Sediment Transport in Open Channels

Sediment Transport in Open Channels - 35 Sediment Transport...

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© 2003 by CRC Press LLC 35 Sediment Transport in Open Channels 35.1 Introduction 35.2 The Characteristics of Sediment Density, Size, and Shape • Size Distribution • Fall (or Settling) Velocity • Angle of Repose 35.3 Flow Characteristics and Dimensionless Parameters; Notation 35.4 Initiation of Motion The Shields Curve and the Critical Shear Stress • The Effect of Slope • Summary 35.5 Flow Resistance and Stage-Discharge Predictors Form and Grain Resistance Approach • Overall Resistance Approach • Critical Velocity • Summary 35.6 Sediment Transport Suspended Load Models • Bed-Load Models and Formulae • Total Load Models • Measurement of Sediment Transport • Expected Accuracy of Transport Formulae 35.7 Special Topics Local Scour • Unsteady Aspects • Effects of a Nonuniform Size Distribution • Gravel-Bed Streams 35.1 Introduction The erosion, deposition, and transport of sediment by water arise in a variety of situations with engi- neering implications. Erosion must be considered in the design of stable channels or the design for local scour around bridge piers. Resuspension of possibly contaminated bottom sediments have consequences for water quality. Deposition is often undesirable since it may hinder the operation, or shorten the working life, of hydraulic structures or navigational channels. Sediment traps are specifically designed to promote the deposition of suspended material to minimize their downstream impact, e.g., on cooling water inlet works, or in water treatment plants. A large literature exists on approaches to problems involving sediment transport; the following can only introduce the basic concepts in summary fashion. It is oriented primarily to applications in steady uniform flows in a sand-bed channel; problems involving flow nonuniformity, unsteadiness, and gravel-beds, are only briefly mentioned and coastal processes are treated in the section on coastal engineering. Cohesive sediments, for which physico-chemical attractive forces may lead to the aggregation of particles, are not considered at all. The finer fractions (clays and silts, see Section 35.2) that are susceptible to aggregation are found more in estuarial and coastal shelf regions rather than in streams. A recent review of problems in dealing with cohesive sediments is given by Mehta et al. (1989 a, b). D. A. Lyn Purdue University
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35.2 The Characteristics of Sediment Density, Size, and Shape The density of sediment depends on its composition. Typical sediments in alluvial water bodies consist mainly of quartz, the specific gravity of which can be taken as s = 2.65. The specific weight is therefore g s = 165.4 lb/ft, 3 or 26.0 kN/m. 3 In many formulae, the effective specific weight, which includes the effect of buoyancy, is used, i.e., ( s - 1) g , where g is the specific weight of water.
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