predict the effects and magnitude of future human activities; and (4) applying a knowledge of
geology, soils, hydrology, and hydraulics of alluvial rivers.
To predict river response to channel modifications is a very complex task. A large number of
variables are involved in the analysis. These variables are interrelated and can respond to
changes in a river system in the continual evolution of river form. The channel geometry, bars,
and forms of bed roughness all change with changing water and sediment discharges.
Because such a prediction is necessary, methods have been developed to predict the
response of channel systems to changes both qualitatively and quantitatively.
5.5.1 General River Response to Change
Quantitative prediction of response can be made if all of the required data are known with
sufficient accuracy. Usually, however, the data are not sufficient for quantitative estimates, and
only qualitative estimates are possible. Examples of studies that have been undertaken by
various investigators for qualitative estimates follow. Lane (1955) studied the changes in river
morphology caused by modifications of water and sediment discharges. Similar but more
comprehensive treatments of channel response to changing conditions in rivers have been
presented by Leopold and Maddock (1953), Schumm (1971, 1977), Santos-Cayado (1972),
Richards (1982), ASCE (1983), Thorne et al. (1997), Thorne (1998), and Knighton (1998).
Research results support the following general statements:
1. Depth of flow is directly proportional to water discharge and inversely proportional to bed
material discharge,
2. Width of channel is directly proportional to water discharge and to bed material discharge,
3. Shape of channel expressed as width-depth ratio is directly related to bed material
discharge,
4. Meander wavelength is directly proportional to water discharge and to bed material
discharge,
5. Slope of stream channel is inversely proportional to water discharge and directly
proportional to bed material discharge and grain size, and
6. Sinuosity of stream channel is proportional to valley slope and inversely proportional to bed
material discharge.
It is important to remember that these statements pertain to natural rivers and not necessarily
to artificial channels with bank materials that are not representative of sediment load. In any
event, the relations will help to determine the response of water conveying channels to change.
Bed material sediment transport (Qs) can be directly related to stream power (τoV) and
inversely related to the fall diameter of bed material (D50).
τ o V W Cf
Qs ~
(5.14)
D50
Here τo is the bed shear stress, V is the cross-sectional average velocity, W is the width of
the stream and Cf is the volumetric concentration of fine sediments. Equation 5.14 can be
written as:
5.29
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