海岸动力学英文课件CoastalHydrodynamics51

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,1/39,Physical properties of coastal sediment,Chapter 5,5.1 Characteris

2、tics of Coastal Sediment,2. Modes of coastal sediment movement,3. Threshold of coastal sediment motion,2/39,Chapter 5,Beach materials mainly consist of the sand and gravel transported by rivers, the sand composing dunes located in the vicinity of the beach, the debris from nearby coastal cliffs.,1.

3、Physical properties,3/39,Chapter 5,The sediment physical properties, which must be known in order to investigate coastal change and littoral drift, are distribution of grain size, shape, roundness, mineral composition, porosity, permeability, etc. Among them, grain size distribution and mineral comp

4、osition are generally more important.,4/39,Chapter 5,Grain size distribution can be studied by sieve analysis of sampled material. It is known to be expressible through a log-normal distribution function. The distribution of grain size d itself is not a normal distribution, but the distribution of l

5、ogd can be described by a normal distribution curve.,5/39,Chapter 5,The observation of how the bed material grains are distributed along a beach pro quite interesting for the purpose of understanding the influence of waves and currents on sediment movement.,6/39,Chapter 5,In order to determine the s

6、ource of littoral drift, the predominant direction of littoral transport sometimes must be predicted. In such cases, the mineral composition and spatial distribution of gravel can be used. For that purpose, coastal topography data and information on the depositional sediment state in the vicinity of

7、 coastal structures such as breakwaters, jetties, and groins are taken as valuable indices for this judgment.,7/39,Chapter 5,Coastal sediment particles are transported by the influence of waves and nearshore currents in the onshore or offshore directions, or parallel to the shoreline. There are two

8、modes of sediment movement: suspended sediment movement and bed load movement.,2. Modes of movement,8/39,Chapter 5,Incoming waves reach a certain water depth (offshore region), then bed material sand particles there begin oscillatory motion due to wave action. In a slightly more shallow area, waves

9、produce a net motion of sand particles in the onshore or offshore direction. The interesting feature in this region is the generation of sand ripples, which seem to have a strong influence on sediment movement.,9/39,Chapter 5,The inshore region is the place where distinct sediment movement appears,

10、and where longshore bars are generated by breaking waves. In this region, breaking wave action predominates to intensify the turbulent intensity of fluid motion, thus putting a large amount of sediment in suspension. The suspended sediment in this region is easily transported parallel to the shoreli

11、ne by longshore currents, or offshore by rip currents.,10/39,Chapter 5,The lower and upper limits of the foreshore region are defined by the shoreline at mean low tide and the uprush limit of the breaking waves. In this region, either suspended movement or bed load movement is dominant, depending on

12、 the breaking wave characteristics. For waves approaching the shoreline obliquely, sediment particles move foreshore by taking a zigzag path, and finally move parallel to the shoreline.,11/39,Chapter 5,The threshold of sediment motion under wave action will govern whether shingle is being transporte

13、d on a beach 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,12/39,Chapter 5

14、,As the orbital velocity of water flow over a bed of sediment is increased, a stage is reached when the water exerts a force or stress on the particles sufficient to cause them to move from the bed and be transported. This stage is generally known as the critical stage for erosion or entrainment.,13

15、/39,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,14/39,Chapter 5,For grain diameters greater than 0.5mm (coarse sands and coarser), the threshold is best predicted with,15/39,Chapter 5,The a

16、bove 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 departures from the established relationships. The latter gives good results for grain sizes larg

17、er than 0.5mm and as coarse as 5cm.,16/39,Chapter 5,For a given grain density and diameter, the threshold under waves can therefore be established by a certain wave period and orbital velocity or semi-diameter. Only two of these three parameters need to be established in defining the threshold since

18、,17/39,Chapter 5,Wave period and near-bottom orbital velocity required for threshold of motion,18/39,Chapter 5,Once the threshold wave period and orbital velocity are determined, there are of course many combinations of water depth and wave height that could yield the required orbital velocity. The

19、linearized relationship for the orbital velocity can be expressed as,19/39,Chapter 5,Water depth to which sediments can be set in motion by surface waves of periods T=15s,20/39,Chapter 5,The threshold evaluated with these two equations have been used to calculate water depths to which a range of sed

20、iment grain sizes could be set in motion by 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.,21/39,Chapter 5,For engineering application

21、s, knowledge of the water depth where sediment particles move significant distances due to wave action is important for determining the initiation point for the beach pro of the offshore region. Numerous research efforts have been conducted during the last five decades concerning the critical water

22、depth for the inception of sediment movement.,22/39,Chapter 5,Numerous research efforts have been conducted during the last five decades concerning the critical water depth for the inception of sediment movement. Various formulae for determining the critical water depth of sand movement inception ca

23、n be expressed in the following common form:,23/39,Chapter 5,The reasons why different values of coefficient are taken in each formulae are that different criteria were used for the inception of motion, and the relationship were established using data obtained under certain limited conditions.,24/39

24、,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 surface layer movement, which is defined as the state where almost all sand particles in the first layer

25、move due to wave action.,25/39,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 glass sand particles:,26/39,Chapter 5,The surface layer movement defined here corresponds to the st

26、ate 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 a water depth variation.,27/39,Quasi-steady approach 准稳定流方法,Chapter 5,5.2 Bed-load Transport,2. En

27、ergetics model 能量模型,28/39,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 troughs is slower but of longer duration.,1. Quasi-steady approach,29/39,Chapter 5,The asymmetry of th

28、e 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.,30/39,Chapter 5,Madsen and Grant (1976) introduced a formula applicable to oscillatory flow, by adapting from Browns formula for s

29、teady flow. The bed load transport rate averaged over one-half wave period, is given in dimensionless by,31/39,Chapter 5,In 1956 Bagnold introduced the idea of the bed load sediment transport based on the concept of the work done by the flow in moving the grains. This introduced an efficiency factor

30、 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,32/39,Chapter 5,Schematic of model for sand transport wherein the orbital velocity due to waves places the sand in motion, and the current provides a