Environmental Engineering Reference
In-Depth Information
One of the reasons this distinction is important is that for a supercritical low, disturbances due to
an obstruction in the stream can only propagate downstream. So, if you change the structure of the
stream, for example, it cannot have an impact upstream. However, if the low is subcritical, then
an obstruction may have an impact upstream. For example, a boulder or debris low may create a
backwater effect or increase the water-surface elevation upstream. Channel slopes or gradients that
result in supercritical lows are called steep slopes in hydraulics, while those producing subcritical
slopes are called mild . A natural low in these regions also gradually varies , but in a zone where the
low changes from steep to mild, or supercritical to subcritical, there is generally a rapid and large
increase in water-surface elevations, and a large loss in energy. These transitions are referred to as
hydraulic jumps (Figure 2.13), and the low is categorized as rapidly varied . Hydraulic jumps may
occur naturally or they may be induced downstream of man-made structures such as spillways; they
are important because of the energy that is dissipated and the increased mixing that results from the
highly turbulent lows.
The slope or gradient, and the resulting stream velocities, also has a large impact on whether the
low will erode the channel, carry or transport materials suspended in the low, or allow those mate-
rials to be settled out of the low and deposited. Lunetta et al. (1997) categorized stream reaches,
based on the stream slope, as:
>12%: source reaches since any material falling into a channel of that steepness in a storm
event will immediately move downstream.
4%-12%: transport reaches since materials still tend to move through these areas because of
high hydraulic energy.
<4%: response reaches where the energy of the stream drops and sediment tends to remain
in residence for longer periods.
2.2.1.4.2 Valley Width and Coninement
The transport and transport characteristics of rivers will vary as a function of the balance between
a particular gradient low, the sediment supply, and the valley width and coninement (Figure 2.14).
Take, for example, the wetted perimeter of a river. The wetted perimeter is the perimeter of the
cross section that is wet, or the perimeter over which the bed is in contact with water. So, a wide and
shallow river would have a greater wetted perimeter, more contact with the bed and therefore more
friction. Entrenched rivers would have less friction because they have a lower wetted perimeter.
EGL
Slower turbulent
flow region
h L
V 2
2 g
V 1
Hydraulic
jump
2 g
Rapid laminar
flow region
h 2
V 2
V 1
h 1
FIGURE 2.13
Illustration of a hydraulic jump.
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