Geotechnical Manual for Slopes

100
A = wetted cross-sectional area (m 2 ),
P = wetted perimeter (m), and
S = gradient of channel
Stepped channels are not particularly effective as energy dissipators.
However, there would seem to be no practicable alternative. The flow in
stepped channels is turbulent, and sufficient freeboard must be allowed for
splashing and aeration. The stepped channel details shown in Figure 8.8 make
some allowance for splashing and are the most effective used in Hong Kong to
date for reducing the velocity of flow. In the absence of any experimental
data, the size of the stepped channel and gradient of the invert may be
determined using Figure 8.7 by assuming a velocity of 5 m/sec through the
minimum section (at the top of each step). At the top of slopes, the velocity
is lower and the cross-sectional area of flow greater, but splashing and
aeration is less. Therefore, the section adopted for the stepped channel may
also be used to cross narrow berms.
8.3.5 Changes in Direction
At any change in direction, the pattern of flow in the channels is affected.
Channels in which the velocity is approximately 2 m/sec should change direction
through bends of radius not less than three times the width of the channel.
This radius should be increased where the velocity is greater than 2 m/sec or,
alternatively, sufficient freeboard should be provided to contain the
superelevation of the water surface (Chow, 1959).
Where a stepped channel crosses a berm, a hydraulic jump may form which
must be contained within the channel. The splash allowance provided for the
stepped channel may therefore be extended across the berm.
8.3.6 Junctions of Channels
Junctions of channels pose the greatest problem when designing slope
drainage. They inevitably cause turbulence and splashing, and any chamber
constructed to contain this is vulnerable to blockage by debris. Avoidance
of such chambers is recommended, except possibly at the base of the slope, where
the deep channels required to contain the splashing would cause a hazard if
left uncovered.
At junctions, the smaller channel or channels should be brought in at
half the width of the main channel above the invert of the main channel. The
channels should be deepened with an added freeboard allowance to contain the
turbulence, splashing and backwater effects.
Where channels are to discharge into a stepped channel crossing a berm,
they should be curved into the stepped channel.
If excessive splashing and turbulence is expected at a particular
junction or change in direction, consideration should be given to providing
a baffle wall, as shown on Figure 8.9, or a catchpit, as shown on Figure 8.10.
The tops of all channels should be flush with the slope surface
(Figure 8.11). Where possible, an apron that drains towards the channel