With highways in the sandbed river environment, care must be taken in analyzing the
crossing in order to foresee possible changes that may occur in the bed form and what these
changes may do to the resistance coefficient, to sediment transport, and to the stability of the
reach and its structures.
3.4.13 Alluvial Processes and Resistance to Flow in Coarse Material Streams
The preceding discussion of alluvial channel flow is mainly related to sandbed channels; that
is, channels with noncohesive bed materials of size less than 2 mm. The analysis of
coarse-material channels is also pertinent to highway engineering. This classification
includes all channels with noncohesive bed materials coarser than 2 mm size.
The behavior of coarse material channels is somewhat different from sandbed channels.
The main distinction between the two channels lies in the spread of their bed material size
distribution. In sandbed channels, for example, the bed may consist of particles from 0.02 to
2 mm; i.e., a 100-fold size range. In coarse-material channels, even if the maximum size is
limited to cobbles (250 mm), the size range of particles may be 0.10 to 250 mm, which is a
2,500-fold size range. In general, coarse-material channels are less active and have slower
rates of bank shifting than sandbed channels. However, the tendency for channel armoring is
more pronounced in coarse material channels as discussed next.
Armoring. The phenomenon of armoring in mobile bed channels occurs by the
rearrangement of bed material during movement. The bed is covered by a one particle thick
layer of the coarser material underlain by the finer sizes. The absence of finer sizes from the
surface layer is caused by the winnowing away of these sizes by the flow. As the spread of
particle sizes available in the bed of coarse-material channels is large, these channels can
armor their beds and behave as rigid boundary channels for all except the highest flows.
The bed and bank forming activity in these channels is therefore limited to much smaller
intervals of the annual hydrographs than the sandbed channels.
The general lack of mobility in coarse material channels also means the bed forms do not
change as much or as rapidly as in sandbed channels. The roughness coefficients in coarse
material channels are therefore more consistent during the annual hydrographs than in
sandbed channels. Most of the resistance to flow in coarse material channels comes from
the grain roughness and from bars. The river bed forms (dunes) are less important in the
hydraulic behavior of coarse bed channels.
Sampling. The purpose of bed material sampling in coarse-material channels is: (1) to
determine the conditions of incipient movement; (2) to assess the bed roughness related to
the resistance to flow; (3) to determine the bed material load for a given flow; and (4) to
determine the long and short time response of the channel to specific activities. For
objectives (1) to (3), the properties of the surface layer are needed. If it is anticipated that
the bed layer will be disrupted at any given stage, it is necessary to take an adequate sample
of both the surface and subsurface material.
The surface sampling can be easily done on the channel bed by counting particles on a grid,
as explained earlier in this chapter. However, special effort should be made to obtain an
objective sample. There is a tendency to select too many large particles. The scoop sample
with bed material sizes larger than 25.4 mm (an inch) or so is difficult to obtain and such
samples may have to be collected from bars and other exposed areas on channel perimeter.