changes in a variable can change the planview and profile of a river, adversely affecting a
highway crossing or encroachment. Conversely, the highway crossing or encroachment can
inadvertently change the planview or profile, adversely affecting the river environment. In this
section, stream form is classified and channel processes are discussed.
5.4.1 Classification of River Channels
Brice and Blodgett (1978) developed a simple classification scheme oriented primarily toward
lateral stability of rivers. The common geomorphic terms for the various types of streams (e.g.,
meandering, braided) are shown in Figure 5.11. Each term is defined on the small sketches.
This classification is also used in HEC-20 (Lagasse et al. 2001) as a basis for identifying
geomorphic factors important to stream stability analyses.
Additional information on specific channel features is illustrated in Figure 5.12. Classification
based on oxbow lakes is illustrated in Figure 5.12a. In Figure 5.12b, types of meander scroll
formations are shown. By studying scroll formations in terms of age of vegetation the rate and
direction of channel migration can be quantified. The sinuosity index is the ratio of the length of
the watercourse over the valley length between the same points. Classification based on
natural levees is illustrated in Figure 5.12c. Well developed levees are associated with older
rivers. The floodplain that is broad in relation to the channel width is indicative of an older river.
Conversely, when the river valley is narrow and confined by terraces or valley walls, the river is
usually mature. In general, the growth of vegetation (tree cover) is indicative of the presence of
silts and clays in the river banks and the floodplain. This is particularly true if the floodplain is
well drained. With good drainage, the silt and clay are essential to the growth of vegetation
because of their water holding capability.
Natural levees are a characteristic of alluvial river systems. Levees form during floods as the
river stage exceeds bankfull conditions. Sediment is then deposited on the floodplain due to
the reduced velocity and transporting capacity. The natural levees near the river are rather
steep because coarse material drops out quickly. Farther from the river the gradients are
flatter and the finer materials drop out. Beyond the levees are the swamp areas. On the lower
Mississippi River, natural levees on the order of 10 ft in height are common. The rate of growth
of natural levees is slower after they reach a height equal to the average annual flood stage.
A detailed knowledge of the hydraulic characteristics of different types of streams is of great
value when dealing with the location of highway crossings and encroachment, training works,
flood control works and other river structures. A channel classification in Figure 5.13 shows the
relative stability and types of hazards encountered. Figure 5.13 is also useful in making a
qualitative assessment of stream stability based on stream characteristics. It shows that
straight channels are relatively stable only where flow velocities and sediment load are low. As
these variables increase, flow meanders in the channel causing the formation of alternate bars
and the initiation of a meandering channel pattern. Similarly, meandering channels are
progressively less stable with increasing velocity and bed load. At high values of these
variables, the channel becomes braided. The presence and size of point bars and middle bars
are indications of the relative lateral stability of a stream channel. Bed material transport is
directly related to stream power, and relative stability decreases as stream power increases as
shown by Figure 5.13. An example is given in Section 5.9 (Problem 2) on how to use this
classification.
5.13
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