Geoscience Reference
In-Depth Information
(a)
Figure 14.14
(a) Geometry and (b) hydraulic efficiency in
stream channels. Principal parameters are
identified in the text; h
1
-h
2
and S
1
-S
2
represent the height decline in bed and
water surface respectively along segment
length L. (b) shows the combined flow of
two tributary streams, A and B, downsteam
in C, assuming a uniform velocity of 1 m
sec
-1
. C has a greater channel efficiency,
reflected by the increase in R; R is still
greater than 2.0 even if the stage (depth)
falls towards 4 m.
w
1
h
1
flow
L
w
w
2
Ad
h
2
w
p
(b)
2 m
4 m
A
B
Q = 24
cumecs
1
Q = 24 cumecs
R
= 1.50
3
0
2
0123456789 0 1 2 m
R
= 1.71
1
0
0123456 m
6 m
C
5
4
Q = 48 cumecs
3
R
= 2.40
2
1
0
012345678 m
for declining channel slope and potential energy.
Discharge varies at-a-point with the hydrographic
response to quickflow and delayed flow and increases
downstream as tributaries contribute water. However,
these parameters cannot be considered in isolation from
channel form, which interacts with river flow. We look first
at the water behaviour in channels before considering
their geomorphic development.
valley floor briefly becomes part of the channel. Whilst
regular flooding is indicative of channels poorly adjusted
to discharge, there are excessive 'costs' in maintaining
channels which never flood. It is easy to see this in the case
of large, engineered channels with higher
financial
costs
in land and construction, but it is also evident in the
energy
costs of natural channels. In bedrock, rivers often
cut
channel-in-channel
forms and shrink into the smaller
channel during low flows. River flow cannot sustain large
channels in soft sediments at low flows, and
bank caving
effectively reduces channel dimensions (
Plate 14.8
).
Deep water, gliding smoothly down a channel as
tranquil flow, may enter a steeper segment over a visible
fall or
hydraulic drop
in the water surface. Water depth falls
as rapid flow develops and the water surface becomes
disturbed (
Plate 14.9
).
If the river then enters another less
steep segment, the transition back to deeper tranquil flow
is marked by a
hydraulic jump
or standing wave. This also
clarifies the links between the shape of the water surface
and bed forms in sandy channels, both of which can
change over short distances and time scales. As
F
increases
from values well below 1, small sand bed ripples are
transformed to larger dunes out of phase with surface
River channels
Links between stream flow and channel morphology,
which
conserve
discharge from one segment to another in
the example given (see
Figure 14.14
),
must also embrace
changes
in discharge over time. It might be argued
that ideal channels should accommodate most, but not
all, peak discharges and that
bankfull discharge
leading
to flooding should be exceeded either annually or only
after abnormally wet periods or high rainfall events.
Channel efficiency decreases and friction losses increase
as discharge falls in fixed-geometry channels. Similarly,
wetted perimeter increases and efficiency decreases
dramatically in overbank conditions when the flooded
