CHAPTER 3
FUNDAMENTALS OF ALLUVIAL CHANNEL FLOW
3.1 INTRODUCTION
Most streams that a highway will cross or encroach upon are alluvial. That is, the rivers are
formed in cohesive or non-cohesive materials that have been, and can still be, transported by
the stream. The non-cohesive material generally consists of silt, sand, gravel, or cobbles, or
any combination of these sizes. Silt generally is not present in appreciable quantities in
streams having non-cohesive boundaries. Cohesive material consists of clays (sizes less
than 0.004 mm) forming a binder with silts and sand. Because of the electro-chemical
bonding between clay particles, clays are more resistant to erosion than silts.
In alluvial rivers, bed configuration and resistance to flow are a function of the flow and can
change to increase or decrease the velocity and water surface level. The river channel can
shift its location so that the crossing or encroachment is unfavorably located with respect to
the direction of flow. The moveable boundary of the alluvial river adds another dimension to
the design problem and can compound environmental concerns. Therefore, the design of
highway crossings and encroachments in the river environment requires knowledge of the
mechanics of alluvial channel flow.
This chapter presents the fundamentals of alluvial channel flow. It covers properties of
alluvial material, methods of measuring properties of alluvial materials, flow in sandbed
channels, prediction of bed forms, Manning's n for sandbed and other natural streams, how
bed-form changes affect highways in the river environment, beginning of motion, flow in
coarse-material streams, and physical measurement and determination of sediment
discharge in the field. These fundamentals of alluvial channel flow are used in later chapters
to develop design considerations for highway crossings and encroachments in river
environments.
In Chapter 4, sediment transport and physical and computer modeling of sediment transport
will be covered.
3.2 SEDIMENT PROPERTIES AND MEASUREMENT TECHNIQUES
A knowledge of the properties of the bed material particles is essential, as they indicate the
behavior of the particles in their interaction with the flow. Several of the important bed
material properties are discussed in the following sections (U.S. Interagency Subcommittee
1941, 1943, 1957; Richardson 1971).
3.2.1 Particle Size
Of the various sediment properties, physical size has by far the greatest significance to the
hydraulic engineer. The particle size is the most readily measured property, and other
properties such as shape, fall velocity and specific gravity tend to vary with size in a roughly
predictable manner. In general, size represents a sufficiently complete description of the
sediment particle for many practical purposes.
3.1
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