Fundamentals of Fluvial Geomorphology and Channel Processes
by accumulation of failed bank materials at the toe of the slope and by accumulation of berm materials.
The sequence of channel evolution is based on the assumption that the observed changes in channel
morphology are due to the passage of time in response to a single base level lowering without changes in
the upstream land use and sediment supply from the watershed. Application of the sequence assumes that
the materials forming the channel perimeter are erodible and all degrees of the channel adjustment are
possible. The sequence is applicable only in a system context, and local erosion such as in bends or caused
by deflection of flow by debris may cause difficulty in application of the sequence.
The primary value of the sequence is to determine the evolutionary state of the channel from a field
reconnaissance. The morphometric characteristics of the channel reach types can also be correlated with
hydraulic, geotechnical, and sediment transport parameters (Harvey and Watson, 1986; Watson et al.,
1988). The evolution sequence provides an understanding that reaches of a stream may differ in
appearance, but channel form in one reach is associated with other reaches by an evolving process. Form,
process, and time relate dissimilar reaches of the stream.
The USACE (1990) used the channel evolution sequence in developing regional stability curves
relating the bed slope of Type V reaches as a function of the measured drainage area. Quasi-equilibrium,
Type V reaches were determined by field reconnaissance of knowledgeable personnel. Figure 3.15 is an
example of the empirical bed slope and drainage area relationship for Hickahala Creek, in northern
Mississippi. The 95% confidence intervals of the regression line are shown. The slope-area curve is an
example of an empirical relationships that does not explicitly include the primary factors of water and
sediment discharge, sediment load, hydraulic roughness, and channel morphology.
Watson et al. (1995b) stated that stream classification is an essential element in transferring
knowledge and experience pertaining to channel design from location to location. A computer program
was developed to record a comprehensive data set for a watershed and for channel sites, and to present
alternative classification of each based on three classification systems: Schumm (1977), Rosgen (1994),
or Montgomery and Buffington (1993). A goal of the program was to develop understanding between
groups who are most familiar with only on or two of the classification systems compared. Watson et al.
(1995a) found that the improvement in stability of the incised reaches has resulted in lower channel slope
and sediment yield. Use of a previous slope-area curve data based on generally less stable channel
characteristics, results in the design of a channel that would be stable at higher sediment yields than are now
present in the more stable DEC streams. The slope-area curve must be constantly updated, or a design
method that specifically includes sediment yield should be used.
3.3
QUANTIFICATION OF THE EVOLUTIONARY SEQUENCE
The parameters of the Channel Evolution Model, Section 3.2, are difficult to quantify and to
incorporate in design guidance. The parameters can be compressed into two dimensionless
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