Table 5.4. Change of Variables Induced by Changes in Sediment
Discharge, Size of Bed Sediment and Wash Load.
Relationship
Tendency to Braid or Meander
Qs D50 / Cf ~ S+ V + y o W +
+
-
B
Qs D50 / Cf ~ S- V - y o W -
-
+
M
Qs D50 / Cf ~ S+ V + y o W +
+
-
B
Qs D50 / Cf ~ S- V y o W
-
M
Qs D50 / Cf+ ~ S- V yo W
M
Qs D50 / Cf- ~ S+ V + yo W +
-
B
Qs D50 / Cf ~ S+ V + y o W +
+
+
-
B
Qs D50 / Cf ~ S- V yo W -
-
-
M
Qs D50 / Cf- ~ S+ V + yo W +
+
+
-
B
Qs D50 / Cf+ ~ S- V y o W
-
-
-
M
Qs D50 / Cf+ ~ S+ V + yo W +
+
+
-
B
Note: An increase in the value of the variable is denoted by a +;
and a decrease is denoted by a -. As an example, in the
first line, if the value of Qs increases, the slope, velocity,
and width will increase, the depth of flow will decrease and
the channel may tend toward a braided form.
5.5.2 Prediction of Channel Response to Change
In Section 5.5.1, it was illustrated that the proportionality of Equation 5.16 could be used to
redict changes in channel profiles caused by changes in water and sediment discharge. It is
now possible to talk qualitatively about changes in channel profile, changes in river form and
changes in river cross section both at-a-station and along the river channel using the other
relations presented above.
This can be best illustrated by application. Referring to Table 5.5, consider the effect of an
increase in discharge indicated by a plus sign on line (a) opposite discharge. The increase in
discharge may affect the river form, energy slope, stability of the channel, cross-sectional area
0.25
and river regime. Equations 5.1 and 5.2 or Figure 5.18 (which illustrates SQ ) show that an
increase in discharge could change the channel form in the direction of a braided form.
Whether or not the channel form changes would depend on the river form prior to the increase
in discharge. With the increase in discharge the stability of the channel would be reduced,
which indicates an increase in velocity. On the other hand, this prediction could be affected by
changes in form of bed roughness that dictate resistance to flow.
The information presented in Chapter 4 can be used to determine the direction of change of
hydraulic variables when sediment characteristics or discharges are varied. It is important to
notice that Einstein, Colby, and Manning's equations apply to a cross section or reach and
differ from some of the available geomorphic equations that have been derived by considering
5.32
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