changes to C'A, indicating a decrease in bed elevation and slope in the downstream channel
with time. Note, however, if the dam fills with sediment so that the incoming sediment
discharge passes through, that, except for local scour at the dam, the grade line C'A would
return to the line CA. Also upstream of the dam the grade would return to the original
equilibrium grade but would be offset vertically by the height of the dam. Thus small reservoirs
(storage capacity small in relation to annual discharge) may cause degradation below the dam
and then aggradation over a relatively short period of time.
Figure 5.22. Changes in channel slope in response to a dam at point C.
The engineer is also interested in quantities in addition to qualitative trends. The geomorphic
relation QS ~ QsD50 is only an initial step in analyzing long-term channel response problems.
However, this initial step is useful because it warns of possible future difficulties related to
channel modifications and flood protection works. The prediction of the magnitude of possible
errors in flood protection design, because of changes in stage with time, requires the
quantification of changes in stage. To quantify these changes, it is necessary to be able to
quantify future changes in the variables that affect the stage. In this respect, knowledge of the
future flow conditions is necessary.
In many instances it is important to assess the effects of changes in water and sediment
discharge on specific variables such as depth of flow, channel width, characteristics of bed
materials and velocity. For this type of analysis, we can use Equation 5.15 as follows:
QS D50 / Cf ~ SV yo W
(5.17)
Using this form, the response of a river system to changes in specific variables is given in
Table 5.4. A plus (+) sign signifies an increase in the value of the variable and a minus (-) sign
signifies a decrease in the value of the variable. The letter B indicates an increase in the
1/4
product SQ and a shift toward a braided condition and the letter M indicates a reduction in
1/4
SQ and a shift toward the meandering condition (see Section 5.4.5). No attempt is made
here to determine whether or not the channel actually braids or meanders.
5.31
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