed load sediment transport based on the concep
14、t of the work done by the flow in moving the grains.This introduced an efficiency factor relating the work done on the grains to the energy available.In 1963,He extended his concept of work done by water in moving sediment particles to include wave effects.,2.Energetics model,19/48,Chapter 5,Schemat
15、ic of model for sand transport wherein the orbital velocity due to waves places the sand in motion,and the current provides a net transport of sand,20/48,Chapter 5,He assumed that a mean mass of sediment is supported over a unit bed area by the mean velocity of the to-and-fro motion.Under the orbita
16、l motion of waves,sediment is moved with a back-and-forth motion with no net transport even though wave energy is expended.This expenditure of energy supports and suspends the sand above the bottom,so that no additional stress is required to transport it.,21/48,Chapter 5,The presence of any unidirec
17、tional current superimposed on the to-and-fro motion therefore could produce some net drift of sediment.The magnitude of this current does not matter in that it does not have to provide the stress to support the sediment above the bottom,this having already been accomplished by the waves.,22/48,Chap
18、ter 5,With the energetics model,Bagnold derived therelationship for the sediment transport per unitwidth,q,in the direction determined by theunidirectional current u.,23/48,Chapter 5,5.3 Suspended Sediment Transport,1.Sediment suspension in the vicinity of ripples,2.Suspended sediment transport rate
19、,24/48,Chapter 5,It has been known for many years that sand ripples are generated on the seabed under the influence of waves.The shape of a wave-generated ripple can be simply expressed by its height and length.Sand ripples are of special interest for coastal sediment transport studies because their
20、 influence on the boundary layer structure and the sediment transport mechanism is very strong.,1.Sediment suspension,25/48,Chapter 5,Suppose we place sand on a flat bed in a wave flume and carefully observe the phenomena appearing under the influence of wave action.It will be found that as the wave
21、 height and period increase,the flow in the vicinity of the sand bed changes from laminar to turbulent,and sand ripples appear on the bottom.,26/48,Chapter 5,When a wave crest reaches the location of a sand ripple,the bed materials move along the bottom surface in the direction of wave propagation a
22、s bed load material.Hence the sand ripples advance in the same direction.When a trough approaches,the vortex formed behind the sand ripple is lifted upward with the suspended sediment,transported in the opposite direction to wave propagation,disperses,and finally drops to the bottom.,27/48,Chapter 5
23、,Vortex is formed between the ripples during the onshore and offshore semi-orbital motions.At times of reversal in the direction of orbital motion the vortex is thrown upward off the bottom,the sand being placed in suspension and therefore most susceptible to the current and a net transport.,28/48,C
24、hapter 5,The vortex generated on the down-wave face during onshore semi-orbits becomes by far the stronger,and when thrown upward from the bottom is directed in the offshore direction,accounting for the reduction in transport and the one case of an offshore net transport.,29/48,Chapter 5,Therefore,t
25、he difference between the amount of sediment transported as bed load in the direction of wave propagation and that transported as suspended sediment in the opposite direction determines the direction and amount of net transported.It is apparent that any solution for onshore-offshore transport of sed
26、iments under wave actions is very complex.,30/48,Chapter 5,The suspended sediment transport rate is usually given with the following equation.,2.Transport rate,The horizontal velocity of a sediment particle can generally be assumed equal to the horizontal velocity of the immediately surrounding flui
27、d.The key point is to determine the reference concentration and the sediment diffusivity.,31/48,Chapter 5,It should be noted that both wave irregularity and breaker have profound and highly diverse effects on the magnitude and distribution of suspended sediment concentrations.,32/48,Chapter 5,5.4 Lo
28、ngshore Sediment Transport,Wave power approach 波能流法,33/48,Chapter 5,Littoral drift is defined as sedimentary materialmoved in the nearshore zone under the influenceof waves and currents.The movement associated with littoral drift is called littoral transport.In order to predict future coastal change
29、,the littoral transport rate must be determined,which means the rate of sedimentary material parallel or perpendicular to the shore in the littoral zone.,34/48,Chapter 5,In the treatment of littoral transport,it is quite common to consider separately sediment movement perpendicular and parallel to s
30、hore.Sediment transport perpendicular to the shore(cross-shore or onshore-offshore transport)is a significant factor controlling short-term variations in beach profile,whereas transport parallel to the shore(long-shore transport)is more significant for both long-term and large-scale variations in th
31、e beach.,35/48,Chapter 5,In order to estimate the longshore transport rate,the rate at which sediment moves alongshore,from a knowledge of the wave and current parameters that cause the transport,coastal engineers have relied most heavily on empirical correlations between the rate of sand drift and
32、the expressions so-called“longshore component of wave energy flux”.,36/48,Chapter 5,The immersed-weight transport rate and the longshore component of wave energy flux can be related by,37/48,Chapter 5,The above correlation between the sand transport rate and the longshore component of wave energy fl
33、ux,although apparently successful,is basically empirical,and its formulation involved no real considerations of the mechanics of sand transport.A more fundamental examination of the processes involved is therefore needed.,38/48,Chapter 5,Bagnold developed an energetics model of sediment transport.Ac
34、cording to the model,the stress exerted by the wave motion supports and suspends sediment above the bottom.Superimposed upon this to-and-fro motion is any unidirectional current which produces a net transport of the sediment,the direction of transport being the same as the current.Since waves have a
35、lready supplied the power to put the sand into motion,the unidirectional current can cause a net transport no matter how weak the flow,even if the current taken alone is below the threshold of sediment transport.,Chapter 5,According to the model,the immersed-weightsediment transport rate per unit be
36、d width,isgiven by the following relationship,A portion of the wave energy flux is dissipated in placing the sand in motion,40/48,Chapter 5,u0 is the mean speed of the wave motion relative to the bed within the surf zone and is assumed to be proportional to the orbital velocity near the bottom just
37、before the wave breaks.Once the sediment is in motion,it becomes available for transport by the longshore current.So that the total immersed weight of sediment transported in unit time past a section of beach becomes,41/48,Chapter 5,The above equation was tested by Komar&Inman(1970)with the field me
38、asurements theyobtained.They suggested that the coefficient Kshould be taken as 0.28,and the u0 be replacedby umb,which is the maximum orbit velocityunder the breaking wave.,42/48,Chapter 5,The mean velocity of the longshore current is,The expression based on the energetics model is the same as the
39、empirical correlation.,43/48,Chapter 5,Il is the immersed-weight transport rate.There are two distinct advantages in relating Il and Pl.The first is that Il has units of work(energy flux,power),the same as Pl.The second advantage in using the immersed-weight transport rate is that it takes intoconsi
40、deration the density of the sediment grains.Thus a correlation between Il and Pl could beused for beaches composed of coral sand and soforth,as well as the more usual quartz sand.,44/48,On a north-south-trending beach,the sand may move northward for a time due to waves arriving from the south and th
41、en later move to the south under waves coming from the north.The net transport of sediment under these two wave trains will be the difference between the north and south movements.This net transport is generally small,much smaller than the total transport up and down the beach,and on some beaches ma
42、y be essentially zero.This change in direction may be seasonal.,45/48,Some indirect approach must be taken in evaluating the budget of sediments.Measuring rates of accretion or bypassing of sand at a littoral barrier;Computing the littoral drift from statistical wave data utilizing an equation which relates the two;Measuring the rate of dilution of heavy minerals within the beach sands.,46/48,47/48,THANK YOU,“Coastal Hydrodynamics”chapter 5,ZHENG Jinhai Apr 2008,
