TAMU OCEN 682 - Chapter3_usbr (112 pages)

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Chapter3_usbr



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Chapter3_usbr

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Pages:
112
School:
Texas A&M University
Course:
Ocen 682 - Coastal Sediment Procs
Coastal Sediment Procs Documents
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Chapter 3 Noncohesive Sediment Transport Page 3 1 Introduction 3 1 3 2 Incipient Motion 3 1 3 2 1 Shear Stress Approach 3 2 3 2 2 Velocity Approach 3 7 3 3 Sediment Transport Functions 3 12 3 3 1 Regime Approach 3 12 3 3 2 Regression Approach 3 14 3 3 3 Probabilistic Approach 3 16 3 3 4 Deterministic Approach 3 17 3 3 5 Stream Power Approach 3 23 3 3 5 1 Bagnold s Approach 3 23 3 3 5 2 Engelund and Hansen s Approach 3 25 3 3 5 3 Ackers and White s Approach 3 25 3 3 6 Unit Stream Power Approach 3 28 3 3 7 Power Balance Approach 3 32 3 3 8 Gravitational Power Approach 3 34 3 4 Other Commonly Used Sediment Transport Functions 3 36 3 4 1 Schoklitsch Bedload Formula 3 36 3 4 2 Kalinske BedloadFormula 3 37 3 4 3 Meyer Peter and Miiller Formula 3 39 3 4 4 Rottner Bedload Formula 3 40 3 4 5 Einstein Bedload Formula 3 41 3 4 6 Laursen Bed Material Load Formula 3 41 3 4 7 Colby Bed Material Load Formula 3 42 3 4 8 Einstein Bed Material Load Formula 3 44 3 4 9 Toffaleti Formula 3 44 3 5 Fall Velocity 3 45 3 6 Resistance to Flow 3 47 3 6 1 Einstein s Method 3 49 3 6 2 Engelund and Hansen s Method 3 54 3 58 3 6 3 Yang s Method 3 7 Nonequilibrium Sediment Transport 3 63 3 8 Comparison and Selection of Sediment Transport Formulas 3 63 3 8 1 Direct Comparisons with Measurements 3 64 3 8 2 Comparison by Size Fraction 3 73 3 8 3 Computer Model Simulation Comparison 3 77 3 8 4 Selection of Sediment Transport Formulas 3 83 3 8 4 1 Dimensionless Parameters 3 85 3 8 4 2 Data Analysis 3 86 3 8 4 3 Procedures for Selecting Sediment Transport Formulas 3 102 3 104 3 9 Summary 3 10 References 3 104 Chapter 3 Noncohesive Sediment Transport by Chih Ted Yang 3 1 Introduction Engineers geologists and river morphologists have studied the subject of sediment transport for centuries Different approaches have been used for the development of sediment transport functions or formulas These formulas have been used for solving engineering and environmental problems Results obtained from different approaches often differ drastically from each other and from observations in the field Some of the basic concepts their limits of application and the interrelationships among them have become clear to us only in recent years Many of the complex aspects of sediment transport are yet to be understood and they remain among the challenging subjects for future studies The mechanics of sediment transport for cohesive and noncohesive materials are different Issues relating to cohesive sediment transport will be addressed in chapter 4 This chapter addresses noncohesive sediment transport only This chapter starts with a review of the basic concepts and approaches used in the derivation of incipient motion criteria and sediment transport functions or formulas Evaluations and comparisons of some of the commonly used criteria and transport functions give readers general guidance on the selection of proper functions under different flow and sediment conditions Some of the materials summarized in this chapter can be found in the book Sediment Transport Theory and Pmctice Yang 1996 Most noncohesive sediment transport formulas were developed for sediment transport in clear water under equilibrium conditions Understanding sediment transport in sediment laden flows with a high concentration of wash load is necessary for solving practical engineering problems The need to consider nonequilibrium sediment transport in a sediment routing model is also addressed in this chapter 3 2 Incipient Motion Incipient motion is important in the study of sediment transport channel degradation and stable channel design Due to the stochastic nature of sediment movement along an alluvial bed it is difficult to define precisely at what flow condition a sediment particle will begin to move Consequently it depends more or less on an investigator s definition of incipient motion They use terms such as initial motion several grain moving weak movement and critical movement In spite of these differences in definition significant progress has been made on the study of incipient motion both theoretically and experimentally Figure 3 1 shows the forces acting on a spherical sediment particle at the bottom of an open channel For most natural rivers the channel slopes are small enough that the component of gravitational force in the direction of flow can be neglected compared with other forces acting on a spherical sediment particle The forces to be considered are the drag force FD lift force FL submerged weight W y and resistance force FR A sediment particle is at a state of incipient motion when one of the following conditions is satisfied Err sionand Secimerzt itionManual w 3 Figure 3 1 Diagram of forces acting on a sediment particle in open channel tlow Yang 1973 where Mo overturning moment due to FD and FL and MR resisting moment due to FL and W Most incipient motion criteria are derived from either a shear stress or a velocity approach 3 2 1 Shear Stress Approach One of the most prominent and widely used incipient motion criteria is the Shields diagram 1936 based on shear stress Shields assumed that the factors in the determination of incipient motion are the shear stress r the difference in density between sediment and fluidp p the diameter of the particle d the kinematic viscosity v and the gravitational acceleration g These five quantities can be grouped into two dimensionless quantities namely and Chclpter 3 Noncohesive Sedirnenf Transport where p and pf y densities of sediment and fluid respectively specific weight of water U shear velocity and t critical shear stress at initial motion The relationship between these two parameters is then determined experimentally Figure 3 2 shows the experimental results obtained by Shields and other investigators at incipient motion At points above the curve the particle will move At points below the curve the flow is unable to move the particle It should be pointed out that Shields did not fit a curve to the data but showed a band of considerable width Rouse 1 939 first proposed the curve shown in Figure 3 2 Although engineers have used the Shields diagram widely as a criterion for incipient motion dissatisfactions can be found in the literature Yang 1973 pointed out the following factors and suggested that the Shields diagram may not be the most desirable criterion for incipient motion x I 1 00 0 80 0 60 x Sand U S WES A Sand Gilbert j 2 f 0 30 g 0 20 2 65 2 65 Sand in air White 2 10 9 a P j 0 06 0 05 0 04 0 03 0 2 0 4 0 6


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