5.4.8 River Profiles and Bed Material
The slope of a river channel or a river system is usually steepest in the headwater regions.
The river profile is concave upward and the slope of the river profile can be represented by the
equation:
Sx = Soe- α x
(5.12)
where:
Sx
=
Slope at any station a distance x downstream of the reference station
So
=
Slope at the reference station
α
=
Coefficient
Similarly, the bed sediment size is coarser in the upper reaches where the channel slopes are
steep and the bed sediment size becomes finer with distance downstream. Generally, the size
of the bed material reduces with distance according to the relationship
D50 x = D50o e- β x
(5.13)
where:
D50x
=
Median size of bed material at distance x downstream of reference station
D50o
=
Median size of bed material at the reference station
=
Coefficient
β
The hydraulic geometry relations are applicable to continuous channel behavior. In some
cases, this behavior (in this case the slope) may become discontinuous as the channel pattern
changes from meandering to braided by the formation of cutoffs. Application of Equation 5.13
is illustrated with field data in Section 5.9 (Problem 7).
5.5 QUALITATIVE RESPONSE OF RIVER SYSTEMS
Many rivers have achieved a state of practical equilibrium throughout long reaches. For
practical engineering purposes, these stable reaches can be also called "graded" streams by
geologists and "poised" streams by engineers. However, this does not preclude significant
changes over a short period of time or over a period of years. Conversely, many streams
contain long reaches that are actively aggrading or degrading. These aggrading and
degrading channels pose a definite hazard to any highway crossing or encroachment, as
compared to poised streams.
Regardless of the degree of channel stability, local human activities may produce major
changes in river characteristics locally and throughout the entire reach. All too frequently the
net result of a river improvement is a greater departure from equilibrium than that which
originally prevailed. Good engineering design must invariably seek to enhance the natural
tendency of the stream toward stable conditions. To do so, an understanding of the direction
and magnitude of change in channel characteristics caused by human activity and natural
processes is required. This understanding can be obtained by: (1) studying the river in a
natural condition; (2) having knowledge of the sediment and water discharge; (3) being able to
5.28
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